Formation mechanism of layered Ti(C, N) during bidirectional freeze casting and its effect on the mechanical properties of nacre-inspired Ti(C, N)/AZ91 composites

A large-sized Ti(C, N) skeleton is prepared using an environmentally friendly and low-energy bidirectional freeze casting technology. In the horizontal direction, the ice crystals grow stably since the nucleation temperature line is always located at the front end of the ice crystals; thus, a long-range ordered layered morphology is formed on the cross section. Due to the extremely low temperature gradient in the vertical direction, the nucleation temperature line exceeds the front end of the ice crystal growth, resulting in the simultaneous nucleation and growth of multiple ice crystals and the formation of a dendritic lamellar morphology on the longitudinal section. In addition, due to the increase of undercooling and the settlement of ceramic particles, the ceramic content increases gradually from the upper to the lower part of the ceramic skeleton, while the layer thickness and pore width decrease accordingly. Subsequently, AZ91 is filled into the Ti(C, N) skeleton via gas pressure infiltration to obtain Ti(C, N)/AZ91 layered composites. Due to the strong support provided by the bottom-up continuous ceramic layer, the lower part of the composites has the highest compressive strength (395 MPa), while their middle part exhibits the best fracture initiation toughness (14.2 MPa·m^1/2), crack propagation toughness (32.6 MPa·m^1/2), and bending strength (421 MPa). The high strength and toughness in the middle portion of the composites can be attributed to the fewer defects in the ceramic layer, the plastic deformation of the metal layer, and a variety of external toughening mechanisms. Due to the combined effect of various non-inherent toughening mechanisms, the R curve of the composites presents an upward behavior, i.e., the fracture resistance is improved during the fracture process.