Non-uniform Solute Segregation at Semi-Coherent Metal/Oxide Interfaces.

The properties and performance of metal/oxide nanocomposites are governed by the structure and chemistry of the metal/oxide interfaces. Here we report an integrated theoretical and experimental study examining the role of interfacial structure, particularly misfit dislocations, on solute segregation at a metal/oxide interface. We find that the local oxygen environment, which varies significantly between the misfit dislocations and the coherent terraces, dictates the segregation tendency of solutes to the interface. Depending on the nature of the solute and local oxygen content, segregation to misfit dislocations can change from attraction to repulsion, revealing the complex interplay between chemistry and structure at metal/oxide interfaces. These findings indicate that the solute chemistry at misfit dislocations is controlled by the dislocation density and oxygen content. Fundamental thermodynamic concepts ­ the Hume-Rothery rules and the Ellingham diagram ­ qualitatively predict the segregation behavior of solutes to such interfaces, providing design rules for novel interfacial chemistries.

Shock-induced phase transformation in tantalum.

A TEM study of pure tantalum and tantalum-tungsten alloys explosively shocked at a peak pressure of 30 GPa is presented. While no omega phase was found in shock-recovered pure Ta and Ta-5W which mainly contain a cellular dislocation structure, a shock-induced omega phase was found in Ta-10W which contains evenly distributed dislocations with a density higher than 1 × 10(12) cm( - 2). The shock-induced [Formula: see text] (hexagonal) transition occurs when the dynamic recovery of dislocations becomes largely suppressed in Ta-10W shocked under dynamic-pressure conditions. A dislocation-based mechanism is proposed for the shock-induced phase transformation.

Deformation twinning in a creep-deformed nanolaminate structure.

The underlying mechanism of deformation twinning occurring in a TiAl-(γ)/Ti(3)Al-(α(2)) nanolaminate creep deformed at elevated temperatures has been studied. Since the multiplication and propagation of lattice dislocations in both γ and α(2) thin lamellae are very limited, the total flow of lattice dislocations becomes insufficient to accommodate the accumulated creep strains. Consequently, the movement of interfacial dislocations along the laminate interfaces, i.e., interface sliding, becomes an alternative deformation mode of the nanolaminate structure. Pile-ups of interfacial dislocations occur when interfacial ledges and impinged lattice dislocations act as obstacles to impede the movement of interfacial dislocations. Deformation twinning can accordingly take place to relieve a stress concentration resulting from the pile-up of interfacial dislocations. An interface-controlled twinning mechanism driven by the pile-up and dissociation of interfacial dislocations is accordingly proposed.

Depth-dependent mechanical properties of enamel by nanoindentation.

Nanoindentation has recently emerged to be the primary method to study the mechanical behavior and reliability of human enamel. Its hardness and elastic modulus were generally reported as average values with standard deviations that were calculated from the results of multiple nanoindentation testing. In such an approach, it is assumed that the mechanical properties of human enamel are constant, independent of testing parameters, like indent depth and loading rate. However, little is known if they affect the measurements. In this study, we investigated the dependence of the hardness and elastic modulus of human enamel on the indent depth. We found that in a depth range from 100 to 2000 nm the elastic moduli continuously decreased from approximately 104 to 70 GPa, and the hardnesses decreased from approximately 5.7 to 3.6 GPa. We then considered human enamel as a fiber-reinforced composite, and used the celebrated rule of mixture theory to quantify the upper and lower bounds of the elastic moduli, which were shown to cover the values measured in the current study and previous studies. Accordingly, we attributed the depth dependence of the hardness and modulus to the continuous microstructure evolution induced by the nanoindenter tip.

Dislocation multi-junctions and strain hardening.

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.