A high-resolution synchrotron-based diffraction technique for in situ characterization of deformation behaviour in magnesium alloys

Abstract

To promote accurate lattice-strain measurement and twinning observation during in situ deformation of age-hardenable lightweight magnesium alloys, a high-resolution X-ray diffraction technique was applied using medium-energy synchrotron X-rays (≤21 keV) coupled with a fast Mythen strip detector. This technique allows data collection in transmission geometry, with sufficient grain-sampling statistics achieved by rocking the samples during each measurement under step-wise uniaxial tensile/compressive loads. The capabilities of the method are demonstrated on a model age-hardenable Mg–Sn-based alloy in compression. The measurements confirm that this technique offers high angular resolution and a wide angular range, minimizing the problem of peak overlap, which is advantageous for accurate lattice-strain determination of both the α-Mg matrix and strengthening precipitate phases. The absolute strain resolution is approximately ±2 × 10−4. Lattice-strain partitioning and anisotropy in the α-Mg phase reveal the occurrence of microplasticity due to the activation of basal dislocation slip in Mg alloys and provide experimental information for characterizing the plastic anisotropy of the materials. The initiation and growth of {10 {\overline 1} 2} tension twins are identified and quantified from the changes in the integrated intensities of 10 {\overline 1} 0/0002 reflections as a function of stress. The critical resolved shear stresses (CRSSs) for the activations of basal slip and tension twin modes in both non-aged and aged materials were estimated. The results reveal that, after the ageing treatment, the CRSS value for basal slip increases from 18 to 33 MPa, an increase of ∼83%, and that for tension twinning increases from 32 to 52 MPa, an increase of ∼63%. The methodology also enables further microstructural data to be probed in situ. This includes the apparent area-weighted twin size and dislocation density during twin onset, and the precipitate volume fraction.