THE TERNARY-SYSTEM AU-CU-SN

The phase relations in the Au-Cu-Sn system have been examined by powder X-ray diffraction, metallography, scanning electron microscopy and thermal analysis. The range of homogeneity of the various phases, tie-lines and tie-triangles is shown in an isothermal (horizontal) section of the phase diagram at 360-degrees-C. Au and Cu can to a large extent be mutually exchanged in all phases. A continuous range of homogeneity is found between AuSn (delta) and Cu6Sn5 (etha) with a maximum variation of approximately 3.5 atom % Sn at equal amounts of Au and Cu. The three genuine ternary phases, labelled A, B and C, have, respectively, the homogeneity ranges Au42Cu38Sn20-Au47Cu33Sn20, Au11Cu69Sn20-Au38Cu42Sn20 (with a maximum broadening of approximate 2.5 atom % Sn) and Au330Cu337Sn333-Au370Cu297Sn333 at 360-degrees-C. The difference between Au and Cu atomic volumes show a systematic increase with increasing Sn content, and to is qualitatively interpreted as evidence for transfer of electron density from Sn to Au. Alloys with 20 atom % Sn are examined at different temperatures, and a vertical (pseudo-isoplethal) section through the phase diagram is presented for this Sn content. A number of isothermal cross-sections are advanced for the Au-rich part of the phase diagram. Attention is paid to the phase equilibrium in an invariant four-phase region.