Microstructure-mechanical property correlation in shot peened and vibro-peened Ni-based superalloy

In this experimental research, shot peening (SP) and vibro-peening (VP) processes were employed to modify the surface and sub-surface of a Ni-based superalloy Udimet (R) 720Li. The effect of these treatments on the surface topography, microstructure evolution, and mechanical properties was investigated using instrumented tests. The compressive residual stresses and plastic strain generated through surface treatment were characterized using X-ray diffraction (XRD). The depth of plastically strained region was estimated using a quantitative analysis of grain orientation spread (GOS) data from electron back-scattered diffraction (EBSD) technique. It was observed that the magnitude of compressive residual stresses induced by VP is relatively low compared to SP, but the converse is true for depth of influence (DOI) for the same peening intensity. After thermal exposure at 650 degrees C (similar to 0.4T(m)) for 5 h, the relaxation of compressive residual stresses and plastic strain was less pronounced in vibro-peening when compared to shot peening representing an improved thermal stability of the former process. Furthermore, 5.24 GPa surface micro-hardness of the untreated sample (O) was increased by similar to 30% and similar to 25.2% following SP and VP respectively. This resultant micro-hardness enhancement is due to grain refinement, plastic strain effects, and the introduction of low angle grain boundaries (LAGBs). Finally, a positive correlation among microstructural features and mechanical properties was fitted from the experimental data.