Grain Boundary Sliding in Isothermal and Thermal Fatigue of 304 Stainless Steel

Abstract

Grain boundary sliding (GB sliding) and grain boundary cracking in both isothermal fatigue under saw-tooth wave strain cycling and thermal fatigue were investigated on 304 stainless steel, in order to elucidate the physical meanings of the damage acceleration under these types of fatigue conditions found in the previous papers.In isothermal fatigue under saw-tooth wave and thermal fatigue, GB sliding was found to be accumulated in nearly proportion to the number of strain cycles, contrary to the case of isothermal fatigue under triangular wave where the accumulation of GB sliding was negligibly small. The direction of GB sliding accumulation both in isothermal fatigue under slow-fast saw-tooth wave and in-phase thermal fatigue was characterized as to induce the tensile residual strain of the specimen, while the direction both in isothermal fatigue under fast-slow saw-tooth wave and out-of-phase thermal fatigue was contrary.Grain boundary cracking was frequently observed in the vicinity of triple points or ledges on the grain boundary which accommodated the strain concentration due to GB sliding, and the higher amount of accumulation of GB sliding caused the earlier initiation of grain boundary cracking. Based on these metallurgical findings, it is concluded that the acceleration of fatigue damage is closely related to the accumulation of GB sliding.