J. C. LaCombe

CROSSOVER SCALING IN DENDRITIC EVOLUTION AT LOW UNDERCOOLING

We examine scaling in two-dimensional simulations of dendritic growth at low undercooling, as well as in three-dimensional pivalic acid dendrites grown on NASA’s USMP-4 isothermal dendritic growth experiment. We report new results on self-affine evolution in both the experiments and simulations. We find that the time-dependent scaling of our low undercooling simulations displays a crossover scaling from a regime different than that characterizing Laplacian growth to steady-state growth. [S0031-9007(99)09307-2]

Implications of the interface shape on steady-state dendritic crystal growth

Experimental results obtained as part of a series of experiments on dendritic growth in microgravity, the isothermal dendritic growth experiment (IDGE), indicate that Ivantsovs transport solution to the dendrite problem is valid to first order. The data reveal a Peclet (Pe) number vs. supercooling relationship that is both systematically lower than expected, and exhibits larger scatter than is accounted for by the uncertainties of the individual data points. A range of explanations have been offered, including residual convection, far-field chamber effects, and dendrite self-interaction. Although some of these explain certain features of the experimental data, quantitative evidence remains lacking concerning the cause of the two issues mentioned above. This investigation examines the dendritic growth data in the context of how the actual shape of the dendrite tip and its trailing side-branch structure may affect the transport process. The method of moving heat sources is used to describe the diffusive thermal transport processes around a crystalline body of revolution growing into its quiescent melt at a constant rate. Results indicate that when Ivantsovs assumed paraboloidal tip shape is modified to better reflect the actual observed tip shape, enhanced agreement can be obtained between the model and the experimental data. Additionally, these results support earlier work by Schaefer (J. Crystal Growth 43 (1978) 17) in predicting that under the conditions of the IDGE experiment, the side-branch region of a dendrite can contribute significantly to the thermal field at the tip. This suggests that the scatter in the IDGE data can be explained by stochastic variations in this influential side-branch structure. With these observations in hand, it is reasonable to claim that the basic transport solution describing dendritic growth is correct, provided adequate account has been taken of details such as container wall effects and dendrite self-interaction. Until now, these conclusions have not been completely supported by quantitative evidence.

Use of microgravity to interpret dendritic growth kinetics at small supercoolings

The Isothermal Dendritic Growth Experiment (IDGE), first performed in low-earth orbit in March of 1994 showed variation in the growth data beyond that due to measurement uncertainties, and a significant deviation from predictions of diffusive transport theory with boundary conditions at infinity. Recently, two models described in the J. Crystal Growth suggested modifications from the Ivantsov model to describe the heat transfer of a dendrite growing into a supercooled melt. One model, by Sekerka et al. [J. Crystal Growth 154 (1995) 370], describes how convection resulting from the residual micro-accelerations present on orbit could enhance the heat transfer. Another, by Pines et al. [J. Crystal Growth 167 (1996) 383], describes the observed differences as a thermal boundary layer effect arising from the proximity of the growth chamber wall to the dendrite. Recent in-situ telemetry of dendrite images received during the second flight of the IDGE in March 1996 showed no correlation between the variations in the crystal growth velocities and the quasi-static microgravity environment.

The Clapeyron effect in succinonitrile : applications to crystal growth

Abstract This paper describes experiments that measure the Clapeyron effect, i.e., the change in melting temperature with static pressure in succinonitrile (SCN), a common model material used in crystal growth experiments. The Clapeyron results also yield information about the density change upon solidification – a parameter that is difficult to measure accurately and directly in SCN due to void formation in the solid. Furthermore, making a pressure change in an experimental environment presents researchers with a way to change more rapidly and uniformly the supercooling of a melt than by using conventional heating and cooling techniques.

The Isothermal Dendritic Growth Experiment

The growth of dendrites is one of the commonly observed forms of solidification encountered when metals and alloys freeze under low thermal gradients, as occurs in most casting and welding processes. In engineering alloys, the details of the dendritic morphology directly relates to important material responses and properties. Of more generic interest, dendritic growth is also an archetypical problem in morphogenesis, where a complex pattern evolves from simple starting conditions. Thus, the physical understanding and mathematical description of how dendritic patterns emerge during the growth process are of interest to both scientists and engineers. The Isothermal Dendritic Growth Experiment (IDGE) is a basic science experiment designed to measure, for a fundamental test of theory, the kinetics and morphology of dendritic growth without complications induced by gravity-driven convection. The IDGE, a collaboration between Rensselaer Polytechnic Institute, in Troy NY, and NASAs Lewis Research Center (LeRC) was developed over a ten year period from a ground-based research program into a space flight experiment. Important to the success of this flight experiment was provision of in situ near-real-time teleoperations during the spaceflight experiment.

THE EFFECT OF CONVECTION ON DENDRITIC GROWTH UNDER MICROGRAVITY CONDITIONS

The Isothermal Dendritic Growth Experiment (IDGE) is an orbital space flight experiment, launched by NASA, in March, 1994, as part of the United States Microgravity Payload (USMP-2). The IDGE provided accurately measured dendritic growth rates, tip radii of curvature, and morphological observations of ultra-pure succinontrile obtained at supercoolings in the range 0.05-2.0 K. Data were received in the form of pairs of digitized binary images telemetered to the ground from orbit in near-real-time, and as 35mm photographic film received 3 months after the flight. The IDGE flight data has now been analyzed, permitting a comprehensive comparison between dendritic growth under terrestrial and microgravity conditions. The measured growth kinetics, in the form of velocity versus supercooling, is markedly different from those observed in terrestrial experiments. Above 0.4 K supercooling in microgravity, the process of dendritic growth is diffusion controlled, i.e., thermal conduction is the rate limiting process....

Measurement of thermal expansion in liquid succinonitrile and pivalic acid

Abstract We present here, the experimental measurement of the thermal expansion of 99.99% pure, liquid succinonitrile (SCN), and pivalic acid (PVA). The optical transparency and convenient melting temperatures of these materials, allows in situ observation of solidifying morphological structures, which makes these substances valuable for use as “model” materials in the experimental investigation of solidification processes in bcc (SCN) and fcc (PVA) systems. The thermal expansion coefficient in these pure melts provides information which is needed in the development of complimentary numerical and analytical models describing the solidification process and the influence of natural convection.

Space flight data from the isothermal dendritic growth experiment

Abstract The Isothermal Dendritic Growth Experiment (IDGE) is a NASA space flight experiment which flew as part of the United States Microgravity Payload (USMP-2), in early 1994. The IDGE measured dendritic growth rates, tip radii, and crystal morphologies of ultra-pure succinonitrile [CN-(CH 2 ) 2 -CN] at supercoolings in the range from 0.05–2.0 K. Data taken in the form of slow-scan binary digitized images telemetered to the ground from orbit in near-real time, combined with the IDGE terrestrial data set, provide the first quantitative assessment of various theories on dendritic solidification and the effects of convection on dendritic growth.

Evidence for Eigenfrequencies in Dendritic Growth Dynamics

Microgravity dendritic growth experiments, conducted aboard the space shuttle Columbia, are described. In-situ video images reveal that pivalic acid dendrites growing in the diffusion-controlled environment of low-earth orbit exhibit a range of transient or non-steady-state behaviors. The observed transient features of the growth process are being studied with the objective of understanding the mechanisms responsible for these behaviors. Included in these observations is possible evidence for characteristic frequencies or limit cycles in the growth behavior near the tip of the dendrites. These data, and their interpretations, will be discussed.

Time-Dependent Behavior of Dendrites Under Diffusion-Controlled Conditions

Dendrites interact with hydrodynamic flows during solidification. In the presence of gravity, thermal and solutal buoyancy forces induce convective motion in the melt. The basic theories of dendritic growth, however, are best tested under diffusion-controlled conditions, where, ideally, gravitational acceleration and convection are absent, or at least drastically reduced. Microgravity experiments of the Isothermal Dendritic Growth Experiment (IDGE) were designed to measure convection-free dendritic growth. IDGE experiments to accomplish this were flown on United States Microgravity Payload Missions: USMP-2 (March 1994), USMP-3 (March 1996), and USMP-4 (December 1997).