Yao Shen

Surface effect on the screw dislocation mobility over the Peierls barrier

The effect of the image force on the Peierls stress (τ p ) of a screw dislocation below a free surface is studied via a self-consistent semidiscrete variational Peierls–Nabarro model. The consequence of reduction in elastic energy and increase in stacking fault energy by the presence of the free surface is found to additively increase the Peierls stress (τ p ). This model gives a physical interpretation of the same tend observed in a recent molecular dynamic study, while previous continuum analysis predicted the opposite.

Grain Size Dependence of Uniform Elongation in Single-Phase FCC/BCC Metals

We studied the dependence of uniform elongation on grain size in the range of submicron to millimeter for single-phase FCC/BCC metals by reviewing recent experimental results and applying crystal plasticity finite element method simulation. In the order of increasing grain size, uniform elongation can be divided into three stages, namely low elongation stage, nearly constant elongation stage, and decreased elongation with large scatters stage. Low elongation stage features a dramatic increase near the critical grain size at the end of the stage, which is primarily attributed to the emergence of dislocation cell size transition from ultrafine to mid-size grain. Other factors can be neglected due to their negligible influence on overall variation trend. In nearly constant elongation stage, uniform elongation remains unchanged at a high level in general. As grain size keeps growing, uniform elongation starts decreasing and becomes scattered upon a certain grain size, indicating the initiation of decreased elongation with large scatters stage. It is shown that the increase is not linear or smooth but rather sharp at the end of low elongation stage, leading to a wider range in nearly constant elongation stage. The grain size dependence of uniform elongation can serve as a guiding principle for designing small uniaxial tensile specimens for mechanical testing, where size effect matters in most cases.

Strength and Ductility with Dual Grain-Size and Texture Gradients in AZ31 Mg Alloy

Using surface rotation rolling (SRR) treatment with various vertical loads (120 to 280xa0N) while maintaining other processing parameters (rotation speed and horizontal velocity), the SRR-treated AZ31xa0Mg alloy sheets exhibit through-thickness gradients of both grain size and basal texture, as revealed by cross-sectional hardness tests and microstructure analysis. An optimal strength–ductility combination is achieved as the vertical load is around 200xa0N. The corresponding microstructure exhibits two characters: (1) The affected depth reaches the very center of the sheet and (2) the gradients approach the allowable maximum. Texture gradient is found to be the major contributor to the significant ductility enhancement.

Two-Step Intercritical Annealing to Eliminate Lüders Band in a Strong and Ductile Medium Mn Steel

Lüders band usually apprears in medium Mn steel, and they are difficult to be eliminated without sacrificing the mechanical properties. In this study, a tailored two-step intercritical annealing approach is proposed to eliminate Lüders band and maintain excellent mechanical behavior. A strong ferritic matrix and austenite with appropriate stability are obtained, achieving excellent mechanical behavior. Lüders band is fully removed by the early stress-induced martensitic transformation.

Influence of Residual Stresses on the Interfacial Toughness Measurement by Cross-Sectional Nanoindentation

Cross-sectional nanoindentation (CSN) is a new method for measuring interface adhesion of thin films. The interfacial energy release rate (G), characterizing interfacial adhesion, is calculated from the material and geometrical parameters relevant to the test. Effects of residual stresses on G and crack tip phase angle Ψ, have been studied by finite element simulation in this study. The results show tensile residual stresses increase G and compressive stresses reduce it, and they have similar effects on the magnitude of Ψ.