Elimination of Elongated Bands by Clock Rolling in High-Purity Tantalum
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lectron beam melting (EBM) is a popular method for the manufacture of high-purity Ta products due to its enhanced refining capability. Unfortunately, EBM produces ingots with a microstructure consisting of very coarse grains.[1–4] In order to break down such coarse-grained microstructure, plastic deformations, like swaging or rolling, are commonly employed in combination with heat treatments. Compared to fine-grained microstructure, the coarse-grained microstructure would display a more heterogeneous subdivision characterized by deformation and shear bands.[2,4,5] These heterogeneities tend to act as preferential nucleation sites for recrystallization. Thus, banded grains are expected to develop in further recovery and recrystallization processes. At the same time, a few deformation bands with elongated shape are also observed. These elongated bands at very large stains are difficult to be eliminated even at high annealing temperatures. The existence of elongated bands is deleterious to the material performance. For example in Ta sputtering targets, the elongated bands often possessing {100} texture
C. DENG, Engineer, S.F. LIU and Z.Q. ZHANG, Associate Professors, H.Y FAN, X.B. HAO, and J.L. JI, Master Students, and Q. LIU, Professor, are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China. Contact e-mail: [email protected] Manuscript submitted February 20, 2014. Article published online September 9, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
components are sputter-resistant areas, leading to unacceptable thickness uniformity in deposited Ta thin film.[6] Such elongated bands have been also reported in many other materials, such as Fe-Si steel,[7] ferritic stainless steel,[8] and interstitial-free (IF) steel.[9] By now it has been proved that the elongated bands belong to a-fiber (h110i//RD) or h-fiber (h100i//ND). This type of orientations has low Taylor factors and would possess low stored energy during deformation; thus the weak driving force for nucleation leads to sluggish recrystallization behavior. Due to the stored energy largely depends on grain subdivision, enhancement of the subdivision in these grains may promote the ability of recrystallization and therefore eliminate the elongated bands. It has been observed by Quadir and Duggan that in heavily rolled IF steel (rolling reductions above 85 pct), a grains begin to split their orientations by deformation bands, thus producing lattice curvature which allows recrystallization of a-fiber at the expense of other a components in the same original grain.[10] In this study, clock rolling is employed to process Ta plates, aiming to enhance the nucleation ability of a/h grains and thus eliminate the elongated bands in unidirectional rolling. The clock rolling method is adopted in that changing the rolling path has significant effect on texture, deformation substructure, and recrystallization behavior of materials,[11–16] but for Ta it has not been reported yet in detail.[17,18] Two high-purity Ta plates (99.95
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