Platinum Metals Rev., 1968, 12, (4), 134
Phase Equilibria in the Platinum-Molybdenum System
Significance in the Glass Industry
The practical significance of the high temperature phase relationships in the platinum-molybdenum binary system has been discussed in two recent articles in this journal, (1), (2), which described the way in which platinum-clad molybdenum glass handling equipment can fail at temperatures above 1300°C by creep of the protective sheath if high temperature intermetallics are allowed to form on its inner surface. The interpretation of the interfacial diffusion zone microstructures observed during these investigations was made more difficult by the lack of sound information on the binary system between the two metals, and it is pleasing to note, therefore, that a thorough investigation recently carried out by H. Ocken and J. H. N Van Vucht in the Philips Research Laboratories at Eindhoven (3) has resulted in the presentation of a phase diagram for the platinum-molybdenum system which serves to bring into perspective the many partial investigations which have previously been made.
Seven single-phase regions are shown in the new diagram, which was determined by conventional metallurgical techniques using arc-melted alloy buttons. The molybdenum-rich A 15 phase, whose formation is thought to be largely responsible for the high interfacial strains developed in the platinum claddings of molybdenum glass stirrers, is shown to form via a peritectoid reaction between the b.c.c. Mo solid solution and a disordered form of the hexagonal intermediate phase, originally observed by Raub (4), at approximately 1800°C, and to decompose eutectoidally at approximately 1300°C, forming a duplex mixture of the terminal molybdenum-rich solid solution and the ordered hexagonal phase.
It must surely be more than a coincidence that the decomposition temperature of this phase corresponds closely with what is regarded as the maximum long-term operating temperature of a conventional platinum-clad molybdenum component in the glass industry.
The investigators at Philips paid particular attention to the formation and properties of this molybdenum-rich compound. An alloy containing 28 atomic per cent platinum was shown to be superconducting. The highest superconductive transition temperature for this eutectic composition, 5.6°K, was observed after annealing the alloy at 1800°C.
Other significant features of the diagram include a proposed orthorhombic phase of equiatomic composition, stable at temperatures below 1300°C, and an extensive solubility of platinum in molybdenum. The latter finding is of interest in that it was originally predicted on theoretical grounds by means of the Engel-Brewer correlation (5), and was confirmed experimentally some time later (1).
At the platinum-rich end of the system, the new diagram indicates the existence of a second orthorhombic phase, of approximate composition Mo Pt2. This structure appears to be an ordered form of the terminal face-centred cubic platinum solid solution, and occupies the position formerly ascribed to the tetragonally distorted phase denoted α1 by Raub (4).
- 1G. L. Selman, Platinum Metals Rev., 1967, 11, (4), 132
- 2A. S. Darling and G. L. Selman, Platinum Metals Rev., 1968, 12, (3), 92
- 3H. Ocken and J. H. N. Van Vucht, J. Less-Common Metals, 1968, 15, (2), 193
- 4E. Raub, Z. Metallk., 1954, 45, 23
- 5W. Hume-Rothery, Progress in Materials Science, 1967, 13, (5), 231