Physical processes which occur at, or near, interphase boundaries during solidification, or other phase transformations, play a major role in the determination of many of the technologically important properties of solids. To-date, interfacial morphologies and particle-interface interactions in the respective metallic, optically opaque systems have been deduced from post-process metallographic analyses of specimens. Thus, little information is obtained about the detailed dynamics of the processes. This shortcoming impedes fundamental research and important technology developments, since the phenomena which occur at interphase boundaries during solidification determine many of the important properties of solids.
We have developed a high resolution x-ray microscope to view, in-situ and in real time, interfacial processes in metallic systems during freezing. The X-ray Transmission Microscope operates in the hard x-ray range (10 to 100 keV) and achieves magnification through projection. We obtained, for the first time in a metal system, in-situ records of the evolution of interface morphologies with characteristic lengths as small as 35 µm, interfacial solute accumulation and formation of droplets (70 µm).
With metallic and semiconducting samples, the penetration of macroscopic layers requires photon energies in excess of 10 keV. This precludes the use of optical approaches for the focusing. Hence, only contact or projection radiography can be employed in this energy range. Projection radiography uses the divergence of the beam from a very small source. The ultimate resolution is limited by the diameter of the source. Hence, x-ray projection radiography requires micro-focus tubes.
Such observations require sufficient contrast (difference in absorptance) between features to be resolved and the retention of this contrast by the imaging devices (x-ray converter, image intensifier, camera, recording device). In monocomponent metallic systems, contrast between solid and melt is determined by the (electron cloud) density of the two phases, typically only 2-5%.
The figure below schematically indicates the components of the system and their placement. A metal sample (thickness of order 1 mm) is contained in a high transmittance crucible. A high temperature furnace on a translation stage imposes a temperature gradient onto the sample. The solid-liquid interface is positioned in close proximity to the focal spot of a micro-focus x-ray source. The diverging x-ray beam permeates the sample and the resulting shadow falls on an x-ray image converter. The resulting visible image is converted to a digital image by a CCD camera and stored in a computer. This image is displayed on a high resolution monitor, either in real time or after further processing (contrast enhancement, filtering, etc.).
The development of the microstructure of metal alloys during solidification processing can now directly observed in a way that has never been done before. This technique has been extended to examining the particulate structure of metal matrix composites with particulate reinforcing particles. Many other studies are already planned but the instrument is still being improved.