Microstructure evolution and deformation behavior of Au–20Sn eutectic alloy during hot rolling process

Abstract

Au–20Sn preforms such as microscale thin foils or wires can be widely used as lead-free solder in the high-power optoelectronics and hermetic sealing applications. However, as-cast Au–20Sn eutectic alloy with Au 5 Sn and AuSn phases is intrinsically brittle at room temperature due to limited slip systems associated with their complicated trigonal structure and hexagonal packed structure. In this study, we successfully made an Au–20Sn foil with thickness of 48 μm by multi-pass hot rolling on as-cast alloy with an initial thickness of 4 mm. The microstructure, texture and recrystallization evolutions of the Au–20Sn eutectic alloy during hot rolling process were investigated in detail. The results indicated that plastic instability occurs in the eutectic lamellar structure when the nanoscale layers were subjected to the rolling strain, and promoted the microstructural spheroidization. Besides, the spheroidizing behavior in AuSn phase was found more preferential compared with that in the Au 5 Sn phase. After dynamic recrystallization, <0001>∥TD deformation texture was introduced in the AuSn phase while <0001>∥ND deformation texture was formed in the Au 5 Sn phase. These phenomena were attributed to the difference in slip systems activated in the two phases associated with their different c/a ratios. A transition of dynamic recrystallization mechanism from the initial lamellar phase to the later equiaxed phase (in other words, from continuous dynamic recrystallization mechanism to discontinuous dynamic recrystallization) was observed, as evidenced by the evolutions of in-grain misorientation accumulation and grain boundary migration. Additionally, an interesting recrystallization phenomenon occurred that the AuSn grains could nucleate in the Au 5 Sn phase during hot rolling, which was considered as a strain induced phase decomposition behavior. The present studies can provide insight into understanding microstructural evolution of Au–20Sn eutectic alloy during hot rolling, which enables a further optimization of processing parameters to fabricate Au–20Sn alloy foils with desired microstructure and properties. • Au–20Sn foil with thickness of 48 μm is successfully made by hot rolling process. • Plastic instability occurs in the eutectic lamellar structure during hot rolling. • Different textures are introduced in the AuSn and Au 5 Sn eutectic phases. • Transition of recrystallization from CDRX to DDRX is found in the eutectic alloy. • AuSn phase can precipitate in Au 5 Sn grains which subjected to severe local strain.