domain: spi phys

text:
Mainak Saha
email: mainaksaha1995@gmail.com

Oxidation of ZrB 2 -MoSi 2 ultra high-temperature Ceramic Matrix Composites: A brief review

Keywords: Ultra High-Temperature Ceramic Matrix Composite (UHTCMC), Microscopy, Microstructure, Oxidation

The structure-property(S-P) correlation, especially in the context of high-temperature oxidation, in ZrB 2 -MoSi 2 based Ultra High-Temperature Ceramic Matrix Composites (UHTCMCs) have been extensively investigated for quite some time since the last 25 years and a countless amount of published data is presently available in this field. On the other hand, emergence of "Correlative Microscopy" [1] has completely revolutionised the world of "Material Science Research", in a number of ways. However, owing to the challenges of sample preparation, there has hardly been a literature aimed at understanding the aforesaid phenomenon of understanding S-P correlation, based on oxidation in ZrB 2 -MoSi 2 UHTCMCs, which, in future, may open up new frontiers of research in UHTCMCs. The present review intends to discuss some of the most interesting data published existing in this field and intends to provide a brief overview of the challenges associated along with some of the currently unexplored avenues in this field, especially in terms of fundamental research.

However, in view of the enormous amount of research already done in understanding oxidation-based S-P correlation in these materials, the author does not claim to address all the issues which may be associated with understanding the same.

Introduction

Since the early 2000s, the search for materials which may withstand extreme conditions of temperature, chemical reactivity, mechanical stress, radiation, and ablation, especially in the context of hypersonic and space aviation (Re-entry vehicles, most presently) [START_REF] Ma | Studying the effect of grain boundaries in dislocation density based crystal plasticity finite element simulations[END_REF][START_REF] Raabe | Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: a pathway to ductile martensite[END_REF][START_REF] Raabe | Nano-precipitate hardened 1.5 GPa steels with unexpected high ductility[END_REF][START_REF] Herbig | Atomic-scale quantification of grain boundary segregation in nanocrystalline material[END_REF][START_REF] Pj Felfer | A reproducible method for damage-free sitespecific preparation of atom probe tips from interfaces[END_REF][START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF][START_REF] Gault | Atom probe crystallography[END_REF][START_REF] Singh | Decomposition in multicomponent AlCoCrCuFeNi high-entropy alloy[END_REF][START_REF] Raabe | Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces[END_REF][START_REF] Wuchina | UHTCs: ultra-high temperature ceramic materials for extreme environment applications[END_REF], has been a driving force for an enormous volume of research based on the borides and carbides of a number of transition metals (such as, Zr, Hf etc.). ZrB 2 can be considered a leading material in this field of research owing to a unique combination of properties such as melting point above 3000˚C, relatively low density (than the other diborides), high thermal conductivity coupled with good mechanical strength and remarkable refractoriness at elevated temperatures [START_REF] Wuchina | UHTCs: ultra-high temperature ceramic materials for extreme environment applications[END_REF][START_REF] Van Wie | The hypersonic environment: required operating conditions and design challenges[END_REF][START_REF] Monteverde | Processing and properties of zirconium diboride-based composites[END_REF][START_REF] Chamberlain | High strength ZrB 2based ceramics[END_REF][START_REF] Sonber | Sythesis and consolidation of zirconium diboride: Review[END_REF][START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF], ZrB 2 has been the centre of attraction in this field. On the contrary, Sinterability of ZrB 2 is extremely low, owing to the presence of strong covalent bonds and low self-diffusivity [START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF], and therefore it requires pressure-assisted sintering techniques at extremely high temperatures (nearly~ 1900˚C) in order to eliminate porosity [START_REF] Guo | Densification of ZrB 2 -based composites and their mechanical and physical properties: a review[END_REF].

Furthermore, the ZrB 2 exhibits poor oxidation resistance [START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF] and mechanical properties [START_REF] Guo | Densification of ZrB 2 -based composites and their mechanical and physical properties: a review[END_REF][START_REF] Monteverde | Oxidation of ZrB 2 -based ceramics in dry air[END_REF] owing to the fact that powder surface is always associated with a number of oxides, mainly: B 2 O 3 and ZrO 2 , leading to an evaporation-condensation mechanism, which subsequently causes mass transfer without effective densification at low temperature and abnormal grain growth at high temperatures, both resulting in poor mechanical properties 17, 18 . This is due to the reason that formation of glassy B 2 O 3 and crystalline ZrO 2 upon exposure in air at temperatures above 800˚C [START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF][START_REF] Guo | Densification of ZrB 2 -based composites and their mechanical and physical properties: a review[END_REF][START_REF] Parthasarathy | A model for the oxidation of ZrB 2 , HfB2 and TiB2[END_REF], due to reaction (1):

ZrB 2 + 5/2O 2 = ZrO 2 + B 2 O 3 (1)
With increase in temperature above 1100˚C, the evaporation of protective glassy B 2 O 3 layer on ZrO 2 occurs, thereby leaving behind a porous ZrO 2 scale. With further increase in temperature, the porous ZrO 2 scale shows a change in grain morphology: from equiaxed to columnar grains, thus aiding the oxidation process [START_REF] Tripp | Thermogravimetric study of the oxidation of ZrB 2 in the temperature range of 800˚C to 1500˚C[END_REF][START_REF] Talmy | Oxidation of ZrB 2 ceramics modified with SiC and group IV-VI transition metal diborides[END_REF]. Effective approaches for simultaneous improvement in sinterability, mechanical properties [START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF][START_REF] Monteverde | Oxidation of ZrB 2 -based ceramics in dry air[END_REF][START_REF] Parthasarathy | A model for the oxidation of ZrB 2 , HfB2 and TiB2[END_REF][START_REF] Tripp | Thermogravimetric study of the oxidation of ZrB 2 in the temperature range of 800˚C to 1500˚C[END_REF] and oxidation resistance of these borides, based on the incorporation of additives have been developed [START_REF] Guo | Densification of ZrB 2 -based composites and their mechanical and physical properties: a review[END_REF][START_REF] Monteverde | Oxidation of ZrB 2 -based ceramics in dry air[END_REF][START_REF] Parthasarathy | A model for the oxidation of ZrB 2 , HfB2 and TiB2[END_REF][START_REF] Talmy | Oxidation of ZrB 2 ceramics modified with SiC and group IV-VI transition metal diborides[END_REF][START_REF] Silvestroni | Oxidation behavior of ZrB 2 composites doped with various transition metal silicides[END_REF][START_REF] Panasyk | Interaction between ZrB 2 -MoSi 2 cermets and oxide melts, Powder Metall[END_REF][START_REF] Hu | Effect of various additives on the oxidation behavior of ZrB2-based ultra-high temperature ceramics at 1800?C[END_REF][START_REF] Sciti | Microstructure and mechanical properties of ZrB 2 -MoSi 2 ceramic composites produced by different sintering techniques[END_REF] among which MoSi 2 has been reported to be one of the most beneficial, to enhance the densification of ZrB 2 at ~1750˚C, owing to the formation of transient liquid phases [START_REF] Silvestroni | Transmission electron microscopy on Zr-and Hf-borides with MoSi 2 addition: densification mechanisms[END_REF][START_REF] Silvestroni | Effects of MoSi 2 additions on the properties of Hf-and ZrB 2 composites produced by pressureless sintering[END_REF][START_REF] Sciti | Long-term oxidation behavior and mechanical strength degradation of pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF]. The formation of glassy SiO 2 layer [START_REF] Panasyk | Interaction between ZrB 2 -MoSi 2 cermets and oxide melts, Powder Metall[END_REF][START_REF] Opeka | Oxidation-based materials selection for 2000˚C hypersonic aerosurfaces: theoretical considerations and historical experience[END_REF] as an effective oxygen diffusion barrier, coupled with the formation of phases such as MoB and Mo 5 Si 3 , and the suppression of volatile species [START_REF] Panasyk | Interaction between ZrB 2 -MoSi 2 cermets and oxide melts, Powder Metall[END_REF], may be attributed to be responsible for the beneficial effect of MoSi 2 during oxidation. These aforementioned factors have been reported to be additionally responsible for hindering the formation of columnar ZrO 2 grains in the subsurface. This hinders the inward oxygen diffusion into the ceramic matrix [START_REF] Tripp | Thermogravimetric study of the oxidation of ZrB 2 in the temperature range of 800˚C to 1500˚C[END_REF][START_REF] Sciti | Microstructure and mechanical properties of ZrB 2 -MoSi 2 ceramic composites produced by different sintering techniques[END_REF]. The oxidation reactions of ZrB 2 -MoSi 2 UHTCMCs have been shown [START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF][START_REF] Panasyk | Interaction between ZrB 2 -MoSi 2 cermets and oxide melts, Powder Metall[END_REF][START_REF] Opeka | Oxidation-based materials selection for 2000˚C hypersonic aerosurfaces: theoretical considerations and historical experience[END_REF][START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Sonber | Sythesis and consolidation of zirconium diboride: Review[END_REF][START_REF] Fahrenholtz | Refractory diborides of zirconium and hafnium[END_REF]:

ZrB 2 + 5MoSi 2 + 19/2 O 2 = ZrO 2 + 7SiO 2 + Mo 5 Si 3 + B 2 O 3 (2) 2MoSi 2 + B 2 O 3 + 5/2O 2 = 4SiO 2 + 2MoB (3)
However, unfortunately, the influence of different interfaces, for instance, Grain boundaries (GBs), Interphase Boundaries (IBs) etc. on the grain growth kinetics and during change in grain morphology at elevated temperatures in ZrB 2 , still needs a lot of experimental understanding, in order to be able to come up with a novel methodology of interfacecontrolled engineering of microstructures in these UHTCs and UHTCMCs during oxidation at elevated temperatures (> 1900˚C). The novel concept of "Correlative Microscopy", developed a few years back, may be a tool for future investigation of 'structure-property (S-P)' correlation in these materials and coupled with the 1 st principle calculations, may be extensively used to address a number of presently unexplored fundamental issues, associated with high-temperature oxidation of these materials. This will be discussed in greater details in the upcoming section.

Oxidation behaviour of pressureless sintered ZrB 2 -MoSi 2 ceramics based on Sciti et al. [33]

Between 800 and 1000˚C

Based on the report of Sciti et al. [START_REF] Sciti | Oxidation behavior of a pressureless sintered ZrB2-MoSi2 ceramic composite[END_REF], the rate of formation of protective silica (from reaction (4)), between 800 and 1000 °C, is not high enough to prevent rapid oxidation. Therefore, the oxidation of ZrB 2 to zirconia (reactions ( 5) and ( 6)), along with the formation of volatile MoO 3 by reaction (4), has been reported to be the main phenomenon in this temperature regime [START_REF] Tripp | Effect of an SiC ad-dition on the oxidation of ZrB2[END_REF][START_REF] Mchale | Phase Diagram for Ceramists[END_REF].

MoSi 2 (s) + 7/2O 2 (g) = SiO 2 (s) + MoO 3 (s) (4)
At these temperatures, ZrO 2 and liquid B 2 O 3 are semi-protective and protective, respectively, as confirmed by the kinetic curves, which resemble those for the oxidation of pure ZrB 2 [START_REF] Tripp | Effect of an SiC ad-dition on the oxidation of ZrB2[END_REF] in this temperature regime. The consequent volume expansion due to the two aforesaid phenomenon, combined with the escape of gases, leads to formation of voids and cracks in the oxide layer. The presence of crystalline MoB during oxidation at 1000˚C, indicates the occurrence of reaction (5) besides reactions (3) and (4) [START_REF] Mchale | Phase Diagram for Ceramists[END_REF].

ZrB 2 (s) + 5/2O 2 (g) = ZrO 2 (s) + B 2 O 3 (l) (5) B 2 O 3 (l) = B 2 O 3 (s) (6)
ZrB 2 (s) + 2MoSi 2 (s) + 5O 2 (g) = ZrO 2 (s) + 2MoB(s) + 4SiO 2 (s) [START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF] Reaction [START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF] has been reported to be thermodynamically favoured at all temperatures between 700 and 1400˚C, with highly negative values of ΔG at 700 and 1400 °C [START_REF] Mchale | Phase Diagram for Ceramists[END_REF]. However, owing to the slow formation of SiO 2 , especially at 1000 °C, molybdenum oxide [START_REF] Mckamey | A study of pest oxidation in polycrystalline Mo Si2[END_REF][START_REF] Meschter | Low-temperature oxidation of molybdenum disilicide[END_REF][START_REF] Chou | Mechanism of MoSi2 pest during low temperature oxidation[END_REF][START_REF] Natesan | Oxidation behaviour of molybdenum silicides and their composites[END_REF][START_REF] Zhu | Kinetics and products of molybdenum disilicide powder oxidation[END_REF] and liquid B 2 O 3 may be expected to expected to undergo some amount of vaporisation.

Between 1000 and 1200˚C

Based on Sciti et al. [START_REF] Sciti | Oxidation behavior of a pressureless sintered ZrB2-MoSi2 ceramic composite[END_REF], the production of SiO 2 due to oxidation of MoSi 2 has been reported

to occur at a rate, high enough to prevent inward oxygen transport from the atmosphere. At ~ 1200 °C, formation of cracks in ZrB 2 , evaporation of liquid B 2 O 3 has been reported to occur.

However, this phenomenon is highly time-dependent viz. after ~ 100 min, a drop in the weight gain, caused due to formation of a continuous silica-rich layer, exhibiting protective action, has been reported [START_REF] Sciti | Oxidation behavior of a pressureless sintered ZrB2-MoSi2 ceramic composite[END_REF] wherein, formation of ZrO 2 and oxides of Mo, favouring the penetration of oxygen and, consequently, its contact with ZrB 2 along MoSi 2 particles also in the bulk, have been reported to be responsible for the formation of crack in ZrB 2 at ~1200˚C.

At the interface between the oxide and the unreacted bulk, owing to low partial pressure of oxygen, selective oxidation of Si occurs; resulting in MoSi 2 getting depleted of Si and reaction of Mo with B from ZrB 2 to form MoB. A very little amount of borosilicate glass, however, has been reported to be observed in the oxide layer, unlike that in ZrB 2 -SiC composites [START_REF] Mchale | Phase Diagram for Ceramists[END_REF] and besides, formation of MoB through reaction ( 5) has also been reported to be thermodynamically and kinetically more favourable than the formation of borosilicate glass. According to previous studies [START_REF] Mckamey | A study of pest oxidation in polycrystalline Mo Si2[END_REF] when ZrO 2 grains are embedded in an SiO 2 layer, diffusion and dissolution of interstitial silicon into crystalline zirconia occurs until the solubility limit is attained; after which zircon precipitates out, thereby leading to the formation of zirconia grains, surrounded by a ZrSiO 2 shell, further, embedded in the amorphous silica, as shown in reaction (8) [START_REF] Tripp | Effect of an SiC ad-dition on the oxidation of ZrB2[END_REF]:

ZrO 2 (s) + SiO 2 (s) (amorphous) = ZrSiO 4 (s) (8)
The nearly parabolic kinetics of oxidation at 1200 and 1300 °C and the pure parabolic kinetics found at 1400 °C have been reported to be the prime indication for diffusion of oxygen through the silica-rich oxide layer, similar to ZrB 2 -SiC UHTCMCs.

Other notable reports on oxidation of ZrB 2 -MoSi 2 ceramics

Between 700 and 1200˚C

ZrB 2 is well-known to oxidize according to the reaction [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF][START_REF] Yt Zhu | Thermal Oxidation Kinetics of MoSi 2 -Based Powders[END_REF]:

ZrB 2 + 2.5O 2 (g) = ZrO 2 + B 2 O 3 (l) (9)
The low melting point (450 ˚C) and high vapor pressure reported for B 2 O 3 make it undergo vaporisation at low temperatures:

B 2 O 3 (l) = B 2 O 3 (g) (10)
At temperature below 1100 ˚C, the presence of B 2 O 3 in the form of a liquid has been reported

to hinder oxygen transport [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF], thereby following parabolic kinetics [START_REF] Tripp | Thermogravimetric study of the oxidation of ZrB 2 in the temperature range of 800˚C to 1500˚C[END_REF], while at temperatures higher than 1100˚C, the protective action of B MoO 3 with high vapor pressure is volatile between 500 and 800 ˚C and melts at ~ 801 ˚C [START_REF] Yt Zhu | Thermal Oxidation Kinetics of MoSi 2 -Based Powders[END_REF]. At temperatures higher than 1200 ˚C, SiO 2 tends to act as a barrier against oxidation, whereas, at lower temperatures, formation of MoO 3 , at a much higher rate than that of SiO 2 , [START_REF] Yt Zhu | Kinetics and products of molybdenum disilicide powder oxidation[END_REF][START_REF] Natesan | Oxidation behaviour of molybdenum silicides and their composites[END_REF] tends to dominate the oxidation of monolithic MoSi 2 , as a result of which there is a volume expansion leading to the build-up of a huge amount of internal stresses at grain boundaries (GBs).

The production of silica between 700 and 1000 ˚C has been reported to be slow enough for protecting the surface [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF]. With an increase in oxidation temperature above 1000 ˚C, the formation of MoB and ZrC secondary phases, mainly concentrated near the sample surface, has been reported during sintering [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF]. Oxidation of MoB has been reported to occur, according to Reaction (11) [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF]45]:

2MoB + 3.5O 2 (g) = B 2 O 3 + 2MoO 2 (l) (11)
The oxide growth leads to ZrC undergoing catastrophic oxidation at relatively low temperatures, resulting in high stress levels along GBs [46]. Oxidation of ZrC has been reported to occur based on Reaction [START_REF] Van Wie | The hypersonic environment: required operating conditions and design challenges[END_REF] ZrC + 2O 2 (g) = ZrO 2 + CO 2 (g) ( 12)

Fig

Between 1200 and 1400˚C

Based on Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF] and Sciti et al. [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF], the formation of SiO 2 , as shown in reaction (4), may be expected to be rapid enough to protect the ZrB 2 -MoSi 2 UHTCMC. It was hypothesized in the reports [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF] that at the early stage of oxidation of the UHTCMC, SiO 2 may not have been completely formed and that the Reactions ( 4) and ( 9)-( 12) led to an extremely rapid rate of degradation and this process is also aided by the presence of fastoxidizing species such as MoB and ZrC. The formation of cracks, in a way, is detrimental, not only from the mechanical point of view but also because cracking leads to rapid transport of oxygen to the internal regions in the sample and this subsequently, leads to an increase in the area exposed to oxidation. Liquid B 2 O 3 has been reported to react with SiO 2 , thereby forming a borosilicate glass, similar to that observed for ZrB 2 -SiC composites [START_REF] Jang | Processing of molybdenum disilicide[END_REF][START_REF] Monteverde | Advances in microstructure and mechanical properties of zirconium diboride based ceramics[END_REF][START_REF] Monteverde | Beneficial Effects of AlN as Sintering Aid on Microstructure and Mechanical Properties of Hot-pressed ZrB 2[END_REF] and which, owing to low viscosity, may get easily penetrated into the bulk of ZrB 2 -MoSi 2 sample, through voids left behind by the oxidation of ZrB 2 .

Based on the work of Sciti et al. [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF], formation of cracks and the presence of fast oxidizing species such as ZrC and MoB in the sub-surface of the sintered samples leading to rapid subsurface oxidation have been reported to be the main factors for controlling the degradation of ZrB 2 -MoSi 2 UHTCMC between 1200 and 1400 ˚C.

On the other hand, based on the work of Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF], consumption of of B 2 O 3 content in the subsurface liquid through reaction with MoSi 2 to form solid MoB [START_REF] Gulbransen | Oxidation of molybdenum 550˚ to 1700˚C[END_REF], may be attributed to the retardation in formation of ZrO 2 , on adding MoSi 2 . In addition, the dissolution of a small amount of Mo into the SiO 2 scale, which has actually been reported to act as a reticulating agent in glass structures [START_REF] Panasyk | Interaction between ZrB2-MoSi2 cermets and oxide melts, Powder Metall[END_REF][START_REF] Silvestroni | Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500-1650?C temperature range[END_REF], may increase the viscosity of the liquid phase, thereby acting as an effective barrier against inward diffusion of oxygen from the atmosphere to the sample surface. Besides, the pronounced bubbling activity on the surface of the oxidized sample at 1800 ˚C, reported by Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF], indicates that various gaseous species are formed in the subsurface layers. Interestingly, during the oxidation of ZrB 2 -MoSi 2 UHTCMC, MoB has been reported to occur in 2 morphologies, [START_REF] Silvestroni | Effects of MoSi 2 additions on the properties of Hf-and ZrB 2 composites produced by pressureless sintering[END_REF][START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF] depending on the oxidation temperature. At ~1650 ˚C MoB is homogeneously spread and presently mostly in nano-inclusions within zirconia grains, whereas, at 1800 ˚C, a thick cordillera ~5mm is observed.

From the mechanical point of view, degradation caused due to oxidation of ZrB2 has been reported to be responsible for the massive drop in flexural strength in the material at 1300 and 1400˚C [START_REF] Sciti | Long-term oxidation behaviour and mechanical strength degradation of a pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF]. Till date, there is limited data available for ZrB 2 -MoSi 2 UHTCMCs, for oxidation at temperatures exceeding 1400 ˚C, although considerable amount of work has been done in investigating phase evolution during oxidation at 1650 and 1800 ˚C by Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF].

The major conclusion from the work of Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Silvestroni | Method to improve the oxidation resistance of ZrB2-based ceramics for reusable space systems[END_REF] is that a homogeneous distribution of Mo-rich phases, i.e., a continuous (Zr, Mo)B 2 solid solution without large Mophases, is desirable for enhancing the oxidation resistance of ZrB 2 -MoSi 2 UHTCMCs.

Criterion for design of UHTCMCs with improved oxidation resistance

The choice of cation to be dissolved into the boride matrix is of extreme importance. As long as the nano-inclusions within ZrO 2 grains are located at oxygen partial pressure values of below 10 4 -10 6 MPa between 1650-1850°C (Fig. 4), they may be considered to be stable compounds [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF]. Hence, the main criterion, to be focussed on for the design of UHTCMCs should be based on using suitable additives (Me) with ZrB 2 , in order to form a homogeneous (Zr, Me)B 2 solid solution with an extremely high phase stability in the region of the highest partial pressure of oxygen, without any amount of GB segregation, whatsoever. Based on the Predominance diagrams at different temperatures, Mo, Cr and W have been reported to possess the highest stability field at the highest oxygen partial pressures, [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF][START_REF] Silvestroni | Method to improve the oxidation resistance of ZrB2-based ceramics for reusable space systems[END_REF] in-line with the experimental findings of Kazmedashi et al. [START_REF] Kazemzadeh Dehdashti | Effects of transition metals on the oxidation behavior of ZrB2 ceramics[END_REF] and Talmy et al. [START_REF] Talmy | High-temperature chemistry and oxidation of ZrB2 ceramics containing SiC, Si3N4,Ta5Si3, and TaSi2[END_REF].

Anisotropy in oxidation of hot-forged (textured) ZrB 2 -MoSi 2 UHTCMCs

Liu et al. [START_REF] Ht Liu | Anisotropy oxidation of textured ZrB2-MoSi 2 ceramics[END_REF] investigated the anisotropic oxidation behaviour of textured ZrB 2 -MoS i2

UHTCMCs, prepared by reactive hot pressing and subsequent hot forging, for different exposure times(ranging from 0.5h to 12h) at 1500˚C and reported that on exposure in air for a relatively short period of time (0.5 h and four h), the ceramics, textured as a result of the aforesaid deformation process, exhibit a good amount of anisotropy in oxidation i.e., the Vdirection surface has a much better oxidation resistance than the P-direction surface (Fig. 5).

The V-direction surface of the texture ceramics also shows a better oxidation resistance, in contrast to the V-direction surface in non-textured ceramics, which may be certainly attributed to the higher atomic density of ZrB 2 grains and a much higher diffusion path length in the V-than in the P-direction. However, with passage of time, the formation of thick SiO 2 layer, preventing the inward diffusion of oxygen from the outer atmosphere, has been reported to weaken the anisotropy in oxidation. Liu et al. [START_REF] Ht Liu | Anisotropy oxidation of textured ZrB2-MoSi 2 ceramics[END_REF] have finally concluded that the combination of good oxidation resistance and flexural strength in the V-direction surface for textured ZrB 2 -MoSi 2 UHTCMCs may enable these materials to act as promising candidates for future applications, especially for improving the performance of nosecones of the most trending hypersonic re-entry vehicles.

Mechanical properties of ZrB 2 -MoSi 2 ceramics due to oxidation

Grohsmeyer et al. [START_REF] Grohsmeyer | ZrB 2 -MoSi 2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties[END_REF] have reported that the Elastic properties (Fig. 6(a)) and Vickers microhardness (Fig. 6(b)) of ZrB 2 -MoSi 2 ceramics follows rule of mixture trends based on the constituent phases and that the Young's modulus also decreases with decreasing ZrB 2 average particle size due to the presence of higher amounts of low modulus tertiary oxide phases such as SiO 2 in compositions prepared from finer ZrB 2 powders and is highly coherent with the previous experimental investigations [START_REF] Lavrenko | Electrochemical oxidation of ZrB2-MoSi2 ceramics in a 3% NaCl solution[END_REF][START_REF] Sciti | Suitability of ultra-refractory diboride ceramics as absorbers for solar energy applications[END_REF][START_REF] Chamberlain | Characterization of zirconium diboride-molybdenum disilicide ceramics[END_REF][START_REF] Neuman | Strength of zirconium diboride to 2300°C[END_REF][START_REF] Fahrenholtz | Thermodynamic analysis of ZrB2-SiC oxidation: formation of a SiC-Depleted region[END_REF][START_REF] Guo | Oxidation behavior of ZrB2-MoSi2-SiC composites in air at 1500°C[END_REF][START_REF] Silvestroni | Transmission electron microscopy on Zr-and Hf-borides with MoSi2 addition: densification mechanisms[END_REF][START_REF] Srinivasan | Ductile-to-brittle transition in MoSi2[END_REF][START_REF] Evans | Deformation mechanisms in the intermetallic compound MoSi2[END_REF][START_REF] Unal | Dislocations and plastic deformation in molybdenum disilicide[END_REF][START_REF] Guo | Quantitative Electron microprobe characterizations of oxidized ZrB2 containing MoSi2 additives[END_REF]. Besides, the Flexural strength at 1500 °C for MoSi 2 contents above 25 vol.% has been reported to vary slightly between 420 and 460 MPa for all compositions, whilst for lower contents, two trends were reported [START_REF] Grohsmeyer | ZrB 2 -MoSi 2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties[END_REF]. For fine and medium ZrB 2 grades, flexural strength was reported to increase with MoSi2 content due to a probable transition from unprotective to protective mechanism of oxidation. Besides, for these compositions, a continuous borosilicate glassy scale was observed and this attributed as the main factor for limiting the depth of oxidation damage. For coarse ZrB 2 particles, flexural strength was reported to decrease with increasing MoSi 2 content owing to the ductilization of the disilicide, thereby leading to the partial masking of the beneficial effect of absorbing fracture energy possessed by the solid solution shell [START_REF] Grohsmeyer | ZrB 2 -MoSi 2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties[END_REF].

A few presently unexplored avenues in this field and outlooks for the future

The onset of Correlative Microscopy [START_REF] Ma | On the consideration of interactions between dislocations and grain boundaries in crystal plasticity finite element modelingtheory, experiments, and simulations[END_REF] has completely revolutionised the field of research in structural materials. Ranging from the study of segregated atoms [START_REF] Raabe | Nano-precipitate hardened 1.5 GPa steels with unexpected high ductility[END_REF][START_REF] Herbig | Atomic-scale quantification of grain boundary segregation in nanocrystalline material[END_REF][START_REF] Raabe | Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces[END_REF] to characterising nanoscale precipitates at different interfaces [START_REF] Herbig | Atomic-scale quantification of grain boundary segregation in nanocrystalline material[END_REF], the novel technique has enabled material scientists to correlate the properties of a material with its bulk and interfacial structure, simultaneously. From the fundamental viewpoint, there still remain a lot of unexplored avenues, in the context of oxidation studies in the UHTCMCs, which may be potentially addressed by this novel technique. For instance, in the context of oxidation of MoSi 2 in ZrB 2 -MoSi 2 based UHTCMCs at temperatures lower than 1200 ˚C, formation of MoO 3 , has been reported by Silvestroni et al. [START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF], to occur at a rate which is much lower than that of SiO 2 , thereby leading to a volume expansion and a consequent piling up of a huge amount of internal stresses at grain boundaries (GBs). Unfortunately, at present, there is limited understanding of the effect of volume expansion associated with the formation of MoO 3 at temperatures lower than 1200 ˚C on the stress pile-up at GBs . Hence, its the GB cohesion [START_REF] Raabe | Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces[END_REF] and GB energy [START_REF] Herbig | Atomic-scale quantification of grain boundary segregation in nanocrystalline material[END_REF] which needs to be addressed, for a detailed understanding of the influence of the same on the stress pile-up associated with volume expansion, due to formation of MoO 3 , in the present context. Besides, adhesion between the oxide layer and the surface of the base material (ZrB 2 -MoSi 2 , in the present context) is also something which needs to be addressed as a function of the local chemistry at the substrate-oxide interface before subjecting these materials to service at elevated temperatures.

Although a number of theoretical studies based on the kinetics of oxidation as a function of the diffusivity of oxygen, have illustrated the properties of the oxides, formed during oxidation of ZrB 2 -MoSi 2 at different temperatures [START_REF] Silvestroni | Transmission electron microscopy on Zr-and Hf-borides with MoSi 2 addition: densification mechanisms[END_REF][START_REF] Silvestroni | Effects of MoSi 2 additions on the properties of Hf-and ZrB 2 composites produced by pressureless sintering[END_REF][START_REF] Sciti | Long-term oxidation behavior and mechanical strength degradation of pressurelessly sintered ZrB 2 -MoSi 2 ceramic[END_REF][START_REF] Opeka | Oxidation-based materials selection for 2000˚C hypersonic aerosurfaces: theoretical considerations and historical experience[END_REF][START_REF] Silvestroni | The oxidation behaviour of a ZrB 2 -MoSi 2 composite at ultra-high temperatures[END_REF], but however, there exists a huge possibility of understanding the same experimentally, through detailed study of oxygendefect interaction, using extensive defect and orientation based microscopy techniques such as Electron Backscatter Diffraction (EBSD) [START_REF] Raabe | Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces[END_REF], Electron Channeling Contrast Imaging (ECCI) [START_REF] Raabe | Alloy design, combinatorial synthesis, and microstructure-property relations for low-density Fe-Mn-Al-C austenitic steels[END_REF] e.t.c. and correlating the structural information, obtained henceforth with the atomic-level chemistry using Atom Probe Tomography (APT) [START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF] technique, using Focussed Ion Beaming (FIB) [START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF] as a lift-out technique to prepare samples for APT. This, in a way, may also be used a means to understand the change in GB character [START_REF] Herbig | Atomic-scale quantification of grain boundary segregation in nanocrystalline material[END_REF] of different crystalline oxides formed during oxidation at different temperatures and determining there is a localised phase formation at GBs, due to the same, as recently reported by Meiners et al. [START_REF] Meiners | Observations of grain-boundary phase transformations in an elemental metal[END_REF]. Addressing a couple of fundamental issues, as these may, henceforth be used as a pre-requisite to design ZrB 2 -MoSi 2 based UHTCMCs, in future, with even better mechanical properties during oxidation at elevated temperatures.

Conclusions

There is no doubt that ZrB 2 -MoSi 2 UHTCMCs will find numerous applications in future, owing to the excellent combination of physical, mechanical, and electrical properties of these materials, and that the research associated with these materials, would be aimed at further improvement in different properties of these materials, however, exploring the currently unaddressed avenues, would be the pre-requisite for aiming at property enhancement based on understanding structure-property correlation at different oxidation conditions in these materials.

  2 O 3 fades owing to the significant vaporisation of B 2 O 3 , thereby following paralinear kinetics [20]. Formation of MoO 3 from MoSi 2 has already been shown in Reaction (4). Figs. 1, 2 and 3 show the Scanning Electron Microscopy (SEM) images of ZrB 2 -20 vol.% MoSi 2 oxidised at 700, 1000 and 1200°C, respectively.

. 1

 1 Fig. 1 SEM images showing (a, b) Surface and (c) Cross-section of ZrB 2 -20 vol.% MoSi oxidised at 700°C [41].

Fig. 2

 2 Fig. 2 SEM images showing (a, b) Surface and (c) Cross-section of ZrB2 -20 vol.% MoSi oxidised at 1000°C [41].

Fig. 3

 3 Fig. 3 SEM images showing (a, b) Surface and (c) Cross-section of ZrB2 -20 vol.% MoSi oxidised at 1200°C [41].

Fig. 4

 4 Fig. 4 Left-hand side: Schematic of microstructural evolution starting from (a) as-sintered state up to oxidation at (b) 1650 and (c) 1800 °C. Right-hand side: phase stability diagrams for the Zr-B-O (black) and Mo-B-O (red) system as a function of partial pressures of oxygen and boron oxide at (b) 1650 °C and (c) 1850 °C [30].

Fig. 5 .

 5 Fig. 5. Schematic representation of both V-and P-direction cross sections in textured ZrB 2 -MoSi 2 ceramics oxidized for four h at 1500 • C, illustrating anisotropy in oxidation behaviour [56].

Fig. 6 (

 6 Fig. 6 (a) Young's modulus and (b) Vickers microhardness as a function of MoSi 2 content in ZrB 2 -MoSi 2 ceramics. N, M and C indicate coarse, medium and fine powder grades of ZrB 2 , respectively. X indicates vol.% of MoSi 2 . In part (a), ROM refers to volumetric rule of mixtures [57].

Acknowledgement

Mr. Mainak Saha is grateful to the Department of Metallurgical and Materials Engineering, NIT Durgapur for extending its valuable support while carrying out the present review.

Conflict of Interest

The author hereby declares no conflict of interest.


Mainak Saha
email: mainaksaha1995@gmail.com

Understanding the role of Al2O3 formed during isothermal oxidation in a dual phase AlCoCrFeNi 2.1 Eutectic High-Entropy Alloy

Keywords: Oxidation, Eutectic High-Entropy Alloys (EHEAs), Optical microscopy (OM), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM)

In recent times, there has been a significant volume of work on Eutectic High Entropy Alloys (EHEAs) owing to their remarkable castability combined with excellent mechanical properties, which aids in clearing obstacles for their technological applications. One of the most common EHEAs, which has been of huge interest, at present, primarily owing to its solidification and tensile behavior, is AlCoCrFeNi 2.1 . However, in order to aim for high temperature applications, oxidation behaviour of material is one of the major aspects which needs to be extensively investigated. To this end, the present work aims to study the phases evolved during oxidation at elevated temperatures as high as 950 and 1000 ⁰ C in AlCoCrFeNi 2.1 using XRD and also to determine the rate law followed for isothermal oxidation of this alloy at 950 and 1000 ⁰ C, in order to understand the role of Al2O3 phase formed during isothermal oxidation at 950 and 1000˚C.

Introduction

High-entropy alloys (HEAs), or multi-principal-element alloys, differentiate with conventional alloys in that they have at least four principal elements, instead of one or at most, two in the latter. This has led to a paradigm shift in alloy design strategies, and opened a new path for exploration of new materials.

A significant volume of work has been done on understanding solidification behaviour of Eutectic HEAs (EHEAs), especially, due to the excellent castability of these alloys [START_REF] Wani | Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy[END_REF][START_REF] Yiping | Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[END_REF][START_REF] Wani | Effect of severe cold-rolling and annealing on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Wani | Cold-rolling and recrystallization textures of a nano-lamellar AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF], arising due to meagre chances of segregation and shrinkage cavities during solidification .

Accordingly, if eutectic HEAs with the dual phase FCC + BCC microstructure may be synthesised, they may be expected to possess the combined advantages of excellent mechanical properties and castability of eutectic alloys coupled with a remarkable strength-ductility optimization arising out of a dual phase FCC+BCC microstructure, especially when both FCC and BCC phases are ordered [START_REF] Wani | Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy[END_REF][START_REF] Yiping | Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[END_REF][START_REF] Wani | Effect of severe cold-rolling and annealing on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Wani | Cold-rolling and recrystallization textures of a nano-lamellar AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Xuzhou | Microstructural origins of high strength and high ductility in an AlCoCrFeNi2.1 eutectic highentropy alloy[END_REF][START_REF] Yaoli | Deformation mechanism during high-temperature tensile test in an eutectic high-entropy alloy AlCoCrFeNi2.1[END_REF][START_REF] Ding | Preparation, characterization and properties of multicomponent AlCoCrFeNi2.1 powder by gas atomization method[END_REF][START_REF] Bhattacharjee | Effect of low temperature on tensile properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Jichao | High temperature oxidation behaviour of AlCuTiFeNiCr high-entropy alloy[END_REF]. This has primarily been the ideology behind designing EHEAs, as reported by a number of recent studies. In this context, AlCoCrFeNi 2.1 is a popular EHEA, in which quite a significant volume of work has been done on correlating the dual phase microstructure comprising of ordered FCC (L12) and ordered BCC (B2) phases with the overall solidification behaviour and mechanical response of the alloy, synthesized using different techniques [START_REF] Wani | Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy[END_REF][START_REF] Yiping | Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[END_REF][START_REF] Wani | Effect of severe cold-rolling and annealing on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Wani | Cold-rolling and recrystallization textures of a nano-lamellar AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Xuzhou | Microstructural origins of high strength and high ductility in an AlCoCrFeNi2.1 eutectic highentropy alloy[END_REF][START_REF] Yaoli | Deformation mechanism during high-temperature tensile test in an eutectic high-entropy alloy AlCoCrFeNi2.1[END_REF][START_REF] Ding | Preparation, characterization and properties of multicomponent AlCoCrFeNi2.1 powder by gas atomization method[END_REF][START_REF] Bhattacharjee | Effect of low temperature on tensile properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF]35]. With regard to oxidation, there have been a number of publications which have reported the oxidation behaviour of HEAs. For instance, Daoud et al. [START_REF] Daoud | Oxidation behavior of Al8Co17Cr17Cu8Fe17Ni33, Al23Co15Cr23Cu8Fe15Ni15, and Al17Co17Cr17Cu17Fe17Ni17 compositionally complex alloys (high-entropy alloys) at elevated temperatures in air[END_REF] [START_REF] Holcomb | Oxidation of CoCrFeMnNi high entropy alloys[END_REF] investigated the oxidation behaviour of 8 CoCrFeMnNi-based HEAs, along with different conventional alloys and reported that the Cr and Mn containing HEAs preferentially form both Cr2O3 and Mn oxides. Senkov et al. [START_REF] Senkov | Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy[END_REF] examined the isothermal oxidation behaviour of an arc-melted NbCrMo 0.5 Ta 0.5 TiZr refractory HEA (RHEA) at 1000 • C ad reported a superior oxidation resistance of the aforementioned alloy as compared to that of similar Nb-based refractory alloys. Similarly, Gorr et al. [START_REF] Gorr | Phase equilibria, microstructure, and high temperature oxidation resistance of novel refractory highentropy alloys[END_REF][START_REF] Gorr | High temperature oxidation behaviour of an equimolar refractory metal-based alloy 20Nb-20Mo-20Cr-20Ti-20Al with and without Si addition[END_REF] investigated the influence of Si on the high temperature oxidation behaviour of a 20Nb-20Mo-20Cr-20Ti-20Al HEAand reported that, in general, for HEAs with no Si content, a linear oxide growth rate law is followed with the formation of porous and non-protective oxides whereas, the addition of even 1 at % Si promotes parabolic oxide growth kinetics, to a certain extent, and leads to formation of a thin, continuous and protective Al and Cr rich oxide scale.

However, in view of limited understanding on the oxidation behaviour of AlCoCrFeNi 2.1

EHEA upto 1200⁰C, present work is aimed at understanding the nature of Al2O3 phase formed during isothermal oxidation of AlCoCrFeNi 2.1 EHEA at 950 and 1000 ⁰ C.

Experiment

Initially, a single sample (in the shape of a button) of AlCoCrFeNi 2.1 is prepared from Suction Casting of pure components (purity > 99.9%) (on a water-cooled Cu hearth, under Ar inert atmosphere and using Ti as getter). The bar with a rectangular cross-section, so obtained was flipped and remelted multiple times to promote homogeneity and then cut into 8 samples of average dimensions ~10 by 6 mm 2 (approximate thickness ~ 1.2 mm) using EDM (Electrode Discharge Machining). After standard metallographic polishing, the 1 st and 2 nd samples were subjected to isothermal oxidation at 1000 ⁰ C, 500h in a muffle furnace (CWF-1300). The 3 rd sample was subjected to isothermal oxidation at 1000 ⁰ C, 100h in Al2O3 crucible, using Kanthal wire as heating element and a static oxidation environment. The 4 th and 8 th sample were similarly put in the same furnace at 1000 and 950 ⁰ C, respectively and each of them was taken out after 4, 18, 25, 50 and 100h . Similarly, samples 5 and 6 were oxidised at 950 ⁰ C, 500h and sample 7 at 950 ⁰ C, 100h. Phase analysis of the base and oxidized samples was carried out using XRD (X-Ray Diffraction, Bruker) using Cu kα radiation at 45 mV and 30 mA and, at a scanning rate of 0.02⁰/min between the scanning range of 20 to 80 ⁰ and peak fitting was done using Xpert

Hi-score analysis software, in order to determine the phases present in the oxidised layer, at different temperatures and time of oxidation. For microstructural characterization, the base sample was etched with aqua regia (90% C2H5OH+10% HNO3). The cross sectional area and difference in weight (before and after oxidation) of every sample were measured to determine the oxidation rate law at different temperatures of oxidation. In terms of angular Wavenumber (k), with the exception of 1000 ⁰ C, 100h sample, is observed from 1 to 5. Whereas, the variation in the same quantity is observed between 2 and 5 in 1000 ⁰ C, 500h sample. Moreover, It is evident (from Fig. 1) that in as-cast sample, there is a dual-phase microstructure comprising of B2 (ordered BCC) and L12 (ordered FCC) phases (lattice parameters: 3.6011Ǻ and 5.9375 Ǻ, respectively), with the most intense (111) superlattice peak at 2θ = 43.5 ⁰ , corresponding to L12 phase. Besides, it is also observed that in as-cast sample, there are more peaks of L12 phase as compared to that of B2. After isothermal oxidation at varying temperatures and times of oxidation till 1000 ⁰ C, 500h, there are three phases detected by XRD analysis: L12, Al2O3 and Fe2O3. After 25h of isothermal oxidation at 950⁰C, Cr2O3 phase is also observed. However, after 100h of oxidation, it is clearly observed that there are 2 oxides present (similar to that observed after isothermal oxidation at 950⁰C for 25h) namely, Al2O3 and Fe2O3 phases, with the most intense (113) peak of Fe2O3 at 2θ = 43.5 ⁰ , indicating that Fe2O3 is the predominant oxide phase at 950 ⁰ C, 100h sample. Unlike Al2O3, Fe2O3 phase forms a non-protective oxide layer but the exact information about oxides may be gained only by looking at cross-sections using SEM [START_REF] Butler | Influence of Annealing on the Microstructures and Oxidation Behaviors of Al8(CoCrFeNi)92, Al15(CoCrFeNi)85, and Al30[END_REF][START_REF] Butler | Oxidation behaviour of arc melted AlCoCrFeNi multicomponent High-Entropy Alloy[END_REF][START_REF] Butler | High-temperature oxidation behavior of Al-Co-Cr-Ni-(Fe or Si) multicomponent high-entropy alloys[END_REF] which will be taken up in subsequent publications. After 500h of isothermal oxidation at 950˚C, the most intense ( 113 Both optical and SEM images reveal eutectic colonies with alternate lamellae (of various thickness) of L12 and B2 phases. Based on image analysis using ImageJ software, the average thickness of L12 phase is found to be nearly 1 μm whereas that of B2 lamella is found to be nearly 1.2 μm . Besides, the phase faction of L12 and B2 phases (based on variation in contrast) in the as-cast microstructure is also found to be 0.89 and 0.11 respectively from Figs. 2(a-c), similar to that inferred from the XRD of as-cast sample in Fig. 1. From the previous studies of oxidation behaviour of AlCoCrFeNi2.1 EHEA, [START_REF] Wani | Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy[END_REF][START_REF] Yiping | Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[END_REF][START_REF] Wani | Effect of severe cold-rolling and annealing on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Wani | Cold-rolling and recrystallization textures of a nano-lamellar AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Xuzhou | Microstructural origins of high strength and high ductility in an AlCoCrFeNi2.1 eutectic highentropy alloy[END_REF][START_REF] Yaoli | Deformation mechanism during high-temperature tensile test in an eutectic high-entropy alloy AlCoCrFeNi2.1[END_REF][START_REF] Ding | Preparation, characterization and properties of multicomponent AlCoCrFeNi2.1 powder by gas atomization method[END_REF][START_REF] Bhattacharjee | Effect of low temperature on tensile properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF], it has been reported that at elevated temperatures between 900 and 1300⁰C, the weight loss curve for oxidation initially shows a parabolic nature, which saturates with higher periods of oxidation. Combining the observation on the phase fraction of Al2O3 and Fe2O3 phases (Fig. 1) with the weight loss curves (Figs. 3(a-c)), it may be inferred that a slightly higher volume fraction of Al2O3 phase (in a dual phase Al2O3 + Fe2O3 microstructure) after isothermal oxidation at 1000˚C for 500h (that that after oxidation at 950˚C for 500h) leads to a significantly higher amount of weight loss than that after isothermal oxidation at 950˚C for 500h. This further provides an indication that Al2O3 scale formed during oxidation at elevated temperatures in AlCoCrFeNi2.1 EHEA aids in further oxidation unlike the reported nature of Al2O3 scale [START_REF] Wani | Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy[END_REF][START_REF] Yiping | Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[END_REF][START_REF] Wani | Effect of severe cold-rolling and annealing on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF][START_REF] Wani | Cold-rolling and recrystallization textures of a nano-lamellar AlCoCrFeNi2.1 eutectic high entropy alloy[END_REF].

However, the detailed microstructural investigation supporting the present claim on the nature of Al2O3 scale formed during prolonged periods of high temperature oxidation of AlCoCrFeNi2.1 EHEA, will be discussed in subsequent publications.

Conclusions

1.

During isothermal oxidation at 950 and 1000˚C, for oxidation periods ranging from 25 to 500h, AlCoCrFeNi2.1 EHEA (with a dual phase L12 + B2 microstructure) shows a dual phase Al2O3 + Fe2O3 microstructure.

2.

At 950˚C, there is phase reversion from Al2O3 to Fe2O3 when oxidation is performed till 50h.

However, Fe2O3 reverts back to Al2O3 when oxidation is performed at 950˚C for 100 and 500h.

Similarly, At 1000˚C, there is phase reversion from Al2O3 to Fe2O3 when oxidation is performed till 100h followed by a re-reversion from Fe2O3 to Al2O3 phase for oxidation till 500h at 1000˚C.

3.

Irrespective of considerable fluctuations in weight loss curves due to reasonable amounts of spallation, a linear rate law is observed at both 950 and 1000˚C, with increase in time of oxidation at both these temperatures from 4h till 500h.

4.

Al2O3 scale formed during oxidation at elevated temperatures in AlCoCrFeNi2.1 EHEA aids in further oxidation unlike the reported nature of Al2O3 scale.

3. Results and discussions 3 . 1 Fig 1 .

 311 Fig 1. Stacked XRD plots of as-cast (at the bottom, marked as black) to oxidised samples at different temperatures and time of isothermal oxidation (marked as red, blue, pink, green, deep blue, violet indigo and maroon).

Fig. 1

 1 Fig.1shows the X-Ray diffractograms of as-cast along with oxidised samples. The variation

3 . 2 Fig. 2

 322 Fig. 2 As-cast sample: (a) Optical image (100x), (b) Optical image (500x) of the region highlighted with a red outlined enclosure in part (a); and (c) SEM image (3000x) showing eutectic colonies with alternate lamellae (of various thickness) of L12 and B2 phases (enclosed within orange outlined rectangular box in part (c))

3. 3 Fig. 3 (

 33 Fig. 3 (a) Comparative Isothermal oxidation plots of AlCoCrFeNi2.1 EHEA at 950 and 1000⁰C between 4h and 500h of oxidation, (b) Isothermal oxidation plot of AlCoCrFeNi2.1 EHEA at 950⁰C between 4h and 50h of oxidation (drawn as subplot of part (a)) and (c) Isothermal oxidation plot of AlCoCrFeNi2.1 EHEA at 1000⁰C (plotted separately for a higher level of clarity) between 4h and 500h of oxidation.

Fig. 3 (

 3 Fig.3(a)), its subplot (Fig.3(b)) and Fig.3(c) indicate that irrespective of considerable fluctuations in these weight loss curves due to reasonable amounts of spallation, a linear rate law is observed at both 950 and 1000˚C, with increase in time of oxidation at both these temperatures.

  examined the oxidation behaviour of Al 8 Co 17 Cr 17 Cu 8 Fe 17 Ni 33 , Al 23 Co 15 Cr 23 Cu 8 Fe 15 Ni 15 , and Al 17 Co 17 Cr 17 Cu 17 Fe 17 Ni 17 HEAs at 800 • C and 1000 • C in air and reported that at 800 • C, that the alloy with lower Al content forms NiO along with other oxides, namely, Fe2O3, Cr 2 O 3 , and Al 2 O 3 , unlike at 1000˚C, where the same alloy preferentially forms Cr 2 O 3 over Al 2 O 3 , in contrast to the oxidation behaviour of HEAs with higher Al content, forming Al 2 O 3 scales at both temperatures (800 ˚C and 1000˚C). Holcomb et al.

Acknowledgement

MS is thankful to the Department of Metallurgical and Materials Engineering, NIT Durgapur, for carrying out the work.


text:
Mainak Saha

Fly ash composites: A review

Keywords: Fly ash composites, stir casting, hand lay-up, mechanical properties, correlative microstructural characterisation

The present decade has witnessed numerous investigations focussed on the determination of the mechanical properties of Fly ash composites. These composites have attracted attention, especially in the form of reinforcements owing to their excellent tensile and compressive properties coupled with impact and hardness response. A number of techniques viz. in-situ deposition, hand-up lay and compo-casting techniques have been reported to fabricate these composites. However, in the context of these composites, a systematic structure-property correlation has not been established till date. The present review is aimed at highlighting the current state of research in the avenue of Fly ash composites from two different viewpoints, viz. (i) fabrication technique and (ii) mechanical properties of the fabricated composites.

Moreover, the necessity of establishing systematic structure-property correlation in these materials has also been briefly discussed from the author's viewpoint.

Introduction

Composites find a number of engineering applications owing to the enhancement of physical and chemical properties obtained by the combination of two or more materials of which one is a matrix and the other acts as a reinforcement [START_REF] Mallik | Carbon-Based Nanocomposites: Processing, Electronic Properties and Applications[END_REF]. A number of matrices have been reported, common examples of which include polymer [START_REF] Gowda | Polymer matrix-natural fiber composites: An overview[END_REF], ceramic [START_REF] Binner | Selection, processing, properties and applications of ultra-high temperature ceramic matrix composites, UHTCMCs-a review[END_REF], metals [START_REF] Prabu | Influence of stirring speed and stirring time on distribution of particles in cast metal matrix composite[END_REF], and non-metals [START_REF] Mallik | Carbon-Based Nanocomposites: Processing, Electronic Properties and Applications[END_REF].

Common examples of reinforcements used in composites are silicon carbide (SiC), boron (B), alumina (Al2O3), fibers, particulates, graphite, rice husk, and fly ash [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF]. Composites are very strong in specific strength, specific modulus, damping capacity, and excellent wear resistance [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF]- [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF]. Zhang et al. [START_REF] Zhang | Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites[END_REF] have reported that the mechanical properties of metal matrix composites (MMCs) may be improved by the addition of high strength and high modulus particles like Al2O3, TiC, TiB2, SiC, etc. as reinforcements.

Two types of fly ash particles have been reported, namely (i) cenosphere-hollow particles (density< 1g/cc) and (ii) precipitator-spherical particles (density range: 2-2.5 g/cc). The former is used in the preparation of ultra-light composites due to their low density when compared to the density of metal matrix whereas, the latter is used for the enhancement of properties which include density, wear resistance, stiffness and strength [START_REF] Matsunaga | Crystallinity and selected properties of fly ash particles[END_REF], [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. Fly ash is a major industrial waste from thermal power plants and is highly abundant in nature owing to its low cost coupled with low density [START_REF] Rao | Machining Behavior of Al6061-Fly Ash Composites[END_REF], [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]- [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF]. Table. 1 shows chemical composition of fly ash particles (in wt.%) [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF]. Fig. 1 shows Scanning Electron micrograph of fly ash particles [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF]. Table . 2 shows chemical composition (in wt.%) of A356 composite [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF]. Table. [START_REF] Binner | Selection, processing, properties and applications of ultra-high temperature ceramic matrix composites, UHTCMCs-a review[END_REF] shows the Si content (in wt.%) of A356 alloy and that of A356 based C6S, C12S and C12AR composites. Prabu et al. [START_REF] Prabu | Influence of stirring speed and stirring time on distribution of particles in cast metal matrix composite[END_REF] have reported that Fly ash may be used as a potential reinforcement for the fabrication of low-cost composite with enhanced physical and mechanical properties for use in a number of industrial applications. Correlative microscopy, on the other hand, has emerged as a powerful tool for in-depth characterisation of a number of different microstructural features across varying length scales, both structurally and chemically [START_REF] Herbig | Atomic-Scale Quantification of Grain Boundary Segregation in Nanocrystalline Material[END_REF], [START_REF] Raabe | Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite[END_REF]. In other words, the aforementioned tool has the capability to provide both structural and chemical information from the same region in a given microstructure [START_REF] Toji | Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy[END_REF]. The present review highlights some of the techniques commonly used for fabricating Fly ash composites. This has been followed by a discussion on the mechanical properties (compressive, tensile and hardness) of these composites. In addition, the importance of correlative microstructural characterisation in terms of establishing a systematic structure-property correlation in these materials has also been discussed from the author's point of view.

Fabrication of Fly ash composites

A number of different techniques have been reported for the fabrication of Fly ash composites.

These commonly include stir casting and hand lay-up techniques.

Stir casting

In this method, the dispersed phase is mixed with molten metal matrix by means of mechanical stirring [START_REF] Manimaran | Mechanical properties of fly ash composites-A review[END_REF]. Large sized composites can be manufactured in a highly economical way [START_REF] Manimaran | Mechanical properties of fly ash composites-A review[END_REF]. A number of factors such as reinforcement material distribution, wettability, porosity of cast composites and chemical reaction between matrix and reinforcement material are the main process parameters during this process [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF].

Fabrication of MMC (by stir casting) using Al356 alloy as a matrix along with the reinforcement of size < 100 μm has been reported by Kulkarni et al. [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. In addition, fabrication of MMC through combination of (2, 4, 6, and 8) wt% fly ash, (2 and 6) wt% Eglass, and Al6061 composite has been reported [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. Preparation of three types of Al6061 composites using fly ash as reinforcement for reinforcement weight fractions of 10, 15, and 20% and particle sizes of 4-25, 45-50, and 78-100 μm has been reported using stir casting technique [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF]. Al matrix composites in the form of rod and slab have been fabricated by using hexachloroethane tablets, Al6061, and heat-treated fly ash [START_REF] Manimaran | Mechanical properties of fly ash composites-A review[END_REF].

Hand lay-up

This technique involves the use of hand to combine the resin and reinforcement on the surface of a mold followed by curing of the final laminate without any treatment [START_REF] Rohatgi | Low-cost, fly-ash-containing aluminum-matrix composites[END_REF]. The main advantage of this technique is to fabricate large and complex part in a user-friendly manner [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF].

For instance, turbine blades of high strength are manufactured at a low cost using this technique. Using this technique, fabrication of glass fiber reinforced fly ash composites with (0, 5,10, and 15 wt.%) reinforcements has been reported [START_REF] Rao | Machining Behavior of Al6061-Fly Ash Composites[END_REF]. In addition, fabrication of Eglass/epoxy-based composite slabs has been reported with Al2O3, magnesium hydroxide, hematite powder [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. Moreover, different concentrations of fly ash (0, 10, and 15 vol.%) have been used to determine the epoxy, filler materials, and fiber fraction [START_REF] Devendra | Strength Characterization of E-glass Fiber Reinforced Epoxy Composites with Filler Materials[END_REF].

Density

Fly ash composites have been reported to possess a lower density as compared to that of Al2O3 reinforcement [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. In the context of fly ash based composites, density decreases with increasing fly ash content. On the other hand, density increases with increasing Al2O3 content for Al2O3 reinforced composites [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. Boopathi et al. [START_REF] Boopathi | Evaluation of mechanical properties of Aluminium alloy 2024 reinforced with silicon carbide and fly ash hybrid metal matrix composites[END_REF] have studied the densities of Al2024 composites reinforced with SiC and fly ash particles using Archimedes principle. It has been reported that density of Al-SiC, Al-fly ash, and Al-SiC-fly ash composites undergoes a linear decrease with increasing fly ash and SiC contents. This may be attributed to the lower density of fly ash and SiC particles with respect to that of Al (matrix) [START_REF] Boopathi | Evaluation of mechanical properties of Aluminium alloy 2024 reinforced with silicon carbide and fly ash hybrid metal matrix composites[END_REF]. Table. [START_REF] Prabu | Influence of stirring speed and stirring time on distribution of particles in cast metal matrix composite[END_REF] shows the theoretical and measured densities and porosities of A356 alloy and A356 based C6S, C12S, and C12AR composites in both as-cast and extruded conditions. Density of composites was experimentally measured using Archimedes principle [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

Mechanical properties

Tensile properties

The tensile strength of the fly ash composites has been reported to be increased by the addition of fly ash content and E-glass fiber in Al6061 [START_REF] Manimaran | Mechanical properties of fly ash composites-A review[END_REF]. While applying tensile load, the presence of high-strength fly ash particles leads to an increase in tensile strength [START_REF] Basavarajappa | Mechanical properties of mmc's-An experimental investigation[END_REF]. The increase in ash content in Al7075 alloy leads to an enhancement in the tensile strength of the alloy up to a certain limit beyond which it decreases [START_REF] Shanmughasundaram | Synthesis of Al-Fly Ash Composites by Modified Two Step Stir Casting Method[END_REF]. Fig. 2 shows the tensile fracture surface of fly ash reinforced A356 composites [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

It has also been observed that the tensile strength of fly ash and alumina reinforced LM25

MMC increases with increasing fly ash content [START_REF] Qiao | Tensile properties of fly ash/polyurea composites[END_REF]. The variation of the tensile strength of polyphenylene oxide composite with fly ash reinforcement has been experimentally investigated [START_REF] Zaichenko | Composite material based on the polyethylene terephthalate polymer and modified fly ash filler[END_REF]. It has been shown that for higher size fly ash content, tensile strength (ultimate tensile strength (UTS)) is independent of fly ash content, whereas for smaller particle sizes, UTS decreases due to the agglomeration for high fly-ash concentration [START_REF] Zaichenko | Composite material based on the polyethylene terephthalate polymer and modified fly ash filler[END_REF].

Fig. 2 SEM image of fracture surfaces (after tensile fracture) of fly ash reinforced A356 composites [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

Compressive properties

In all types of fly ash reinforced composites, the compressive strength increases with increase in fly ash content. However, composites reinforced with Al2O3 show higher compressive strength than the one reinforced with fly ash [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. Besides, reinforced with E-glass fiber and fly ash lead to an increase in the density (of the composite) which enhances the compressive strength of these composites [START_REF] Kulkarni | Effect of Fly Ash Hybrid Reinforcement on Mechanical Property and Density of Aluminium 356 Alloy[END_REF]. In the context of Al6061, reinforcement with fly ash particles has been reported to lead to an increase in compressive strength of the composite [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF]. The compressive strength of the composite has also been reported to decrease with increasing fly ash particle size . In the context of concrete composite reinforced with fly ash and nano-SiO2, compressive strength is found to be higher than that of concrete composite without the presence of steel fiber reinforcement [START_REF] Zhang | Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites[END_REF]. Fig. 3 shows SEM image of fly ash reinforced A356 composite showing fractured particles of fly ash after compression [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

Fig. 3 SEM image of fly ash reinforced A356 composites showing fractured particles (after compression) of fly ash [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF]. The white arrow in the image represents loading direction.

Hardness

In the context of Al6061 composites, there is a conversion from ductile to brittle nature with increasing fly ash content [START_REF] Rohatgi | Low-cost, fly-ash-containing aluminum-matrix composites[END_REF]. This leads to an increase in the hardness of these composites [START_REF] Shanmughasundaram | Synthesis of Al-Fly Ash Composites by Modified Two Step Stir Casting Method[END_REF], [START_REF] Zaichenko | Composite material based on the polyethylene terephthalate polymer and modified fly ash filler[END_REF]. In the case of polymers, resistance to plastic deformation is increased by the addition of fly ash. This leads to an increase in the hardness value. For instance, the 300-nm-size fly ash filler in polymer matrix composites results in higher hardness when compared to other fly ash filler sizes [START_REF] Mahendra | Fabrication of Al-4.5% Cu alloy with fly ash metal matrix composites and its characterization[END_REF]. This has been attributed to a larger phase contact between the filler and matrix phases [START_REF] Raja | Effect of fly ash filler size on mechanical properties of polymer matrix composites[END_REF]. The hardness of these composites has also been reported to increase with increasing silicon carbide (SiC) particle (as reinforcement). For the combination of Al/(10%SiC+10%fly ash), maximum hardness is achieved [START_REF] Boopathi | Evaluation of mechanical properties of aluminium alloy 2024 reinforced with silicon carbide and fly ash hybrid metal matrix composites[END_REF].

Wear

In the context of Al MMCs reinforced with fly ash, Sharma et al. [START_REF] Sharma | Effect of flyash particles with aluminium melt on the wear of aluminium metal matrix composites[END_REF] have reported that wear resistance of the fabricated composites increases with increasing fly ash content. In addition, composites with high fly ash contents have been reported to result in ~13.6% less wear as compared to that of the composites with low fly ash content [START_REF] Sharma | Effect of flyash particles with aluminium melt on the wear of aluminium metal matrix composites[END_REF]. For medium fly ash content (~ 4%) average coefficient of friction was reported to be the lowest (~0.12) whereas, for high fly ash contents (~6%), the average coefficient of friction was reported to be the maximum (~0.161). Based on this work, it was concluded that the amount of fly ash content in the Al matrix must not be > 4%, so as to prevent a high value of the coefficient of friction. Sudarshan

and Surappa [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF], [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF] and Rohatgi et al. [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF] have studied the dry sliding wear of fly ash particle reinforced A356 Al composites. It was reported that incorporation of 6 vol. % of fly ash particles into A356 Al alloy leads to a decrease in dry sliding wear rates at low loads (~10 and 20 N) [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. 12 vol.% of fly ash reinforced Composites reinforced with ~12 vol.% of fly ash were reported to show lower wear rates as compared to that of non-reinforced alloy between 20 and 80 N [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF], [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. For composites with 12 vol. % fly ash, wear rate was observed to decrease with particle size [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. An increase in the magnitude of friction co-efficient (from ∼0.49 to ∼0.58) with increasing fly ash content from 6 to 12 vol.%) was reported [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF], [START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

Adhesive wear was reported to be dominant in non-reinforced alloy, whereas abrasive wear was prevalent in composites [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF], [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. Moreover, at higher load, subsurface delamination was reported to be the main mechanism for both alloys and composites [START_REF] Rohatgi | Friction and abrasion resistance of cast aluminum alloy-fly ash composites[END_REF]. Fig. 4 shows the SEM image of worn surfaces (at various loads) of fly ash reinforced A356, C6S, C12S, and C12AR composites [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. In the context of Al-10 wt.% fly ash reinforced composite, Desai et al. [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF] have reported that high load results in high pressure at the point, of contact subsequently resulting in a high rubbing action [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF]. As a result, an increase in the applied load leads to an increase in the wear rate [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF]. It is worth mentioning that wear mechanisms involving adhesion are predominant in the base metal, while abrasion with micro-cutting and oxide formation dominate in Al based fly ash composites [START_REF] Desai | Mechanical properties and wear behavior of fly ash particle reinforced Al matrix composites[END_REF]. (f) 80 N; and C12AR composite: (g) 10 N and (h) 80 N [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF]. In parts (a-h), AR and S abbreviate for as-receive and sieved respectively. Besides, C6S represents 6 vol.% of fly ash (as a reinforcement). Similarly, C12S represents 12 vol.% of fly ash (as a reinforcement) [START_REF] Sudarshan | Dry sliding wear of fly ash particle reinforced A356 Al composites[END_REF].

Future perspectives

The present state of research in the field of fly ash composites is focussed on the fabrication techniques and mechanical properties of these materials. However, a systematic structureproperty correlation based investigation is missing. This is because research in the field of these composites, till date, has not been dedicated to: (i) understanding the influence of process parameters (associated with the different fabrication techniques, as discussed in section 2) on the microstructure evolution, and (ii) the understanding of microstructural defects and their associated influence on the mechanical properties of these materials. This may be attributed to the fact that at present, most research groups involved with fly ash composite research are not primarily focussed on metallurgical research.

The mechanical properties of composites are highly sensitive towards microstructural defects, especially developed during fabrication [START_REF] Asl | Effects of carbon additives on the properties of ZrB2-based composites: A review[END_REF]. Moreover, these properties are anisotropic along longitudinal and transverse section of these composites [START_REF] Saha | ɤ-TiAl alloy: revisiting tensile creep deformation behaviour and creep life at 832[END_REF], [START_REF] Saha | ɤ-TiAl alloy: revisiting tensile creep deformation behaviour and creep life at 832 °C[END_REF]. In the last 10 years, correlative microscopy has emerged as a powerful tool for a structural cum chemical characterisation from the same region in a given microstructure [START_REF] Mallik | Carbon-Based Nanocomposites: Processing, Electronic Properties and Applications[END_REF], [START_REF] Schwarz | Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se2 grain boundaries[END_REF]. This has been largely utilised for understanding the influence of microstructural defects on the mechanical properties of metallic materials [START_REF] Raabe | Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces[END_REF]. However, till date, this methodology has not been utilised towards establishing systematic structure-property correlation in these fly ash composites. The challenge primarily lies in terms of the expertise and cost associated with operating Focussed Ion Beam (FIB)-based sample preparation technique for preparing samples for correlative microstructural analysis across different length scales and the reproducibility of the results obtained. The other challenge lies in understanding the complexity of crystal structures associated with these composites. Understanding crystal structures may be considered as the first step towards addressing microstructural defects associated with processing of these composites. Hence, the aforementioned methodology shows extensive potential for future research in the avenue of fly ash research.

Conclusions

The present need of the hour is to design composites with high specific strength. Fly ash particles have shown tremendously high potential as reinforcements with lightweight Al alloys leading to enhanced mechanical properties. More specifically, hardness, tensile and compressive strength of these composites increase with increasing fly ash content. On the other hand, density decreases with increasing fly ash content. However, extensive microstructure analysis aimed at systematic structure-property correlation in these materials is necessary before using these materials for large-scale industrial applications. Correlative microscopy, as a characterisation methodology, shows a high potential towards extensive structural cum chemical characterisation of these materials. This methodology may be used to obtain an indepth understanding of the microstructure evolution in these materials.

Fig. 4

 4 Fig. 4 SEM image showing worn surface of fly ash reinforced A356 composite at various loads: (a) 10 N, and (b) 80 N; C6S composite: (c) 10 N and (d) 20 N; C12S composite: (e) 10 N, and

Table . 1

 . Chemical composition (in wt.) of fly ash particles[START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

	Fig. 1 SEM image of fly ash particles [14].
	Compound	Composition (wt.%)
	SiO2	64.80
	Al2O3	24.01
	Fe2O3	5.23
	CaO	2.76
	MgO	0.90
	TiO2	0.50
	Loss on ignition (LOI)	0.87-1.33

Table . 2

 . Chemical composition (in wt.%) of A356 alloy[START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

	Element	Composition (wt.%)
	Si	7.083
	Mg	0.409
	Cu	0.059
	Ti	0.132
	Mn	0.003
	Cr	0.012
	Al	Balance
	Table. 3 Si content (in wt.%) of A356 alloy and C6S, C12S, and C12AR composite [14].
	Material	Si (in wt.%)
	A356Al	7.08
	C6(S)	8.33
	C12(S)	15.83
	C12(AR)	15.31

Table . 4

 . Densities and porosities of A356 alloy and A356 based C6S, C12S, and C12AR composites[START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

	Material	Theoretical	Measured density	Porosity (vol. %)
		density (g/cc)
			As-cast	Extruded	As-cast	Extruded
	A356 Al	2.680	2.665	2.676	0.560	0.149
	C6(S)	2.645	2.616	2.640	1.096	0.189
	C12(S)	2.609	2.442	2.591	6.401	0.690
	C12(AR)	2.611	2.458	2.602	5.860	0.345

Table . 5

 . shows Vickers microhardness and Brinell microhardness values of A356 alloy and its composites.

Table . 5

 . Vickers and Brinell hardness of A356 alloy and its composites[START_REF] Sudarshan | Synthesis of fly ash particle reinforced A356 Al composites and their characterization[END_REF].

	Alloy/composite	Vickers hardness number	Brinell hardness number
		(VHN)	(BHN)
	A356 Al	51.1 ± 1.2	48.5 ± 1.0
	C6S	59.5 ± 1.1	54.8 ± 0.5
	C12S	55.6 ± 2.5	54.2 ± 0.4
	C12AR	54.3 ± 0.8	52.4 ± 0.9

Acknowledgement

MS would like to thank the research scholars in the Department of Metallurgical and Materials

Engineering, NIT Durgapur, for detailed discussions during the scripting of the review paper.