Visualizing the three-step freezing process and three-phase reaction not predicted by the (NH4) 2SO4/H2O phase diagram

According to the conventional phase diagrams, aqueous solutions freeze at the liquidus and are frozen/solid below the eutectic solidus. Herein, using differential scanning calorimetry (DSC) and optical cryo-microscopy (OC-M), we demonstrate that hy-poeutectic, eutectic 40 wt% (NH4)2SO4 and hypereutectic (NH4)2SO4/H2O remain liquid well below the eutectic solidus before freezing in three steps: fast-slow-fast. The first fast freezing produces a ramified ice microstructure (IM) and freeze-concentrated solution (FCS) containing up to ~70 wt% (NH4)2SO4. As temperature decreases further, the slow freezing of FCS precedes its fast freezing, which produces a striped IM and (NH4)2SO4 microcrystals. Videos recorded upon warming of frozen (NH4)2SO4/H2O reveal a new three-phase reaction, which is the recrystallization of ice and (NH4)2SO4 microcrystals into the lamellar eutectic ice-(NH4)2SO4 superlattice. This work demonstrates limitations of the (NH4)2SO4/H2O phase diagram and pro-poses an effective strategy for studying other deeply supercooled solutions whose behavior is not predicted by the phase dia-gram.

Effect of High Pressure and Temperature on the Evolution of Si Phase and Eutectic Spacing in Al-20Si Alloys

The microstructure of the Si phase in Al-20Si alloys solidified under high pressure was investigated. The results demonstrate that the morphology of Si phase transformed (bulk→short rod→long needle) with the increase of superheat temperature under high pressure. At a pressure of 3 GPa and a superheat temperature of 100 K, a microstructure with a uniform distribution of fine Si phases on the α-Al matrix was obtained in the Al-20Si alloy. In addition, a mathematical model was developed to analyze the spacing variation of the lamellar Al-Si eutectics under the effect of pressure. The lamellar Al-Si eutectics appeared at 2 GPa and superheat temperatures of 70–150 K, and at 3 GPa and superheat temperatures of 140–200 K. With the increase of pressure from 2 GPa to 3 GPa, the average spacing of lamellar Al-Si eutectics decreased from 1.2–1.6 μm to 0.9–1.1 μm. In binary alloys, the effect of pressure on the eutectic spacing is related to the volume change of the solute phase from liquid to solid. When the volume change of the solute phase from liquid to solid is negative, the lamellar eutectic spacing decreases with increasing pressure. When it is positive, the eutectic spacing increases with increasing pressure.

Rod eutectic growth in bulk undercooled melts

This article proposes an analytical model to understand the rod-growth of eutectic in the bulk undercooled melt. Based on the previous derivations of the lamellar eutectic growth models, relaxing the assumptions of small Peclet numbers, the model is derived by considering melt kinetic and thermal undercoolings. The intent of this model is to predict the transitions in eutectic pattern for conditions of the low and high growth velocity. In addition to investigation of the transition between lamellar and rod eutectic pattern, mathematical simplifications of solving Bessel function are presented as well, which is the most important priority to model calculation.