Grain size dependence of strain rate sensitivity in a single phase FCC high entropy alloy Al0.3CoCrFeNi

High-entropy alloys (HEAs) can offer exceptional strain-rate sensitivity (SRS) due to high lattice-friction and strong solid-solution strengthening, and in some cases, low stacking fault energy. In this study, SRS of a single phase FCC HEA- Al0.3CoCrFeNi was investigated at two grain sizes. Due to its outstanding Hall-Petch coefficient, grain size becomes a crucial microstructural feature in determination of strength and SRS. Change in SRS due to grain refinement was derived to be inversely proportional to associated strength gain, in coarse microstructures where grain size >> dislocation forest cell size. This correlation was proven using two microstructures with grain sizes of 12 µm and 150 µm, with yield strengths of 313 MPa and 145 MPa, and demonstrated SRS of m = 0.029 and 0.064, respectively. SRS was also derived to increase linearly with strength contribution from thermal short-range obstacles. The slope reflects the maximum upper limit on SRS possible upon elimination of all other obstacles of long-range nature. This limiting value of SRS was derived to be 0.118 for this HEA.