Brittle fracture of polycrystalline tungsten

Abstract

A comparative study of the brittle fracture of three varieties of polycrystalline tungsten has been carried out using mainly impact tests, scanning electron microscopy and Auger spectroscopy. It indicates the occurrence of two modes of brittle fracture, namely cleavage and intergranular fractures, whose proportions depend on temperature and on phosphorus segregation at the grain boundaries. Scanning Auger images indicate unambiguously that phosphorus is only located on the intergranular surfaces. No phosphorus is observed on cleavage planes. Fracture surfaces are the result of the propagation of a crack through the whole sample. As the propagation follows the path which requires the lowest expenditure of energy, it depends both on a geometrical factor and on the respective values of the cleavage (γcl) and intergranular (γl) works of fracture. Our results indicate an increase in γcl and γl with temperature (the effect being more marked in the case of γcl) and a large decrease in γl with phosphorus segregation at the grain boundaries. This impurity produces an intergranular embrittlement of the tungsten.