Grain growth and fatigue behaviors of GH4169 superalloy subjected to excessive ultrasonic surface rolling process

Abstract

In this study, the fatigue behaviors of GH4169 superalloy, which possessed a gradient nanostructure layer introduced by an ultrasonic surface rolling process (USRP) and characterized using scanning electron microscopy and transmission electron microscopy, were investigated. Rotary bending fatigue test results verified that the fatigue resistance of the alloy was enhanced due to the improved surface integrity, such as refined grains, improved microhardness and the great compressive residual stress field. However, excessive USRP treatment induced grain growth (from 58.23 to 280.7 nm) at the top surface of the sample accompanied by residual stress relaxation and decreased microhardness, leading to the degradation of fatigue resistance. In addition, the mechanism of the grain growth observed in the alloy subjected to excessive USRP treatment was revealed. It was found that the collective migration of neighboring grain boundaries caused by excessive cyclic plastic deformation promoted the formation of nanotwins, and such process played a key role in accelerating grain growth.