Exploring the coupled effects of interfaces and grain size gradients in heterogeneous laminate and gradient structures via a multiple mechanism based constitutive model

As two promising types of heterogeneous materials, gradient structures and heterogeneous laminates possess superior mechanical properties, such as a combination of high strength and exceptional ductility, which can be attributed to back stress strengthening and strain hardening arising from their heterogeneous microstructures. Recent efforts have been made to combine these two unique microstructures to further improve their properties and performance. However, how heterogeneous interfaces and grain size gradients interplay with each other remains unknown. Therefore, this work aims at exploring the coupled effects of interfaces and grain size gradients in copper-bronze heterogeneous laminate and gradient samples via multiple mechanism-based constitutive modeling incorporating statistically stored dislocations, geometrically necessary dislocations, back stress and deformation twinning. It is revealed that different grain size gradients have profound influence on the deformation mechanisms and mechanical behaviors of the material: a monotonic grain size gradient in the copper layers strengthens the material yet reduces its ductility as a result of diminished back stress hardening due to weakened interface effects; a symmetric grain size gradient in the bronze layers leads to an increase in ductility and a decrease in strength; only by introducing a symmetric grain size gradient into the copper layers can the strength and ductility of the material be improved simultaneously, which is accredited to stronger back stress and grain boundary strengthening due to the synergistic effects between the interfaces and the grain size gradients. In addition, the distinct propensities for twinning in the copper and bronze layers can be attributed to both their different stacking fault energies and grain sizes.