To characterize the dynamical formation of three-dimensional (3-D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile–0.24 wt.% camphor model alloy was carried out under low gravity in the Device for the Study of Critical Liquids and Crystallization (DECLIC) Directional Solidification Insert on board the International Space Station (ISS). The present paper focuses on the study of the transient solid–liquid interface recoil. Numerical thermal modeling led us to identify two thermal contributions to the interface recoil that increase with the pulling rate and add to the classical recoil associated with the solute boundary layer formation. As a consequence of those additional contributions, the characteristic front recoil is characterized by a fast initial transient followed by stabilization to a plateau whose location depends on pulling rate. The analysis of comparative experiments carried out on the ground shows the absence of stabilization of the interface position, attributed to longitudinal macrosegregation of the solute induced by convection. This behavior is surprisingly also observed in space experiments for low pulling rates. An order of magnitude analysis of the mode of solute transport reveals that for these conditions, the effective level of reduced gravity on board the ISS is not sufficiently low to suppress convection so that the interface recoils with longitudinal macrosegregation in a similar way as in ground experiments.
Research Containing: Directional solidification
Structures in directionally solidified Al–7 wt.% Si alloys: Benchmark experiments under microgravity
Microgravity offers a unique opportunity to achieve solidification in the limit of diffusive transport. Directional solidification experiments in Al–7 wt.% Si alloys were carried out under such conditions on board the International Space Station in orbit around the Earth. Microstructural characterizations include the elongation factor and equivalent diameter of the dendritic grains, together with the dendrite arm spacing and the percentage of eutectic. The experimental investigations reveal that coarse randomly oriented dendritic grains promote non-uniform distribution of eutectic and enhance intergranular segregation. The columnar-to-equiaxed transition (CET) observed in the dendritic grain structure of the refined alloys, sharp or progressive, is defined and characterized based on the profile of the averaged elongation factor. Progressive CET is revealed by an intermediate zone where elongated and equiaxed dendritic grains coexist, sandwiched between the columnar and the equiaxed zones. Fragmentation is also observed in non-refined alloy experiments by electron backscattered diffraction analyses. Capillary-driven detachment during coarsening is suggested to explain this finding, while dendrite fragments cannot cause CET under microgravity because of the absence of convection. This unique set of well-characterized experiments serve as benchmark data for direct numerical simulation of structures and segregations.
Disruption of an Aligned Dendritic Network by Bubbles During Re-melting in a Microgravity Environment
The Pore Formation and Mobility Investigation (PFMI) utilized quartz tubes containing succinonitrile and 0.24 wt% water “alloys” for directional solidification (DS) experiments which were conducted in the microgravity environment aboard the International Space Station (ISS; 2002–2006). The sample mixture was initially melted back under controlled conditions in order to establish an equilibrium solid-liquid interface. During this procedure thermocapillary convection initiated when the directional melting exposed a previously trapped bubble. The induced fluid flow was capable of detaching and redistributing large arrays of aligned dendrite branches. In other cases, rapidly translating bubbles originating in the mushy zone dislodged dendrite fragments from the interface. The detrimental consequence of randomly oriented dendrite arms at the equilibrium interface upon reinitiating controlled solidification is discussed.
Dynamics of interface pattern formation in 3D alloy solidification: first results from experiments in the DECLIC directional solidification insert on the International Space Station
One of the critical microstructures in directional solidification of alloys is the cellular/dendritic pattern that governs the properties and reliability of the solidified material. Our quantitative understanding of the solid–liquid interface pattern has come mainly from experiments in thin samples where the microstructure selection is shown to occur during the dynamical growth process. A more realistic configuration is to examine the evolution of microstructure in three-dimensions, which is not possible terrestrially since convection effects dominate in bulk samples and prevent precise characterization of microstructure dynamics. Recently, experiments under low gravity conditions have been carried out jointly by NASA and CNES in the model transparent system succinonitrile–camphor on board the International Space Station using the Directional Solidification Insert in the DECLIC facility. After a brief description of the experimental setup and methods, the first results on dynamics of interface pattern formation obtained in microgravity will be presented and the information extracted from their quantitative analysis will be discussed.