Influence of Ni and Al Elements on Microstructure and Mechanical Properties of Low Carbon Cocrmo Alloy Coatings

Abstract

Low-carbon cobalt-based alloy coatings with high ductility and low thermal fatigue crack propagation rates were prepared using laser cladding (LC) technology. The microstructure of the coatings was characterized using SEM, EBSD, and TEM, while the mechanical properties were tested and analyzed. The CoCrMo alloy primarily consists of γ-Co and a minor amount of ε-Co. The addition of Ni increases the stacking fault energy (SFE), resulting in the retention of only γ-Co in the alloy. Increasing Al content decreases the SFE, leading to the precipitation of the Al3Ni intermetallic compound, forming a eutectic region with Al3Ni, Cr23C6, and ε-Co. More ε-Co phase appeared after tensile deformation due to the stress-induced transformation induced plasticity (TRIP) effect. Ni addition increased the content of face-centered cubic (fcc) γ-Co, enhancing the coating’s ductility by approximately 85% compared to CoCrMo. With higher Al content, the ε-Co phase increased, and the dispersion of Al3Ni improved the coatings' strength. The jagged structure of the eutectic region impedes thermal fatigue cracks from crossing it, causing them to expand along the grain boundary. Optimal mechanical properties were achieved with 13 wt.% Ni and 7 wt.% Al, resulting in a 53% increase in ductility and a 40% reduction in the thermal fatigue crack propagation rate compared to CoCrMo.

Keywords: CoCrMo alloy coating, Al3Ni, Thermal fatigue crack propagation, Mechanical Properties