Hetero-zone boundary affected region: A primary microstructural factor controlling extra work hardening in heterostructure

Heterostructured metals possess superior mechanical properties exceeding the prediction by the rule-of-mixtures. However, it remains a challenge to understand the key microstructural factor that controls the extra work hardening. Here aided by a newly developed mechanism-based plasticity model that incorporates the constitutive law of the back stress induced by zone-scale deformation heterogeneity, we reveal that the hetero-zone boundary affected region (Hbar) plays the key role in controlling the synergistic mechanical responses of heterostructure. Specifically, the Hbar, characterized by high strain gradient with a constant characteristic width, is formed to coordinate inter-zone deformation heterogeneity. The extra work hardening originates primarily in the Hbar, where the accumulation of geometrically necessary dislocations develops back stress and forest hardening. Importantly, the extra work hardening increases proportionally with Hbar volume fraction, and the best strength-ductility combination is reached when Hbar approaches saturation. In addition, the influences of zone configuration, mechanical incompatibility, and zone volume fraction on Hbar effect are analyzed, which sheds light on potential strategies to enhance the Hbar effect for optimizing strength-ductility combination.