Dislocation structures in single-crystal tungsten and tungsten alloys

Abstract

Deformation of tungsten single crystals as a function of strain, temperature, and alloying was studied by transmission electron microscopy. Single crystals oriented for (−101)[lll] slip were grown by electron beam zone refining. Compression specimens of tungsten, W-l and 3 pct Re and W-l and 3 pct Ta were deformed to 2 pct strain at 150°, 300°, and 590°K (0.04, 0.08, and 0.16T _m). Specimens were also strained to 0.5 and 5.0 pct strain at 300°K. Transmission microscopy revealed that the dislocation substructures in single-crystal tungsten are similar to substructures in other refractory metals when compared on a homologous temperature basis. At temperatures greater than 0.1T _m, the substructure is characterized primarily by edge dipoles. At temperatures less than 0.1T _m, long screw dislocations lying parallel to the primary [111] slip direction characterize the substructure. Rhenium additions to tungsten promote formation of edge dipoles at temperatures of 300° and 150°K and increase dislocation density at all three temperatures. In addition, dislocations consistent with (1−12)[−111] slip were observed in the W-Re single crystals after deformation at 150°K. Tantalum additions had a lesser effect on the dislocation substructure compared to rhenium additions. The W-l and 3 pct Ta alloys exhibited higher dislocation densities than unalloyed tungsten after similar strains and, at 150°K, W-3 pct Ta contained a few dislocations consistent with (1−12)[−111] slip. It is concluded that the reduction in ductile-brittle transition temperature of poly crystalline tungsten containing dilute rhenium additions, 1 to 5 pct, can be attributed to an increase in dislocation mobility at temperatures less than 0.1 T_m.