Patrice Berthod
email: patrice.berthod@centraliens-lille.org

Zohra Himeur

Pierre-Jean Panteix

Influences of the Co content and of the level of high temperature on the microstructure and oxidation of cast {Ni, Co}-based Cr-rich TaCcontaining cast alloys

Keywords: Cast superalloys, Nickel, Cobalt, Tantalum Carbides, Microstructure, High temperature oxidation

A series of six alloys derived from a Ni-25Cr-0.4C-6Ta (wt.%) base one was developed by substituting nickel by cobalt. They were synthesized by casting and exposed to oxidative environment at two high temperatures. Their bulk microstructures were studied in as-cast condition and in two high temperature aged states. Their surfaces after oxidation during aging were characterized. The cobalt enrichment succeeded in avoiding chromium carbides formation and in stabilizing the TaC carbides at high temperature. As the high temperature morphologic stability of TaC was not perfect, it was much better than the one of the chromium carbides, but can be improved by the total removal of nickel. Unfortunately, at the same time, the oxidation behavior, initially good, shows increased rate of the oxides formation. The room temperature hardness was also significantly increased by the substitution of Ni by Co, and decreased after aging when carbides became rounder or fragmented.

Introduction

In parallel with many totally different applications such as medical ones [1], nickel-based superalloys have been used for many years in components working at high temperature [START_REF] Bradley | Superalloys: A Technical Guide[END_REF]. Among them the ones able to be exploited at very high temperature are surely the single-crystalline ones [START_REF] Donachie | Superalloys: A Technical Guide[END_REF][START_REF] Szczotok | [END_REF]. These ones demonstrate superior behavior in very hot conditions in oxidative environment, even if their microstructures tend to evolve during exposure to high temperature with simultaneous application of mechanical stresses [START_REF] Bradley | Superalloys: A Technical Guide[END_REF][START_REF] Donachie | Superalloys: A Technical Guide[END_REF]5,6]. On the other hand, here too in parallel with many other applications such as prosthetic ones [7] or cutting tools [8], another important family of superalloys is constituted of the cobalt-chromium alloys, prepared by casting processes followed by forging or not [START_REF] Sims | The Superalloys[END_REF]. Cobalt-based alloys rich in chromium can be met as constitutive materials of aeronautical components working at high temperature, but also in the glass industry [START_REF] Berthod | [END_REF], for example.

In such nickel-based or cobalt-based alloys high mechanical strength may be achieved at elevated temperature following different ways, for example by gamma prime precipitates for the single-crystalline Ni alloys or by solid solution strengthening for the Co alloys. However, in both cases, significant reinforcement may also be obtained by taking benefit from dispersed hard particles as carbides [START_REF] Bradley | Superalloys: A Technical Guide[END_REF][START_REF] Donachie | Superalloys: A Technical Guide[END_REF]. Particularly efficient carbides for elevated temperatures of work are tantalum carbides, which may precipitate during solidification (primary carbides) or thanks to judicious heat treatment (secondary carbides). Their very interesting script-like shape inherited from solidification and their rather high temperature morphological stability allows high performances in term of creep resistance at high temperature. For example some polycrystalline cobalt-nickel superalloys rich in chromium and containing TaC as single carbide, are successfully used at temperatures of 1200°C and more. Lifetimes of several hundreds of hours can be reached under constant tensile stresses (several tens of MPa) in severe conditions of oxidation by gases and of corrosion/erosion by melts [START_REF] Berthod | [END_REF]. The formation of TaC logically requires the presence of carbon and of tantalum, the latter one being also able to act as a solid solution strengthener. It is also known to influence the high temperature behavior of refractory alloys in the microstructure stability field [START_REF] Donachie | Superalloys: A Technical Guide[END_REF][START_REF] Ritter | [END_REF] or in the hot oxidation one [START_REF] Young | High Temperature Oxidation and Corrosion of Metals[END_REF][START_REF] Ren | [END_REF].

When the alloy must contain high quantity of chromium to well resist to high temperature oxidation and corrosion, tantalum carbides can be difficult to obtain. This seems to depend on the base element as seen earlier in 30wt.%Cr-containing cast alloys based on Co, Fe or Ni [14][15][16], for instance. TaC easily appeared during solidification in {Co-30Cr, wt.%}-based [14] and {Fe-30Cr, wt.%}-based [15] alloys and were thereafter stable during exposure at high temperature, while it was all the contrary in {Ni-30Cr, wt.%}-based alloys [16]. The TaC which can be obtained without any problem cannot significantly reinforce Fe-based alloys rich in chromium, since the Body Centered Cubic matrix of these alloys is much too weak to allow any benefit from the presence of TaC carbides. Co-based alloys take benefit from both the intrinsic high mechanical strength of their matrix and the stable TaC carbides, but they are often hard to machine at room temperature. This difficulty is enhanced by the presence of the tantalum carbides. In addition their behavior in oxidation at high temperature is far from perfect. Indeed they are often threatened by the appearance, in addition to Cr 2 O 3 (chromia), of CoCr 2 O 4 spinel oxide and of the CoO cobalt oxide, both not protective and leading to the loss of the chromia-forming behavior. Nickel-based alloys rich in chromium appear as being a good compromise of reasonable high temperature strength and satisfying oxidation behavior. But their high temperature mechanical properties cannot be improved by TaC carbides as efficiently as for cobalt-base alloys. Inspired by what was observed in the case of addition of sufficiently high content in iron to nickelchromium alloys [START_REF] Berthod | 3Ni2Co'' alloy Tantalum carbides: Chromium carbides: Dendrites core: Interdendritique spaces: Scale bar for all: 25µm ''0Ni5Co'' alloy Tantalum carbides: Chromium carbides: Dendrites core: Interdendritique spaces: Scale bar for all: 25µm[END_REF], as is to say the stabilization of TaC carbides with favorable fractions and morphologies, one can think to apply this principle to cobalt.

In this work cobalt was effectively added to a {Ni(bal.)-25Cr-0.4C-6Ta, wt.%}base. The effect of such additions on the TaC fractions obtained during solidification was observed. The hardness at ambient temperature, the TaC characteristics after exposure at two different high temperatures and the apparent behavior in oxidation at high temperature were characterized.

Experimental details of the study

Choice and synthesis of the alloys

A series of six alloys was chosen for the study. From an initial base consisting of the Ni(bal.)-25Cr-0.4C-6Ta (wt.%) alloy, a second alloy was defined by replacing a fifth of the nickel quantity by the same amount of cobalt. A third alloy was derived from the second one by replacing a second fifth of the initial nickel quantity, again by the corresponding amount of cobalt… and so on until finishing with a sixth alloy based only on cobalt. These six alloys were named "5Ni0Co", "4Ni1Co". "3Ni2Co", "2Ni3Co", "1Ni4Co" and "0Ni5Co" respectively. Their theoretical compositions are given in Table 1.

The alloys were elaborated by using a CELES high frequency induction furnace. Appropriate amounts of pure elements (Alfa Aesar, purity > 99.9%) were first weighed using a precision balance, to have finally about 40 grams of each of the six alloys. These parts of pure elements were placed in the metallic crucible of the furnace (copper made, cooled by internal circulation of water during operation). A silica tube was placed around to isolate the fusion chamber from the laboratory air. Pumping was carried out until obtaining about 5 × 10 -2 millibars as internal pressure. Vacuum was done again and pure argon was injected until reaching 800 millibars. Pumping until 5 × 10 -2 millibars was then done again. After three cycles an internal atmosphere of about 300 millibars Ar was obtained. Heating was carried out by injecting the 100 kHz alternative current corresponding to 2500 Volts. After thermal homogenization voltage was increased to reach 4000 Volts and was then maintained for three minutes at this value. Cooling down to room temperature was followed by a second thermal cycle, identical to the first one, in order to ensure the melting of all solids and good homogenization of the melt before final solidification and cooling.

Ingot cutting, sample preparation, high temperature exposures

Each obtained ingot was cut using a Buelher Delta Abrasimet metallographic saw. Four quarters were thus obtained.

One was kept for preparing a metallographic sample devoted to the examination and characterization of the microstructure of the alloy in its as-cast state. For that this first quarter was cut again, in two parts. These parts were embedded in a cold resin mixture (ESCIL) and ground. This was done using paper from 120-grit to 1200-grit, in water. After washing and ultrasonic cleaning, samples were polished using textile disk containing 1µm hard particles until a mirror-like surface state was obtained.

Two other quarters were considered for the exposure at high temperature in a Nabertherm muffle furnace: one at 1127°C for 24 hours, and the other one at 1237°C for 24 hours (measured temperatures). After high temperature exposure they were ground with papers up to 1200-grit, with smoothing of the edges. Gold was cathodically deposited all around the oxidized samples in order to obtain electrical conductivity of the surface despite the presence of oxides, and thus to allow subsequent nickel electrolytic deposition. The gold-coated high temperature aged samples were covered all around by a sufficiently thick nickel layer to protect the oxide scales from deterioration during cutting. The parameters of this electrolytic deposition were: Watt's bath maintained at 50°C, current density: 1.6 A/dm², duration: at least two hours. Careful cutting in two halves was achieved using the Buehler Delta Abrasimet saw again, and these two halves were prepared as earlier described for the samples for ascast microstructure examination.

Bulk microstructure and oxidized surfaces examinations, hardness measurements

The bulks of the six alloys in their as-cast state and in their two aged states were examined by scanning electron microscopy (SEM: JEOL JSM6010LA) in back scattered electrons mode (BSE). The chemical compositions were controlled using the Energy Dispersive Spectrometry device (EDS) equipping the SEM and spot analyses were tried on the seen particles. For each alloy and each metallurgical state three ×1000 SEM/BSE micrographs were subjected to image analysis (Photoshop CS software) to evaluate the surface fractions of the different phases.

The cross-sectional samples prepared from the aged alloys were examined to characterize the formed oxidation products and the subsurface changes. The oxides were specified by EDS analysis and, when possible (i.e. when allowed by a not too severe oxide spallation during cooling), the thickness of the external oxide scales was measured in several locations. The depth from the alloy/scale interface in which carbides have disappeared was also measured, in several locations too. To understand the apparent oxidation behaviour the chemical composition in alloy very close to the alloy/scale interface was measured by EDS spot analysis.

The hardness of the as-cast alloy and its two versions aged at 1127°C or 1237°C was measured by indentation using a Testwell Wolpert apparatus. The Vickers method was chosen, and the applied load was 10 kg. Five indentations per sample were carried out.

Results

Qualitative bulk examinations

The microstructures of the six alloys were studied for their three metallurgical states: as-cast, after aging at 1127°C and after aging at 1237°C. To facilitate the comparison between these different states representative SEM/BSE micrographs (magnification: ×1000) are presented together in Fig. 1 for the "5Ni0Co" alloy, in Fig. 2 for the "4Ni1Co", in Fig. 3 for the "3Ni2Co" alloy, in Fig. 4 for the "2Ni3Co", in Fig. 5 for the "1Ni4Co" alloy and in Fig. 6 for the "0Ni5Co".

On these micrographs it first can be observed that all alloys started their solidification by the development of dendrites of matrix and finished it by the eutectic precipitation of matrix + carbides mix. The carbides are present only in the interdendritic spaces, in the as-cast alloys. These carbides can be of two types which were subjected to EDS analyses. Despite that they were generally too small to be analyzed using this technique, the results obtained on the coarsest carbides suggest that the black particles are really chromium carbides, with stoichiometry close to Cr 7 C 3 . Due to the uncertainty about the exact stoichiometry, it was preferred to simply note these phases as "Cr x C y ". The white particles are TaC carbides. The same particles are observed in the aged alloys but one can also remark that their morphology and location may have changed.

Concerning the as-cast microstructures it appears judicious to allocate the six alloys into three groups: a first one gathering the two Ni-richest alloys ("5Ni0Co" and "4Ni1Co") which contain chromium carbides and tantalum carbides in equivalent quantities, the second one consisting in the two alloys with almost equivalent quantities in Ni and Co ("3Ni2Co" and "2Ni3Co") which contain mainly tantalum carbides but also a little part of chromium carbides, and the third one concerning the two Co-richest alloys ("1Ni4Co" and "0Ni5Co") in which only tantalum carbides are present.

The three groups also exhibit different microstructure evolution during aging. The alloys of the first group remain with the two types of carbides after the isothermal stage, whatever the aging temperature is. But their shapes significantly evolved as coarsening of the chromium carbides and fragmentation/coarsening for the tantalum carbides are observed. These changes are more marked after heat-treatment at 1237°C than at 1127°C. The two alloys of the second group are less affected by the coarsening and fragmentation phenomena. In as-cast state they mainly contain TaC carbides and these ones seem to be less fragmented and coarsened than the two previous alloys, even after aging at the highest temperature. The chromium carbides, initially in low quantities in the as-cast microstructures are still present after aging, whatever the temperature is, and their morphologies did not seemingly evolve. Another difference with the first group is the higher tendency to precipitate numerous little secondary tantalum carbides during aging, especially when the aging temperature is 1127°C. After aging at 1237°C it seems that the TaC precipitate in the matrix are less numerous and with a greater size. Concerning the alloys of the third group, fragmentation occurred again (in higher proportions for the "1Ni4Co" alloy than for the other one) but with less coarsening of both primary and secondary tantalum carbides. Secondary TaC precipitation and coarsening were obviously hindered for the "0Ni5Co" alloy while the two phenomena are more visible for the "1Ni4Co" alloy. This is observed for the two aging temperatures.

Quantitative microstructure analysis; results of indentation

The surface fractions of the chromium carbides and of the tantalum carbides were measured on three ×1000 SEM/BSE pictures taken in zones randomly chosen in the bulk of the metallographic samples, using an image analysis tool of the Photoshop CS software of Adobe. This was done for all alloys for their three metallurgical states. The obtained results are displayed in Table 2 and their evolution versus the alloy chemical composition illustrated in Fig. 7. After having confirmed that the chromium carbides surface fraction really decreases when substitution of nickel by cobalt is increasing, it can be noticed that the quantity of chromium carbides tends to become higher during aging (Fig. 7 A). Furthermore it increases when the aging temperature increases. Concerning the tantalum carbides (Fig. 7 B), the results globally confirm that the TaC surface fraction increases when the substitution of nickel by cobalt is greater. The results concerning the 1127°C-aging are rather scattered, which does not reveal any clear dependence of the TaC fraction on the Co content. In contrast there is a very regular increase of the TaC fraction versus the Co content after 1237°C-aging. Despite the rather curious results obtained for 1127°C, it seems that aging at high temperature induced an increase in TaC quantity for all alloys.

For all alloys and their three metallurgical states five Vickers indentations under a 10kg-load were carried out. The obtained values are displayed in Table 3 and their evolution versus the chemical composition of alloy is illustrated in Fig. 7 C. These graphs clearly show a hardness increase when the Co-substitution of Ni is greater, this for all states (as-cast or aged). Knowing that cobalt is intrinsically harder than nickel, it appears rather logical to observe an increase in hardness when progressively moving from a nickel base to a cobalt one. Globally the alloys tend to be less hard after aging than before. This difference in hardness is more visible between the as-cast alloys and the 1237°C-aged ones, this seeming to point out an effect of the coarsening of the chromium carbides and of the fragmentation + coarsening of the TaC. One can notice that the intermediate alloys ("2Ni3Co" and "3Ni2Co") tend to be a little harder after 1127°C-aging than in their as-cast condition and in their 1237°C state: this may point out a strengthening effect of the numerous secondary carbides precipitated in matrix during the 1127°C exposure.

Qualitative cross-sectional examinations

The surface (and subsurface) states of the six alloys aged at 1127°C and aged at 1237°C were characterized on the same cross-sectional samples as previously. Again, to facilitate the comparison between these different states per alloy representative SEM/BSE micrographs (magnifications: ×250 and ×1000) are presented together in Fig. 8 for the "5Ni0Co" alloy, in Fig. 9 for the "4Ni1Co", in Fig. 10 for the "3Ni2Co" alloy, in Fig. 11 for the "2Ni3Co", in Fig. 12 for the "1Ni4Co" alloy and in Fig. 13 for the "0Ni5Co".

The surface and subsurface behavior of all alloys, for all temperatures, can be described as follows:

-oxidation took place externally with the formation of an outer continuous oxide scale covering the alloy, involving chromium and also possibly the base elements -oxidation also occurred internally with appearance of CrTaO 4 oxides in the subsurface close to the alloy/external oxide scale interface -carbides of all natures (Cr x C y , TaC) and all origins (primary or secondary) disappeared over a depth of several tens of micrometers. They are present again deeper.

Considering the high temperature oxidation point of view the six alloys may be gathered into groups. The first group, including the three nickel-richest alloys ("5Ni0Co", "4Ni1Co" and "3Ni2Co"), corresponds to the outer presence of a Cr 2 O 3 (chromia) scale, unfortunately lost during cooling in some cases (1237°C); when observable, this oxide is obviously compact and efficiently protected the alloys from too rapid and general oxidation. The second group, containing the three cobalt-richest alloys ("2Ni3Co", "1Ni4Co" and "0Ni5Co"), is distinguished by the deterioration of the chromia scale (irregular, presence of porosities) and/or the upcoming loss of the chromia-forming behavior (presence of nickel and/or cobalt oxides).

Quantitative metallographic results about oxidation

After these first observations, the thickness of the external scale was measured in about five different locations on the cross-sections, as well as the depth of the carbidefree zone. The results are displayed in Table 4 and plotted versus the chemical composition of the alloy in Fig. 14 (Fig. 14 A for the oxide scale thickness and Fig. 14 B for the carbide-free depth). To complete the observations concerning the lasting quality or inversely the imminence of the loss of the chromia-forming behavior, three to five spot EDS measurements of the local chemical composition of the alloy were performed close to the oxidation front. The results are displayed in Table 5 and plotted versus the alloy chemical composition in Fig. 14 C).

It appears that the oxide scale thickness increases with the Ni by Co substitution in alloy and with the temperature. For the first point, no evolution may be noticed as long as nickel remains the major element (from "5Ni0Co" to "3Ni2Co"), but the oxide thickness dramatically increases when the alloy contains more cobalt than nickel. The evolution of the carbide-free depth seems being inversed, notably at 1237°C. Indeed, by adding cobalt the carbide-free zone decreases in depth until cobalt becomes more present than nickel. The carbide-free depth becomes stabilized for further cobalt increases. Concerning the samples aged at 1127°C, the evolution is not so monotonous. Furthermore the three nickel-richest alloys seem to exhibit carbide-free zones a little thinner than the three cobalt-richest ones. The evolution of the chromium content in extreme surface versus the Co content in the alloy is the same for the two temperatures: it regularly decreases between the alloy only based on nickel and the alloy only based on cobalt.

Discussion

For 25Cr, 0.4C and 6Ta (wt.%), whatever the Co/Ni balance for the base element is, the obtained alloys all contain, in the as-cast condition, a dendritic matrix and primary carbides on a eutectic nature and located in the interdendritic spaces, without any other particle/phase such as pre-eutectic carbide for example. A microstructure thus constituted of imbricated matrix dendrites and carbides closely mixed with the periphery of these dendrites is favorable to high mechanical properties at elevated temperature. Unfortunately the morphology of the carbides evolves rapidly at high temperature, faster for 1237°C than for 1127°C, faster for chromium carbides than for tantalum carbides and, for the later ones, faster when the matrix is rich in nickel than when it is rich in cobalt. When initially present, as is to say when the alloy is richer in nickel than in cobalt, the chromium carbides coalesce and become coarser and rounder. The tantalum carbides become fragmented into alignments of round carbides. This firstly resulted in a decrease of room temperature hardness for all alloys, from initial hardness levels which become higher when cobalt substitutes more nickel in the alloy. Secondly, this reveals highly probable mechanical weakening of the alloy at high temperature.

The tendency to coarsening or fragmentation of the carbides may be easily explained by the thermodynamic reduction of the alloy energy due to the decrease of the cumulative matrix-carbides interface area. After 24 hours of exposure at 1127°C this morphologic evolution is not sensibly advanced but some first little changes can be observed. It can be assumed that the carbides morphology is significantly transformed after much longer exposure. Fragmentation and/or coarsening occurred faster at 1237°C, with significant modifications after only 24 hours. Since this affects essentially the nickel-richest alloys specifically for their chromium carbides, it clearly appears that these alloys can be expected to be used under stress only at 1100°C and lower temperatures than at around 1200-1250°C. At this later temperature the cobalt-richest may be still usable. The intermediate alloys, containing almost cobalt as much as nickel, seem to be the best candidates to resist mechanical stresses and creep at around 1100-1150°C. Indeed, their primary carbides (all of tantalum) are almost unaffected, and they exhibit particularly dense and homogenous fine TaC carbides in the matrix, which obviously precipitated spontaneously during the 24 hours at 1127°C. So, concerning the bulk microstructure, it can be concluded that progressively substituting nickel by cobalt has a triple effect: (i) hardening the matrix, (ii) replacing the part of primary carbides which were initially chromium carbides by more morphologically stable TaC carbides, and stabilizing at high temperature the morphology of TaC carbides which were already more stable than the chromium carbides. On the microstructure point of view, and consequently for the resulting mechanical properties, the temperature at which the M-25Cr-0.4C-6Ta alloy may be usable under stresses increases of about one hundred Celsius degrees.

Concerning the behavior in high temperature oxidation the evolutions of the three considered parameters (oxide scale thickness, subsurface carbide-free depth, chromium content in extreme surface) are consistent and demonstrate a significant decrease in oxidation resistance by replacing nickel by cobalt. By crossing the {Co=Ni} limit the external scale becomes clearly deteriorated in quality (porosity) and purity (not only chromia but additional presence of cobalt oxide). Due to the poorer protection of alloy, oxidation becomes faster, this resulting in thicker external oxide. In the same time the chromium diffusion in the bulk becomes more difficult, and its diffusivity decreases with the progressive replacement of the nickel-based matrix by a cobalt-based ones. This limits the depth of alloy supplying the oxidation front in Cr: chromium carbidefree zone is not so deep, and a more intensive solicitation of the thinner outer part of the alloys involves locally low chromium contents. Tantalum and tantalum carbides are also concerned: indeed, for the nickel-richest alloys, the limit between the carbide-free zone and the inner part of alloy still containing TaC carbides obviously corresponds to the same concerning chromium and the chromium carbides. When TaC are the single carbides initially present (i.e. case of the cobalt-richest alloys) the tendency goes on: the limit between alloy with TaC and the carbide-free zone progressively approaches the oxidation front although the oxidation of tantalum in subsurface seems to increase (seemingly more CrTaO 4 oxides). Diffusion of tantalum to meet oxygen diffusing in the alloy in the other direction seems then more difficult. This double increasing difficulty for Cr and Ta to diffuse through the matrix more and more enriched in cobalt (to the detriment of nickel) appears to be the origin of the lower oxidation resistance (Cr) but also of the persistence of an interdendritic network of TaC still close to the surface and favorable to a mechanical resistance of this outer zone of the alloy.

Conclusion

By exploring the as-cast microstructures, the microstructures aged at different high temperatures, the evolution of the corresponding room temperature hardness and by having a look at the oxidized states after aging treatment, this work pointed out several significant consequences of the progressive substitution of the base element by cobalt until total replacement in an initial chromium-rich nickel-based alloy containing carbon and tantalum in equivalent atomic quantities. As expected this evolution of chemical composition had positive effects (chromium carbides deleted in the microstructure and simultaneous increased TaC densities, progress in carbide stability at high temperature, potentially better mechanical behavior at high temperature) but also negative ones (increase of hardness and probably lower machinability, deterioration of the high temperature oxidation behavior). After these first observations and the consequent hypothesis, further work may consist in really characterizing the high temperature mechanical behavior of this series of alloys, as well as longer exposures at high temperatures, to confirm the tendencies revealed here about the bulk microstructures changes and the transition from good behavior to catastrophic oxidation. Fortunately, it remains possible to enrich in chromium the surface and subsurface of the alloys containing TaC as single carbide, by using pack-cementation. Indeed it was demonstrated that such coating operation may be successful for polycrystalline cast cobalt-nickel containing only TaC [18,19]. Thus, alloys with both high strength and high chemical resistance may be considered for work under stress and aggressive environment at temperatures between 1100°C and almost 1250°C, thanks to efficiently reinforcing TaC particles and to available chromium leading to protective chromia scale. This latter might be subject to volatilization at so high temperatures, but it can also provide several improvements, as a better resistance against both oxidation by gases and corrosion by melts.

Fig. 1 .'Fig. 2 .Fig. 5 .Fig. 7 .Fig. 8 .

 12578 Fig. 1. SEM/BSE micrographs illustrating the as-cast (bottom), 1127°C-24h aged (middle position) and 1237°C-24h aged (top) microstructures of the "5Ni0Co" alloy

Fig. 9 .

 9 Fig. 9. SEM/BSE micrographs illustrating the surface (A, B) and subsurface (A, C) deteriorations of the "4Ni1Co" alloy after 24 hours spent at 1127°C (A) or 1237°C (B & C) in hot laboratory air

Fig. 10 .

 10 Fig. 10. SEM/BSE micrographs illustrating the surface (A, B) and subsurface (A, C) deteriorations of the "3Ni2Co" alloy after 24 hours spent at 1127°C (A) or 1237°C (B & C) in hot laboratory air

Fig. 11 .

 11 Fig. 11. SEM/BSE micrographs illustrating the surface (A, B) and subsurface (A, C) deteriorations of the "2Ni3Co" alloy after 24 hours spent at 1127°C (A) or 1237°C (B & C) in hot laboratory air

Fig. 12 .

 12 Fig. 12. SEM/BSE micrographs illustrating the surface (A, B) and subsurface (A, C) deteriorations of the "1Ni4Co" alloy after 24 hours spent at 1127°C (A) or 1237°C (B & C) in hot laboratory air

Fig. 13 .Fig. 14 .

 1314 Fig. 13. SEM/BSE micrographs illustrating the surface (A, B) and subsurface (A, C) deteriorations of the "0Ni5Co" alloy after 24 hours spent at 1127°C (A) or 1237°C (B & C) in hot laboratory air

Table 1

 1 Chemical compositions of the studied alloys; theoretic and obtained*

		Ni	Co	Cr	Ta	C
		68.6	0	25	6
	5Ni0Co	Bal.	/	25.5 ±0.1	6.5 ±0.2
		54.88	13.72	25	6
	4Ni1Co	Bal.	14.3 ±0.2	25.4 ±0.4	5.6 ±0.9
	3Ni2Co 2Ni3Co	41.16 Bal. 27.44 25.4 ±0.3	27.44 27.2 ±0.2 41.16 Bal.	25 25.4 ±0.1 25 25.7 ±0.4	6 6.5 ±0.2 6 6.9 ±0.2	Expected 0.4wt.% for all alloys
		13.72	54.88	25	6
	1Ni4Co	12.7 ±0.5	Bal.	26.2 ±0.2	6.6 ±0.8
	0Ni5Co	0	68.6	25	6
		/	Bal.	25.3 ±0.2	7.2 ±0.5
	*: average and standard deviation values from the SEM/BSE full frame measurements
			three randomly chosen ×250 areas

Table 2

 2 Surface fractions of the tantalum carbides and of the chromium carbides present in the alloys in their different metallurgical states

Alloy designation Metallurgical state Surf.% TaC Surf.% Cr x C y average value standard deviation average value standard deviation 5Ni0Co

		Aged @1237°C	2.9	±0.3	4.8	±1.7
		Aged @1127°C	2.6	±0.2	4.0	±0.7
		As-cast	1.3	±0.3	3.3	±0.5
		Aged @1237°C	4.7	±0.1	3.1	±0.3
	4Ni1Co	Aged @1127°C	5.8	±0.7	2.4	±0.8
		As-cast	1.6	±0.0	1.2	±0.0
		Aged @1237°C	5.8	±0.9	0.4	±0.3
	3Ni2Co	Aged @1127°C	4.1	±0.2	0.9	±0.4
		As-cast	3.8	±1.0	1.4	±0.3
		Aged @1237°C	6.8	±0.9	0.1	±0.0
	2Ni3Co	Aged @1127°C	5.7	±0.4	0.1	±0.1
		As-cast	6.8	±1.0	0.1	±0.0
		Aged @1237°C	7.2	±0.1	0.0	±0.0
	1Ni4Co	Aged @1127°C	3.5	±0.7	0.0	±0.0
		As-cast	5.6	±2.2	0.0	±0.0
		Aged @1237°C	7.0	±0.8	0.0	±0.0
	0Ni5Co	Aged @1127°C As-cast	4.7 6.2	±0.7 ±1.4	0.0 0.2	±0.0 ±0.2

Table 3

 3 Hardness of the alloys in their different metallurgical states

	Alloy	Metallurgical	Hardness / Hv 10kg
	designation	state	average value	standard deviation
		Aged @1237°C	179
	5Ni0Co	Aged @1127°C	193
		As-cast	205
		Aged @1237°C	184
	4Ni1Co	Aged @1127°C	204
		As-cast	196
		Aged @1237°C	202
	3Ni2Co	Aged @1127°C	223
		As-cast	199
		Aged @1237°C	218
	2Ni3Co	Aged @1127°C	270
		As-cast	260
		Aged @1237°C	270
	1Ni4Co	Aged @1127°C	299
		As-cast	296
		Aged @1237°C	306
	0Ni5Co	Aged @1127°C As-cast	337 339

Table 4

 4 Thickness of the external oxide scales and depth of the carbide-free zones after oxidation for 24 hours at 1127°C or at 1237°C

	Alloy	Temperature	Scale thickness (µm)	Carbide-free depth (µm)
	designation	of oxidation	average	standard	average	standard
			value	deviation	value	deviation
	5Ni0Co	1237°C 1127°C	/ 5.9	/ ±1.6	128 28	±8 ±3
	4Ni1Co	1237°C 1127°C	17 6.0	±6 ±2.3	98 18	±8 ±5
	3Ni2Co	1237°C 1127°C	/ 4.9	/ ±0.7	85 19	±5 ±2
	2Ni3Co	1237°C 1127°C	21 16	±4 ±4	88 42	±10 ±7
	1Ni4Co	1237°C 1127°C	// 24	// ±5	90 36	±12 ±8
		1237°C	//	//	84	±7
	0Ni5Co	1127°C	//	//	29	±6
		/: oxide scale spalled off	//: oxide scale not regular

Tantalum carbides: Chromium carbides: Dendrites core: Interdendritique spaces:


Nathalie Morel
email: nathalie.morel@sciencespo.fr

Chloé Touzet
email: chloe.touzet@spi.ox.ac.uk

Michaël Zemmour
email: michael.zemmour@univ-lille1.fr

Research Group "Socio-Fiscal Policies" Fiscal welfare in Europe: a state of the art

Keywords: Social tax expenditures, Fiscal welfare, Fiscal welfare reform, incentives, hidden welfare state

Engaging with fiscal welfare leads to a reassessment of existing understandings of welfare state trajectories in Europe, with regard to their political, institutional or distributional dimensions. Therefore, it is a necessary exercise. Yet most of the recent literature on social tax expenditures focuses on the US, and both literature and data on European cases are sparse. The present article reviews the various concepts used to operationalize the analysis of fiscal welfare and argues that the "Social Tax Expenditures" concept is best adapted to the exercise of measuring the size and defining the properties of fiscal welfare. It presents the data currently available, demonstrates its limits, and identifies pressing gaps in terms of data generation in the field. Finally, based on a survey of the emerging literature on the topic, it presents the potential drivers and effects of the use of Social Tax Expenditures, some of them specific to the European context.

I. Introduction

In 1958, British scholar Richard Titmuss highlighted what he termed the 'social division of welfare', distinguishing between three sources of welfare: social, occupational and fiscal welfare. He noted that most scholarship on the welfare state restricted itself to the world of social welfare, that is the direct public provision of welfare, failing to note the growing scale and distributive tendencies of occupational and fiscal systemsand the ways in which they often ran counter to the distributive directions of the social welfare system. While US scholars have highlighted the importance of tax expenditures in the American welfare state, in Europe the importance and role of fiscal welfare still remains a blind spot in the welfare state literature, despite the growing acknowledgement of the significant use of social tax expenditures, notably through OECD research conducted in the last 20 years. Indeed, fiscal welfare is no longer an attribute of the liberal welfare states alone (Esping-Andersen, 1990). It also contributes to organising the household services and care markets in various Nordic or Continental countries; it is a key driver of the development of private pension and health insurance in France; it played a key role in "making work pay" policies in various countries. This development of fiscal welfare across a range of welfare regimes needs to be taken into account, to obtain a better descriptive picture of welfare states'distributive outcomes, and to understand changes in the politics, the governance, and the institutional architecture of specific national welfare arrangements.

To do so, however, certain prerequisites need to be met: a stabilized and comparable concept is necessary to empirically operationalize the notion of fiscal welfare; a comprehensive description of existing schemes, as well as comparable quantitative data, are also needed. Eventually, analysing European fiscal welfare implies to partially depart from the lens used in analysing fiscal welfare in the last twenty years (as this was mainly done in the very specific US context 1 ), and to develop an analytical framework adapted to the European cases.

This article aims to tackle these three challenges, opening the 'black box' of European fiscal welfare, by providing a state of the art of the existing literature (mainly US-based, but also including the sparse European literature), and by highlighting the limitations of the existing scholarship on the uses and impacts of fiscal welfare in Europe. In doing so, this state of the art opens up an agenda for further research. Specifically, three questions are addressed in each of the three parts of this article. First, we inquire into the state of the conceptual discussion on fiscal welfare and look for the most relevant concepts to use, from a political economy of the welfare state perspective. Second, we ask what data on social tax expenditures in Europe is available, and what methodological issues need tackling in this regard. Finally, we ask what is known about the uses of fiscal welfare in Europe.

II.

Setting the conceptual scene: what are "fiscal welfare" and "social tax expenditures"?

The concept of fiscal welfare, defined as the state's use of the tax system in order to provide subsidies for social purposes, was developed by [START_REF] Richard | Essays on 'the Welfare state[END_REF] as part of his theory of the "social division of welfare" (SDW). This theory argued that, contrary to the commonly held assumption, middle and higher income earners substantively benefit from the welfare state, which does not merely transfer from rich to poor. This common misrepresentation stems from a truncated understanding of welfare. For Titmuss, a comprehensive definition of the welfare state should not be reduced to social welfare (direct public transfers and services) but should include "all collective interventions to meet certain 1 Thus, one of the most striking results of the literature on the US case is that taking social tax expenditures into account there shows that the Federal state is much more interventionist in the social policy field than suggested by figures on direct social expenditures. We cannot expect this to be the most important part of the diagnosis in the European case.

needs of the individual and/or to serve the wider interests of society (Titmuss, 1958:42). It thus encompasses three major categories: social welfare, occupational welfare, and fiscal welfare.

While the conceptualization of Titmuss remains relevant to expose the general phenomenon of SDW, it does not provide any tools to operationalize it in the same way that we do for social welfare (especially to identify and quantify its component). In the literature engaging with this topic, several different and partly overlapping concepts have spread. In the present section we map the relation between these categories.

The scholarship on fiscal welfare includes, e.g., the study of tax breaks for mortgages [START_REF] Rune | The hidden welfare state in comparative perspective, tax expenditures and social policy in different welfare models[END_REF], income deduction for insurance premium, tax-free child allowances, reduced taxation for the elderly, or tax cuts subsidizing occupational benefits (see e.g. Greve 1994, Kvist and[START_REF] Jon | Comparing tax routes to welfare in Denmark and the United Kingdom[END_REF]. Scholars in the SDW tradition (e.g. [START_REF] Sinfield | Analyses in the social division of welfare[END_REF][START_REF] Sinfield | Comparing divisions of welfarea suitable framework for the analysis of welfare in transition[END_REF][START_REF] Rose | Re-reading Titmuss: the sexual division of welfare[END_REF][START_REF] Mimi | Everyone is on Welfare: The Role of Redistribution in Social Policy Revisited[END_REF][START_REF] Kirk | The social division of welfare: a class struggle perspective[END_REF], 2009) analyse the division of welfare in the light of prevailing social divisions based on occupation, or gender, and establish a "close correspondence between class, power, and the SDW" (Mann, 1991:11): while means-tested social welfare goes to the least well-off, fiscal welfare is welfare for the rich (see e.g. [START_REF] Sinfield | Tax privileges: a suitable subject for social policy?[END_REF], 1997[START_REF] Orton | Exploring neglected dimensions of social policy: the SDW, fiscal welfare and the exemplar of local taxation in England[END_REF].

The realization that the well-off benefited disproportionately from provisions of the US tax code is also what led tax scholars to draw attention to "tax expenditures" (TEs) in the late 1960s. TEs were defined as "departures from the normal tax structure (…) designed to favour a particular industry, activity or class of persons [in the form of] permanent exclusion from income, deductions, deferrals from tax liabilities, credits against tax, or special rates" equivalent to "government spending for favoured activities or groups" (Surrey and McDaniel, 1985:3).

Despite facing numerous criticisms at first2 , the concept spread, both in academic and administrative circles, in the US [START_REF] Surrey | Tax expenditures analysis: the concept and its use[END_REF][START_REF] Toder | Tax Cuts or Spending -Does it Make a Difference?[END_REF] as well as in other national contexts (Owens 1983, Surrey and McDaniel 1985 3 ). Most OECD countries now publish yearly lists of their main tax expenditures. TEs are not necessarily fulfilling a social policy purpose; they constitute an object larger than social tax expenditures (STEs). STEs have been defined as "social spending delivered as tax breaks" (Stebbing and Spies-Butcher, 2010:586); they can be conceived of as an operationalization of fiscal welfare, or the general set of tax expenditures (departures from the fiscal norm) that contribute to social policy4 .

A neighbouring concept is that of the OECD's "tax breaks for social purpose", or TBSPs (Adema, 1997:154;see also, 2011[START_REF] Silvia | The distributional effects of personal income tax expenditure[END_REF]. TBSPs are benefits granted through the tax system. They can perform "the same policy function as transfer payments which, if they existed, would be classified as social expenditures" (this is the case, for instance, of tax credits for families); they can also aim "at stimulating the private provision of benefits"e.g., tax cuts for private health insurance (Adema et al, 2011:29).

Finally, US scholars have developed the concept of "hidden welfare state" to describe the role of "indirect tools of social policy such as loans, loan guarantees, and tax expenditures" (Howard, 1997:5) in the American welfare state, typically in sectors like pensions, occupational health insurance, housing, health care, childcare, employment or income support. This "constellation of more indirect or 'hidden' government interventions" (Hacker, 2002:12) was perceived as a blind spot in also Shaviro, 2003); Wildavsky (1985:414) criticized the concept for attributing default ownership of all individual income to the state. [START_REF] Kleinbard | Rethinking Tax Expenditures[END_REF] argued that TE analysis was often "a thinly veiled agenda for a specific form of tax reform", with the "normal tax system" being "not simply an analytical tool, but (…) also an aspirational goal of the process" (Kleinbard, 2008:6). 3 The term has lost its controversial power, and is also now routinely included in taxation textbooks (see e.g. Gravelle, 2005).

the US social policy literature 5 (Blank 2009). The US hidden welfare state is described as rather substantial, "38 to 50 percent as large as the visible welfare state" according to Howard (1997).

These various concepts are rarely articulated together, yet we attempt to put them on the same theoretical map. The "hidden welfare state" includes the largest range of tools (i.e. regulation, tax incentives, payments to private organizations and households), limited to the welfare sector; the TE concept is the most encompassing in terms of policy sectors spanned, limited to one type of policy instrument. In this article, we focus on the area of welfare (comprising both social and employment policies), and on a definite set of policy tools, namely tax instruments aiming not to raise revenue but to perform social policy objectives. Thus this article's object of study seems best described by the concepts of STEs and fiscal welfare. Indeed the perimeter of TBSPs, based on discretionary choices at the country level 6 and on additional criteria 7 , is too restrictive; some STEs (e.g. some employment policies targeting firms which we consider to be an important part of fiscal welfare) are excluded from it.

Two particular conceptual critiques should be addressed when analysing these types of instruments. Prasad (2011) criticized the tendency to equate TEs and traditional instruments of social policy. Considering revenue foregone in the form of tax expenditures as similar to welfare spending leads to the paradoxical idea that the more tax revenue foregone, the larger the welfare state (Prasad, 2011:254), and thus that the US welfare state and some of the more generous European ones are in fact similar, once the hidden part of the US welfare state is ack-5 An insight shared with the work on TBSPs is that traditional social policy instrument "do not exhaust the available strategies for intervention" (Hacker, 2002:11); closely-related concepts include the "divided" [START_REF] Hacker | The Divided Welfare State: The Battle over Public and Private Social Benefits in the United States[END_REF], "submerged" (Mettler, 2011), or "delegated" (Morgan andCampbell, 2011) welfare state. 6 Thus Germany does not report the preferential taxation of families and couples as a TBSP, while France does. Data on TBSPs for the UK only contains a very small portion of expenditures on tax credits, because the largest part (the refundable part paid out to beneficiaries) is counted as a direct expenditure. 7 See note 11.

nowledged (cf. [START_REF] Alber | What the European and American welfare states have in common and where they differ: facts and fiction in comparisons of the European Social Model and the United States[END_REF]. This is problematic, because while "redistribution and collectivization of risk (…) are the primary achievements of the welfare state", it is unclear whether the "hidden welfare state" delivers on them (Prasad, 2011:261). To avoid such pitfalls, Prasad argues that these instruments should not be analysed as part of the "welfare state"8 , but as a form of tax cut.

Yet, in excluding STEs from the realm of welfare, one risks missing the realization that all different income groups in society benefit from some form of state intervention (Titmuss, 1958:52); such artificial divisions risk undermining support for the visible welfare state; Prasad, in contrast, warns that wrongly-assumed equivalences risk concealing"the reprivatization of risks that had previously been met collectively, and the waning of state capacity" (Prasad, 2011:263).

We argue that using the concepts of STEs (when referring to precise policy instruments) and fiscal welfare (for more general references to the phenomenon), is best adapted to research on the political economy of welfare states, and allows avoiding both pitfalls. We situate both concepts in the analytical framework of "the welfare state"; yet, the many ways in which fiscal welfare differs from traditional welfare, as well as how it might affect the latter, are precisely the subject of our analysis.

We exclude from the scope of the present article the discussion on the general tax structure (size, progressivity and predominance of different types of taxation), as well as the level of taxation of social benefits (the claw-back effect). These dimensions are certainly of relevance in order to depict and understand national welfare systems, since they determine the economic, distributive and political properties of a national welfare system. Social tax expenditures, however, are an additional dimension of the analysis, situated at the crossroad between the tax structure and the benefit system.

III. Where do we find STEs in Europe? An overview of available data

In addition to developing conceptual tools, quantitative measurement of STEs is necessary to analyse fiscal welfare, and in particular to assess their financial and political importance and to draw comparisons over time and across countries. As will be demonstrated below, so far, reasonably comprehensive and comparable data is still missing in most of countries, and even more so at the international level. Here we draw a panorama of available sources and we make some recommendations as to how they should be used; we also highlight directions for future data generation.

The issue of tax expenditures has been increasingly prominent in OECD research since the late 2000s. In 2010, data on tax expenditures in six countries (namely the US, the UK, Spain, the Netherlands, Germany, and Canada) was collected, and harmonized to some extent9 (OECD, 2010). As far as STEs in particular are concerned, the OECD dataset on Net Social Expenditures (Adema et al., 1997 and following) is the only dataset10 with comparative, cross-sectorial data on STEs in OECD countries. Since 2001, information is collected every two years through a questionnaire sent to relevant administrations in 33 countries. A simple comparison between these two OECD dataset show dramatic discrepancies, probably due to the lack of stabilized statistics in national administrations, but also to different methodologies used in building the datasets (Figure 1). Among both sources, the Net Social Expenditures dataset is the most useful for welfare scholars. It allows substantiating the claim that STEs are a substantial phenomenon in Europe. It also allows identifying some of the main policy domains in which STEs are prevalent, across countries. Yet, caution should apply when it comes to using this data for descriptive or comparative purposes, as there is a good chance that it captures only a truncated picture of the phenomenon under study.

First, the division of the dataset in three parts (part A is concerned with the direct taxation of social benefits, part B documents the indirect taxation of consumption out of benefit income, and part C is made of estimations of Tax Breaks for Social Purposes) leads to an underestimation of the data on TBSPs, since some STEs such as reduced tax rate on pensions or unemployment benefits, or reduced VAT for drugs, might be recorded in part A and B, and excluded from part C. For our purpose of estimating the sums spent on various STEs, this information is thus lost, because part A and B record itemized tax rates, and do not provide estimates of revenue foregone.

In the same manner, some STEs might be already recorded in the OECD social expenditures database (Socx), and thus be excluded from part C of the net social expenditure database: this is the case, for some countries (like the United Kingdom), of the part of tax credits that is paid out to beneficiaries, and not offset against tax liabilities (which, in many case, represents the lion's share of tax credits). In this way, a significant part of STEs is likely to be left out of the picture.

In part C itself, 27 % of the data is missing; it is not clear whether these missing data points correspond to non-answers from national governments, or simply to the absence of any estimation at the national level.

Data on pensions are systematically reported separately, as "memorandum items", because of comparability concerns (since different calculation rules are used in different countries).

Second, some specific STEs are also missing, which could potentially be included, such as tax breaks for firms, intended as incentives for job creation, mortgage-friendly policies, or some STEs subsidizing optional occupational welfare schemes. This is due both to the restricted definition of TBSPs adopted by the OECD11 , and to arbitrary decisions on reporting at the national level. Indeed, the questionnaire sent to governments contains very little guidance, and ultimately leaves to national administrations the decision of what to include in the report. There is no harmonization a posteriori either. The result of this is an important variation in the data reported, both between countries, and between years for a given country.

Thus, beyond systematically underestimating the size of the phenomenon, this data is likely to lead to flawed cross-country comparisons, and to bias longitudinal analyses: indeed, apparent changes in the data might reflect arbitrary modifications of the perimeter, changes in estimation techniques, or improvement of tracking systems at the national level, rather than an actual increase in spending.

Overall, while this dataset is useful to get a preliminary picture of the phenomenon, we argue that it should not be used for comparative purposes. The likely underestimation inherent in the data should also be kept in mind when using it for descriptive purposes.

The following graph (figure 2) represents STEs as a percentage of total public social spending in 15 OECD countries, highlighting the two policy sectors concentrating most spending on STEs in each country. We include it mainly to illustrate the starting point of this paper, i.e. that STEs are used, for a variety of welfare policy purposes, across European countries; all the limitations highlighted above apply to the data presented in this graph. There are two reasons why the existing data on STEs gathered so far is still plagued by the limitations highlighted above, beyond the sheer lack of attention from researchers and governments alike, until relatively recent times. First, the cost of STEs cannot be measured: it has to be estimated. Producing accurate estimations of the budgetary costs incurred by states has proven a difficult enterprise [START_REF] Altshuler | Reconsidering Tax Expenditure Estimation[END_REF]Dietz, 2011, Kleinbard 2008). Three main methods are used to estimate the cost of TEs in the literature [START_REF] Swift | The Role of Fiscal Welfare in an Illiberal Democracy. The Case of Flat Tax and Family Tax Allowances in Hungary since 2010[END_REF], each with its own challenges and limitations: the revenue gain method estimates the expected gain for public finances if a particular TE was to be abolished; hard-to-estimate behavioural changes and interaction effects need to be factored in. The outlay equivalence method estimates the net cost of the direct expenditure which would be necessary to achieve the same post-tax distribution, if replacing the TE in question; it is a more distant estimation of costs really incurred. The revenue foregone method measures the government's lost revenue as the difference between the respective tax liabilities of a taxpayer getting the TEs, and a similar taxpayer not affected; but this method (recommended by the OECD) does not take into account chain reactions in the tax and benefit system, or behavioural change. This could lead to either over or under12 estimations.

The complexity of these estimation techniques, coupled with a relative lack of attention until fairly recent times, means that national administrative data on the subject is still largely incomplete. Some cases of "departure from the fiscal norm" are not recorded as tax expenditures and their cost is not regularly estimated (this is typically the case for social security contribution exemptions). More generally, the quality of estimations (both ex ante and ex post) is uneven. In France, in 2009, more than half of the tax expenditures were not evaluated. In 2010, only "estimates" were available for a large number of them [START_REF] Pollard | L'action publique par les niches fiscales, l'exemple du secteur du logement[END_REF]. Estimations are often exactly the same from year to year (with no account taken of the effect of either economic activity or inflation). In the UK, [START_REF] Sinfield | Tax benefits and their impact on the social division of welfare[END_REF] has shown that while the Office for Tax Simplification has identified 1140 forms of tax relief, Her Majesty's Revenue and Customs (HMRC) acknowledges 400 tax reliefs, only 200 of which are costed.

The second difficulty is linked to the production of cross-country data in particular. Namely, the task of defining a consensual common "normal tax benchmark", against which to define what an STE is, is deemed near impossible. Reliably comparable data is extremely difficult to produce because, beyond the complexity inherent in finding a "norm" to multiple idiosyncratic tax systems, doing so would also necessarily involve a degree of arbitrariness, which sits uneasily with the desired neutrality of the researcher.

Taking into account both the limitations of existing data and the difficulties in generating better data, we argue that future research should focus on producing two types of data: country-specific data, and comparative data at the policy level. Focusing on the first type would allow guaranteeing better internal consistency over time, and getting a clearer picture of fiscal welfare in specific national contexts. Such data is already emerging in some countries (see e.g. Houssoy and Zemmour, 2016, for France), and should be pursued further. For comparative purposes, focusing on producing reliably comparable data of similar policies would allow bypassing many of the issues linked to the absence of a common benchmark, and limiting the degree of arbitrariness involved.

In this endeavour, micro-simulation models are likely to be particularly useful. In particular, the EU-wide model EUROMOD, which simulates the effects of national tax and benefits systems for the 28 EU member states using household income survey [START_REF] Sutherland | EUROMOD: the European Union tax-benefit microsimulation model[END_REF], is particularly adapted to generating data on European fiscal welfare. EU-ROMOD automatically includes certain type of STEs (such as in-work tax credit, and family tax credit), and can also be used to simulate data for some STEs conditional on private expenditures (such as, e.g. STEs incentivizing the purchase of private pension plans) provided complementary data on consumption of these services is available. But some forms of STEs, that are not linked to income tax or personal social security contributions, but to other forms of taxes (such as, e.g., corporate tax, or employer's social security contributions) cannot be simulated using EUROMOD.

Thus, although the model has already been used to include specific STEs in distributive analysis on the welfare state (e.g. [START_REF] Avram | Income redistribution in the European Union[END_REF], EUROMOD only allows drawing an incomplete picture of the distributive effect of fiscal welfare as whole. Yet, an important reason why producing more reliable data is needed, beyond obtaining a complete and more fine-grained description of the welfare state, is linked to the distributive effect of fiscal welfare. As explained above, available data is likely to be skewed (with some forms of tax expenditures, e.g. in-work tax benefits more visible and well-monitored than others in administrative data); thus even the few existing evaluations of the distributive impact of STEs draw an incomplete picture. In order to better assess the distributive profile of different welfare states, better and more comprehensive data on fiscal welfare needs to be included.

IV. The uses and potential effects of STEs in European welfare states

Having dealt with the conceptualization of fiscal welfare, and having drawn a road map for its better quantification, a third important question needs to be addressed, namely why governments use STEs (as opposed to benefit in cash or in kind), and how this use transforms national welfare arrangements. This question has already received significant attention in the US context; yet the emerging literature on European fiscal welfare tends to demonstrates that different trends are also at play there, that are specific to European contexts.

Insights from the US-based literature

The question of why and how STEs are used has been the subject of extensive studies in the American context, this literature highlighting some of the specificities of STEs, and pointing to some of the reasons why STEs can be attractive instruments in the social policy field.

Tax instruments are described as discreet and less traceable than traditional social welfare instruments. Democratic and popular scrutiny might be lighter (Hacker, 2002:44). Legislative and bureaucratic processes of the "hidden welfare state" are also described as less constraining than those characterizing direct social expenditures (Howard, 1997:30). Fewer veto points characterize the policy process in the US context when using "hidden welfare" instruments.

Political processes also appear to be different: the ambiguity that is described as inherent to fiscal welfare instruments (Howard, 1997:141), means that they can be embraced on multiple grounds. In that sense, they are an ideal policy tool to cut deals and reach compromises, because they can be framed in many different ways that can suit broader coalitions than traditional "visible" spending.

STEs would be an ideal tool for Conservative governments to mobilize in a "layering technique" in order to transform well-entrenched policies: private programmes, publicly subsidised through tax expenditures, can be discretely added on top of direct spending programmes, in order to attract beneficiaries to the new layers and away from the former ones.

Conservatives thus manage to "curb the demand" for traditional direct spending programs. According to Howard, this has been "the general pattern" of social policy making in the US in the twentieth century (Howard, 2007:90). STEs can also be mobilized in "starve the beast" (Bartlett 2007) strategies, in order to limit the future growth of direct spending programs, through reducing available public revenues (Howard, 1997:4).

STEs would be used to foster privatization and the development of occupational forms of welfare, "altering the balance between public and private power in society" (Faricy, 2011:74). Thus, Hacker shows how third-party providers might be empowered through tax expenditures, and subsequently able to mobilize very large resources to maintain these policies (Hacker, 2002:56).

Beyond privatization, tax expenditures are associated with a free-market logic, concepts like individual choice or self-reliance (Howard, 1997:11). Working through individual incentives rather than top-down regulations of social entitlements, they would be a better fit with the self-image of the middle class than direct spending [START_REF] Barbara | The Enabling state :Modern Welfare Capitalism in America[END_REF]. They are described as a "less intrusive, less bureaucratic alternative to government regulations or direct expenditures" (Howard, 1997:8).

Finally, due to their specific distributive propensities, STEs would be used to appeal to higher and middle-class voters. Indeed, progressivity means that upper-income taxpayers generally benefit more from tax expenditures (Howard, 1997:31). STEs also reward behaviours that "less affluent taxpayers cannot afford to engage in (e.g. owning a home)" (Howard, 1997:31). Occupational tax benefits are also more likely to concern workers in large companies, with secured employment and above-the-median wages. At the same time, some tax expenditures are also explicitly targeted at low-income people: the implementation and development of the Earned Income Tax Credit (EITC) are described as the result of party competition for the vote of the working poor (Howard, 1997:188).

2. Descriptive trends emerging from the still patchy literature on the uses of STEs in European welfare states Both the descriptive and analytical contents of this literature can feed into analyses of the European case in a relevant manner. Yet there are many reasons to think that the political economy of STEs in Europe might be different, not least due to the fact that European welfare states are larger, more mature and legitimate institutions. Political processes are also very different, with generally fewer veto points and less formal opposition to social spending. The ideological direction of reforms in the US and Europe cannot be simply assimilated either; thus the use of similar instruments cannot be assumed to mean that similar aims are pursued and similar reform logics followed. Thus, we need more empirical insights into the particular uses of social tax expenditures in European welfare states, and possibly new theories to account for and understand the effects of the development of STEs in the European context. Although it is still in its early development, a literature on fiscal welfare in Europe is emerging, with particularly informative national case studies, partially filling in the data gap highlighted above 13 . Surveying this literature allows identifying common trends in the uses of fiscal welfare in European contexts, which enables us to put forward a number of hypotheses on the uses and effects of STEs in a European context. More precisely, we identify four trends which would warrant further analysis. a. STEs as a social policy instrument adapted to times of fiscal austerity

First, several accounts tend to show that STEs would be particularly well adapted to social policymaking in contexts of austerity. This assertion is linked to two different types of uses of STEs of which we find several examples in the literature; namely, in fiscally constrained contexts, STEs could allow bypassing accountability rules, and could represent means of affordable credit claiming.

In institutional contexts characterized by the implementation of legally binding expenditure ceilings, STEs seem to be a way to maintain spending on social policy. Indeed, although in theory, a deficit-adverse government should avoid both direct and indirect forms of spending 14 , in practice, STEs often constitute loopholes in government's hand-tying devices. This is due to the fact that budgetary control instruments almost always consist of a formal control of public expenditures, rather 13 In an attempt to foster research on this understudied area, we have organised a workshop in Paris in May 2016 and set up a website for collaborative and dissemination purposes: www.fiscalwelfare.eu 14 Additionally, European treaties (Maastricht Treaty, TSCG) are concerned with overall levels of fiscal (im-)balance, not by absolute levels of expenditure and/or revenue.

than a control of the overall balance15 . For instance, France has a binding ceiling of public expenditures, voted each year, which cannot be breached unless a specific corrective bill is passed. In contrast, the level of tax revenue considered in the budgetary bill has the legal status of a mere prevision. Consequently, if an STE happens to be much costlier than expected, the unforeseen cost will not entail any automatic reaction and the STE will benefit every eligible taxpayer. Moreover, an increase in spending in the form of STEs is likely to simply go unnoticed, because the monitoring of tax expenditures is often incomplete, as explained above (see part II).

Consequently, the skyrocketing cost of a tax expenditure may well remain unnoticed for several years; the revenue loss will simply be considered lost in the gap between tax prevision and tax collection. In contrast, the traceability of public expenditures is excellent: the unforeseen drift of a social benefit cannot remain unaddressed for more than a few months. This lack of cost estimation means that STEs might also be easier to introduce. An example of this is the 2003 pension reform in France in 2003, which main purpose was to balance the pension system in the long term. All measures included had to be examined ex-ante and their budgetary impact estimated. However, one scheme had no estimated cost attached: a social security contribution exemption for employers subscribing to certain types of occupational pensions. Yet, this cost was far from negligible: ten years later, it was estimated to be worth at least 1.3 billion euros [START_REF] Houssoy | Tax expenditures in France in 2014[END_REF]. Consistently with the argument brought up in the American literature, according to which STEs do not mobilize the same public attention as direct social expenditures, the introduction of this measure was barely noticed by commentators and did not trigger debate or opposition, while the overall reform was generating public outrage.

STEs can also been used as a means of "affordable credit claiming"; they are at first sight cheaper to implement as they do not necessitate the setting up of a specific bureaucracy to examine claimants' applications and to manage the payments. Beneficiaries simply indicate in their tax file the deductions or reductions they are entitled to (for instance for the purchase of childcare or household services). Likewise, income support for low-wage earners is handled by the tax authorities rather than a social service. Because no additional layer of administration is necessary, the use of STEs can be portrayed as part of a larger effort to make the welfare bureaucracy leaner and more efficient. Additionally, STEs help governments to appear as more responsible since they can be portrayed as tax cuts rather than increased spending. Thus, using STEs "allowed New Labour to be modestly expansionary while still respecting (…) their 1997 election pledge (…) to remain within the very tight spending plans of their Conservative predecessors" (Clegg, 2015:9). In other words, it allowed New Labour to appear "responsible", even according to Conservative standards.

b. STEs as "modernized" social policy: new risks, new actors, shorter processes Second, existing empirical descriptions of fiscal welfare in European countries tend to show that the use of STEs would be part of a phenomenon of "modernization" of the welfare state. They are used to answer new types of social risks, and they tend to be implemented by actors and through processes that are different to those traditionally characterizing social policymaking.

A survey of the welfare state reform literature16 indicates that STEs have been used across Europe in the fields of active labour market policy, as well as in family and childcare policy, that is to say in relation to what has been termed 'new social risks' (Bonoli, 2005). In the field of family policy and childcare, tax allowances or tax credits for families have emerged in a number of countries, especially since the 1990s (cf. [START_REF] Tommy | The Fiscalization of Child Benefits in OECD Countries[END_REF]. STEs are also present in reforms favouring the development of private providers of childcare services, through tax deductions for formal care expenditures. In the field of active labour market policies, STEs are used in reforms that seek to 'make work pay' by providing wage subsidies to low-income earners through income tax credits. The Working Families Tax Credit introduced in the UK in 1997, the Prime Pour l'Emploi in France in 2001, or the Jobbskatteavdrag in Sweden in 2010 are examples of such instruments. STEs are also used as incentives designed to encourage job-creation, for instance, in the form of exemptions of employers' social security contributions on specific jobs (low-wage jobs, jobs in particular sectors) or specific categories of individuals (for instance young or senior jobseekers).

In accordance with the point made above, the use of tax expenditures in these areas can be understood as offering room for manoeuvre for governments to respond to these new needs in a context of budgetary austerity. Beyond, they also seem to fit in well with the political agenda of activating people rather than keeping them "dependent" on social transfers. They are also aligned with the dominant contemporary drive towards "individualisation" and "free choice" (Ervik, 2000:245), which seem to constitute the new horizons of social policy. With respect to childcare for example, tax expenditures enable parents to purchase the types of services they wish between competing providers, rather than imposing a 'one-size fits all' public childcare service for instance. This does not mean that STEs are more technically prone to enhance free choice: "cash-for-care" schemes in the form of a benefit (i.e. a direct expenditure) in effect allow beneficiaries to exercise free choice when purchasing care services. Still, STEs seem to be more strongly associated with this new social policy repertoire. They can be used to foster the development of a variety of non-state service providers which development is associated with the drive towards free choice (see below).

Since they aim not at decommodifying labour but rather at enabling more people to participate in the labour market, STEs are more likely to benefit from the support of non-traditional welfare constituencies, such as employers and Right-wing parties. Non-traditional actors are also involved because STEs are discussed in specific institutional arenas (the ones traditionally used for tax policy discussions), different from the traditional social policy arenas. Traditional social policy elites (like trade unions representatives, officials from social security institutions, etc.), usually associated to formal processes of social policy negotiations in many countries17 , are likely to be side-lined when STEs are used. Instead, relatively new actors like finance ministries administration are often involved in the design and management of STEs (see e.g. [START_REF] Daniel | Fiscal or Social? The Instrumentation of 'Make Work Pay' Policies in the United Kingdom[END_REF]. This is likely to bear consequences, as finance ministry officials are often primarily sensitized to the issue of budget balancing, while social policy objectives come into consideration only as secondary and even contradictory aims. Thus, in Poland, a tax expenditure for families was introduced in 2006, and proved to benefit mostly higher income families. In 2007, discussions to include a negative income tax mechanism to remedy that problem were cut short, partly because Ministry of Finance officials objected that the amount of foregone revenue would be too high, especially as the country was entering the Eurozone's Excessive Deficit Procedure [START_REF] Polakowski | Hidden welfare state in Poland? Fiscal welfare in the context of permanent austerity[END_REF].

Using STEs also potentially simplifies the legislative process, as implementing STEs do not require voting a dedicated text: they can be implemented in any budget bill. This means that the window of opportunity to create, modify or suppress STEs opens at least once a year. Using STEs is thus a way to intervene quickly on social issues. For instance, in France, two schemes are designed to support the cost of elderly care, a means-tested in-kind benefit ("allocation personnalisée d'autonomie") and a targeted tax reduction (mostly used by the wealthiest, [START_REF] Clément | Job creation, public cost and distributive profile of tax allowances for household services in France[END_REF]. Between 1991 and 2014, there were three legal changes to the in-kind benefit (concerning its scope, eligibility, etc.) in 1997, 2001 and 2003; these three reforms were each included in a dedicated bill and voted in Parliament during a process that lasted at least three months. During the same period, there were eight comparable changes to the targeted tax reduction (ceiling, eligibility), mostly through simple articles in the budget bill (1992, 1995, 1997, 2002, 2003, 2005 and 2011) [START_REF] Clément | Job creation, public cost and distributive profile of tax allowances for household services in France[END_REF]. c. STEs as a tool to foster the partial privatization of welfare A third trend identified in the literature on European cases is the association of fiscal welfare with a "privatization logic" [START_REF] Sinfield | Fiscal welfare[END_REF]. OECD data shows that pensions stand for a large share of STEs in many countries; other sources also underline the importance of healthcare. These policy sectors also represents very large shares of total public social expenditure, making them the focus of governments' efforts to curb spending.

In this context, it seems that STEs are sometimes used to encourage the development of private (pension and healthcare) insurances and to reduce the reliance on public insurance programs, through stimulating either the supply or demand of private welfare services. In particular, in countries where the demand for private insurances is crowded out by generous mandatory public insurances, STEs are a way to encourage employers to supply occupational welfare schemes, thus shaping previously inexistent private markets. Another example, on the demand side, is the introduction of tax credit schemes in a number of Continental and Nordic countries, with the explicit aim of fostering the development of a personal household services sector for both care and non-care needs [START_REF] Morel | Servants for the knowledge-based society? The political economy of domestic services in Europe[END_REF]Carbonnier and Morel, 2015).

STEs are also used to organize newly created markets. Governments have an opportunity to shape the services produced by the market, through STEs conditional on private consumption, which are associated to a legally defined service. STEs can be restricted either to approved providers or to services fulfilling certain criteria. Thus, other services, in a business close to but excluded from the target of the tax breaks, are disadvantaged. The market for private health insurance in France provides a good example (e.g. [START_REF] Kerleau | Le nouveau cadre institutionnel de la protection sociale complémentaire d'entreprise: quels enjeux pour les mutuelles santé?[END_REF]. The introduction of a payroll tax exemption, restricted to i) compulsory occupational schemes, regulated by a collective agreement, ii) offering a minimal "care basket" and excluding certain forms of risk selection, has strongly disadvantaged other forms of health insurances (such as individual and voluntary contracts), which used to dominate the field. It also created a strong incentive for providers to redefine their offer accordingly. This is significant in explaining the ongoing reorganisation of this sector (e.g. [START_REF] Batifoulier | Mutation du patient et construction d'un marché de la santé. L'expérience française[END_REF][START_REF] Brigitte | Refonder l'assurance-maladie[END_REF]. Far from signalling a rollback of public intervention in favour of a "free" market, the use of STEs in this case appears to be a (costly) means of publicly subsidizing and organizing a private market.

Beyond creating and steering markets, the use of STEs also results in the diverting of earmarked public resources towards private schemes. In France, employers' participation to collective private social insurance (health and/or pensions funds) can be partly exempted from contributions to mandatory social insurances. This allows a partial transfer of public resources to private schemes: a share of labour cost, which should accrue to public social security funds through social contributions, is diverted to a private insurance. In 2011, the resources diverted from social security to private insurances amounted to at least 4.5 billion euros for health insurance and 2.5 billion euros for pensions (while the market for private insurance amounted to 30 and 10 billion euros respectively) (Zemmour 2013). d. Beyond social policy: governing through incentives, defining norms of deservingness Fourth, and finally, a survey of the emerging European literature reveals that STEs might be used, beyond their social policy function, as a specific governance tools designed to alter economic and behavioural incentives, and thus defining and concretely embodying social norms of deservingness.

STEs are a means of governing through incentives. An important way in which they differ from direct social expenditures is that they do not necessarily primarily aim to redistribute, but rather to alter incentives. This can relate to economic incentives (encouraging individuals to purchase private goods or servicesa home; private insurance plans; household services; or encouraging employers to finance private insurance schemes for their employees). But incentives can also be directed towards fostering a desired behaviour with respect to labour market participation or to marriage and fertility (e.g. tax advantages for married couples; tax benefits linked to the number of children). This mode of governance through incentives is likely to entail a greater degree of targeting, and could be used to more discreetly target specific demographics or electoral constituencies. [START_REF] Branco | The golden age of tax expenditures. Fiscal welfare and inequality in Portugal (1989-2011)[END_REF] for instance suggest that the development of STEs in Portugal can be understood in light of the transition to democracy in a context where electoral volatility has led political parties to target specific segments of the population amongst the middle class and the elite as a way to mobilize these constituencies.

As incentives, STEs can also be used as a means to (re)define norms of deservingness. For instance, an interesting development is the way in which family benefits have become increasingly tied to labour market incentives in a number of countries. This has typically been the case in the UK for the Working Families Tax Credit, and has become the norm in some countries of Central Eastern Europe also, where family benefits in the form of direct transfers have been replaced by tax benefits conditional on employment. This is the case notably in Poland, the Czech Republic and Hungary (cf. [START_REF] Polakowski | Hidden welfare state in Poland? Fiscal welfare in the context of permanent austerity[END_REF][START_REF] Jahoda | Distributional impacts of family policy reforms in the Czech Republic. Case study: tax reliefs[END_REF]Szrika, 2016). The aim there was notably to reinforce the distinction between the deserving and non-deserving citizens. This idea was made explicit by the Ministry of Labour and Social Affairs in the Czech Republic, who stated: "By taking this step, the state wants to strengthen incentives to encourage employment and thus reduce the number of people dependent on social benefits" (quoted in Jahoda & Godarovà, 2016 :6). Likewise, Szrika notes how in the case of Hungary, "the more generous, post-1990 social policy efforts that aimed at the creation of a welfare state […] and promoted the development of certain social transfers as universally available to all citizens who need them, has been fading away. In the understanding of the Orbán-government social rights are not any more based on ??Szrika, 2106:21).

The same holds true for STEs mobilized for the purpose of 'activation' and of 'making work pay'. According to Clegg tax expenditures introduced under New Labour precisely aimed at signalling that "recipients of in-work support had a different, and superior standing to recipients of 'welfare'" (Clegg, 2015:10). This particular use of STEs would thus amount to a means of (re)drawing of the boundaries of welfare and social citizenship, with possibly important distributive consequences.

Conclusion

Based on the above discussion, it is safe to conclude that the issue of STEs in European welfare states is a subject that needs to be more thoroughly researched. STEs are present in Europe, and represent substantial amounts of public monies. They seem to be used for a variety of reasons, from social policymaking under austerity, the modernization of welfare and its partial privatization, to economic and political objectives, beyond social policy. They appear to be mobilized because of some distinctive intrinsic features, which make them technically attractive in the face of certain institutional or regulatory constraints, as well as because of their association with certain policy trends and discourses, which make them politically attractive.

Yet this paper also leads to the conclusion that the literature as it stands still lacks accounts of both the ways in which STEs are mobilized and the rationales behind their uses in specific national contexts, as well as their longer term effects on national welfare states. This article allows acknowledging that much, and constructing a basis on which to build future research. We mapped out the existing conceptual discussion, and concluded that the concept of Social Tax Expenditures was the best adapted to research on the political economy of welfare; we identified empirical gaps in the available data, and exposed the challenges involved in generating better data; this led us to argue in favour of the development of cross-country comparisons at the policy level, as well as the development of country-specific analyses. Finally, we identified empirical trends emerging from the patchy European literature on fiscal welfare, which can be used as cues in building a theory of fiscal welfare in European countries; doing so, we also revealed remaining theoretical gaps, in particular when it comes to the effects of STEs. In that sense, the present article is only the first building block of a larger endeavour.

It remains to be added that the emerging trends identified in the second part of this article might very well be particular to a specific context and time on which the existing literature is concentrating, namely the 1990s and early 2000s. Yet this context is changing. Thus, the renewed fiscal surveillance applied in the EU (and especially in EMU countries) since the 2012 crisis means that tax expenditures are increasingly taken into account in budgetary processes as well: the new European System of Account issued in 2010 prescribes that the refundable part of tax credits be reported as public expenditures. A European Regulation adopted in 2011 prescribes yearly detailed reporting of tax expenditures and their impact on state revenues in Member-States. This renewed attention to tax expenditures in accounting systems could explain the multiple instances, since 2010, of suppression of tax expenditures in order to broaden the tax base (examples are the United Kingdom 2010, Greece 2010, Portugal 2010, Ireland 2010, and France 2010 18 ). Future research should take this changing context into account, and look in particular at the issue of whether some types of STEs are more likely to be cut than others.

Figure 1 -

 1 Figure 1 -Spending on STEs in 5 countries according to two different OECD sources (% of GDP)

Figure 2 -

 2 Figure 2 -Spending on STEs in OECD countries in 2011 (in % of total public social spending)

Bittker (1969Bittker ( , 1973) ) prominently criticized the notion as based on a biased definition of income; Le Pan (1979) highlighted the inevitable role of value judgement in defining a "normal" baseline tax structure (see

Toder offers another definition more immediately applied to the US case: "tax expenditures (…) to promote social goals (…) include provisions that promote education, health, housing, retirement security, and income security for low-income families"(Toder, 1999:1).

Indeed, "if we accept the definition of a welfare state as different from the absence of a welfare state because it redistributes and pools risks, then tax preferences that do not do these things do not make the welfare state larger"(Prasad, 2011:263).

The author proceeded to some reclassification, based on prevailing accounting rules in the majority of countries. Yet, most of the limitations highlighted (see infra) concerning the data compiled in[START_REF] Willem | How much do OECD countries spend on social protection and how redistributive are their tax/benefit systems?[END_REF] apply to the data in OECD (2010) as well.

Since 2011, Eurostat has also started systematically collecting data on STEs in Member States. Yet, this information is not yet available as a single dataset. In addition, Eurostat does not produce its own estimates, but relies on national estimates, so this data is likely to be similarly plagued by issues of comparability

To be considered a TBSP, schemes have to "be intended to address one or more social purposes", and to involve either inter-personal redistribution or compulsion(Adema et al, 2011:90). Following this definition, some STEs incentivizing voluntary occupational schemes are likely to be left out.

Actual costs could be magnified through behavioural reactions, for instance if the TE is used for tax optimization purposes, or through interactions with the rest of the tax and benefit system, if the TE pushes some people in lower tax brackets(Burman et al 2008)

In the same way, the European Commission, which theoretically only looks at the deficit size, pays in practice a particular attention to the total amount of public expenditures (notably through the European Semester budgetary review procedure).

Available in the Working paper version of this article[START_REF] Morel | Fiscal welfare and welfare state reform: a research agenda[END_REF]

For instance, in France social partners control the use of public subsidies to collective childcare structures, since they are in charge of administering the social security fund dedicated to family policy (CNAF). On the other hand, they have no direct control on the variation of childcare tax credits (they can at best express an opinion through the Haut Conseil à la Famille, a broad consultative body).

In France since 2010, we witnessed a general policy of reducing the generosity of certain tax benefits (reduction of ceiling on Quotient Familial, freeze of the Prime Pour l'Emploi…)