Microstructure and hardness of solution treated and cold-rolled Co75Cr25 alloy

Abstract

In this study, the effects of solution treatment and plastic deformation on the microstructure and hardness of a Co75Cr25 alloy were investigated. The microstructures of the solution-treated and deformed specimens were investigated through X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results indicated that the Co75Cr25 alloy cooled by air (AC) during solution treatment has a single hexagonal close-packed (HCP) phase structure, while the alloy quenched by water (WQ) has a dual phase structure with the mixture of HCP phase and face-centered cubic (FCC) phase. Fast cooling rate inhibited the FCC to HCP martensitic phase transformation and led to more FCC phase retained at room temperature. During subsequent cold rolling (CR), stress-induced HCP to FCC martensitic transformation further increased the fraction of FCC phase in the WQ-CR specimen, through gliding of Shockley partial dislocations. Besides, stress-induced 101¯2 and 101¯1 twins were also widely observed in the WQ-CR specimen. Both martensitic transformation and twinning refined the microstructure of the WQ-CR specimen. However, for the AC-CR specimen, the deformation did not introduce obvious martensitic phase transformation. Some 101¯2and 112¯1 twins were activated to accommodate deformation. The hardness values of specimens after cold rolling were higher than those of the specimens before cold rolling. Deformation induced grain refinement played a very important role for improving the hardness.