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June 18, 2009

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Researchers find “a touch of glass” in metal

June 18, 2009
Courtesy National Institute of Standards and Technology
and World Science staff

Met­als and ce­ram­ics have more in com­mon with glass than has been pre­vi­ously rec­og­nized, a new study in­di­cates.

The find­ings could lead to bet­ter pre­dic­tions of how many in­dus­t­ri­ally val­u­a­ble ma­te­ri­als be­have un­der stress, ac­cord­ing to the sci­en­tists who car­ried out the re­search, from the U.S. Na­tional In­sti­tute of Stan­dards and Tech­nol­o­gy. 

Be­tween a pol­y­crys­tal­line ma­te­ri­al's grains (saf­fron lay­ers) ex­ist dis­or­der­ly ar­eas called grain bound­aries, the be­hav­ior of which has been dif­fi­cult to un­der­stand. The green and blue ob­jects in the bound­a­ry are string-like col­lec­tions of atoms that sci­en­tists have found be­have like glass-forming liq­uids, a sim­i­lar­i­ty that should help sci­en­tists an­a­lyze a wide range of ma­te­ri­als. (Cred­it: NIST)

Most met­als and ce­ram­ics used in ma­n­u­fac­tur­ing are known as poly­crys­tals. The steel in a bridge gird­er, for in­stance, con­sists of count­less ti­ny met­al crys­tals that grew to­geth­er in a patch­work as the mol­ten steel cooled and so­lid­i­fied. Each crys­tal, or “grain,” is very or­derly on the in­side, but in the thin bound­aries it shares with the grains around it, the mo­le­cules are disor­derly. 

Be­cause these grain bound­aries pro­foundly af­fect the me­chan­i­cal and elec­tri­cal prop­er­ties of polycrys­talline ma­te­ri­als, en­gi­neers would like a bet­ter un­der­stand­ing of grain bound­aries’ forma­t­ion and be­hav­ior. Un­for­tu­nate­ly, grain bound­a­ry forma­t­ion in most tech­nic­ally use­ful met­als has elud­ed ef­forts to ob­serve it for a cen­tu­ry.

“Y­ou’d like to have sim­ple en­gi­neer­ing rules re­gard­ing how a ma­te­ri­al’s go­ing to break,” said ma­te­ri­als sci­ent­ist Jack Doug­las at the in­sti­tute. “For ex­am­ple, cor­ro­sion typ­ic­ally trav­els along grain bound­aries, so poly­crys­tals usu­ally frac­ture along them. But met­als melt and de­form at very high tem­per­a­tures, so ob­serv­ing them un­der those con­di­tions is a chal­lenge.”

Some sci­en­tists had spec­u­lat­ed that the mo­le­cules in grain bound­aries be­have si­m­i­larly to the way mo­le­cules do in glass-forming liq­uids, whose prop­er­ties are well un­der­stood. But none had found con­clu­sive ev­i­dence to back up such a claim, Doug­las said.

That started to change when the­o­rist James War­ren at the in­sti­tute saw a con­fer­ence pre­s­enta­t­ion by the Un­ivers­ity of Al­ber­ta’s Hao Zhang con­cern­ing some odd “strings” of atoms in his sim­ula­t­ion of grain bound­a­ry mo­tion us­ing a sim­ula­t­ion tech­nique. The col­lec­tive atom­ic be­hav­ior ob­served in grain bound­aries re­minded the team of pri­or find­ings about glass-forming liq­uids, whose atoms al­so form strings. 

The team lat­er found that the strings of atoms aris­ing in grain bound­aries are strik­ingly si­m­i­lar in form, dis­tri­bu­tion and tem­per­a­ture de­pend­ence to the string-like col­lec­tive atom­ic mo­tions gen­er­ally found in glass-forming liq­uids—and that prop­er­ties for both types of sub­stances change with tem­per­a­ture in vir­tu­ally the same way. 

“All the im­por­tant qual­i­ties re­lat­ing to atom­ic mo­tion in both of these types of ma­te­ri­als—the de­vel­op­ment of these string-like atom­ic mo­tions, or the am­pli­tude [size of the vibra­t­ions] at which their atoms rat­tle—are strik­ingly sim­i­lar,” Doug­las said. “For all in­tents and pur­poses, grain bound­aries are a type of glass.”

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Metals and ceramics have more in common with glass than has been previously recognized, a new study indicates. The findings could lead to better predictions of how many industrially valuable materials behave under stress, according to the scientists who carried out the research, from the U.S. National Institute of Standards and Technology. Most metals and ceramics used in manufacturing are known as polycrystals. The steel in a bridge girder, for instance, is formed from countless tiny metal crystals that grew together in a patchwork as the molten steel cooled and solidified. Each crystal, or “grain,” is very orderly on the inside, but in the thin boundaries it shares with the grains around it, the molecules are disorderly. Because these grain boundaries profoundly affect the mechanical and electrical properties of polycrystalline materials, engineers would like a better understanding of grain boundaries’ formation and behavior. Unfortunately, grain boundary formation in most technically useful metals has eluded efforts to observe it for a century. “You’d like to have simple engineering rules regarding how a material’s going to break,” said materials scientist Jack Douglas at the institute. “For example, corrosion typically travels along grain boundaries, so polycrystals usually fracture along them. But metals melt and deform at very high temperatures, so observing them under those conditions is a challenge.” While some scientists had speculated that the molecules in grain boundaries behave similarly to the way molecules do in glass-forming liquids, whose properties are well understood, none had found conclusive evidence to back up such a claim. That started to change when theorist James Warren at the institute saw a conference presentation by the University of Alberta’s Hao Zhang concerning some odd “strings” of atoms in his simulation of grain boundary motion using a simulation technique. The collective atomic behavior observed in grain boundaries reminded the team of prior findings about glass-forming liquids, whose atoms also form strings. The team later found that the strings of atoms arising in grain boundaries are strikingly similar in form, distribution and temperature dependence to the string-like collective atomic motions generally found in glass-forming liquids—and that properties for both types of substances change with temperature in virtually the same way. “All the important qualities relating to atomic motion in both of these types of materials—the development of these string-like atomic motions, or the amplitude [size of the vibrations] at which their atoms rattle—are strikingly similar,” Douglas said. “For all intents and purposes, grain boundaries are a type of glass.” He added that the findings could permit progress in predicting the failure of many materials important in construction and manufacturing and could improve our understanding of how crystals form boundaries with one another.