Pure platinum is too soft to be used for jewellery and scratches easily. Alloying platinum increases its hardness significantly. However, platinum alloys used in jewellery do need to be easy to work and thus the alloy should be sufficiently soft, but not so soft that their wear resistance is low. A good compromise would be to work with a soft alloy during jewellery manufacture, then harden the alloy so the final finished properties were improved. In order to identify platinum alloys suitable for hardening, platinum with different alloying additions was studied. Platinum alloys with additions of less than 7 wt.% of Ag, Au, Cu, Co, Cr, Fe, Ga, Ge, In, Mg, Mn, Mo, Ni, Si, Sn, Ta, Ti, V, W or Zr were examined, and the merits of each system were assessed for commercial viability. The platinum-titanium system was deemed to show the most promise.

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The TiPt phase with the B2 structure has been reported to undergo a reversible displacive transformation to B19 martensite at about 1000 ЊC. 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 ЊC lower than previously accepted. The hardness of B19 martensite shows a minimum at a stoichiometry of ϳ50 at. pct Pt. Between 45 and ϳ50 at. pct 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 ЊC. No evidence for this intermediate phase was found for Pt contents from 50 to 56 at. pct Pt. It is clear that a Pt content of about 50 at. pct 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 ЊC.

Alloys of the Ti-Pt system between 30 and 61 at.% Pt were studied using metallography with optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential thermal analysis (DTA). A phase of nominal composition Ti 4 Pt 3 , in the range 41.7-43.4 at.% Pt was found in samples containing between 30 and 47 at.% Pt, and was apparently formed by a peritectoid reaction between ␤-TiPt and Ti 3 Pt at 1205±10 • C. The eutectic reaction between Ti 3 Pt and ␤-TiPt was found to be at ∼35 at.% Pt and 1424±10 • C, and the Ti 3 Pt phase melted at circa 1500 • C. The other phase boundaries agreed with the published literature, except that a slightly wider range was found at 1200 • C for ␤-TiPt.

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