Influence of initial ingot breakdown on the microstructural and textural development of high-purity tantalum
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I.
INTRODUCTION
A. Processing Procedure 1. Initial ingot breakdown
H I S T O R I C A L L Y , commercial processing of pure tantalum plate has not required tight control on the microstructural uniformity or crystallographic texture. Electrical properties for lamp filaments and the capacitor industry, corrosion resistance for chemical equipment, and oxidation resistance for high-temperature shielding applications have been the driving requirements.tq Therefore, annealed tantalum plate has typically possessed a banded microstructure, exhibiting a mixture of recrystallized and unrecrystallized material, a highly variable grain size, and texture and microhardness gradients, as discussed by the authors in a previous article, t2J It appears that the banded microstructure is due to insufficient deformation needed to break down the large columnar as-cast microstructure of the electron beam melted (EBM) and the vacuum arc remelted (VAR) tantalum ingots. Although little recent work has been performed on texture development in tantalum, its body-centered cubic (bcc) structure is the same as steel, so the texture development is expected to be similar. Since the early work of Lankford et al.t3] on steels, it is clear that texture and microstructure have an important influence on the ability of a material to be deep drawn. In low-carbon steels, Emren et al.[4~ showed that improved deep-drawing characteristics can be achieved with a fine grain size and a texture consisting of {111} < 112> and {111} < 110> orientations. Pokross t51 showed that it was possible to obtain a fine grain size and {111} texture components in commercially processed tantalum. The goal of this study is to understand the effect of the initial ingot breakdown process on the development of the {111} components in final recrystallized tantalum plate.
The starting material came from three 250-mm-long x 76-mm-diameter VAR tantalum ingots, produced by Cabot Corporation, Boyertown, PA. As-cast microstructures were characterized by analyzing 25-mm-thick sections cut from each of the VAR ingots. Four different ingot breakdown processes were utilized to produce the 32-mmthick rolling bars. Figure 1 provides a schematic view of each of the four breakdown processes. Two ingots were processed using processing steps that simulate current commercial practices. Process 1 included side forging the ingot into a roiling bar, which is similar to the commercial process employed at Cabot. In process 2, the ingot was upset forged 50 pct and then side forged into a rolling bar. Process 2 is similar to the Fansteel process described by Pokross. I51 The third ingot was sectioned into two parts, each being approximately 114-mm long, and the initial ingot breakdown included two processing approaches not previously used for tantalum. In process 3, the half ingot was completely upset forged into a disk 32-mm thick and trimrned into a square plate for ease of subsequent rolling. The other half of the ingot was extruded into a rolling bar, process 4, with the extrusion axis parallel to
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