High Entropy (HEA) or multicomponent alloys consist of four - six elements in equiatomic (or close to equiatomic) ratios [1]. In these systems, solid solution with simple crystallographic structures (e.g. Face Centered Cubic (FCC) or Base Centered Cubic (BCC)) is promoted instead of the formation of intermetallic compounds due to the high mixing entropy effect. HEA exhibit exceptionally high hardness, good wear and oxidation resistance [2]. These qualities make HEA attractive candidates for variety of engineering applications.
In the current work, equiatomic AlCoCrFeNi HEA was studied thoroughly. Previously, it was reported that AlCoCrFeNi solidifies in a dendritic regime, forming two areas: dendrite (DR) and interdendrite (ID) [3] differing in their microstructure. Understanding how such differences affect the properties of each are and the evolution of the microstructure upon heating is essential for the production of an homogenous alloy.
By analyzing the microstructure of the as cast AlCoCrFeNi HEA, we noted differences in the phase content between the DR and ID areas, fact that explains the different morphology and hardness measured in these areas. Investigation of this alloy after heat treatments (followed by quenching) performed at 850°C and 1200°C has shown that these variations dictate the evolution of different microstructures in the DR and ID areas. At 850°C sigma phase was formed in both areas but with a different morphology, along with the growth of the FCC phase. At 1200°C the microstructure consisted of FCC, BCC and ordered BCC, the same phases obtained in the as cast condition, yet with very complex morphology, requiring high-end microscopy methods for investigating their structure and chemical composition. Conclusions were drawn regarding phase transformations occurring in the AlCoCrFeNi system.
References:
[1] J.W. Yeh, S.K. Chen, Advanced Engineering Materials 6 (5), (2014), 299-303.
[2] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Progress in Materials Science 61, (2014), 1-93.
[3] A. Manzoni, H. Daoud, R. Volkl, U. Glatzel, N. Wanderka, Ultramicroscopy 132, (2013), 212-215.
