Kc. Russell, EFFECTS OF PRECURSOR MATRIX EVENTS ON SUBSEQUENT NUCLEATION, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 25(9), 1994, pp. 1933-1939
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.