MICROSTRUCTURAL STUDY OF TI50PD50 AND TI50NI50-XPDX ALLOYS PRODUCED BY COMBUSTION SYNTHESIS

The microstructure of the Ti-Ni-Pd ternary shape memory alloys (SMAs) produced by combustion synthesis was studied by scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) and X-ray diffraction methods. It was found that, apart from the parent phase, Ti50Ni50-xPdx, there also exists a second phase with the stoichiometry of (Ni1 y,Pdy)Ti2. Higher palladium and lower nickel contents were found in the parent Ti-Ni-Pd phase compared with the theoretical, ideal solution values. Lower palladium and higher nickel contents were found in the second (Ni1 y,Pdy)Ti2 phase. These discrepancies were explained in that palladium has a greater affinity for titanium than for nickel. The X-ray diffraction data show that the structure of the parent phase is orthorhombic (B19), and the orthorhombic unit cell contracts with increasing nickel content.