Effect of heat and cryogenic treatments on the microstructure and properties of YF3 modified NiTi-Si laser cladding coatings

Abstract

YF3 modification NiTi-Si coatings were prepared on Ti-6Al-4 V substrate by laser cladding. On this foundation, heat and cryogenic treatments were conducted respectively to explore the effect of after-treatment procedures. The consequence of post-treatment methods on the phase constituent, microstructure, mechanical property, and phase transition characteristics of post-treatment coatings was studied. The phase constituent of the three coatings was roughly the same, composed of NiTi and NiTi2. The results show that heat treatment led to the coarsening of NiTi phase grain size and increased Si content in the NiTi2 phase. In addition, due to the rapid cooling rate provided by cryogenic treatment, the spinodal decomposition of the Si element occurred in the NiTi2 phase. Furthermore, the post-treatments also affected the phase transformation characteristics of the coatings. As the temperature of martensitic transformation is sensitive to the content of Ni element in the NiTi phase, the Ms and Mf points of the heat treatment coating were slightly higher. In contrast to the heat treatment, the Ms and Mf points of the cryogenic treatment coating decreased. The microhardness of the three coatings is as follows: cryogenic treatment > original coatings > heat treatment. The cryogenic treatment coating has the highest microhardness due to grain refinement and spinodal decomposition. In addition, the cryogenic treatment coating showed the best wear morphology.