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论文题目： Effects of rolling reduction on Burgers orientation relationship and slip behavior of a Tie5.5Moe7.2Ale4.5Zre2.6Sne2.1Cr alloy

发表时间：Available online 1 October 2021

刊源：Journal of Materials Research and Technology2021；15：3099-3109 [ 点击下载PDF ]

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Prior to hot rolling, the titanium alloy (Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr) was forged and subsequently heat-treated （880 ℃/0.5 h/FC + 740 ℃/2 h/FC）. A schematic of the hot rolling process is shown in Fig. 1. The β-transus temperature of this alloy was ~895 ℃. Three specimens with dimension of 40 mm × 40 mm × 20 mm (RD × TD × ND) were cut and heated at 850 ℃ for 1 h in an air furnace. Afterward, the specimens were unidirectionally hot rolled in the α+β phase field to 20%, 40%, and 60% reductions in thickness (per pass: ~10%). The specimens will be reheated to the preset temperature for 2 min after two rolling passes. The rolling speed was 0.16 m/s. Consequently, the rolled sheets at the corresponding thickness of 14.4 mm, 10.8 mm, and 7.2 mm along the ND were obtained.

Fig. 1 A schematic of the hot rolling process.

Fig. 2 shows the initial microstructure and crystallographic texture of the Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy via EBSD. Fig. 2(a) and (b) show the crystalline orientation maps of the α and β phases along the ND, respectively (comprising their histograms of grain size distribution, see the inset). The microstructure feature of equiaxial primary α phase and intergranular β matrix can be observed. The volume fraction of the α phase is 63.9% and that of 36.1% for the β phase. The considerable grain aggregates for the β phase exhibit similar crystallographic orientation, thus forming the macrozones. The equivalent circle diameters of the α and β grains are 4.88 μm and 5.29 μm, respectively (see the corresponding inset). Fig. 2(c) show three representative pole figures for each phase, i.e., {0001}, {101-0}, and {1-21-0} for the α phase; {001}, {011}, and {111} for the β phase. The normal axis of the pole figures is the ND of the sheet. The initial alloy develops a main texture component that c-axes tilt ~40° from the ND towards the RD. Compared with the α phase, there is more random texture component in the β phase. Furthermore, the texture components in the {0001} or {1-21-0} pole figure overlap with that in the {011} or {111} pole figure (see the rectangles or triangles with different colors), which indicates that the BOR relation between the α grains and adjacent β matrix is always satisfied.

Fig. 2. Initial microstructure and crystallographic texture: crystalline orientation maps of the α phase (a) and β phase (b), the corresponding texture pole figures and the 3D ODF distributions (c) and (d), respectively.

To further analyze the heterogeneous distribution of the intragranular Schmid factor in Fig. 8, Fig. 10 shows the local orientation of three typical α grains at different thickness reductions. The existing colors are dependent on the Euler angle sites in the unit triangle. Fig. 10(a) shows the crystalline orientation map along the ND and the corresponding {0001} pole figure of a α grain at 20% reduction. The substantially uniform Euler angle of (171.4,82.5,30.6) and a unique orientation-associated position in {0001} pole figure can be observed, indicating the limited localized grain rotation. As shown in Fig. 10(b), when rolling reduction increases to 40%, a new orientation (60,137.6,49.3) in a special α grain with an initial Euler angle of (53.3,139.9,39.9) appears gradually (see the bottom left-hand corner). This intragranular misorientation gradient results in a slight deviation of ~5° in {0001} pole figure. Fig. 10(c) shows the crystalline orientation map along the ND and the corresponding {0001} pole figure of a special α grain with an initial Euler angle of (32.8,44,73) at 60% reduction. The localized distribution diversity of intragranular orientation is very noticeable under more severe deformation condition. As a result, three new orientations of (29.5,55.8,65.9), (49.4,48.3,37.8), and (32.5,62.5,74.5) are expressed precisely, thus facilitating the significant difference (~30°) among directions in {0001} pole figure. The accumulated intragranular misorientation satisfies the high-angle grain boundaries criterion (>15° misorientation, marked by the black lines), and the microstructural characteristics of continuous dynamic recrystallization is visualized, as also reported in the literature.

Fig. 10. Intergranular orientation of three typical α grains at different thickness reductions: (a) 20%, (b) 40%, and (c) 60%.