The constitution of the copper-rich copper-aluminium-tin alloys, with special reference to ternary compound formation

Abstract

The constitution of copper-rich copper-aluminium-tin alloys has been examined by metallographic and X-ray methods in order to investigate the nature of a suspected ternary phase stable at temperatures below that of the eutectoid decomposition in the copper-ahiminium system. The existence of a ternary phase T has been established, and isothermal diagrams have been established at various temperatures between 650 and 505° C. The characteristic of the ternary phase is a constant copper content of approximately 75 atomic %, which is maintained despite a wide variability in the tin and aluminium contents. At 505° C its structure is body-centred cubic, but at lower temperatures the phase splits up into two phases of different crystal structure (β´ and T). The structure of the low-temperature β' phase formed below a tin content of 6·3 atomic % is ordered body-centred cubic, and the unit cell contains 16 atomic sites. The T phase has a small range of homogeneity in the region of 11 to m13 atomic % of tin, and its crystal structure is very similar to that of the δ phase in the coppertin system. At intermediate compositions β´ and T exist together at the lower temperatures. Accurate density measurements show that the T phase contains vacant atomic sites, which increase in number as the tin content increases. The difference between the low-temperature structures formed from T is to be attributed to this development of defects. The results are discussed with particular reference to the factors affecting the formation of the T phase, and the conditions under which ternary compounds may be expected in alloys of copper and silver with elements of the B subgroups of the periodic table.