The influence of strain rate on the low cycle fatigue properties of single crystals and polycrystals of two ferritic alloys

Abstract

The influence of strain rate on the cyclic plastic deformation behaviour of two high purity body-centered cubic iron-based alloys (Fe-3% Si and Fe-26% Cr-1% Mo) was studied paying particular attention to the relation between the behaviour of polycrystalline specimens and that of single crystals of various orientations. At high strain rates (about 10 −1 s −1) most single crystals exhibit stress asymetries and pronounced shape changes as a consequence of the observed slip asymmetry. At low strain rates (about 10 −6 s −1) such single-crystal effects are markedly reduced. For polycrystalline material the strain rate dependence of the single-crystal fatigue behaviour is correlated with the crack initiation mode and the relative values of the internal and effective stresses σ i and σ ∗ . At intermediate strain rates (about 4 × 10 −3 s −1 for the FeSi alloy, and at 4 × 10 −5 s −1 for the Fez.sbnd;Si alloy) σ i ≈ σ ∗ and both transgranular and intergranular crack initiation are observed. At lower strain rates σ ∗ < σ i and crack initiation takes place transgranularly along slip bands. At higher strain rates σ ∗ > σ i and crack initiation is intergranular owing to the large crystal shape changes.