The Phase Composition and Microstructure of AlxCoCrFeNiTi Alloys for the Development of High-Entropy Alloy Systems

Abstract

Abstract:Alloying aluminum offers the possibility of creating low-density high-entropy alloys (HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination. The effect of aluminum on the phase composition and microstructure of the compositionally complex alloy (CCA) system Al x CoCrFeNiTi was studied with variation in aluminum content (molar ratios x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the different domains in the microstructure. The crystal structure was determined using X-ray diffraction (XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with associated orientation distribution was performed using electron backscatter diffraction. This made it possible to correlate the chemical and structural conditions of the phases. The phase formation strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum amounts. The hard η-(Ni, Co) 3 Ti phase in the x = 0.2 alloy was detected via metallographic investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc) with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural and chemical properties as well as microstructure formation contribute to a better understanding of the alloying effects concerning the aluminum content in CCAs. Especially in the context of current developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach to the development of new alloy systems.

Influence of Titanium on Microstructure, Phase Formation and Wear Behaviour of AlCoCrFeNiTix High-Entropy Alloy

“…DSC results also revealed the formation of an additional phase, which solidifies as a remainder after the two major bcc phases and forms cell walls in the microstructure. This phase has been detected in the same alloy system in preliminary studies by Lindner et al and for a high aluminium content (AlCoCrFeNiTi 1.5 ), the formation of this phase could be suppressed [19]. The diffractogram of the alloy with the highest titanium content (x = 1.5) exhibited diffraction peaks which can be ascribed to the bcc (A2), bcc (B2) and an fcc phase.…”

“…This phase corresponded to the second exothermic reaction in the DSC measurements, and hence to the interdendritic phase (IP) in the alloys x ≥ 0.2. The stabilisation of bcc phases due to a high aluminium content has been reported elsewhere in detail [19]. For the alloy with the lowest titanium content (x = 0.2), no additional phases were be detected.…”

“…Further diffraction peaks of this phase overlapped with the bcc (B2) phase. Previous investigations of the alloy system Al x CoCrFeNiTi revealed the solidification of the fcc phase as a remainder in the cell walls [19]. The diffraction diagram of the alloy with a titanium content of x = 0.8 exhibited several additional diffraction peaks, which can be assigned to a tetragonal σ phase.…”

“…For a low aluminium content, fcc phases are stabilised, whereas high contents of aluminium act as a strong bcc phase stabiliser [17,18]. In addition, the formation of complex phases can be suppressed for a high aluminium content [19].…”

Microstructural Evolution from Dendrites to Core-Shell Equiaxed Grain Morphology for CoCrFeNiVx High-Entropy Alloys in Metallic Casting Mold

“…Therefore, it seems acceptable that the V 0.9 and V 1.0 melts followed the described solidification path with the equiaxed crystal growth. A similar result was also observed in the copper-mold cast Al x CoCrFeNiTi HEAs, wherein large equiaxed grains solidified first from the melt and grew up [34]. However, it is rather difficult to understand the nature of the distinct core-shell structure.…”