Largely alleviating the orientation dependence by sequentially changing strain paths Haiyang Fan, Shifeng Liu ⁎, Lijuan Li, Chao Deng, Qing Liu ⁎ College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China abstract article info Article history: Received 9 November 2015 Received in revised form 19 February 2016 Accepted 20 February 2016 Available online 23 February 2016 Grains with γ-fiber texture (〈111〉 direction // normal direction (ND)) and θ-fiber texture (〈100〉 direction // ND) show different subdivision behaviors during unidirectional rolling, which leads to orientation-dependent stored energy. This orientation dependence in tantalum can be largely alleviated by a novel approach named 135° clock rolling, which is attributed to two reasons. One is that the clock rolling can weaken the micro-shear bands and destroy the parallel dislocation boundaries in γ-fiber grains, thus reducing their stored energies; and the other is that the clock rolling changes the stability of θ-fiber orientations and introduces plenty of “veins” into θ-fiber grains, thereby increasing corresponding stored energies. Therefore, 135° clock rolling narrows the stored energy difference between these two types of grains, which is beneficial for homogenizing the annealing microstructure of tantalum. © 2016 Elsevier Ltd. All rights reserved. Keywords: 135° clock rolling Strain path change Orientation dependence Stored energy Shear bands Tantalum 1. Introduction Since the activity of slip systems depends on orientations, the defor- mation substructure and stored energy also differ from grain to grain [1], which is called the orientation dependence. This kind of dependence has adverse effects on the microstructural and textural uniformity of annealed samples [2,3]. For years, systematic studies about this topic have been done on some face-centered cubic (fcc) metals like copper and aluminum [4–6], while the body-centered cubic (bcc) transition metals with high melting points, e.g. niobium, molybdenum and tanta- lum, have received less attention than the above fcc metals, even though these refractory metals also show some degree of orientation depen- dence [7–9]. In these transition metals, representative components with orientation dependence are grains with {100} and {111} orientation, respectively [10–12], which own entirely different deformation sub- structures, stored energies and even recrystallization tendencies [8,13, 14]. In this study, tantalum (Ta) is chosen as a model material for its application of sputtering target [15]. It is commonly accepted that orientation dependence of Ta is detrimental to the recrystallization ho- mogeneity since it often leads to residual deformation bands and texture clusters during annealing [14]. As a result, the sputtering perfor- mance of Ta targets is largely damaged by this uneven recrystallization structure [16]. Unfortunately, relevant work about the orientation dependence of Ta is so lacking that only few studies based on single crystals and coarse columnar grains could be referred to [8,13,14,17]. Nevertheless, most of the available literatures including those above have just focused on characterizing the orientation dependence rather than alleviating it. Strain path change should be an approach to weaken the orientation dependence as it activates slip systems from different directions and then destroys the substructure developed in preceding deformation courses [6,18]. Although some severe deformation modes like equal channel angular pressing (ECAP) can introduce a fine and uniform mi- crostructure [19–21], these modes are not particularly applicable in in- dustry for the limitation to billet's dimensions [19,20,22,23], especially for materials with a high hardness, e.g. Ta. Considering that rolling has long been popular in many industrial applications to produce sheet ma- terials, this study focuses on rolling instead of some severe deformation modes. The previous studies have reported that the 45° clock rolling cannot only weaken the basal texture and generate a uniform annealed micro- structure of magnesium alloy [24], but also enhance the isotropy of zir- conium [25]. The clock rolling is also a desirable industrial technique to produce sputtering targets [15]. Therefore, this study focuses on a clock rolling schedule with a rotation angle of 135°. Subdivision behaviors of θ-fiber and γ-fiber grains under unidirectional rolling (UR) and 135° clock rolling (CKR) were analyzed by techniques of electron back- scattered diffraction (EBSD), X-ray line profile analysis (XLPA), trans- mission electron microscope (TEM) and electron channeling contrast imaging (ECCI). Materials and Design 97 (2016) 464–472 ⁎ Corresponding authors. E-mail addresses: liusf06@cqu.edu.cn (S. Liu), qingliu@cqu.edu.cn (Q. Liu). http://dx.doi.org/10.1016/j.matdes.2016.02.084 0264-1275/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes