Plastic instability and spheroidization in a nanolayered Au-20Sn alloy during the hot rolling process

Abstract

Au-20Sn eutectic alloys are an important lead-free solder applied in the welding temperature range from 280 to 350 °C. However, the poor formability originating from the brittle eutectic structure (AuSn + Au5Sn) restricts its applications because it is difficult to deform this alloy to the specifications required for industrial applications. In this study, the hot rolling process was carried out on an as-cast Au-20Sn alloy, and the deformation behaviour as well as the spheroidization evolution of the nanolayered structure was investigated in detail because the goal was to evaluate and optimize the thermoforming process of the Au-20Sn alloy. The results indicated that plastic instability occurred across the multilayers during hot rolling, which showed a gradual transition from geometric bending to the “S” type shear mode as the rolling strain increased. As a result, nanostructure spheroidization in a lamellar colony could take place preferentially based on these shear instability regions owing to a high strain accumulation in these places. Even so, the formation of high-angle grain boundaries in the AuSn lamellae occurred faster than that in the Au5Sn lamellae, resulting in a higher spheroidizing velocity in the AuSn phase. After spheroidization, a (0001)//transverse direction deformation texture formed in the AuSn phase, while a (0001)//normal direction deformation texture was introduced in the Au5Sn phase. Such differences in texture formation were primarily due to the different deformation mechanisms activated in these two phases due to their very different c/a ratios. This study can provide insights to improve the formability of Au-20Sn alloys during thermoforming.