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Deformation of Ni3Al Polycrystals at Extremely High Pressures John K.Vassiliou1, J.W. Otto2, G. Frommeyer3, A. J. Viescas1, K. Bulusu1 and H. Bellumkonda1 Dept. Physics, Villanova University, Villanova, PA 19085, USA 2 Joint Research Center of the European Commission, Brussels, Belgium 3 MPI Eisenforschung, 40237 Dusseldorf, Germany Corresponding author: [email protected] 1

ABSTRACT The compression behavior in a multi-anvil apparatus of a foil of Ni3Al embedded in a pressure medium of NaCl has been studied by energy-dispersive X-ray diffraction (EDX). At ambient temperature, the pressure and stresses, determined from line positions of NaCl, were constant throughout the sample chamber. Line positions and line widths of NaCl reflections were reversible on pressure release. Ni3Al polycrystals, in contrast, undergo extensive (ductile) plastic deformation above 4 GPa due to the onset of high non-hydrostatic stresses and the introduction of stacking faults and dislocations. Plastic deformation due to stacking faults leads to a volume incompressibility followed by elastic compression of a fully plastically deformed state. The compression of a fully plastically deformed material is elastic and isotropic, independent of the presence and type of pressure medium. A discontinuity in the compressibility at the transition back from plastic to elastic compression is due to the yield strength of the plastically deformed material and corresponds to the Hugoniot elastic limit. INTRODUCTION Deviatoric stresses in high-pressure experiments can develop because of a non-isotropic macroscopic stress field set-up by the pressure device, substantial viscosity of the pressure transmitting medium or local stresses set up at the grain boundaries of a polycrystalline elastically anisotropic sample. For ductile fcc-based metals at ambient conditions, the critical resolved shear stress for the glide of dislocations in a single crystal, causing plastic deformation by slip, is of the order of only 10-4-10-3 GPa . The yield stress in polycrystals is higher than this value by a factor of 3 for fcc and bcc based materials and may increase substantially with decreasing grain size1. Although the pressure effect on the yield stress is controversial2, stresses of only a small fraction of the applied nominally hydrostatic pressure are required to cause plastic deformation of many materials. Such stresses may occur even in some liquid pressure media. The general compression behavior of ductile elastically anisotropic polycrystals under increasing non-hydrostatic stress has recently been studied in detail with energy-dispersive X-ray diffraction of a foil of disordered Cu3Au in a diamond anvil cell3,4. The sample was chosen to be a foil in order to illustrate the limiting case of non-ideal powders in which significant microstrains develop under compression. It was found that an initial region of elastic compression is followed by a pressure region over which the volume does not change, terminated by a discontinuity in the compression curve. Elast