Tuning strain-induced -to- martensitic transformation of biomedical Co-Cr-Mo alloys by introducing parent phase lattice defects

森 真奈美*; 山中 謙太*; 佐藤 茂男*; 椿 真貴*; 佐藤 こずえ*; 熊谷 正芳*; 今福 宗行*; 菖蒲 敬久  ; 千葉 明彦*

Mori, Manami*; Yamanaka, Kenta*; Sato, Shigeo*; Tsubaki, Shinki*; Sato, Kozue*; Kumagai, Masayoshi*; Imafuku, Muneyuki*; Shobu, Takahisa; Chiba, Akihiko*

In this study, we examined the effect of pre-existing dislocation structures in a face-centered cubic gamma-phase on strain-induced martensitic transformation (SIMT) to produce a hexagonal close-packed epsilon-phase in a hot-rolled biomedical Co-Cr-Mo alloy. The as-rolled microstructure was characterized by numerous dislocations as well as stacking faults and deformation twins. SIMT occurred just after macroscopic yielding in tensile deformation. Using synchrotron X-ray diffraction line-profile analysis, we successfully captured the nucleation of epsilon-martensite during tensile deformation in terms of structural evolution in the surrounding gamma-matrix: many dislocations that were introduced into the gamma-matrix during the hot-rolling process were consumed to produce epsilon-martensite, together with strong interactions between dislocations in the gamma-matrix. As a result, the SIMT behavior during tensile deformation was accelerated through the consumption of these lattice defects, and the nucleation sites for the SIMT epsilon-phase transformed into intergranular regions upon hot rolling. Consequently, the hot-rolled Co-Cr-Mo alloy simultaneously exhibited an enhanced strain hardening and a high yield strength. The results of this study suggest the possibility of a novel approach for controlling the to SIMT behavior, and ultimately, the performance of the alloy in service by manipulating the initial dislocation structures.