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 Å. 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/σ_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 μN 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 b³ (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.