Translated Abstract
The gradient nanostructured materials exhibit good combination of strength and ductility, and the engineering application potential is being explored. The gradient nanostructured materials are characterized with the gradient strength, microstructure and residual stress in different regions along the cross section, and the deformation mechanism is different with the homogeneous materials. Meanwhile, the deformation modes of the metals are strongly depended on the crystal structure. The investigations of the gradient nanostructured metals have mainly concentrated on face-centered cubic (FCC) and body-centered cubic (BCC) metals, little work on close-packed hexagonal (HCP) metals. In this paper, the HCP pure titanium (Ti) was selected for the research material. A surface nanocrystallization (SNC) device was designed and processed to fabricate a series of gradient nanostructured Ti solid bars and tubes. The fatigue properties and microstructural evolution of the gradient nanostructured Ti under different applied stress conditions were comparatively studied to clarify the deformation mechanism of gradient nanostructured materials.
The formation mechanism of the gradient nanostructured Ti was firstly investigated. The gradient structure sequentially consists of nano/ultrafine grain surface, the deformed grain subsurface and the coarse grain matrix from the surface to the center. In the depth of 100 μm, nanograins with an average grain size of ~100 nm are achieved. In the depth span of 100-800 μm, a large number of deformation twins are identified, and the density of deformation twins decreases as the depth increases. The grain refinement mechanisms include twinning refinement, grain refinement resulting from dislocation accumulation effect and the transformation of low angle grain boundaries into high angle grain boundaries.
Under tension-compression loading, the applied stress is uniform distribution on the cross section of SNC Ti samples. The fatigue strength is improved by 6.8% in comparison with the coarse-grained Ti (CG Ti) samples. The free-surface cracking is suppressed by the surface nano-gradient structure in the SNC Ti. Thus, the fatigue crack initiation sites change from the free-surface to the subsurface. Furthermore, the fatigue crack propagation path in SNC Ti is more tortuous than that of CG Ti. This implies the lower fatigue crack propagation rate in SNC Ti. It mainly contributed to the barriers of twin boundaries to the propagation of the fatigue crack and the residual compressive stress on the fatigue crack closure effect.
Under cyclic torsional loading, the applied stress is gradient distribution on the cross section in the SNC Ti samples. The fatigue strength is improved by 45%, which is more significant than that under tension-compression loading. Both CG and SNC Ti display cyclic softening during cyclic torsional loading. The cyclic softening rate of SNC Ti is lower than that of CG Ti. Microstructural analysis reveals that, fatigue crack initiation is suppressed in the SNC Ti since the nano-gradient structure restrains strain localization. As a result, the mechanism of crack initiation changes from grain boundary and twin boundary cracking to shear band cracking. Parallel dislocation lines, embryonic cells and twins are observed in the fatigued CG Ti. In comparison, no obvious variation of substructures can be distinguished in the SNC Ti before and after torsion fatigue. The relative stable microstructure in SNC Ti facilitates the improvement of fatigue lives.
The biaxial tension-torsion fatigue behavior of the tubular SNC Ti was systematically investigated under different loading paths. The SNC Ti has the improved in-phase (IP) biaxial fatigue lives in comparison with the CG Ti. The fatigue strength is improved by 10%, and improved degree is in the range of the tension-compression loading and the torsion loading. The improved fatigue lives are attributed the superposition of the residual compressive stress and the applied stress, which results in the maximum shear stress and maximum normal stress in SNC Ti are smaller than that of CG Ti at the same cyclic equivalent stress amplitude. The loading paths have a significant influence on the fatigue life and macrostructure. The fatigue lives of SNC Ti under 90° out-of-phase (OP) loading are shorter than those under IP loading at the same cyclic equivalent stress amplitude. The IP fatigue lives increase with the biaxial stress ratio increasing, and the OP fatigue lives decrease with phase angle. After biaxial fatigue tests, the stress-induced nanograin growth was observed on the top surface. Due to the rotation of maximum shear stress plane under 90° OP loading, the typical dislocation substructure in coarse grained region changed from parallel dislocation lines for IP loading to dislocation tangles under 90° OP loading. Finally, the growing direction of microcracks and macrocrack were experimentally observed and theoretically predicted using the critical plane approaches. The results indicate that the measured direction under IP loading is consistent with the predicted results.
It is found that the improvement degree of fatigue lives in SNC Ti increases with the matching degree between the applied stress and the gradient strength in SNC Ti increasing. Meanwhile, the microstructural evolution maps and fatigue crack initiation and propagation model of SNC Ti under different applied stress condition are established.
Translated Keyword
[Crack initiation, Deformation mechanism, Gradient nanostructure, Pure titanium, Tension-torsion biaxial fatigue, Uniaxial fatigue]
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