Effect of Zn, Au, and in on the polymorphic phase transformation in Cu ₆Sn ₅ intermetallics

Cu ₆Sn ₅ is a critical intermetallic compound in soldering operations. Conventional equilibrium phase diagrams show that this compound is of either a hexagonal or monoclinic structure at temperatures above and below 186 °C, respectively. Under nonequilibrium conditions, the crystal structure is dependent on composition, temperature, and processing history. The effect of Zn, Au, and In on the hexagonal to monoclinic polymorphic transformation in Cu ₆Sn ₅ intermetallics is investigated using variable temperature synchrotron powder x-ray diffraction and differential scanning calorimetry. It is revealed that, as in the case of trace Ni additions, the alloying elements Zn and Au completely stabilize the hexagonal Cu ₆Sn ₅ and prevent the phase transformation. In contrast, In additions only partially stabilize the hexagonal Cu ₆Sn ₅.