Three-dimensional residual stress mapping of an aluminum 2024-T3 arcan specimen, butt-welded by the friction stir technique, was performed by neutron diffraction. Results indicate that the residual stress distribution profiles across the weld region are asymmetric with respect to the weld centerline, with the largest gradients in the measured residual stress components occurring on the advancing side of the weld, with the longitudinal stress, σL, oriented along the weld line, as the largest stress. Within the region inside the shoulder diameter, the through-thickness stress, σZ, is entirely compressive, with large gradients occurring along the transverse direction just beyond the shoulder region. In addition, results indicate a significant reduction in the observed residual stresses for a transverse section that was somewhat closer to the free edge of an Arcan specimen. Microstructural studies indicate that the grain size in the weld nugget, is approximately 6.4 microns, with the maximum extent of the recrystallized zone extending to 6 mm on each side of the weld centerline. Outside of this region, the plate material has an unrecrystallized grain structure that consists of pancake shaped grains ranging up to several mm in size in two dimensions and 10 microns in through-thickness dimension.

Radial collimators have been recently introduced to de®ne the sampling volume during neutron diffraction stress and texture mapping experiments. This paper presents both analytical and Monte Carlo numerical models for the calculation of the spatial distribution of neutron transmission through a radial collimator. It is shown that the effective size of the scattered neutron beam as seen by detectors behind the collimator is quite sensitive to the collimator length and the number of blades. For a given radius of a collimator, the effective beam width increases sharply as the length is shortened. Due to the ®nite blade thickness, the center of gravity of the sampling volume is shifted away from the collimator. In contrast, attenuation of the neutron beam by the sample brings the center of gravity of the sampling volume closer to the collimator.

In a previous paper , J. Appl. Cryst. 34, 343± 357], the phase-space analysis of neutron imaging by Bragg re¯ection from thick bent perfect crystals or multi-wafer assemblies resulted in the derivation of various imaging conditions. An array of new applications becomes possible, including dispersive and non-dispersive neutron imaging at a sub-millimetre spatial resolution. This paper outlines the experimental test results on nondispersive imaging with thick packets of silicon wafers. The experimental results are compared with Monte Carlo simulations.