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Materials Science MATERIAL MATTERS behavior (explanation of discontinuous yield and strain-aging in steels), random- Materials Science: walk theory applied to diffusion and later also to rubberlike elastidty, band theory of conduction in solids, cohesion of solids viewed in quantum-mechanical terms, and Parepistemes theory of color centers in ionic crystals. It is instructive to consider the attitudes of metallurgists and physidsts to disloca- tions. Many metallurgists were fiercely Rampant resistant; they thought the dislocation, pos- tulated in 1934, an unnecessary hypothe- Robert W. Cahn sis—and it was a hypothesis only, since its reality was not demonstrated unfil the end of the 1940s. Among physidsts there were positivists, like Percy Bridgman, who did This article is based on a plenary address which Robert W. Cahn, Cambridge University, deliveredno t take an interest in dislocations. Most in at the International Union of Materials Research Societies International Conference on Advancedbof h camps, however, were convinced by Materials (IUMRS-ICAM'99) hosted by the Chinese Materials Research Society (C-MRS) in the rigorous quantitative treatment, espe- Beijing, on June 14,1999. A Version ofthis article is also being published in Materials Today, the dally by Alan Cottrell. A particularly con- quarterly magazine published by Elsevierfor the European-MRS. A report ofthe IUMRS-ICAM'99 vincing example of the power of the strictly meeting will be published in afuture issue o/MRS Bulletin. quantitative approach was J.L. Snoek's linkage, 1939-1941, of internal friction in Disdplines emerge either by the fission science as distinct from a craft, scarcely wires of iron containing dissolved carbon or the fusion of older disdplines. The rea- goes back before the middle of the 19th or nitrogen to the solute concentration. son for such emergence may be profound Century. The fusion of metallurgy and That linkage allowed minute interstitial or eise trivial. Fission is the more common: solid-state physics came about because of concentrations to be measured by purely thus, physical chemistry was created and perceived educational needs in the late physical means. This marked the begin- split resolutely from organic chemistry in 1950s, and that fusion was cemented by ning of this century's conversion of compo- the 1880s because of the founders' passion the invention of the Materials Research sitional analysis of materials from a chemi­ to establish generalizations about reaction Laboratories in the United States in the cal into a physical skill. equilibria and kinetics as distinct from col- 1960s. The rearguard fight of those who lecting facts about particular reactions. did not want to be fused was long and bit­ That was a profound event. Chemical ter; great research metallurgists such as I have just used the physics split from physical chemistry in Robert Mehl fought hard against these the 1930s mainly because of many people's developments but they were defeated in word parepisteme, which exasperation with a physical chemist who the end. Others, in the first half of the 20th I have invented to denote a was also a particularly troublesome Jour­ Century, poured comprehensive scorn on self-contained subsidiary nal editor; practitioners still have great dif- the whole enterprise of sdentific metallur­ ficulty in pinpointing the difference gy: Thus the British inventor of stainless field of research. between physical chemistry and chemical steel, Harry Brearley, who was an excel- physics. That was a trivial event. Colloid lent practitioner but had no sdentific edu- In addition to the vital quantitative sdentists have made repeated attempts to cation, as late as the 1930s wrote, "To approach, one other factor brought wide- split away from physical chemistry on the know the ingredients of a rice pudding spread acceptance of novel concepts like basis of their focus on tiny particles, but and the appearance of a rice pudding dislocations and Guinier-Preston zones. the split has never firmly taken hold. when well made does not mean, dear This was an emphasis on Visual demon- The field of materials sdence and engi­ reader, that you are able to make one." stration: Seeing is believing. Earlier in the neering, MSE, is different. 1t came into That deeply entrenched conviction (and Century, the invention by Charles (C.T.R.) being by the fusion of metallurgy and all that flowed from it) was one which Wilson in Cambridge University of the solid-state physics, much later comple- materials scientists had to combat for cloud Chamber, side by side with the mented by the infusion of materials chem­ many decades. micrographic demonstration and measure- istry and even, to a limited extent, chemi­ Solid-state physics was in its nature ment of Brownian motion, had dispelled cal engineering. Solid-state physics is itself much more quantitative than was metal­ the last doubts of the anti-atomists, apart a relatively recent concept: It was not real- lurgy in the middle of the 20th Century. from a very few irrecondlable diehards. In ly recognized as a discipline before the From 1935 to 1955, approximately, a the 1950s, the developments of improved feats of physicists like Frederick Seitz, "quantitative revolution" occurred in the methods of optical microscopy (e.g., deco- Nevill Mott, William Hume-Rofhery, and borderlands of the two sdences, induding ration, etching, and infrared microscopy) Rudolf Peierls, some still living and some for example the elastic theory of dislocation led to a proper demonstration of the geom- only recently dead. (Hume-Rothery was etry of dislocations. The real visual break- through came from the introduction of in fact a chemist, with a PhD degree in transmission and scanning electron micros- metallurgy, who positioned himself in the Material Matters is a forum for copies in the mid-1950s. This, together with undefined area between physics, physical expressing personal points of the crucial invention of electron micro­ chemistry, and metallurgy and did more view on issues of interest to the probe analysis (the most important post­ to bring them closer than anyone eise materials Community. war instrumental invention, originally except Seitz.) Metallurgy itself, as a proper I MRS BULLETIN/SEPTEMBER 1999 3 Downloaded from https://www.cambridge.org/core. IP address: 170.106.51.11, on 04 Oct 2021 at 11:11:49, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S0883769400052957 MATERIAL MATTERS made by Raimond Castaing in France), However, once it acddentally became clear about as credible as if you had fired a 15- transformed the power of the new integrat- how to make large crystals, especially of inch shell at a piece of tissue paper and it ed science of MSE. The new Instruments, iron, they were used for fundamental came back and hit you." The point was jointly with new theories that were fed by research on metal plasticity, and incidental- that the Observation was wholly incompat- them, led to dramatic advances in the ly led to the perfection of a crystal growth ible with the then-current "currant-bun" understanding of phase transformations, method which was later used to grow Sili­ model of the atom. This quotation has the factors governing microstructure and con crystals for microelectronics. Other always seemed to me the perfect Illustra­ its quantitative characterization (stereolo- parepistemes, very detailed and subtie, did tion of Pasteur's principle. gy), and the identification of property grow directly from urgent practical needs: ■ Pierre Weiss and the recognition that the changes due to trace elements. The study of radiation damage in nuclear only way to Interpret the phenomenon of Much more recently, chemists have fuels is a prime example, while deforma- ferromagnetism was to postulate the exis- begun to accelerate their infiltration into tion maps constitute another. tence of magnetic domains, which were only MSE. "Materials chemistry" has become a demonstrated visually many years later. recognized concept and various new Jour­ ■ A.A. Griffith and the need to postulate nals include the term in their titles, where- The field of materials the existence of invisible microcracks in the as "materials physics" is not used in Jour­ surfaces of brittle materials in Order to nal titles (although one Journal is entitled science and engineering account for their low resistance to fracture. Applied Physics A—Materials Science and came into being by the ■ Hermann Staudinger and his insistence, Processing, and edited by someone work- fusion of metallurgy and against resolute Opposition, on the reality ing in an instirute for biophysical chem­ of covalently bonded macromolecules of istry). Materials chemists have wrought solid-state physics. variable molecular weights. This presump- remarkable developments in processing, a tion was extremely surprising at the time broad aspect of MSE which was identified In my view, the birth and flowering of because of the variable molecular weights, by the U.S. Academies, a few years ago, as parepistemes is one of the key progenitors but inescapable. one requiring concentrated attention. of modern MSE, and they are vital to its ■ Egon Orowan, Geoffrey Taylor, and Techniques such as self-sustaining high- further development because several Michael Polanyi and their Joint invention temperature synthesis, and sol-gel proce- parepistemes are integrated in the Solution of of the dislocation to explain the weak resis­ dures (and, more generally, what French
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