Grain-size effects on the deformation of tantalum at low temperatures

Abstract

The dependence of yield stress on grain size in high-purity powder metallurgy tantalum has been investigated by tensile tests on specimens having a wide range of grain sizes at 298°K, 203°K, and 77°K. The variation of lower yield stress, σLY, with grain diameter, d, has been found to obey the Petch equation, σ LY = σ 0 + k yd −11 2 ; values determined for k y are 2.3 · 10 7 dynes/cm 3 2 at 298°K, 3.8·10 7 dynes/cm 3 2 at 203°K, and 5.2·10 7 dynes/cm 3 2 at 77°K. The effective surface energy for cleavage crack propagation, γ, has been estimated to be > 1.8·10 5 erg/cm 2. Comparisons of the values of k y and γ for tantalum with those determined for other b.c.c. metals suggest that the exceptionally high ductility of tantalum at low temperatures is mainly due to the large effective surface energy required for cleavage crack propagation. A strong dependence of deformation twinning on grain size at 77°K has been observed. Profuse twinning occurs in all coarse-grain specimens even at low strain rates, but not in fine-grain specimens. The failure to induce twinning by high strain rates in fine-grain specimens cannot be attributed to the heat generated during rapid loading.