Strain-induced martensitic transformation in biomedical Co–Cr–W–Ni alloys

The nucleation, variant selection, and orientation dependence of the strain-induced martensitic transformation (SIMT) process in biomedical Co–Cr–W–Ni alloys were investigated. The experimental results show that the ε-hexagonal-close-packed phase was preferentially formed at the Σ3 twin boundaries and high-angle grain boundaries during the tensile process. The theoretical analysis shows that the variant selection of SIMT is governed by Schmid’s law. However, the SIMTed ε-phase did not form equally on the two sides of the annealing twins, even though they had the same Schmid factor. This phenomenon is related to the mechanical work developed by the formation of the ε-phase. Only the side which has both high Schmid factor and high mechanical work can initiate the SIMT process. A strong 〈111〉 fiber texture was formed, and the ε-variants tended to appear in grains with orientations close to the 〈111〉 and 〈100〉 directions during the tensile process. These results can provide theoretical guidance for controlling the SIMT process of Co–Cr–W–Ni alloys to fabricate more reliable stents.