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论文题目：Influence of twins found in adiabatic shear bands on dynamic recrystallization of a near β Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy

发表时间：Available online 2 April 2022

发表期刊： Materials Science & Engineering A [ 点击下载PDF ]

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It can be seen from Fig. 1 (a) that the microstructure is composed of transformed β region (β_t) and equiaxed primary α phase (α_p) with an average grain size of 1.0 μm. Fig. 1(b) shows the high magnification view of β_t regions, which comprise acicular secondary α phase (α_s) and β matrix separated by α_s. The thickness of acicular α_s is 30 nm–50 nm, and the average grain size of β matrix is about 60 nm.

Fig. 1. The microstructure of the titanium alloy utilized in the present study. (a) The SEM image and (b) the corresponding high magnification view of β_t regions.

In order to reveal the details of the microstructure in the HRs, the microstructures of α grains and β grains were characterized by HAADFSTEM. The microstructure and composition distribution maps near α grains in the HRs are shown in Fig. 6. As shown in Fig. 6(a), the lamellar phase, which exhibits the raft-like distribution characteristics, is observed in α grains. Fig. 6(b) shows the detailed microstructure of the lamellar phase. It can be seen that there is an obvious demarcation line between the lamellar phase and the parent phase (α phase). Simultaneously, as indicated by the red arrows in Fig. 6(b), a great number of dislocations are accumulated at the interface of the lamellar phase and the parent phase, and these dislocations tend to form dislocation walls. In order to identify the lamellar phase, the corresponding Selected Area Electron Diffraction (SAED) pattern is obtained, as shown in Fig. 6(c). It indicates that the lamellar phase in α grains is [-1100] twin. In fact, the [-1100] twin is one of the six variants of {11–22}<11-2-3> twin systems in Hexagonal Close-Packed (HCP) microstructure. Previous research has reported the {11–22}<11-2-3> twin observed in pure titanium after deformation under high strain rate.

Fig. 6. The microstructure and composition distribution maps of α phase in the ASB central region characterized by HAADF-STEM. (a) The microstructure near α phase, (b) the microstructure characteristics, (c) the corresponding SAED pattern of (b), and (d)–(i) the corresponding composition distribution maps of the lamellar phase.

As shown in Fig. 7(a), the lamellar phase, which is similar to that in α grains shown in Fig. 5, is also found in β grains. Similarly, in order to identify the lamellar phase, the detailed microstructure of the lamellar phase is obtained and shown in Fig. 7(b). It is found that there are a great number of dislocations at the interface of the lamellar phase and the parent phase (β phase), and dislocation walls are formed in some local regions, as indicated by the red arrows in Fig. 7(b). According to the corresponding SAED pattern shown by Fig. 7(c), it can be determined that the lamellar phase in β grains is [3-3-2] twin, which is a typical one of the {332}<113> twin in Body-Centered Cubic (BCC) microstructure. In addition, as shown in Fig. 7(c)–7(h), the EDS results of Ti, Al, Zr, Sn, Cr and Mo also support the above inference about the lamellar phase in β grains.

Fig. 7. The microstructure and composition distribution maps of β phase in the ASB central region characterized by HAADF-STEM. (a) The microstructure near β phase, (b) the microstructure characteristics of the lamellae pahse, (c) the corresponding SAED pattern of (b), (d) the microzone utilized for TEM-EDS tests, and (e)– (j) the corresponding composition distribution maps.

Based on the above analysis, the corresponding diagram of microstructure evolution during the formation of ASBs is shown in Fig. 10. As shown in Fig. 10(a), the microstructure of the titanium alloy utilized in the present study is composed of α_p regions and equiaxed β_p regions, and the detailed microstructure evolution processes of different regions are shown in Fig. 10(b)–10(e) and (f)–10(i), respectively. In equiaxed α_p regions, the original microstructure consists of single α_p, as shown in Fig. 10(b). In the second deformation stage (Stage 2), α_p deforms under dynamic loading, which makes [-1100] twin be formed in α phase, as shown in Fig. 10(c). In the third deformation stage (Stage 3), once twinning occurs, a large number of dislocations will be preferentially accumulated at the interface of twins and their parent phases, thus the dislocation walls are further formed, as shown in Fig. 10(d). In the fourth deformation stage (Stage 4), partial dislocation walls develop into new grain boundaries (GBs) of DRX grains, which promotes the DRX process in ASBs. However, for the residual dislocation walls, only the continuous increase of dislocation density occurs, as shown in Fig. 10(e).

Fig. 10. The schematic diagram of the microstructure evolution during the formation of ASBs. (a) The microstructure of the titanium alloy, the microstructure evolution processes of (b)–(e) α_p regions and (f)–(i) β_t regions, respectively.