Microstructures and recrystallization behavior of severely hot-deformed tungsten

Abstract

When coarse-grained (CG) tungsten (W) is heavily worked by equal-channel angular extrusion (ECAE), the grain size is reduced to the ultrafine-grained/nanocrystalline regimes (UFG/NC) and the strength and ductility increase. Because of the brittle nature of CG W, the material must be hot-extruded, and, if the temperatures are near the recrystallization temperature ( T rc), gains in properties may not be maximized. In this study, the recrystallization behavior of ECAE-processed CG W is examined as a function of the imparted strain (i.e., number of extrusions) and the hot-working extrusion temperature. Up to four ECAE passes were performed in tooling with a 90° channel intersection, and at temperatures of 1000 °C or 1200 °C. Subsequent 60 min annealing of the worked material to 1600 °C allowed for the determination of T rc. Vickers microhardness measurements and scanning electron microscopy, were used to characterize the microstructures in the as-worked and recrystallized states. The ECAE-processed W shows increased microstructural break-up and refinement with increasing strain and decreasing hot-working temperature in the fully worked state. T rc was determined to be ∼1400 °C, which is nearly independent of the number of extrusions and the working temperature. These results show that if ECAE is accomplished below 1400 °C (i.e., at 1000 °C or lower) the attractive properties of the UFG/NC-worked W may be retained. Specifically, below 1000 °C, with increasing strain imparted to the material, high hardness values with a concomitant grain size refinement (∼350 nm) could be expected.