Castaing’s Electron Microprobe and Its Impact onMaterials ScienceDale E. NewburySurface and Microanalysis Science Division, National Institute of Standards and Technology, MS 8371, 100 Bureau Drive,Gaithersburg, MD 20899-8371Abstract: The development of the electron microprobe by Raymond Castaing provided a great stimulus tomaterials science at a critical time in its history. For the first time, accurate elemental analysis could beperformed with a spatial resolution of 1 µm, well within the dimensions of many microstructural features. Theimpact of the microprobe occurred across the entire spectrum of materials science and engineering. Contri-butions to the basic infrastructure of materials science included more accurate and efficient determination ofphase diagrams and diffusion coefficients. The study of the microstructure of alloys was greatly enhanced byelectron microprobe characterization of major, minor, and trace phases, including contamination. Finally, theelectron microprobe has proven to be a critical tool for materials engineering, particularly to study failures,which often begin on a micro-scale and then propagate to the macro-scale with catastrophic results.Key words: electron microprobe, electron probe microanalyzer, failure analysis, materials science, phase dia-gram, X-ray microanalysisINTRODUCTIONRaymond Castaing, in conjunction with his thesis supervi-sor, Professor A. Guinier, presented the first paper on theelectron microprobe, entitled “Application of electronprobes to metallographic analysis,” at the First InternationalCongress of Electron Microscopy held in Delft, the Neth-erlands, in 1949. They described an instrument in which astatic electron beam was focused to form a probe approxi-mately 1 µm in diameter that was directed at a thick targetto locally excite characteristic X-rays for spectrometric mea-surement. The area of interest could be selected with the aidof an optical microscope. This seminal paper marked thepractical beginning of the field of electron beam microanal-ysis, and indeed it laid the foundation of the much broaderfield of instrumental microanalysis. As such, it is interestingto note that the focus of the paper was directed to electronprobe applications in metallography, the study of the mi-crostructure of metals and alloys, a branch of the field weknow today as materials science. In 1949, the term “mate-rials science” would not have been recognized, rather thefield was known instead by the distinct and separate disci-plines of metallurgy, ceramics, glass, and polymers. (Com-posite materials, a materials topic of great importance to-day, were not yet recognized.) Workers in any of these fieldswould have readily understood the importance of Casta-ing’s breakthrough in establishing practical microanalysis.Beginning in the 1880s, the techniques of metallographydeveloped by Sorby in England, Martens in Germany, Os-Received November 1, 1999; accepted January 19, 2000.This publication is a contribution of a United States government employee.Microsc. Microanal. 7, 178–192, 2001DOI: 10.1007/s100050010082Microscopy ANDMicroanalysismond and Le Chatelier in France, and others, revealed themicroscopic structure of steel and other industrial alloys.These advances were enabled by the development of theoptical microscope to very nearly its modern level of per-formance. Researchers pursuing the techniques that came tobe known as “metallography” learned how to perform care-ful mechanical polishing of metallic alloys followed by se-lective chemical etching to produce differential relief onchemically distinct phases or at grain boundaries. With suchspecimens, reflection optical microscopy revealed structureswith micrometer and even finer dimensions. Examples ofthe excellent quality of their images of metallographically-prepared iron-based and Cu-Ag alloys are shown in Figures1 and 2. The microstructural world that was found provedto be highly complex, and most alloys were observed to bechemically differentiated into two or more distinct phases.In a short time, what has become the central paradigm ofmodern materials science was recognized, namely that themicrostructure of a material had a profound, often control-ling, impact upon macroscopic properties and behavior.Control of the microstructure meant control of criticalproperties such as strength, hardness, ductility, corrosionresistance, etc. In his introduction to his 1912 book, Metal-lography of Iron and Steel, Albert Sauveur, Professor of Met-allurgy and Metallography at Harvard University, noted:To realize the practical importance of metallogra-phy, it should be borne in mind that the physicalproperties of metals and alloys—that is, thoseproperties to which those substances owe their ex-ceptional industrial importance—are much moreclosely related to their proximate than to their ul-timate composition, and that microscopical ex-amination reveals, in part at least, the proximatecomposition of metals and alloys, whereas chemi-cal analysis seldom does more than reveal theirultimate composition. (Sauveur, 1912)Here the “ultimate composition” is what we todaywould refer to as the bulk composition, while the “proxi-mate composition” refers to the local microstructural com-position. Sauveur continued in his enthusiastic and colorfulprose:Unfortunately the chemist too often is able to giveus positive information in regard to the proportionof the ultimate constituents only, his reference toproximate analysis being of the nature of specula-tion. Ultimate analysis has reached a high degree ofperfection in regard to accuracy as well as to speedof methods and analytical chemists have built up amarvelous structure calling for the greatest admi-Figure 1. a: Structure of pearlite at high magnification as revealedby metallographic preparation, chemical etching, and optical mi-croscopy (Sauveur, 1912). b: MnS (dark) and pearlite in low car-bon steel viewed at high magnification as revealed by metallo-graphic preparation, chemical etching, and optical microscopy(Sauveur, 1912).Figure 2. Ag-Cu alloys: (a) Ag-28 wt% Cu; (b) Ag-65 wt% Cu asrevealed by metallographic preparation, chemical etching, and op-tical microscopy (Sauveur, 1912).Impact on Materials Science179ration, their searching methods never failing to laybare the ultimate composition of substances. Buthow much darkness still surrounds the proxi-mate composition of bodies and how great thereward awaiting the lifting of the veil!