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

A Cr₄₀Co₄₀Ni₂₀ (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 (σ_TRIP = σ_TRIP,0 + k_TRIPd^-0.5), with the Hall-Petch slope being approximately the same for yield stress and TRIP effect (k_y ≈ 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 Cr₄₀Co₄₀Ni₂₀ alloy provide high work-hardening capacity, which improves ductility and toughness. Here, a refined FCC grain size (d = 1.3; c = 0.7 μm) was shown to increase the yield stress by at least 100 % (417 → 834 MPa), compared to a coarser grain material (17; 6.8 μm), 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.