Nucleation and propagation of cleavage fracture in iron

Abstract

The effect of oxygen segregation and of oxide precipitation on the initiation and propagation of cleavage cracks at 77 K has been studied in binary iron-oxygen alloys. Results indicate that fracture is initiated by intergranular cracks, which are nucleated by the intersection of slip initiating from the surface with the segregated grain boundary. It has been shown that for low oxygen contents, the fracture is coincident with general yield and is classified as initiation controlled cleavage. The nucleation of cracks in alloys containing oxide inclusions takes place by the growth of voids along associated grain boundaries. The dispersed oxide particles (~ 1 μm) promote local plasticity which effectively raises the plastic work associated with crack propagation. It has been shown that fracture at high oxygen levels is growth controlled cleavage and can only be achieved at stresses exceeding the general yield stress. A direct correlation has been established between the macroscopic behaviour characterized by the ratio σf σy and the micromechanisms of cleavage fracture. Based on these observations a fracture identification diagram for iron at 77 K has been proposed.