Kinetics Of Phase Changes Research Articles

Effect of Hydrostatic Pressures on the Crystallization Kinetics of Natural Rubber

The crystallization kinetics of natural rubber were investigated, utilizing dilatometric techniques, over an extended temperature range under hydrostatic pressures up to 800 kg/cm2. Irrespective of pressure, the isothermal crystallization displayed good adherence to the general mathematical formulation of the kinetics of phase changes as modified for the crystallization of chain-like molecules. Nucleation control is indicated at the higher temperatures. The rate of crystallization increases with increasing pressure at constant temperature as a consequence of the increased supercooling. At lower temperatures, transport terms dominate with the rate increasing with temperature at constant pressure. Over the temperature range accessible to study, the data are quantitatively explained by utilizing an activation energy for transport which is independent of temperature but which increases slightly with pressure.

Effect of Noncrystallizable Components on the Crystallization Kinetics of Polymers

The effects of chain irregularities, treated as randomly introduced copolymeric units and of added monomeric diluents, on the nucleation rate of crystallizing polymeric systems have been calculated. A marked attenuation in the nucleation rate is expected under certain conditions as a consequence of the varying composition of the melt. The decrease in the nucleation rate becomes more pronounced at the lower undercoolings and as the concentration of the noncrystallizing component increases. The Avrami mathematical formulation for the kinetics of phase changes can be generalized to allow for nucleation and growth rates which vary with the amount of material transformed. The resulting isotherms for copolymers are no longer superposable, superposability being characteristic of crystallization under conditions of constant nucleation and growth. Possible error in the interpretation of crystallization kinetics utilizing the simplified version of Avrami's equation is pointed out, as is the extreme sensitivity of the crystallization process to the presence of small amounts of chain irregularities.

Accelerated recrystallization of low carbon steel

1. The possibility of considerably accelerating recrystallization (i. e. process) annealing of low-carbon steel by heating at above 1000°C (1800°F)/second to temperatures around the critical has been established. 2. Holding for a few seconds after rapid high temperature heating completes recrystallization, produces a fine-grained structure and high mechanical properties.Rapid cooling (e.g. water quenching) after such holding prevents quench-aging effects and the steel preserves high ductility and strength. 3. In organizing rapid process annealing of low-carbon steel, due regard must be paid to the kinetics of phase changes in rapid heating and cooling.