Development of a laboratory-based transmission diffraction technique forin situdeformation studies of Mg alloys

Abstract

A laboratory‐based transmission X‐ray diffraction technique was developed to measure elastic lattice strains parallel to the loading direction during in situ tensile deformation. High‐quality transmission X‐ray diffraction data were acquired in a time frame suitable for in situ loading experiments by application of a polycapillary X‐ray optic with a conventional laboratory Cu X‐ray source. Based on the measurement of two standard reference materials [lanthanum hexaboride (NIST SRM 660b) and silicon (NIST SRM 640c)], precise instrumental alignment and calibration of the transmission diffraction geometry were realized. These results were also confirmed by the equivalent data acquired using the standard Bragg–Brentano measurement geometry. An empirical Caglioti function was employed to describe the instrumental broadening, while an axis of rotation correction was used to measure and correct the specimen displacement from the centre of the goniometer axis. For precise Bragg peak position and hkil intensity information, a line profile fitting methodology was implemented, with Pawley refinement used to measure the sample reference lattice spacings (do(hkil)). It is shown that the relatively large X‐ray probe size available (7 × 7 mm) provides a relatively straightforward approach for improving the grain statistics for the study of metal alloys, where grain sizes in excess of 1 µm can become problematic for synchrotron‐based measurements. This new laboratory‐based capability was applied to study the lattice strain evolution during the elastic–plastic transition in extruded and rolled magnesium alloys. A strain resolution of 2 × 10−4 at relatively low 2θ angles (20–65° 2θ) was achieved for the in situ tensile deformation studies. In situ measurement of the elastic lattice strain accommodation with applied stress in the magnesium alloys indicated the activation of dislocation slip and twin deformation mechanisms. Furthermore, measurement of the relative change in the intensity of 0002 and 103 was used to quantify {102}〈011〉 tensile twin onset and growth with applied load.

Deformation modes during room temperature tension of fine-grained pure magnesium

“…Beyond this elongation, the increase of elastic lattice strain plateaued with increased elongation. This deflection suggested the relaxation of elastic strain, reportedly caused by the onset of basal dislocation slip [40,47,48]. In contrast, in the FG-Mg (Fig.…”

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

“…Experimental techniques based on X-ray, neutron and electron probes are commonly used to study the deformation behaviour of Mg alloys and many other materials, such as steels, or Al, Ti and Zr alloys (Lee et al, 2014;Lentz et al, 2015;Kada et al, 2015;Ferná ndez-Castrillo et al, 2008;Hounkpati et al, 2016;Liss & Yan, 2010;Jain et al, 2013;Stanford et al, 2015;Bettles et al, 2011). Electron microscopy and electron backscattered diffraction (EBSD) approaches are restricted to probing very limited volumes or only thin layers of samples, which limit the grain-sampling statistics and may not represent or fully describe the deformation response in the bulk of the material.…”

<i>In Situ</i> Synchrotron Analysis of Twinning Stresses in an Aged Mg-4.5Zn Alloy

“…This precipitate is in the form of rods aligned perpendicular to the basal plane [7,8]. In recent studies by Stanford and Barnett [5] and Jain et al [6,9] the effect of rod shape precipitates in Mg-Zn alloys on the strengthening of basal slip, prismatic slip and (10)(11)(12) twinning has been investigated using viscoplastic self-consistent modelling and compared with the Orowan hardening equation. However, there is a lack of direct observation of strengthening effect of a separate deformation (especially (10)(11)(12) twinning) in the presence of precipitates.…”