Length scale (thickness) controlled ductile versus brittle transition in layered materials

The length scale relationship of ductile versus brittle behavior in layered materials is examined in terms of the competition between atomic decohesion and dislocation nucleation at the interface crack tip. A model of one metal layer sandwiched between other layers is presented, and a straight line dislocation is assumed to emit from the crack tip at the lower interface. Considering the image force due to the upper interface, the total force on the emitted dislocation is obtained, from which the critical energy release rate, Gdisl, for dislocation emission is expressed as a function of the middle layer thickness. The role of the length scale (thickness) on Gdisl is analyzed and compared with some previous works by Rice et al. The results show that ductile versus brittle behavior is controlled by the layer thickness, through the image force from the upper interface and the atomic phase angle. When thickness is larger than a critical value, ductile versus brittle transition is insensitive to the layer thickness.