EFFECTS OF PRECURSOR MATRIX EVENTS ON SUBSEQUENT NUCLEATION

A precursor transformation may actually increase the chemical driving force for subsequent nucleation of certain phases while decreasing the free energy of the system. Spinodal decomposition of Fe-C austenite h as been proposed as a means of providing C-poor regions in which subse quent nucleation of ferrite or martensite is easier. However, the C-C interaction potential in austenite precludes such a reaction. Statisti cal fluctuations have also been proposed as a means of providing C-poo r regions in austenite. However, the activity coefficient of C in aust enite varies only slightly with C content, so that the scale of these fluctuations is far too small to provide attractive sites for martensi te or ferrite nucleation. Statistical concentration fluctuations obser ved above the theta solvus in Al-Cu alloys bear a striking resemblance to Guinier Preston (GP) zone critical nuclei which will form only at temperatures that are hundreds of degrees lower. The role played by th ese fluctuations in subsequent critical nucleus formation is as yet un clear. A proposal that solute segregation to dislocations increases th e driving force for nucleation is shown to be rendered invalid by the requirement that the chemical potentials in the system be uniform.