Phase transformations and equilibria of titanium platinum alloys in the composition range 30-50 atomic percent platinum

Abstract

The Ti-Pt phase binary phase diagram and the corresponding phase transformations in the composition range 30-50 at.percent Pt have been investigated using a variety of characterization methods (DTA, SEM and TEM). This study was inspired by ongoing work on some experimental Ti-Ni-Pt and Ti-Pt-Ni-Hf high temperature shape memory alloys that were found to contain unexpected phases not reported previously in such alloys. Furthermore, close analysis of the peritectoid invariant proposed by Biggs et al. revealed a range of confusing and somewhat contradictory results and, as a result, it was decided to attempt to determine the true nature of the diagram in this composition range and to understand the complicating effects of interstitial contamination on the observed microstructures and phase equilibria. The microstructure of as-cast and heat treated alloys contains more than two phases after equilibration treatments suggesting interstitial contamination. In addition, the microstructures revealed that the peritectoid transformation (Ti3Pt plus TiPt Ti4Pt3) proposed in the literature exists but, because of sluggish transformation kinetics, the actual peritectoid reaction is limited and does not account for the observed DTA peaks that Biggs et al. used to estimate the invariant temperature. Rather, it will be shown that the peaks are due to the transformation of TiPt to a lamellar TiPt plus Ti4Pt3 structure at approximately 1230 degrees C. In addition, a modification to the phase diagram is proposed based on other experimental evidence. Characterization of the various phases observed in the microstructures (using SADP and CBED in the TEM) confirmed the presence of the known phases Ti3Pt and TiPt. In addition, a new phase with stoichiometry Ti5Pt3 was observed in both as-cast and heat treated samples. This phase is shown to be stabilized by oxygen and to have a hexagonal structure with lattice parameters a approx. 8.0 nm and c approx. 5:0 nm (space group P63/mcm). The Ti4Pt3 phase appears to be a true binary phase that tends to be highly faulted and be structurally related to the Ti4Pt3 phase with a pseudo-hexagonal structure with a approx. 7.96 nm and c approx. 23.6 nm. Detailed electron diffraction evidence indicates that the crystal structure is probably triclinic although it was difficult to determine the actual point and space group.