Strengthening mechanisms in equiatomic ultrafine grained AlCoCrCuFeNi high-entropy alloy studied by micro- and nanoindentation methods

A single phase fcc based nanocrystalline solid solution in equiatomic AlCoCrCuFeNi high-entropy alloy (HEA) has been synthesized using ball milling. The milled powders were of "plate-like" morphology and possessed a precise lattice parameter of 3.641 angstrom. Compaction of ball milled powders into bulk components using spark plasma sintering (SPS) at 1023 K led to the precipitation of ordered bcc (B2). Detailed structural and microstructural investigations on the sintered alloy indicate the presence of bimodal grain size distribution with average grain sizes of 112 +/- 46 nm and 1550 +/- 500 nm, solid solutions (fcc and B2 phases), dislocations and twin boundaries. A high hardness value of 6.5 +/- 0.1 GPa was measured for the sample sintered at 1023 K/15 min using Vickers microindentation. Comprehensive analysis on probable strengthening mechanisms suggests that frictional stress, Taylor hardening, Hall-Petch strengthening, solid solution strengthening and twin boundary strengthening mechanisms are responsible. The Taylor hardening arising from intersection of dislocations and grain boundary (Hall-Petch) strengthening arising from grain boundary-dislocation interactions together account for 85% of the observed flow stress. The Tabor's ratio, (H/sigma(flow)) attained a value of 2.7 which is in close agreement with that for conventional polycrystalline materials. Nanoindentation at a peak force of 8000 mu N yielded a high hardness value of 8.13 +/- 0.15 GPa and an elastic modulus of 172 +/- 10 GPa. A low strain rate sensitivity of 0.0084 and an activation volume of 13 b(3) (b is 0.23 nm) were measured, suggesting that grain boundaries, twin boundaries and interphase boundaries (fcc/B2) are influential in governing the deformation kinetics. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.