In recent years, development of advanced lightweight materials to reduce fuel consumption in vehicles has received increasing attention from researchers. However, these same applications require high levels of mechanical reliability, so it is critical to achieve a balance of high strength-to-weight ratio without sacrificing fracture toughness or fatigue life. Many biological materials, such as nacre in shells, exhibit such properties already, so exploring routes to engineer bio-inspired microstructures is promising.
The researchers of this article report on a nacre-like ceramic and metallic composite, with alumina “bricks” in a zirconium-based bulk metallic glass matrix “mortar”. The relationship between materials processing, microstructure, and properties were then explored. Specifically, the alumina is freeze casted into two structures, lamellar and brick-and-mortar, then infiltrated with metallic glass at two different temperatures 1153 and 1273 K. All four materials exhibit different mechanical properties with regards to bulk flexural strength and fracture toughness. Of most interest, the brick-and-mortar structure infiltrated at 1273 K possessed a rising R curve, indicating stable slow crack growth and extrinsic toughening up to 14 MPa-m1/2. This was attributed to crack bridging, deflection, and pullout of the ceramic bricks from the matrix. They also were able to perform in-situ SEM notched microcantilever tests, isolating the ceramic-metallic interfaces for the two infiltration temperatures. The 1273 K infiltration temperature resulted in interfacial fracture versus BMG fracture, due to a rougher interface from erosion of the alumina. So even though the microscale test indicted lower toughness due to the weak interface, at the bulk scale, forcing the failure along the interface activated extrinsic toughening mechanisms. This is a nice demonstration of the complementary information that can be gathered from micromechanical and bulk scale mechanical testing that is necessary for engineering new advanced structural materials.