Nucleation of the isothermal martensitic transformation

Abstract

By means of quantitative metallography and electrical-resistivity measurements, the initial nucleation rates of the isothermal martensitic transformation were determined in a series of three iron, 23–25 wt.% nickel, 2–3 wt. % manganese alloys as a function of austenitic grain size and reaction temperature, taking autocatalytic effects into account. Typical C-curve kinetics are observed, with the temperature of maximum nucleation rate occurring at—125°C. The conventional initial nucleation rate (averaged up to 0.2% transformation, which is the first reliably detectable amount of marteusite), decreases with decreasing austenitic grain size; this trend arises because the smaller the martensitic plate size, the larger is the number of nucleation events required to generate a given amount of martensite. Thus, for a fine-grained austenite, a much larger fraction of the pre-existing embryos has to be activated to produce 0.2% martensite than in the case of a coarse-grained austenite. When this difference is taken into account, the true initial rate of nucleation is found to be independent of the grain size and austenitizing temperature within the limits studied, thereby indicating that grain boundaries are not dominant nucleation sites for the martensitic transformation in these alloys. The resulting activation energies for nucleation vary with reaction temperature in a manner predicted by the Kaufman—Cohen model, and the agreement becomes quantitative if an effective embryo radius of 180–200 Å is assumed. The conditions leading to athermal nucleation are also rationalized. Self-consistent results are obtained when the number of pre-existing embryos is taken to be 10 7/cm 3. The probability of encountering such embryos in transmission electron microscopy is extremely small, even though they may be large enough to be resolved.