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 mu m 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 gamma 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 epsilon 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 gamma phase can provide a potent strategy to obtain superior combination of high strength and adequate ductility.