Crystallographic Analysis of Lower Bainite Transformation in Fe-0.7%C Alloy

Lower bainite transformation in an Fe-0.7 mass% C alloy has been investigated by means of transmission electron microscopy. The results obtained can be explained consistently in terms of the Bowles-Mackenzie theory assuming the [101]a(111)a lattice invariant shear and the dilatation parameter δ=0.990. This contraction can equally be replaced by the austenite lattice expansion.Cementite platelets nucleate on the ferrite ledges probably formed by lattice invariant shears at the austenite/ferrite interphase boundaries. In addition to this, the lattice expansion of austenite in the vicinity of a bainite plate corresponding to δ=0.990 suggests that partitioning of carbon atoms occurs and the carbon concentration in the austenite layer surrounding the bainite is enriched up to 1–2 mass% during the transformation.Since the cementite particles grow in size within the bainite plates, the initially formed bainites are thought to be significantly super-saturated in carbon.