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论文题目：In-situ investigation via high energy X-ray diffraction of stress-induced (0002)α→(110)β transformation in a Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy

发表时间：Available online 26 February 2020

刊源：Materials Science & Engineering A 779 (2020) 139154 Available [ 点击下载PDF ]

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The equilibrium phase diagram of the Ti-5. 5Mo-7. 2A1-4.5Zr-26Sn-2.1Cr titanium alloy is constructed using JMatPro software, as illustrated in Fig. 2（a）. The amount of β-Ti will increase monotonically with increasing temperature, whereas α-Ti disappears completely at a temperature of -895℃. Therefore, the β-transus temperature （Tβ）is-895℃. The initial scanning electron microscopy of the alloy in Fig. 2（b）reveals the bi-modal microstructure which consists of equiaxial primary α phase with a mean diameter of -3 μm and precipitated secondary α lamellae （length：-3-4μm）in β matrix. Furthermore, the microstructure is accurately reproduced at the center of the dog-bone-shaped specimen via EBSD. Fig. 2(c) shows the 400 x 400μm2 phase distribution map, collected at a step size of 0.5μm. The volume fractions of the hexagonal-close-packed α phase (red) and the body-centered-cubic β phase (blue) are 44% and 56%, respectively.

Fig.2. Equilibrium phase diagram and initial microstructure of the heat-treated Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr titanium alloy: (a) equilibrium phase diagram, constructed using JMatPro software, (b) scanning electron microscopy morphology, (c) phase distribution map obtained via EBSD.

Fig. 3 shows the crystalline orientation maps and the histograms of grain size distribution. Grain boundaries are defined as having >15° misorientation. The crystalline orientation maps of the α and β phases along the LD are shown in Fig. 3(a) and (c), respectively. In the test area, the texture distribution of the α phase is relatively random, whereas grains with LD//<001> (red) and LD//<111> (blue) occur preferentially for the β phase. Furthermore, the texture distribution of the β phase is inhomogeneous, with a higher intensity of the LD//<111> texture component in the bottom right-hand corner than in other regions. The average grain sizes of the α-phase (2.89 μm) and β-phase (4.43 μm) are evaluated based on the equivalent circle diameters, as shown in Fig. 3(b) and (d), respectively. In addition, most (32.48%) of the α-phase grains have sizes of 2–3 μm and the grain size distribution is more uneven than that of the β phase.

Fig. 3. Crystalline orientation maps and histograms of grain size distribution for the: (a) and (b) α phase, (c) and (d) β phase.

Fig. 11(a) shows a TEM-BF micrograph of the specimen subjected to 3% strain. In the early stage, irreversible plastic deformation is retained in the form of dislocation lines at the grain boundaries (indicated by the arrows in Fig. 11(a)). As the strain increases further to 5% (Fig. 11(b)), the multiplication of dislocations leads to an increase in the accumulated dislocation density. Fig. 11(c) shows a high-magnification image of the region enclosed in the circle (see Fig. 11(b)). A mass of dislocations accumulates in local regions, thereby resulting in dislocation tangles, as indicated by the arrows in Fig. 11(c). Therefore, the interaction between dislocations is considerable, and acts as an obstacle to further dislocation motion. The strain hardening behavior at a strain of ~5% (Fig. 5 inset) confirms this hypothesis. A TEM-BF micrograph obtained at 8% strain (Fig. 11(d)) shows that dislocation tangles are quite prominent at this level of strain. Moreover, in the cellular dislocation structure (indicated by a red dashed box in Fig. 11(d)), low-density dislocation regions are surrounded by cell walls with a relatively high dislocation density. Fig. 11(e) shows a high-magnification image of the region enclosed in the circle (see Fig. 11(d)). The dislocation tangles are gradually released and cross-slipping occurs, as indicated by the arrows in Fig. 11(e). This results in dislocation annihilation between opposite-type dislocations. The softening behavior induced by this annihilation and the strain hardening behavior will reach a dynamic equilibrium, corresponding to the steady-state flow stage in Fig. 5. In addition, the lack of a special BOR and stress states between the (0002) and (110) reflections (see Fig. 10) may have prevented the stress-induced (0002)α→(110)β transformation during the plastic deformation stage.

Fig.11. TEM micrographs obtained at various strains: (a) BF image at 3% strain, (b) BF image at 5% strain, (c) dislocation tangles, magnified view of the region enclosed in the circle shown in (b), (d) BF image at 8% strain, (e) cross-slipping dislocation, magnified view of the region enclosed in the circle shown in (d).