Abstract
The Cu-Al eutectoid was chosen for investigation of pearlite growth and morphological evolution behaviour. The two pearlitic phases were found to obey the Kurdjumov-Sachs orientation relationship precisely. The only micro-habit plane observed in the isothermally transformed pearlite was the (111)fcc||(110)bcc, which is different to that reported for directionally grown Cu-Al pearlite. In common with other ferrous and non-ferrous lamellar eutectoids the Cu-Al pearlite possesses direction steps at the interlamellar interface. The observed planar growth defects, twins in the fcc α-phase and low angle boundaries in the bcc γ-phase, are shown to be consistent with a ledge mechanism of pearlite growth. The long term stability of direction steps is demonstrated by TEM. It is shown that prolonged ageing brings about a change in interface structure from one with direction steps and other defects to one with misfit compensating dislocations. A modified ledge growth mechanism is proposed to account for experimental growth velocities of pearlite. This, along with the classic Zener-Hillert approach and that of Lee, Spanos, Shiflet and Aaronson are tested against available growth velocity data for the Cu-Al pearlite and other non-ferrous and ferrous pearlites. It is shown that a ledge mechanism of pearlite growth with interfacial interdiffusion rate control is consistent with the experimental data. It is proposed that the spacing of pearlite is controlled entirely by local ledge phenomena. The morphological evolution of the Cu-Al pearlite is different to that of ferrous pearlites and includes a new mechanism of termination generation along with classic termination migration and coarsening. Termination generation, which makes only a small contribution to the overall shape change, is asymmetric and requires γ-phase low angle boundaries. Termination migration is important only in the first 100 h of ageing, after this time conventional coarsening is the dominant mechanism. Modelling of the time dependent morphology in terms of specific interfacial area is outlined. The role of crystallography/interfacial structure during shape evolution is considered to be more important in determining which mechanisms operate rather than being directly rate controlling. The implications of these results for the general understanding of morphological developments in lamellar microstructures are also considered.