Enhanced Mechanical Properties of As-Forged Co-Cr-Mo-N Alloys with Ultrafine-Grained Structures

The ultrafine-grained (UFG) microstructures of Ni-free Co-29Cr-6Mo (mass pct) alloys, which are designed for biomedical applications, have been successively fabricated by the conventional hot-forging process. The grain size decreased with increasing hot-forging reduction, and the equiaxed UFG structures with a mean grain size less than 1 μm were obtained in 83 pct (true strain of 1.8) hot-forged specimens. Significant grain refinement drastically enhanced tensile strength; dislocations residual in the grains also play a crucial role for strengthening of the UFG-structured specimen. The elongation decreased with the reduction in grain size. However, we revealed that the addition of nitrogen, which is one of the nontoxic γ phase (face-centered cubic [fcc] structure) stabilizer, improves the ductility of the UFG alloys remarkably with maintaining high strength. It was deduced that the enhanced ductility in the UFG material by N doping was related to constituent phase and strain-induced martensitic transformation behavior: the addition of nitrogen eliminated athermal ε martensite detrimental to tensile elongation, and strain-induced martensitic transformation effectively increased work-hardening rate to avoid the plastic instability at the early stage of deformation. The present method characterized by ultragrain refinement in conjunction with nitrogen addition to stabilize the γ phase can provide a potent strategy to obtain superior combination of high strength and adequate ductility.