Study on shallow grain induced micro-region surface topography instability of bulk Sn alloys upon internal stress conditions

• Internal stress conditions in bulk Sn-1wt.% Bi were created via casting and forging. • Depth-wise stress distribution induces shallow grain (SG) formation near grain boundaries, causing micro-topography instability during aging. • Cast Sn-1wt.% Bi showed a more rapid increase in surface peak height near SG than forged Sn-1wt.% BiC. • Uniform stress distribution suppresses SG-induced surface topography instability in bulk Sn alloy. Bulk Sn-alloy surface spontaneously develop topographical changes at room temperature due to the time-varying shallow grain (SG) transformations, significantly impacting surface roughness and arising surface instability concerns for its industrial applications such as fixed abrasive lapping plates. However, the underlying case behind SG formation under different internal stress conditions is less investigated especially in bulk alloys, limiting surface quality control during manufacturing. This work focuses on the internal residual stress induced SG transformations and its resultant surface and microstructural features. Two distinct internal stress conditions were fabricated with industrial standard casting and forging respectively, to create varied subsurface stress fields for elucidating SGs’ behavior. Results reveal that SG formation is highly affected and facilitated by stress distribution via active recrystallization near grain boundaries, resulting in new grain boundaries and dynamic crystallographic reorientations. Cast Sn-1 wt% Bi samples exhibited a more pronounced stress gradient near surface, resulting in a rapid increase in surface peak height around SGs, which conversely credits forging as a better method in suppressing the unwanted surface topographic variation. This work summarizes the fundamental understanding of SG behavior under varying internal stress conditions, elevating the function-oriented manufacturing process design on bulk Sn-alloy for ensuring the surface quality.