R.T. Qu

Elasticity dominates strength and failure in metallic glasses

Two distinct deformation mechanisms of shearing and volume dilatation are quantitatively analyzed in metallic glasses (MGs) from the fundamental thermodynamics. Their competition is deduced to intrinsically dominate the strength and failure behaviors of MGs. Both the intrinsic shear and normal strengths give rise to the critical mechanical energies to activate destabilization of amorphous structures, under pure shearing and volume dilatation, respectively, and can be determined in terms of elastic constants. By adopting an ellipse failure criterion, the strength and failure behaviors of MGs can be precisely described just according to their shear modulus and Poissons ratio without mechanical testing. Quantitative relations are established systematically and verified by experimental results. Accordingly, the real-sense non-destructive failure prediction can be achieved in various MGs. By highlighting the broad key significance of elasticity, a “composition-elasticity-property” scheme is further outlined for better understanding and controlling the mechanical properties of MGs and other glassy materials from the elastic perspectives.

Anisotropic mechanical behaviors and their structural dependences of crossed-lamellar structure in a bivalve shell

Crossed-lamellar structure is one of the most common organizations found in mollusk shells and may serve as a natural mimetic model for designing bio-inspired synthetic materials. Nonetheless, the mechanical behaviors and corresponding mechanisms have rarely been investigated for individual macro-layer of such structure. The integrated effects of orientation and hydration also remain unclear. In this study, the mechanical behaviors and their structural dependences of pure crossed-lamellar structure in Saxidomus purpuratus shell were systematically examined by three-point bending and compression tests. Mechanical properties and fracture mechanisms were revealed to depend strongly on the orientation, hydration state and loading condition. Three basic cracking modes of inter-platelet, trans-platelet, and along the interfaces between first-order lamellae were identified, and the interfacial separation was enhanced by hydration. Macroscopic compressive fracture was accomplished through axial splitting during which multiple toughening mechanisms were activated. The competition among different cracking modes was quantitatively evaluated by analyzing their driving stresses and resistances from fundamental mechanics. This study helps to clarify the mechanical behaviors of naturally occurring crossed-lamellar structure, and accordingly, aids in designing new bio-inspired synthetic materials by mimicking it.

Precisely predicting and designing the elasticity of metallic glasses

We reveal that the elastic moduli of metallic glasses (MGs) invariably vary in a much steeper manner than that predicted by the conventional “rule of mixtures” in individual alloy systems. Such deviations are proved to originate fundamentally from their disordered atomic structures and intrinsic local heterogeneities. By treating the MGs as atomic-level dual phase hybrids, we further propose universal relations to be capable of precisely predicting and designing the elastic constants of MGs. This may contribute to the development of MGs with intended properties and behaviors, and allow new understandings on the structures and properties as well as their relationships in MGs.

Intrinsic Strength Asymmetry Between Tension and Compression of Perfect Face-Centered-Cubic Crystals

The strength asymmetry between tension and compression is a typical case of mechanical response of materials. Here we achieve the intrinsic strength asymmetry of six face-centered-cubic perfect crystals (Cu, Au, Ni, Pt, Al and Ir) through calculating the ideal tensile and compressive strength with considering the normal stress effect and the competition between different crystallographic planes. The results show that both the intrinsic factors (the ideal shear strength and cleavage strength of low-index planes) and the orientation could affect the strength asymmetry, which may provide insights into understanding the strength of ultra-strong materials.

Anisotropy of tensile strength and fracture mode of perfect face-centered-cubic crystals

This study presents an effective method to calculate the ideal tensile strength of six face-centered-cubic (fcc) crystals (Cu, Au, Ni, Pt, Al, and Ir) along an arbitrary tensile direction by considering the coupling effect of normal stress and shear stress on a given crystallographic plane. Meanwhile, the fracture modes of the six crystals can also be derived from the competition between shear and cleavage fracture along different crystallographic planes. The results show that both the intrinsic factors (the ideal shear strength and cleavage strength of low-index planes) and the orientation may affect the tensile strength and fracture modes of ideal fcc crystals, which may give the reliable strength limit of fcc metals and well interpret the observed high strength in nano-scale mechanical experiments.