Fibre-loading Mechanism for the Fracture of Cementite Films in Ferrite

Abstract

GRAIN boundary cementite films are known to have an important influence on the cleavage fracture of low carbon steels^1–6. Plastic deformation in the ferrite matrix results in the formation of cracks within the cementite which can lead to grain-size microcracks^1,5 or complete failure of a tensile specimen. The cracks in the carbides are widely believed to be nucleated by slip or twinning dislocation pile-ups and Smith^7,8 has described this process by a model developed from those of Cottrell⁹ and Petch¹⁰. This model is consistent with many experimental observations, in particular with the influence of microstructure and superimposed hydrostatic stress on fracture behaviour^2,11. There are, however, certain inconsistencies between the experimental observations and fracture theories based solely on the micro-mechanistic approach. These include the observed limited propagation of cementite cracks into ferrite^1,5 and the increased tendency for carbides to crack as the film thickness is reduced. Furthermore, carbide orientation influences the tendency to crack (Fig. 1) and there is a higher probability for carbides to fracture at the mid-length position (Fig. 2). It is proposed that these observations can be explained if, once plastic yielding has occurred in the matrix, fibre loading can lead to stresses in carbide films sufficient to cause fracture.