表面机械研磨处理对纯铜棘轮行为的影响:宏微观试验与本构模拟

Effect of the Surface Mechanical Attrition Treatment on the Ratcheting of Pure Copper:Macro/Micro-tests and Constitutive Modeling

目的通过金属表面纳米化试验机制备出梯度结构纯铜,提升纯铜材料的疲劳寿命,并揭示其背后的机理。方法通过系统的宏观力学性能测试、微观组织表征以及本构模拟探究了表面机械研磨处理(Surface Mechanical Attrition Treatment,SMAT)对T2纯铜棘轮行为的影响。结果循环变形试验结果表明SMAT纯铜样品的循环失效圈数明显多于未处理纯铜样品的循环圈数,且SMAT纯铜样品在循环过程中的累积塑性变形明显小于未处理纯铜样品的累积塑性变形,即棘轮应变明显小于未处理纯铜样品的棘轮应变。电子背散射衍射(Electron Back Scattered Diffraction,EBSD)和X射线衍射分析(X-Ray Diffraction,XRD)表征发现:经过SMAT后,材料的晶粒尺寸均呈现由处理表面到材料芯部逐渐减小的梯度分布。且SMAT时间越长,样品的总位错密度越大。此外,基于应变梯度塑性理论模型对SMAT前后纯铜的单拉及循环变形响应进行了有限元模拟,模拟结果显示累积塑性应变沿深度方向(SMAT冲击方向)呈梯度分布,最大几何必需位错密度以及最大等效应力均出现在模型的次表层。同时,当模拟的循环圈数相同时,代表SMAT样品的梯度结构模型的棘轮应变明显低于代表未处理样品的均匀模型的棘轮应变。结论循环变形试验结果表明SMAT对于T2纯铜的棘轮应变有抑制作用,有限元模拟进一步揭示了SMAT对于棘轮应变的抑制效应以及背后的机理。

Most of the damage of metal materials originates from the surface,so surface treatment plays a vital role in improving the service life of metal materials.The traditional materials with uniform microstructure show a tradeoff in strength and ductility.Coarse-grained materials usually maintain considerable ductility,but the strength is relatively low;while the strength of ultra-fine-grained is elevated,the ductility is dramatically decreased.Materials with gradient nano-grained(GNG)structure,where grain size changes gradually from the treated surface to the core,are promising for overcoming the strength-ductility contradiction.Moreover,the GNG materials also possess other outstanding mechanical properties,such as enhanced wear resistance,improved fatigue life and fracture resistance.In this paper,the GNG copper was prepared using the SMAT technique,and uniaxial tensile tests were performed on both GNG and coarse-grained copper.The test results showed that the SMAT greatly improves the yield strength,without compromising too much ductility.Furthermore,Cyclic deformation tests show that the number of cycles of SMAT samples to failure is significantly higher than that of as-received samples,and the ratcheting strain of SMAT samples is significantly lower than that of as-received ones.In other words,the SMAT effectively inhibits the ratcheting strain of pure copper and enhances its fatigue life.Microscopic characterization was carried out,including electron backscatter diffraction(EBSD)analysis and X-ray diffraction(XRD)analysis.It is found that the average grain size of the material after SMAT decreases,while the average dislocation density increases.In addition,the grain size and total dislocation density show a spatial gradient from the surface to the core.In the end,based on a modified version of the conventional theory of mechanism-based strain gradient plasticity(CMSG),the mechanical responses of pure copper before and after SMAT are simulated and compared with the results of the tension-compression cyclic test.The study found that the constitutive model can well describe the uniaxial tensile response of the material after SMAT,and can qualitatively capture the inhibition effect of the SMAT technique on the ratcheting strain evolution.