Elastic and plastic deformation of NaCl and Ni3Al polycrystals during compression in a multi-anvil apparatus

The compression behaviour in a multi-anvil apparatus of pure NaCl and of a foil of Ni3Al embedded in a pressure medium of NaCl has been studied by ene rgy-dispersive Xray diffraction. At ambient temperature, the pressure and s tresses, determined from line positions of NaCl, were constant throughout t he sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. A saturation of microstrains observed in Na Cl at 2 GPa is thus attributed to brittle fracture setting in at uniaxial s tresses of around 0.3 GPa. Ni3Al polycrystals, in contrast, undergo extensi ve (ductile) plastic deformation above 4 GPa. The compression behaviour of both Ni3Al and NaCl is identical to that previously determined in a diamond anvil cell. While a multi-anvil device thus has the advantage, compared wi th a diamond anvil cell, of constant pressure and stress throughout the sam ple chamber, microstrains in polycrystalline samples arise in both devices. Samples in a multi-anvil apparatus thus need to be mixed with a pressure m edium and to consist of essentially single crystals just as in a diamond an vil cell. Annealing experiments at high pressures confirm that the release of the uniaxial stress component in the pressure medium does not cause a re lease of microstrains in the embedded sample if the latter has been plastic ally deformed. Annealing for the purpose of attaining hydrostatic condition s in compression studies thus has to be carried out with care.