Cyclic deformation mechanisms of a two-phase stainless steel in various environmental conditions

Abstract

The physical mechanisms which control fatigue damage in air and in a 3.5% NaCl solution of a two-phase (ferritic (α)-austentic (γ)) stainless steel with a 50% ferrite content are examined. The study focuses on the damage mechanisms leading to crack initiationat an imposed plastic strain amplitude in various environmental conditions: dry laboratory air, conditions of passivity, conditions of pitting and at cathodic potentials in the 3.5% NaCl solution. The fatigue properties of the two-phase alloy in air are related to a mechanical coupling effect between the two phases. At plastic strain amplitudes higher than 10 −3, the fatigue behaviour of the α-γ alloy is related to the basic cyclic deformation mechanisms of the α phase (which exhibits twinning and pencil glide) and at lower plastic strain amplitudes to the cyclic properties of the γ phase (which exhibits planar slip). Finally the better resistance to corrosions fatigue of the α-γ alloy in comparison with those of its individual phases is emphasized and explained by both mechanical and electrochemical coupling effects between the α and γ phases of the two-phase alloy.