Brittle–ductile transitions in polycrystalline tungsten

Abstract

The strain rate dependence of the brittle-to-ductile transition (BDT) temperature was investigated in notched and un-notched miniature bars made of high-purity polycrystalline tungsten and in notched bars of less-pure sintered material. The activation energy, E _BDT, for the process controlling the BDT in pure tungsten was equal to 1.0 eV both in un-notched and notched specimens, though the brittle–ductile transition temperature, T _BDT, was ≈ 40 K lower at each strain rate for the un-notched samples, indicating that the activation energy, E _BDT, is a materials parameter, independent of geometrical factors. The experimental data obtained from pure tungsten are described well by a two-dimensional dislocation-dynamics model of crack-tip plasticity, which is also discussed. For sintered tungsten, E _BDT was found to be 1.45 eV; T _BDT at a given strain rate was higher than in the pure tungsten by ≈ 90 K, suggesting that the BDT in tungsten is very sensitive to impurity levels.

Keywords:

• tungsten
• fracture
• dislocation
• brittle–ductile transition
• plasticity
• purity

Acknowledgements

The authors thank Dr Sergio Lozano-Perez for help with TEM analysis. Support by UKAEA Culham laboratory and the EU project “ExtreMat”, contract NMP-CT-2004-500253, is gratefully acknowledged.