Crack growth in ordered alloys

Abstract

The influence of order on fatigue crack propagation in an (Fe,Ni)₃V alloy (which orders into an Ll₂ structure) and an FeCo-2%V alloy (which orders into a B2 structure) has been studied. By virtue of decreased cross slip and increased planarity of slip, ordering would be expected to enhance the resistance of alloys to fatigue crack propagation. Ordering did lead to a decrease in the rate of crack propagation in the ductile (Fe,Ni)₃V alloy whereas crack propagation rates were enhanced by the onset of order in the FeCo-V alloy. This behavior is explained on the basis of order-induced dislocation structure and consequent slip behavior in these alloys.

The effect of ordering on the susceptibility of both the alloys to hydrogen embrittlement were examined. Tensile, delayed failure and crack growth tests revealed an enhancement in the susceptibility of these alloys to hydrogen embrittlement with order. However, the effect was not quite clear in the ordered condition in FeCo-V that was inherently brittle. Hydrogen embrittlement is explained on the basis of hydrogen-dislocation interaction and the characteristic dislocation structure of the ordered lattice. It is emphasized that although there is a correlation between ordering and susceptibility to hydrogen embrittlement, the manifestation of this phenomenon could be affected by grain boundary segregation, hydrogen-induced dislocation activity and slip processes that change with ordering.

Crack growth rates increased with temperature in the (Fe,Ni)₃V alloy contrary to the variation of yield stress with temperature. This behavior is attributed to an environmental effect and possible effects of localized stress assisted disordering at high temperatures.