The development of an X-ray-diffraction strain-measurement technique and its application to micromechanical-deformation modes

When a material is subjected to an external load, the resultant mechanical deformation is a complex three-dimensional response function best captured by a second-rank strain tensor. In addition the resultant deformation is comprised of several micro-mechanical modes. Traditionally these modes are classified as reversible (elastic) or irreversible (plastic), but in fact there are several more subclasses of micromechanical deformation and to understand the mechanical response of an advanced material (such as superelastic Nitinol) it is often necessary to partition the global (macromechanical) deformation into its constituent micromechanical modes.;The three-dimensional mechanical response of a component under external load can be visualized as an ellipsoidal distortion of a spherical surface centered at that point. The local strain in any direction is then measured as a deviation from sphericity. Traditional strain measurements, which depend on measuring the deformation of an externally applied one or two dimensional gauge, can only sample a limited portion of the strain ellipsoid. Further, the spatial resolution of these measurements is several mm and it is impossible to distinguish between the different modes of deformation. A technique for obtaining the deviatoric portion of the second-rank strain tensor from white-beam Laue diffraction is well established when the crystallite size of the material is comparable or larger than the beam size. However, because of extreme difficulty in obtaining a nanometer beam size with sufficient flux this technique fails for nanocrystalline materials.;In this dissertation, I show the derivation for a multiwavelength focused-beam x-ray powder diffraction technique using an area detector that overcomes all of the shortcomings of traditional strain determination in nanomaterials and yields the full (deviatoric + hydrostatic) second-rank strain tensor. I apply this technique to characterize the deformation behavior of superelastic Nitinol which allows the exploration of the different micromechanical deformation modalities associated with this 'smart' material. I also give a brief view of the anisotropie effects in nanocrystalline materials and the steps required to predict deformation behavior.