Author: Wolfgang Schmahl

Abstract
One of the major advantages of neutron diffraction over competing technologies is the ease of construction of sample environments and the possibility to sample sufficiently representative sample volumes of bulk samples which are penetrated by the beam. The use of photons, on the other hand, suffers from absorption and - notably for highly collimated beams - from grain statistics. Thus many natural and engineering materials are not well-suited for x-ray diffraction in their actual state, as they are too coarse grained and/or diffraction information is limited to near-surface regions, which are influenced by sample preparation. Neither for rocks nor for engineering materials “making a powder” or “making a single crystal” is an option if real materials properties need to be studied.
For example, considering materials in stress fields, such as deforming rocks or engineering materials in stress/strain testing or other mechanical load, the materials response is naturally anisotropic. Rocks and most advanced engineering materials are polycrystalline, textured multi-phase systems, and many of those phases are of low-symmetry or have large-unit-cells. The power of the Rietveld method has made it possible to investigate those materials by diffraction, due to its elegant way of treating many overlapping peaks. Conventional constant wavelength diffractometers, however, use detectors in one particular scattering plane. Due to the diffraction condition, the longitudinal response of a material to uniaxial load can be measured only for one particular diffraction peak at a time, which makes it almost unfeasible to do such an experiment on a complex material, where e.g. load transfer from one phase to another during plastic yielding, domain switching or phase transitions is associated with simultaneous changes in texture (as will be demonstrated with the example of shape memory alloys). Here only full diffraction profile measurement offers sufficient information to recognize these effects.
A TOF diffractometer offers the advantage that using two detectors, both the longitudinal and the transverse effects can be measured simultanously without moving the sample (which always has detrimental effects on the stability of temperature, stress, etc.) in full diffraction profiles if the direction of the field is applied at 45° to the incoming beam. A detector ring is capable of logging the materials response in many directions with respect to the field. Moreover, while for TOF the orientation of each observed lattice plane is constant with respect to the field, it does vary in constant wavelength experiments within a diffractogram for any near-longitudinal configuration.

The new ‘Osthalle’ at FRM-II offers the chance to build a new TOF-diffractometer for in-situ materials experiments both for the geomaterials and the engineering materials science community. If a flight path of about 35 m can be realized, resolution an flux should be indeed sufficient for the study of the anisotropy of materials response to applied fields, which require parametric measurement series.
The concept is complementary to the constant-wavelength diffractometers STRESS-SPEC and SPODI at FRM-II.

Author Description
Wolfgang W. Schmahl

Sektion Kristallographie, Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians Universität München, Theresienstr. 41, D-80333 München