Orientation dependence of stored energy release and microstructure evolution in cold rolled tantalum
Introduction
When a metal is deformed, a fraction of energy is stored in the form of lattice defects, such as dislocations. Such stored energy will release during subsequent annealing process, and facilitate recovery and recrystallization to occur. Generally, the release of stored energy during recovery refers to annealing out of point defects, dislocation annihilation and rearrangement, and subgrain growth, while during recrystallization involves the formation of new strain-free grains and the subsequent growth of these to consume the deformed or recovered microstructure. Although the microstructure feature during the energy release process is identifiable, the borderlines between the various stages are often unclear [1]. However, understanding microstructure evolution is necessary and will be very helpful for optimizing heat treatments during material processing.
In this study, the stored energy of cold rolled tantalum (Ta) is measured and related microstructure evolution during stored energy release is investigated. Ta is selected as a research material for the following two reasons: (i) The stored energy of most of the metals has been measured in the early years, seeing the review by Bever et al. [2]. These materials have low melting points and relatively low recrystallization temperatures. In contrast, the data of the stored energy for high melting points metals, especially refractory metals (W, Mo, Ta), can be seldom found. The latest research on the stored energy measurement is carried out on Fe by Scholz et al. [3]. (ii) Ta is a refractory metal with a bcc structure. Due to unique properties, Ta has been widely used in many fields, such as electronics industry, cutting-tool industry, and chemical industry, medical and military fields [4], [5], [6]. Unfortunately, its fundamental studies drop behind. Specially, refining the grain size from a cast microstructure is notoriously problematic and thermo-mechanical processing is commonly employed in the manufacture of Ta products [7], [8], [9], [10]. Producing a uniform fine grain structure largely relies upon multiple annealing steps between the mechanical deformation steps. Multiple annealing steps are considered to be costly and inefficient and it is necessary to shorten such process flow. Giving an insight into microstructure evolution during annealing is essential for us to find out optimal heat treatment conditions.
Section snippets
Experimental
The starting material in this study was high purity Ta ingot in the diameter of 97 mm with 99.99% minimum purity level. The chemical composition was determined by glow discharge mass spectrum and shown in Table 1. Such ingot was up-forged (with 50% deformation) followed by side-forging (with 50% deformation) to break down initial coarse grain structure. The forged Ta had 20 mm in thickness and then was annealed at 1250 °C for 2 h in a vacuum environment to get a fully recrystallized microstructure
Deformation microstructure
Fig. 1 shows the deformation microtexture and microstructure of cold rolled Ta. After 87% thickness reduction, the deformed sample consists of a typical texture mixed with {1 1 1} texture (< 1 1 1 > // ND) and {1 0 0} texture (< 1 0 0 > // ND). The microtexture from EBSD results is consistent with the macrotexture measured by X-ray diffraction method [10]. The deformed sample contains large numbers of low angle boundaries (LAB, 2°–10°), as revealed by red lines in the Fig. 1b. Further qualitative
Discussion
In this study, two interesting questions need to be further discussed. The first one is the difference in energy release content between recovery and recrystallization process. In previous studies on several metals, such as Cu, Ni, and Fe, recystallization is the primary process during heating, which dominates the release of the stored energy [3], [14]. However, the present DSC results (Fig. 2) reveal that recovery rather than recrystallization lasts a long time and releases the most, amounted
Concluding remarks
In this study, the stored energy of refractory metal Ta with 87% thickness reduction and related microstructure evolution during stored energy release were investigated. This study has extended our general knowledge about processes such as recovery and recrystallization. For example, the stored energy as well as subsequent recovery and recrystallization processes in the deformed polycrystalline Ta was orientation dependent. The common findings that {1 1 1} grains tend to nucleate readily and {1
Acknowledgment
The present work was supported by the Major National Science and Technology Projects of China (No. 2011ZX02705), and the Chongqing Science and Technology Commission in China (CSTC, 2012jjA50023).
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