Orientation relations during the α-ω phase transition of zirconium: in situ texture observations at high pressure and temperature.

Transition metals Ti, Zr, and Hf have a hexagonal close-packed structure (α) at ambient conditions, but undergo phase transformations with increasing temperature and pressure. Of particular significance is the high-pressure hexagonal ω phase which is brittle compared to the α phase. There has been a long debate about transformation mechanisms and orientation relations between the two crystal structures. Here we present the first high pressure experiments with in situ synchrotron x-ray diffraction texture studies on polycrystalline aggregates. We follow crystal orientation changes in Zr, confirming the original suggestion by Silcock for an α→ω martensitic transition for Ti, with (0001)(α)||(1120)(ω), and a remarkable orientation memory when ω reverts back to α.

Deformation experiments in the diamond-anvil cell: texture in copper to 30 GPa.

The combination of the diamond-anvil cell, synchrotron x-ray diffraction in radial geometry and simultaneous Rietveld refinement with texture analysis allows us to quantitatively investigate the plastic deformation behaviour of materials at very high pressures. Our study of copper to 30 GPa shows in ideal experimental geometry a [110] fibre texture component, as is typical for axial compression of soft face centred cubic metals. Locally a plane strain texture develops which is energetically favoured (curling). A transition from compressional to plane strain/pure shear texture can be monitored by analysing individual images taken at different positions in the diamond cell.

Deformation textures produced in diamond anvil experiments, analysed in radial diffraction geometry.

Diamond anvil cells may not only impose pressure upon a sample but also a compressive stress that produces elastic and plastic deformation of polycrystalline samples. The plastic deformation may result in texture development if the material deforms by slip or mechanical twinning, or if grains have a non-equiaxed shape. In radial diffraction geometry, texture is revealed by variation of intensity along Debye rings relative to the compression direction. Diffraction images (obtained by CCD or image plate) can be used to extract quantitative texture information. Currently the most elegant and powerful method is a modified Rietveld technique as implemented in the software package MAUD. From texture data one can evaluate the homogeneity of strain in a diamond anvil cell, the strain magnitude and deformation mechanisms, the latter by comparing observed texture patterns with results from polycrystal plasticity simulations. Some examples such as olivine, magnesiowuestite, MgSiO(3) perovskite and ε-iron are discussed.

Crystallographic texture for tube and plate of the superelastic/shape-memory alloy Nitinol used for endovascular stents.

The superelastic/shape-memory material, Nitinol, an approximately equiatomic alloy of Ni and Ti, is rapidly becoming one of the most important metallic implant materials in the biomedical industry, in particular for the manufacture of endovascular stents. As such stents are invariably laser-machined from Nitinol tubes or sheets rolled into tubes, it is important to fully understand the physical phenomena that may affect the mechanical behavior of this material. With tubing and plate, one major issue is crystallographic texture, which can play a key role in influencing the mechanical properties of Nitinol. In this article, we present a study on how geometry and heat treatment can affect the texture of Nitinol, with specific quantification of the texture of Nitinol tube used for the production of endovascular stents.

The plastic deformation of iron at pressures of the Earth's inner core

Soon after the discovery of seismic anisotropy in the Earth's inner core, it was suggested that crystal alignment attained during deformation might be responsible. Since then, several other mechanisms have been proposed to account for the observed anisotropy, but the lack of deformation experiments performed at the extreme pressure conditions corresponding to the solid inner core has limited our ability to determine which deformation mechanism applies to this region of the Earth. Here we determine directly the elastic and plastic deformation mechanism of iron at pressures of the Earth's core, from synchrotron X-ray diffraction measurements of iron, under imposed axial stress, in diamond-anvil cells. The epsilon-iron (hexagonally close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity theory predicts an alignment of c-axes parallel to the compression direction as a result of basal slip, if basal slip is either the primary or a secondary slip system. The experiments provide direct observations of deformation mechanisms that occur in the Earth's inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.