Mechanism of brittle-to-ductile transition in tungsten under small-punch testing

Abstract

Exceptional high brittle-to-ductile transition temperature (BDTT) is a critical issue that limits the application of tungsten as structural materials. The controlling factors in BDT have been the focus of debate in past decades. In this work, small-punch testing were performed in a wide temperature range to explore the mechanism of BDT in as-rolled tungsten. A transition in failure mode from cleavage cracking to delamination fracture with increasing temperature marks the BDT of the as-rolled tungsten. Numerous (001) planar triangle-shaped shear tongues are produced on fracture surface. The number density and distribution of shear tongues show a clear temperature and orientation dependence. Analysis of the slip line angle on the shear tongues reveals that profuse dislocation activities on the {110}, {112} and {123} planes at and above BDTT. Temperature and orientation dependent slip activities are decisive factors controlling the BDT. Below BDTT, fewer freely moved edge dislocations are insufficient to blunt the crack tip, thus as-rolled tungsten displays brittleness. Once above the BDTT, the mobility of screw dislocations is increased, and a large fraction of mixed dislocations glide could effectively suppress cracking and make as-rolled tungsten ductile.