Superplastic deformation mechanism of the friction stir processed fully lamellar Ti-6Al-4V alloy
Superplastic deformation mechanism of the friction stir processed fully lamellar Ti-6Al-4V alloy
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DOI：
10.1016/j.msea.2020.139390
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发表时间：
2020-05
期刊：
影响因子：
6.4
通讯作者：
Wenjing Zhang;Huihong Liu;H. Ding;H. Fujii
中科院分区：
文献类型：
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作者：
Wenjing Zhang;Huihong Liu;H. Ding;H. Fujii
采用高热输入参数，在325 rpm的旋转速度和50 mm/min的横移速度下，对轧制后的Ti-6Al-4V合金板材进行摩擦搅拌加工（FSP），在搅拌区（SZ）获得了全层状组织。然后在850-900 °C温度范围内以3 × 10-2-3 × 10-4s-1的应变速率对该全层片组织进行高温拉伸试验。在合适的应变速率下，合金在所有试验温度下均获得了伸长率大于400%的超塑性，其中在875 °C和1 × 10-3s-1下获得的最大伸长率为553%，这归因于低应变阶段的动态球化和高应变阶段的边界滑移。动态球化的主要机制为不连续动态再结晶（DDRX）和连续动态再结晶（CDRX），并伴有β相沿小角度晶界（LAGB）向槽内沿着生长。由于α和β两相的共存可以有效地抑制晶粒的严重长大，有利于BS的连续运行，因此，该球化组织具有良好的热机械稳定性。此外，β相从压缩边界向拉伸边界的转变以及α相向β相转变所引起的应变/应力可以作为额外的调节机制，缓解应力集中，抑制空洞的形成，从而促进超塑性的增强。
The rolled Ti-6Al-4V alloy sheets were subjected to friction stir processing (FSP) using high heat-input parameters at a tool rotation speed of 325 rpm with a traverse speed of 50 mm/min, and a fully lamellar microstructure was obtained in the stir zone (SZ). The high-temperature tensile tests were then conducted on this fully lamellar microstructure in the temperature range of 850–900 °C at the strain rates of 3 × 10-2–3 × 10-4s-1. The superplasticity with elongations of above 400% was achieved at all the testing temperatures with the appropriate strain rates, and the maximum elongation of 553% was achieved at the temperature of 875 °C and 1 × 10-3s-1, which was attributed to the dynamic globularization at the low strain stage and subsequent boundary sliding (BS) at the high strain stage. The main dynamic globularization mechanisms were considered as discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) accompanied with theβphase growing towards the inside of the groove along the low angle grain boundaries (LAGBs). The present spheroidized microstructure shows an excellent thermal-mechanical stability because the co-existence of the two phases ofαandβcould effectively inhibit the severe grain growth and facilitate the continuous operating of the BS. In addition, theβphase transferring from compressive boundaries to tensile boundaries and the strain/stress induced theαtoβphase transformation can act as the additional accommodation mechanisms to relax the stress concentration and inhibit the formation of the cavities, which can facilitate the achievement of the enhanced superplasticity.