Cleavage crack-growth-resistance of grain boundaries in polycrystalline Fe-2%Si alloy: experiments and modeling

Experiments were carried out on the modes of advance of cleavage cracking through a field of randomly oriented grains at -30 degreesC in the pure cleavage range (lower shelf) in very large grain size Fe-2%Si alloy, permitting detailed observations of the chronology of percolation of the cleavage crack front across a field of grains. Utilizing previously developed model-based information on the peak resistance to fracture of high angle grain boundaries, with a given tilt and twist misorientation across them, and detailed measurements of orientations of individual grains in the samples, it was possible to develop percolation maps of cleavage through the field of grains that for the initial phases of fracture through grains predicted the actual path with about 70% accuracy when compared with experimental observations. On the basis of these experiments and modeling a simple expression is projected for the effect of grain boundaries on the overall cleavage fracture resistance of low-carbon steel in the ower shelf region. The model gives a nearly 3-fold rise of cleavage resistance of the polycrystal over the single crystal substantially independent of grain size. Additional experiments on decarburized 1010 steel of 50 mum grain size with an unusually low brittle-to-ductile transition temperature gave very similar results on the mode of percolation of cleavage cracking through grains even though in this case the cleavage propagation was under upper shelf conditions. (C) 2002 Elsevier Science Ltd. All rights reserved.