Deformation pseudo-twinning of the L21-ordered intermetallic superlattice

Ordered intermetallic materials are promising candidates for applications in extreme environments due to the strong localized bonding schemes that stabilize the structures against degradation. Here, we investigate the propensity for deformation twinning modes in the L2₁-ordered MnCu₂Al intermetallic by inducing a state of severe plastic deformation. Post-mortem microstructural analysis reveals the presence of meso-scale deformation twins accompanied by a high degree of nano-scale crystalline heterogeneity. The orientation relationship between the parent and twin structures is consistent with activation of the $〈111〉\left\{112\right\}$ deformation twinning mode. Ab initio generalized stacking fault energy curve calculations corresponding to $〈111〉\left\{112\right\}$ twinning shear suggest that the magnitude of the twinning partial is $\frac{1}{4}〈111〉\left\{112\right\}$ and results in the local formation of a metastable orthorhombic phase within the pseudo-twin. This work highlights the discrepancy between the deformation modes of ordered/disordered structures and promotes an increased awareness for the role that long-range chemical order plays in determining deformation modes.