Balancing the strength-plasticity dilemma via hierarchical interfaces in crystalline Cr/amorphous CrAlN nanolaminates

Heterogeneous materials composed of ductile and hard phases are considered to have the potential to enhance plasticity while maintaining high strength. Here, interface engineering was employed to balance the strength-plasticity dilemma by constructing hierarchical structures in the crystalline/amorphous Cr/CrAlN nanolaminates, which refer to the layered structure bounded by heterointerfaces and the columnar structure bounded by homointerfaces (i.e., grain and glassy boundaries). The column width remains essentially constant as the layer thickness increases from 20 to 500 nm. The plastic deformability of the nanolaminates is found to greatly enhance with raising the layer thickness, without sacrificing strength and fracture toughness. The dislocation slip in Cr and shear band activation in amorphous CrAlN are dominantly contributing to the strengthening of the nanolaminates, while both events are controlled by the column width rather than layer thickness, thereby resulting in the layer thickness-independent strength. Meanwhile, the layer thickness-dependent plasticity is controlled by the kinking process in Cr and the heterointerface-induced crystallization in amorphous CrAlN. The former allows for a larger plastic strain due to the massive dislocation interactions, yet the latter may lead to the splitting in CrAlN. A larger layer thickness is beneficial for the kinking formation and can reduce the crystallization ratio in CrAlN layers, further resulting in higher plasticity. These mechanisms endow the Cr/CrAlN nanolaminates with tunable plasticity while maintaining non-degraded strength.