Abstract
Rolled and annealed Mg-1wt. %Zn-1wt. %Gd-0.6wt. %Zr (ZEK110) alloys were subjected to room temperature in-plane compression along the rolling direction, followed by isochronal annealing treatments for 1 h. Despite a starting orientation favoring c-axis extension, the as-deformed microstructure revealed a hierarchical network of twins with profuse quantities of second and third generation twinning appearing within primary tension and compression twins. Complex twin-twin and twin–particle interactions were accompanied by the activation of both basal and non-basal dislocation slip in the neighborhood. While the population density of twins nucleated was significantly high, twin growth was severely retarded due to the presence of secondary phases. In terms of the overall distribution of grain orientations, the as-deformed texture displayed a rather weak basal component with a large portion of the basal poles aligned towards the longitudinal direction with an angular spread of ± 30°. Recrystallization commenced within twins, at twin-twin intersections at lower annealing temperatures and occurred additionally near secondary phases at higher annealing temperatures, giving rise to diverse orientations of both basal and off-basal character. Subsequent growth led to favorable coarsening of the off-basal orientations resulting in an overall texture weakening. The findings provide critical insights with respect to engineering high-strength, high-ductility lean magnesium alloys, comprising hierarchical microstructures that are not only associated with favorable crystallographic textures but additionally display a multi-scale strengthening behavior.
Highlights
Designing high-strength and high-cold-formability magnesium alloys remains a considerable challenge owing to the inherent anisotropy in the hexagonal crystal structure, wherein strain accommodation along the longer c-axis is difficult owing to significantly larger critical resolved shear stresses in this direction (Yoo, 1981; Mordike and Ebert, 2001; Hirsch and Al-Samman, 2013; Wu and Curtin, 2015; Wang et al, 2018)
Since the primary motivation behind such a deformation set-up was to activate deformation twinning in the material, the final compressive strain was limited to a value: εlogarithmic∼6%, which typically corresponds to the value where twinning is known to exhaust and further deformation ensues primarily by dislocation slip
Plane-strain compression (PSC) experiments in the present work was ideally designed to generate extension along the caxis, since it is well-known that pure Mg undergoes {1012} extension twinning under such circumstances
Summary
Designing high-strength and high-cold-formability magnesium alloys remains a considerable challenge owing to the inherent anisotropy in the hexagonal crystal structure, wherein strain accommodation along the longer c-axis is difficult owing to significantly larger critical resolved shear stresses in this direction (Yoo, 1981; Mordike and Ebert, 2001; Hirsch and Al-Samman, 2013; Wu and Curtin, 2015; Wang et al, 2018). While the presence of twin boundaries promote strengthening through dynamic grain refinement as well as lead to substantial latent hardening inside the twins; their role in terms of ductility is often seen as unfavorable (El Kadiri and Oppedal, 2010; Qiao et al, 2017) This is owing to the fact that twinning can either give rise to regions of severe shear incompatibility near the twin-parent interface (such as for compression twins) or generate crystallographically hard orientations (in case of extension twins) that adversely affect the overall elongation response (Aydiner et al, 2009; Molodov et al, 2014, 2017; Basu and Al-Samman, 2015). In this regard, promoting twin formation is often avoided when it comes to designing ductile Mg alloys, resulting in employing alternative routes such as designing fine-grained microstructures thereby abating twinning activity and increasing slip contribution (Yamashita et al, 2001; Hofstetter et al, 2015; Lin et al, 2016; Trang et al, 2018)
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