Title: Microstructural changes in additively manufactured Co-Cr-Mo alloy during cyclic loading

Abstract

Cobalt-Chromium-Molybdenum (CoCrMo) alloys are used in applications that require high strength and wear resistance. Examples in biomedical area include artificial hip and knee joints implants that are subjected to repetitive loads during the service. The cyclic loading implies high fatigue strength as fundamental mechanical property, besides those already pointed out. In this regard, it is crucial to understand the mechanism associated with the crack propagation. Therefore, in this work, microstructural changes associated with crack propagation during cyclic loading of a CoCrMo alloy were investigated. Tensile test specimens were manufactured using laser powder bed fusion (LPBF) additive manufacturing technique and examined in the as-build condition. The sample was subjected to cycling loading with constant tension load applied (above the yield strength of the material) and subsequent tension release, Figure 1a. Figure 1a shows an initial linear increase in strain hardening based on the deformation-induced martensitic transformation commonly present in materials with low or negative stacking fault energy. Microstructural changes were followed using electron backscatter diffraction (EBSD) technique. Figure 1b shows a typical as-build microstructure with a single-phase face-centered cubic (γ) structure, consistent to reported previously. After 5100 cycles, it was possible to trace deformation-induced phase transformation from γ to hexagonal close-packed (ε) structure, Figure 1c. Our investigation also revealed that cracks nucleated at the ε-phase formed at grain boundaries. In addition, it was observed that the crack tip was deflected when encountered a grain boundary unfavorably oriented to the crack propagation. Then the crack tip propagated further at the ε-phase inside the grain. Therefore, γ → ε deformation-induced martensitic phase transformation serves as a preferential path to the crack propagation.