Origin of Significant Grain Refinement in Co-Cr-Mo Alloys Without Severe Plastic Deformation
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GRAIN refinement is a promising technique for strengthening metallic materials. Recently, ultrafinegrained (UFG) materials with grains smaller than 1 lm and nanocrystalline structures have been attracting considerable interest, because they have superior mechanical properties to conventional materials. Severe plastic deformation (SPD) processes[1–8] such as equalchannel angular pressing,[1–4] high-pressure torsion,[1,3] accumulative roll bonding,[2,5,6] and multidirectional forging[7,8] are generally employed to produce UFG microstructures. However, SPD processes require an accumulated equivalent strain of over 4,[9] which sometimes necessitates complicated procedures. In addition, the limited dimensions of SPD processed specimens restrict the use of UFG materials in practical applications. Several methods were developed for fabricating UFG structures without SPD, especially for steels.[10,11] Co-Cr-Mo–based alloys are used as biomedical implants, such as artificial hips and knees,[12] wearresistant tools, and superalloys. We have investigated various approaches for improving the mechanical properties of these alloys. We found that grain refinement can be realized on a submicron scale by hot forging to a KENTA YAMANAKA, Postdoctoral Student, and AKIHIKO CHIBA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: a.chiba@ imr.tohoku.ac.jp MANAMI MORI, formerly Graduate Student with Institute for Materials Research, Tohoku University, is now Researcher with NISSAN ARC, Ltd., Yokosuka 237-0061, Japan. Manuscript submitted February 28, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
reduction of 60 pct (true strain, e = 0.92) as a result of dynamic recrystallization (DRX).[13] Since grain refinement drastically enhances the tensile strength of Co-CrMo alloys,[14] it is important to elucidate the mechanism of microstructure evolution during DRX. DRX is generally categorized as discontinuous and continuous DRX based on the deformation conditions. At relatively high temperatures, discontinuous DRX characterized by nucleation and growth of new grains operates.[15] In this type of DRX, serration of the original grain boundaries, which is attributed to strain-induced grain boundary migration, usually occurs in the early stages of deformation and results in new grains being formed. This nucleation mechanism is manifested by bulging of grain boundaries, and it was observed in many metals and alloys.[16–18] In contrast, continuous DRX is considered to cause microstructural refining during SPD processes; in other words, the original grains are subdivided into small, highly misoriented domains by deformation-induced dislocation boundaries and dynamic recovery (DRV) increases the misorientation of these boundaries during further straining.[19] This type of DRX exhibits very little pronounced grain growth after nucleation. Consequently, a remarkable reduction in grain size is more likely to be achieved by continuous DRX rather than discontinuous DRX, although this require
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