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Two well-known aging effects on the martensite phase, rubber effect and stabilization of martensite, are closely related to each other. They appear in the same specimen after the same aging treatment. It has been shown that the time constants for these effects are quite similar. However, the causes to bring about these effects may not be the same. For the rubber effect, a short-range-order model has been proposed (K.Marukawa and K.Tsuchiya, Scripta Met. Mater., 32, 77, (1995)). Although somemodifications on the model has been suggested, the original one seems to be most appropriate. In this report, it is shown that the principal cause for the stabilization is the change in the long-range-order of the alloy, while the long-range-order has no relation to the rubber effect. There are two cases for the stabilization in Cu-Zn-Al alloys. In one case, the parent phase has B2 order and the B2 order disintegrates to the disordered state during the aging after the martensitic transformation. In the other case, the matrix phase has L21 structure and the ordered structure transforms to a different ordered structure, D022. Both these changes in the long-range order result in a stabilization of martensite phase. The relation between these effects and the change in the long-range or short-range order is examined quantitatively with the help of the Monte Carlo simulation. It is shown that the transformation temperature is quite sensitive to the change in the long range order, so that a few percent change is enough to give rise to a change in the transformation temperature of tens of degrees. It is also shown that the changing rates of the long range order and the short range order are comparable with each other in most cases. These results are compared with recent experimental results.

In this paper we describe how to record on-line filtered diffraction patterns rapidly with a serial electron energy loss spectrometer. A numerical treatment then yields the RDF for structural identification. Differents treatments ways of improving the accuracy of the results are proposed and examined in detail.

La redistribution atomique induite par irradiation a ete etudiee par mesures de diffusion diffuse de neutrons dans des alliages NiFeCr prepares a partir de l'isotope 62 Ni: Ni 50 Fe 42,5 Cr 7,5 ,Ni 48 Fe 38 Cr 14 et Ni 46 Fe 34 Cr 20 . Les mesures ont ete effctuees a la temperature ambiante sur des poudres cristallines, irradiees a 310 °C par des electrons de 2,5 MeV a une fluence de 1 x 10 18 e - cm -2 . Les parametres d'ordre a courte distance de Warren-Cowley ont ete determines sur trois spheres de coordination. Le degre d'ordre dans les alliages irradies est plus eleve que celui obtenu par des traitements thermiques effectues a plus haute temperature. Le modele propose pour la redistribution atomique est un ordre local du type Ni(Cr,Fe) avec une tendance a la formation preferentielle de paires Ni-Cr.

I propose a double-twist texture with local smectic order, which may have been seen in recent experiments. As in the Renn-Lubensky TGB phase, the smectic order is broken only through a lattice of screw dislocations. A melted lattice of screw dislocations can produce a double-twist texture as can an unmelted lattice. In the latter case I show that geometry only allows for certain angles between smectic regions. I discuss the possibility of connecting these double-twist tubes together to form a smectic blue phase., Comment: 12 pages, plain TeX (macros included), 5 postscript figures (included). Revised version has some more text and a new figure. To appear in J. Phys. II France (1997)

A general model is presented which allows one to calculate the average number of neighbours in small clusters as a function of the cluster size. A formula is derived for spherical clusters and applied to small Cu clusters made by ion implantation in AIN. EXAFS was used to determine the atomic short range order and size of the clusters.

EXAFS spectra of NiMn alloys annealed at various conditions have been measured at Ni and Mn K-edges. A simple method to estimate the degree of short-range order of atomic arrangements in NiMn alloys is presented. It is found that the short range order in NiMn obtained from EXAFS corresponds to the magnetic properties.

Optical transmission spectrum measurements in an electric field are used to study the short-range structure of the blue phase III (BPIII). The data are compared with the theoretical models for BPIII. Our results can be explained by the models in which BPIII has the simple-cubic local structure or cubic bond orientational order.

Ohba, Otsuka and Sasaki [1] have established that the rubber-like behavior in the martensitic phase of Au-47.5at%Cd alloys cannot be caused by the change in the long-range order during the room temperature annealing. The rubber-like behavior should be understood in terms of the mechanical relaxation associated with the change in the orientation of the local short range order. It is proposed that, while the short-range order plays a prominent role in the rubber-like behavior in the martensite, it should play a comparably important role also in the austenite.

The stability of the martensitic transformation in Ni.35at% Al was investigated as a function of processing and heat treatment conditions. Material was produced by HIPing or sintering of as-received, attritor milled or ball-milled powder. A reversible martensitic transformation is obtained in mechanically milled and consolidated Ni-35at% Al. Whereas Ni 5 Al 3 suppresses the martensitic transformationin the material made from as-received powder, the Ni 5 Al 3 transformation from the L1 0 martensite itself is retarded by the formation of Ni 2 Al in the mechanically milled material. This could be explained by the fine grain size and microstructural disorder. The activation energy for short range order is lowered, thereby facilitating the formation of Ni 2 Al and the martensitic transformation. The present results are promising for the further development of NiAl and its alloys as high temperature shape memory alloys.

Due to the non-existence of long-range order, the diffraction peaks of 2D-solids are considered to have a power-law shape q p η-2 . Taking into account the finite size effects and calculating the powder average, we show that this power-law behaviour appears only for high q p and then for very small intensities. It is therefore quite difficult and hazardous to characterise the quasiorder by using this asymptotic behaviour. Although the shape of the central part of the peak cannot be used to characterise the quasiorder, we show that, for a fairly good resolution, it is possible to determine η using this central part. This determination can be done irrespectively with the other details of the system by comparing the peak width to its value at low temperature, i.e., at low value of η. By using two diffraction peaks, we propose the simple relation : η(Q B1 )/Q 2 B 1 = η(Q B2 )/Q 2 B 2 as a check of the two-dimensional quasiorder.

Cette étude concerne la caractérisation de l'ordre local dans une phase Ni2CoCr modèle de la phase γ des superalliages. Des mesures de diffusion diffuse anomale ont été effectuées à l'aide du rayonnement synchrotron du LURE-DCI. Etant donné la faible intensité du signal dû à l'ordre local, une méthode de dépouillement plus complexe que celle précédemment prise en compte à été utilisée. L'ordre local caractérisé montre que seule la paire NiCr présente une mise en ordre significative, cet ordre étant de type Ni3Cr (DO22).

Calcul microscopique de l'ordre chimique a courte distance dans le cas d'alliages amorphes binaires (2 metaux simples ou 2 metaux de transition). Technique de pseudopotentiel pour les metaux simples et modele d'hamiltonien de liaisons fortes avec approximation des amas de Bethe pour les metaux de transition. Utilisation des hypotheses variationnelles thermodynamiques

An extension of the « D.L.P. » theory of Stringfellow taking clustering into account has been applied both to pseudobinary, pseudoternary and pseudoquaternary III V alloys. For alloys with two sites sublattices, this clustering tendency is introduced in each sublattice of the zinc blende through a positive short-range order parameter αi-j depending on temperature and composition. It is shown that clustering increases in the GaxIn 1-xX serie (X = Sb, As, P) from the antimonide (αGa-In(1/2, 700°C) = 0.04) to the phosphide (αGa-In(1/2, 700°C) = 0.08) whereas GaASyP1-y and mainly InAs yP1-y have quite random As-P distribution. For pseudoquaternaries the model predicts a linear variation of α i-j versus the composition of the other sublattice. The calculated miscibility gaps are in good agreement with known experimental results for GaxIn1- xP and GaxIn1-xAs yP1-y (the critical temperatures are respectively 750 and 880 °C), whereas the model does not explain an asymmetrical shape of miscibility gap as in the case of GaAsySb 1-y if the excess entropy reduces only to a configurational one.

Etude de l'ordre atomique a courte distance dans les alliages CuMn (5-30%at. Mn) a l'equilibre et de son influence sur son comportement magnetique et la chaleur specifique a basse temperature

57Fe Mössbauer spectroscopy has been combined with bulk results to determine the electronic configuration of iron, the short-range structural order and the magnetic properties of glasses in the system (Fe2O3)x(BaO)y(B 2O3)z. The average coordination of the iron atoms is determined by the Fe/O atomic ratio while the valence state of Fe seems to be affected essentially by the preparation conditions. Structural short-range order around Fe atoms is concluded to be very strong, with random bonding angle and distance fluctuations. At low Fe2O3 content, the glass is paramagnetic down to 1.5 K. Local moment measurements reveal the coexistence of isolated Fe ions and of atomic scale clusters. At intermediate Fe2O3 concentration (~ 25 %), the glass presents a mictomagnetic transition. At large Fe2O3 content (~ 50%), the glass behaves like a superparamagnet, indicating inhomogeneous distribution of the Fe atoms in the solid. Mössbauer spectroscopy is also applied to the investigation of the crystallization of a glass with intermediate Fe2O3 concentration.

Two Ni- and Co-containing manganese oxides, a lithiophorite and an asbolane, which are representative of continental ore deposits have been studied by XAS. The distinct behaviours of Ni and Co are emphasized and concern both oxidation States and local structures. The electronic structure and short range order around Co atoms do not depend on the nature of the bearing phase. Co atoms are found to be trivalent and 6-fold coordinated. Results lead to rejection of a substitution of cobalt in manganese sites and strongly support a segregation of Co atoms in a locally ordered structure. Unlike cobalt, nickel exhibits distinct surroundings in both phases : it builds partial Ni(OH)2 layers in asbolane and is located within the hydrargillite layer Al(OH)3 in lithiophorite.

Recently new smectic phases have been discovered: the SmF and Sml phases. They appear in some coumpounds when the length of the aliphatic chains reaches a sufficient value to minimize the interlayer coupling. They are characteristic of new stages between the three-dimensionaly (3D) ordered SmB and SmG and the “2D liquid” SmC phase. These phases are made of stacked uncorrelated layers and exhibit either a topological 2D order in Sml or a short range order within a layer in SmF (hexatic phase). These main features are established from different experimental techniques: X-rays, neutrons, NMR and DTA.

On observe par spectrometrie Mossbauer, mesures d'aimantation et de resistivite, des modifications des proprietes physiques de Fe 20 Ni 60 B 20 amorphe, induites par la relaxation thermique. Ces variations peuvent etre expliquees par des modifications de l'ordre chimique a courte distance autour de Fe, favorisant la formation de paires Fe−Ni

Mesures de susceptibilite magnetique jusqu'a 2200 K. Etude de la dependance en temperature et concentration de l'ordre atomique a courte distance dans Co-B fondu

Etude experimentale dans les alliages Ni-16,6% at. W et Ni-9% at. Mo-9% at. W. On observe des regions riches en solute (environ 50% at. de solute). Description de ces regions. Analyse de la repartition des atomes Mo et W a l'interieur et a l'interface des regions riches en (Mo+W) dans l'alliage ternaire

A model for Al(Si)Mn quasicrystals is proposed. It is characterized by pure quasiperiodic long range order with icosahedral symmetry. The density and concentrations fit to the experimental values. The interatomic distances are compatible with crystallographic data and compare well to the related α phase. A large number of icosahedral clusters (Mackay icosahedra) are present in the model Nous proposons un modele, pour la phase quasicristalline de l'AIMn(Si), caracterise par un ordre a longue distance purement quasiperiodique et la symetrie isoaedrique. La densite et les concentrations coincident avec les valeurs experimentales. Les distances interatomiques sont compatibles avec les donnees de la cristallographie et sont proches de celles de la phase α voisine. Le modele comprend une forte proportion d'icosaedres de Mackay

La section efficace de diffusion diffuse elastique de neutrons par un monocristal TiN 0,82 a ete mesuree a l'equilibre thermodynamique a 700, 800 et 900 o C. L'intensite diffuse est maximale aux points de type (1/2 1/2 1/2) du reseau reciproque. Les lacunes d'azote se mettent preferentiellement en positions de 3emes voisins sur le sous-reseau metalloide c.f.c. Des energies effectives de mise en ordre de paires d'atomes ont ete calculees, par l'approximation de champ moyen, par simulations de Monte Carlo et par la Methode de Variation d'Amas; les deux dernieres techniques ont donne des energies tres proches, independantes de la temperature: V 1 ≃82 et V 2 ≃62 meV pour les premiers et seconds voisins azote respectivement; V 3 et les energies d'interaction plus lointaines sont tres faibles. Les atomes de titane s'eloignent, et les atomes d'azote se rapprochent de leurs lacunes premieres voisines, de 0,042 et 0,024 A respectivement

In this paper we discuss four interesting features of the Cu(Mn) alloy systems : 1) The character and temperature dependence of the diffuse neutron scattering and its relation to electronic structure. 2) The persistence of the short-range atomic ordering over a broad temperature range. 3) The high critical concentration for antiferromagnetism in fcc systems. 4) Mictomagnetic behavior in these fcc alloys.

Etude par diffraction de neutrons polarises des fluctuations de spin dans le systeme antiferromagnetique Y(Mn 1−x Al x ) 2 passant d'un comportement itinerant a un comportement localise en fonction de la concentration en Al

Local order in Au 75 Cu 25 and Au 70 Cu 30 has been investigated by X-ray diffuse scattering from single crystals quenched from T/Tc≃1.03 and 1.2 (Tc: transition temperature between long range ordered and disordered phases). Data analysis has been made by the Borie-Sparks method (Acta Cryst. A 27 (1971) 198-201). Short range order and first order displacement parameters are determined. A fourfold splitting of the diffuse intensity contours is observed at 100 special positions in reciprocal space in all studied cases. The recalculated short-range-order intensity maps show that long-range two-site correlation functions (SRO parameters) are necessary for recovering the fine structure of experimental intensity. The determination of pair-interaction potentials with two thermodynamic approximations (Bragg-Williams and Cluster variation Method with quadruple tetrahedron-octahedron for maximum cluster) is given. The probabilities of local atomic configurations (not measurable by experimental techniques) are calculated by two techniques: a) A simulation method leading to a configuration where short range parameters are as close as possible to the experimental ones (Gehlen and Cohen, Phys. Rev. A 139, 844-855). b) An analytical CVM approach with quadruple tetrahedron-octahedron for maximum cluster (Cenedese et al., Progress in X-ray Studies by Synchrotron radiation, Strasbourg, 1985). The diffuse intensi Etude experimentale des monocristaux trempes depuis une temperature T telle que T/Tc≃1,03 et 1,2 (Tc temperature de transformation ordre-desordre. Determination des parametres d'ordre a courte distance et de deplacement du premier ordre. Les cartes d'intensite recalculee d'ordre a courte distance montrent que les fonctions de correlation de paires lointaines sont essentielles pour reconstruire la structure fine de l'intensite experimentale. Determination des potentiels d'interactions de paires a l'aide de deux approximations thermodynamiques

Des informations structurales concernant la phase icosaedrique T2 du systeme AlCuLi sont obtenues par diffraction des neutrons, avec substitution isotopique sur le cuivre et le lithium. Des fonctions de distribution de paires partielles sont alors determinees et comparees a celles que l'on peut deduire de la structure connue d'une phase (R) cubique voisine. Cette etude permet de degager les analogies et les differences de l'ordre a courte distance des deux composes

Analyse de la distribution des champs hyperfins sur le cobalt obtenue par RMN dans les alliages amorphes Co-Sn ferromagnetiques en relation avec l'environnement en Sn des atomes resonnants de Co. Mesure de l'effet quadripolaire sur l'etain par spectroscopie Mossbauer dans les alliages paramagnetiques

We present experimental and theoretical results for geometrical properties of 2D packings of disks. We were mainly interested in the study of mixtures with disk size distribution which are of more practical interest than equal disks. Average geometrical properties, such as packing fraction of coordination number do not depend on the composition of the mixture, contrary to what would be expected from 3D experiments. We show the existence of a local order in the relative positions of grains with different sizes; this local order may modify the physical properties of the packing. An approximate theoretical expression for the packing fraction c of 2D close packings is given. It implies the knowledge of the average area of quadrilaterals of the network drawn from the real contacts only. For equal disk disordered packings, it yields the limit c=π 2 /12∼0.822 Presentation de resultats experimentaux et theoriques concernant les proprietes d'empilement bidimensionnels de disques, en particulier dans le cas de melanges avec distribution de taille des disques. Les proprietes moyennes, telles que compacite et coordinence, ne dependent pas de la composition du melange, contrairement a ce que l'on pourrait attendre au vu des experiences a 3-D. Mise en evidence de l'existence d'un ordre local dans la position relative de grains de tailles differentes, ordre pouvant modifier les proprietes physiques de l'empilement. Une expression theorique approchee pour la compacite d'empilements 2-D compacts est donnee

We discuss the effects of in-plane ordering of amphiphilic membranes on the statistics of their shape fluctuations. Effects of flexo- and ferroelectricity as well as of several kinds of orientational order are considered. We close by a discussion of the fluctuations of a membrane immersed in an anisotropic fluid Nous discutons les effets de l'ordre local des membranes d'amphiphiles sur la statistique de leurs fluctuations de forme. Nous considerons en particulier les effets de la flexo- et ferro-electricite et de differentes especes d'ordre orientationnel. Nous discutons enfin les ondulations d'une membrane immergee dans un fluide anisotropie