Comprehensive experimental design for the effects of asymmetric rolling on the microstructure and properties of pure tantalum

Tantalum (Ta) is a refractory metal with a body-centered cubic crystal structure. It exhibits outstanding physical and chemical properties, including a high melting point, high density, excellent ductility, superior corrosion resistance, and favorable biocompatibility. In recent years, it has been widely applied in the aerospace, chemical engineering, and medical device industries, and is a common sputtering target in the electronics industry. Ta targets are typically fabricated by plastic deformation followed by annealing of high-purity Ta ingots. A fine and uniform grain structure, together with a preferred crystallographic orientation, is critical for achieving high sputtering efficiency and improved film uniformity. However, due to inherent microstructural inhomogeneities in the initial ingot and nonuniform plastic deformation during processing, obtaining a refined and homogeneous microstructure—especially one with consistent crystallographic texture—remains a substantial challenge using conventional processing techniques. To address this issue, this study systematically investigates the influence of various rolling processes on the microstructure of high-purity Ta and proposes a novel processing strategy that integrates asymmetric rolling(AR) with subsequent recrystallization treatment.

High-purity Ta ingots are prepared by vacuum arc-melting in a Ti-gettered argon atmosphere. The ingots are processed by AR for a total thickness reduction of 90%, achieved by setting the speed ratio of the top and bottom rollers to 1.2∶1. To ensure homogeneous deformation, samples are rotated 135° in the normal direction and flipped between consecutive rolling passes. For comparison, synchronous rolling (SR) is also conducted under the same total reduction, with the upper and lower rolls operating at a speed ratio of 1∶1. The rolled specimens are then annealed at 1000 ℃, 1100 ℃, and 1200 ℃ for 1 h. Prior to all heat treatments, samples are sealed in vacuum quartz tubes to prevent oxidation. The microstructures and mechanical properties of the annealed specimens are characterized using electron backscatter diffraction, microhardness testing, and room-temperature tensile testing, respectively.

AR greatly enhances the equivalent strain by introducing additional shear deformation, thereby promoting more uniform deformation penetration through the sheet thickness and forming a distinct shear zone. This results in more pronounced grain fragmentation compared to SR. Furthermore, the AR sample exhibits a markedly lower maximum texture intensity(24.76 mud) than the SR sample (74.12 mud), along with a correspondingly reduced total texture fraction (54.2% vs. 74.8%). After annealing at 1200 ℃, both samples achieved nearly complete recrystallization. The average grain size of the AR sample (50.70 μm) is finer than that of the SR sample (64.90 μm). As the annealing temperature increases, the texture intensity decreases for both samples, with the AR samples consistently exhibiting lower texture intensity than the SR samples. Rolling induced notable hardening, increasing the microhardness to 201.9 HV in the AR sample and 196.3 HV in the SR sample. The yield strength of the AR sample reached 378 MPa, which is 63 MPa higher than that of the SR sample (315 MPa). Although the mechanical properties of both rolled specimens exhibited decreasing trends with increasing annealing temperature, the strength and hardness of the AR samples remained consistently higher than those of the SR samples.

AR is an effective technique for refining the grain size of Ta, weakening its texture, and enhancing its mechanical properties. This experiment involves the preparation, characterization, and application of pure Ta, thereby not only improving students' experimental design skills but also promoting their scientific thinking and innovative awareness.