Abstract
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 106 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.
Highlights
The gradient nanostructured (GNS) layer is induced in the surface of Zr-4 alloy by the surface mechanical grinding treatment (SMGT) process, and the results show that biaxial fatigue strength increases when compared to that of coarse-grained samples [28]
The results indicate that fatigue crack initiation is more difficult in the GNS surface layer than the coarse-grained surface
The gradient nanostructured surface layer in the Zr-4 samples by the SMGT process is stable after annealing at 400 ◦ C for 2h, while the compressive residual stress is apparently relaxed
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Metals with gradient structures (GS) have attracted great attention in recent years due to a superior combination of strength and ductility [1,2]. A gradient nanostructure is introduced in the surface layer with hundreds of microns by means of surface mechanical attrition treatment (SMAT) or surface mechanical grinding treatment (SMGT), and is characterized by nanoscale to ultra-fine grains and dislocation structures with increasing depth from the surface [3,4,5]. K. Lu’s group has reported that gradient nanograined (GNG)
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