Catherine Perron
email: catherine.perron@sciencespo.fr

Reimagining German identity through the politics of memory: Changing interpretations of German past migrations during

Keywords: Germany, memory, migration, flight and expulsion, politics of history Expanding German Identity Boundaries

This paper brings memory and migration studies together. It focuses on the way the past was used in the context of the "refugee crisis" in Germany in 2015/2016. The analysis concentrates on how politicians and journalists used the memory of Germans' own migrations to legitimise rhetorically the political decision to open the borders and let more than a million people into the country, as well as to call for a welcoming attitude (Willkommenskultur) towards the refugees. It shows how, by doing so, they have contributed to the reframing of one of the founding identity narratives of the FRG, namely the one about "flight and expulsion", and thus helped to redraw Germany's identity boundaries in a more inclusive way.

In the interplay between migrants and their hosting societies, the focus of research is often on how migrants can identify with a society that has markedly different histories and memories from their own, or on "how they have succeeded in appropriating it [the dominant culture] for themselves often by modifying it to the point at which the latter is enriched through contact with foreign contributions" (Noiriel 2007, 15). 1 Or it is about the break-up of homogenic national identity constructions through the pluralisation of identity discourses and their underlying memory (Baussant et al. 2017). However, as Irial Glynn and Olaf Kleist have pointed out, "scholars have rarely combined Migration Studies and Memory Studies to consider how perceptions of the past affect the incorporation of immigrants in their host societies" (2012, 4). This is the focus I intend to adopt in this paper.

The collective process of remembering is essential to the self-definition of societies (Renan, 1882, Castoriadis, 1975, Anderson 1983). It provides the nation with a sense of continuity and a shared image of the group. But memory as "vis"/ force also plays a significant role in political change (Assmann, 2009, pp. 27-32, Assmann & Shortt 2012).

Memory is an active social process carried out by different actors that originates in present needs and is tightly bound to forgetting. Remembering or, in other words, shaping past events into a narrative entails silences, renewals, distortions, shifts and reevaluations (Assmann, 2009, p. 29). This power to revise the past, to question ongoing representations, values and attitudes inherent to memory opens new frames of action. Starting from these assumptions, my aim is to analyse the uses of the past in Germany during what has been called the "refugee crisis" of the early twenty-first century.

First, it is necessary to address the matter of which past was mobilised, since there have been several waves of migration to and from Germany historically. Significantly, it was neither the experience of the most recent massive arrival of asylum seekers, at the beginning of the 1990s, 2 nor the arrival of the Gastarbeiter in the 1950s and 1960s that were most often referred to during the 2015/2016 "crisis", 3 but rather an older experience dating back to the Second World War, namely the "flight and expulsion" of Germans from Eastern and Central Europe at the end of and immediately following the war.

1 All translations are mine. 2 A time not only when East Germans left the GDR/New Bundesländer, but also when hundreds of thousands of late re-settlers (Spätaussiedler) arrived from the USSR and Romania, along with hundreds of thousands of asylum seekers from Yugoslavia.

3 This is especially noteworthy since this last "refugee crisis" is much more recent and since the solidarity movement that developed around 2015 is rooted in the mobilisation for refugees that emerged in the 1990s. (Ulrike Hamann, Serhat Karakavali 2016, 74).

After reviewing how this still controversial chapter of German history was used by politicians and journalists to legitimise rhetorically the present political decision to open the borders and to call for a welcoming attitude, I show the simultaneous resistance to this comparison of fate (Schicksalsvergleich)4 revealing the power struggles over the interpretation and uses of history. I explain why referring to the Germans own experience of migration is anything but obvious in this context. Resistance can be explained by how the memory of "flight and expulsion" has been built (Bazin, Perron 2018). Analysing a few of the narratives on which this memory is based helps elucidate why the comparison has been contested, and why, despite massive waves of immigration, the country long continued to define itself as not a country of immigration.

I then demonstrate that the current political and media uses of this past change the frame in which "flight and expulsion" is remembered. I consider how, by deconstructing some myths attached to "flight and expulsion" the opinion leaders reframed one of the founding identity narratives of the FRG, so as to transform the representation of who a German is and who a German can be, and how this, by softening the rigidity of the borders between newcomers and natives (Bertossi, Duyvendak, Foner in this volume) -aimed at enforcing a more political vision of citizenship and deconstructing culturalist conceptions that used to be predominant until recently in German politics as well as in German law. 5 The analysis is based on a systematic study of the political and media discourse during the so-called refugee crisis: of Chancellor Angela Merkel, President Joachim Gauck, the federal minister of the interior, and the minister-presidents (Ministerpräsidenten) of the sixteen states (Bundesländer), as found in government declarations and speeches delivered in national and regional parliaments, as well as interviews in the media6 and federal and regional parliamentary debates. It is also based on observations gathered in Germany between mid-2013 and the end of 2017.

The "Refugee Crisis" of 2015/2016

Over night on September 4, 2015, Chancellor Merkel unilaterally decided to open the border between Germany and Austria, suspending de facto the Dublin Agreement. Tens of thousands of refugees who had come through the "Balkan route" via Turkey crossed this border in the next days.7 By the end of the year about 1.1 million refugees had entered Germany (of which about two thirds had arrived between September and December), causing the so-called refugee crisis (see Oltmer 2016).

However impressive this influx was in the autumn of 2015, it did not come completely unexpectedly. Since 2010 Germany had had to cope with a steadily growing number of refugees. Before 2015, most were from the Balkans, but that summer the situation changed and the number of migrants from Syria, Afghanistan, and Iraq, all of them asylum seekers, dramatically rose (Migrationsbericht 2014, 37).

The federal government and the governments of the Bundesländer acted during the next months following the maxim: "Those who need and are seeking for protection, shall obtain protection" in Germany, 8 a position that was widely backed. Throughout the political spectrum only a few voices openly criticised the decision to open the borders. In the media, the tone was overwhelmingly positive in favour of welcoming the refugees. Even Bild Zeitung, Germany's leading tabloid, usually known for its right-wing populist positions against foreigners, called loudly for a welcoming attitude. 9

The purportedly cold-hearted Germans who had just been blamed for their lack of solidarity with the Greeks during the Euro crisis demonstrated an impressive generosity and readiness to help. Migrants stepping out of trains coming from Hungary were acclaimed at Munich Central Station (and elsewhere) in a way that was puzzlingly reminiscent of the arrival of trains full of East-German refugees coming from Prague in the summer and autumn of 1989. An immense mobilisation took hold of the country. Thousands of volunteers spent days and nights helping to relieve the basic needs of the refugees, making a humanitarian arrival in the unprepared FRG possible and compelling admiration of the German "welcoming culture". All this seemed to acknowledge Angela Merkel's mantra "Wir schaffen das" (we will manage it).

However, the influx of refugees in 2015 represented a huge challenge for German politics and society at all levels, and the general welcoming attitude and empathy towards the refugees' fate could not mask the strengthening of a new national-populism, which manifested itself on German streets and in public places in more and more violent forms of rejection of foreigners (like in the activities of the Patriotic Europeans against the Islamisation of the Occident [PEGIDA]), 10 and through the electoral successes of the Alternative for Germany AfD (Party). Radical anti-immigration discourse and attitudes moved from the margins of German society to its core. The "uncontrolled" migration flow gave the demonstrations of right-wing groups a fresh impetus and a perfect theme around which to crystallise the anger of the so-called Wutbürger coming from the middle classes.

"Flight and Expulsion" (Flucht und Vertreibung) and the "Refugee Crisis"

In this context, references to German experiences of "flight and expulsion" were pervasive. In its popular understanding "flight and expulsion" refers to the fact that "after WWII the German population of German territories and of East central European countries [usually a number of 12-14 million people is mentioned] were expelled by their enemies" (Hahn and Hahn 2010, 9) and settled in what remained of the German Reich (the four occupation zones and later the FRG and GDR). It became one of the founding narratives of the FRG's national self-understanding, the memory of it being officially supported through commemorations and by financial means allocated for preserving the culture of the territories of expulsion (Vertreibungsgebiete) in the consciousness of the German people, following section 96 of the 1953 Federal Law on Refugees and Exiles (Bundesvertriebenengesetz) (Perron 2016).

This past was repeatedly referred to in 2015/2016 in connection with the fate of the refugees at all levels (local, regional, and national) by actors comparing "then and now" (Foner 2006).

The Ministerpräsidenten of the western 11 Bundesländer, whatever their political colour, used this comparison in speeches given in reaction to the opening of the border and the arrival of tens of thousands of people every week. For example, on September 10, 2015, 10 PEGIDA was created in autumn 2014 and organised weekly demonstrations -mainly in Dresden -against immigration and asylum policies in Germany and in Europe. See the Chronik Flüchtlingsfeindlicher Vorfälle of the Antonio-Amadeo Stiftung and the maps done in common with the Stiftung PRO-ASYL. https://www.mutgegen-rechte-gewalt.de/chronik-karte and https://www.mut-gegen-rechte-gewalt.de/service/chronik-vorfaelle (accessed October 4, 2017). 11 In the New Bundesländer this reference was strikingly absent. Two explanations can be found to this. First, the memory of "flight and expulsion" has been silenced in the GDR, and therefore it is much less present there. Second, 2015 was the 25th anniversary of the unification and of the (re)creation of the New Bundesländer.

Stephan Weil (SPD), Ministerpräsident from Lower Saxony concluded his government declaration with the words:

From the very beginning Lower Saxony was the Land of the flight. After the Second World War, more than a quarter of our population were expellees (Vertriebene). There is numerous evidence for the fact that these people were not welcomed with open arms in Lower Saxony at the end of their flight in 1945, but on the contrary often with distance and rejection. (…) Many expellees felt their new homeland as a cold homeland. At the same time, we know that it was in particular expellees who made an essential contribution to reconstruction.

Today we are again confronted with humans that came to us out of deepest distress, that seek protection. Today we can show in Lower Saxony, that we can do better. (…) I am certain: We will manage it (Wir schaffen das).

His colleague from Hessen, Volker Bouffier (CDU), on September 19, 2015, at the occasion of the 70 Years of Proclamation of Hessen, similarly finished his speech by saying: I would like to use the opportunity to once point [out] the fact that this land, Hessen, had received until 1950 one million refugees. One million. In a completely devastated country, in which people were indeed fighting for survival. This one million was an immense challenge. The people that came to us, the refugees, the expellees, they had not only lost their homeland (Heimat) but also many relatives. They came into a situation in which they were not at all welcomed with open arms. What we witness today as a refugee welcoming culture (Flüchtlingswillkommenskultur), this did not exist at the time. Nobody was standing at the train station shouting "welcome to Germany". They were very different times. […] In some ways it can be compared with today in some ways of course not. The people that came at the time were Germans. It is also because of this personal experience that I know about the difficulties that we will have to face, but I also know about the opportunities.

We want to integrate the people that will stay permanently here. This will be demanding for us, but it also will be very demanding for the refugees. […] Baden Wuerttemberg has always been a land of immigration and has proved that it can successfully integrate people: expellees, Gastarbeiter, refugees from the Balkans. We are able to manage this (Wir können es schaffen) if we go about it with courage, with pragmatism, and with realism.

A little later, even in the eastern state of Thüringen, Ministerpräsident Bodo Ramelow (Die Linke) said in a government declaration on November 26, 2015 dedicated to "Thuringia in the 25th year of German unity": Thuringia counted on the basis of the official forecast of the federal government for the whole year 2015 with about 8,000 refugees. Almost as many came in October alone. In total, we expect around 30,000 refugees to arrive in Thuringia this year.

There is little to suggest that the influx will quickly subside. […] It is precisely the broad look back at the 25 years since Reunification that opens up a valuable perspective for us.

Valuable firstly, because it imposes on us a little more prudence with regard to our current tasks, in view of what we have achieved under initially difficult conditions.

Here I like to quote the former Ministerpräsident of Thuringia, who said […] "A people that has managed reunification will also manage that".

Valuable, secondly, because placing the refugee crisis in its historical context provides an important change of perspective: from uncertainty, even fear, to couragefrom burden to opportunity. The integration of hundreds of thousands of people will, if it succeeds as it did more than half a century ago with the integration of 12 million expellees, become a building block for the completion of inner unity.

We will change together and move together anew. In Europe, in Germany, in Thuringia.

The heads of other western Bundesländer also used this parallelism: Olaf Scholz (SPD), Regierender Bürgermeister of Hamburg, in a speech on September 12, 2015, Thorsten Albig (SPD) of Schleswig-Holstein in his governmental declaration of September 18, 2015, and Malu Dreyer (SPD), from Rhineland-Palatinate, also in a governmental declaration on September 23, 2015. [START_REF] See | Regierungserklärung: Flüchtlinge in Schleswig-Holsteinzusammenstehen, helfen, gemeinsam Heimat schaffen[END_REF] However, although it was especially prevalent in the months after the opening of the border, the parallelism was used earlier, including by Thorsten Albig 14 , Malu Dreyer 15 , Stefan Weil 16 , Bodo Ramlow 17 , Rainer Haseloff 18 , Winfried Kretschmann 19 , Katrin Göring-Eckhard, 20 and even Horst Seehofer [START_REF]Museum der Bayrischen Geschichte[END_REF] . The most prominent politician, and maybe the first to use this "comparison of fate", was President Gauck at the federal level, who did so repeatedly from 2013 on. 22 The first National Remembrance Day for the victims of flight and expulsion on June 20, 2015, offered him the occasion (the more so because this day is coupled with World Refugee Day) to develop the parallelism at length. Among other things he made the following plea:

Today we want to talk about those who have been uprooted. About refugees and expellees, who have been forced into emigration. About those who were driven out of their homes in the past and those who will be forced to leave their homes today and tomorrow. [...] For they belong together in a quite essential manner -the fate of people back then and the fate of people today, the grief and the expectations back then and the fears and hopes for the future of today. [...] I wish the memory of those who fled or were expelled back then could enhance our understanding for those who have fled or been displaced today. And vice versa: our encounters with those who are uprooted today could enhance our empathy with those who were uprooted back then. […] 70 years ago, a poor and ruined Germany managed to integrate millions of refugees. Let us not think ourselves capable of too little today. Let us trust in the energies this country has.

A few months later, during a visit to a Jewish community in Oranienburg, on the anniversary of the Reichskristallnacht, November 9, 2015, he was questioned by journalists about recent political developments and repeated "that flight and expulsion have marked the fate of the German nation during decades". He continued calling the Germans to "remember how it was in many families, when we were homeless (Heimatlos) and we needed protection". And he finished saying "This means that this country will stay open and that it will talk with dignity and decently about the burden it can withstand and about its welcoming capacity. That we are ready to welcome is a fact, it's a part of our culture". These declarations, like the speech of June 20, were relayed by numerous media and were to be seen in the evening journal of the ARD the same day.

The abundant use of this parallelism, which may be explained as resulting from a "commemoration effect", 23 was not limited to politicians. There are also innumerable examples in the media in which the historical episode of "flight and expulsion" was contrasted to the contemporary challenges posed by the massive arrival of refugees. 24 The parallelism has been established as a kind of topos. Municipalities like Leipzig, for instance, linked both experiences, hanging banners with similar pictures of refugees fleeing then and now (Scholz 2016, 40). Exhibitions contrasted then and now, for example, in the Westpreussisches Landesmuseum in Warendorf, 25 a museum dedicated to a "lost" territory in the East from which Germans had been expelled. Organizations such as the Catholic Church 23 In relation with the first National Remembrance Day for the Victims of Flight and Expulsion. I thank Jan Willem Duyvendak for suggesting this idea to me during our discussion at NIAS in Amsterdam in July 2018. 24 A few examples amongst many others: in the Zeit Online, the much read historian Andreas Kossert, wrote an article entitled "Böhmen, Pommern, Syrien", on February 12, 2015, Der Tagesspiegel, "Frau Kieswetters Liste", 24.04.2015, Zeit Online April 25, 2015, "Meine Großeltern waren auch Flüchtlinge",; June 16, 2015, Süddeustche Zeitung "Vertriebene 1945, Flüchtlinge 2015, Stolz und Vorurteil", Die Zeit, n°27/2015, (July 2, 2015) "Bundesstiftung Flucht, Vertreibung Versöhnung : zu deutsch gedacht. Noch nie gab es so viele Flüchtlinge -die Vertriebenenlobby aber kreist um sich selbst". Stuttgarter Zeitung, on October 7, 2015: "Wie sich die Geschichte wiederholt. On radio: Deutschlandfunk « Das Leid ist das Gleiche », 18.11.2015 25 Here on September 19, 2015, the Westpreussisches Landesmuseum in Warendorf organised from July 25 to September 27, 2015, a special exhibition called "Integration der Vertriebenen" and organised an encounter between expellees and refugees: "Ankommen. Flüchtlinge und Vertriebe: Damals -und heute. Begegnungen über Ländergrenzen und Generationen hinweg".

in Erfurt coordinated encounters between refugees of yesterday and today,26 while others such as the Protestant Church in the Rhineland ran campaigns and numerous cultural projects such as readings (in literature this topos is widely used27 ) theatre plays, and discussions were planned throughout Germany..

The functions of using the past and the resistance to these uses

Recalling the past experience of "flight and expulsion" served different functions in the political debates about the refugee crisis.

First, drawing a parallel between the current situation of the refugees and the experience of "flight and expulsion" legitimises the government's decision to open the borders. Recalling how it felt years before to be offered shelter after having had to flee acknowledges the soundness of the principle of receiving and welcoming refugees. It also recognises the rightness of the huge spontaneous mobilisation of the Germans to help the refugees.

Referring to this past experience also serves to reiterate trust. It supports Chancellor Merkel's claim, "we will manage it", recalling that "we have already managed it" in a situation which was much more difficult. Then, about 11.5 million people had to be welcomed into a devastated country that lacked the administrative, financial, material, and human resources to do so. And this has created no major political, economic, or social instability.

A third function relates to the historicisation of the crisis: by referring to past challenges, politicians and journalists take the long-term view, which helps to minimise the exceptionality of the phenomenon as perceived by contemporaries. It contextualises it and makes the situation less alarming, not only in practical terms (registration, health care, sheltering, costs, etc.), but also in terms of potential dangers linked to the number of people who have arrived. It normalises the situation, showing that the encounter with people from elsewhere is more ordinary than is usually admitted in Germany (see Beer 2014, 11). A look back to the groups of people that have settled in Germany or left the country over time challenges the widespread "metaphor of being rooted" as the proper way of living (Bausinger 2014, 40). It is therefore also a way to make a plea for accepting the foreigners in their foreignness and for acknowledging that integration takes time.

Perhaps most significantly, it offers a change of perspective that calls for a paradigmatic shift in the way immigration is conceived. For instance, recalling the fact that Winfried Kretschmann -first Green Ministerpräsident, and a very popular politician -is himself a refugee's child, shows that migration and migrants are not a problem per se. On the contrary, it highlights the opportunities migration bears, especially the fact that refugees can importantly contribute to the development of the country in the future, the same way the expellees made an essential contribution to the Wirtschaftswunder in the 1950s and 1960s.

And last but not least, the "comparison of fate" works as a call for an empathetic attitude towards people arriving, to have an eye for their needs, to give them protection and help. President Gauck's discourse on the first Day of Remembrance for Refugees and Expellees illustrates this use. The former pastor used this occasion dedicated to a specific instance of German suffering (for which the associations of expellees had fought for years) to transform it into an exemplary memory in Tzvetan Todorov's sense: "without denying the singularity of the event", he used it "as a model to understand new situations with different agents", making it an exemplum so that "the past thus becomes a principle of action for the present" (Todorov 2004, 30-31). In doing so, he deconstructed its monological orientation turned only towards the German victims and their suffering. He opened perspectives that go beyond the event itself and created space for the recognition of the suffering of others (and not the measuring up of one's own suffering against that of the others).

However, the use of the "flight and expulsion" narrative for legitimising the welcoming of hundreds of thousands of refugees and calling for empathy towards them was highly contested. Resistance emerged first from the Bavarian Ministerpräsident Horst Seehofer from the CSU, a political party close to the associations of expellees. Seehofer reacted to Gauck's speech, in an interview with the Münchner Merkur about the tense situation with migration: "I know from many conversations with expellees that they don't like to hear such comparisons. The causes are now different, now it is also a matter of mass abuse of asylum. I don't think this discussion is appropriate (Merkur 2015, n.p.) 28 Even though the president of the Federation of Expellees (Bund der Vertriebenen -BdV) and CDU member of the Bundestag, Bernd Fabritius, supported Gauck's view at first, in December 2015 he expressed his disapproval: I am also concerned about the confusion which is again evident […] between the themes of "flight" and "expulsion" and questions of simple, plain migration.

It should be precisely the task of responsible politicians to differentiate and clearly separate these issues. 29 Many expellees were much more radical:

The moral extortion of the German expellees (Heimatvertriebene) as chief witnesses of the necessity of an uncritical "welcoming culture" (Willkommenskultur) is one of the most perfidious brainwashing arguments, with which the political and medial class try to suppress any skeptical thought about abuse of Asylum and illegal immigration. [START_REF] Paulwitz | Vergleich 'Flüchtlinge'/ Heimatvertriebene hinkt[END_REF] This reaction of a far-right publicist close to the Federation of Expellees, entitled "Comparison Refugees/Expellees (Heimatvertriebene) Does Not Work", exemplifies the points of view circulating among expellees' circles, to be found for instance in many discussion fora around the articles about Gauck's speeches in the German media on the web. 31 Such points of view insist on the ethnic boundary between yesterday's expellees and today's migrants (those are not Germans, not Christians, and do not speak German, unlike the expellees). They reject the idea that acculturation processes were necessary for German expellees coming from the East and, by doing so, invalidate the fact that "we will manage" the integration of the refugees into the FRG the way "we" managed the integration of the expellees. They consider that the integration of the expellees was not an issue.

They further question the soundness of the categories of comparison, calling into doubt the legitimacy of the motives for departure of today's migrants. In their eyes these people are economic migrants and not people fleeing war or being persecuted in their home countries. This is a clear distinction that Erika Steinbach, at the time president of the Federation of Expellees (and speaker for human rights of the CDU-parliamentary Group!) had always stressed, 32 saying expulsion is a "heavy violation of human rights", whereas migration is "something very voluntary" (quoted in Ohlinger 2005, 432). She reiterated this position in the course of the refugee crisis in her tweets 33 , and left the CDU in January 2017 in protest against its refugee politics.

In their own way the late re-settlers (Spätaussiedler), mostly from Russia or the ex-Soviet Union, who are considered by law as "expellees", also resisted the comparison of expellees and today's migrants. They expressed their defiance against migrants, Islam, and Germany's welcoming policy through demonstrations against "the violence of foreigners" throughout Germany (especially in the south-west) and criticised Chancellor Merkel's policy of asylum. 34 Fearing that "the wrong ones are coming", 35 they drew a clear boundary between themselves and the incoming refugees. 36

Reimagining "Flight and Expulsion": Revising Germany's National Myths

Resistance to comparisons between contemporary migrants and historical German expellees can be explained by how "flight and expulsion" has been regarded and used in Germany's memory landscape until recently. Not only did the codification of what an expellee (Vertriebener) and a refugee (Flüchtling) is, by the 1953 Federal law 37 rest on ethnicity (Stoke 2019), but a critical appraisal of the construction of the memory of "flight and expulsion", its shape, content, and function in setting German identity boundaries, clarifies why using it as an argument for integration or for legitimising the government's liberal welcoming policy can be considered as a rupture.

In fact, it is less its use that is noteworthy -the memory of "flight and expulsion" had already been mobilized to protest against ethnic cleansing (especially during the Balkan wars)

32 She said this in 2003 during a discussion about the eventuality of making out of the Zentrum gegen Vertreibung planned by the Association of expellees, a migration museum. 33 Such as mid-November 2015, when she tweeted "we can not absorb more foreigners, this will lead to murder and killing", a quotation from the former President Helmut Schmidt, who had died a few days earlier and that was extracted from its context. This was followed a few months later (on Februarry 27, 2016) by another tweet with the image of a blonde girl child surrounded by dark-skinned people, with the title "Germany 2030", and the text: "Where are you coming from?". 34 Those demonstrations were a reaction to the news of the kidnapping and raping of Lisa, a thirteen-year-old girl from a Russian German family in Berlin, which rapidly proved to be a fake news story relayed to the public by Russian state TV. 35 "Die Wut der Russlanddeutschen", Frankfurter Allgemeine Zeitung, February 11, 2016. 36 See "Russlanddeutsche im Zwiespalt", Stuttgarter Zeitung, March 12, 2016. See also the organisation of vigilante groups against alleged threats from refugees in several neighbourhoods (in Stuttgart/ Pforzheim etc.), or the support to the AfD: cf. results in Haidach/Pforzheim where out of 8,000 inhabitants 5,500 are late re-settlers (Spätaussiedler) (and where the AfD had its best result in European, local, regional, and federal elections). 37 This law was the legal framework for the economic and social integration of the expellees that had arrived in the aftermath of Words War II in the Western Occupation Zones, but it also laid the provisions for further migrations of ethnic Germans. May 1953 Federal Law on Expellees and Refugees (Bundesvertriebenengesetz) (Stoke 2019) -than the intention with which it was used -for advocating tolerance and the welcoming of migrants into Germany -and the very wide range of politicians that used it, well beyond the usual CDU/CSU representatives linked to the associations of expellees. Until recently, the frame in which "flight and expulsion" was referred to, namely "ethnic cleansing", enabled it to be used to legitimise German claims of being a victim of the Second World War. But it was built in such a way that it wasn't useful for advocating tolerance and the welcoming of migrants into Germany. On the contrary.

As the historians Eva Hahn and Hans Henning Hahn have shown, the narrative linked to "flight and expulsion" "was less the result of the free 'play' of memories, than of a very concrete politics of memory" (2001, 339). Its core, which until recently had proven extremely stable since the 1950s, was built under the first Adenauer government by the leaders of the expellee organizations for revisionist reasons: documenting the injustice and the sufferings experienced by the Germans in order to legitimize the claims for compensations and the recovering the lost territories in the East (Lotz 2007; Ahonen 2003). It was framed by the wish to draw a thick line under the past through silencing the National-Socialist era and downplayed German guilt by considering Germans as (the main -at least by numbers) victims of the war. "Flight and expulsion" was a simplifying narrative whose function was largely to unify ex-post a very heterogeneous group of people, 38 whose migration experiences strongly differed in timing and context. 39 This was important for recognising the huge injustice the expellees considered had been enacted upon them. It gave more weight to their claims of a right to return, 40 and of material and moral compensations. This memory narrative worked well because it was rooted in the ethnonationalist/racist (völkisch) discourse of the pre-war period and based on tenacious political myths. 41 I will pick up three important ones -the myth of origins combined with the myth of 38 Some were German citizens before the end of the Second World War, like the Silezian or the Eastern Prussian, others had never had German citizenship and were minorities like the Swabians from the Banat, the Germans from Bohemia, the Transylvanian Saxons or Germans from Bessarabia -not all even used German as their primary language. 39 People who were repatriated to the Reich by the Nazi regime, people who fled in front of the Red Army in the course of the war, people who were expelled violently at the end of the war during the so-called period of "wild" expulsion, people who were expelled by the governments of Central European countries following the Potsdam agreements, Aussiedler/re-settlers that left their home countries during Communism or late re-settler /Spätaussiedler who took the advantage of the collapse of the communist regimes to leave their home countries. 40 To understand the stability of the memory and the claims it nourishes, one can mention the fact that the Sudetendeutsche Landsmannschaft (association of Sudeten Germans) has only renounced its territorial claims in February 2016! Die Zeit, "Sudetendeutsche verzichten auf 'Wiedergewinnung der Heimat", February 28, 2016. 41 I use here the definition of Heidi Hein-Kirchner of the political myth, as "an emotionally charged narrative that reads certain historical facts 'mythically', that is, interprets reality not according to the facts, but in a selective and stereotyped manner, thus giving it a semblance of historicity" (2006, 408).

successful integration and the myth of victimhood -to show how this combination at the core of German identity construction made it difficult to use the memory of "flight and expulsion" as an example of the capacity for integration of migrants within German society, preventing it from becoming an "exemplary memory" in Todorov's (2004, 29-33) sense, a memory that uses the lessons from endured past injustice to raise empathy for the similar fate of others and to combat today's injustices.

Myth of the Origins and Civilising Mission of the Germans in the East

In most of the founding narratives about the origins of the expellees, the German presence in the East is presented in missionary terms. It is referred to as a civilising mission (Bausinger 1971; Wild 2005; Seewan 2014). The pioneering spirit of the Germans that left to populate the "East" is highlighted, whereas the sheer poverty and distress that pushed many people to emigrate is left unspoken (Bausinger 2014, 27). Their cultural achievements and their contributions to the manufacturing, technical, and agricultural development of allegedly depopulated and abandoned wastelands are elements that can be found in historical narratives about the emigration of the Donauswabians, Transylvanian Saxons, Eastern Prussians, and Germans from Bohemia or the Volga until today. Not only were the Germans often depicted as the schoolmasters (Lehrmeisters) of other ethnic groups, but they were also described as acting as a bulwark, having had to defend themselves against other ethnic groups (the non-Christian or Orthodox East), or to protect German language, customs, and traditions. Thus, not only was Germanness associated with the virtue of being hardworking, but also the demarcation with other ethnic groups was often overemphasised whereas cultural transfers were mostly silenced (Bausinger 2014, 25-47).

The Myth of the Victim

In the narratives about "flight and expulsion" the end of the so-called "German East" has usually been depicted as a radical historical break that brought German presence in the East to an end. It is described as something that came over Germans and German-speaking minorities brutally, without a warning, like a natural catastrophe (Hahn and Hahn 2001, 379-396) or the doom of Atlantis. 42 In these narratives, "flight and expulsion" are imputed to the will of the Central-Eastern European people, dating back to the nineteenth century, to build "ethnically pure" nation states. Thus, it is understood as an example -certainly the worst by virtue of numbers -of "ethnic cleansing". Neither the attitude of German minorities in the East in the 42 A metaphor used among others by Karl Schlögel (2002, 239-245).

interwar period, including their attempts at destabilizing the new nation states they were living in (even when those were democracies like Czechoslovakia), and their often very active support for the Nazis, nor the disastrous experience of the Second World War in this part of Europe are mentioned. By virtue of the sheer number of expellees and casualties (which are usually exaggerated) the Germans in the East appear in these narratives as the main victims of the war. This victimhood, conveyed by the narratives about "flight and expulsion", gives expellees a special status43 in German society, a status, like all victim status', which needs to be defended against possible competitors (see Lefranc, Mathieu, and Siméant 2008).

The Myth of Successful Integration into (West) German Society

An essential function of memory is the selection it operates on past facts. Here, interestingly, an episode that found no place in the original narratives about "flight and expulsion" was the one about the expellees' arrival in the West. If the sufferings of the flight and the ones caused by the loss of home are extremely present, the hardships of the arrival and of the first years in the "West" are silenced in the "official" accounts (see Fendl 1995). Material misery is the only element that is mentioned in these narratives of "flight and expulsion", overshadowing other sufferings like moral deprivation, the lack of compassion of the locals, the cold welcome from their so-called fellow-citizens, humiliations of all kinds but notably the one of being treated like a foreigner (because of differences in religious beliefs or practices, customs, clothing, dialects, cooking, religious practices, etc.), and the lingering racial prejudices of the Third Reich against people from Eastern Europe and Slavs, homesickness and a feeling of not being welcome. Last but not least, the fact that the expellees not only were instrumentalised politically, but also served as scapegoats for German guilt is silenced. To face this, the expellees highlighted their achievements (Leistung) and insisted on their economic performance as well as their adaptability (Angepasstheit). However, as Klaus Bade, historian of migration, states, if in the mid-1950s economic and social integration can be considered resolved for German expellees, "this was not the case from the point of view of the problems of identity and mentality. In this regard integration represented for many the traits of a regular immigration prospect inside the nation itself" (Bade 1992, 7-8).

Altogether these three myths of the original narrative of "flight and expulsion" not only silence the numerous conflicts that existed between locals and refugees from the East in the first decades after their arrival, but also suggest that integration was not an issue for the expellees. Because of the way it is framed, this memory could hardly be used "to keep alive or to raise the consciousness in the public sphere, that this century it [Germany] has already managed to master successfully the situation of a partly peaceful, partly conflictual cultural contact in the context of a mass migration in the interest of both groups", as the cultural anthropologist Albrecht Lehmann (1995, 30) concluded it should in his plea for a historical comparison between "the conditions then and now of migration and acculturation" more than twenty years ago (28).

Conclusion: Expanding German Identity Boundaries

A closer analysis of the references to "flight and expulsion" made during the "refugee crisis" reveals a significant evolution of its public narrative. Beyond the fact that reference to the topic of "flight and expulsion" had moved beyond the usual narrow circles to be mentioned by a very broad range of politicians, the striking thing is that in most speeches a new kind of narrative was associated with it. German politicians of all parties tended to historicise the topic of "flight and expulsion" as a history of migration, and in doing so, to deconstruct the above-mentioned myths. Watering down the distinction between "flight and expulsion" and migration in public discourse allowed them a reappraisal of integration prospects for today's migrants and to loosen German identity boundaries.

The new official discourse challenges the myth of the unproblematic integration of the expellees. The journalists and politicians who use the comparison of fate acknowledge the "crucial contribution to the reconstruction" of the country of the expellees after the war as well as their contribution to the economic miracle (Wirtschaftswunder), but at the same time, they precisely highlight the previously obliterated episode of the harshness of the arrival in the new homeland, if not the outright rejection by local populations and the difficulties of integration. "These people were not welcomed with open arms" as Volker Bouffier (CDU) and Stephan Weil (SPD) stressed.44 "Wasn't it so, that at the time our Western Germany was almost overburdened with so many refugees? […] Our country has learned then openness.

Sometimes laboriously; sometimes it took long" Malu Dreyer (SPD) recalled. 45 Winfried Kretschmann (B90/Grüne) confirmed this when he mentioned the fact that his older brother

  They were ethnic Germans (Volksdeutsche), they all spoke German. The people coming today don't speak German. They too have lost their homeland (Heimat), they look for a new future. […] This a challenge […]. But, Ladies and Gentlemen, we can manage it. […] What we can do from looking back is taking the power and confidence and say: If we do it intelligently, then we have the chance to overcome such challenges. 12 For his part, Winfried Kretschmann (B90/Grüne), Baden-Württemberg's Ministerpräsident, concluded his government declaration about the politics of the state in the refugee crisis on October 1, 2015 in this way: My parents were refugees themselves. My older brother died as an infant during the flight.(…) Baden-Wuerttemberg has become my home (Heimat), throughout.

I borrow this term from StephanScholz (2016, 41).

Here the differences with the Greek case, a country that had also received millions of Greek refugees in the aftermath of a population exchange in the early twenty century, as described in this issue by Georgios Kritikos, are noteworthy.

I analysed the Plenary minutes of the Bundestag, the 16 Landtage (Bürgerschaft/Senat), between 2014 and 2016, as well as the personal, party, or official websites and/or Facebook accounts of the Ministerpräsidenten (or Regierender/Erster Bürgermeister in the case of the city-states Berlin, Hamburg, and Bremen), and of Chancellor Merkel, President Gauck, and the federal minister of the interior, De Maizière.

On Saturday, September 5, one day after this announcement, 6,780 refugees arrived at Munich Central Station; on Sunday, this number increased to about 11,000(Die Zeit, 35, August 18, 2016,

See : https://www.hessen.de/sites/default/files/media/staatskanzlei/09-19-15_70_jahre_proklamation_hessen.pdf (accessed September 20, 2018)

See the report about the encounters in the Saint Martin's Church in Erfurt entitled "Flüchtlinge Gestern und Heute, Das Leid ist das gleiche", Deutschlandfunk, September 19, 2016

See the novel Das Schicksal der Sterne, by Daniel Höra and published in 2017 by Bloomoon, Munich.

See Münchner Merkur, June 25, 2015, "Seehofer zu Asyl: Merkel hat den Ernst der Lage nicht erkannt", https://www.merkur.de/politik/horst-seehofer-asyl-merkel-ernste-lage-erkannt-5165954.html (accessed September 20, 2018).

See Debate "Fluchtursachen Bekämpfen", Deutscher Bundestag, Plenary session, 146 session, 18th legislative period, December 17, 2015.

In fact, a legally recognized status, included in the law of 1953 on expellees (BVFG), and a presence guaranteed by law in numerous instances such as for instance in public broadcasting boards.

Discourse of Ministerpräsident Volker Bouffier, 70 Jahre Proklamation Hessen, Festakt, September 19, 2015, Stefan Weil, Government declaration, Niedersächsischer Landtag, September 10, 2015.

Perspektiven -Flüchtlingsaufnahme in Rheinland Pfalz".

Acknowledgements

I would like to thank Tibor Dessewffy, the anonymous referees, and all the participants to the Amsterdam-meeting of July 2018 for their comments and advice.

could not become an altar boy, because he spoke no Swabian (the local dialect). 46 Or President Gauck in his speech on the first Day of Remembrance of "flight and expulsion": "The refugees and expellees after the end of the war were often discriminated against and abused as Polacks, gypsies, backpack Germans or have-nots, were branded as backward and had allegedly offered themselves to the Nazi Reich in a special way. In this way, the lack of solidarity found a cynical justification."

But even more ground-breaking is the fact that these discourses openly break with the previously general understanding that "flight and expulsion" and migration are two distinctive subjects. President Gauck, spoke of people then and now "who have the same painful experiences [of emigration] etched in their souls". More explicitly, Olaf Scholz (SPD) declared that "Migration and immigration are part of Hamburg's history. For example, after the war, in times of hunger, unemployment and total exhaustion Hamburg has welcomed 275,000 expellees (Heimatvertriebene) and refugees". 47 Winfried Kretschmann claimed that "Baden Württemberg has always been a Land of immigration", mentioning different groups of "immigrants": namely, "Expellees, Gastarbeiter, refugees from the Balkans".

These assertions are noteworthy in that they bring together the history of "flight and expulsion" and the integration of expellees into German society with migration history, a parallelism that has been vigorously combatted by the representatives of the expellees for years, that was anything but obvious until recently in public discourse, and that was contradicted by the law and the Constitution until the end of 1990. 48 They publicly challenge the belief that used to be the official position in Germany until the turn of the millennium -"Germany is not a country of immigration" (Deutschland ist kein Einwanderungsland)endorsing a shift that law and social sciences had taken only recently (see Meier-Braun 2014). 49 In this case, in a country that used to be considered an anti-model in Europe because of its ethnocultural definition of the nation, in which migrants were long denied access to citizenship, a country that thus thought of itself as having no experience of migrants integration (Sebaux, 2016, Wilhelm 2017), past experiences could neither function in a legitimizing way by situating the present state of society in a shared continuity (perpetual grace) nor could it be used in a deligitimizing way (rebirth now or never) (see Bertossi, Duyvendak, Foner in this volume), pointing the deficiencies of the present state of things through contrasting it with better times that have past. In the German case, rememoration was used to shed a new light on past events so as to reframe the Flight and expulsion narrative and by doing so, to soften the borders that had been constructed (also with the help of the numerous political myths attached to this past) between native Germans and all newcomers, deconstructing nativist perceptions about immigration.

To conclude, if one can agree that from a heuristic point of view it is questionable whether it is possible to draw lessons and knowledge from the historical comparison between refugees and migration now and "flight and expulsion" then, yet, what I have shown in this paper is that the way the past experience of the Germans' own forced migrations has been referred to by politicians and the media during the "refugee crisis" reframed one of the founding German identity narratives. Through defining "flight and expulsion" as a migration it reminds the German that migration has been an integral part of German society for decades and reinforces the new self-perception of Germany as a country of immigration. It reasserts the legitimacy of the recent changes in the citizenship law, disclosing new prospects of integration of the migrants (and in an understated way, deflates the still problematic question of their naturalisation).


Alexander Donchev
email: donchev@dechema.de@49697564386

Lukas Mengis

Alain Couret

Svea Mayer

Helmut Clemens

Mathias Galetz

Effects of tungsten alloying and the fluorination on the oxidation behavior of intermetallic titanium aluminides for aerospace applications

Current limitations to a wider use of intermetallic TiAl alloys in aircraft and automotive engines arise from an insufficient oxidation resistance at temperatures above approximately 800°C. In this paper, the high temperature oxidation behavior of three engineering -TiAl-based alloys at 900°C in air is reported. The performance of the TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B), the 4822 alloy (Ti-48Al-2Cr-2Nb), and the Nb-free IRIS alloy (Ti-48Al-2W-0.08B) is compared (all chemical compositions are given in at.%). During testing in air non-protective mixed oxide scales developed on all untreated samples, but with different compositions and thicknesses. These different oxide layers are characterized and their formation mechanisms are discussed. The presence of W in the IRIS alloy leads to a better oxidation behavior compared to untreated TNM and 4822. This behavior was changed in the direction of a protective alumina layer formation via the so-called "fluorine effect". The above-mentioned alloys were treated with fluorine via a liquid phase process by evenly spraying a fluorine containing polymer on all faces of the specimens. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated specimens. Due to the fluorination all treated test coupons exhibited slow oxidation kinetics. The results of isothermal as well as thermocyclic exposure tests are presented and discussed in the view of the chemical composition and processing conditioned microstructure of the three investigated γ-TiAl-based alloys.

Introduction

The human impact on the climate change triggers the demand for advanced automotive or jet engines with higher efficiency, i.e. reduced fuel consumption and lower CO 2 and NO x emissions, than the ones currently used. In addition to new and improved design concepts, a possibility to reach this goal is the use of light-weight materials which allow faster rotating speeds and shorter ramp-up times. Hence, the use of innovative structural materials, such as intermetallic γ-TiAlbased alloys, may play a decisive role since they can replace currently used heavier Ni-or Cobased alloys with only half of their density (ca. 4 g/cm³) [1][START_REF] Boyer | Ti 2011[END_REF][START_REF] Clemens | [END_REF][4]. Cast TiAl blades already have been introduced in the low-pressure turbine of General Electric's GEnX jet engine in 2011 [1] and recently by Pratt & Whitney in the as-forged condition in the Geared Turbofan engine [5]. An industrial exploitation of these alloys on a larger scale requires addressing two major issues. One of them concerns the room temperature (RT) ductility along with high temperature creep strength. The other one is the resistance against environmental damage at elevated temperatures. Research and development activities have led to high-strength alloys with a fraction of  o -phase besides the and  2 -phase, i.e. complex multi-phase alloys of the 4 th generation, which represent the class of process-adapted TiAl alloys, see [START_REF] Clemens | [END_REF]6] and the references cited in this review paper. They possess an improved hot-workability and balanced mechanical properties which allow long-term use as advanced high-temperature material up to temperatures of about 750°C. In the future an increase of the operation temperature of TiAl components is planned but currently the use is impeded by an insufficient oxidation resistance [7]. Despite an Al content of above 40 at.%, TiAl alloys do not form an alumina layer. Reasons for this are the similar thermodynamic stabilities of Ti-and Al-oxides and the low Al-activity within the alloy system, e.g. see [8]. Hence, a mixed oxide scale develops during hightemperature exposure in air, which is non-protective and grows fast due to the high kinetics of the TiO 2 formation [9]. The mixed scale can spall if a certain stress level is reached [START_REF] Schütze | Protective oxide scales and their breakdown[END_REF]. This leads to an accelerated material loss and a shortage of TiAl component lifetime. In the course of alloy development, further element additions, which chiefly affects the mechanical properties of the bulk material [START_REF] Shida | [END_REF], can improve the oxidation behavior of TiAl alloys, but cannot prevent environmental damage during long term exposure at temperatures above 800°C. For example, Nb additions up to about 10 at.% can improve the oxidation resistance of these alloys, but do not impede the growth of a mixed scale [START_REF] Leyens | Titanium and Titanium Alloys[END_REF]. Mo additions up to 2 at.% are also beneficial, but with the same restrictions [START_REF] Perez | [END_REF]14]. A higher Mo content shows an inferior oxidation behavior [15,16]. The results concerning the effect of Cr on the oxidation behavior revealed a detrimental behavior [17] while for W a positive effect was found [18]. However, there are also other ways to overcome the oxidation problems, e.g. the use of coatings [19].This is not an easy task for TiAl due to the formation of brittle intermetallic phases. The socalled "fluorine effect", which is a surface modification technique, is an alternative solution. It has been proven to dramatically increase the oxidation resistance of technical TiAl alloys with an Al content above 40 at.% during high temperature exposure up to temperatures of 1050°C [20,[START_REF] Donchev | Intermetallic Compounds[END_REF]. Hereby, very small amounts of fluorine at the metal/oxide interface lead to the formation of a protective alumina barrier [START_REF] Donchev | [END_REF]. It protects the material even in water vapor containing environments as well as under thermocyclic conditions [23]. Nevertheless, the effect of chemical composition and phase constitution on the efficiency of the fluorine effect has not been systematically studied up to now. In this paper three engineering -TiAl-based alloys for aerospace applications are investigated, namely the TNM [START_REF] Clemens | [END_REF], the 4822 [24] and the Nb-free IRIS alloy [25]. The results of isothermal and thermocyclic high temperature exposure experiments on untreated and F-treated specimens of the three TiAl alloys with different chemical compositions and microstructures are presented. The obtained results are discussed in view of a potential use of such multi-phase alloys at operating temperatures higher than 750°C.

Experimental

Specimens of the TiAl alloy variants TNM and 4822 were produced by ingot metallurgy via vacuum arc remelting and centrifugal casting in permanent molds by GfE Metalle und Materialien GmbH, Nuremberg, Germany [6]. Subsequently, the ingots were hot-isostatically pressed (HIPed) and slowly cooled to RT. The third alloy variant was produced by powder metallurgy via spark-plasma-sintering (SPS) at CEMES, Toulouse, France [26]. The nominal chemical compositions are as following: Ti-43.5Al-4Nb-1Mo-0.1B (TNM), Ti-48Al-2Nb-2Cr (4822) and Ti-48Al-2W-0.08B (IRIS) (in at.%).

High temperature oxidation tests were performed on coupons of 15 x 10 x 1 mm³ (TNM and 4822) and on cubes of 7 x 7 x 7 mm³ (IRIS). The samples were ground to 1200 grit using SiC paper, ultrasonically cleaned with ethanol, rinsed with distilled water and dried in air at RT prior to further studies. Subsequently, a fluorine-containing organic compound based on (C 2 F 4 ) n which was sprayed in a homogeneous manner over the whole surface of the specimens at RT. The samples were sprayed on all sides and dried overnight in a fume hood. The mass change due to fluorine application was measured by weighing the specimens. The amount of the F-containing compound deposited per square centimeter was comparable for all examined samples [START_REF] Donchev | Structural Aluminides for Elevated Temperatures[END_REF]. All not fluorinated and fluorinated samples were isothermally exposed at 900°C in dry synthetic air for 120 h. The mass gain was measured by thermogravimetric analysis (TGA). More specifically, the specimens were connected to a balance and heated up to 900°C under a steady stream of dry Ar. After the temperature was reached, the gas was switched to dry synthetic air and the TGA measurement was started. After the end of the envisaged testing period, the furnace was switched off and the specimens were slowly cooled to RT within the furnace (FC) under dry air. Thermocyclic exposure tests were performed by exposing untreated and F-treated specimens in a furnace at 900°C for 24 h in laboratory air, cooling to RT within 15 min outside of the furnace, weighting with a balance having an accuracy of less than 0.01 mg, and finally putting them back into the furnace after one hour. In total 25 total test cycles were performed on each alloy variant. Post experimental investigations including the examination of the surface with a Leica magnifier, a Philips XL 40 scanning electron microscope (SEM) coupled with an EDAX energy dispersive X-ray spectroscopy (EDX) and employing a Bruker D8 Advance X-Ray diffractometer (XRD). After surface analysis, all exposed samples were sputtered with gold and galvanically plated with Ni prior to the preparation of metallographic cross-sections [START_REF] Berghof-Hasselbächer | [END_REF]. This led to a better visibility of the very thin oxide scales grown on the oxidized samples and protected the scales during metallographic preparation. Furthermore, the cross-sections were analyzed with the SEM mentioned above and an electron probe micro analysis (EPMA) JXA-8100 from Jeol.

Results

Microstructure

The microstructures of the investigated alloys differ from each other. The following images (Figs. 1a-c) show SEM micrographs taken in back-scattered electron (BSE) mode of the initial material states. The TNM alloy reveals a multi-phase structure with α 2 /γ-lamellar grains and globular γ-grains and β o -phase at the colony boundaries (Fig. 1a). The β o -phase appears bright, the α 2 -phase grey and the γ-phase dark grey. The 4822 alloy reveals a two-phase structure with predominantly large lamellar α 2 /γ-grains (Fig. 1b). The IRIS alloy, see Fig. 1c, exhibits also a nearly lamellar microstructure with  grains and bright small W-enriched β o -precipitates, also occurring within the lamellar colonies. However, the grain size within the IRIS alloy is smaller.

Mass change behavior

The thermogravimetric mass change curves of the untreated alloys during isothermal exposure at 900°C for 120 h in dry synthetic air are presented in Fig. 2. The mass change behavior of the three tested materials during isothermal exposure is similar showing a steady mass gain throughout the exposure (Fig. 2a). The mass gain curves of TNM and 4822 are almost equal while the mass gain for the IRIS alloy is lower over the whole time of exposure. The mass changes of the materials follow the parabolic kinetics. Therefore, the final average mass gain values and k p are listed in Table 1. The fluorine treatment reduces the oxidation kinetics for all three tested materials, as shown in Fig. 2b. The mass gain of all fluorine treated samples stays below 1 mg/cm². The mass gain of the fluorinated TNM sample is slightly higher than that of the other two materials. For all specimens in the untreated as well as F-treated condition, no signs of spallation, i.e. mass losses, are visible during isothermal exposure at 900°C. In this case, too, the final mass gain and k p values are listed in Table 1.

The mass change after thermocyclic exposure of the untreated samples is different as shown in Fig. 3. The untreated TNM sample reveals a mass gain for the first 6 cycles of exposure which is similar to the isothermal test but after that sudden and abrupt mass losses are recorded which are caused by spallation of larger parts of the oxide scale. Such spallation occurs only during cooling to RT. In comparison, the untreated 4822 sample also exhibits an increase in mass for the first day of about 1.5 mg/cm², followed by slight mass gains during the next cycles and then by mass loss after seven days of exposure with some minor spallation. After about 10 days the mass loss of small parts of the oxide scale by spallation overcomes the mass gain by oxidation leading to a final mass change of about -1 mg/cm² after 25 cycles. The untreated IRIS alloy does not show any spallation and exhibits the lowest mass gain of the three untreated materials, which is steady with a final mass gain of about 2 mg/cm² after 25 cycles.

In contrast, the F-treated samples of all three alloys reveal a steady and very low mass gain. All three final mass gains are below 1 mg/cm² after 25 cycles. This indicates the benefit of the fluorine effect for all tested alloys, independent of the chemical composition, the microstructure of the base material as well as the exposure conditions, i.e. isothermal or thermocyclic.

Surface analysis

Optical analysis of the untreated specimens confirms different behavior of the materials during thermocyclic exposure (Fig. 4). The TNM alloy exhibits more spallation (Fig. 4a), which is in accordance to the mass change data (Fig. 3), when compared to the 4822 material (Fig. 4b). On the TNM sample surface residues of the oxide scale after the last cycle are still present at the outer areas of the specimen, while for the 4822 sample the oxide scale spalled off at the edges and corners. The IRIS alloy shows the lowest spallation rate. Only a small spalled area is found at one edge (Fig. 4c). These results correlate with the mass change data shown in Fig. 3. All fluorine treated samples look similar, hence, only one representative image is shown in Fig. 4d. They appear grey without any signs of spallation. Taking a closer look at higher magnification of the oxide scale residues by means of SEM, similar structures of coarse crystals on all untreated alloy variants are detected. Hence, only one image of the untreated TNM sample is exemplarily presented in Fig. 5a. The image was taken in secondary electron (SE) mode to obtain a better impression of the surface topography. The crystals consist of TiO 2 , which was proven by EDX (see inset). The measurements were performed with energies from 10 keV up to 20 keV so that also deeper zones of the oxide scale were measured, but the peak ratio always stays the same. It should be kept in mind, that no Nb, Cr, Mo or W from the base material was detected, i.e. the oxide scales on the untreated materials are thicker than the penetration depth, even at 20 keV (Tab. 2). For lack of space, no image and measurement of a spalled area are shown. The fluorine treated samples totally differ in their surface quality. They all also look similar despite their different starting microstructure and chemical composition. Thus, exemplary the corresponding image of the F-treated TNM sample is presented. No spallation can be observed even after thermocyclic exposure for 600 h, i.e. 25 h cycles, at 900°C in air. The surfaces are smooth and homogeneous, see Fig. 5b, and no coarse oxide crystals are observed. The EDX analysis, which was performed within the same intensity range as the untreated samples (10 -20 keV), reveals the peaks of Nb and Mo so that the substrate was detected underneath the thin oxide scale, too (Tab. 2). XRD analyses of the exposed samples reveal similar results. Hence, as an example, only the patterns of the exposed TNM samples are presented (Fig. 6). The main peaks arise from the oxides TiO ing a thinner oxide layer while for the untreated sample the peak with the highest intensity is correlated to TiO 2 .

Oxide scale analysis

Metallographic cross-sections of the specimens after exposure were prepared in order to investigate the scales that have formed during exposure. LOM investigations provide a first impression of the formed oxide scales, but do not allow a closer look on the composition and morphology of the scales formed. Therefore, no LOM images are presented. Detailed insight on the composition of the oxide scales is given by SEM investigations and further understanding is gained by additional EPMA measurements. The following SEM images in BSE mode allow to distinguish phases containing light or heavy alloying elements. Here, phases with more dense alloying elements show a brighter contrast. The remaining parts of the mixed scale which stayed attached on the surface of the untreated TNM alloy are about 10 µm thick (Fig. 7a). Outer coarse rutile crystals, which have been identified by EDX, are followed by a darker alumina enriched zone. This thin Al-rich layer is clearly visible. This is the case for all three alloys (Figs. [8][9][START_REF] Schütze | Protective oxide scales and their breakdown[END_REF]. Underneath this zone a porous mixed oxide scale can be found. This mixed scale forms the thickest part of the total oxide scale and contains some Nb (Fig. 8f). However, no Nb is found within the Al-rich layer. At the interface oxide/substrate some enrichment of Nb (bright contrast) and N is detected. This scale morphology is characteristic for -TiAl based alloys oxidized in air. The elemental distributions reveal the formation of a mixed TiO 2 /Al 2 O 3 scale along with a thin nitride layer (Fig. 8e). The results for the untreated 4822 alloy are similar (Fig. 7b). The oxide scale morphology is comparable, but the nitride layer is more pronounced (Fig. 9e). At the interface towards the substrate an almost continuous nitride layer 2

θ [deg]

Intensity [] can be found on both samples. Additionally, Nb enrichment at the interface is visible. The elemental intensities in the EPMA elemental distributions are the same for all samples so that a comparison of the prevailing amounts can be made. The scale on the untreated IRIS alloy appears different. The scale is thinner (Fig. 7c). The alumina enriched zone is more compact and the mixed scale underneath is also thinner. Additionally, less outer rutile crystals are visible. Some W is found in the mixed scale but not in the alumina rich layer (Fig. 10f). The W distribution is similar to that of Nb. In the IRIS alloy some areas reveal local attack into the depth of the material. This attack reaches about 15 µm below the oxide scale into the material. EPMA again reveals a continuous nitride layer underneath the mixed oxide scale at the interface towards the substrate (Fig. 10).

As a conclusion, qualitatively similar oxide scales are observed in all three F-untreated alloys, with the successive observation of rutile, alumina and mixed oxide layers. In all cases, the oxides are separated from the alloy by a thin nitride layer. The differences are mainly due to the thickness of the mixed oxide scale which is thicker in case of the TNM and 4822 alloys, to some spallation in these two alloys and to the presence of a local attack in the IRIS sample which is crossing the oxide layer. Interestingly, similar repartitions are measured for Nb and W, indicating the same oxidation mechanisms. Thermocyclic exposure for 600 h, i.e. 25 h cycles, leads to pronounced spallation on the untreated TNM and 4822 samples (Figs. 4a,b). This was not the case for the IRIS alloy (Fig. 4c).

The oxide scale morphologies on the three materials are mostly the same as after isothermal exposure, but the scales are thicker (Figs. 11a-c) which can be explained by the longer exposure time, i.e. 600 h vs. 120 h. As can be seen in Fig. 11a spallation does not lead to the total exfoliation of the oxide scale. Some oxides/nitrides remain on the surface as a thin layer. Here the access of the atmosphere towards the substrate is much easier when compared to areas where a thick scale has remained. Consequently, at these spots an accelerated attack can take place. The scales are porous and crackly. At the interface towards the substrate enrichments of the heavy additional elements, i. e. Cr, Nb or W, can be seen in the SEM/BSE images where phases containing heavy elements appear brighter than those bearing light elements. The remained scale on the 4822 sample looks similar but is thicker (Fig. 11b) than the scale on the IRIS sample (Fig. 11c) which exhibits the thinnest one of the three alloys. The dark alumina layer within the scale is broad and compact, but at the interface towards the substrate there are cracks and pores. Nevertheless, it has to be mentioned, that in some spots an accelerated attack was observed on the untreated IRIS sample, which was not the case for the other two materials. Such spots were found on several areas of the specimen. A closer look on such a spot reveals a thick and porous mixed oxide underneath the outer alumina (Fig. 12a) and a fine scale at the interface towards the substrate which is enriched with very bright particles, see Fig. 12b. EDX analysis of these spots disclose a high amount of nitride formation adjacent to the oxides based of Al and Ti within this very thick mixed scale (Fig. 12d) and a roughly 2 µm thick dense nitride layer at the interface towards the substrate (Fig. 12e). The total thickness of the scale in such a spot can reach more than 100 µm in depth (Fig. 12c). Fluorination totally changes the oxidation behavior of all three alloy variants. Instead of a mixed oxide scale, a thin alumina rich layer was formed after high temperature exposure. The observed behavior shows that in case of the investigated alloys the F-process is independent of chemical composition, initial microstructure and the processing history of the alloys as well as the exposure conditions (isothermal vs. thermocyclic or dry synthetic air vs. laboratory air). In the following section only images of the fluorinated samples after thermocyclic exposure at 900°C for 600 h in laboratory air will be shown. Minor differences between the materials become visible by means of SEM examination. The BSE image of the F-treated TNM sample shows a slightly thicker oxide (roughly 4 µm) when compared to the two other alloys, with some pores at the oxide/metal interface (Fig. 13a). At the interface also a continuous brighter zone is visible. This is caused by Al-depletion via alumina formation. This zone is more pronounced on the exposed and treated 4822 sample (Fig. 13b). Here, the alumina layer is smooth and thinner (< 2 µm) when compared to the TNM sample and appears more compact. Again, the scale on the Ftreated IRIS specimen looks different, i.e., the scale is not so even, see Fig. 13c. The surface of the alumina layer is rough and the subsurface zone of the substrate is enriched in W, while Al is depleted. The EPMA results prove these findings. On top of the alumina layer some rutile crystals can be found on the fluorinated TNM sample (Fig. 14) and depletion of Al is obvious. Some Nb enrichment underneath the oxide scale is visible, too. The signals of Mo and B are too low to make such a similar conclusion. For the 4822 alloy the alumina layer is thinner compared to the TNM sample (Fig. 15). Also, the Al depleted zone appears thinner and no enrichments of Nb and Cr are detectable. The scales on the F-treated IRIS specimen are different again (Fig. 16). The alumina layer is thin and homogeneous, but the Al depleted zone is thicker. Additionally, several W-enriched spots are found in the Al depleted zone.

Discussion

The results presented in this paper describe the high temperature oxidation behavior of three engineering -TiAl-based alloys at 900°C in air. In the following the influence of the initial microstructure, the chemical composition and the fluorine effect on the oxidation behavior will be discussed.

The TNM alloy has the lowest Al content (43.5 at.%) of the examined alloys, but its oxidation behavior is similar to the two other alloy variants, both showing an Al content of 48 at.%. Furthermore, the TNM alloy contains a significant amount of  o -TiAl phase besides -TiAl and  2 -Ti 3 Al phase, while the other alloys predominantly consist of  and  2 . The -phase undergoes a phase transition from the disordered high temperature -Ti(Al) with A2 structure towards the ordered  o with B2-structure above service temperature [29]. The globular β o -phase in the initial microstructure is located at the boundaries of lamellar / 2 -colonies, where also some globular phase can be found. The two other alloys consist mainly of lamellar / 2 -colonies with almost no globular -grains at their boundaries.

In general, the Al content of the  2 -phase is lower when compared to the -phase, so that preferential Ti oxide formation is observed during oxidation of this phase, especially if it is present in globular form as reported in [START_REF] Smialek | MRS Symp. Proc[END_REF]. Globular  o -grains at the surface, however, negatively influence the oxidation behavior. Therefore, the microstructure and, more specifically, the phase distribution affect the oxidation resistance and thus have to be taken into account [START_REF] Gil | [END_REF]32].

Next to the microstructure the alloying elements present have a significant impact, for example, a certain amount of Nb improves the oxidation resistance of -TiAl-based alloys by several combining mechanisms, see ref. [33]. This is also the case for the TNM alloy which contains 4 at.% Nb. The alloy forms a slightly thinner and more homogeneous mixed oxide scale when

high low a) b) c) d) e) f) g)

Ni plating

compared to the one found on the surface of the untreated 4822 sample with a higher Al content, but exhibiting a lower Nb content of 2 at.%. Both Nb and Mo are known to be beneficial for the oxidation behavior, while Cr shows a detrimental effect [34]. However, the negative effect of Cr within the 4822 alloy is compensated by the higher Al content along with the addition of Nb, while for the TNM alloy Nb and Mo determine the overall oxide scale formation. The influence of W which has also a positive effect on the oxidation behavior [35] in the IRIS alloy is discussed further below.

It is tempting to speculate that the higher spallation rate of the TNM alloy is a consequence of the microstructure which, in contrast to the other alloys, consists of three phases. The phase transformation from the disordered  to the ordered  o should be no reason for stresses within the substrate because it happens above 1000°C where the material is ductile enough to release such stresses. Due to higher solubilities, the uptake of O into the phases is higher for the  2 -and  ophase when compared to the -phase, which in turn can change their lattice parameters [START_REF] Smialek | MRS Symp. Proc[END_REF]. This happens within the subsurface zone of the material directly underneath the oxide scale. Such O uptake might cause a higher stress level within the TNM alloy compared to the other two alloy variants, especially when globular  o -grains are located at the interface with the oxide, explaining the onset of spallation if a certain stress level is reached [START_REF] Schütze | Protective oxide scales and their breakdown[END_REF]. The 4822 alloy is also susceptible for spallation, especially during thermocyclic exposure but to a lesser extent when compared to the TNM alloy. The mixed scale consists of TiO 2 , Al 2 O 3 and at the interface towards the substrate TiN which have similar coefficients of thermal expansion (CTE) [33][34][35].

The CTEs for TiO 2 depend on the crystal axis, i.e. 7.14 (parallel to a) and 9.19 (parallel to c). The CTE for Al 2 O 3 is 8.1 and the one for TiN 9.4 (all multiplied by 10 -6 K). However, these are lower than those of the  2 -and -phase, neglecting the effects of additional elements [START_REF] Kumpfert | Titanium and Titanium Alloys[END_REF]. Since the scales on all untreated materials are similar in composition the CTE mismatch seems not to be the most determining reason for the differences in the observed spallation tendency. The differences in the nitride layer formation can also play a role, which has been the focus of various studies [START_REF] Zheng | [END_REF]40], however, its definite role in the oxidation behaviour is not unambiguous.

As far as spallation is concerned the results suggest that stronger generation of nitrides can cause enhanced spallation. The presence of N leads to the formation of a segmented nitride/oxide layer at the interface towards the substrate [41]. Such a structure causes stresses, which finally can trigger spallation.

The addition of 2 at.% W in case of the IRIS alloy leads to a lower mass gain during isothermal exposure when compared to the 4822 alloy containing the same Al content and an equal amount of Nb. The scale on the IRIS alloy is thinner than the scale on the other two alloys. Different to the other alloys is the presence of a compact alumina layer near the surface on the exposed IRIS sample, which is not so dominant on the 4822 material and even less distinct on the TNM specimen. For example, the alumina layer on the TNM specimen is more intersected by rutile than it is the case for the other two alloys. The element Cr is known to deteriorate the oxidation resistance of TiAl alloys. However, in combination with Nb this negative effect does not play a role as shown by comparison of the oxidation behavior of the 4822 alloy with that of a Nb-free Cr containing alloy [START_REF] Huang | Proc. Int. Symp. Intermet. Comp. JIMIS[END_REF]. Therefore, Cr can be excluded as the main reason for the differences between the 4822 alloy and the IRIS alloy, whose oxidation behavior is quite astonishing. Despite the lack of Nb, the mass gain of the IRIS alloy during isothermal or thermocyclic oxidation is the lowest when compared to the other two alloys. This is attributed to the dense formed alumina layer, which is stable even during thermocyclic exposure. This layer hinders the inward diffusion of O so that the substrate does not get oxidized in the same way as the TNM or 4822 alloy. This lower inward diffusion of O explains why the mixed scale underneath the alumina enrichment on the IRIS sample is the thinnest. Moreover, W can form oxidation states up to +6 and the incorporation of such a high valence cation within the oxide scale reduces the anion diffusion of oxygen through the oxide and, hence, increases the oxidation resistance. Additionally, the ionic radius of W is higher than the ones of Ti and Al. This fact can also influence the oxygen diffusion through the oxide scale. The positive effect of W to form a dense alumina layer and alumina enrichment near the substrate [START_REF] Shida | [END_REF], however, vanishes after a certain period of time when the oxide scale gets too thick. Then, during long thermal cycling cracks will form within the scale, which leads to enhanced O ingress. This causes accelerated damage of the substrate due to kinetic reasons of a fast TiO 2 formation. This is why a marked inward attack was observed on some spots after 600 h of oxidation. Hence, the positive effect of 2 at.% W addition on the oxidation behavior does not last longer than 600 h at 900°C. Fluorine on the other hand triggers a long-term protection. Preliminary results have shown that the alumina layer after fluorine treatment is stable under thermocyclic conditions for several thousand hours even with a continuous  2 -Ti 3 Al zone underneath [23]. The  2 -phase usually forms a TiO 2 rich mixed scale. In contrast, only a very small amount of TiO 2 is formed on the fluorine treated samples during the incubation period. The alumina formation via the fluorine mechanism is described to be based on the predominant formation of gaseous Al-fluorides [START_REF] Donchev | Intermetallic Compounds[END_REF]. These compounds evaporate through micropores and microcracks at the interface metal/oxide and along the grain boundaries of the oxides. Their transformation towards alumina is at the end forced by the increased oxygen partial pressure, which has a very steep gradient towards the outer surface, i.e. 0.2 bar at the surface if oxidization takes place in air. Diffusion of O plays a minor role while Al diffusion is not involved in the Al 2 O 3 formation. The following reactions describe the fluorine effect (eqs. 1-4). The fluorides form at the metal/oxide interface where the O partial pressure is very low. The partial pressures of the Al fluorides are several orders of magnitude higher than those of the Ti-fluorides if a defined amount of fluorine is applied [START_REF] Donchev | [END_REF]. The monovalent AlF (g) is the predominant compound. This fluoride is finally oxidized to Al 2 O 3 during to its outward diffusion due to the increasing O partial pressure (3). The enthalpies have been calculated with FactSage [44]. It can be seen that the enthalpy of fluoride formation is similar for Al and Ti but the enthalpy of oxide formation from the fluorides is higher for AlF (g) (eq. 3) compared to TiF 2(g) (eq. 4). 2 Al (s) + F 2(g)  2 AlF (g) ; ΔG 0 900°C = 709 kJ/mol (1) Ti

(s) + F 2(g)  TiF 2(g) ; ΔG 0 900°C = 743 kJ/mol (2) 4 AlF (g) + 3 O 2(g)  2 Al 2 O 3 +2 F 2(g) ; ΔG 0 900°C = 1190 kJ/mol (3) TiF 2(g) + O 2(g)  TiO 2 + F 2(g) ; ΔG 0 900°C = 13 kJ/mol (4)
Fluorine is still present at the metal/oxide interface, which maintains the fluorine effect mechanism in a circular process [45]. Thus, alumina is even stable on top of the Al-lean  2 -Ti 3 Al phase. The conditions for the formation of gaseous Al-fluorides totally differ from those of "normal" alumina formation, which is only based on cation and anion diffusion within the lattice [START_REF] Kofstad | High Temperature Corrosion[END_REF]. The treatment was the same for all materials and the protective effect of fluorine was proven for all investigated alloys. The alumina layer on the TNM alloy is only slightly thicker than the layers found on 4822 and the IRIS alloy.

Summary

The oxidation kinetic of three engineering -TiAl-based alloys was investigated during isothermal tests using thermogravimetric analysis. The untreated alloys, i.e. no prior fluorinetreatment was conducted, TNM and 4822 reveal similar mass gain behavior, while the IRIS alloy shows a significant lower mass gain. The first two alloys exhibit also similar behavior during thermocyclic exposure, but only for the first 120 h. After this duration, spallation is observed that is more pronounced for the TNM alloy than for the 4822 alloy. On the other hand, the IRIS alloy reveals only very slight spallation. At all, the IRIS alloy exhibits a quite astonishing oxidation behavior, which is due to the formation of a dense alumina layer which is caused by the presence of 2 at.% W. However, after a fluorine treatment all alloys exhibit the so-called positive "fluorine effect". The presence of fluorine leads to the formation of a slow-growing alumina Al 2 O 3 layer instead of a non-protective mixed TiO 2 /Al 2 O 3 /TiN scale. This alumina layer has the ability to protect the substrate against further oxidation and to suppress the mixed oxide scale formation. The results prove that the F-induced beneficial change in the oxidation mechanism can be applied to various intermetallic -TiAl based alloys, independent of the appearing phases, their chemical composition as well as processing history.

Tables:

Table 1: Final mass gain values of the isothermally exposed specimens.

Table 2: Elemental quantification (at.-%) of the EDX spectra in Fig. 5.

Graphical Abstract:

SEM/BSE images of untreated (left) and F-treated (right) 4822 samples after thermocyclic exposure at 900°C for 600 h in air.

µm µm

Figure 1 :

 1 Figure 1: Microstructure of the investigated alloys: a) TNM, b) 4822 and c) IRIS. SEM images taken in BSE mode. Here, the γ-TiAl phase appears dark, whereas  2 -Ti 3 Al and  o -TiAl show a grey and white contrast, respectively.

  Figure 2a, b: TGA curves obtained during isothermal exposure at 900°C in dry synthetic air for 120 h for the untreated material variants (a) and F-treated material variants (b).

Figure 3 :

 3 Figure 3: Mass change curves of the three untreated and F-treated -TiAl-based alloys during thermocyclic exposure at 900°C in laboratory air for 600 h. Here, so-called 25 h cycle tests, see text, were conducted.

Figure 4a- d :

 d Figure 4a-d: Magnified images of the untreated materials TNM (a), 4822 (b), IRIS (c) and the fluorine treated TNM sample (d) after thermocyclic exposure at 900°C in laboratory air for 600 h.

  2 and Al 2 O 3 in addition to the substrate phases, γ-TiAl, 2 -Ti 3 Al and β o -TiAl. For the fluorinated sample the peaks from the substrate have the highest intensity indicat-

Figure 5 :

 5 Figure 5: SEM/SE images and embedded EDX spectra (20 keV) of the surface of the untreated a) and F-treated b) TNM samples after thermocyclic exposure at 900°C under laboratory air for 600 h (25 h-cycle tests).

Table 2 :

 2 Figure 6a, b: XRD patterns of the surface of the exposed TNM samples a) without treatment and b) with fluorine, both after 120 h of exposure at 900°C in dry synthetic air (1 = Al 2 O 3 / 2 = TiO 2 / 3 = γ-TiAl / 4 =  2 -Ti 3 Al)

Figure 7 : 3 Figure 8 :

 738 Figure 7: SEM/BSE images of the mixed oxide scales on the a) untreated TNM sample, b) untreated 4822 sample and c) untreated IRIS sample after isothermal exposure at 900°C in laboratory air for 120 h.

Figure 9 :Figure 10 :

 910 Figure 9: a) SEM/BSE image and corresponding EPMA elemental distributions of b) O, c) Ti, d) Al, e) N, f) Nb, and g) Cr of the mixed oxide scale of untreated 4822 sample after isothermal exposure at 900°C in laboratory air for 120 h.

Figure 11 :

 11 Figure 11: SEM/BSE images of the mixed oxide scales on the untreated alloys after thermocyclic exposure for 600 h at 900°C in laboratory air: a) TNM, b) 4822 and c) IRIS.

Figure 12 :

 12 Figure 12: SEM/BSE images of a) an area with accelerated attack and b) corresponding EDX spectrum; c) interface towards the substrate underneath the zone with accelerated attack, d) corresponding EDX spectrum, and e) overview image of the untreated IRIS sample after 25 h cycles, i.e. 600 h exposure at 900°C in laboratory air.

Figure 13 : 3 Figure 14 :

 13314 Figure 13: SEM/BSE images of metallographic cross-sections of the F-treated material variants: a) TNM sample, b) 4822 sample and c) IRIS sample after thermocyclic exposure at 900°C in laboratory air for 25 h cycles, i.e. 600 h.

Figure 15 :

 15 Figure 15: a) SEM/BSE image and corresponding EPMA elemental distributions of b) O, c) Ti, d) Al, e) N, f) Nb, g) Cr, and h) F of the oxide scale of F-treated 4822 sample after thermocyclic exposure at 900°C in laboratory air for 600 h.

Figure 16 :

 16 Figure 16: a) SEM/BSE image and corresponding EPMA elemental distributions of b) F, c) B, d) Ti, e) W, f) O, and g) Al of the oxide scale of the F-treated IRIS sample after thermocyclic exposure at 900°C in laboratory air for 600 h.

Figure captions:Figure 1 :

 1 Figure captions:

Figure 3 :

 3 Figure 3: Mass change curves of the untreated and F-treated alloys during thermocyclic exposure at 900°C in laboratory air for 600 h (25 h-cycle test).

Figure 4a- c :

 c Figure 4a-c: Magnified images of the untreated materials after thermocyclic exposure at 900°C in laboratory air for 600 h a) TNM, b) 4822 and c) IRIS.

Figure

  Figure 5a, b: SEM/SE images and embedded EDX spectra (20 keV) of the surface of the untreated (a) and F-treated (b) TNM samples after thermocyclic exposure at 900°C under laboratory air for 600 h (25 h-cycle test).

Figure 6a, b :

 b Figure 6a, b: XRD patterns of the surface of the exposed TNM samples a) without treatment and b) with fluorine both after 120 h of exposure at 900°C in dry synthetic air (1 = Al 2 O 3 / 2 = TiO 2 / 3 = γ-TiAl / 4 =  2 -Ti 3 Al)

Figure 8 :

 8 Figure 8: EPMA elemental distributions of the mixed oxide scale of untreated TNM sample after isothermal exposure at 900°C in laboratory air for 120 h.

Figure 9 :

 9 Figure 9: EPMA elemental images of the mixed oxide scale of untreated 4822 sample after isothermal exposure at 900°C in laboratory air for 120 h.

Figure 10 :

 10 Figure 10: EPMA elemental images of the mixed oxide scale of untreated IRIS sample after isothermal exposure at 900°C in laboratory air for 120 h.

Figure 11a- c :

 c Figure 11a-c: SEM/BSE images of the mixed oxide scales on the untreated alloys after thermocyclic exposure for 600 h at 900°C in laboratory air, a = TNM, b = 4822, c = IRIS.

Figure 12a -

 12a Figure 12a-e: SEM/BSE images of an area with accelerated attack (a) and corresponding EDX spectrum (c), the interface towards the substrate underneath the zone with accelerated attack (b) with corresponding EDX spectrum (d) and an overview image (e) of the untreated IRIS sample after 25 cycles, i.e. 600 h, at 900°C in laboratory air.

Figure 13a -

 13a Figure 13a-c: SEM/BSE -images of metallographic cross sections of the F-treated materials (a) TNM sample, (b) 4822 sample and (c) IRIS sample after thermocyclic exposure at 900°C in laboratory air for 25 cycles, i.e. 600 h.

Figure 14 :

 14 Figure 14: EPMA elemental images of the mixed oxide scale of F-treated TMN sample after thermocyclic exposure at 900°C in laboratory air for 600 h.

Figure 15 :

 15 Figure 15: EPMA elemental images of the mixed oxide scale of F-treated 822 sample after thermocyclic exposure at 900°C in laboratory air for 600 h.

Figure 16 :

 16 Figure 16: EPMA elemental images of the mixed oxide scale of F-treated IRIS sample after thermocyclic exposure at 900°C in laboratory air for 600 h.

Table 1 :

 1 Final mass gain and k p values of the isothermally (900°C, dry synthetic air, 120 h) exposed specimens in the untreated and fluorinated (+ F) condition.

	Sample	TNM	TNM+F	4822	4822+F	IRIS	IRIS+F
	Δm [mg/cm²]	2.3	0.7	2.3	0.3	1.4	0.4
	k p [mg²/cm 4 s] 1.22 x 10 -5 1.13 x 10 -6 1.22 x 10 -5 2.08 x 10 -