Influence of grain size on strain-induced phase transformation in a CrCoNi multi-principal element alloy

A Cr(40)Co(40)Ni(2)0 (at.%) alloy with different grain/crystallite sizes was analyzed through in-situ synchrotron X-ray diffraction during tensile testing. The FCC starting structure underwent a partial strain-induced transformation to HCP (TRIP effect) and the percent transformed was measured throughout the deformation. The critical stress required to form a certain HCP fraction was shown to follow a Hall-Petch relation (sigma(TRIP) = sigma(TRIP,0 )+ k(TRIPd)(-0.5)), with the Hall-Petch slope being approximately the same for yield stress and TRIP effect (ky approximate to k(TRIP)). Furthermore, this work developed a Hall-Petch-based model that correlates the applied stress, the transformed phase fraction, and the initial FCC grain/crystallite size. It predicts the stress required to form a certain HCP fraction, or the fraction formed when a certain stress is applied, for different grain/crystallite sizes. We also proposed a mechanism to explain the grain/crystallite size dependence of the TRIP effect and discuss how the TRIP effect and its early activation in the Cr(40)Co(40)Ni(2)0 alloy provide high work-hardening capacity, which improves ductility and toughness. Here, a refined FCC grain size (d = 1.3; c = 0.7 mu m) was shown to increase the yield stress by at least 100 % (417 -> 834 MPa), compared to a coarser grain material (17; 6.8 mu m), while maintaining a high ductility of 41 %. This work contributes to a better understanding of the deformation mechanisms, mainly the strain-induced phase transformation (TRIP), highlighting their impact and importance on mechanical properties.