Microstructure and mechanical anisotropy of CoCrW alloy processed by selective laser melting

Abstract

We have investigated the correlation between the microstructure and mechanical anisotropy of biomedical CoCrW alloy processed by selective laser melting (SLM). A phase analysis revealed that the as-SLM CoCrW alloy consisted of mostly the γ-fcc phase and little ε-hcp phase. The as-SLM CoCrW microstructure featured the rotated cube texture, a strong <100> texture along the building direction (BD) and a weaker <110> texture along the front direction. To investigate the mechanical anisotropy, the external loading was applied either perpendicular or parallel to the BD (i.e., horizontal loading (⊥BD) or vertical loading (//BD), respectively). The rotated cube texture leads to a higher elastic modulus (~ 2.4 times), plastic flow stress, and microhardness in the horizontal loading. Macroscopic plastic anisotropy was elucidated by the Taylor factor analysis of polycrystalline aggregate. Strain-induced martensite transformation (SIMT) occurring in the horizontal loading (ef = 6.9%) is responsible for significantly reduced elongation to fracture compared to the vertical loading (ef = 32.8%). In the fatigue crack growth experiment, the threshold stress intensity range in the vertical loading is ~ 32% higher compared to the horizontal loading, leading to a retarded fatigue crack growth rate. A vertically elongated columnar granular structure due to the epitaxial grain growth along the BD results in anisotropic behavior of the fatigue crack growth.