Microstructure evolution and crack propagation feature in thermal fatigue of laser-deposited Stellite 6 coating for brake discs

摘要

In order to reveal the mechanism of microstructure evolution and crack propagation in laser-deposited Stellite 6 alloys, a quenching thermal fatigue test was conducted. Various detection methods were applied to observe differences between the coatings as deposited and after thermal fatigue. The results showed that the gamma - epsilon martensitic transformation occurred in the as-deposited gamma-Co matrix during the thermal fatigue process, driven by a fast cooling and thermal stress. The generated epsilon-Co phase presented variant selection, obeying Schmidt's law. In the epsilon-Co phase, the slip activity derived from different {1 1 1}(gamma) 1 1 (2) over bar (gamma) slipping systems that produced stacking faults and planar defects during the phase transformation. In addition, the stacking faults on {1 1 1}(gamma) planes promoted the precipitation of directional M7C3 fine particle carbides. The net-like eutectic structures and gamma/epsilon interfaces acted as paths for thermal crack propagation.