Effect of cyclic loading on microstructure and crack propagation in additively manufactured biomaterial Co–Cr–Mo alloy

Cobalt–Chromium–Molybdenum (Co–Cr–Mo) alloys are commonly used for artificial hip and knee joint metallic implants. These components are subjected to repetitive loads during service. Therefore, materials used for such applications must exhibit a high fatigue crack resistance. In this research, Co–28Cr–6Mo (wt.-%) powder was utilized as a feedstock in a laser powder bed fusion process to produce test coupons. The coupons were then subjected to load-controlled cyclic material tests in the low cycle fatigue regime to study mechanical response and microstructural changes of the material. With the progressing number of cycles, a continuous increase in macroscopic plastic strain was observed. The electron backscattered diffraction analysis revealed that cyclic loading caused deformation-induced face-centered cubic (FCC) → hexagonal close-packed (HCP) phase transformation. In addition, the phase transition generated an accumulation of plastic strain at the FCC/HCP interface giving rise to crack nucleation. The crack propagation path along HCP orientation variants with high mechanical work and strain hardening mechanism is discussed.