Linli Zhu

Dual-phase nanostructuring as a route to high-strength magnesium alloys

It is not easy to fabricate materials that exhibit their theoretical ‘ideal’ strength. Most methods of producing stronger materials are based on controlling defects to impede the motion of dislocations, but such methods have their limitations. For example, industrial single-phase nanocrystalline alloys and single-phase metallic glasses can be very strong, but they typically soften at relatively low strains (less than two per cent) because of, respectively, the reverse Hall–Petch effect and shear-band formation. Here we describe an approach that combines the strengthening benefits of nanocrystallinity with those of amorphization to produce a dual-phase material that exhibits near-ideal strength at room temperature and without sample size effects. Our magnesium-alloy system consists of nanocrystalline cores embedded in amorphous glassy shells, and the strength of the resulting dual-phase material is a near-ideal 3.3 gigapascals—making this the strongest magnesium-alloy thin film yet achieved. We propose a mechanism, supported by constitutive modelling, in which the crystalline phase (consisting of almost-dislocation-free grains of around six nanometres in diameter) blocks the propagation of localized shear bands when under strain; moreover, within any shear bands that do appear, embedded crystalline grains divide and rotate, contributing to hardening and countering the softening effect of the shear band.

Computer simulation of strength and ductility of nanotwin-strengthened coarse-grained metals

The superior strength-ductility combination in nanotwin (NT)-strengthened metals has provided a new potential for optimizing the mechanical properties of coarse-grained (CG) metals. In this paper computer simulations based on the mechanism-based strain gradient plasticity and the Johnson–Cook failure criterion have been carried out to uncover the critical factors that serve to provide this dual function. Our results indicate that both the distribution characteristics of the NT regions and the constitutive relations of the NT phase can have a significant impact on the strength and ductility of the CG Cu strengthened by the NT regions. In particular, twin spacing, distribution characteristics such as arrangement, shape and orientation, together with volume fraction of the NT regions, can all have significant effects. Along the way, we also discovered that microcrack initiation, coalescence and deflection constituted the entire failure process. Significant insights into the morphology of NT regions that could deliver superior strength and ductility combination for CG metals have been established.

Two softening stages in nanotwinned Cu

This study focuses on the deformation mechanisms of nanotwinned Cu with hierarchical twins (HTs) by virtue of a series of large-scale molecular dynamics (MD) simulations. For the same grain size dG and the same secondary twin spacing L2, two softening stages and one strengthening stage are discovered, accompanied by the reduction of the primary twin spacing L1. This special phenomenon is firstly found in nanotwinned Cu with HTs by MD simulations. In addition, totally different deformation mechanisms are observed along with the variation of L1: (1) The transition of full dislocation to partial dislocation-dominated plastic deformation as the primary twin spacing L1 decreases until L1 reaches the first critical primary twin spacing in the first softening stage. (2) The transition of partial dislocation to dislocation blockage-dominated plastic deformation leads to strengthening when the primary twin spacing L1 is between the first critical primary twin spacing and second critical primary twin spacing and in the strengthening stage. (3) Partial dislocations parallel to the twin boundaries (TBs), detwinning and partial dislocation motion-induced thickening/thinning of TBs result in the second softening stage when the primary twin spacing L1 is smaller than the second critical primary twin spacing .

Effects of Surface Stress on the Phonon Properties in GaN Nanofilms

This work investigates the phonon properties such as phonon dispersion relation, average group velocity, and phonon density of state (DOS) theoretically in GaN nanofilm under various surface stress fields. By taking into account of the surface energy effects, the elasticity theory is presented to describe the confined phonons of nanofilms with different surface stresses. The calculation results show that the influence of surface stress on the phonon properties depends on the thickness of nanofilm. The negative surface stress leads to a higher average group velocity and corresponding lower phonon DOS. The positive surface stress has the opposite effect. The significant modification of thermal properties, e.g., phonon thermal conductivity, in GaN nanofilms is mostly stemmed from the change of phonon average group velocity and DOS by surface stress. These results suggest that the thermal or electrical properties in GaN nanofilms could be enhanced or reduced by tuning the surface stress acting on the films.

Nature-Inspired Hierarchical Steels

Materials can be made strong, but as such they are often brittle and prone to fracture when under stress. Inspired by the exceptionally strong and ductile structure of byssal threads found in certain mussels, we have designed and manufactured a multi-hierarchical steel, based on an inexpensive austenitic stainless steel, which defeats this “conflict” by possessing both superior strength and ductility. These excellent mechanical properties are realized by structurally introducing sandwich structures at both the macro- and nano-scales, the latter via an isometric, alternating, dual-phase crystal phases comprising nano-band austenite and nano-lamellar martensite, without change in chemical composition. Our experiments (transmission and scanning electron microscopy, electron back-scattered diffraction, nano-indentation and tensile tests) and micromechanics simulation results reveal a synergy of mechanisms underlying such exceptional properties. This synergy is key to the development of vastly superior mechanical properties, and may provide a unique strategy for the future development of new super strong and tough (damage-tolerant), lightweight and inexpensive structural materials.

Tensile Failure Modes in Nanograined Metals with Nanotwinned Regions

Nanotwinned (NT) regions can compensate the lower ductility of nanograined (NG) matrix so that NG metals with NT regions can achieve high strength and modest ductility. Main factors affecting the strength and ductility of the NG metals with NT regions have not been systematically and numerically investigated. Based on the strain gradient plasticity and Johnson–Cook failure criterion, computer simulations are carried out to clarify the effects of twin spacing together with shape and distribution of NT regions on their strength and ductility. Our calculations indicate that these attributes have significant effects on the overall ductility. In particular, it is discovered that a critical twin spacing marks the reversal of the overall ductility, that is, the overall ductility decreases and then increases with the continuous increase of twin spacing. Compared with the circular NT regions, the square and oblique square ones are found to provide higher overall strength and ductility. For the circular and oblique square NT regions, array arrangement tends to perform better in strengthening and toughening, while for the square NT regions, staggered arrangement is advisable. We have also uncovered three distinct failure modes, including fracture of matrix, fracture of NT regions, and interface debonding. Furthermore, fracture of NT regions can enhance the overall ductility and lead to the reversal of the overall ductility. It is believed that this study has provided significant insights into the roles of twin spacing together with shape and distribution of NT regions on the overall strength and ductility of this novel class of metals.