Enhanced Mechanical Properties of As-Forged Co-Cr-Mo-N Alloys with Ultrafine-Grained Structures
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IT is well known that cobalt-chromium-molybdenum (Co-Cr-Mo) alloys, austenitic stainless steels, titanium, and its alloys are widely applied as biomedical implant materials. In particular, Co-Cr-Mo alloys have been used for artificial hip and knee joints because of their excellent corrosion and wear resistances as well as good mechanical properties and biocompatibility.[1–4] Although as-cast or annealed Co-Cr-Mo alloys are frequently subjected to the practical use, there are possibilities to exhibit insufficient mechanical properties due to a coarse microstructure and solidification defects such as microvoids, segregation of solute atoms, and so on.[5] The r phase, Co(Cr, Mo), which is known as a brittle intermetallic compound, sometimes precipitates during heat treatment. Therefore, the improvement of tensile properties as well as fatigue properties is eagerly anticipated because further mechanical reliability is required for providing long-term use as hard tissue replacements. Enhanced mechanical properties are generally obtained by thermomechanical processing on the basis of microstructure control, involving in plastic deformation, KENTA YAMANAKA, Ph.D. Student, and AKIHIKO CHIBA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: [email protected] MANAMI MORI, formerly Graduate Student, Institute for Materials Research, Tohoku University, is now Researcher, Nissan Arc, Ltd., Yokosuka 237-0061, Japan. Manuscript submitted November 26, 2011. Article published online July 20, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
recrystallization, precipitation, and phase transformation. Combining plastic deformation with subsequent recrystallization heat treatment is one of the most practical techniques used for conventional structural metallic materials. However, such a method is difficult to apply for Co-Cr-Mo alloys owing to their poor workability, especially at room temperature. This is because hexagonal close-packed (hcp) e martensite, which is detrimental to cold workability, forms during both quenching after solution treatment and plastic deformation due to low stability of face-centered cubic (fcc) c phase.[6,7] A large amount of nickel (10 to 37 mass pct) is, therefore, added to these alloys to improve their ductility by stabilizing the c phase, in spite of the fact that Ni possibly causes skin allergies or cancer in living organisms. Hence, novel methods to obtain optimized microstructures of Ni-free Co-Cr-Mo alloys must be developed for this purpose. Grain refinement is a promising approach to improve the mechanical properties of the present alloys, as well as other metallic materials. Recently, ultrafine-grained (UFG) and nanocrystalline materials, of which grain sizes are ranging from 100 nm to 1 lm and smaller than 100 nm, respectively, have received considerable attention because of their superior strength,[8] good fracture toughness at low temperature,[9] and excellent superplasticity[10] compared with conventional coarse-grained
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