X-ray Video Microscopy Research Articles

In-situ observation of transient columnar dendrite growth in the presence of thermo-solutal convection

The time evolution of tip radii and velocities of growing columnar dendrites have been extracted from an image sequence measured in situ by synchrotron X-ray video microscopy during directional solidification of Al-15%Cu-9%wtSi alloy. At the scale of the primary dendrites, a highly transient growth was observed in conjunction with liquid flow caused by modest thermo-soutal convection. The nominal spatial and temporal resolutions employed in the experiment were 1.4 μm at approximately 100ms frame rate, respectively. A very prominent mutual inter-relation between the solute field and flow velocity field and the dendrite growth rate and morphology is evident in these observations. Image processing allowed extraction of the corresponding projection averaged solute field in each frame, and an estimate of the solute boundary velocity field by frame-to-frame tracking of its motion. These measurements may eventually support development of statistical dendrite kinetics models for transient states in alloy solidification processes that could be applied in microscopic scale solidification models such as phase field.

In Situ X-Ray Video Microscopy as a Tool in Solidification Science

Over the last 20 years, time-resolved high-resolution x-ray imaging techniques have become established as methods of choice for in situ studies of solidification microstructure formation in metallic alloys. Using high-brilliance x-radiation delivered at modern synchrotrons, combined with the most recent advances in x-ray cameras and optics, it is now possible to capture real-time video sequences with projection image resolutions down to 100 nm at frame rates of 20 Hz. This article presents some illustrative examples of the current capabilities of in situ x-ray video microscopy in terms of radiographic and topographic imaging as a platform for studying microstructure pattern formation, growth dynamics, and other topics related to metallic alloy solidification.

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.

Analysis of an Equiaxed Dendrite Growth Model with Comparisons to <i>In-Situ</i> Results of Equiaxed Dendritic Growth in an Al-Ge Alloy

The Lipton Glicksman Kurz (LGK) growth model is commonly used to predict growth rates for equiaxed dendrites in solidifying mushy zones. However, the original LGK method treats an isolated dendrite growing in an infinite volume of liquid. In an equiaxed mushy zone, with multiple nucleation events, thermal and solutal interactions take place between the equiaxed dendrites. A modified version of the LGK model was developed that allows for measurement of the solute build-up ahead of the dendrites. To investigate the validity of the model, comparisons are made with results obtained from in-situ synchrotron X-ray videomicroscopy of solidification in a Bridgman furnace of an Al-12wt.%Ge alloy inoculated with Al-Ti-B grain refiner. Comparisons between the original LGK and modified LGK models are presented for discussion. The modified LGK model shows realistic tip temperature trends.

<i>In Situ</i> Investigation of Liquid-Liquid Phase Separation in Hypermonotectic Alloys

Hypermonotectic alloys are distinguished by a temperature region for which the homogeneous melt decomposes into two liquid phases. In Al-based hypermonotectics, the minority phase is much higher in density than the matrix melt phase and consequently macro-segregation due to sedimentation is an inherent problem when casting these alloys. However, under the correct solidification conditions, it may be feasible to counteract sedimentation by thermocapillary forces that arise due to the thermosolutal dependence of the surface tension between the two liquid phases. The current investigation involved in-situ X-ray video microscopy studies during directional solidification of Al-Bi samples of various compositions employing a Bridgman furnace. It was found that large undercoolings were required to initiate the L  L1 + L2 reaction and L2 nucleation occurred heterogeneously on the monotectic front. L2 droplets were then set in collective size-dependent motion by hydrodynamic forces coupled to external fields (i.e. temperature gradient and gravity) and internal fluctuations. The resulting flow fields are superimposed onto the short range coagulation mechanisms, such as diffusion coupling, and are all crucial in determining the final size distribution and dispersion of the Bi-rich phase in the cast material.

Mesoscopic Simulation of Dendritic Growth Observed in X-ray Video Microscopy During Directional Solidification of Al–Cu Alloys

A mesoscopic model is developed to simulate microstructures observed in situ by X-ray video microscopy during directional solidification of Al–Cu alloys in a Hele–Shaw cell. In the model, a volume-averaged species conservation equation is solved to obtain the solute concentration and solid fraction fields, and an analytical stagnant film model is used to predict the motion of the dendrite envelopes. The model is carefully validated in several test cases. Then, the model is applied to simulate the columnar dendritic microstructures observed in the X-ray video microscopy experiments for two different alloy compositions. Reasonable agreement is found between the measured and predicted dendrite envelope shapes, solid fractions, and solute concentration fields. The predicted size of the mushy zone and the extent of the undercooled melt region ahead of the columnar front agree well with the in situ experimental observations. The simulation results show quantitative agreement with the internal solid fraction variations measured from the radiographs. The present model is also able to realistically simulate a primary dendrite trunk spacing adjustment that was observed in one of the experiments. Overall, the present study represents the first successful validation of a solidification model using real time, in situ data from an experiment with a metallic alloy. Considerable additional research is needed to account in the model for the effect of gravity driven melt convection.

X-Ray Video Microscopy Studies of Irregular Eutectic Solidification Microstructures in Al–Si–Cu Alloys

In-situ studies of Al-Si eutectic growth has been carried out for the first time by X-ray video microscopy during directional solidification of Al-Si-Cu alloys with and without Sr-addtions. The unmodified eutectics showed distinctive non-isothermal growth dynamics, where Si-crystals attained needle-like tip morphologies and progressed under significantly higher undercooling than Al, leading to formation of an irregular eutectic with Si as the leading phase and subsequent nucleation of Al on the Si-surfaces. In the Sr-modified alloys, the eutectic reaction was found to be strongly suppressed, occurring with low nucleation frequencies at undercoolings in the range 10-18 K. In the Cu-enriched melt, the eutectic front was found to attain meso-scale interface perturbations, sometimes evolving into equiaxed cellular rosettes in order to accommodate to the long-range redistribution of Cu from the composite eutectic interface. The eutectic front also attained shortrange microscale interface perturbations consistent with characteristics of a fibrous Si growth. However, further improvements in spatial resolution are required in order to study the microscale structure formation in greater detail. Evidence was found in support of Si-nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form on the Si-particles. To what extent nucleation mechanisms in the Cu-enriched systems are transferable to binary or commercial Al-Si alloys remains uncertain. © 2010 ISIJ.

Impact of buoyancy on the growth of equiaxed grains in Al–Cu

Equiaxed dendritic growth in grain refined Al–Cu (15, 20, 25 wt-%Cu) has been studied in-situ during directional solidification by means of high resolution synchrotron X-ray video microscopy. At these compositions, the α-Al grains have a lower density than the surrounding melt and experience buoyant forces which affect their growth rates and morphologies. A model has been derived, based on Scheil conditions to account for solute transport across the solid-liquid interface, and a spherical envelope approximation to the dendrite morphology in order to simplify both the interface geometry of the growing crystals and the Stokes drag exerted upon them during motion. The model was compared with experimental results to evaluate its present merits and to devise possible routes for further improvement and development.

The real-time, high-resolution x-ray video microscopy of solidification in aluminum alloys

The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.

Crystal fragmentation and columnar-to-equiaxed transitions in Al-Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

Stray crystal formation in Al–20 wt.% Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered at 3rd generation synchrotron sources have opened the field for in situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching true video microscopy. Here, observations on dendrite fragmentation from columnar fronts in Al–20 wt.% Cu and subsequent transport phenomena will be presented. From directional solidification experiments, it has been found that the frequency for crystal fragmentation, resulting from remelting of dendrite arm roots, depends strongly upon the buoyant forces. At 20 wt.% Cu concentration, primary aluminium dendrites are lighter than the melt and solidification experiments parallel- and anti parallel to gravity show significant differences in stray crystal formation tendency. Under particular conditions, it has been found that crystal fragmentation could lead to a meso-scale liquid segregation.