The effect of nickel on the martensitic-type transformations of Pt₃Al and TiPt

Abstract
The effect of nickel on two classes of martensitic-type transformations in platinum systems has been studied. The first transformation is Ll₂ to DO'_c in the Pt₃Al system and the second is B2 to B19 in the TiPt system. The microstructures after transformation in the two systems are very different. The product of the Pt₃Al transformation has a twinned microstructure, typical of cubic-to-tetragonal transformations. The product of the TiPt transformation is lath-like, although the morphology can be altered using heat treatments. The parent phase in the TiPt system is not retained at room temperature, whereas the parent phase in the Pt₃Al transformation can be stabilised to room temperature. A great variation in hardness and transformation temperature is seen in each system as the composition is varied about the stoichiometric ratio, which has the lowest hardness. The Pt₃Al transformation temperature has been reported to range from around room temperature to 1000°C. The TiPt transformation temperature can range from 1000 to 1080°C. The effect of nickel additions on these alloys also has a marked effect on the parent and product phase stability, and hence the microstructure and resulting hardness. The effect on the Pt₃Al phase is complex, as nickel appears to stabilise the parent phase. The hardness varied in the region of 350 to 500 HV₁₀. For the TiPt phase, the hardness values were generally found to increase with the nickel additions increasing from 250 to about 600 HV₁₀. The addition of 20 at.% nickel decreases the transformation temperature from around 1000°C to about 600°C.