The Role of Mechanical Twinning in the Fatigue of an Iron–4% Silicon Alloy

Abstract

The part played by mechanical twinning in the initiation and propagation of fatigue cracks in an iron–4% silicon alloy has been observed by performing interrupted tests on polished specimens in both the annealed and the pre-twinned condition. In annealed specimens, mechanical twinning occurs only after a fatigue crack has been nucleated by slip-band cracking. The twins then play a significant role in the subsequent growth of fatigue cracks, the twin/matrix interface offering preferred paths for both surface- and interior-crack propagation. In pre-twinned material, surface twin/matrix interfaces are preferred sites for fatigue-crack initiation, but aid crack propagation only if the twin is oriented favourably with respect to the advancing fatigue crack and the principal fatigue stress.