A comparison of the phase evolution in Ni, Al, and Ti powder mixtures synthesized by SHS and MA processes

The purpose of this study was to investigate the phase evolution of the Ni, Al, and Ti powder mixtures with a composition of Ni60.1Al27.2Ti12.7 during both self-propagating high-temperature (SHS) and mechanical alloying (MA) processes. The DSC thermal analysis of this powder mixture, from 20 to 1200 °C, shows that there are two exothermic and two endothermic reactions in the SHS process. The phases presented in heat-treated powders and in MA samples were then identified by using X-ray diffraction and were also examined metallographically. Data obtained from thermal analysis, XRD experiments, and microstructure examination may be well-interpreted in terms of the Ni–Al binary and Ni–Al–Ti ternary phase diagrams. The growth of the γ′ phase at the expense of the β phase during heat treatment was analyzed kinetically, and the results show that grain boundary diffusion is responsible for the γ′ growth in samples heat treated at lower temperatures (670 or 875 °C), whereas volume diffusion becomes a dominant mechanism for the higher heat-treatment temperature (1165 °C) in the SHS process. The sequence of formation of stable intermetallic phases was β–Ni(Al,Ti), γ′–Ni3(Al,Ti), and β′–Ni2AlTi when annealed from room temperature to 1200 °C, and this sequence of formation of phases was the same as that revealed in the MA process.