Time and spatial resolution of slip and twinning in a grain embedded within a magnesium polycrystal

Abstract

Plastic yielding in magnesium alloys frequently involves the initiation of both slip and twinning events. A proper understanding of the phenomenon at the grain level requires knowledge of how these two mechanisms progress and interact over both time and space and what the local resolved stresses are. To date, simultaneous collection of such information has not been achievable. To address this shortfall, we have developed a modified Laue based in situ micro X-ray diffraction technique with an unprecedented combination of time and spatial resolution. A ten-fold reduction in data collection times is realized by the refinement of rapid polychromatic Laue “single-shot” mapping. From single Laue patterns, we extract grain depth information, detect onset of yielding and achieve 2 × 10⁻⁴ lattice strain resolution. The technique is employed to examine yielding and twinning in a magnesium grain embedded ∼200 μm below the sample surface. We examine 13 time steps and reveal the following behaviour: initial onset of basal slip, subsequent onset of twinning, development of further accommodation slip and evolution of twin shape and size; along with the corresponding values of local resolved shear stresses.

Introduction

Plastic yielding in magnesium alloys frequently involves the initiation of both slip and twinning events. A proper understanding of the phenomenon at the grain level requires knowledge of how these two mechanisms progress over both time and space and what the local stresses are [1], [2]. To date, experimental identification of active deformation mechanisms along with simultaneous monitoring of their variation over time and space has proved challenging. To address this shortfall, we develop herein a modified Laue in situ micro X-ray diffraction technique (XRD) with the required combination of time and spatial resolution. The technique is employed to determine the onset and progress of slip and twinning in a magnesium grain embedded ∼200 μm below the sample surface.

In situ neutron diffraction has been widely employed to probe the average response of similarly diffracting grains in metals yielding by twinning [3], [4], [5], [6], [7], [8]. Magnesium has become somewhat of a model hexagonal close packed (hcp) material for a such study since Gharghouri et al. [8] showed how changes in the diffracted intensity reveal the onset of deformation twinning and used this to demonstrate that $\{10\overline{1}2\}〈\overline{1}01〉$ twinning in magnesium follows a critical resolved shear stress law. This agrees well with the theoretical treatment by Lebensohn and Tome [9] and is an important point for the present study because resolved shear stresses can be adequately obtained from the deviatoric strains that are provided by Laue type techniques. Neutron diffraction has also been combined with crystal plasticity by a number of workers to extract effective mean critical resolved shear stresses for slip and twinning modes [4], [7], [10]. The stresses for the onset of twinning were found to exceed those for basal slip by a factor of 2–7. It is therefore quite likely that for a wide class of grain orientations, “micro” yielding due to basal slip will precede twinning. This is likely to impact significantly on subsequent twinning events via a number of mechanisms; for example, by stimulating nucleation [11], by facilitating initial growth [12], [13], [14] and ultimately by impeding growth [15], [16]. Bulk neutron methods have also shown that newly formed twins display relaxed [3] and even reversed [7] internal stresses. The magnitude of these stresses will control the activation of subsequent slip modes but because of their transient nature, diffraction data averaged over many grains are not able to reveal the sequence of deformation with necessary precision.

Steps towards addressing this problem have been taken by Aydiner et al. [17], who employed a high-energy synchrotron experiment to isolate radiation diffracting from a single embedded grain in a deforming magnesium AZ31 alloy. Seven time steps – levels of imposed stress – were examined and full stress tensors were obtained at the grain level. Two twins were seen to form at a resolved stress somewhere between ∼22 and 42 MPa, a range set by the chosen loading increment. Resolved shear stresses in the twins were seen to be relaxed compared to the parent and ranged between −10 and 14 MPa. But no information was obtained on any prior, concurrent or subsequent basal slip, nor is spatial intragranular information available with this technique. To access intragranular information, Balogh et al. [18] applied differential aperture X-ray diffraction (DAXM) [19] to study an embedded twinned grain under load in a magnesium AZ31 alloy. The technique is attractive because it provides unprecedented spatial resolution in three dimensions. Analysis was restricted to a single component of the stress tensor and this revealed significant gradients over the grain and in the twin domain. However, the development of the stresses and structure with increasing stress is not practically accessible using DAXM due to multiple scans required at each time (stress) step.

So it is seen that while considerable progress has been made, to date it has not been possible to simultaneously correlate spatial and time resolution to the extent desired to observe the variation of slip and twinning events in time and with increasing stress during the yielding of magnesium. The present study addresses this problem and reports observations of the progress of slip and twinning in an embedded magnesium grain during tensile testing. The paper is organized as follows: first the modified Laue technique developed for the study is described in some detail, then findings are presented in terms of measured diffraction intensities, peak broadening and micro-strains. The paper concludes with our interpretation of how slip and twinning interact in the present material.