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论文题目： Achieving super-high strength and acceptable plasticity for a near β-type Ti- 4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy through manipulating hierarchical microstructure

发表时间：Available online 16 August 2021

刊源：Materials Science & Engineering A xxx (xxxx) 141907 [ 点击下载PDF ]

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Figure 2 shows the microstructure of the hot-rolled Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn -2.5Cr-2.9Zn alloy. It can be seen from Figure 2(a) that the microstructure contains equiaxed α_p (identified by the dotted line) and βt regions, and the average grain size of α_p is about 1.8 μm. Figure 2(b) indicates that the microstructure of β_t regions is composed of β phase and acicular α_s embedded in β matrix. Acicular α_s was formed in the cooling process after hot-rolling. Figure 2(c) shows the high magnification view of α_p regions, and some white precipitates are observed in α_p regions as indicated by the yellow arrow. In order to further identify the white precipitates, the bright-field TEM image near αp regions was obtained and shown in Figure 2(d). The selected area electron diffraction (SAED) pattern of the region in the red circle of Figure 2 (d) is shown in Figure 2 (e). It can be confirmed that the white precipitates are β phase. The phenomenon that β grains distribute in α_p regions may be attributed to some relatively enriched regions of β stable elements in α_p regions. During hot-rolling at high temperature, the β stable elements near these regions diffuse and aggregate to promote the nucleation of β phase. Finally, the nucleated β grains are retained in α_p regions during rapid cooling in water after hot-rolling.

Fig. 2. The microstructure of the hot-rolled titanium alloy. (a) The OM image, (b) the SEM image, (c) the SEM image showing the high magnification view of α_p regions, (d) the bright-field TEM image of α_p regions and (e) the SAED of the red circle in (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

In the present study, a lot of β grains discontinuously distributed in α_p regions of the hot-rolled titanium alloy, as shown in Fig. 6(a). It can be determined that there must be a discontinuity in the concentration of β stable elements. Therefore, α/α grain boundaries and α/β phase boundaries can be eliminated, and the CBs will be formed during rapid heating. In the subsequent process of rapid cooling in water, a large number of ultra- fine β grains will precipitate in α_p regions, as shown in Fig. 6(b) and (c).In addition, in order to prepare different hierarchical microstructures in α_p regions, the solution temperature is set at 900 °C and 920 °C, respectively. At a higher solution temperature of 920 °C, β stable elements relatively fully diffuse and continuously concentrate to form a hierarchical microstructure, which is composed of equiaxed α_ps and β phase embedded between α_ps, as shown in Fig. 6(c). At a lower solution temperature of 900 °C, β grains dispersed in α_p regions are not connected with each other to separate α phase, thus no equiaxed α_ps is observed, as shown in Fig. 6(b). Simultaneously, increasing the solution temperature from 900 °C to 920 °C results in severe α→β phase transformation, which promotes β phase to grow. Consequently, 2# titanium alloy acquires a higher β phase fraction compared with 1# titanium alloy. Furthermore, severe α→β phase transformation leads to the sharp decrease of α_p grain size in 2# titanium alloy.

Fig. 6. The schematic diagram for the formation process of the hierarchical microstructures. The microstructures of the (a) hot-rolled, (b) 1# solution treated, (c) 2# solution treated, (d) and (e) 3# completely heat-treated, and (f) and (g) 4# completely heat-treated titanium alloys, respectively.

The quasi-static tensile tests were carried out on 1#, 2#, 3# and 4# titanium alloys, respectively. The test results are shown in Fig. 7. Fig. 7 (a) shows the true stress-strain curves of the titanium alloys under different conditions. It can be seen that 1# titanium alloy acquires the lowest YS of 885 MPa and UTS of 1056 MPa, and the elongation is about 10.6%. Compared with 1# titanium alloy, the YS and UTS of 2# titanium alloy increase to 969 MPa and 1142 MPa, respectively, and the elongation is larger (about 12.8%). 3# titanium alloy exhibits moderate strength and the lowest elongation of only 4.1%. 4# titanium alloy acquires the highest YS of 1255 MPa and UTS of 1420 MPa. Compared with other titanium alloys, the elongation of 4# titanium alloy remains at an acceptable level (about 6%). The bar chart of the mechanical properties is shown in Fig. 7(b). It can be summarized that after aging- treatment, the strength of titanium alloy increases greatly, while the elongation decreases sharply. Compared with 3# titanium alloy, 4# titanium alloy exhibits higher strength and plasticity, which overcomes the limitation of the strength-ductility trade-off to a certain extent.

Fig. 7. The quasi-static mechanical properties of the titanium alloys under different conditions. (a) The true stress-strain curves and (b) the bar chart of the mechanical properties.