The Mechanism of Low-cycle Fatigue Crack Formation Related to Annealing Twin Boundaries in Austenitic Stainless Steels

Abstract

Crack along annealing twin boundary lines can frequently be observed on the surface of low-cycle-fatigued specimens of austenitic stainless steels. In this study, a mechanism for the formation of such fatigue cracks is proposed.
When cracks are initiated along twin boundary lines in a crystal grain which contains many twin boundary planes parallel to each other, the cracks are initiated only along alternate twin boundary lines. These cracks do not propagate along the twin boundary planes, but deviate from the twin boundary plane with increasing depth.
The mechanism of the formation of these cracks is discussed from a crystallographic point of view. Such cracks are initiated in crystal grains where the strain concentrated on a {111} plane including a twin boundary line at the surface. Two slip systems along {111} planes, one of which is parallel to the twin boundary plane, are then activated alternately. The crack propagates approximately along a {101} plane, which is an intermediate plane between the {111} planes.