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 , 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 . 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 . 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 and That linkage allowed minute interstitial or eise trivial. Fission is the more common: solid-state 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 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) , 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 , 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 . 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 ■ , Geoffrey Taylor, and Techniques such as self-sustaining high- further development because several 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 clearly defined, difficult practical prob- tance to plastic deformation in various chemists have dubbed chimie douce, soft lems, such as Greg Olson's approach to crystals. Again, invention was followed by chemistry) are examples. More recently, the first-principles design of high-strength demonstration only many years later. techniques for self-organizing periodic steels. The present stage of development ■ Albert Bradley and Abe Taylor in the microstructures have been perfected by of MSE is characterized by the prevalence 1930s and the recognition of a large con- materials chemists such as George of review papers and entire books devot- centration of structural vacancies in an Whitesides at Harvard University; this ed to a single parepisteme. This could not intermetallic Compound. This insight is the field is regarded as so important that a have happened a Century ago. object of renewed skepticism at present. new materials encyclopedia currently in As MSE became ever more quantitative ■ Charles Frank and the recognition that preparation has an editor solely responsi- and less handwaving in its approach, one crystal growth at small supersaturations ble for articles on "self-assembling materi­ feature became steadily more central: the required the participation of screw dislo- als chemistry." The curious new parepis- power of surprise. Scientists learned to cations. Here, visual proof came instantly. teme of "supramolecular chemistry"—the recognize when they had observed some- spontaneous self-arrangement of groups ■ Andrew Keller and the necessity of thing mystifying—that is, surprising—or of large molecules—is closely related to assuming chain-folding in the formation when an Observation was wildly at vari- self-assembling materials chemistry. of polymer crystals, although it took ance with the relevant theory. The impor- many years for this notion to be accepted. I have just used the word parepisteme, tance of this "surprise factor" goes back to He was vehemently challenged at first. which I have invented to denote a self- Louis Pasteur, who defined the origin of In these examples of Pasteur's principle contained subsidiary field of research. The scientific creativity as being "savoir s'eton- in action, surprise was occasioned by a term derives from the Greek "episteme," a ner ä propos" (to know when to be aston- mismatch between the initial theory and field of knowledge, and "para-," here ished). He applied this principle first as a the results of measurement. This kind of meaning "subsidiary." Many of the hun- young man to his precocious observations mismatch, combined with Pasteur's prin­ dreds of parepistemes attached to MSE are on optical rotation of the plane of polar- ciple, was and remains a powerful incen- fields of research which were not, initially, ization by certain transparent crystals. A tive to intellectual Innovation in MSE, and directed to the Solution of any practical contemporary corollary of Pasteur's prin­ the kind of discovery based on this princi­ problem (e.g., diffusion in solids; high- ciple was, and remains, "accident favors ple only became possible once the quanti­ pressure research; the vast domain of crys- the prepared mind." Because the feature tative approach had become habitual in tallography, including but not restricted to that occasions surprise is so unexpected, MSE. In this respect, Griffith's prediction crystal-structure determination; micro- scientists who have drawn the unavoid- of microcracks in the early 1920s was dis- structural coarsening [Ostwald ripening]; able conclusion offen have a sustained tinctly precocious. It may well be that the and piezoelectric behavior). Sometimes, a fight on their hands. A few exemplifica- most significant conceptual innovations parepisteme has grown indirectly out of an tions are provided in outline form and in arising from such mismatches have most- attempt to solve a practical problem; thus, chronological sequence: ly been made, and that this mode of the preparation of metallic Single crystals ■ Ernest Rutherford and the structure of advance in MSE may henceforth be sub- for research purposes emerged, paradoxi- the atom in which he found that a very ject to the law of diminishing returns. cally, from steel metallurgists' attempts, few of the alpha particles used to bombard I come now to the most striking of all early in the 20th Century, to avoid the a metal foil came straight back toward the changes in the practice of MSE, which will generation of large grains in steel objects. source, as he later commented, "It was certainly have a major impact on the

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future of our science. This is Computer Simu­ One central advantage of Computer Simu­ In this essay I have steered clear of dis- lation. The first stirrings of this approach, lation is that it can be used like a spread- cussion of new materials in terms of cate- in the form of the Monte Carlo method, sheet in finance or industrial management. gories. Recent populär texts on materials came just after the Second World War, at If a model for, say, grain-boundary prop- have focused on functional materials of Los Alamos, and it was applied to neu- erties or anomalously fast diffusion various kinds at the expense of structural tron physics. This development owed depends on numerous variables, any one materials, and this change of emphasis everything to pure mathematicians like of these variables can be changed and the has led to errors in historical understand- Stanislaw Ulam (his little daughter com- consequent change of predicted structure, ing, such as the fantasy that the steel plained at the time that her father spent all or properties, can be very rapidly comput- industry is a "smokestack" domain his time thinking instead of playing with ed. This kind of one-thing-at-a-time devoid of any serious modern science. As her): Their involvement led to a pro- change in a Computer Simulation is intel- with all Swings of the pendulum, this one longed, abstruse debate whether a table of lectually akin to combinatorial chemistry, will repair itself in due time. truly random numbers can ever be gener- as practiced originally in pharmacology Some of the giants who drove materials ated. The origins of Monte Carlo simula- (drug development) and now also in the science forward in past years have been tions and the difficult problems which improvement of (functional) electronic mentioned. That is not to say that the era of had to be solved en route have been memo- materials where the effects of numerous great materials scientists has passed. In rably surveyed by the Harvard science small compositional variations are rapidly fact, to quote a chai/s remark retailed by a historian Peter Galison in his book, Image tested in an automated experimental science historian, Gerald Holton, "Today and Logic (University of Chicago Press, setup. In my view, this "spreadsheet" we are privileged to sit side-by-side with Chicago, 1997). At one point, Galison aspect of Computer Simulation justifies the giants on whose Shoulders we stand." quotes an early practitioner asking, "How regarding the technique as a form of quasi- The whole raison d'etre of materials sci­ should one class this type of data-to-model experiment; I believe it is more productive ence is to achieve cross-fertilization Simulation? As experimental theory? The- to regard Simulation in this way rather between understanding of different cate- oretical experiment? Is it a case of induc- than as a second-class (non-analytical) gories of materials. This is at last beginning tion from data? Deduction from theory?" form of theory. to become explicit in materials science edu- Galison continues, "Each such attempt to cation: A very recent undergraduate text- force the argument back into older cate- book by S.M. Allen and E.L. Thomas of gories strikes me as awkward, a rearguard One central advantage of Massachusetts Institute of Technology, action unable to capture the novelty of Structure of Materials (Wiley & Sons, New procedure....I would suggest that the Computer Simulation is that it York, 1999), is predicated on this approach. Monte Carlo method is best seen as can be used like a spreadsheet It is even gradually becoming possible to expanding the spectrum of persuasive in finance or industrial integrate the study of polymers, hitherto evidence, a tertium quid." resistant to such integration, with the rest As the power and speed of Computers management. of MSE; such concepts as phase diagrams, improved and as practitioners enhanced dislocation motion, and crystal perfection the subtlety of their programming meth- Early in the development of Computers, are making their appearance in some poly­ ods, the ränge of issues treated by Monte a senior executive of IBM opined that half mer research. I hope and believe that this Carlo and molecular dynamics "experi- a dozen or so large installations would kind of integration will become more and ments" increased apace. In retrospect, the saturate the market; this was akin to some more widespread in the years to corne. choice of themes to be examined by these early prophecies by military experts, for methods sometimes seems odd; for instance, about the certain uselessness of Suggested Readings instance, for decades Computer Simulation air power. Computers are effectively used 1. R.W. Cahn, Physics of Materials, chapter in was applied to the process of grain today in many different ways to perform Twentieth Century Physics, edited by L.M. Brown, growth during the annealing of polycrys- a variety of tasks. First, many forms of A. Pais, and B. Pippard (Institute of Physics talline metals. That process is not particu- processes, such as plastic deformation, Publishing, Bristol, UK, and American Institute larly important in itself, but it is perfectly sintering, and polymer extrusion, have of Physics Press, New York, 1995), p. 1505. adapted for comparing the validity of been simulated and thereby improved in 2. E. Braun, Mechanical Properties of Solids, and S.R. Weart, The Solid Community, chapters in competing Simulation strategies. Such detail. This use is likely to continue to grow. A particularly fruitful approach is Out of the Crystal Maze, edited by L. Hoddeson, comparisons have for years been a major E. Braun, J. Teichmann, and S. Weart (Oxford Obsession of materials scientists who use to predict properties of polymers as a function not only of chemical composition University Press, New York, 1992), pp. 317,617. Computer Simulation because the accep- 3. N. Mott, ed., The Beginnings of Solid State tance of the Simulation approach depends and molecular weight, but also of "meso- Physics, a special issue of Proc. Roy. Soc. (London) entirely on confidence in its validity. For scopic" or medium-range structure, that 371 (1980) pp. 1-177. example, CALPHAD, or the calculation of is, the shapes of the long-chain molecules 4. D. Raabe, Computational Materials Science and Statistical modeis of their interaction. binary or ternary phase diagrams from (Wiley-VCH, Weinheim, 1998). This represents a prime example of the thermochemical measurements used as 5. D. Frenkel and B. Smit, Understanding Molecular "spreadsheet" approach. The study of input, is one form of Computer Simulation Simulation (Academic Press, San Diego, 1996). interface structure and properties is 6. N. Saunders and A.P. Miodownik, dating back to the early 1960s. For rnost of another interesting example. CALPHAD — Calculation of Phase Diagrams, A the intervening years, its practitioners Comprehensive Guide (Pergamon, Oxford, 1998). have compared CALPHAD predictions Computer Simulation is on the way to 7. P. Ball, Made to Measure: New Materials for the with experimentally determined phase becoming the prime exemplar of a 21st Century (Princeton University Press, diagrams. At last, today, customers are parepisteme, and I predict that as confi­ Princeton, 1997). mostly prepared to accept the validity of dence in its methodology grows, so its such predictions even without immediate role (relative to traditional theory and Robert W. Cahn was trained at Cambridge experimental backup. Confidence is all. experiment) will steadily expand. University as a physical metallurgist and all

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his personal research has been in this field. Research, and since 1993 he has been co-edilor ly involved with three materials encyclopedias, Front 1965 to 1981 he held thefirst chair in of a new Journal, Intermetallics. He is now - one of these now in preparation. He has been Britain, at Sussex University, designated in attached, in nominal retirement, to his alma editor of two successive monograph series in materials science and there he helped to define mater, Cambridge. Since 1967, he has written materials science, first for Cambridge the nature of the nascent discipline, in part by approximately 100 articles of scientific popu- University Press and nowfor Elsevier Science becoming founder-editor in 1966 ofa new peri- larization in materials science for the weekly Ltd. Cahn is currently writing a historical odical, Journal of Materials Sdence. Before Journal Nature. He was Joint editor-in-chief book, The Coming of Materials Science, that, he was founder-editor of the Journal of of a series of 18 multi-author volumes which will appear next year in the Pergamon Nuclear Materials; later, in 1985-1992, he (1991-1999) encompassing most of materials Materials Series, published by Elsevier. Cahn helped develop MRS's Journal of Materials science and technology, and has been editorial- is a Fellow ofthe Royal Society of London.

RESEARCH/RESEARCHERS

Bravman to Lead Engineering. He will also continue to con- Their discovery is based, among other Undergraduate duct research with his group of PhD stu­ things, on a study in which crystals of a dents. His research focuses on the related material, copper indium dise- Education at mechanical behavior of thin film materials lenide, were exarnined using high-energy Stanford University and the reliability of the microscopic x-rays. In that study, conducted by Cahen John C. Bravman, the structures found in Computer chips and and other colleagues at the European chair of the Department other microelectronic circuitry. His group Synchrotron Research Fadlity in Grenoble, AV of Materials Science and has pioneered techniques for imaging and it was shown that in sorne cases the bonds Engineering and the Bing tracking the movements of the microscop­ between certain atoms of copper indium Centennial Professor at Stanford ic voids that appear in the metal contact diselenide can be broken relatively easily. University and 1994 president of the lines used in such circuits. The group also Cahen's group had also shown that Materials Research Society, has been is developing methods to test and analyze copper atoms can move inside these semi­ named vice provost for undergraduate the and that develop in conductor crystals. He said that this find­ education. Bravman's appointment thin films. Such methods are needed to ing was surprising because such move­ became effective September 1. identify highly reliable films that can be ment is uncommon in solid, nonliving "I'm thrilled and honored to take on used in the construction of devices that materials, and extremely unusual in mate­ the important task of ensuring that combine microscopic electronic circuitry rials used in electronic devices where Stanford's unparalleled efforts at the and microscopic mechanical components. atomic mobility is viewed as anathema. Moreover, he said, seeing it in a semicon­ undergraduate level continue to move After receiving his BS, MS, and PhD ductor known for its stability was particu- forward," Bravman said. degrees in materials science and engineer- larly unexpected. Provost John Hennessy said that ing at Stanford, Bravman was named Bravman "brings a real wealth of experi- assistant professor in the department in The researchers furthermore found that once some atomic bonds have been bro­ ence as well as a passion for working for 1985, achieving füll professorship in 1995. ken, the copper atoms, which are capable students that I think is truly extraordinary." Among other honors, he received the of moving throughout the crystal, wander While the Stanford Introductory Studies Walter J. Gores Award for Excellence in (SIS) developed during the term of the around until they reach the damaged spot Teaching in 1989. He also has served as a and undo the effects of the damage. This previous vice provost has concentrated on resident fellow and on a wide variety of the first two years of the undergraduate "self-repair" mechanism stems from the committees and boards, including the material's tendency to try and stay close experience, Bravman will look toward Commission on Undergraduate Education. enhancing the major programs as well as to equilibrium. making possible changes in the Science, Cahen said, "Now we understand how Mathematics, and Engineering Core, Copper-Indium-Gallium- solar cells made of copper indium gallium which is an Option for nonscience majors Diselenide Semiconductor diselenide manage to survive and func- in fulfilling the undergraduate general Heals Itself tion effectively in hostile environments education requirements. An international team consisting of such as those encountered on satellites: Once damaged, for example by radiation, Bravman also will be involved with David Cahen of the Weizmann Institute's this 'smart' material simply 'heals' itself developing a solid financial basis to sus- Materials and Interfaces Department, and restores its previous function." tain the recent improvements in under­ working with Consultant Leeor Kronik of graduate offerings at Stanford. He espe- Tel Aviv University and colleagues from cially will be seeking funds to endow France's CNRS and Germany's Stuttgart Microwave-Sintered Metal Parts undergraduate research initiatives. "I University have discovered a self-healing Demonstrate Finer Grain Size think perhaps as many as a third of under- process that can occur in a copper-indium- than Conventional Sintering graduates are looking for a substantial gallium-diselenide semiconductor. This A team of materials scientists at The research experience either over the Sum­ finding, presented in June at the European Pennsylvania State University is micro- mer or during the year or both," he said. Materials Research Conference in waving a wide ränge of powder metals Bravman, who will serve a five-year Strasbourg and scheduled for publication and producing machine components with term, will continue as senior associate in Advanced Materials, may help create bet­ improved properties over those produced dean for Student affairs in the School of ter solar cells and other electronic devices. through traditional sintering methods.

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