Influence of grain size on plastic anisotropy in low-carbon steels

Abstract

The plastic anisotropy of textured polycrystalline materials has often been described by the r-value – the ratio of the width and thickness strains. The measured –-values of low-carbon steels for larger grain sizes are found to agree reasonably well with values calculated by the authors on the basis of the Taylor theory. In steels with small grain sizes the absolute level of r-value is not predicted particularly well, but the variation with testing direction (planar anisotropy) is predicted well. Calculations showed that in all cases the instantaneous anisotropy parameter ρ as a function of strain, when extrapolated to zero strain, agreed with the value predicted from the measured texture. With small grain sizes a sudden drop in ρ was observed with increasing strain, this being complete within 2–3% of strain. The influence of grain size on the plastic anisotropy can be understood if it is assumed that the grain boundary resistance parameter K in the Hall–Petch relation depends on the contraction ratio – in a different way from the Taylor factor M(q). Measurements of the Hall–Petch constant in plane-strain compression support this view.MST/84