ar X iv : 0 70 7 . 00 56 v 1 [ co nd - m at . o th er ] 3 0 Ju n 20 07 The equation of state of solid nickel aluminide

The pressure-volume-temperature equation of state of the intermetallic compound NiAl was calculated theoretically, and compared with experimental measurements. Electron ground states were calculated for NiAl in the CsCl structure, using ab initio pseudopotentials and density functional theory (DFT), and were used to predict the cold compression curve and the density of phonon states. It was desirable to interpolate and smooth the cold compression states; the Rose form of compression curve was found to reproduce the ab initio calculations well in compression but exhibited significant deviations in expansion. A thermodynamically-complete equation of state was constructed for NiAl, which overpredicted the mass density at standard temperature and pressure (STP) by 4%, fairly typical for predictions based on DFT A minimally-adjusted equation of state was constructed by tilting the cold compression energy-volume relation by ∼7GPa to reproduce the observed STP mass density. Shock waves were induced in crystals of NiAl by the impact of laser-launched Cu flyers and by launching NiAl flyers into transparent windows of known properties. The TRIDENT laser was used to accelerate the flyers, 5mm in diameter and 100 to 400μm thick, to speeds between 100 and 600m/s. Point and line-imaging laser Doppler velocimetry was used to measure the acceleration of the flyer and the surface velocity history of the target. The velocity histories were used to deduce the stress state, and hence states on the principal Hugoniot and the flow stress. Flyers and targets were recovered from most experiments. The effect of elasticity and plastic flow in the sample and window was assessed. The ambient isotherm reproduced static compression data very well, and the predicted Hugoniot was consistent with shock compression data.