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Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt was made to relate the results to the controllable parameters of the system. A microscope-video system recorded the mushy zone-melt interface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates, and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface. Fragmentation rates appear to correlate well with the magnitude of particle velocities near the interface, with increasing fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation are discussed. Although it appears that either constitutional remelting or capillary pinching are likely of importance, hydrodynamic shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings.

Precision adiabatic recalescence experiments on both pure and binary undercooled melts have been conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable under-cooled state (creating a mushy zone) and also sets bounds on the microstructural parameters of the mushy zone, namely the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: a short time scale (less than ∼1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with the propagation of the dendrites and release of latent heat; an intermediate time scale (∼103–104 s) of slowly rising temperature, associated with coarsening of the mushy zone; and a long time scale (days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface.

A series of experiments were performed to investigate the effects of a thick wall on the freezing process occurring inside a cylindrical container. Two cylinders with different thermal diffusivity (k/ϱc) values were compared. To isolate the effects of the wall sensible energy, experiments were run for both a subcooled wall and the wall initially at the phase change material (PCM) fusion temperature. The PCM was initially at its fusion temperature for all cases. The base of the cylinder was maintained at a constant temperature in all but one case, where the lower boundary was approximately adiabatic. At several time steps, the mass fraction solidified and the phase front profile were determined. Measurements of the base heat flux and mean cylinder temperature were also obtained as functions of time. The fraction frozen vs time curves and the shape of the phase front profiles are compared for each set of conditions. The energy content of the wall can significantly affect both results. The data are presented in such a way as to encourage comparison with numerical computations of a similar complex-boundary freezing problem.

A key element of mushy zone modeling is the description of the microscopic evolution of the lengthscales within the mushy zone and the influence of macroscopic transport processes. This article describes some recent progress in developing a mean-field statistical theory of phase coarsening in adiabatic mushy zones. The main theoretical results are (1) temporal scaling laws that indicate that the average lengthscale increases as time1/3, (2) a self-similar distribution of mushy zone lengthscales based on spherical solid particle shapes, and (3) kinetic rate constants which provide the dependences of the coarsening process on material parameters and the volume fraction of the solid phase. High precision thermal decay experiments are described which verify aspects of the theory in pure material mushy zones held under adiabatic conditions. The microscopic coarsening theory is then integrated within a simple macroscopic heat-transfer model of one-dimensional alloy solidification. The method demonstrates an ability to predict the influence of macroscopic heat transfer on the evolution of primary and secondary dendrite arm spacings in Al-Cu alloys.