The development of a twinned microstructure in hexagonal close-packed rolled magnesium compressed in the in-plane direction has been monitored in situ with neutron diffraction. The continuous conversion of the parent to daughter microstructure is tracked through the variation of diffraction peak intensities corresponding to each. Approximately 80% of the parent microstructure twins by 8% compression. Elastic lattice strain measurements indicate that the stress in the newly formed twins (daughters) is relaxed relative to the stress field in the surrounding matrix. However, since the daughters are in a plastically “hard” deformation orientation, they quickly accumulate elastic strain as surrounding grains deform plastically. Polycrystal modeling of the deformation process provides insight about the crystallographic deformation mechanism involved.

Internal strain and texture evolution during deformation twinning in magnesium

A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Neutron diffraction methods for the study of residual stress fields

The development of high grain-to-grain interactions in Zircaloy-2 polycrystals is due to the anisotropy of the thermal, elastic and plastic properties of the constituent grains. These residual stresses modify the mechanical performance of the materials as well as its response under neutron irradiation. Here we simulate the evolution of internal stresses in Zircaloy-2 with rod texture, when the aggregate is cooled down from the annealing temperature, and also when it is subjected to tensile and compressive deformation. We implement for that purpose an elasto-plastic self-consistent scheme that accounts for grain interactions, and compare our predictions with experimental determinations of residual strains obtained by neutron diffraction. Our results are in good quantitative agreement with the experimental evidence, and represent a qualitative improvement over the predictions of the more rigid upper-bound approach. We discuss the role that slip and twinning systems play in the macroscopic response of the aggregate and in the evolution of internal stresses.

A study of residual stresses in Zircaloy-2 with rod texture

Internal stresses in tension and compression in an extruded and artificially aged Mg-7.7at.% Al alloy have been determined by in-situ neutron diffraction. Measurements were made on grains having the c axis normal to, parallel to and at 62° from the stress axis, and on the reinforcing phase. The results are consistent with basal slip and {1012} twinning. The second-phase particles of Mg17Al12 bear much higher stresses than the magnesium matrix does. A simple mean stress model proposed by Brown and Clarke is shown to describe adequately the strengthening due to the particles. Twinning manifests itself clearly through variations in the integrated intensity of diffraction peaks during loading. Most of the observed variations in scattered peak intensity can be explained by referring to the lattice reorientation produced by {1012} twinning. The calculated stress tensors corresponding to these intensity variations have been used to show that a critical resolved shear stress criterion is applicable for {...

J. Faber

Papers

The use of time-of-flight neutron diffraction to study grain interaction stresses

DILATION OF h.c.p. ZIRCONIUM BY INTERSTITIAL DEUTERIUM

The influence of texture on the interpretation of diffraction data to determine residual stress