MICROSTRUCTURAL DEVELOPMENT OF EUTECTIC BI-SN AND EUTECTIC IN-SN DURING HIGH-TEMPERATURE DEFORMATION

Eutectic Bi-Sn and In-Sn solder joints were subjected to high temperat ure deformation in shear in order to determine whether microstructural instabilities are generated during testing. Dynamic recrystallization had previously been observed in Sn-Pb solder joints during creep and fatigue in shear. The current study shows that Bi-Sn can recrystallize during deformation in creep or at constant strain rate, whereas no mi crostructural changes are observed in In-Sn. Recrystallization of Bi-S n is concentrated in a narrow band along the length of the sample, par allel to the direction of shear strain, similar to behavior in Sn-Pb. The recrystallization appears to proceed by migration of interphase bo undaries rather than by a nucleation and growth mechanism. A minimum t otal strain is required to induce obvious recrystallization in Bi-Sn, independent of applied stress or strain rate. This value of strain is much higher than the strain at initiation of tertiary creep or at the maximum shear stress. Onset of tertiary creep and strain softening occ ur as a result of nonuniform deformation in the samples that is indepe ndent of the microstructural instabilities. The creep behavior of In-S n is relatively straightforward, with a single creep mechanism operati ng at all temperatures tested. The creep behavior of Bi-Sn is temperat ure-dependent. Two mechanisms operate at lower temperatures, but there is still some question as to whether one or both of these, or a third mechanism, operates at higher temperatures.