Microstructures and properties of ultrafine-grained tungsten produced by equal-channel angular pressing at low temperatures

The ultrafine-grained tungsten was fabricated by equal-channel angular pressing (ECAP) at 800 °C and 950 °C, respectively, and was characterized by electron microscopy and Vickers microhardness measurement. The average grain size of W decreases with the increasing number of ECAP extrusion passes and can be refined from the initial value of about 6 μm to 0.9 μm after two ECAP extrusion passes at 800 °C, while it is refined to 1.5 μm after three ECAP extrusion passes at 950 °C. Small angle tilt grain boundaries consisting of edge dislocations are formed in the 950 °C extrusion. Intergranular fracture mode dominates the fracture failure of the W after 800 °C extrusion while transgranular facture mode together with intergranular fracture mode play an important role in the fracture failure of the W after 950 °C extrusion. The Vickers microhardness increases from about 510 HV for the as-received W to about 560 HV for the W extruded at both 800 °C and 950 °C. However, the thermal conductivity was affected only a little by the ECAP processing.