RECRYSTALLIZATION OF TANTALUM PROCESSED BY EQUAL CHANNEL ANGULAR PRESSING

Current ingot refinement and solidification techniques used in commercially pure tantalum processing often result in a product with a heterogeneous microstructure and associated inconsistencies in mechanical properties. An example of the heterogeneous structure is the texture banding observed in forged tantalum disks. These bands alternate between regions dominated by <100> or <111> oriented grains relative to the plate normal. The current study examined if severe plastic deformation of pure tantalum via Equal Channel Angular Pressing (ECAP) reduces the texture banding in the recrystallized microstructure. The materials of interest are 63 mm (2.5-inch) diameter round bars of tantalum supplied by H.C. Starck and Cabot. These bars were processed eight times through a 135-degree ECAP die using route C (180 degrees rotation between each pass) and then forged into 6.3 mm (0.25-inch) thick disks. A series of annealing coupons were sectioned from the disks and vacuum annealed for one hour at temperatures ranging from 700 degrees C to 1250 degrees C. Hardness measurements taken on the coupons revealed a 150 degrees C difference in annealing response of the tantalum obtained from the two vendors. The annealing coupons were sectioned and examined using optical microscopy, X-ray diffraction and Electron Back-Scattered Diffraction (EBSD). These data were used to analyze structural features on the meso and macro scale. The role of high and low angle grain boundaries on the recrystallization process will be discussed.