From Social Relevances to Design Issues

From Social Relevances to Design Issue

Brittle-to-Ductile Transition in Metallic Glass Nanowires

When reducing the size of metallic glass samples down to the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure

Ideal shear banding in metallic glass

<p>As the most fundamental deformation mechanism in metallic glasses (MGs), the shear banding has attracted a lot of attention and interest over the years. However, the intrinsic properties of the shear band are affected and even substantially changed by the influence of non-rigid testing machine that cannot be completely removed in real compression tests. In particular, the duration of the shear banding event is prolonged due to the recovery of the stressed compliant frame of testing machine and therefore the temperature rise at the operating shear band is, more or less, underestimated in previous literatures. In this study, we propose a model for the ‘ideal’ shear banding in metallic glass. The compliance of the testing machine is eliminated, and the intrinsic shear banding process is extracted and investigated. Two important physical parameters, the sliding speed and the temperature of shear band, are calculated and analysed on the basis of the thermo-mechanical coupling. Strain-rate hardening is proposed to compensate thermal softening and stabilise the shear band. The maximum value of the sliding speed is found to be on the order of 10 m/s at least, and the critical temperature at which strain-rate hardening begins to take effect should reach as high as 0.9<i>T</i>_g (<i>T</i>_g is the glass transition temperature) for a stable shear banding event in metallic glass according to the early experimental data. This model can help to understand and control the shear banding and therefore the deformation in MGs.</p

New Lithium Copper Borates with BO₃ Triangles: Li₆CuB₄O₁₀, Li₃CuB₃O₇, Li₈Cu₇B₁₄O₃₂, and Li₂Cu₉B₁₂O₂₈

Crystal structures of three new lithium copper borates, Li₃CuB₃O₇, Li₈Cu₇B₁₄O₃₂, and Li₂Cu₉B₁₂O₂₈, and a new Li₆CuB₄O₁₀ polymorph were solved by single-crystal X-ray diffraction. In all of the structures, the boron cations form BO₃ triangles, which are connected with each other and with copper polyhedra only via corners in Li₆CuB₄O₁₀ and Li₃CuB₃O₇ and via both corners and edges in Li₈Cu₇B₁₄O₃₂ and Li₂Cu₉B₁₂O₂₈. The Li₃CuB₃O₇ and Li₈Cu₇B₁₄O₃₂ compounds were synthesized as pure samples with only trace amounts of impurities; hence, their magnetic properties could be investigated and analyzed in terms of underlying magnetic couplings. Other compositions always represented multiphase mixtures. Li₃CuB₃O₇ features infinite Cu,O chains formed by Cu₂O₆ units consisting of edge-shared CuO₄ squares. Together with two apical oxygen atoms with long interatomic Cu–O distances of 2.7–2.8 Å, the Cu₂O₆ units form chains extended along the <i>a</i> axis. These pseudochains are responsible for strong anisotropic thermal expansion behavior. The temperature dependence of the magnetization between 4 and 380 K for Li₃CuB₃O₇ could be fit well by a spin-dimer model. The magnetic susceptibility of Li₈Cu₇B₁₄O₃₂ showed a more complex temperature dependence, with two different Curie–Weiss regimes in the temperature range of 2–380 K

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

<p>The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_{65–<i>x</i>}Cu₂₅Al₁₀Fe_<i>x</i> (<i>x</i> = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.</p