[What We Can Do]

[IMAGE] We ( Dr. Peter A. Curreri of National Aeronautics and Space Administration at Marshall Space Flight Center and Dr. William F. Kaukler of the Center for Materials Research at The University of Alabama in Huntsville) are using a high resolution x-ray microscope to view, in-situ and in real time, interfacial processes in metallic systems during freezing. Studies of this type are not being performed anywhere else. The instrument is in the background of the picture.

~~~~ Picture gallery of microradiographs are below ~~~~

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For bigger images, click on the pictures.

In the field of materials science it is well known that many of the properties of a material stem from the structure of that material. During the last century, by examining metal alloys with an optical microscope after polishing and etching the surface, it was discovered that the microstructures influenced the material's properties. The initial microstructure forms during the casting process where the melted alloy becomes a crystalline solid. In various ways, 80% of ALL industry involves a casting or solidification process of a material. Clearly it is important to understand this subject.

Physical processes which occur at, or near, the solid-liquid interface during solidification lead to a variety of patterned structures or morphologies. These interfacial morphologies account for the various microstructures found in alloys partly due to the composition and partly to the solidification conditions. Certain tricks were required to see these interfacial structures while they formed since people could not see inside a metal as it solidified. One trick was to quench the material and rapidly freeze the interface shape to be later seen upon sectioning (metallography). Another trick was to decant (pour off) the molten metal that had not yet solidified and leave behind the solid part of the interface structure. The best trick was to not use metal at all, but instead study the solidification of transparent organic models for metallic solidification. Every trick used still couldn't show exactly what's going on during this transformation of liquid to crystalline solid. In addition, little information was obtained about the dynamics of the processes.

The X-ray Transmission Microscope (XTM) operates in the hard x-ray range (10 to 100 keV) and achieves magnification through projection. (explained later). We have obtained, using aluminum alloys, real-time images of the evolution of interface morphologies with characteristic lengths as small as 10 µm (micrometers), interfacial solute accumulation and formation of droplets (5 µm). We are addressing the complex issues of resolution, contrast and minimal exposure time and improving the capability of the XTM.

Some radiographs of alloys captured during solidification are presented below.

The next web page shows how it is done.

[IMAGE]

Interface of Aluminum-Lead (Al-Pb) alloy showing liquid (dark left side) and solid. Solidification is progressing from right to left. The solid portion on the right shows three regions with different growth rates as marked. The band structure in the solid (zebra stripes vertical) changes spacing with this growth rate change. The liquid near the solid is quite dark due to the accumulation of the dense lead (Pb) in the liquid. This is a solute layer formed by the rejection of the lead by the solid and can only diffuse away into the liquid away from the interface. This rejection is a normal solidification phenomenon but has never been photographed before in a metal alloy.
For bigger images, click on the pictures.
[IMAGE]

Interface of Aluminum-2% Silver (Al-Ag) alloy showing liquid (melt) on the left side and solid Al 'fingers' on the right. Solidification is progressing from right to left. This interface has what is called a cellular morphology. Constitutional undercooling (solidification phenomenon in alloys) brought on by the solute (Ag) buildup during solidification lead to the formation of these fingers of Al surrounded by Ag-rich melt. These cells are about 75 µm across. Since the cells are so small, they overlap in layers and it is not easy to delineate them except when you are lucky find a clean area like here. The growth rate is 2 µm/sec. The image was taken with a 2 second exposure with 55 kV acceleration and 200 µA input current.
For bigger images, click on the pictures.
[IMAGE]

Interface of Aluminum-Indium (Al-In) alloy showing how the Indium solute is collecting ahead of the solid in the liquid region during growth as solidification progresses from right to left. The dark cloud to the left of the interface is this solute layer in the liquid (melt). The darker portion on the right was solidified at a slower rate than the light colored part between the interface and this darker region. It is this higher growth rate that is causing the more solute to be collected ahead of the interface in the liquid instead of being deposited in the solid thus making the solid 'lighter' or less absorbing to x-rays.

For bigger images, click on the pictures.
[IMAGE]

Interface of Aluminum-2% Silver (Al-Ag) alloy about to engulf a gas bubble that formed in the melt. The interface itself is growing with a cellular structure due to the build-up of silver in the melt resulting in constitutional supercooling that leads to the interface breakdown. (See also image above.) The extra silver in the liquid accounts for the darker appearance of the melt and in the cell walls. This is the first image of its kind showing the growing cellular interface AND the engulfment of the void. The growth rate is 2 µm/sec and the thermal gradient is 47 degrees C per centimeter.

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Last Updated October 2006