Understanding the grain refinement and residual stress formation mechanisms of a Ni-based alloy during machining processes

Ni-based alloys are important materials for many industries, microstructure of the sub-surface regions of these alloys can be refined during machining processes, affecting both their machinability and service performance. Several mechanisms have been proposed to explain the machining-induced microstructure refinement of these alloys, however, sound evidences to support them are still lacking. In this research, microstructure characterization with an emphasis on the micro-texture and defects was conducted on milled surfaces of a Ni-based alloy, to promote in-depth understanding of the grain-refinement process. A nano-crystalline layer consisted a topmost region with randomly-orientated grains and annealing nano-twins and a textured region beneath it with deformed microstructure was observed in the cross-section of the alloy with the help of high resolution orientation determination techniques. These observations provide additional evidences to support the recrystallization induced grain-refinement mechanism of the nano-grains. It is also suggested that recrystallization was a by-product of the shear deformation occurred during the milling process, supported by the observation of the clear boundary between the nano-grain layer and its beneath deformed region. The deformed region was refined as a result of mechanical twining and twin-intersections, leading to significant fluctuations of the residuals stresses.