Intergranular fracture at low temperatures at polycrystalline materials

The proportion of cleavage and intergranular fracture which occurs when high purity α-iron and ferritic steels are tested at temperatures below the ductile-brittle transition has been investigated using both theoretical models and high resolution scanning electron microscopy. Some intergranular fracture is necessary because cleavage cracks in adjacent grains do not meet in a line in their common grain boundary. If it is assumed that cleavage occurs on the three variants of the {100} plane, several different models indicate that the proportion of intergranular fracture should be about 30%. Experiments show that in pure α-iron the proportion may approach 20% but in a C-Mn steel it less than 10%. This discrepancy can be explained if it is assumed that, at least in some grains, cleavage fracture occurs on both {100} and {110} planes. There is some experimental evidence for this suggestion and it is also supported indirectly by preliminary experiments on polycrystalline zinc.