Enhancing strength, ductility, and thermal fatigue resistance in Stellite 21 coatings via Mn alloying and Transformation-Induced Plasticity

This study investigates the effect of Mn on the microstructure and mechanical properties of Stellite 21 cobalt-based coatings prepared by laser cladding on 24CrNiMo steel substrates, focusing on utilizing the Transformation-Induced Plasticity (TRIP) mechanism to enhance toughness and thermal fatigue performance. The results indicate that the microstructure of the Stellite 21 alloy coating is primarily composed of gamma-Co (FCC) with a minor presence of epsilon-Co (HCP), M23C6, and M7C3 carbides, which predominantly precipitate at or near the grain boundaries. Upon the addition of Mn, it dissolves into the gamma-Co matrix, causing lattice distortion and increasing dislocation density, thereby contributing to solid solution strengthening. Compared to the Mn-free Stellite 21 alloy coating, the coating with 5 % Mn exhibits an approximately 21 % increase in tensile strength (similar to 1135 MPa) and a 74 % increase in elongation (similar to 14.5 %). This improvement is mainly attributed to the solid solution strengthening caused by lattice distortion due to Mn addition and the synergistic enhancement from the TRIP mechanism facilitated by the lower stacking fault energy. Moreover, the thermal fatigue crack propagation rate in the coating with 5 % Mn is reduced by approximately 42.5 % compared to the Mn-free Stellite 21 alloy coating. The enhancement in thermal fatigue performance is primarily due to the Mn addition, which stabilizes the epsilon-Co phase during thermal fatigue processes, enhancing plastic deformation capacity. The phase transformation process alleviates stress concentration, effectively reducing crack propagation rates and ultimately improving the material's thermal fatigue performance.