Two state model for critical points and the negative slope of the melting-curve

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states - one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed

Interface structure between Nb thin film and MgO(112) substrate: A first-principles prediction

The crystal orientation of ceramic substrates is an important factor affecting the interface structure of metal/ceramic composite materials. However, there is little information about the interface composed of metal films and ceramic substrates with a high-index plane. In this work, we predicted the interface structure between a Nb film and a MgO(112) substrate by calculating the interface separation works of different interface models by using the first-principles calculation method. The results showed that the preferred growth direction is Nb [120], and that the value of the interface separation work is 0.35 eV/Å2. The lattice mismatch between the film and substrate is less than 3%, implying that a coherent interface type is highly realizable in Nb/MgO(112). Furthermore, we analyzed the interface structures of Nb/MgO(100), Nb/MgO(110), Nb/MgO(111), and Nb/MgO(112) and found that the unique atomic configuration of the MgO substrate is the main factor determining the preferred interface structure of Nb/MgO

Collective nature of plasticity in mediating phase transformation under shock compression

An open question in the behavior of metals subjected to shock is the nature of the deformation that couples to the phase transformation process. Experiments to date cannot discriminate between the role of known deformation processes such as twinning or dislocations accompanying a phase change, and modes that can become active only in extreme environments. We show that a deformation mode not present in static conditions plays a dominant role in mediating plastic behavior in hcp metals and determines the course of the transformation. Our molecular dynamics simulations for titanium demonstrate that the transformation is preceded by a 90° lattice reorientation of the parent, and the growth of the reoriented domains is accompanied by the collective action of dislocations and deformation twins. We suggest how diffraction and transmission electron microscopy experiments may validate our findings.United States. Dept. of Energy (Contract DE-AC52-06NA25396