Hot creep of single crystals by bending: procedure, theory and pilot experiment on enstatite
Carrying out hot creep experiments by three-point bending of thin single crystals has three main advantages: (1) it permits plastic deformation in a soft machine under very small loads; (2) the forces and resolved stresses are well defined relative to the glide systems in oriented crystal bars; and (3) the deformation can be analysed without perturbing preparations by X-ray diffraction topography. Thus a complete creep experiment consists of strain and strain rate measurements during high temperature deformation followed by separate X-ray diffraction studies. The three-point bending rig is operated in high vacuum or controlled ambient atmospheres. Creep strains are determined with a displacement transducer from the central deflections of the crystal plate. Variable loads corresponding to resolved shear stresses, computed for chosen orientations of the glide systems, are applied electromagnetically to the central knife edge. Load or stress can be maintained constant by on-line feedback control. Elastic and plastic beam theory are used to obtain creep strains and strain rates from the maximum bending deflections. Calibration of the temperature dependence of the elastic deflections makes it possible to isolate elastic and plastic strains and to determine exact strain rates as functions of load, stress and time. The distributions of lattice strains, rotations, and dislocation concentrations identifiable with different creep stages are analysed by X-ray diffraction topography of the bent crystal plate in its entirety. Sets of images of both translation and section topographs as well as intensity scans are taken using different Bragg reflections. Particularly attractive features of the interpretation of creep experiments by bending, illustrated by the pilot experiment on enstatite, are: (a) the power n obtained for the strain dependence on the stress (in work-hardening) or the strain rate dependence on the stress (in steady flow) is identical to the power n describing the curve of the plastically deflected beam as a function of beam dimensions and load; (b) elastic and plastic strains are directly obtainable from the beam curvature; and (c) for single crystals of known orientations the plastic lattice curvature is related to the dislocation density.
|Journal||Physics of the Earth and Planetary Interiors|
Schloessin, H.H. (H. H.), & Ranalli, G. (1988). Hot creep of single crystals by bending: procedure, theory and pilot experiment on enstatite. Physics of the Earth and Planetary Interiors, 52(1-2), 132–149. doi:10.1016/0031-9201(88)90062-3