基于同步辐射X射线多尺度测量的镁合金拉压不对称性研究

Due to the strong fiber texture formed in the forging process, the macroscale mechanical behavior of wrought magnesium alloys shows obvious tension-compression asymmetry, which limits the wide application of magnesium alloys to a certain extent, especially when they are used as structural parts bearing tension and compression loads. Therefore, it is of practical significance to fully understand the tension-compression asymmetry mechanism of magnesium alloys to broaden their industrial applications. The material testing system and split Hopkinson tension/pressure bar are utilized to load the wrought magnesium alloy specimen under uniaxial tension and compression, and synchrotron radiation X-ray is irradiated to the specimen simultaneously. The transmission and scattering X-rays are incident on imaging and diffraction scintillators and recorded by high-speed cameras to achieve in-situ X-ray phase contrast imaging and diffraction measurements on the tension-compression asymmetric deformation of magnesium alloy, respectively. The macroscale stress-strain curve is obtained by strain/stress gauge. The X-ray imaging sequences are processed by digital image correlation to obtain mesoscale displacement and strain fields, and the X-ray diffraction data are analyzed to quantify the microscale lattice deformation by using an X-ray diffraction simulation. The macroscale-mesoscale-microscale deformation of magnesium alloy across multiple spatial scale under wide strain-rate tension and compression establishes a one-to-one correspondence over time, which provides a new way to study the tension-compression asymmetry of magnesium alloy, and is helpful to further reveal the essence of asymmetric tension-compression deformation of magnesium alloy.

变形镁合金在成型过程中通常会形成较强的丝织构，宏观力学行为表现出明显的拉压不对称性，这一定程度上限制了镁合金的广泛应用，尤其是用作同时承受拉压载荷的结构件时。因此，充分了解镁合金拉压不对称性机理对拓宽其应用有现实意义。本项目采用材料试验机与霍普金森拉/压杆对变形镁合金进行单轴拉伸和压缩加载，加载的同时同步辐射X射线照射试样，透射和散射X射线经闪烁体转化为可见光后被高速相机记录，进而实现镁合金拉压不对称变形的X射线相衬成像和衍射原位测量。应变/应力计提供宏观尺度的应力-应变曲线，X射线成像序列经数字图像相关方法处理得到细观尺度的位移场和应变场；运用X射线衍射模拟分析X射线衍射数据，用以量化微观晶格尺度的变形。至此，宽应变率拉伸和压缩加载下镁合金空间跨尺度的宏观-细观-微观变形实现了与时间演化的一一对应，为研究镁合金拉压不对称性提供新思路，有助于进一步揭示镁合金拉压不对称变形的本质。

针对变形镁合金的拉压不对称变形行为，设计了一种狭缝尺寸可动态调节的X射线快门，实现了镁合金拉压变形过程中大光斑X射线相衬成像和小光斑X射线衍射测量，提升X射线衍射数据质量的同时保持了较大的X射线相衬成像视场。基于同步辐射X射线多尺度诊断和电子背散射衍射技术获取材料试验机和霍普金森杆拉伸/压缩作用下镁合金的宏观应力-应变曲线、细观变形场和微观变形机制。研究表明镁合金的拉压不对称变形呈现出一定程度的应变率敏感特性。当拉应力平行或压应力垂直于镁合金的晶体学<c>轴时，{10-12}拉伸孪晶易于形成。拉应力下变形孪晶的增长速率高于压应力下形成的孪晶，根据粘塑性自洽模型拟合结果中拉应力下变形模式中锥面滑移占比较高，推断平行晶体学<c>轴拉伸时，协调晶体学<c>轴方向的变形更易形成。拉应力和压应力下形成的变形孪晶均可以引起去局域化变形，进而促进应变硬化率的增加。