 研究文献发布相关信息

论文题目： Simultaneously enhancing strength and ductility of Ti-6Al-4V alloy with the hierarchical structure via a novel thermal annealing treatment

发表时间：Available online 15 April 2021

刊源：Materials Characterization 176 (2021) 111112 [ 点击下载PDF ]

 研究文献内容展示

The representative optical micrographs (OMs) and SEM images of the as-fabricated Ti-6Al-4V sample and the STA Ti-6Al-4V sample are presented in Fig. 4, respectively. As shown in Fig. 4(a), the as-fabricated Ti-6Al-4V alloy has a typical Widmanstatten structure, consisting of grain boundary α (called as α_GB) located at β grain boundaries and α colonies within β matrix. Besides, the β grain size of the as-fabricated Ti-6Al-4V sample was calculated to be 117 ± 40 μm by counting more than about 300 β grains in many OMs (more than tens of OMs).For the STA Ti-6Al-4V sample, as shown in Fig. 4(b), the size of βgrains was 126 ± 20 μm, which have almost no significant increase compared with that of the as-fabricated Ti-6Al-4V sample. This indicates that the influences of the STA heat treatment on the grain size of β grain are negligible. Meanwhile, it is easily seen that the microstructure of the STA Ti-6Al-4V sample had made a great change after the heating treatment process.

Fig. 4. Optical micrographs and SEM images of (a) and (c) as-fabricated Ti-6Al-4V sample, and (b) and (d) STA Ti-6Al-4V sample

Fig. 7 shows the tensile fractography of the as-fabricated Ti-6Al-4V sample and the STA Ti-6Al-4V sample. As shown in Fig. 7(a), the asfabricated Ti-6Al-4V sample mainly had a mixed fracture morphology of both brittle cleavage fracture with cleavage surface and ductile fracture with shallow dimples. The river patterns marked by the red arrow with numerous shallow and parallel tearing ridges were also observed in the fracture surface of the as-fabricated Ti-6Al-4V sample.Though some dimples also existed in the fracture surface of the as-fabricated Ti-6Al-4V sample, these dimples were very shallow and the number of these dimples was not great. On the contrary, the fracture morphology of the STA Ti-6Al- 4V sample with a ductile fracture mode is shown in Fig. 7(b). There were a large number of dimples (marked by blue arrows) in the fracture surface of the STA Ti-6Al-4V sample. These dimples were deeper than that of the as-fabricated Ti-6Al-4V sample. This also demonstrates that the Ti-6Al-4V sample by the STA heat treatment has higher ductility,which was in agreement with the results of the tensile tests.

Fig. 7. Fracture surfaces of (a) as-fabricated Ti-6Al-4V sample and (b) STA Ti-6Al-4V sample.

As shown in Fig. 8(a), some twins were found at the α/β interface, which can be formed during the STA heat treatment. The formation of annealing twins could result from the low stacking fault energy of the Ti-6Al-4V alloy. Therefore, these annealing twins present in the STA Ti-6Al-4V sample can be beneficial for increasing ductility. Meanwhile, as shown in Fig. 8(b), surprisingly, there are some fine particles in the grain interiors, which is in accord with the results of Fig. 4(b) and Fig. 4(d). The selected area electron diffraction patterns shown in Fig. 8(c-d) confirmed that these finely dispersed phases and the ambient equiaxed phases were corresponding to the diffraction spots of the β phase and the α phase, respectively. These equiaxed α phases should be formed during the furnace cooling process. Fig. 8(e) exhibits the region of energy dispersive spectroscopy (EDS), in which the β phase were embedded in the equiaxed α phase. As shown in Fig. 8(f) and Fig. 8 (g), large amounts of V element (β stabilizer) are detected in the β phases, and the composition of V is higher than that in the equiaxed α phases. While Al as the α stabilizer was enriched in the α phase, and the composition of Al is higher than that in the β phase.

Fig. 8. TEM micrograph of the STA Ti-6Al-4V alloy: (a) Twins distributing in the matrix; (b) Fine particles β surrounded by the equiaxed α grains; (c) and (d) The corresponding selected area electron diffraction patterns of (b); (e) The EDS regions of the STA Ti-6Al-4V alloy; The corresponding EDS mapping images of (f) V element and (g) Al element.