Grain refinement and recrystallization of heavily worked tantalum

Ingots of vacuum arc remelted (VAR) tantalum containing as-cast and highly textured microstructures were deformed by multipass equal channel angular extrusion (ECAE) to strains of 9.2 via different extrusion routes at room temperature through 90° die-angle tooling. The ability of ECAE to break up the as-cast microstructure and produce a homogeneous, fine-grained microstructure in the as-worked state and after a recrystallization heat treatment is reported. It is shown that two extrusions correspond to a level of strain (ɛ = 2.3) where the average as-worked and recrystallized grain sizes become independent of the initial grain size or grain morphology. Also at this strain level, microstructural regions are observed which indicate the near minimum grain size attainable with further thermo-mechanical processing. Microstructural uniformity in the recrystallized material can be achieved after four extrusions with a route which incorporates 90° rotations about the billet axis.