In situ study of plastic deformation in a Mg–Ce alloy using polychromatic micro X-ray diffraction

Abstract

Abstract Synchrotron micro X-ray diffraction (micro-XRD) has been applied to map, at an intragranular length scale, the evolution of deviatoric strain and dislocation distribution across an embedded grain in a polycrystalline Mg(0.2 wt.%)Ce alloy sample during uni-axial tensile loading. Employing a normal distribution, elastic strain accumulation was quantified by calculation of the mean axial ( ɛ ¯ xx ) and normal ( ɛ ¯ zz ) components, parallel and perpendicular to the loading direction, respectively. In the axial direction, tensile strain increased from 2.3× 10 −3 at an applied macroscopic load of 15 MPa to 6.6× 10 −3 at140 MPa. Whereas, in the normal direction an initial compressive strain of −1.1× 10 −3 at 15 MPa increased to −4.2× 10 −3 at 140 MPa. A noticeable increase in the strain distribution (variance) with load was also accompanied by dislocation slip, identified by Laue spot streaking. Simulation of observed lattice rotation streaking of individual Laue spot patterns was used to determine the active dislocation slip mechanism. At the onset of plastic deformation (90 MPa) relatively uniform streaking was identified. After comparison of observed and simulated patterns, streaking was attributed to ensembles of edge geometrically necessary dislocations (GNDs) undergoing a >-type basal slip 1 1 2 ¯ 0 > (0 0 0 2). At 140 MPa non-uniform streaking and splitting of individual spots was prevalent. It is anticipated the inhomogeneous nature of the streaking arises from distinctly different loading conditions imposed by neighbouring grain boundaries. Onset of Laue spot splitting was attributed to ordering of dislocations into geometrically necessary boundaries (GNBs). Up to 4 individual sub-grains were identified at some locations while in the grain center only 2 distinct sub-grains were identified. Newly formed small-angle tilt boundaries produced a maximum misorientation of up to 1.4 ° within the original parent grain.