Strain rate sensitivity of a novel refractory high entropy alloy: Intrinsic versus extrinsic effects

In recent years, there is great interest in development of reduced activity refractory alloys for next generation nuclear applications for safe and efficient post-service recycling. Here, the strain rate sensitivity of a refractory high entropy alloy, namely HfTaTiVZr, was investigated consisting of all reduced activation elements. The intrinsic versus extrinsic effects in rate sensitivity were evaluated for fundamental understanding of the differences in loading condition. Uniaxial bulk and micro-pillar compression tests were performed along with multi-axial nano-indentation in a wide range of strain rates. The extrinsic strain rate sensitivity obtained from micro-pillar compression and bulk compression was higher than its intrinsic characteristic determined from nano-indentation. The low value of strain rate sensitivity obtained from nano-indentation was attributed to greater degree of dislocation entanglement in confined volume beneath the indenter. On the other hand, the high value of strain rate sensitivity in case of micro-pillar compression tests was attributed to the extrinsic effect of larger surface to volume ratio which helped in annihilation of dislocations through the free surface. A pronounced indentation size effect of strain rate sensitivity was also observed. At smaller depth, higher mobility and diffusion of dislocations near the free surface resulted in higher strain rate sensitivity compared to that at larger indentation depth.