Investigation of the mechanisms of reactive sintering and combustion synthesis of NiTi using differential scanning calorimetry and microstructural analysis

The mechanisms of reactive sintering in a Ni + Ti powder compact were investigated using differential scanning calorimetry and microstructural analysis. Heating these mixtures up to 900 °C involves the slow growth of the three intermetallic compounds and the transformation of α- to β-Ti followed by its rapid saturation with Ni. When samples were heated above 942 °C a thermal explosion (TE) mode of self-propagating high-temperature synthesis (SHS) was ignited by the melting of the (β-Ti) solid solution at 944 °C. Increasing hold time at 900 °C prior to SHS decreased the volume fraction of (β-Ti) in the powder compact and reduced the magnitude of combustion. The amount of (β-Ti) was quantitatively determined using DSC and found to decay according to a two-stage parabolic law. In addition, the magnitude of the exothermic reaction occurring during SHS was found to decrease linearly with a decrease in the volume fraction of (β-Ti) developed at 900 °C.