Isothermal Solidification Kinetics Research Articles

Simulation of solidification structures of binary alloys: a multi-particle diffusion-limited aggregation model with surface rearrangement

The thermodynamics and kinetics of isothermal solidification in binary-alloy melts were simulated in a two-dimensional model incorporating the effects of nearest and next-nearest neighbours at the solid-liquid interfaces, taking into account the simultaneous diffusion in the melt of all mobile particles (atoms) and allowing rearrangement at the solid-liquid interface on a local scale. The next-nearest-neighbour interaction and the surface rearrangement were shown to have large effects on the resulting solidification morphologies. Additionally, the influences of the operating diffusion coefficient and the sticking rule for the solidifying particle were analysed. A wide range of growth modes, dendritic, massive, equiaxed, eutectic, and transitions thereof during solidification, involving change of time dependence of the growth velocity, as observed in practice, could be placed on a rationalistic basis.

Polymorphism and solidification kinetics of the binary system POS‐SOS

AbstractAs a first approach to modeling the crystallization behavior of cocoa butter, the binary system of its two major components, 1‐palmitoyl‐2‐oleoyl‐3‐stearoyl‐s n‐glycerol (POS) and 1,3‐distearoyl‐2‐oleoyl‐s n‐glycerol (SOS), was studied. Differential scanning calorimetry, coupled with polarized light microscopy, was used to determine the phase diagrams of the various polymorphic forms (the most metastable sub‐α and α, intermediate δ and β′, and stable β). Associated theoretical phase diagrams were also built. The presence of a solid solution for β and the ideal behavior of α were confirmed. Kinetics of isothermal solidification as a function of temperature were studied for three compositions of the POS‐SOS system. Results were displayed as time‐temperature‐transformation (TTT) diagrams and crystal morphology maps. Dependence of the crystallization kinetics on composition of the binary system was interpreted in terms of nucleation and growth mechanisms. The asymmetry of the POS molecule induces a slower growth rate of the mixture when the concentration of this triacylglycerol is increased. Solidification kinetics during continuous cooling were studied at various cooling rates for the system POS‐SOS 25:75. Experimental results were compared with numerical predictions of a solidification model based upon TTT data and an additivity principle. The calculated and measured volume fractions of the different phases formed and times of onset and finish of the solidification were in good agreement.

Effects of composition on isothermal solidification kinetics of highly undercooled alloys

A modified theory incorporates the effects of composition on homogeneous nucleation and isotropic crystal growth in isothermal, massive solidification of alloys. Simple, realistic values for the needed kinetic and thermodynamic quantities facilitate calculation of grain size and solidification times as a function of temperature and concentration. Reasonable agreement is found between the isothermal theory and results of continuous cooling experiments on pure Al and some Al-based alloys. The conditions for producing a vitreous, non-crystalline solid are also given. We propose that the extremely fine grain sizes obtained in AlGe and AgCu alloys rise from quenching to form a glass, followed by isothermal crystallization at or near room temperature.