bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhancement of the Icosahedral Quasicrystalline Phase Forming Ability with Be Addition to an Al62.5Cu25.5Fe12 Alloy
Authors: Lee S.M. , Kim B.H. , Kim W.T. , Kim D.H.
Published: April 2011

The quasicrystalline phase forming ability, solidification structure, thermal stability, and phase relationships of a series of conventionally-cast Al62-xCu25.5Fe12.5Bex (x=0,1,3,5,7) alloys were investigated. Single icosahedral quasicrystalline phase was successfully obtained by partial replacement of Al with Be (7 at.%) in an Al62Cu25.5Fe12.5 alloy during crucible cooling. The I-phase in an Al55Cu25.5Fe12.5Be7 alloy has the same face-centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy, with less phason strain. Hardness of the I-phase increases with x, i.e. from Hv 710 for x=1 to Hv 809 for x=7.
Modulated Structures, Nanocrystals and Quasicrystals in Mg-Zn-Y Alloys
Authors: Subramaniam Anandh , Ranganathan S.
Published: April 2011

The formation of the icosahedral phase, BCC (Body centred cubic) phase and nanocrystals are seen in the as-cast alloy with nominal composition of Mg4Zn94Y2. FCC (face centred cubic) phase and modulated structures are formed in the alloys with higher Y content (10% and 25% Y respectively). These phases are analysed keeping in view their relation to the quasicrystals of the Mg-Zn-Y system.
Modelling Quasicrystal Plastic Deformation By Means of Constitutive Equations
Authors: Feuerbacher M. , Schall P. , Estrin Y. , Bréchet Y. , Urban K.
Published: April 2011

The interpretation of plastic deformation experiments on quasicrystals is a challenging task due to the occurrence of changes of the structure during deformation. In this paper, we present a quantitative model for quasicrystal plasticity on the basis of a constitutive-equations Ansatz, which takes these effects into account. A single-internal-variable model of the kind commonly used for describing crystal plasticity, is adapted for the description of the dislocation density evolution in a quasicrystal. In addition, we introduce a structural parameter that accounts for the evolution of order in the course of plastic deformation. The numerical solution of the resulting set of evolution equations yields the flow stress and the dislocation density as a function of strain, which can be directly compared to corresponding experimental curves obtained on icosahedral Al-Pd-Mn. An excellent agreement between experiment and the calculated curves obtained using our model is found.
The Thermal Stability of a Single-Grain Mg-Zn-Y Icosahedral Quasicrystal
Authors: Luo Z.P. , Tang Y.L. , Miller D.J. , Kramer M.J. , Fisher I.R. , Canfield P.C.
Published: April 2011

The stability of the Mg-Zn-Y icosahedral quasicrystal (IQC) has been studied by long-term annealing of a single grain IQC in quartz tubes. Decomposition of the IQC was observed after annealing at high temperatures (T≥773 K) sealed in Ar. During the decomposition process, the quasilattice parameteraRwas found to decrease, associated with a decrease in Mg content of the IQC phase as confirmed by quantitative x-ray energy dispersive spectroscopy analyses. In addition, a new cubic approximant has been found in the annealed samples. This cubic approximant has a face-centered cubic (fcc) structure with lattice parameter ofa= 1.276 nm, which is about (1/τ) times smaller than that of the fcc W'-(MgZnY) witha= 2.05 nm reported previously (where τ is the golden ratio).
The Model for Icosahedral Al-Pd-Mn phase based on theT*(2F)Canonical Tiling
Authors: Kasner Gerald , Paradopolos Zorka
Published: April 2011

The icosahedral canonical tiling of the three-dimensional space by six golden tetahedraT*(2F)[1] is decorated for physical applications by the Bergman polytopes [2]. The model can be also formulated as the “primitive) tilingTP[3] decorated by alternating Bergman symmetry axis of and icosahedron, there appear the plans on three mutual distances following the rule of a decorated Fibonacci sequence. All these three distances among the terraces (mutually scaled by a factor τ) have been recently observed by shenet al. [5]. In particular they have measured also the shortest distance of 2.52Å that breaks the Fibonnaci-sequence of terrace like surfaces measured previously by schaubet al. [6]. We predict the frequencies for the appearance of the terraces of different heights in the model under the condition that the model of Boudardet al. [7.8], we decorate the atomic positions by Al, Pd and Mn. We present images of the predictedpossibleterrace-like surfaces on three possible distances in the fully decorated model by the atomic species.
Internal Friction and Elastic Modulus Change Associated with Structural Relaxation and Crystallization in Amorphous Mg-Ni-Y Alloys.
Authors: Soifer Y. M. , Kobelev N. P. , Korin E. , Soifer L. , Mogilyanskii D. , Bettelheim A.
Published: April 2011

Mechanical spectroscopy technique has been applied to investigation of structural relaxation and crystallization of Mg-Ni-Y amorphous alloys of different composition. The composition of the alloys was varied due to change of yttrium concentration from 0 to 7 at.%. The internal friction (IF) and Young's modulus have been measured by a vibrating reed method at frequency of ∼200 Hz at heating and cooling runs in the temperature range from 300K to 650K. Crystallization kinetic studies of the alloys by DSC and XRD showed that the transition from amorphous to crystalline state in the Mg-Ni-Y alloys has gone through several stages with origination of intermediate phase. IF spectra and Young's modulus of the alloys have been analyzed and the effect of yttrium content on the stability of intermediate phase was investigated.
Structural and Phase Transformations During Crystallization of Pd43Ni10Cu27P20 Metallic Glass
Authors: Pekarskaya Evgenia , Schroers Jan , Johnson William L.
Published: April 2011

Crystallization of the Pd43Ni10Cu27P20 amorphous alloy during isothermal annealing in the undercooled temperature region is studied by electron microscopy and diffrential scanning calorimetry (DSC). It is established that different crystallization processes take place above and below the nose temperature of the time-temperature-transformation (TTT) diagram. Detailed analysis of the microstructural evolution at the early stages of the crystallization is performed. In addition, the stable phases in the Pd-Ni-Cu-P system are identified.
Molecular Dynamics Simulations of Supercooled Liquid Metals and Glasses
Authors: Lee Hyon-Jee , Qi Yue , Strachan Alejandro , Cagin Tahir , Goddard William A. , Johnson William L.
Published: April 2011

The thermodynamic, transport and structural properties of a binary metallic glass former in solid, liquid, and glass phases were studied using molecular dynamics simulation. We used a model binary alloy system with a sufficient atomic size mismatch and observed a glass transition in a quenching process. The diffusivity and viscosity were calculated in the liquid state and the super-cooled liquid state. The smaller atom showed higher diffusivity and more configurational randomness compared to the larger atom. The viscosity increased abruptly around the glass transition temperature. The solvent/solute concentration effect on the glass transition was examined in terms of a packing fraction. We find that the glass forming ability increases with the packing fraction in the liquid state because the densely-packed material requires more time to rearrange and crystallize.
Crystallization of Bulk Zr48Nb8Cu14Ni12Be18Metallic Glass
Authors: Zhuang Y.X. , Gerward L. , Jiang J.Z. , Olsen J.S. , Zhang Y. , Wang W.H.
Published: April 2011

The crystallization of bulk Zr48Nb8Cu14Ni12Be18metallic glass has been investigated by differential scanning calorimetry (DSC) and X-ray powder diffraction. The activation energies of glass transition and crystallization for the glass obtained using Kissinger analysis from the shift of the peak temperature in the DSC curve are 470 and 235 kJ/mol, respectively. The effect of applied pressure on crystallization is studied by in situ high-pressure and high-temperature X-ray powder diffraction using synchrotron radiation. It is found that the crystallization temperature increases with pressure having a slope of 9.5 K/GPa in the range of 0-4.4 GPa.
Spatial Localization of the Nucleation Rate in Deeply Undercooled Liquids
Authors: Bossuyt S. , Johnson W. L.
Published: April 2011

In bulk metallic glass forming alloys cooled close to the critical cooling rate for glass formation, the nucleation density is observed to be spatially inhomogeneous; in an amorphous matrix there are spherical clusters with a high density of nanocrystals. This is attributed to the combined effect of recalescence due to the heat of crystallization and the fact that in deeply undercooled liquids the nucleation rate increases with increasing temperature.

A linear stability analysis of the non-linear differential equations describing nucleation and growth reveals that in the early stages of crystallization, stability is determined by the temperature dependence of the nucleation rate. When the nucleation rate increases with increasing temperature, fluctuations are amplified, resulting in “hot spots” where both the nucleation rate and the growth rate are higher.
Changes in Crystal Structure and Microhardness During Thermal Treatment of a NiCoCrAlY Alloy.
Authors: Rehfeldt Thomas , Schumacher Gerhard , Klingelhoeffer Hellmuth
Published: April 2011

The stability of microstructure and the microhardness of a NiCoCrAlY alloy was studied after thermal treatment at high temperatures and subsequent quenching into ice water. The alloy revealed mainly two ordered phases, a γ' phase with L12 crystal structure and a β phase with B2 structure. The γ' phase is shown to become unstable in the temperature range between 1073 K and 1373 K where it undergoes an order-disorder phase transformation. The low transformation temperature compared to pure Ni3Al is ascribed to deviations from the Ni3Al stoichiometry and to the additional alloying elements Co and Cr. The γ'-γ order-disorder transformation is shown to be attended by a decrease in microhardness of about 30 %.
Compression Behavior of L12 Modified Titanium Trialuminides Alloyed with Chromium and Iron
Authors: Takahashi Tohru , Hasegawa Tadashi
Published: April 2011

L12 modified titanium trialuminides have been prepared by replacing 9at.% of the aluminum in Al3Ti with chromium and/or iron. The materials were recrystallized into single phase polycrystals after isothermal forging resulting in an average grain diameter of about 40μm. Lattice parameter of the material containing 9 at.% chromium or 9 at.% iron, are 0.3959nm and 0.3939nm, respectively. The lattice parameters varied linearly with composition between these values for additions of both chromium and iron. Uniaxial compression tests were performed at temperatures ranging from 293K to 1300K. The yield strength is not sensitive to chemical composition within the range of compositions tested. Flow stress serrations of a few % were observed at temperatures around 600K, where intermittent drops in flow stress started immediately after yielding and continued to the end. These serrations were observed up to about 800K. At 900K and above the materials became fully deformable. Quasi steady state flow and strain softening were observed at 1200K and 1300K, respectively, due to dynamic recrystallization.
Dislocation Interaction with Point Defects in Transition-Metal Disilicides
Authors: Yoo Man H.
Published: April 2011

Energetics of the formation of jog-pairs and kink-pairs on a straight dislocation are analyzed using anisotropic elasticity theory for the equivalent slip systems of seven transition-metal disilicides. While glide loops of the active slip systems are stable in all cases, having positive line tension, the interaction energies of two opposite segments in kink-pairs and jog-pairs are found to be very anisotropic with respect to dislocation orientation. The anisotropic interaction plays an important role in the glide resistance due to dislocation-point defect interactions. A dislocation model is proposed for the glide resistance on edge and near edge dislocations based on jog-pairs resulting from the contact interaction between dislocations and intrinsic point defects. The available data of yield strength anomaly and dislocation structures of disilicide crystals are discussed in view of the proposed model for jog-pair pinning and dynamic breakaway.
Mechanical Behavior of Molybdenum Disilicide-Based Alloys
Authors: Misra A. , Sharif A.A. , Petrovic J. J. , Mitchell T. E.
Published: April 2011

We have investigated the mechanical behavior of the following single-phase polycrystalline alloys with the MoSi2 body-center tetragonal structure: MoSi2 alloyed with ∼2.5 at.% Re, MoSi2 alloyed with 2 at.% Al, MoSi2 alloyed with 1 at.% Nb, and MoSi2 alloyed with 1 at.% Re and 2 at.% Al. Several anomalies in the mechanical behavior of alloyed materials were observed. For example, (i) addition of only ∼2.5 at. % Re results in an order of magnitude increase in compressive strength at 1600 °C, (ii) additions of Nb and Al cause solution softening at near-ambient temperatures, and (iii) quaternary MoSi2-Re-Al alloys show strengthening at elevated temperatures and reduction in flow stress with enhanced plasticity at near-ambient temperatures in compression. The mechanisms of anomalous solution hardening and softening are discussed.
Multi-Modal “Order-Order” Kinetics in Ni3Al Studied by Monte Carlo Computer Simulation
Authors: Oramus P. , Kozubski R. , Pierron-Bohnes V. , Cadeville M.C. , Pfeiler W.
Published: April 2011

“Order-order” relaxations in γ-Ni3Al previously extensively studied by means of resistometry, are simulated within a model of vacancy mechanism of atomic migration in a superstructure implemented with Monte Carlo technique and the Glauber algorithm. The observed operation of two simultaneous relaxation processes showing different rates, as well as the theoretically predicted effect of vacancy ordering have been definitely reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing L12-type superstructure shows that the experimentally observed features of the “order-order” processes in Ni3Al follow from an interplay between two dominating and coupled modes of long- and short-range ordering: the creation/elimination of nn pairs of antisites (SRO) and the change of the “overall” number of antisites (LRO). High profile of the first process results in a high contribution of the fast component of LRO kinetics.
First Principles Calculation of Cooperative Atom Migration in L12 Ni3Al
Authors: Schweiger H. , Podloucky R. , Wolf W. , Püschl W. , Pfeiler W.
Published: April 2011

Recent Monte-Carlo simulations of order relaxations in L12-ordered Ni3Al reproduced the simultaneous action of two processes as experimentally observed by residual resistometry. It was shown that the fast process is related to the fast annihilation/creation of nearest neighbour antisite pairs. These findings are now strongly corroborated by a new supercell approach of ab initio quantum mechanical calculations describing the simultaneous displacement of Ni and Al atoms on their way to their respective antisite positions. Studies of single jumps suggest that such a cooperative migration of Ni and Al is necessary in order to prevent Al antisites from jumping back into their regular position. Relaxation of neighbouring atoms was taken into account. Thus, a minimum migration barrier of about 3 eV was derived which together with the calculated formation enthalpy of a Ni vacancy of 1.5 eV amounts to 4.5 eV, in remarkable agreement with the high activation enthalpy of 4.6 eV as observed experimentally.
Ni3Al Thin Foil by Cold Rolling
Authors: Hirano Toshiyuki , Demura Masahiko , Kishida Kyosuke , Suga Yozo
Published: April 2011

Thin foils of stoichiometric Ni3Al below 100 μm in thickness were successfully fabricated by cold rolling of the sheets which were sectioned from directionally solidified ingots. Maximum rolling reduction in thickness amounted to 96%, irrespective of the initial orientation or the existence of columnar grains in the starting sheets. The as-rolled foils were characterized in terms of microstructures, textures and dislocation structures. The deformation microstructures were of a dual banded structure composed of two different {110} textures in the case of <001> rolling direction, while a rather homogeneous structure with a single {110} texture resulted in the case of <112> rolling direction. TEM observation revealed homogenous dislocation structures in either case without cell formation, accompanied by very fine grained-regions at higher reduction.
The Influence of some Microstructural and Test Parameters on the Tensile Stress and Ductility Behaviour of a MA FeAl Intermetallic
Authors: Chao J. , Muñoz-Morris M.A. , Gonzalez-Carrasco J.L. , Morris D.G.
Published: April 2011

This study examines the influence of microstructural parameters (grain and dispersoid size, vacancy content) and some test parameters (strain rate, protective oxide coatings, air and water vapour excluding films, and surface geometrical quality) on the tensile behaviour (yield stress, work hardening rate, tensile stress, ductility) of a mechanically-alloyed, fine-grained Fe-40Al intermetallic. Major changes of strength and ductility are obtained by changing grain size (1% and 10% for grain sizes of 100μm and 1μm) and by avoiding premature stress/strain concentrators (ductility increased from 5% to 10% for imperfectly machined to prepolished samples). Ductility variations are interpreted using a slow-crack-propagation-to-instability model, where the roles of environment, surface state, deformation processes, and fracture mechanisms can be distinguished.
Microstructure and Plastic Deformation in Unidirectionally Solidified NbSi2 (C40) /MoSi2 (C11b) Crystals
Authors: Nakano Takayoshi , Nakai Yasuhiro , Umakoshi Yukichi
Published: April 2011

Microstructure and plastic deformation behavior in duplex-phase silicides composed of the C40 and C11b phases were examined using pseudo-binary (Nb,Mo)Si2 crystals with a single set of lamellae. Single crystals of the C40 single-phase were grown from the master ingot with a composition of (Nb0.15Mo0.85)Si2 by a floating zone method and duplex-phase microstructure containing a single set of lamellae was obtained by subsequent heat treatment at 1400°C for no less than 6h. During the heat treatment, the C11b phase was precipitated from the C40 matrix by satisfying the crystallographic relationship of (0001)C40//(110)C11b, <1210]C40//[110]C11b and < [1010]C40//[001]C11b at the lamellar boundary, while randomly oriented C11b grains also appeared at further annealing. As a result, the duplex-phase silicides with a single set of lamellae contained the C40 phase with a single orientation and three variants of the C11b phase. The lamellar spacing and the volume fraction of their phases depended strongly on annealing time.

In compression tests, yield stress and fracture strain of the duplex-phase silicides depended strongly on angle (φ) between the loading axis and lamellar boundaries, similar to TiAl-PST crystals. At φ=0°, specimens fractured just after showing high fracture stress even at 1400°C. In contrast, at φ=45° where shear deformation in the C11b phase of lamellae occurred parallel to lamellar boundaries, low yield stress and significant plastic strain were achieved at 1400°C.
C11b/C40 Lamellar Structure in a Ta-added Molybdenum Disilicide
Authors: Wei Fu-Gao , Kimura Yoshisato , Mishima Yoshinao
Published: April 2011

C11b/C40 fully lamellar microstructures, similar to the well-known TiAl/Ti3Al lamellae, were obtained in Ta- and Nb-added MoSi2 polycrystalline alloys in a previous work. In the present study, the crystallography of the lamellar structure is investigated in a MoSi2-15mol%TaSi2 pseudo-binary alloy after homogenized at 1400°C for 168h, in order to provide some useful parameters for microstructural control to improve mechanical properties. The orientation relationship between C11b and C40 phases and its three distinct variants were identified. Coherency of the lamellar interface is analyzed in comparison with the TiAl/Ti3Al lamellae. Approach to modify the C11b/C40 lamellar microstructure to increase its coherency is discussed based on the results obtained.
Dislocation Processes and Deformation Behavior in <001>-Oriented FeX-Ni60–X-Al40 Single Crystals
Authors: Brenner P. S. , Srinivasan R. , Noebe R. D. , Lograsso T. , Mills M. J.
Published: April 2011

The mechanical properties and dislocation microstructure of single crystals with a range of compositions within the Fex-Ni60–x-Al40 pseudobinary system have been investigated, with the purpose of bridging the behavior from FeAl to NiAl. Experiments are focused on the compression testing of <001> oriented single crystals with compositions where x = 10, 20, 30, 40, and 50 (in atomic percent). Observations of a<111> dislocation morphologies at room temperature and both a<111> and non-a<111> dislocation activity at elevated temperatures are reported and discussed. Measurements of the yield strength, elastic modulus and strain hardening rates are reported, and the variation of strength with composition is correlated with dislocation dissociation and overall dislocation morphology.
Nonstoichiometry and Defect Mechanism in Intermetallics with L12-Structure
Authors: Ipser Herbert , Semenova Olga P. , Krachler Regina , Schweitzer Agnes , Yuan Wenxia , Peng Ming , Qiao Zhiyu
Published: April 2011

A statistical-thermodynamic model was derived which allows to describe thermodynamic activities in intermetallic compounds with L12-structure as a function of composition and temperature. The energies of formation of the four types of point defects (anti-structure atoms and vacancies on both sublattices) were used as adjustable parameters. The model was applied to the three compounds Ni3Al, Ni3Ga, and Pt3Ga, and it permitted to estimate for the first time the defect formation energies for Ni3Ga and to provide initial estimates for Pt3Ga.
Synthesis of Spatially Controlled Nanostructures by Ion Implantation in V-Grooves on (001) Si Surfaces
Authors: Müller Torsten , Heinig Karl-Heinz , Schmidt Bernd , Mücklich Arndt , Möller Wolfhard
Published: April 2011

The synthesis of spatially controlled Ge nanowires and nanoclusters by Ge+ ion implantation in oxidized V-grooves on (001) Si surfaces has been studied experimentally as well as theoretically. The V-grooves were prepared by anisotropic wet chemical etching and thermal oxidation. The SiO2-covered V-grooves were implanted with 70 keV Ge+ ions up to a fluence of 1017 cm−2. Ge accumulates within the SiO2 at the bottom of the V-groove, which has been proven by analytical TEM (EDX-mapping). Theoretical studies have shown that the Ge accumulation is caused by the V-groove geometry, forward sputtering, and re-deposition. During subsequent annealing the redistributed Ge forms a nanowire by precipitation, ripening and coalescence. Kinetic lattice Monte Carlo simulations of the nanowire formation process show growth instabilities and self-organization phenomena.
Mechanical Properties of AlN Thin Films Prepared by Ion Beam Assisted Deposition
Authors: Miyabe Shuichi , Okawa Toshiyuki , Kitazawa Nobuaki , Watanabe Yoshihisa , Nakamura Yoshikazu
Published: April 2011

Aluminum nitride (AlN) thin films were prepared by ion-beam assisted deposition method, and the influence of the nitrogen ion beam energy on their microstructure and mechanical properties was studied by changing the ion beam energy from 0.1 to 1.5 keV. Films prepared with a low-energy ion beam show a columnar structure, while films prepared with a high-energy ion beam show a granular structure. The film hardness is found to decrease with increasing nitrogen ion beam energy. It is also found that the film hardness does not change drastically after annealing in nitrogen atmosphere at 500 °C, yielding the residual stress relaxation. It is proposed that the film hardness is dependent on the film microstructure, which can be controlled with the nitrogen ion beam energy, rather than the residual stress in the films.
synthesis of III-Nx-V1-x Thin Films by N Ion Implantation
Authors: Yu K. M. , Walukiewicz W. , Shan W. , Wu J. , Beeman J. W. , Ager J. W. , Haller E. E. , Ridgway M.C.
Published: April 2011

Dilute III-Nx-V1-x alloys were successfully synthesized by nitrogen implantation in GaAs and InP. The fundamental band gap energy for the ion beam synthesized III-Nx-V1-x alloys was found to decrease with increasing N implantation dose in a manner similar to that commonly observed in epitaxially grown GaNxAs1-x and InNxP1-xthin films. The fraction of N occupying anion sites ("active" N) in the GaNxAs1-x layers formed by N implantation was thermally unstable and decreased with increasing annealing temperature. In contrast, thermally stable InNxP1-x alloys with N mole fraction as high as 0.012 were synthesized by N implantation in InP. Moreover, the N activation efficiency in InP was at least a factor of two higher than in GaAs under similar processing conditions. The low N activation efficiency (<20%) in GaAs can be improved by co-implanting Ga and N in GaAs.
Inverse Ostwald Ripening and Self-Organization of Nanoclusters due to Ion Irradiation
Authors: Heinig K.-H. , Schmidt B. , Strobel M. , Bernas H.
Published: April 2011

Under ion irradiation collisional mixing competes with phase separation if the irradiated solid consists of immiscible components. If a component is a chemical compound, there is another competition between the collisional forced chemical dissociation of the compound and its thermally activated re-formation. Especially at interfaces between immiscible components, irradiation processes far from thermodynamical equilibrium may lead to new phenomena. If the formation of nanoclusters (NCs) occurs during ion implantation, the phase separation caused by ion implantation induced supersaturation can be superimposed by phenomena caused by collisional mixing. In this contribution it will be studied how collisional mixing during high-fluence ion implantation affects NC synthesis and how ion irradiation through a layer of NCs modifies their size and size distribution. Inverse Ostwald ripening of NCs will be predicted theoretically and by kinetic lattice Monte-Carlo simulations. The mathematical treatment of the competition between irradiation-induced detachment of atoms from clusters and their thermally activated diffusion leads to a Gibbs-Thomson relation with modified parameters. The predictions have been confirmed by experimental studies of the evolution of Au NCs in SiO2 irradiated by MeV ions. The unusual behavior results from an effective negative capillary length, which will be shown to be the reason for inverse Ostwald ripening. Another new phenomenon to be addressed is self-organization of NCs in a d-layer parallel to the Si/SiO2 interface. Such d-layers were found when the damage level at the interface was of the order of 1-3 dpa. It will be discussed that the origin of the d-layer of NCs can be assigned to two different mechanisms: (i) The negative interface energy due to collisional mixing gives rise to the formation of tiny clusters of substrate material in front of the interface, which promotes heteronucleation of the implanted impurities. (ii) Collisional mixing in the SiO2produces diffusing oxygen, which may be consumed by the Si/SiO2 interface. A thin layer parallel to the interface becomes denuded of diffusing oxygen, which results in a strong pile up of Si excess. This Si excess promotes heteronucleation too. Independent of the dominating mechanism of self-organization of a d-layer of NCs, its location in SiO2 close to the SiO2/Si interface makes it interesting for non-volatile memory application.
Characterization of Thin Layers of Metal Clusters Embedded in Silica Glass Formed by High Dose Ion Implantation
Authors: Chung P.S. , Wong S.P. , Cheung W.Y. , Ke N. , Lee W.K. , Chan C.W.
Published: April 2011

Thin layers of metal clusters in silica glass were formed by high dose ion implantation of Ag, Ni and Cu using a metal vapor vacuum arc (MEVVA) ion source. Characterization of the implanted layers was performed using Rutherford backscattering spectroscopy, transmission electron microscopy, x-ray diffraction, and optical measurements. The nonlinear optical properties of the implanted samples were studied by the z-scan method, using a self mode-locked Ti: sapphire laser delivering linearly polarized pulses of 130fs long at a wavelength of 790nm at 76MHz. The variation of the intensity-dependent refractive index n2 with the ion species and the implantation conditions were studied. An n2 value of about 0.6 cm2/GW was measured for one of the Cu and Ni co-implanted samples. The correlation between n2 and the cluster size was also analyzed. Ellipsometry spectra of the samples were measured in the visible range from 0.4 to 0.7 [.proportional]m and in the near infrared range from 0.9 to 1.6 [.proportional]m. Preliminary results are reported on our attempt to deduce the effective complex refractive indices of the implanted layers by fitting of the ellipsometry spectra using a simple single uniform layer on substrate model and the Maxwell-Garnett effective medium approximation.
Gold Nanoclusters Formed by Ion-Implantation into Bi2TeO5
Authors: Kling A. , da Silva M.F. , Soares J.C. , Fichtner P.F.P. , Amaral L. , Zawislak F.C. , Földvári I. , Péter Á.
Published: April 2011

Single crystalline samples of bismuth tellurite (Bi2TeO5) were implanted with 800 keV Au+ions to a fluence of 1×1016 cm-2 at room temperature. The samples were subjected to heat treatments in two different ambients (air and high vacuum) at temperatures ranging from 400 - 700°C in a conventional furnace. Strong absorption maxima in the range from 600 - 630 nm in the optical absorption spectra and an intense blue-green color in the samples were observed for annealings performed in air at temperatures between 500 and 700°C indicating the formation of gold colloids. The average radii of the Au clusters formed were estimated to be in the range of 3-4 nm. Samples annealed under vacuum showed distinct changes in color for different annealing temperatures. Studies using the RBS/channeling technique indicate that no full recrystallization of the samples was achieved under both annealing regimes although heat treatment under vacuum provides a significantly better lattice recovery than for air ambient.
Chemical Effects in Ion Implantation Induced Quantum Well Intermixing
Authors: Simpson Todd W. , Piva Paul G. , Mitchell Ian V.
Published: April 2011

Ion implantation followed by rapid thermal annealing is used to induce layer intermixing and thus selectively blue-shift the emission wavelength of InP-based quantum well hetero- structures. The intermixing is greatly enhanced over thermal intermixing due to the supersaturation of defects. The magnitude of the observed blue-shift has been studied previously as a function of ion fluence and ion mass: the dependence on ion mass is well established, with heavier ions producing a larger shift. We show here that chemical effects can also play a significant role in determining the induced blue-shift. Data are presented from the implantation of the similar mass ions; aluminum (m~27), silicon (m~28) and phosphorus (m~31). The P- induced blue shift displays a monotonic increase with fluence, consistent with previous studies; however, the fluence dependence of Al- and Si-induced blue-shifts both deviate significantly from the behaviour for P. These results have important implications for attempts to scale intermixing behaviour with ion mass.
Simulations and Argon-Cluster-Ion Smoothing of Surfaces
Authors: Fenner D. B. , Dean D. W. , DiFilippo V. , Allen L. P. , Hautala J. , Mirkarimi P. B.
Published: April 2011

Bombardment of slightly rough, planar surfaces with an energetic gas-cluster ion beam (GCIB) is known to result in a decrease in roughness. We report comparison of measured surface evolution with simulations based on a simple phenomenological model. The model consists of a continuum rate equation for the surface height undergoing multiple cluster impacts. With suitable assumptions the main features observed in the evolution of an initially nonplanar surface can be simulated. Qualitatively, both experiment and simulation show that sharp features and asperities are rapidly eroded. Examples are given of initial surfaces that are either randomly rough and scratched, or very smooth with added nanoscale engineered features. Both types of surfaces show smoothing in microscopy evaluations of the GCIB and in the simulations, thereby identifying impact-transient surface diffusion as the primary cluster smoothing mechanism.
On the Measurement of Stress-Strain Curves by Spherical Indentation
Authors: Herbert E. G. , Pharr G. M. , Oliver W. C. , Lucas B. N. , Hay J. L.
Published: April 2011

It has been proposed that with the appropriate models, instrumented indentation test (IIT) data can be reduced to yield the uniaxial stress-strain behavior of the test material. However, very little work has been done to directly compare the results from uniaxial tension and spherical indentation experiments. In this work, indentation and uniaxial tension experiments have been performed on the aluminum alloy 6061-T6. The purpose of these experiments was to specifically explore the accuracy with which the analytical models can be applied to IIT data to predict the uniaxial stress-strain behavior of the aluminum alloy.
Nanoindentation Measurements of Mechanical Properties of Polystyrene Thin Films
Authors: Li Min , Carter C. Barry , Gerberich William W.
Published: April 2011

The elastic modulus and work of adhesion of thin polystyrene (PS) films have been evaluated from nanoindentation load-displacement curves. The modulus was calculated using two methods: an unloading stiffness analysis and an elastoplastic unloading analysis. Results indicate that the latter analysis gives better modulus evaluation for the polymers. Two methods were also utilized in determining the work of adhesion, one using the pull-off forces and one using the displacement difference at zero force and pull-off force. The values given by the two methods are close. The effects of surface roughness and maximum load on the adhesion measurements are discussed. Different molecular weights were also chosen to compare the characteristics of the polymers during use under the same conditions. No significant difference in either modulus or adhesion energy was shown between the PSs of very low, moderate, and high molecular weights at room temperature.
Identification of Pressure-Induced Phase Transformations Using Nanoindentation
Authors: Domnich Vladislav , Gogotsi Yury , Trenary Michael
Published: April 2011

A combination of depth-sensing indentation and Raman microspectroscopy has been used for the identification of pressure-induced phase transformations in silicon, germanium, boron carbide and partially stabilized zirconia single crystals. Phase transformations during nanoindentation may be revealed through deviations in the shape of the load-displacement curves from that of a perfect elastoplastic material. Such deviations are often more readily identified if the nanoindentation data are presented as average contact pressure vs. contact depth curves, allowing assessment of the corresponding transformation pressures.
Irradiation-Induced Recrystallization of Cellular Dislocation Networks in Uranium-Molybdenum Alloys
Authors: Rest J. , Hofman G. L.
Published: April 2011

We developed a rate-theory-based model to investigate the nucleation and growth of interstitial loops and cavities during low-temperature in-reactor irradiation of uranium-molybdenum alloys. Consolidation of the dislocation structure takes into account the generation of forest dislocations and capture of interstitial dislocation loops. The theoretical description includes stress-induced glide of dislocation loops and accumulation of dislocations on cell walls. The loops accumulate and ultimately evolve into a low-energy cellular dislocation structure. Calculations indicate that nanometer-size bubbles are associated with the walls of the cellular dislocation structure. The accumulation of interstitial loops within the cells and of dislocations on the cell walls leads to increasing values for the rotation (misfit) of the cell wall into a subgrain boundary and a change in the lattice parameter as a function of dose. Subsequently, increasing values for the stored energy in the material are shown to be sufficient for the material to undergo recrystallization. Results of the calculations are compared with SEM photomicrographs of irradiated U- 10Mo, as well as with data from irradiated UO2.
Influence of Interatomic Potentials in MD Investigation of Ordering in a-SiC
Authors: Yuan Xianglong , Hobbs Linn W.
Published: April 2011

Molecular dynamics (MD) simulations of a-SiC using several Tersoff potentials have been performed and their influences on structure ordering were studied. It was found that using different potential cutoffs leads to remarkably different structures. An abrupt cutoff at 2.5 Å greatly increases the chemical ordering of a-SiC by disfavoring the formation of Si-Si bonds. In addition, annealing of SiC cascades embedded in β-SiC was simulated, and the final structures were compared. Again, much stronger topological and chemical ordering was observed in the structure modeled with the 2.5 Å potential cutoff.
Computer Simulation of Energy Dependence of Primary Damage States in SiC
Authors: Devanathan R. , Gao F. , Weber W. J.
Published: April 2011

The primary damage state in 3C-SiC has been comprehensively studied by molecular dynamics using a modified Tersoff potential. The simulations examined damage produced by Si and C primary knock-on atoms (PKA) with energies from 0.25 to 30 keV. The study also generated statistics of defect production by simulating a number of PKAs at each energy. The defect production efficiency decreases with increasing PKA energy, as observed previously in metals. However, the cascade lifetime is very short (less than 1 ps), localized melting does not occur, the defect arrangements are highly dispersed, and the tendency for defects to form clusters is much less compared to the case of metals. Frenkel pairs on the C sublattice are more numerous than Si Frenkel pairs, and 10-20% of the displacements are in the form of anti-site defects.
Hydrogen Passivation of Si Nanocrystals in Silica
Authors: Cheylan Stephanie , Elliman Robert G.
Published: April 2011

This paper explores the effect of hydrogen on the luminescence properties of silicon nanocrystals formed in silica by high-dose ion-implantation and thermal annealing. For samples implanted to low fluence (small nanocrystals), passivation is shown to result in a uniform enhancement of the PL emission for all wavelengths. However, for samples implanted to high fluence, preferential enhancement of the emission from larger nanocrystals is evident, resulting in a red-shift of emission spectra. Both the intensity enhancement and the red-shift are shown to be reversible, with spectra returning to their pre-passivation form when H is removed from the samples by annealing. The luminescence lifetime is also shown to increase after passivation, confirming that defect-containing nanocrystals luminesce.
Irradiation-Induced Amorphization Of Titanite
Authors: Meldrum A.
Published: April 2011

Titanite (CaTiSiO5) is a widely occurring accessory mineral that contains ppm-level concentrations of U and Th. Radiation effects in titanite are important because this phase is commonly-used for U-Pb age dating of rock formations, and it is also the main crystalline constituent of certain glass-ceramic nuclear waste forms. Previous work suggested that titanite is highly susceptible to natural alpha-decay-induced amorphization, but ion irradiation experiments have so far been reported only at room temperature. In this work, the first temperature-dependent amorphization data for titanite are reported. High-purity single-crystal specimens from the Khan Mine, Namibia were characterized by analytical electron microscopy and powder XRD. Suitable specimens were then irradiated in-situ at the IVEM Facility using 800 keV Kr ions at temperatures ranging from 30 to 1100 K. Conventional imaging and diffraction techniques were used to monitor the transformation to the amorphous state. Titanite was amorphized at a relatively low dose over the entire temperature range investigated. Limited crystallization of ion-beam-amorphized titanite was observed at temperatures above 1100 K.
A Method to Improve Activation of Implanted Dopants in SiC
Authors: Holland O. W. , Thomas D. K.
Published: April 2011

Implantation of dopant ions in SiC has evolved according to the assumption that the best electrical results (i.e., carrier concentrations and mobility) are achieved by using the highest possible processing temperature. This includes implantation at > 600°C followed by furnace annealing at temperatures as high as 1750°C. Despite such aggressive and extreme processing, implantation suffers because of poor dopant activation, typically ranging between < 2%–50% with p-type dopants represented in the lower portion of this range and n-types in the upper. Additionally, high-temperature processing can led to several problems including changes in the stoichiometry and topography of the surface, as well as degradation of the electrical properties of devices. A novel approach for increasing activation of implanted dopants in SiC and lowering the activation temperature will be discussed. This approach utilizes the manipulation of the ion-induced damage to enhance activation of implanted dopants. It will be shown that nearly amorphous layers containing a small amount of residual crystallinity can be recrystallized at temperatures below 900°C with little residual damage. It will be shown that recrystallization traps a high fraction of the implanted dopant residing within the amorphous phase (prior to annealing) onto substitutional sites within the SiC lattice.
Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of Nanostructural Features in Irradiated Fe-Cu-Mn Alloys
Authors: Wirth B. D. , Asoka-Kumar P. , Howell R. H. , Odette G. R. , Sterne P. A.
Published: April 2011

Radiation embrittlement of nuclear reactor pressure vessel steels results from a high number density of nanometer sized Cu-Mn-Ni rich precipitates (CRPs) and sub-nanometer matrix features, thought to be vacancy-solute cluster complexes (VSC). However, questions exist regarding both the composition of the precipitates and the defect character and composition of the matrix features. We present results of positron annihilation spectroscopy (PAS) and small angle neutron scattering (SANS) characterization of irradiated and thermally aged Fe-Cu and Fe-Cu-Mn alloys. These complementary techniques provide insight into the composition and character of both types of nanoscale features. The SANS measurements indicate populations of CRPs and VSCs in both alloys. The CRPs are coarser in the Fe-Cu alloy and the number densities of CRP and VSC increase with the addition of Mn. The PAS involved measuring both the positron lifetimes and the Doppler broadened annihilation spectra in the high momentum region to provide elemental sensitivity at the annihilation site. The spectra in Fe-Cu-Mn specimens thermally aged to peak hardness at 450°C and irradiated at 288°C are nearly identical to elemental Cu. Positron lifetime and spectrum measurements in Fe-Cu specimens irradiated at 288°C clearly show the existence of long lifetime (∼500 ps) open volume defects, which also contain Cu. Thus the SANS and PAS provide a self-consistent picture of nanostructures composed of CRPs and VSCs and tend to discount high Fe concentrations in the CRPs.
Characterizations of Lamellar Interfaces and Segregations in a PST-TiAl Intermetallic Alloy by an Analytical Scanning Transmission Electron Microscope
Authors: Zhao Wei , Luzzi David E.
Published: April 2011

Polysynthetically-twinned titanium aluminide (PST-TiAl), a fully lamellar γTiAl + α2-Ti3Al dual-phase alloy, is under evaluation for applications in rotary components in aircraft and automobile industries due to its high specific strength, and a high strength-retention capability at elevated-temperatures. However, the low ductility at room- to mid-high temperatures of the material hinders its application. Additions of certain tertiary elements to the binary TiAl system appear to improve the ductility at room- to mid-high temperatures, thus a balance among strength, ductility, and fracture toughness can be expected. In this article, segregation of tertiary elements to the lamellar interfaces is investigated. Single crystals of a TiAl with 0.6% atomic percentage tertiary additions are grown by an optical float-zone method. Segregation to the lamellar interfaces and the microstructure of the interfaces are investigated. Structures of the lamellar interfaces are characterized, and microchemistry and distribution habits of these elements along the γ+α2 lamellar boundaries as well as the γ-γ lamellar and domain boundaries are analyzed.
High-Resolution Electron Microscopy of Grain Boundary Migration
Authors: Merkle K. L. , Thompson L. J. , Phillipp Fritz
Published: April 2011

Atomic-scale grain boundary (GB) migration has been directly observed by high-resolution transmission electron microscopy (HREM). Atomic-scale motion of high-angle tilt GBs as well as twist and general GBs at gold island grains with a number of planar facets has been studied at ambient and elevated temperatures. GB migration mechanisms depend on GB structure and geometry. Strong indications for cooperative effects has been found. In this case, as has been proposed before, atoms may undergo small shifts in their lattice positions to be incorporated into the growing grain in a collective mode. At high temperature and in the absence of a strong driving force such small lattice regions are observed to fluctuate back and forth between the two grains. Faceted GBs typically move in spurts. This appears to be inherent to GB migration, whenever the motion is controlled by different structural entities. For some GB geometries the motion was found to proceed by the lateral propagation of atomic-scale steps.
Evolution of Nanostructure of FeNi(Ti/Cr)N Alloys During Phase Cycling
Authors: Chechenin N. G. , Bronsveld P. M. , Chezan A. R. , Craus C. B. , Boerma D. O. , de Hosson J. Th.M. , Niesen L.
Published: April 2011

A possibility to manipulate the microstructure of cold rolled (down to less than 1% of the initial thickness) Fe94Ni4Ti2 and Fe93Ni4 Cr3 foils via inter-phase cycling by nitriding and de-nitriding is investigated. The grains in the as-rolled material had a dominating (001)[110] texture and contained a complicated internal nanostructure due to a dislocation network. The interphase cycling α↔γ'-Fe4N and α↔ε-FexN (x∼3) was investigated as a tool to change or destroy the texture. We observed that the α↔γ'-Fe4N phase transformations weaken but do not erase the texture. A stronger reduction of the texture was obtained in the α↔ε-cycling, where grains with a new orientation appear in XRD scans. During the α↔γ' phase transformation a lamella structure is formed which is coarsening with the number of cycles. It was found that the rates of the α↔γ' and α↔ε interphase transitions were slowing down with the number of cycles. The observation is explained by an increase of kinetic barriers while removing/transforming the rolling-induced defects during the cycling.