Study of 3-D stress development in parent and twin pairs of a hexagonal close-packed polycrystal: Part I – in-situ three-dimensional synchrotron X-ray diffraction measurement

Abstract

High anisotropy in the elastic and plastic properties of hexagonal close-packed (hcp) structured metals not only results in drastic stress variation across grain boundaries, but also heterogeneous distributions within grains. Understanding the mechanism of load sharing between different grains becomes more complicated when deformation twinning plays a significant role in accommodating an externally applied load. In this paper, a comprehensive study of stress development in a coarse grained strongly textured hcp polycrystal Zircaloy-2, is given using three-dimensional X-ray diffraction (3DXRD) microscopy. In-situ uniaxial straining was carried out at seven steps up to 2.7% in the macroscopic direction that favors twin formation, while center-of-mass position, crystallographic orientation, elastic strain, stress, and relative volume of each grain were measured. This information was used to reconstruct the 3D microstructure and statistically study neighborhood effects on the load sharing. The investigated volume of the sample contained 6132 grains initially, yet as a result of twin formation, 9724 grains were measured in the same volume at the last loading step. It is shown that the most favored (highest Schmid factor) twin variant contributes the most to the twin number fraction; however, if the measured local stress within each grain is used for the calculation of Schmid factor, the contribution of other variants is relatively independent of Schmid factor.