1. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 2. State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China 3. Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing 100084, China 4. Tianjin Research Institute for Advanced Equipment, Tsinghua University, Tianjin 300304, China
Cite this article:
Qingdong XU, Kejian LI, Zhipeng CAI, Yao WU. Effect of Pulsed Magnetic Field on the Microstructure of TC4 Titanium Alloy and Its Mechanism. Acta Metall Sin, 2019, 55(4): 489-495.
In this work, the effect of pulsed magnetic treatment (PMT) on the microstructure of TC4 titanium alloy was investigated. TC4 titanium alloy is widely used in the manufacture of the blade of aviation engine. The microstructure of TC4 titanium alloy determines its property. PMT is a novel method used to modify the microstructures of alloys and has been explored in several papers recently. PMT has many advantages in the aspect of efficiency, energy-saving, non-deformation, etc. Therefore, the effect of PMT on the microstructures of TC4 titanium alloy was explored in this work. The variation of the dislocation density and the grain boundary angle of TC4 titanium alloy was observed after PMT. In the experiment, the magnetic induction density is 2 T, the pulse frequency is 5 Hz and the pulse number is 100. According to XRD tests, the dislocation density in TC4 alloy after PMT increased about 10.9%. KAM maps in EBSD test were used for evaluating the same area's dislocation density of the TC4 alloy before and after PMT. The dislocation distribution of TC4 titanium alloy changes notably: the in-grain dislocation density became more homogeneous and some local high-density areas disappeared, the distribution of dislocation near grain boundaries caused the angles of the grain boundaries altered and the fraction of low-angle grain boundaries decreased while the fraction of Σ11 grain boundaries (CSL grain boundary) increased. The motivation mechanism of the dislocation in TC4 titanium alloy under PMT was speculated based on the experimental results and some previous researches. The PMT may change the energy state of the electrons in pinning area of dislocations, which accelerates the electrons transformation from singlet state to triplet state and then increases the mobility of the vacancy or impurity atoms so that the dislocation de-pinning could occur under the original stress field and thus leads to dislocation movement and transformation of microstructure.
Fig.4 Low (a, b) and locally high (c, d) magnified KAM distributions of the TC4 sample before (a, c) and after (b, d) pulsed magnetic treatment (The KAM distribution after treatment is more homogeneous than that before treatment totally. Positions 1, 2 and 3 in Figs.4a and b as an example indicate the variation of KAM near the grain boundaries. The boxes in Figs.4c and d forcefully indicate the reduction of KAM in the specific grain while the ellipses in Figs.4c and d indicate the increase of KAM in the specific grains. Positions 4, 5 and 6 in Figs.4c and d as an example indicate the variation of KAM and especially the variation of the shape of the grain boundaries)
Fig.5
Fig.6 Transforming relationship of the spin energy state of the electron pair (The solid arrows indicate the transformation between these two states happens frequently while the hollow arrows indicate the transformation happens rarely)
Fig.7 Transformation of the state of electron spin under pulsed magnetic field (Due to the pulsed magnetic field (B), the Larmor precession angular frequency of the electron pair changes faintly (ωp→ωp?), which produces a chance for the electron pair to transform their state from singlet to triplet)
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