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Lamellar eutectic growth was investigated in the transparent organic alloy neopentylglycol-(D)camphor of eutectic composition (NPG-45.3 wt% DC) using bulk (3D) and thin (2D) samples. Two types of eutectic grains were observed in the polycrystalline samples, either with lamellae well aligned to the direction of solidification or inclined at an angle of 21.5±1.5°. The well aligned grains were used for determining lamellar spacing as function of growth velocity V and temperature gradient G . Based on these data the Jackson–Hunt constant was evaluated to be K JH =1.60±0.15 μm 3 s −1 . For low growth velocity experiments the contact angles for (DC) and (NPG) lamellae at eutectic triple junctions were also evaluated, being θ (DC) =50.9±4.1° and θ (NPG) =41.8±4.7°, respectively. Using these values, as well as phase diagram data and the Gibbs–Thomson coefficients, the chemical coefficient of diffusion of (D)camphor in the eutectic liquid at eutectic temperature 53 °C was estimated to be D L =97±15 µm 2 s −1 .

Alloys from the succinonitrile–(D)camphor (SCN–DC) system are widely used as model alloys for the in situ investigation of solidification using light optical microscopy, but literature on the binary phase diagram is contradictory with respect to the solubility limit of DC in (SCN). Phase equilibria of the system were therefore revisited experimentally and critically assessed in the present work. The results prove that the maximum solubility of DC in the succinonitrile solid solution (SCN) is far less 1 wt% and the volume fraction of the (DC) phase in the eutectic solid is 23.3%. On this basis and on recently reported experimental data the CALPHAD description of the SCN–DC system was re-optimized.

An experimental method for the in-situ observation of solidification in eutectic metallic alloys growing at moderate temperatures, i.e. 500–600 °C was developed. It is based on unidirectional solidification of thin samples (200 to 700 μm thick) contained in rectangular quartz glass tubes allowing for in-situ observation of growth on the sample surface by means of light microscopy using a long-distance microscope. The method is illustrated for two-phase eutectic growth in Al–17.5Cu–1.0Ag, Al–11.8Si and Al–11.8Si–0.025Sr as well as for three phase growth in Al–13.3Cu–5.7Si, all at%. Coupled growth of lamellar Al–Al2Cu eutectic in Al–17.5Cu–1.0Ag was investigated in more detail with regard to spacing selection: experiments were performed in a temperature gradient of 38±1 K cm−1 using different, but constant withdrawal velocities ranging from 0.025 to 1.8 μm s−1. Measured spacing was compared with literature data and discussed with reference to the Jackson–Hunt theory and the recent amendment proposed by Akamatsu, Faivre, Karma and Plapp. In-situ observations were complemented by post-mortem metallographic analysis using SEM and EBSD in transverse sections.

A new experimental method for in-situ observation of microstructure formation in top view during unidirectional solidification of bulk, transparent, organic alloys is presented. This method allows observing growth patterns over an extended interface area with high resolution and minimal optical aberrations. With (D)camphor – neopentylglycol – succinonitrile (DC–NPG–SCN) alloys a series of unidirectional solidification experiments were performed in order to validate the set-up. By means of multi-focus exposition eutectic cells were observed over a depth of several millimeters, followed by 3D reconstruction of their shape. The method also allows capturing the integral interface dynamics and measuring its relative undercooling.

Several key experiments were performed to determine the temperature of transformations of organic alloys from the ternary systems 2-amino-2-methyl-1,3-propanediol–(D)camphor–succinonitrile (AMPD-DC-SCN) and 2-amino-2-methyl-1,3-propanediol–neopentylglycol–succinonitrile (AMPD-NPG-SCN) by means of differential scanning calorimetry (DSC). The phase diagrams of these ternary systems were assessed via the CALPHAD approach using Thermo-Calc and the experimental data on phase equilibria were determined by the above key experiments. Good agreement between the experimental and the calculated data was achieved for both ternary systems. Combining the obtained thermodynamic descriptions with those obtained for two other ternary systems, AMPD–DC–NPG and DC–NPG–SCN, published earlier, the thermodynamic description for the entire quaternary system was created. In order to identify the phases involved in solid–liquid equilibria and the nature of coupled eutectic growth in quaternary alloys, several unidirectional solidification experiments were performed: two quaternary alloys with composition 37.5SCN–44.5DC–16.5NPG–1.5AMPD (wt%) and 34SCN–44DC–19NPG–3AMPD (wt%) were used for the experiments on monovariant eutectic growth. We find that in the last alloy the monoclinic NPG phase is fully suppressed by the addition of AMPD and three solid phases show coupled eutectic growth at the solid/liquid interface: the hexagonal phase (DC solid solution), the FCC phase (NPG–AMPD solid solution) and the BCC phase (SCN solid solution). The quasi-2D eutectic patterns obtained in a thin sample show the stacking sequence –BCC–Hex–FCC–Hex–. The volume ratio of Hex:BCC:FCC phases equals 4:3:2. In the quaternary alloy with 1.5 wt% AMPD the monoclinic NPG phase is present at the solid/liquid interface and the eutectic growth is rather irregular.

The temperature and enthalpy of transformations of organic alloys from the ternary system 2-amino-2-methyl-1,3-propanediol–(D)camphor–neopentylglycol (AMPD–DC–NPG) were measured by means of differential scanning calorimetry (DSC). The phase diagram of this ternary system was assessed via the CALPHAD approach using Thermo-Calc based on the thermodynamic descriptions of the constituent binaries published earlier and on the thermodynamic and phase equilibrium data measured in the present work for ternary alloys. Good agreement between the experimental and calculated data was achieved. Experiments and calculations show that the AMPD–DC–NPG exhibits a nonvariant quasiperitectic reaction at 336.1 K and composition AMPD-17.5 mol% DC-42.56 mol% NPG, with all solid phases being nonfaceted. Unidirectional solidification experiments were performed with three different alloys selected such that each exhibits a small amount of the primary BCC (AMPD solid solution), the FCC (NPG solid solution) and the hexagonal (DC solid solution) phase, respectively, in order to verify the nature of the phases involved in solid–liquid equilibria and the nature of coupled growth in this ternary system. We find that in all these cases once steady state growth is reached two solid phases grow at the solid–liquid interface with a rod-like coupled growth pattern: The hexagonal phase (DC solid solution) and the FCC phase (NPG solid solution).

The temperature and enthalpy of transformations of organic alloys from the ternary systems (D)camphor–neopentylglycol–succinonitrile (DC–NPG–SCN) were measured by means of differential scanning calorimetry (DSC). The phase diagram of this ternary system was assessed via the CALPHAD approach using Thermo-Calc based on the thermodynamic descriptions of the constituent binary systems published earlier and the thermodynamic and phase equilibrium data for the ternary alloys measured in the present work. Proper agreements between the experimental and the calculated data were achieved. Experiments and calculations show that the DC–NPG–SCN exhibits a nonvariant eutectic reaction with the eutectic point at 297.0 K, 26.3 mol%DC and 14.8 mol%NPG. This eutectic reaction involves the ordered monoclinic crystals (OCs) of NPG and the orientationally disordered crystals (ODICs) of both, SCN (BCC_A2 phase) and DC (hexagonal phase). Unidirectional solidification experiments were performed with the nonvariant eutectic DC–NPG–SCN alloy in order to verify the phases involved in the solid–liquid equilibrium and the nature of eutectic growth. We find that in the eutectic alloy the eutectic growth is irregular: the extremely facetted monoclinic phase (NPG) grows as thin and long needle-like crystals that protrude far into the liquid and are then decorated with nonfacetted eutectic of BCC_A2 (SCN) and hexagonal (DC) lamellae, which mainly grow perpendicular to the growth direction of the monoclinic (NPG) needles. Few unidirectional solidification experiments were performed with dilute ternary alloys with composition close to the univariant eutectic reactions that extend from the constituent binary systems NPG–DC and SCN–DC, respectively. The destabilisation of planar eutectic interfaces due to constitutional supercooling were qualitatively monitored.

Transparent organic alloys showing a plastic crystal phase were investigated experimentally using differential scanning calorimetry and directional solidification with respect to find a suitable model system for regular ternary eutectic growth. The temperature, enthalpy and entropy of phase transitions have been determined for a number of pure substances. A distinction of substances with and without plastic crystal phases was made from their entropy of melting. Binary phase diagrams were determined for selected plastic crystal alloys with the aim to identify eutectic reactions. Examples for lamellar and rod-like eutectic solidification microstructures in binary systems are given. The system ( d )Camphor–Neopentylglycol–Succinonitrile is identified as a system that exhibits, among others, univariant and a nonvariant eutectic reaction. The ternary eutectic alloy close to the nonvariant eutectic composition solidifies with a partially faceted solid–liquid interface. However, by adding a small amount of Amino-Methyl-Propanediol (AMPD), the temperature of the nonvariant eutectic reaction and of the solid state transformation from plastic to crystalline state are shifted such, that regular eutectic growth with three distinct nonfaceted phases is observed in univariant eutectic reaction for the first time. The ternary phase diagram and examples for eutectic microstructures in the ternary and the quaternary eutectic alloy are given.

Interface pattern evolution during directional solidification has been investigated in the microgravity environment of a space shuttle mission providing diffusive conditions for cellular pattern evolution. A model transparent alloy of succinonitrile–acetone was used in these studies for in situ observation of the dynamic process of pattern evolution. In addition to the microgravity environment, alloy composition and growth condition experiment parameters were selected in such a way that the planar-to-cellular transition was supercritical, and pattern evolution was examined close to the transition velocity that gave rise to very small amplitude cells. Since the supercritical transition shows continuous increase in amplitude with velocity, non-linear effects in wavelength selection are small. This means that linear theory could be used to interpret the results. The evolution of primary spacing and the order of cellular arrangements were characterized as a function of time for three different velocities. No sharp wavelength selection in primary spacing was observed in the experimental results, and the distribution of steady-state spacing was characterized for each velocity. Wavelength selection for supercritical transition near the transition point can also be explained by stability analysis.

Cellular pattern formation in the system succinonitrile–0.075 wt% acetone in a supercritical regime is analyzed on the basis of a set of purely diffusive space experiments in bulk samples. Order–disorder investigations using the minimal-spanning-tree approach yield steady-state patterns for the microgravity experiments of an order which suggests a honeycomb structure disturbed by Gaussian noise. The most important morphology parameter, primary spacing of the cellular array, is critically discussed in comparison to the established theoretical models. It is found that analytical models fail to describe cell sizes in supercritical cellular growth. In contrast, excellent agreement is demonstrated using the numerical model of Hunt. This implies that modelling supercritical cellular growth requires consideration of interacting solute diffusion fields ahead of the cells and free development of cell shapes.

Directional solidification with a cellular solid–liquid interface morphology is characterized by two typical length scales – the size of the cells and the thickness of the solutal boundary layer ahead of the growing interface. The interaction of the solutal field with the morphology is investigated by using the model alloy succinonitrile–acetone which allows by its transparency the in situ observation of the interface structure. To avoid wall effects and decouple the solidification parameters temperature gradient and solidification velocity a rod-like bulk sample and a Bridgman Stockbarger-type gradient furnace are used. In the experiments the development of the interface morphology is affected by buoyancy driven melt convection which stabilizes significantly the planar morphology. At the planar to cellular transition quasi-hexagonal and elongated cells exist simultaneously, both at grain boundaries as well as inside the grains. With increasing solidification length a quasi-hexagonal pattern covers the interface which shows a random rearrangement of the cells. The size of the cells varies in a wide range and no wavelength selection is found.