retaining the nano in nanocrystalline alloys.full

DOI: 10.1126/science.1226724, 921 (2012);337 Science Julia R. WeertmanRetaining the Nano in Nanocrystalline AlloysThis copy is for your personal, non-commercial use only.clicking here.colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to othershere.following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles): September 24, 2014 (this information is current as of The following resources related to this article are available online at/content/337/6097/921.full.html version of this article at:including high-resolution figures, can be found in the online Updated information and services, /content/337/6097/921.full.html#related found at:can be related to this article A list of selected additional articles on the Science Web sites /content/337/6097/921.full.html#ref-list-1, 1 of which can be accessed free:cites 13 articles This article /content/337/6097/921.full.html#related-urls 1 articles hosted by HighWire Press; see:cited by This article has been/cgi/collection/mat_sci Materials Sciencesubject collections:This article appears in the following registered trademark of AAAS.is a Science 2012 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n S e p t e m b e r 24, 2014w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o mPERSPECTIVESRetaining the Nanoin Nanocrystalline Alloys MATERIALS SCIENCEJulia R. Weertman Judicious alloying can prevent grain growth in nanocrystalline metals, preserving their useful properties at high temperatures.W hen nanocrystalline metals first became available, their remark-ably high strength seemed to open up many interesting design applications, but several adverse properties were soon noted.A particularly vexing problem was a micro-structural instability that decreases strength over time. The small crystal grain size cre-ates a large grain boundary area; the associ-ated high interfacial energy drives coarsening (increase in grain size) that leads to soften-ing. For many advanced alloys, the coarsen-ing problem is accelerated by the prolonged high temperatures associated with process-ing. Attempts to prevent grain growth in a nanocrystalline metal by alloying have been hit and miss, and in general they have only retarded coarsening. On page 951 of this issue, Chookajorn et al. ( 1) present a solution to this microstructural instability problem. They performed thermodynamic calculations on a series of candidate binary alloys to deter-mine which are stabilizedby grain boundary segre-gation, not only againstcoarsening but also againstphase separation. A rationalmethod is made availableto design nanocrystallinealloys that meet operational requirements, even at ele-vated temperatures.In 1981, at the annualRisø Materials ScienceConference in Roskilde,Denmark, Gleiter ( 2) pre-sented a process, inert gas condensation, for making nanocrystalline samples.Soon after, researchersworldwide were synthesiz-ing nanocrystalline met-als and measuring theirproperties. As expectedfrom the Hall-Petch equa-tion ( 3, 4), which predictsthat the strength of a metal increases with the inverse of the square rootof its grain size, the nanocrystalline metalswere strong. However, they also were brittleand microstructurally unstable. Nanocrystal-line copper samples left at room temperaturefor several days exhibited substantial coars-ening ( 5). The application of stress greatlyhastens coarsening, even at liquid nitrogentemperatures [see, for example, ( 6, 7)]. Theaddition of solutes segregated to the grainboundaries or even small amounts of impuri-ties in the sample ( 7) slow coarsening but donot prevent it. Attempts to fi nd effective sol-utes to prevent coarsening have been some-what empirical. A large difference in atomicradius between matrix and segregating atomsoften was considered desirable, reminiscentof one of the criteria for constituents in com-posing an amorphous metallic alloy.Although solute additions to nanocrystal-line metals in general cannot eliminate thetendency to coarsen over time, they can slowcoarsening to a point that it is not a practi-cal consideration. As noted by Krill et al.( 8), who invoked the well-known expressionfor grain boundary velocity as the productof interface mobility M and grain boundaryenergy γ divided by grain size d, if γ is low-ered substantially by segregation of a soluteto the grain boundary, grain boundary migra-tion is greatly reduced. Following a different,thermodynamics-based approach, Schuh andcolleagues [see, for example, ( 9– 11)] madeseveral theoretical advances and performeda remarkably extensive and systematic seriesof experiments and simulations on nickel-tungsten (Ni-W) binary alloys. Tungsten isa weakly segregating solute with consider-able solubility in the nickel matrix. Grain sizedecreases with higher solute concentrations,so a full range of nanocrystalline grain sizescould be accessed.Although it appears that γ can be madevery low by a choosing a suitable concen-tration of the solute W, itdoes not go to zero. How-ever, for applications atroom temperature or onlymodestly elevated tempera-tures, the alloy is metasta-ble and can be used in com-mercial applications. Anelectroplated nanocrystal-line Ni-W alloy provides ahard coating with excellentwear and corrosion proper-ties ( 12). It also is widelyused as an undercoated bar-rier for electrical contactsand retains its propertiesat modestly elevated tem-peratures (~300°C). Thefi gure compares the coars-ening in a pure electrode-posited nanocrystalline Niafter annealing for 30 minat 300°C with that in a nanoNi-W alloy (13 atomic per-cent W) after annealing forthe same time at 600°C ( 11,13). In both cases, the ini-tial grain size was 20 nm.The pure nanocrystalline Niunderwent massive coars-Department of Materials Science and Engineering, Northwestern Univer-sity, Evanston, IL 60208, USA. E-mail: jrweertman@A BC D200 nmAs prepared After 600°C, 30 minAs prepared After 300°C, 30 min200 nm50 nm50 nmStaying just fi ne. The benefi t of tungsten alloying for maintaining nanocrystalline grainsizes is illustrated. The 20-nm grains of a pure electrodeposited nanocrystalline Ni fi lm (A)coarsened after heating at 300°C for 30 min (B). A nickel-tungsten alloy (13 atomic percentW) with the same grain size (C) was annealed for the same time at 600°C (D) and coarsenedonly slightly (grain size increased to 28 nm) [Images from ( 11, 13)]. Chookajorn et al. showhow consideration of solute segregation can predict alloys that can actually prevent even thissmall extent of coarsening and maintain favorable materials properties, such as high strength.Published by AAAS SCIENCE VOL 337 24 AUGUST 201292124 AUGUST 2012 VOL 337 SCIENCE 922PERSPECTIVESening, whereas the Ni-W alloy only coars-ened modestly to 28 nm.Of interest here is that the alloy did suf-fer some grain growth. Many nanocrystalline alloys of technological interest are prepared by powder metallurgy routes that involve prolonged periods at high temperatures dur-ing powder consolidation. Thus, it will be of practical value, as well as of scientifi c inter-est, to produce a nanocrystalline alloy that is essentially thermodynamically stable, not just metastable, and considerable effort has gone into achieving this goal. In 1993, Weissmül-ler ( 14) presented a thermodynamic approach to fi nding such alloys. Further developments were carried out by Kirchheim and co-work-ers ( 15), who examined only alloys in the limit of high dilution.An important consideration, the possibil-ity of phase separation, was not addressed previously. Chookajorn et al . developed a theoretical framework, based on thermo-dynamic considerations, that can be used to design stable nanocrystalline alloys. Atype of Ashby map was constructed for W alloys, with axes of enthalpy of segregation and enthalpy of mixing, which divides the space so defi ned into nanocrystalline-stable and bulk-stable regions for W-based binary alloys. After considering about a dozen W alloys, the authors chose W-Ti as a model to test the theories. Samples of a W-Ti alloy with 20 atomic percent Ti annealed for 1 week at 1100°C increased in grain size from 22 nm to only 24 nm, essentially within the error limits. The structure exhibits a nonuniform chemical distribution that appears critical to its stability.The approach described by Chookajorn et al . for producing stable nanocrystalline alloys and its verifi cation open the way for synthesis of an assortment of high-strength nanocrys-talline alloys that can withstand high process-ing as well as high service temperatures. It will be interesting to explore their mechanical properties and behavior under stress, espe-cially time-dependent deformation, which is known to destabilize microstructures.References1. T. Chookajorn et al ., Science 337, 951 (2012).2. H. Gleiter, in Deformation in Polycrystals: Mechanics andMicrostructures, Second Risø International Symposium on Metallurgy and Materials Science, N. Hansen et al ., Eds. (Risø National Laboratory, Risø, Denmark, 1981), pp. 15–21.3. E. O. Hall, Proc. R. Soc. Lond. B Biol. Sci. 64, 474 (1951).4. N. J. Petch, J. Iron Steel Inst. 174, 25 (1953).5. V. Y. Gertsman, R. Birringer, Scr. Metall. Mater. 30, 577(1994).6. M. Jin et al ., Acta Mater. 52, 5381 (2004).7. K. Zhang, J. R. Weertman, J. A. Eastman, Appl. Phys. Lett.87, 061921 (2005).8. C. E. Krill, H. Ehrhardt, R. Birringer, Z. Metallk. 96, 1134(2005).9. J. R. Trelewicz, C. A. Schuh, Phys. Rev. B 79, 094112(2009).10. A. J. Detor, C. A. Schuh, Acta Mater. 55, 4221 (2007). 11. A. J. Detor, C. A. Schuh, J. Mater. Res. 22, 3233 (2007). 12. A. R. Jones, I. J. Hamann, A. C. Lund, C. A. Schuh, Finish-ing 97, 52 (2010).13. U. Erb, K. T. Aust, G. Palumbo, in Nanostructured Materi-als, Processing, Properties, and Applications , C. C. Koch, Ed. (William Andrew, Norwich, NY, ed. 2, 2007), pp. 235–292.14. J. Weissmüller, Nanostruct. Mater 3, 261 (1993). 15. R. Kirchheim, Acta Mater. 50, 413 (2002).The Unusual Case of PorcupineCELL BIOLOGYLawrence Lum 1 and Hans Clevers 2A compound that targets the Wnt signaling pathway in cancer moves into therapeutic trials, 30 years after Wnt was discovered.Earlier this year, with little fanfare, one of the fi rst small molecules tar-geting Wnt-mediated cellular signal-ing, called LGK974, entered a phase I clini-cal trial. Cell signaling that is controlled by secreted Wnt proteins is pivotal in animal development and tissue homeostasis, and has become a high-priority anticancer drug target given its essential role in colorectal cancer and its contribution to a broad range of other cancer types ( 1, 2). The importance of tar-geting this pathway was recently highlighted in a keynote address by Harold Varmus at a meeting in the Netherlands ( 3) marking the 30th anniversary of the discovery of the fi rst Wnt molecule (there are 19 of them). With Roel Nusse, Varmus linked deviant activ-ity of Wnt molecules to cancer. Although the target of LGK974—an acyltransferase called Porcupine (Porcn) that adds fatty acid to Wnt—has been well studied, few of the meeting participants were aware of the drugcandidate, refl ecting the remarkable speed and circumstances whereby this compound was nominated as a clinical candidate. The atypical path to the discovery of LGK974 may also signal changes in the approach to cancer drug discovery that include an increasing reliance on collaboration between government and industry to bring new drug targets to clinical testing.Discovered in screens for genes that affect embryonic patterning in the fruit fl y ( 4), PORCN is the founding member of a 16-gene family with predicted acyltransfer-ase activity. Given their multiple membrane-spanning domains, the proteins encoded by these genes are called membrane-bound O-acyltransferases (MBOATs) to distin-guish them from cytoplasmic acyltransfer-ases such as those that modify the Ras proto-oncogene proteins. Two other family mem-bers have known protein substrates: Hhat modifi es the signaling molecule Hedgehog, and Goat modifi es the appetite-controlling hormone ghrelin. The fatty acyl modifi ca-tion of Wnt, Hedgehog, and ghrelin is essen-tial to their activity. Palmiteoylation of Wnt proteins on a highly conserved serine residue precedes their engagement with a chaperone molecule (Wntless), which then shepherds them through the secretory pathway. Thus, in the absence of Porcn to catalyze this modi-fi cation, Wnt proteins remain trapped inside the cell. This modifi cation is also essential for Wnt binding to their cognate receptors (Frizzled proteins), as revealed in the crystal structure of Wnt and discussed at the recent Wnt anniversary meeting ( 5). The activity dependence of three major signaling mol-ecules on fatty acyl adducts suggests that coordination of cellular behavior in metazo-ans may be directly infl uenced by metabolic status of ligand-producing cells.The chemical tractability of Porcn was fi rst revealed in a cell culture–based screen for small molecules that disable signaling by Wnt at the level of the transcriptional effector of the Wnt pathway, T cell factor (Tcf) ( 6). One class of small molecule identifi ed through this strategy stabilizes Axin proteins by inhibiting Tankyrase (Tnks) enzymes ( 7). A xins scaf-fold a protein complex that destroys β-catenin, a downstream signaling molecule in the Wnt pathway that controls target gene expression. Another class identifi ed by this approach dis-ables Porcn and Wnt protein production ( 6). Porcn affords the more potent and selective1Department of Cell Biology, University of Texas Southwest-ern Medical Center, 5323 Harry Hines Boulevard, Dallas TX 75390, USA. 2Hubrecht Institute, Uppsalalaan 8, 3584 CT, Utrecht and the University Medical Center Utrecht, Netherlands. E-mail: lawrence.lum@; h.clevers@hubrecht.eu10.1126/science.1226724Published by AAAS。