Grain refinement in bulk pure tantalum using equal channel angular extrusion

Abstract

For this study, the effectiveness of equal channel angular extrusion (ECAE) to improve the material properties and processing characteristics of vacuum arc remelted (VAR) pure tantalum was determined. The primary objectives were: 1) determination of recrystallization temperatures for processed material 2) determination of the grain refinement potential of ECAE and 3) determination of the ability of ECAE to produce a homogeneous grain structure. The effects of initial grains size (as-cast: [] 5 mm, large: 500 [u]m - 2 mm, medium: 20 [u]m - 100[u]m) and morphology, annealing temperature (23°C - 1370°C) and level of strain (one, two, or four extrusions) and extrusion route (C and E) on the recrystallized grain size, percent recrystallization, microstructural uniformity, grain morphology and Vickers microhardness were investigated. All extrusions were performed at room temperature in a 90° die using a punch speed of 5 mm/sec. Microstructural uniformity and morphology were observed and characterized using an optical metallograph equipped with a polarizing filter. Grain size measurements were made using the linear intercept method on optical micrographs. Four consecutive passes without intermediate annealing show that pure VAR tantalum is very workable when subjected to ECAE processing. The initial grain size and processing route have little if any effect on the workability or recrystallization temperature after one extrusion pass. Microhardness values are similar to published literature values produced by conventional deformation methods at equivalent strains and tend to increase significantly during the first two extrusions. Routes 2C and 4C result in fine (<22 [u]m), uniform grains after annealing for the large and medium initial grain size materials but do not for the as-cast initial grain size. Route E results in fine-grained, homogeneous, equiaxed microstructures for all initial grain sizes with ~11 [u]m being the smallest produced. The microstructural homogeneity and uniformity, and the fine-grain size resulting from ECAE processing may be advantageous to those produced by the conventional methods such as rolling, swaging, forging and wire drawing, with the added advantage of being a bulk product. The Hall-Petch relationship is found to be valid for ECAE processed tantalum over a grain size range of 10 [u]m to 100[u]m.