Phase formation in mechanically alloyed Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al2.5CoCrCuFeNi and Al5CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al5CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions

Alloying, thermal stability and strengthening in spark plasma sintered Al_xCoCrCuFeNi high entropy alloys

AlxCoCrCuFeNi (x = 0.45, 1, 2.5 and 5 mol) multi-component high entropy alloys synthesized by mechanical alloying were spark plasma sintered to produce high dense compacts. X-ray diffraction and scanning electron microscopy studies reveal that these sintered alloys exhibit varying microstructures from single phase to three phases depending on Al content. The thermal stability studies carried out in the temperature range of 400–600°C for duration of 2–10 h in Ar atmosphere suggest that these alloys exhibit excellent thermal stability in terms of phases and crystallite size. Highest specific hardness of 160 (HV/g cm−3) is achieved in the sintered Al5CoCrCuFeNi alloy and there is no significant change in the hardness after heat treatment of Al0.45CoCrCuFeNi and AlCoCrCuFeNi alloys. Hall–Petch analysis based on hardness measurements carried out on sintered samples reveals that solid solution strengthening seems to increase with increase in Al content

High Entropy Alloys: Criteria for Stable Structure

An effort has been made to reassess the phase predicting capability of various thermodynamic and topological parameters across a wide range of HEA systems. These parameters are valence electron concentration, atomic mismatch (delta), electronegativity difference (Delta chi), mixing entropy (Delta S (mix)), entropy of fusion (Delta S (f)), and mismatch entropy (S (sigma) ). In continuation of that, two new parameters (a) Modified Darken-Gurry parameter (A = S sigma * chi) and (b) Modified Mismatch Entropy parameter (B = delta* S sigma) have been designed to predict the stable crystal structure that would form in the HEA systems considered for assessment. (C) The Minerals, Metals & Materials Society and ASM International 201