## 脉冲磁场对TC4钛合金微观结构的影响及其机理探究

1. 清华大学机械工程系 北京 100084

2. 清华大学摩擦学国家重点实验室 北京 100084

3. 清华大学先进核能技术协同创新中心 北京 100084

4. 清华大学天津高端装备研究院 天津 300304

## Effect of Pulsed Magnetic Field on the Microstructure of TC4 Titanium Alloy and Its Mechanism

XU Qingdong1, LI Kejian1, CAI Zhipeng,1,2,3, WU Yao4

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

 基金资助: 国家科技重大专项项目.  No.2018ZX04042001

Corresponding authors: CAI Zhipeng, associate professor, Tel: (010)62789568, E-mail:czpdme@mail.tsinghua.edu.cn

Received: 2018-06-14   Revised: 2018-10-05   Online: 2019-03-28

 Fund supported: National Science and Technology Major Project of China.  No.2018ZX04042001

Abstract

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.

Keywords： TC4 titanium alloy ; pulsed magnetic field ; dislocation density ; grain boundary angle ; magnetoplasticity

Qingdong XU, Kejian LI, Zhipeng CAI, Yao WU. Effect of Pulsed Magnetic Field on the Microstructure of TC4 Titanium Alloy and Its Mechanism. Acta Metallurgica Sinica[J], 2019, 55(4): 489-495 doi:10.11900/0412.1961.2018.00257

## 2 实验结果

### 图1

Fig.1   Average residual stresses of the samples before and after pulsed magnetic treatment

### 2.2 基于XRD的脉冲磁场处理前后TC4钛合金位错密度统计

$ΔK2K2=12πb2M2C¯h001-qH2ρ$
$H2=h2k2+h2l2+k2l2h2+k2+l2$

### 图2

Fig.2   XRD spectra of TC4 samples before and after pulsed magnetic treatment

### 图3

Fig.3   K)2/K 2~H 2 linear fitting graph (K is expressed as 2sinθ/λ, ΔK is expressed as 2cosθ∙Δθ/λ, θ is the angle of the peak in XRD spectrum, λ is the XRD wave length, H 2 is expressed as equation (2), k1 is the slope of linear fitting line before treatment, k2 is the slope of linear fitting line after treatment)

#### 2.3.1 基于KAM的位错分布

KAM (kernel average misorientation)是一种表征晶内局部平均取向差的手段，常常用于表征材料晶内区域的位错密度[29,30]，KAM值越大，位错密度越高。

### 图4

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)

KAM分布的改变代表了位错分布的改变，总体位错密度呈现均匀化的趋势，晶界附近位错密度的变化显著，推测晶界区域可能存在位错的塞积、穿越及湮没现象，这一推测可从晶界角度变化规律上进一步加以印证。

### 图5

Fig.5   Grain boundary angle distributions of TC4 samples before/after treatment (Significant reduction of the low-angle grain boundaries can be found, while there are also significant increase of the grain boundaries with the angle within the scope of 34°~38°, which may well be the Σ11 boundaries (36.5°, a kind of CSL boundaries of α-phase of TC4 alloy)

## 3 分析讨论

$τT=Gb2πLln(Lb)$

### 图6

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)

$ωp=μgBh$

### 图7

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 (ωpωp΄), which produces a chance for the electron pair to transform their state from singlet to triplet)

## 4 结论

(1) 通过XRD与EBSD手段观察到脉冲磁场作用下TC4钛合金的位错分布发生显著变化。一方面，晶内位错发生显著的均匀化，整体位错密度上升；另一方面，晶界处位错发生塞积、穿越及湮没现象，具体表现在晶界KAM分布的增大、减小及晶界角度的改变。这可能是由于位错运动引起的小角度晶界含量降低和CSL晶界Σ11的含量升高，使得系统能量降低，更稳定。

(2) 应用磁致塑性理论对脉冲磁场作用下位错运动机理做了探讨。针对TC4钛合金，脉冲磁场作用促进了位错钉扎点的电子能态转变，使得位错钉扎点能量升高，钉扎阻力减小，位错在原有应力场作用下，更易产生脱钉扎，从而发生运动。位错的运动引起了晶界角度的变化和残余应力的降低，体系趋于稳定。

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