Mapping of unstressed lattice parameters using pulsed neutron transmission diffraction

Stress measurement by neutron diffraction depends critically on knowledge of the unstressed lattice parameter (a₀) of the specimen under study. As a result, measurement of stress profiles in components where a₀ is not homogeneous throughout the sample, such as welds or carburized surfaces, can be particularly difficult. An efficient solution to this problem is proposed based on the pulsed neutron transmission diffraction technique. This technique exploits the sharp steps in intensity, the so-called Bragg edges, appearing in the transmitted neutron spectra of polycrystalline materials, such steps being produced by coherent scattering from lattice planes. The position of these Bragg edges as defined by the time-of-flight technique is used to determine precisely local interplanar distances. In this work it is shown that the unstressed lattice parameter of thin specimens subjected to plane stress fields can be defined by recording transmission spectra at different sample inclinations, in complete analogy with the sin²ψ technique used in X-ray diffraction. Moreover, by using an array of detectors it is possible to produce a radiographic `image' of a₀ for plane specimens or thin sections out of three-dimensional ones. The capability of the technique is exemplified by mapping the changes in a₀ for a ferritic weld that was used as a round robin sample in an international program for standardization of stress measurements by neutron diffraction.