Effect of Synthesis Mode and Ni Particle Size on Microstructural Aspects and Hardness Properties of Combustion-Synthesized NiTi

NiTi intermetallic compound was fabricated via two modes of the combustion synthesis process, namely, thermal explosion (TE) and self-propagating synthesis (SHS). The effect of combustion synthesis mode as well as Ni particle size on the microstructure and microhardness of the final products was investigated. The phases in the products were determined using XRD technique, and microstructural investigations were performed using optical as well as SEM-EDS in order to make a comparison between microstructural evolutions in each mode, other conditions being constant. The analyses showed that the desired B2(NiTi) was successfully formed as the primary phase in all samples, and no unreacted Ni or Ti powders were present. The main secondary phase was NiTi2 which was invariably present in all samples; however, the percentage as well as the morphology of the detrimental NiTi2 phase was found out to be controllable by the mode of combustion as well as Ni particle size. A comparison between the two modes revealed that samples produced by TE showed coarse dendrites in the microstructure; they also presented higher average microhardness values. The SHS-synthesized samples exhibited microstructures similar to those observed in heat-treated and homogenized NiTi obtained from methods such as vacuum arc remelting and vacuum induction melting, and contained finely dispersed NiTi2 particles. It was shown that a decrease in Ni particle size presented a grain refining effect on NiTi2 in both modes. Graphic