Platinum-based alloys are being developed which have microstructures that are analogous to the ␥/␥ microstructure of the nickel-based superalloys. These Pt-based superalloys have the potential to substitute Ni-based superalloys for even higher temperature applications. The Pt-Cr-Ru system has been studied experimentally as part of this project. The alloys were studied in the as-cast condition, using scanning electron microscope (SEM) and X-ray diffraction (XRD). A solidification projection and liquidus surface were constructed. The major phases were (Ru), (Pt) and ∼CrPt, with ∼Cr 2 Ru (D8 b ), ∼Cr 3 Ru (A15) and ∼Cr 3 Pt (A15) being minor in extent. Both the tetragonal (L1 0 ) and cubic (A1) forms of ∼CrPt were observed. Two invariant reactions were identified in the Pt-Cr-Ru system.

The objective of this work is to mimic the microstructure and strengthening mechanisms of Ni-based superalloys in a new group of high-temperature alloys based on the system Pt-Al. The elements Cr and Ni were chosen as further alloying components. Having a face-centered cubic (fcc) crystal structure with an L1 2 -ordered and coherently embedded phase, these new alloys should increase creep and corrosion resistance beyond Ni-based superalloys. After arc melting and heat treatment, the alloys were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the aged condition, the alloy composition 13 at. pct Al, 3 at. pct Cr, 7 at. pct Ni, and balance Pt showed the most promising microstructure with cubical precipitates, 30 pct precipitate volume fraction, and a lattice misfit of about Ϫ0.1 pct at room temperature.

A series of quaternary platinum-based alloys have been demonstrated to exhibit the same two-phase structure as Ni-based superalloys and showed good mechanical properties. The properties of ternary alloys were a good indication that the quaternary alloys, with their better microstructure, will be even better. The quaternary alloy composition has been optimised at Pt84:Al11:Ru2:Cr3 for the best microstructure and hardness. Work has begun on establishing a thermodynamic database for Pt-Al-Ru-Cr alloys, and further work will be done to enhance the mechanical and oxidation properties of the alloys by adding small amounts of other elements to the base composition of Pt84:Al11:Ru2:Cr3

An investigation was carried out by Mintek to examine the influence of a range of alloying additions (titanium, vanadium, chromium, molybdenum, cobalt, zirconium, boron, aluminium, silicon, zinc, platinum, palladium, rhodium, iridium and ruthenium) on the formability, hardenability and colour of 18 carat gold alloys. This paper provides chemical formulae for yellow 18 carat gold alloys that have hardnesses which are higher than the hardnesses of the corresponding conventional 18 carat alloys, in particular in excess of 300 HV, but with colours similar to conventional 18 carat gold. A preparation method for the alloys will also be described, which includes the steps of melting, annealing, quenching, cold working and age hardening.

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