Applications of Neutron Diffraction Measurements in the Characterization of the Mechanical Properties of Polycrystalline Geological Materials

Conventional deformation experiments on polycrystalline materials are restricted to measurements of whole sample properties. This is a significant limitation for problems where it is important to know how the deformation is accommodated at the grain scale in order to interpret the experimental results and compare them with theoretical treatments. Such problems include (a) characterizing the properties of elastically anisotropic materials, where it is helpful to know the elastic strain in different lattice directions of the constituent minerals, and how this varies with microstructural variables such as the lattice preferred orientation of those minerals; (b) characterizing the mechanical properties of polymineralic materials in terms of the properties of their constituent minerals, where it is helpful to know the contribution which each mineral phase makes to the whole rock properties during deformation and how this varies with microstructural variables such as the spatial distribution of those phases; (c) calibrating stress-induced crystallographic transformations (e.g., mechanical twinning) where it is important to monitor closely the initiation and progress of the transformation as a function of applied stress. By performing deformation experiments in-situ within neutron beam-lines and collecting neutron diffraction patterns at different applied loads, the lattice parameters of all the constituent minerals in the sample may be determined as a function of load. All the requisite information required to address the three problems above may then be obtained. The value of such an approach is much diminished if in obtaining the data, compromises have to be made in the quality of the mechanical measurements. This is particularly so if the diffraction data have to be collected either from small samples or from near surface parts of the sample because the interpretation of the mechanical data in such circumstances is notoriously difficult. In this respect, the penetrating nature of neutrons offers significant advantages over X-rays by permitting the interior of samples of the same size as used in conventional rock deformation experiments to be examined. An experimental procedure for carrying out such experiments has been developed at the ISIS neutron spallation source, Rutherford Appleton Laboratory, U.K., and has been applied successfully to both synthetic and natural samples containing a wide range of rock-forming minerals. The validity of the technique is demonstrated using results from uniaxial deformation experiments performed on olivine + magnesiowustite samples, and current progress in using it to address each of the three problems outlined above is described.