INFLUENCE OF STRAIN RATE ON THE LOW-CYCLE FATIGUE PROPERTIES OF SINGLE-CRYSTALS AND POLYCRYSTALS OF 2 FERRITIC ALLOYS

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 σi and σ*. At intermediate strain rates (about 4 × 10-3 s-1 for the FeSi alloy, and at 4 × 10-5 s-1 for the Fez.sbnd;Si alloy) σi ≈ σ* and both transgranular and intergranular crack initiation are observed. At lower strain rates σ* < σi and crack initiation takes place transgranularly along slip bands. At higher strain rates σ* > σi and crack initiation is intergranular owing to the large crystal shape changes. © 1979.