Plastic Deformation and the Role of Fault Formation in the Equation of State of Micron Size Intermetallic Alloys Under N
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Plastic Deformation and the Role of Fault Formation in the Equation of State of Micron Size Intermetallic Alloys Under Non-Hydrostatic Pressure John K. Vassiliou1, J. W. Otto2, G. Frommeyer3, J. J. Davis1 and P. Pinto1 1 Department of Physics, Villanova University, Villanova, PA 19085 2 Joint Research Center for the European Commission, Brussels, Belgium 3 MPI Eisenforschung, 40237 Dusseldorf, Germany ABSTRACT The elastic and plastic deformation of micron size anisotropic polycrystals of Ni3Al and Cu3Au intermetallic alloys have been studied under non-hydrostatic conditions by energy-dispersive X-ray diffraction (EDX) in a diamond-anvil cell. Compression was achieved by confining the samples in a viscous fluid or directly between the diamond anvils. Deviatoric forces are introduced in the samples as a result of the increasing viscosity with pressure and the eventual glassification of the pressurizing medium or by the contact forces of the diamond anvils. Line shifts and line profiles were used to analyze elastic and plastic strains. Plastic deformation is due to the onset of nonhydrostatic stresses and the introduction of stacking faults and dislocations. A volume incompressibility due to plastic deformation and the saturation of the stacking fault probability is followed by an elastic compression of a fully plastically deformed state. The compression of this state is isotropic and independent of the presence and type of the pressurizing medium. From the measured strains at different crystallographic orientations, the uniaxial stress and the stacking fault probability as a function of the confining pressure are derived and their role in the equation of state is examined. Using finite elasticity, the equation of state is derived in the presence of uniaxial stresses causing stacking faults, defects and dislocations. INTRODUCTION In high-pressure experiments using a diamond anvil cell (DAC), the pressurizing medium is a viscous liquid or a plastically deformable solid totally surrounding the sample. With increasing pressure, deviatoric stresses are induced in the sample because of the progressive increase of the fluid viscosity or the mismatch of the shear moduli of the sample and the pressure-transmitting medium. In a polycrystalline elastically anisotropic sample additional deviatoric stresses develop at the grain boundaries due to the local pressure gradients. The maximum deviatoric stress can be developed when a polycrystalline sample is compressed directly between the diamond cell faces. The uniaxial stresses are equal to a small fraction of the nominally hydrostatic pressure and are functions of the hydrostatic strain. When the uniaxial stresses exceed the yield strength of the material, it plastically deforms by slip, forming stacking faults and dislocations. The yield strength of a polycrystalline material is higher than the yield strength of a crystal and increases substantially with decreasing grain size1-3. Most solids plastically deform at rather low uniaxial stresses. Such deformations may occur e
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