Microstructure and texture evolution in Mg/Nb layered materials made by accumulative roll bonding

In this work, lamellar composite sheets combining one of two fully hexagonal magnesium-lithium alloys (Mg-4Li and Mg-5Li wt%) and pure niobium (Nb) are manufactured via accumulative roll bonding (ARB). With extreme straining of over two true strain, the individual layers were refined to 200 μm. The strength differential between co-deforming phases with strain is characterized to be low enough to facilitate bonding without instabilities. Additionally, homogeneity in deformation was enhanced by intermediate annealing, especially for the Mg-xLi phase. Diffraction methods and polycrystal modeling are employed to study the microstructure and texture evolution of the individual phases after each subsequent ARB pass. Characterization by electron backscatter diffraction and neutron diffraction reveals substantial changes in microstructure and texture in both phases and very little deformation twinning in the Mg-xLi phase. Evolution of grain morphology and the number of grains that span a layer with ARB and annealing are determined and discussed. To link texture evolution to slip-based deformation mechanisms during the processing, a multiscale polycrystalline model of the two-phase Mg-4Li/Nb composite was developed, which included a relative directional compliance (RDC) method to account for anisotropic interactions in the phases, allowing appropriate slip sensitivity to dislocation density based hardening on the multiple slip modes in Mg-4Li and in Nb. The model indicates that the deformation of the Mg-4Li phase during ARB cycles was accommodated by more basal <a> and progressively smaller amounts of prismatic <a>, pyramidal <c+a>, and twinning than previously reported for rolling of Mg-4Li. The experimental textures cannot be entirely explained by grain shape and an increase in pyramidal and prismatic slip resistance are required as expected with a small grain size. Consistent with measurements, the model predicts that c-axis of the Mg-4Li phase tilts from the sheet's normal direction towards the rolling direction, and that the strong γ-fiber and the unusually weak α-fiber should develop in Nb with large ARB straining of the Mg-xLi/Nb composites.