Ruthenium hardens platinum very effectively and the dilute solid solutions thus obtained are strong, ductile, and remarkably resistant to corrosion. In view of their wide application in electrical technology and in the manufacture of jewellery it is surprising that little has hitherto been published on the true nature of these valuable noble metal alloys. Until very recently the widths of the primary solid solutions at each end of the diagram were very uncertain. Whereas early Russian workers had suggested that platinum would dissolve at least 70 atomic per cent (1) and probably 79 atomic per cent (2) of ruthenium, more recent investigations at the Battelle Memorial Institute (3) indicated that the solid solubility of ruthenium in platinum was only about 32 atomic per cent.

Practical experience within the Johnson Matthey Research Laboratories supported the view that the solubility of ruthenium in platinum was very high indeed. Although difficulty was experienced in working alloys containing more than about 30 atomic per cent of ruthenium this was invariably found to be associated with internal and grain boundary oxidation effects, which occurred whenever these alloys were heated in air to temperatures much in excess of 900°C. No evidence of a ruthenium-rich phase was found even in alloys containing 40 and 50 atomic per cent of ruthenium, and it has now been established that platinum at 1000°C will dissolve approximately 62 atomic per cent of ruthenium. This solubility increases to 70 atomic per cent at 1900°C.

The boundaries of the two-phase region separating the cubic platinum and hexagonal ruthenium solid solutions are shown in Fig. 1. The solidus and liquidus lines in this diagram are purely tentative, but in view of the complete absence of intermediate phases in these alloys it seems evident that the system is, as indicated, of the simple peritectic type.

The experimental points defining the limits of the solubility of platinum in ruthenium were obtained without difficulty by micro-probe analysis, and showed little inconsistency. The solubility of platinum in ruthenium increases with temperature very slowly indeed and little precipitation from these solutions occurred on quenching. The solubility of ruthenium in platinum, however, increases fairly rapidly with increasing temperature, and it was found that the high temperature structure of the saturated platinum solutions could not be fully retained by quenching. Considerable precipitation occurred, and this influenced the composition of the retained phases and reflected itself in the high hardness of the quenched duplex alloys. In the hardness curve shown in Fig. 2 a gently ascending line typical of normal solution hardening is followed by a well-defined hardness peak associated with precipitation across the whole duplex region.

Metallography provided supporting evidence for the phase boundaries shown in Fig. 1. Thus, the microstructure of the quenched 70 atomic per cent ruthenium alloy shown in Fig. 3 displays a volumetric distribution of the two phases commensurate with the position of this alloy within the two-phase area. The 80 atomic per cent alloy shown in Fig. 4 contains only a small proportion of the platinum-rich phase, thus confirming that the alloy is just within the duplex boundary at 1000°C.

Acknowledgements are due to Mr J. H. F. Notton for the micro-probe analysis.