Metallic phase with long range phys rev lett.53.1951

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Metallic Phase with Long-Range Orientational Order and No Translational Symmetry

D. Shechtman and I. Blech
Department of Materials Engineering, Israel Institute of Technology Technion, 3200 Haifa, Israel

and
D. Gratias
Centre d'Etudes de Chimie Metallurgique, Centre National de la Recherche Scientiftque, F-94400 Vitry, France

and
J. W. Cahn
Center for Materials Science, National Bureau ofStandards, Gaithersburg, Maryland 20760
(Received 9 October 1984)

We have observed a metallic solid (Al —14-at. /o-Mn) with long-range orientational order, but with icosahedral point group symmetry, which is inconsistent with lattice translations. Its diffraction spots are as sharp as those of crystals but cannot be indexed to any Bravais lattice. The solid is metastable and forms from the melt by a first-order transition.

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Metallic phase with long range phys rev lett.53.1951

  1. 1. VOLUME 53, NUMBER 20 PHYSICAL REVIE% LETTERS 12 NOVEMBER 1984 Metallic Phase with Long-Range Orientational Order and No Translational Symmetry D. Shechtman and I. Blech Department of Materials Engineering, Israel Institute of Technology T-echnion, 3200 Haifa, Israel and D. Gratias Centre dEtudes de Chimie Metallurgique, Centre National de la Recherche Scientiftque, F 94400 try, -Vi France and J. W. Cahn Center for Materials Science, National Bureau of Standards, Gaithersburg, Maryland 20760 (Received 9 October 1984) We have observed a metallic solid (Al — 14-at. /o-Mn) with long-range orientationai order, but with icosahedral point group symmetry, which is inconsistent with lattice translations. Its diffraction spots are as sharp as those of crystals but cannot be indexed to any Bravais lattice. The solid is metastable and forms from the melt by a first-order transition. PACS numbers: 61.50.Em, 61.55.Hg, 64.70.Ew We report herein the existence of a metallic solid rounded by twelve larger atoms arranged like thewhich diffracts electrons like a single crystal but has corners of an icosahedron. The symmetries of thepoint group symmetery rn35 (icosahedral) which is crystals dictate that the several icosqhedra in a unitinconsistent with lattice translations. If the speci- cell have different orientations and allow them tomen is rotated through the angles of this point be distorted, leaving the overall crystal consistentgroup (Fig. 1), selected-area electron diffraction with the well-known crystallographic point andpatterns clearly display the six fivefold, ten three- space groups. Even though icosahedral symmetry isfold, and fifteen twofold axes characteristic of of great importance as an approximate site sym-icosahedral symmetry (Fig. 2). Grains up to 2 p, m metry in crystals, it cannot survive the impositionin size with this structure form in rapidly cooled al- of lattice translations: Crystals cannot and do notloys of Al with 10— at. /0 Mn, Fe, or Cr. We will 14 exhibit the icosahedral point group symmetry.refer to the phase as the icosahedral phase. Micro- Elementary crystallography indicates that fivefolddiffraction from many different volume elements ofa grain and dark-field imaging from various diffrac-tion spots confirm that entire grains have long-range orientational order. If the orientational orderdecays with distance, its correlation length is fargreater than the grain size. We have thus a solidmetallic phase with no translational order and withwith long-range orientational order. The remarkable sharpness of the diffraction spots(Fig. 2) indicates a high coherency in the spatial in-terference, comparable to the one usually encoun-tered in crystals. The diffraction data are qualita-tively well fitted by a model consisting of a randompacking of nonoverlapping parallel icosahedra at-tached by edges. The invariance of the local orien-tational symmetry from site to site and the finitenumber of possible translations between two adja-cent icosahedra seem to be sufficient for insuringhighly coherent interferences. Icosahedra are acommon packing unit in intermetallic crystals with FIG. 1. Stereographic projection of the symmetry ele-the smaller transition element at the center sur- ments of the icosahedral group m35. 1951
  2. 2. VOLUME 53, NUMBER 20 PHYSICAL REVIEW LETTERS 12 NOVEMBER 1984 63.43 I e s:» t»» E I ll * 31.72 36 :K R 79. 2 58. 29 37.38 FIG. 2. Selected-area electron diffraction patterns taken from a single grain of the icosahedral phase. Rotations matchthose in Fig. 1.axes are inconsistent with translational order. not be indexed to any Bravais lattice.Crystals with an apparent fivefold axis do occur as On the basis of these experiments we concludemultiple twins. 5 Icosahedral symmetry could con- that the icosahedral phase does not consist of multi-ceivably occur by multiple t~inning, but at least ply t~inned regular crystal structures.five twins are required in order to obtain an The icosahedral phase forms during rapid coolingicosahedral diffraction set from a conventional crys- of the melt by a nucleation and growth mechanism.tal. Experiments in the electron microscope and This mechanism is characteristic of a first-orderwith x-ray diffraction contradict the twin hypoth- transition because the two phases coexist along aesis: moving interface. Each particle nucleates at a (I) A set of dark-field images taken from any re- center and grows out from there. The atomic rear-flection reveals no twins and in all cases the whole rangements that result in the orientational order ofgrain is illuminated. the icosahedral phase occur at this interface, and (2) A convergent-beam diffraction pattern along the two adjoining phases differ in entropy and, forthe fivefold zone taken from an area 20 nm in di- some alloys, in composition. If the transition werearneter at any point of the grain displays all the re- higher order, ordering would occur everywhere inflections that appear in the selected-area diffraction the liquid instead of being confined to interfaces.pattern. This is also true when the thickness of the Our evidence for the first-order character is mor-specimen is of the order of 10 nm. phological. We examined alloys with 10— at. %14 (3) An x-ray diffraction pattern (Cu Ka source) Mn. Samples with 10% to 12% Mn showed manywas taken from a single-phase sample of the materi- nodular grains separated from each other by crystal-al containing many grains of various orientations. line films of fcc Al. The grains were approximatelyHad the phase consisted of a multiply twinned crys- spherical in shape but were deeply indented with ra-talline structure, it should have been possible to in- dial streaks composed of fcc Al crystals. The mor-dex the powder pattern regardless of the twins. The phology is similar to a commonly observed one inpattern obtained from the icosahedral phase could rapid solidification in which crystals nucleate at1952
  3. 3. VOLUME 53, NUMBER 20 PHYSICAL REVIEW LETTERS 12 NOVEMBER 1984many centers and grow until the remaining liquid icosahedral phase after its formation.solidifies, except that instead of isolated crystals we The icosahedral phase has symmetries intermedi-have isolated icosahedral grains embedded in solidi- ate between those of a crystal and a liquid. Itfied Al. differs from other intermediate phases in that it is Another aspect of the first-order character of the both solid, like a metallic glass, and that it hastransition is that segregation accompanies the long-range orientational order. Many intermediategrowth of the icosahedral phase. In the 10%- and phases do have orientational order, but usually it is12%-Mn samples the growing phase rejects Al into only local, and the transition to such phases is con-the liquid. The indentations in the nodules are tinuous. s 9 The possibility of an icosahedral phasecharacteristic of the cellular morphology caused byan instability in the diffusion layer surrounding the simulation, " with long range order was inferred from a computer and a first-order liquid-to-icosa-solid resulting from segregation during growth.the 14%-Mn alloys the icosahedral grains occupy al-most all of the volume of the specimen. Only small In mean-field theory. " hedral phase transition has been predicted from a We thank F. S. Biancaniello for spinning the al-amounts of fcc A1 occur along some grain boun- loys, C. R. Hubbard for x-ray experiments, anddaries where the grains grew to impingement. B. Burton for a critical review of the manuscript.Some radial streaking still points to the nucleation This work was sponsored by the Defense Advancedcenters from which the grains grew. We conclude Research Projects Agency and the National Sciencethat little segregation occurs in this alloy and that Foundation. This work was performed while one ofthe icosahedral phase contains 14/o Mn. us {D. .) was a guest worker at the National Bureau S The obvious cellular morphology indicates that of Standards and while another of us (D.G. ) was atthe growth of the icosahedral phase is slow enough the Institute for Theoretical Physics, University ofto permit diffusional segregation on the scale of 1 California at Santa Barbara.p, m. With the diffusion coefficient in the liqud oforder 10 m /s this indicates interface velocities of tInternational Tables for Crystallography (Reidel, Hig-order 10 m/s and formation times of 10 s. ham, Mass. , 1983), Vol. A, p. 179.This is 2 to 3 orders of magnitude slower than the 20. Shechtman and I. Blech, to be published. 3F. C. Frank and J. S. Kasper, Acta Crystallogr. 11, 184fastest known crystallization velocities of a metallic (1958), and 12, 483 (1959).liquid during rapid cooling, but it is a typical upper 4M. J. Buerger, Elementary Crystallography (MIT Press,limit of velocity of crystallization with composition Cambridge, 1978).changes. 7 There is thus plenty of time for atomic sG. Friedel, Lecon de Crystallographie (Masson, Paris,rearrangements to occur at the interface between 1921).the melt and the icosahedral phase. 6W. W. Mullins and R. F. Sekerka, J. Appl. Phys. 35, The icosahedral phase in rapidly solidified Al-Mn 444 (1963).alloys is remarkably resistant to crystallization. Pro- W. J. Boettinger, F. S. Biancaniello, G. Kalonji, andlonged heating of 6 h at 300 C and 1 h at 350C J. W. Cahn, in Rapid Solidification Processing Il, edited byproduced no detectable crystallization, but 1 h at R. Mehrabian et al. (Claitors, Baton Rouge, 1980), p. 50. SD. R. Nelson, Phys. Rev. B 2&, 5515 (1983).400C caused crystallization to the stable A16Mn sJ. Sadoc, J. Phys. (Paris), Colloq. 41, C8-326 (1980).phase. We conclude that the icosahedral phase is a &OP. J. Steinhardt, D. R. Nelson and M. Rouchetti,truly metastable phase which nucleates and grows Phys. Rev. Lett. 47, 1297 (1981).for a range of cooling rates which are slow enough »P. J. Steinhardt, D. R. Nelson and M. Rouchetti,to permit its formation but rapid enough to prevent Phys. Rev. B 28, 784 (1983).crystallization, either from the melt or from the t2A. D. J. Haymet, Phys. Rev. B 27, 1725 (1983). 1953

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