Effect of temperature on the brittle fracture of polycrystalline tungsten

Abstract

The effect of temperature on the tensile fracture stress of polycrystalline tungsten of average grain diameter 2 d = 0.03 mm has been investigated in the temperature range 77°–480°K. The onset of the ductile-brittle transition, that is the brittle-semi-brittle transition, occurred at 350°K. Below this transition the fracture stress increased with decreasing temperature. Above 473°K grain-boundary cracking was observed on the specimen surface. The effect of spark-induced microcracks on the fracture stress was studied as a function of microcrack length at 77°K and testing temperatures between 77° and 480°K. The fracture stresses at 77°K followed the Griffith relation to give a value to the surface energy that was close to the theoretical value for the true surface energy. The effect of blunt grain-boundary cracks on the fracture of tungsten was shown by pre-straining the material to 2 and 4% strain at 473°K prior to testing at lower temperatures. The temperature dependence of this fracture stress was less than for sharp spark-induced cracks.