Martensitic transformations, microstructure, and mechanical workability of TiPt

The TiPt phase with the B2 structure has been reported to undergo a reversible displacive transformation to B19 martensite at about 1000 degreesC. This system could, therefore, serve in principle as the basis of a high-temperature shape-memory alloy (SMA). However, very few additional details of the B2 and B19 forms of TiPt have been published. In the present work, the B19 --> B2 transformation temperatures previously reported are confirmed, but the B2 --> B19 temperatures are found to be about 40 degreesC lower than previously accepted. The hardness of B19 martensite shows a minimum at a stoichiometry of similar to 50 at. pet Pt. Between 45 and similar to 50 at. pet Pt, the B2 <----> B19 transformations appear to take place in a complex sequence, with up to two intermediate phases being stable over a range of about 50 degreesC. No evidence for this intermediate phase was found for Pt contents from 50 to 56 at. pet Pt. It is clear that a Pt content of about 50 at. pet marks a significant change in the nature of the TiPt phase and its martensite or martensites. It was also found that TiPt has a slightly wider stability range than is shown in the current phase diagram, extending from 45 to 56 at. pct Pt at 1300 degreesC.