Surface residual stress and microstructure evolutions of Hastelloy X alloy after severe shot peening

Abstract

In this study, severe shot peening (SSP) was performed on the nickel-based Hastelloy X alloy. The effects of SSP treatment on surface properties such as surface topography, microstructure, residual stress and microhardness were investigated by electron microscopy, X-ray diffraction and micro-indentation measurements. The results indicated that SSP induced a high level of compressive residual stress in the surface layer of Hastelloy X. The maximum compressive stress obtained at 125 μm deep was more than 1200 MPa after SSP, and the total depth of the compressed region was ~700 μm, which was nearly two times thicker than the conventionally shot-peened sample. Due to the ultra-high plastic deformation imparting on the material's surface, a gradient nanostructure layer with an average grain size of ~50 nm in the topmost surface was generated after SSP treatment. The grain refinement caused by severe plastic deformation was mainly attributed to the dislocation activities and deformation twins. As a result of the formation of nano-grains, high-density dislocations and deformation twins, the surface microhardness of the SSP sample was significantly increased, which was ~2.2 times higher than that of base material.