Investigation of High-Temperature Recovery and Recrystallization Behavior in a Single Crystal Ni-Based Superalloy Treated with Shot Peening and Laser Peening

In this study, the effects of shot peening (SP) and laser peening (LP) on the high-temperature microstructural evolutions of a CMSX-4 single crystal Ni-based superalloy were scrutinized, with an emphasis on understanding the phenomena of recovery, recrystallization (RX), and oxide layer formation. CMSX-4 specimens were subjected to SP and LP, followed by thermal exposure at 1100 degrees C for 100 h to accelerate dislocation mobility and RX. Quantitative assessments revealed that SP induced significant microstructural changes, including a stark increase in surface hardness by 53% over the untreated specimens, and generated maximum residual stresses of - 1250 MPa. This intense mechanical impact and thermal input facilitated the formation of new grains characterized by high-energy angles of 50-60 degrees with an average diameter of 55 mu m. Furthermore, SP followed by heat treatment was found to promote the growth of a 2.5 mu m thick, continuous Al2O3 oxide layer, providing substantial coverage that could potentially enhance the alloy's resistance to corrosion. In contrast, LP imparted less intense surface modifications, with an increase in hardness of only 26% and lower magnitude residual stresses peaking at - 750 MPa. However, LP preserved the alloy's original crystal structure, exhibiting no evidence of RX.