Geng Donghui, Sun Qiaoyan, Xin Chao, Xiao Lin
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China.
XI'AN Rare Metal Materials Institute Co., Ltd., Xi'an 710016, China.
Nanomaterials (Basel). 2021 Nov 19;11(11):3125. doi: 10.3390/nano11113125.
The gradient nanostructured (GNS) layer forms beneath the surface of Zr-4 samples by the surface mechanical grinding treatment (SMGT) process, which increases the fatigue strength apparently due to the synergistic effect of the gradient nanostructured layer and compressive residual stress. The SMGTed Zr-4 samples are subjected to annealing to remove residual stress (A-SMGT) and the individual effect of the GNS layer and compressive residual stress can be clarified. The results show that the gradient nanostructure in the surface is stable after annealing at 400 °C for 2 h but residual stress is apparently removed. Both SMGTed and A-SMGTed Zr-4 samples exhibit higher fatigue strength than that of coarse-grained (CG) Zr-4 alloy. The fatigue fracture of Zr-4 alloy indicates that the hard GNS surface layer hinders fatigue cracks from approaching the surface and leads to a lower fatigue striation space than that of CG Zr-4 samples. The offset fatigue strength of 10 cycles is taken for SMRT-ed, A-SMRT-ed, and CG Zr-4 samples and the results indicate clearly that the GNS surface layer is a key factor for the improvement of fatigue strength of the Zr-4 alloy with surface mechanical grinding treatment.
通过表面机械研磨处理（SMGT）工艺，在Zr-4样品表面下方形成梯度纳米结构（GNS）层，由于梯度纳米结构层和压缩残余应力的协同作用，该层显著提高了疲劳强度。对经过SMGT处理的Zr-4样品进行退火以消除残余应力（A-SMGT），从而可以阐明GNS层和压缩残余应力的单独作用。结果表明，在400°C退火2小时后，表面的梯度纳米结构是稳定的，但残余应力明显消除。经过SMGT处理和A-SMGT处理的Zr-4样品都表现出比粗晶（CG）Zr-4合金更高的疲劳强度。Zr-4合金的疲劳断裂表明，坚硬的GNS表面层阻碍了疲劳裂纹接近表面，导致疲劳条纹间距比CG Zr-4样品更小。对经过SMRT处理、A-SMRT处理和CG Zr-4样品取10次循环的偏移疲劳强度，结果清楚地表明，GNS表面层是通过表面机械研磨处理提高Zr-4合金疲劳强度的关键因素。
