From Elasticity to Plasticity with a Sinusoidal Stress Field at High P and T

The D-DIA has proven a successful workhorse, high - pressure deformation apparatus capable of providing stress uncertainties of as low as 10 MPa at pressures of 10 GPa and 1600C. The standard mode of operation is to commence a constant strain rate once the pressure and temperature are set. Stress is monitored with time as is the strain. Experiments can last over 24 hours with strains in excess of 50%. Not only is the stress v. strain rate derivable from the data, but also intensities of individual diffraction peaks as a function of angle from the principal stress and the stress supported by different orientations of the crystallites. These data bear directly on the dislocation system that is operating in the sample. The deformation pistons in the D-DIA can be driven in both positive and negative directions. The gives the possibility of applying time varying stress fields to the samples. Here we report a wide range of experiments with a sinusoidal stress field. Small amplitudes of piston displacement may leave the sample in the elastic/anelastic linear regime. Strains of magnitude of 10**-5 can be measured using imaging techniques. While these strains are at the upper limit of expected elastic response, they can provide insights into the behavior of Q at high pressure and temperature. Larger piston displacements bring us into the region of transient creep, and even larger strains encounter full scale plastic behavior. Examples of each of these regions will be examined. Time dependent amplitude of diffraction peaks yields information about developing LPO in the sample, time dependent peak - dependent stress speaks to the dislocation process operating in the sample.