Multi-scale Study Of The Heterogeneity Of Deformation And Annealing Structure In Tantalum

Tantalum owns extensive applications due to its excellent properties, such as high melting point, high hardness and well corrosion resistance etc. However, the inhomogeneity of microstructure and texture is a long-standing issue that brings adverse effects to sputtering targets. Two schedules, unidirectional rolling and 135° clock rolling, were adopted in this study to deform the uneven tantalum. Multi-scale characterization techniques, such as X-ray Diffraction(XRD), Electron Back-Scattered Diffraction(EBSD), X-ray Line Profiles Analysis(XLPA), Transmission Electron Microscopy(TEM) and Electron Channeling Contrast in Scanning Electron Microscopy(SEM-ECC) etc., were utilized to analyze the textural and microstructural uniformity. Main conclusions are shown below:(1) The unidirectional rolling brought severe through-thickness texture gradient, but the 135° clock rolling could weaken this kind of gradient effectively. The forged and then annealed tantalum plates showed obvious heterogeneity: the surface layer included large grains with {100}<uvw> orientations but the center layer showed fine grains with {111}<uvw> orientations. After unidirectional rolling, the as-rolled plate still presented noticeable textural nonuniformity. Conversely, the clock-rolled sample owned relatively random textural distribution, although intense {100} orientations were found in the surface layer.(2) Differently oriented grains have different subdivision behaviors and substructures, especially grains with {100}<uvw> and {111}<uvw> orientations. Under unidirectional rolling, {100} grains could resist the fragmentation and thus formed a structure consisting of dislocation cells, while {111} grains were able to be subdivided readily and then developed a substructure comprising regular Geometrically Necessary Boundaries(GNBs) and Incidental Dislocation Boundaries(IDBs). However, after clock rolling, the stable {100} texture transformed into metastable roles, and thus the {100} grains were fragmented. Meanwhile, the dislocation boundaries in {111} grains were in a state of chaos rather than regularity. Therefore, clock rolling increased the isotropy in {111} grains and also narrowed the stored energy gap.(3) The heterogeneous substructure and uneven distribution of stored energy lead to uneven annealing behaviors. The recrystallization ability of {100}<uvw> grains in unidirectional-rolled Ta was so weak that these grains become residual deformation bands in as-annealed Ta. On the contrary, the recrystallization can occur in {111}<uvw> grains quickly because of their high stored energies. However, clock rolling can weaken the recrystallization ability of {111} grains but increase the ability of {100} grains.(4) The annealing temperature can highly influence the recrystallization texture and grain morphology. Short-time annealing at high-temperature benefited the formation of {111}<uvw> texture, but also introduced elongated coarse grains, while long-time annealing at low-temperature facilitated the formation of {100}<uvw> texture, but the {100} texture cluster was also introduced into the as-annealed Ta.