[en] We investigate solute segregation and transition at grain boundaries and the corresponding drag effect on grain boundary migration. A continuum model of grain boundary segregation based on gradient thermodynamics and its discrete counterpart (discrete lattice model) are formulated. The model differs from much previous work because it takes into account several physically distinctive terms, including concentration gradient, spatial variation of gradient-energy coefficient and concentration dependence of solute-grain boundary interactions. Their effects on the equilibrium and steady-state solute concentration profiles across the grain boundary, the segregation transition temperature and the corresponding drag forces are characterized for a prototype planar grain boundary in a regular solution. It is found that omission of these terms could result in a significant overestimate or underestimate (depending on the boundary velocity) of the enhancement of solute segregation and drag force for systems of a positive mixing energy. Without considering these terms, much higher transition temperatures are predicted and the critical point is displaced towards much higher bulk solute concentration and temperature. The model predicts a sharp transition of grain boundary mobility as a function of temperature, which is related to the sharp transition of solute concentration of grain boundary as a function of temperature. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in both the concentration-temperature plot and the mobility-temperature plot. These predictions agree well with experimental observations

[en] A change in phase stability from hcp Zr to bcc Zr occurs in Nb/Zr multilayers when the bilayer thickness is reduced to the nanometer scale. This phase stability in these multilayers has been described using a model based on classical thermodynamics. Using a previously reported experimental observation of hcp to bcc transformation in these multilayers, a phase stability diagram (referred to as the biphase diagram) has been proposed. Subsequently, a range of multilayers with varying volume fractions and bilayer thicknesses have been sputter deposited. The crystal structures in these multilayers have been determined using X-ray and electron diffraction. The hcp and bcc Zr phases within the Nb/Zr multilayers are in agreement with the predictions afforded by the proposed biphase diagram. The sequence of the Zr bcc phase stability was accomplished by forming its β-Zr (high temperature bcc phase) prior to forming a bcc coherent interface with Nb. First approximations of the structural and chemical contributions to the interfacial energy accompanying the changes in hcp to bcc phase stability for Zr have been evaluated

[en] The microstructural development of Ti:LiNbO₃ optical waveguides as a function of annealing time and temperature was studied using transmission electron microscopy. The morphological evolution of the deposited Ti film can be characterized by three stages: oxidation beginning at low temperatures, coarsening and secondary grain growth of the oxide film at higher temperatures and eventual film breakup and void formation. Secondary grain growth is driven by minimization of interfacial energy of grains which have a special epitaxial relationship with respect to the LiNbO₃ substrate

[en] Highlights: • We have created a high pressure, high temperature material synthesis apparatus. • This apparatus is programmable and capable of various thermal analysis procedures. • Using our apparatus we have established the peritectic transition temperature of MgB₂. • This temperature is 300 °C lower than established values for the same pressure. • We have shown that this discrepancy is likely the result of impurities such as C. - Abstract: We have studied thermodynamic phase stability in the Mg–B system through use of a high-pressure, high-temperature apparatus consisting of a large pressure vessel and an RF induction heater. The incongruent melting temperature for MgB₂ was found to be ∼1450 °C at 10 MPa using thermal analysis data applied to both MgB₂ powder samples and Mg/B mixtures. The experimental temperature is ∼300 °C lower than temperatures shown in calculated phase diagrams of the Mg–B system at the same pressure and the discrepancy demonstrates the need for further experimental investigations of phase stability in binary Mg–B and ternary Mg–B–X systems. In this study C (as an impurity in boron) was found to have a large effect on the peritectic temperature, with a relatively small (0.7 wt% C) impurity concentration resulting in a ∼40 °C elevation of the peritectic temperature. Along with morphological characterization, XRD and EPMA analyses were used to identify phases and confirm the peritectic transformation in the Mg–B phase diagram