Insights into the post-yield microstructure of Mg-Gd-Cu alloy processed by multi-directional forging

The multi-directional forging method has been utilized in improving the yield strength of Mg for several years. However, almost no studies have reported the post-yield behavior which is closely related to the plastic stability after tensile yielding of multi-directional forged Mg with twinned microstructures. For the sake of providing new insights into the post-yield behaviors of pre-twinned Mg, multi-directional forging with different deformation parameters was subjected to one Mg-2.82Gd-0.41Cu (wt%) extruded bar in this work. Through measuring the tensile properties and observing the microstructure evolutions of three typical samples, the preferential multi-directional forging parameters were determined. Specifically, the as-extruded sample with no preset twins owned the lowest yield strength but the continuous strain hardening. The plastic stability was promoted by the cross-slip composed by < a > dislocations and the particle strengthening caused by ternary rods. The sample with grain fragments, processed using liquid-nitrogen temperature, single-pass strain of ε = 0.2 and 3 passes in total, owned the highest yield strength. But unfortunately, plastic instability happened rapidly after yielding, for the lack of continuous strengthening effect during tensile loading. The sample with preset twins, processed using room temperature, single-pass strain of ε = 0.1 and 9 passes in total, owned the best combination of improved yield strength and preserved plastic stability. The excellent performances were realized by the collaboration of sub-grains composed by various types of dislocations and Gd-Cu co-segregations at both grain and twin boundaries. It is believed that these new insights into the post-yield behaviors of Mg manufactured by multi-directional forging method can strongly assist in the development of high-strength Mg alloys without plastic instability. • Twins are widely introduced and yield strength is highly improved by MDF processing. • Evolutions of pre-twinned microstructure are studied in extruded Mg-Gd-Cu. • Single-pass strain is more crucial than temperature and accumulative strain in MDF. • Fragmented grains and high-density twins collaboratively improve yield strength. • Strain hardening can only be maintained by sub-grains and Gd-Cu co-segregations.