Elastic phase transitions in solids. High pressure effect

At high pressures (the pressure is comparable with the bulk modulus) the crystalline lattice may become unstable relative to the uniform shear deformations, and in a result the low symmetric crystalline structures will appear (the so-called “elastic phase transitions”). The order parameters at these transitions are the components of the finite deformations tensor. The stability of the high-pressure phases is defined by the nonlinear elasticity of the lattice (the third, fourth etc. order elastic constants). Here the different cases of the stability loss at hydro-static pressure for the cubic structures are considered. The relation between the second, third and fourth order elastic constants is given, which defines the possibility of the first order deformation phase transition. The jump of the order parameter and the height of the potential barrier are defined by the third and fourth order elastic constants. As an example, the experimentally observed elastic phase transition in vanadium at P ≈ 69 GPa from bcc to the rhombohedral phase is analyzed, and the possible structural transitions in bcc Mo and W at P ≥ 700 GPa are also considered

Effect of Re content on elastic properties of B2 NiAl from ab initio calculations

The effect of substitutional alloying of Re on elastic properties of B2 NiAl has been studied using first-principles electronic-structure calculations by the exact muffin-tin orbitals method and the coherent potential approximation. Our calculations have shown that elastic constants C-12, C-44 and bulk modulus B of (Ni1-xRex) Al alloys increase with Re composition almost linearly, but concentration dependence of elastic constants C-11, Young modulus E, shear modulus G, G/B ratio and the Cauchy pressure P-C is strongly nonmonotonously and has peculiarities near the concentration x = 30 at.% Re. Analyzing the density of states and Fermi surface sections we have a direct connection between the behavior of the elastic constants of (Ni1-xRex) Al alloys and changes in the interatomic bonding and Fermi surface topology.Funding Agencies|RFBR|10-02-00-194a|Goran Gustafsson Foundation for Research in Natural Sciences and Medicine||Swedish Research Council (VR)||Swedish Foundation for Strategic Research (SSF) Program in Materials Science for Nanoscale Surface Engineering||</p