Intermetallic Phase Formation during Combustion Synthesis of Mechanically Activated Ni-Ti Alloy

Both mechanical activation (MA) and pre-heating conditions affect intermetallic phase formation during combustion synthesis (CS) of Ni-Ti equiatomic alloy. Input parameters are pre-heating temperature, activation duration and compaction pressure. Output variables are ignition time, temperature rise, reaction duration, product microstructure, constituent oxidation and intermetallic phase formation. Superelasticity and shape memory behaviors depend on austenite-rhombohedral and rhombohedral-martensite transformation temperatures. These temperatures were detected by differential scanning calorimetry (DSC) measurements. The phase changes were studied by scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis. Production of Ni3Ti was detected when CS was performed on 1800 s milled sample pre-heated to 400°C. Compression tests indicated appearance of superelastic and superelastic/thermoelastic behaviors after CS of (a) 3600 s activated and (b) 7200 s activated samples at 400°C pre-heating, respectively. It was concluded that pre-combustion heating fostered mono-phase formation and titanium oxidation; while mechanical activation lowered ignition-start-time, duration of synthesis and Ni3Ti formation.