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论文题目： Effect of tungsten micro-scale dispersed particles on the microstructure and mechanical properties of Ti-6Al-4V alloy

发表时间：Available online 25 August 2020

刊源：Journal of Alloys and Compounds 851 (2021) 156847 [ 点击下载PDF ]

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The starting materials for mechanical milling were Ti-6Al-4V powders with less than 150 μm in particle size (as shown in Fig. 1(a)) and W powders with ~5 μm in particle size (as shown in Fig. 1(b)). The chemical composition of the Ti-6Al-4V powders was shown in Table 1. The two kinds of powders were mechanically mixed at 250 rpm for 10 h using a SPEX 8000D high energy shaker ball mill (SPEX 8000 mill/mixer, Nanjing University col., Ltd) to avoid the agglomeration ofWpowders and obtain a homogeneous composition during the sintering process. Stainless steel balls with a diameter of 6 mmwere used to mix the powders and the ball-tocharge ratio was 10:1. As a comparison, pure Ti-6Al-4V powders were also prepared for reference under the same conditions. The milled TC4 powders and TC4/W mixed powders were shown in Fig. 1(c) and (d).

Fig. 1. The surface morphology of powders. (a) origin TC4 powders, (b) originWpowders, (c) ball-milled TC4 powders, and (d) ball-milled TC4/W mixed powders. (e) enlarge of (d).

The representative optical micrographs (OMs) of TC4 and TC4- 5%W are shown in Fig. 3, respectively. As shown in Fig. 3(a), TC4 has a typical Widmanstatten microstructure which consists of colonies of α/β lamellae and grain boundary a within b matrix. For colonies of α/β lamellae, the thin white plates represent the α phase and thin dark regions between the plates are the β phase. The thickness of α/β lamellae will be described in the following SEM images due to the limitation of the optical microscope. However, the microstructure of TC4-5%W (Fig. 3(b)) is very different from that of TC4. Fig. 3(b) depicts that many fine W particles are distributed in β grain boundaries and lamellar α/β colony boundaries within b grain interiors. The β grain size distributions of both TC4 and TC4-5%W can be calculated by counting more than 300 β grains in many OMs (more than tens of OMs), and the results are shown in Fig. 3(c-d).

Fig. 3. Optical micrographs of (a) TC4 alloy and (b) TC4-5%W alloy. Inset (c): zoom-in of (b). The b grain distribution of (d) TC4 alloy and (e) TC4-5%W alloy.

Fig. 7 exhibits the fracture surfaces of TC4 and TC4-5%W alloys. As shown in Fig. 7(a), it was found that the fracture surface of TC4 alloy mainly had a brittle cleavage fracture mode. The river patterns were present in the fracture surface of TC4, which contained numerous shallow and parallel tearing ridges. Moreover, A regions (marked by red rectangular area) and B regions (marked by yellow rectangular areas) showed the typical intergranular fracture characteristics, which should be caused by the fracture of the continuous αGB layers. Similar results have been reported by Berg et al. and Cao et al. Besides, therewere still some dimples in the fracture surface of TC4 alloy, but the number of these dimples was very small and most of them were shallow, leading to a decrease of ductility. On the contrary, Fig. 6(b) shows that the fracture surface of TC4-5%W alloy has a ductile fracture mode, which reveals a large number of dimples (marked by arrows). These dimples were deeper than that of TC4 alloy, which also demonstrated that TC4-5% W alloy possessed higher ductility, compared with TC4 alloy. Meanwhile, it can be found that W particles (marked by black arrows in Fig. 6(b) and the pink rectangular frame in Fig. 7(c)) were pulled out of the tensile sample after the tensile test. The region marked by the rectangular frame in Fig. 7(c) was detected by EDS, which suggested that the particle pulled out from the TC4 matrix is W particle. These W particles can effectively inhibit dislocation movement during the tensile process, thus increasing the strength of TC4-5%W alloy.

Fig. 7. Fracture surfaces of (a) TC4 and (b) TC4-5%W. The EDS regions of TC4-5%W alloy (c) and the EDS mapping images of W particle and TC4 matrix (d).