The constitution of the gold-cadmium system

The constitution of the gold-rich region of the gold—cadmium diagram has been etudied by means of thermal analysis, optical microscopy and Debye-Scherrer X-ray films of high-temperature and quenched specimens. Five phases have been identified, α face-centred cubic, α2 close-packed hexagonal, β' ordered body-centred cubic as well as two phases at low temperatures, α1 face-centred tetragonal and α' face-centred cubic. The liquidus and solidus relationships have been found for the α, α2 and β-phases by thermal analysis, the results being in good agreement with the X-ray and microscopical results. A eutectic horizontal representing the reaction α2 + β' ⇌ liquid is found at 626.5°C. The β'-phase maximum is only just detectable, rising to 629°C. From the X-ray evidence a peritectic horizontal, corresponding to the reaction α + liquid ⇌ α2, is believed to exist between 633°C and the eutectic temperature; the thermal arrests place it at 627°C. The α2(α2 + β') and (α2 + β')β boundaries have been determined microscopically. Only a few experimental points have been obtained for the α + α2 boundaries but the high-temperature X-ray films indicate that the boundaries of previous workers are in need of revision. The maximum solid solubility of cadmium in gold is 34 atomic % at 620°C to 630°C. The β'-phase narrows in composition range as temperature falls. The high-temperature X-ray films showed superlattice lines up to 624°C and an estimate of the degree of ordering at 620°C suggests the β-phase to be about 90% ordered at this temperature. The α1-phase at 23 atomic % cadmium is disordered face- centred tetragonal when photographed at elevated temperatures. The absence of superlattice lines is contrary to previous work which describes the unit cell in quenched α1, specimens as ordered (an anti-phase domain structure based on four fundamental tetragonal cells). It is conjectured that quenched α1 specimens may be ordered but as temperature is increased the long-range order is broken down, producing small domains each highly ordered. At about 25 atomic % cadmium the structure is face- centred cubic. A new phase α' is proposed at this composition. A tentative diagram of the Au3Cd region, linking the four phases α, α1, α' and α2 is suggested with peritectoid horizontals at 412°C and 407°C. Factors affecting the behaviour of the liquidus and solidus of the gold-cadmium system are revealed by comparing with other systems of copper, silver and gold. The comparisons show the influence of the electron concentration, modified by lattice distortions and the electrochemical factor. The α2-phase (close-packed hexagonal) is shown to contain numerous stacking faults; this may be a consequence of the α(face-centred cubic) and α2-phases having nearly equal free energies. The occurrence of the 3/2 electron compounds in gold is discussed and suggestions are given for their bearing on maximum solid solubilities in gold.