Fracture in annealed and severely deformed tungsten

Bulk tungsten normally undergoes brittle fracture at ambient temperatures and has a brittle-to-ductile transition in the range 200–300 °C. This limits the use of tungsten for a host of applications. In general, the fracture mode of tungsten at ambient temperature is intergranular whereas at high temperatures it undergoes transgranular fracture. In this work the focus is on the influences of microstructure and temperature on three-point bend fracture of polycrystalline pure tungsten. The samples were processed by equal channel angular extrusion (ECAE) through a 90° die angle via route A in order to produce an elongated microstructure. The mechanical behavior of both unprocessed and processed materials was then characterized by three-point bending at temperatures ranging from −45 °C to 425 °C. The results show that a single ECAE extrusion (strain ~ 1.15) reduces the flexural toughness of the material and increases the brittle-to-ductile transition temperature, while two and four extrusions dramatically increase the flexural toughness with little effect on the transition temperature compared to that of the unprocessed material. The flexural toughness of the material subjected to four extrusions (strains exceeding 4.5) is more than 50 times greater than that of the unprocessed material at ambient temperature. This is mainly due to microstructural changes that increase the resistance to intergranular fracture, enhance plastic dissipation, and activate relatively high fracture toughness crack systems for transgranular fracture. The results show that substantial elongation of grains by deformation processing at a temperature near the brittle-to-ductile transition temperature is an effective method for improving the ambient temperature ductility and toughness of bulk polycrystalline tungsten.