Investigation of microstructure evolution on the fracture toughness behaviour of brass/low carbon steel/brass clad sheets fabricated by cold roll bonding process

The property of fracture toughness is one of the essential parameters in choosing a material that shows resistance to crack growth. In this research, the effect of microstructure evolution on the fracture toughness of Brass (CuZn10)/Low Carbon Steel (St14)/Brass (CuZn10) composite fabricated by the Cold Roll Bonding (CRB) process was investigated. For this purpose, the Brass/Low Carbon Steel/Brass three layered samples were annealed at 450 °C, 600 °C, 750 °C and 850 °C for 2 h. Uniaxial tensile test, microhardness, peeling test, EDS line scan analysis, optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the properties of the composite. The compact tension (CT) test was also used to obtain the fracture toughness of samples. The EDS line scan analysis results showed that no intermetallic compound is formed at the interface of Steel and Brass layers under any temperature. However, increasing the annealing temperature causes the diffusion layer to expand from 5.13 μm to 9.02 μm for samples annealed at 450 °C and 850 °C, respectively, which aids in increasing the bond strength of the layers. Recrystallization changes the layers' microstructure, so the samples' fracture toughness was measured in the roll direction (RD) and transverse direction (TD). It was found that the value of fracture toughness is very different for the as-rolled sample in RD and TD. The most significant decrease in fracture toughness at different temperatures occurs after annealing at 600 °C, wherein the direction of RD compared to the temperature of 450 °C, more than 35% decline was recorded. In fact, after rolling, the microstructure of the layers is elongated in the direction of rolling, which makes the force-crack mouth opening in different directions not the same. Furthermore, as the temperature increases and full recrystallization occurs, the grains become equiaxed, which in addition to reducing the fracture toughness, also reduces the sensitivity to the rolling direction. Based on the obtained results, it was found that the temperature of 600 °C can be chosen as the optimal annealing temperature for three layers of Brass/Steel/Brass composite, in which the appropriate mechanical properties and fracture toughness can be obtained.