NUMERICAL-SIMULATION OF CLUSTER GROWTH DURING THE TRANSIENT NUCLEATION PERIOD IN MELT-SPINNING

Classical nucleation theory proposes that first order phase transforma tions start with the formation of small, unstable clusters (embryos) o f the new phase. Eventually, embryos grow to a critical size, beyond w hich they are stable, i.e. they become nuclei. The nucleation temperat ure T-N and time t(N) are where and when a nucleus reaches a critical size, ie. contains a critical number of atoms. A steady state rate of formation of nuclei per unit volume of the untransformed phase is reac hed after a ''transient'' or ''induction'' time. Various theoretical m odellings of solidification calculate T-N using analytical expressions for the steady state nucleatioin kinetics. In rapid quenching process es, such as the one that occurs in melt-spinning, the ''transient'' pe riod cannot be ignored or approximated. A numerical simulation method is proposed for the reactions involved in embryo cluster formation and growth, at the atomic level. The simulation is adapted to the particu lar now characteristics of the liquid in melt-spinning. II starts at t he liquidus temperature, then goes through the ''transient'' period fo r nucleation, and ends when the critical number of critical nuclei is formed, at the nucleation temperature T-N, and the amount of undercool ing is deduced. The simulation enables the calculation of the nucleati on temperature for each alloy considered. Values for the nucleation pa rameters appear to depend primarily on the number of atoms available, for embryo clusters to grow, on the liquid surface in isothermal conta ct with the substrate. Results for Al ribbons are compared with the an alysis of melt-spinning by steady state nucleation kinetics.