In situ characterization of metals at extremes
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Introduction The need to develop new materials that can perform well under the extreme conditions of high temperature, radiation dose, strain rate, and pressure (either individually or in combination) presents a unique set of challenges for structural characterization methods. Instrumentation must be modified to permit some semblance of the extreme condition(s) to be safely replicated in the experiment, while at the same time allowing the experiments to be performed on the critical time and length scales important for the materials’ function. As methods to understand the long-term aging of components under extreme conditions are well established (simple before-and-after experiments are sufficient provided you can wait long enough), it is the short-timescale interactions that are at the forefront of new developments in in situ experimentation. In this article, the main issues involved in studying metals under extreme conditions are discussed for the current major methods of in situ characterization: transmission electron microscopy (TEM), synchrotron radiation, and short-pulse-laser–driven optical methods. For each of these methods, an example of the type of experiment that can be performed will be described, and the potential of each method for future advancements in the study of extreme conditions will be discussed.
The need for in situ measurements Since post-mortem materials characterization techniques have been in existence for many years, the first thing we need to
address with this article is “Why are in situ studies necessary?” This question takes on even more consequence for metals under extreme conditions that are very difficult to establish in the laboratory. So why should we struggle to do this? It is the very nature of the extreme conditions that makes in situ studies so valuable, even if the analyses do not achieve the same resolution as static experiments. In the extreme conditions where we need metals to function, structural changes are rapid, with many processes often occurring simultaneously (such as phase transitions, secondary phases, voids, and dislocations), and materials may progress through intermediate states that exist for only short periods of time. If we are to control the structure of materials under extreme conditions, we need to know the order in which the structural changes occurred and whether we can change the final outcome of the extreme conditions by changing the kinetics of the intermediate states.
Phase transformations with rapid thermal gradients Since its invention, TEM has excelled at determining and quantifying the structure of materials and the presence of defects at small length scales.1 The development of aberration correctors2,3 over the last 10 years has taken the available direct-imaging resolution in state-of-the-art microscopes into the deep subangstrom regime. Significant, but less publicized, progress is also being made by creating controlled experimental conditions
N.D. Browning, University of California–Davis, One Shields Ave., Davis, CA 95616, USA; nbrowning@ucdav
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