Fatigue Crack Growth Across the Interphase Boundaries in Two-Phase Bicrystals

Abstract

Effect of interphase boundaries (IBs) on transgranular fatigue crack growth was examined paying special attention to IB structures, slip behaviors near a crack tip, dislocation transfer across IBs and strain incompatibilities at IBs. Fatigue crack propagation tests were carried out at room temperature in air by employing two kinds of two-phase (α/γ) stainless steel bicrystals; one is elastically compatible and the other is incompatible. It was found that the fatigue crack growth rates, da/dN, changed complicatedly as the cracks grew and the changes in da/dN were strongly influenced by the direction of crack growth across IBs. Crack arrest at IBs occurred on condition that the cracks propagated from the softer γ -phase into the harder α-phase or dislocation transferring across the IBs was restrained due to the crystallographic orientation of crystals. In the incompatible bicrystal, not only deceleration but also acceleration of the crack growth was recognized as the crack approached to the IB. This interesting phenomenon was demonstrated by an analytical calculation of the stress fluctuation owing to the elastic incompatibility at the IB.