Thermodynamically complete equations of state for nickel-titanium alloy

A thermodynamically complete equation of state for the compression and heating of near-equiatomic Ni-Ti alloy in the CsCl (B2) structure was predicted, based on quantum-mechanical calculations of the electron ground states and a Gruneisen lattice-thermal model. The quantum-mechanical calculations used ab initio pseudopotentials and the local-density approximation; the accuracy of the calculations was investigated for elemental Ni and Ti. These calculations demonstrated that simple averaging techniques do not provide an accurate prediction of the properties of metal alloys, and rigorous treatment of the electron wave functions is needed. Predictions were also made of the behavior of NiTi under uniaxial loading. The pressure-density relation obtained from isotropic compression did not match the mean pressure calculated from uniaxial compression, demonstrating that it is not generally accurate to split the stress response of a material into a scalar equation of state and a stress deviator according to the usual prescription. Polycrystalline NiTi samples were prepared with a range of compositions, in the form of disks from 100 to 400 mu m thick and 5 mm in diameter. Flyer impact experiments were performed using a long-pulse laser drive at the TRIDENT facility to obtain shock wave data on the response of NiTi to around 15 GPa; the new data were consistent with the published results from gas gun experiments. The theoretical equation of state was consistent with the shock wave data. (c) 2005 American Institute of Physics.