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Gasification process is being developed to produce a clean and efficient gas flue from fuels such as coal, biomass, and solid/liquid wastes for power generation. In this work, a biomass gasificatio...

The deflection of cracks in grain interior was observed in a metastable β -phase Ti alloy (Ti-24Nb-4Zr-7.9Sn) with transmission electron microscopy during high cycle fatigue. This peculiar phenomenon is induced by the β → α″ martensitic transformation in front of crack tips in grain interior when cracks propagate along the {011} β slip planes with acute angles between α″ lamellae and cracks approximately of 45°. The α″ product phase has lattice dilation and contraction along two main transformation directions, thus introduces compressive and tensile strains perpendicular and parallel to crack propagating planes, respectively. This exerts beneficial effect on the crack deflection during the subsequent cyclic-loading.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. The Fano parameter $1/{q}^{2}$, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between $1/{q}^{2}$ and the superconducting critical temperature ${T}_{c}$. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable $xy$ orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

This study investigates the differences in the microstructure, defects and mechanical behavior of porous structures from a β-type Ti 24Nb 4Zr 8Sn manufactured by electron beam melting (EBM) and selective laser melting (SLM). The phases, size and shape of melt pool, volume and distribution of defects are analyzed and correlated to the compressive mechanical and fatigue properties. Due to different powder bed temperatures, the microstructure of EBM and SLM samples consists of α+β phases and a single β phase, respectively. The faster cooling rate during SLM promotes the formation of fine β dendrites, which leads to a higher compressive strength (50 ± 0.9 MPa) and lower Young's Modulus (0.95 ± 0.05 GPa) in comparison to the EBM parts (45 ± 1.1 MPa and 1.34 ± 0.04 GPa respectively). The large defects present within solid strut are likely a result of tin vaporization. The tin vapor is more easily trapped during the SLM process due to a smaller laser spot size and a faster cooling rate. This results in a 10 times increase in the number of defects. These defects have a limited influence on both the static properties and low stresses level fatigue strength, but it causes a reduced and variable fatigue life at high stresses level.

This study reports the feasibility to fabricate large size Ti-6Al-4V alloy parts using the retained powder after screened out the finer powder for additive manufacturing. Effects of powder size on the microstructure and mechanical properties of powder compacts were studied. Effects of HIPing cycle on relative density distribution of large size powder compact were also investigated. Hot compression tests of as-HIPed powder compact were conducted. Prediction of temperature field distribution of large size powder compact during ring rolling was obtained though finite element method (FEM). The results showed that the powder size has no obvious influence on the microstructure and mechanical properties of as-HIPed Ti-6Al-4V alloy. The homogeneous HIPed powder compact with large dimensions can be produced through an optimized HIPing schedule. The optimized hot working processing window from the isothermal compression tests is temperature at 940–970 °C and strain rate at 1–10 s− 1. The hot workability of Ti-6Al-4V powder compact is comparable or better than that of as-cast alloys. On the basis of isothermal compression tests and FEM results, a large workpiece of Ti-6Al-4V powder component has been successfully formed by ring rolling, and the mechanical performance of ring-rolled powder compact have some improvements than that of as-HIPed alloys.

This study developed a load forecasting system for electric market participants. Combining the least-square support vector machine (LSSVM) and particle swarm optimisation (PSO), a LSSVM_PSO was proposed for the solving process. The loads, temperature, and relative humidity of the Taipower system were collected in the Excel Database. Data mining techniques is used to discover meaningful patterns, with the PSO applied to adjust learning rates. The forecasting error can be reduced during the training process to improve both the accuracy and reliability, where even the spikes were nicely followed. The support vector regression, LSSVM, radial basis function neural network and the proposed LSSVM_PSO were all developed and compared to check the convergence and performance. Simulation results demonstrated the effectiveness of the proposed method in a price volatile environment.

Two main explanations exist for the deformation mechanisms in Ti-Nb-based gum metals, i.e. the formation of reversible nanodisturbance and reversible stress-induced martensitic transformation. In this work, we used the in situ synchrotron-based high-energy X-ray diffuse-scattering technique to reveal the existence of a specific deformation mechanism, i.e. deformation-induced spatially confined martensitic transformations, in Ti-24Nb-4Zr-8Sn-0.10O single crystals with cubic 13 parent phase, which explains well some anomalous mechanical properties of the alloy such as low elastic modulus and nonlinear superelasticity. Two kinds of nanosized martensites with different crystal structures were found during uniaxial tensile loading along the [11 0](beta) axis at room temperature and 190 K, respectively. The detailed changes in the martensitic phase transformation characteristics and the transformation kinetics were experimentally observed at different temperatures. The domain switch from non-modulated martensite to a modulated one occurred at 190 K, with its physical origin attributed to the heterogeneity of local phonon softening depending on temperature and inhomogeneous composition in the parent phase. An in-depth understanding of the formation of stress-induced spatially confined nanosized martensites with a large gradient in chemical composition may benefit designs of high-strength and high-ductility alloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The change of the lattice dynamics upon the charge disproportionation (CD) transition has been investigated for the ${\mathrm{CaFeO}}_{3}$ crystal by measuring its infrared optical spectra. Across the CD transition, ${\mathrm{CaFeO}}_{3}$ undergoes a metal-insulator transition, and it is found that below ${T}_{CD}\ensuremath{\approx}290$ K the low-frequency optical conductivity gradually decreases to a rather low value and is dominated by a series of infrared-active phonons. Intriguingly, accompanied by the CD transition, two prominent phonon modes at $\ensuremath{\sim}243$ and $\ensuremath{\sim}559$ ${\mathrm{cm}}^{\ensuremath{-}1}\text{associated}$ with the vibrations of Fe-O bonds show obvious redshift and asymmetric line shapes characterized by a Fano profile, suggesting a strong electron-phonon coupling. This coupling behavior reveals an intimate relationship between charge and lattice in the CD transition of ${\mathrm{CaFeO}}_{3}$.

Influence of defects (Ti vacancy and Nb dopant) on the bonding of TiAl/TiO2 interface was studied via first-principles calculations. It was shown that the bonding strength and the stability of TiAl/TiO2 interface were weakened by the presence of Ti vacancy and dopant Nb. The defects could also change the relative stability of the interface with different couplings between the two compounds. Electronic structure of the interface was analyzed and the influence mechanisms of defects on the bonding of interface were presented.

We propose an ω-lattice mechanism that is irrelevant to dislocation behaviors for a popular twinning ({1 1 2}〈1 1 1〉-type) system in body-centered-cubic (bcc) metals and alloys. The twinning process is dependent on the reverse transformation of ω (hexagonal) → bcc. The driving force of the twinning is attributed to the instability of a high density of nanoscale metastable ω precursors, and the mechanism has been experimentally and theoretically confirmed in bcc-Ti alloys with the {1 1 2}〈1 1 1〉-type twin formed under conditions free of external stress and internal strain. The ω-lattice mechanism involves bcc lattice shuffling only, thus can be applied to all bcc metals and alloys.

The formation of prismatic interstitial loops during plastic deformation, as well as their interaction with dislocations, is systematically investigated in aluminum, using molecular dynamics simulations. First, direct dislocation interaction is responsible for producing various types of defects, typically vacancies and their clusters and interstitial loops. Secondly, small interstitial loops act as obstacles to dislocation gliding. When close to each other, a loop either decorates a dislocation with jogs or drags it elastically. The mobility of a dislocation decorated by a loop is significantly reduced. Depending on the relative position of the dislocation and the loop, the Peierls stress can increase several hundred times. The present work shows a complete picture of dislocation–loop interaction with atomic-scale details, which provides reliable information for parameterizing dislocation–debris interactions in constitutive models.

The phenomenon of high twin boundary fractions found experimentally for neighboring gamma lamellae in fully lamellar TiAl alloys was investigated by phase-field simulations. The nucleation and growth processes during alpha(2)' -> alpha(2) + gamma transformation that leads to the lamellar structure were simulated. In particular, the effects of coherency stress and the interfacial energy difference among different types of interfaces and undercooling on the nucleation mechanism were analyzed. It is found that the twin boundary fraction increases with increasing coherency elastic strain energy and increasing interfacial energy difference among different types of interfaces, and with decreasing undercooling. Depending on the relative contributions of these factors, the nucleation events simulated by the Langevin noises could be collective, correlated or independent. These findings could shed light on the control of twin boundary fraction within lamellar structures by adjusting alloy composition and cooling route. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Grain coarsening in the presence of second-phase particles is investigated using a mean-field statistical model. With any given initial grain size distribution and average size and volume fraction of pinning particles as input, the model predicts grain growth kinetics and temporal evolution of the grain size distribution. Simulations are performed for both stable and unstable particle populations with several different initial grain size distributions. It is shown that the kinetics of grain coarsening is sensitive to the initial grain size distribution and pinning force variation as a function of time. Under a constant pinning force (stable particles), the final mean grain size does not vary significantly with different initial grain size distributions, though the kinetics of grain growth differs. On the other hand, a pinning force that decreases with time (associated with, for example, particle dissolution) could induce temporarily abnormal grain coarsening and a wider initial grain size distribution results in an earlier onset of the abnormal grain coarsening and a smaller final grain size.

Single crystals of Ti2448 alloy (Ti–24Nb–4Zr–8Sn in wt.%) were grown successfully using an optical floating-zone furnace. Several kinds of resonant methods gave consistent Young’s moduli of 27.1, 56.3 and 88.1 GPa and shear moduli of 34.8, 11.0 and 14.6 GPa for the 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 oriented single crystals, and C11, C12 and C44 of 57.2, 36.1 and 35.9 GPa respectively. Uniaxial testing revealed asymmetrical elastic behaviors of the 〈1 1 0〉 crystals: tension caused elastic softening with a large reversible strain of ∼4% and a stress plateau of ∼250 MPa, whereas compression resulted in gradual elastic stiffening with much smaller reversible strain. The 〈1 0 0〉 crystals exhibited weak nonlinear elasticity with a large elastic strain of ∼2.5% and a high strength, approaching ∼20% and ∼30% of its ideal shear and ideal tensile strength respectively. The 〈1 1 1〉 crystals showed linear elasticity with a small elastic strain of ∼1%. These elastic deformation characteristics have been interpreted in terms of weakened atomic interactions against crystal distortion to low crystal symmetry under external applied stresses. These results are consistent with the properties of polycrystalline Ti2448, including high strength, low elastic modulus, large recoverable strain and weak strengthening effect due to grain refinement.

beta Phase elasticity stress-induced alpha '' transformation and superelasticity in hot-forged Ti-24Nb-4Zr-8Sn (0 08-0 40)O (wt %) has been investigated by in situ synchrotron X-ray diffraction Rietveld refinement and texture analysis The beta elastic constants were determined by Eshelby-Krone-Kneer self-consistent modelling A micromechanical model based on texture, martensite crystallography and Schmid law nucleation criteria was used to analyse the generation of non linear strain Oxygen increased the e cubic shear constant The beta phase showed Increased compliance along (beta) as precursor to transformation, which occurs above 0 15 wt % O by growth of nanosized nuclei, and is nearly suppressed at 0 40 wt % O by structural convergence of parent/martensite phases The shape of the stress strain curve is due principally to stress induced martensitic transformation with high-symmetry {7 5 5}(beta) habit planes, and is controlled by oxygen through Its effect on the transformation strains and critical shear stress (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved

The present work focuses on stacking fault tetrahedron (SFT) nucleation upon annihilation of edge dislocation dipoles in Al and Cu. SFT nucleation is promoted by relatively low vacancy migration barriers along defected channels, typical of pipe diffusion-like processes, forming small vacancy clusters and elemental SFTs. Within about 1 ns, depending on the material under investigation, the small initial vacancy clusters aggregate into larger clusters, including complete, incomplete and truncated SFTs. Cluster aggregation occurs by diffusion of vacancy complexes whose mutual transformation and energetics are analyzed and compared. It is concluded that the unique presence of deformation-induced SFTs, such as those found experimentally in Al, originates from a local mechanism of forced vacancy injection by dislocation reactions. Subsequent stages of SFT growth by successive vacancy absorption and ledge expansion are deferred to companion papers.

The growth of stacking fault tetrahedra (SFTs) resulting from dipole annihilation is investigated by molecular dynamics (MD) simulations The atomistic processes Involved during growth immediately after nucleation are studied Analyzed for up to three vacancies the site preference of vacancies on a perfect SFT favors vacancy segregation at edge centers and corners in Cu and Al, respectively The formation of small sized SFTs does not require a prior triangular Frank loop Instead, SFT growth involves vacancies and their clusters agglomerating as complex faceted configurations which assisted by accelerated vacancy migration along SFT edges rearrange into near perfect and perfect SFTs SFT growth by the ledge mechanism is investigated in Part III (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved

Ledge expansion and the concomitant growth of a stacking fault tetrahedron (SFT) are investigated in Al and Cu by molecular dynamics (MD) simulations by addition of vacancy rods with selected lengths Ledge expansion is largely governed by the site preference of vacancies on the SFT edges resulting in distinct stable ledged SFTs Both edge- and corner-facing ledge configurations may be adopted The growth of SFTs, especially large ones, is controlled by thermal agitation The mobile part of the ledges consists of a dipole of Shockley partials generally oriented in the 60 degrees mixed orientation that move in a thermally activated manner, reflecting a certain core reorganization of the Shockley dipole (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved

The superelastic behaviour of Ti-30Nb-10Ta-5Zr and Ti-30Nb-8Ta-5Zr (wt.%) biomedical alloys was investigated by tensile testing and in situ synchrotron X-ray diffraction. The phenomenological theory of martensite crystallography and texture prediction based on the Schmid law indicate that alpha '' in Ti-30Nb-10Ta-5Zr forms in untwinned variants. Measurement of single-crystal elastic constants using an Eshelby-Kroener-Kneer self-consistent model shows that the 10-Ta alloy has a C' modulus 3 GPa higher than that of the 8-Ta alloy. That it nevertheless has a proof stress 35 MPa lower can be qualitatively explained by the differences in martensite crystallography. The results suggest that high tensile strength can be maintained for low modulus beta-Ti in spite of martensitic transformation by optimizing transformation strains. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Transformation of vacancy dipoles of dissociated edge dislocations are analyzed in Cu, Ni and gamma-TiAl by molecular dynamics. Dipole heights up to 20 [111] interplanar distances are investigated at temperatures ranging from 0 K to near the melting points of Cu and Ni and slightly below the upper boundary of the single phase gamma-TiAl domain. Three model configurations, hollows, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Except for dipoles one interplanar distance high, hollows are either metastable or unstable and they are never formed by mutually approaching dipolar dislocations. The three configurations transform into a variety of height- and temperature-dependent layouts including cores containing ordered free volumes, zigzagged faulted dipoles and agglomerated stacking-fault tetrahedra (SFT). At the highest temperatures, small individual SFTs are formed by short-range pipe-diffusion along the dipole cores. There is no critical height below which small-height dipoles or their debris would just simply disappear. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The evolution of grain boundaries (GBs) and shear bands in Cu bicrystals during one-pass equal-channel angular pressing (ECAP) was systematically investigated by various techniques. Four Cu bicrystals were designed to make GBs with angles of 0°, 45°, 90° and 135° with respect to the intersection plane (IP) of the ECAP die. After ECAP, the shear bands and the GB orientations in the four bicrystals displayed distinct behaviors due to the difference in the initial GB directions and the special crystallographic orientation of the component grains. Based on the experimental results, it is suggested that the initial GBs have a remarkable influence on the shear deformation behaviors of the adjacent regions, and the deformation regions far from the GBs are mainly controlled by the crystallographic orientations. The present investigations further demonstrate that shear deformation along the normal of the IP plays an important role in the deformation of Cu bicrystals.

Dissolution kinetics of gamma' particles in binary Ni-Al alloys with different initial particle size distributions (PSD) is studied using a three, dimensional (3D) quantitative phase field model. By linking model inputs directly to thermodynamic and atomic mobility databases microstructural evolution during dissolution is simulated in real time and length scales. The model is first validated against analytical solution for dissolution of a single gamma' particle in ID and numerical solution in 3D before it is applied to investigate the effects of initial PSD on dissolution kinetics. Four different types of PSD, uniform, normal, log-normal and bimodal, are considered. The simulatior results show that the volume fraction of gamma' particles decreases exponentially with time, while the temporal evolution of average particle size depends strongly on the initial PSD. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The transformations of dipoles of dissociated edge dislocations in aluminium are investigated using molecular dynamics for dipole heights ranging from one to ten {1 1 1} interplanar distances and at temperatures up to 900 K. Three types of configurations, hollow, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Hollows are unstable at finite temperature. Dipoles neither annihilate nor form stable hollow cores but transform into height- and temperature-dependent layouts including cores containing ordered free volumes, zigzag faulted dipoles and agglomerated truncated stacking fault tetrahedra (SFT). Small individual SFT are formed at the highest temperatures, attesting to short-range pipe diffusion. These rearranged configurations are strong obstacles to mobile dislocations, thence potential nucleation sites for dislocation entanglement and self-patterning under single slip. A new thermally activated mechanism of SFT formation directly from vacancy clusters resulting from local dipole pinching-off is reported.

We report some theoretical and experimental results about a frequency–temperature compensated sapphire loaded cavity for compact hydrogen maser atomic clocks. Based on theoretical calculations, the frequency–temperature coefficients in the TE0 1 1 mode of a sapphire cavity associated with two small rings of SrTiO3 can be reduced. For instance, at 50 °C, when the thickness of the SrTiO3 ring is 5 mm, the frequency–temperature coefficient can be reduced by about five times, and in the meantime, the quality factor can be kept at above 40 000 synchronously. A sapphire loaded cavity and eight small compensated chips of SrTiO3 were prepared and a combined cavity of sapphire with SrTiO3 chips was simulated by the finite element method and measured by experiments. The simulation and the related experimental results agree well. In addition, the frequency–temperature coefficients in two different configurations of eight SrTiO3 chips were also measured and compared.

We have investigated the structural, magnetic, and electrical transport properties of a series of ABO(3)-type perovskite compounds, La1-xNdxMn0.8Cr0.2O3 (x

Rapidly solidified NiAl–28Cr–6Mo–B–Dy prealloyed powder doped with Nb powder was consolidated by hot pressing under 1250 °C for 30 min at 30 MPa. The consolidated material exhibited a different microstructure from the original powder, i.e. the NiAl and Cr(Mo) plates in the eutectic cell tend to break down into short platelets or even particles during hot pressing process. The mechanical behaviors at room temperature and at high temperature of consolidated sample from powder alloy were evaluated by three-point bending technique, tensile test and compressive test. The results showed that the hot pressing alloy possessed a reasonable combination of room temperature ductility and toughness, and elevated temperature strength.

We have studied the magnetocaloric effect (MCE) in a bilayered La4/3Sr5/3Mn2O7 single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the NICE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2 J/kg K. A large low-field magnetic entropy change, reaching 3.2 J/kg K for a magnetic field change of 15 kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane. (c) 006 Elsevier B.V. All rights reserved.

In the present paper, the microstructure, compressive properties at different temperatures and hardness of NiAl–Cr(Mo)/Hf alloy prepared by injection casting were investigated. Compared with the conventionally-cast alloy, the injection-cast alloy exhibited a fine microstructure, i.e. the fine eutectic cell and interlamellar spacing as well as fine primary NiAl phase and Heusler phase Ni2AlHf due to the high cooling rate. In addition, the area fraction of primary NiAl phase at the cell interior or cell boundaries and eutectic cell increased. The ductility and yield strength at room temperature increased by about 100% and 25% over those of conventionally-cast alloy respectively. However, both alloys possessed the similar high temperature strength. The Vickers hardness of injection-cast alloy also increased markedly.

First principles calculations of MgH2 and MgH2-Ti were presented to investigate the influence of titanium on the properties of hydrogen sorption of magnesium hydrides. The heat of formation of MgH2-Ti was enhanced compared with that of MgH2. The bonding between magnesium and hydrogen was weakened, while a stronger interaction between titanium and hydrogen exists if titanium is in solution of the magnesium hydride. (C) 2003 Elsevier B.V. All rights reserved.

A recently developed mean-field model has been combined with first principles calculations of binding energy to investigate the site occupancies of micro-alloying elements and vacancies in NiAl as well as the stability of the micro-alloyed B2 phase with respect to disordering and second-phase formation. The theoretical results suggest that the transition metal elements in the same row of the periodic table increasingly tend to the Ni sublattice with increasing atomic number. Micro-alloying addition tends to decrease the vacancy concentration of NiAl alloys. Alloying with X that substitutes for Ni is predicted to have the sides of its solubility lobe parallel to the Ni-X side of the isotherm, but parallel to the Al-X side if X substitutes for Al. Micro-alloying was shown to raise the ordering temperature of the B2 phase over the corresponding binary alloy, in contrast with the effect of vacancies. Alloying effects on ordering temperature and the formation of point defects appear independent of the site substitution behaviour, and are less significant for 3d than for 4d and 5d transition metal elements. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The precipitation of α2 phase based on Ti3Al from Ti–14Al–2Zr–3Sn–3Mo–0.5Si titanium alloy homogenized at different temperatures in the (α+β) two-phase field has been characterized. Minimal lattice mismatch between the α and α2 phase leads to a massive ordering transformation upon cooling and results in morphologies distinct from the ellipsoidal shape of the α2 particles. Enrichment of Al in front of growing β grains due to the Gibbs–Thomson effect, together with preferential diffusion of Al along 1 1 2 0 produces a layer of acicular α2 particles apparently perpendicular to the α/β phase boundaries. Removal of the capillarity-induced composition inhomogeneity by subsequent aging at 750 °C results in disappearance of the acicular shape of the α2 particles.

As potential candidates for high-temperature applications, TiAl-based intermetallic alloys have been under extensive experimental and theoretical studies in the past two decades. Particularly, simulations from the first principles have offered great insight into the electronic structure and the effects of alloying additions on the intrinsic properties of the materials. This article attempts to highlight the current activities in the field of first principles simulations for the prediction of mechanical properties of TiAl. The preference of sublattice sites of various alloying elements and their effects on lattice distortion were investigated and discussed in relation to experimental findings.

The influence of the interstitial elements H, B, C, N and O on the bulk modulus and the theoretical strength of alpha-Ti was investigated using the discrete variational method under the local density approximation from first principles. The electronic structure and the total and binding energies were first evaluated with and without lattice relaxation and then used to assess the theoretical values of the bulk modulus and the theoretical strength under hydrostatic tension. It was shown that the binding energy was increased when H was in a solution of alpha-Ti but reduced when B, C, N or O was in solution. The bulk modulus and the theoretical strength were increased by C, O and N but reduced by H and B. A charge-density analysis showed that the bonds between Ti atoms were enhanced by the interstitial elements N, C and O but weakened, to some extent, by the incorporation of H.

The electronic structure and binding energy of a number of TiAl-X alloy systems (X = V. Cr. Mn. or Si) were calculated using the discrete variational cluster method. based on the local density approximation of the density functional theory. The influence Of alloying elements on the cleavage strength and the deformability of TiAl "as investigated by means of an atomistically uniaxial loading deformation along the [001] and the [110] directions, respectively. The results show that both the cleavage strength and the deformability of TiAl depend on the substitution behaviour of alloying atoms in TiAl. In general. the 3d transition atoms V. Cr. and Mn enhance the cleavage strength and the deformability of TiAl due to an increased number of d-electrons contributing to the d-d bonding. while the atom Al and Si reduce the cleavage strength and the deformability of TiAl because of relatively strong hybridization between the p- and d-electrons. (C) 2002 Elsevier Science B.V. All rights reserved.

Site substitution of ternary elements in ordered compounds influences the electronic structure and hence the properties of compounds at the continuous level. The electronic structure and binding energy of a number of NiAl-X alloy systems (X=Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, Si, Ga, or Gel were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory. The site preference of the ternary additions to NiAl was investigated by employing the Bra,og-Williams model to analyse the calculated binding energy. The results show that all the considered ternary elements possess stronger preference to the Al sublattice sites than a Ni atom does. A new method of identifying sublattice substitution of ternary additions in NiAl was proposed by comparison of the binding energies per atom of the ternary and the binary clusters involving the fourth nearest neighbours. The analysis suggests that Fe and Co atoms occupy the Ni sublattice sites, whereas Si, Ga and Ti atoms occupy the Al sublattice sites. The remaining elements may substitute for both sublattices: Mn is most likely to go for the Ni sublattice; V, Cr, Zr, Nb, Mo, Hf, Ta, W and Ge have a larger preference for the Al sublattice, but Cr and W do not show significant preference to any sublattice. The densities of states involving alloying additions of Co, Si and Cr were further investigated to clarify the site preference of the alloying additions. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of the hcp metals, Be, Mg, Sc, Y, Ti, Zr, Co, Zn and Cd. The binding energy of these metals was calculated in relation to the volume of a unit cell. The variation in the binding energy with the unit cell volume was obtained by means of a polynomial fit. The theoretical tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding energy calculations. The predicted bulk moduli for these metals are in good agreement with experimental findings and other available theoretical data. A linear relationship between the calculated and the experimental strengths is observed.

The electronic structure and binding energy of a number of TiAl-X alloy systems (X=V, Cr, Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Hf, Ta, W, Ga, Ge, In or Sb) were calculated using the discrete variational cluster method, based on the local density approximation of the density functional theory. The site preference of the ternary additions to TiAl was investigated by means of the binding energy data and the Bragg-Williams model. The results showed that Y, Zr, Nb, Mo and Sb preferentially occupy the Ti sublattice sites, Ga and In occupy the Al sublattice sites, while V, Mn, Cr, Co and Ge may occupy either site, depending on the Ti/Al ratio. Investigation of the total and local densities of states for representative elements showed that the substitution behaviour of ternary additions in TiAl is determined by the electronic structure of the systems. The present predictions of the site preference of alloying elements in TiAl show good agreement with the most recent experimental findings. (C) 2000 Elsevier Science Ltd. All rights reserved.

The influence of ternary additions Cr, Fe, Mn, Ni, Zr, Nb, Mo, Hf Ta, Si, Ga, Ge, In, and Sb, as well as the anti-site defects of both Ti and Al, on lattice parameters of TiAl were studied by the first principles electronic structure calculations with a discrete variational cluster method. The results of the calculation show that the effect of ternary additions on the distortion of TiAl lattice varies with the substitution behavior of the individual alloying element involved The. addition nf alloying elements in TiAl caused. a change in the electronic structure and the density of states of the system and results a change in the electronic structure and the density of states of the system and results in variation of the bond strength between the atoms. The total and partial density of states (DOS) of binary TiAl and of ternary TiAl-M, M = Cr, Zr, and Sb, etc., were comparatively examined. The relationship between the DOS and the bond strength is discussed. The present work suggests that the origin of the lattice distortion of the ternary TiAl-M systems lies in the variation of the electronic structure.

Titanium alloys are favorable implant materials for orthopedic applications, due to their desirable mechanical properties and biochemical compatibility (or bio-inertness). However, current bio-titanium alloys still possess too high an elastic modulus compared with that of the bone, which can lead to premature failure of the implant. Here, a theoretical methodology for the design and development of low modulus Ti alloys and/or structures is provided by means of electronic structural calculations using the discrete variational cluster method (DVM). The preliminary study concentrated on two β-Ti atomic clusters consisting of 15, and 27 atoms, respectively. The binding energies between titanium and various alloying atoms within the clusters were first calculated, from which strength and modulus were then estimated. The results of the calculation suggested that Nb, Mo, Zr and Ta were suitable alloying elements for β-type titanium alloys, capable of enhancing the strength and reducing the modulus of the materials.

Ferromagnetic resonance has been used to investigate the inter-layer coupling in a bilayered manganite La(2-2x)Sr(1+2x)Mn(2)O(7) (x = 0.38) single crystal. The coexistence of a ferromagnetic resonance line and a paramagnetic resonance line was observed over a broad temperature range. Antiferromagnetic coupling between the adjacent MnO(2) bilayers can be identified from the observation of an optical mode in the resonance spectra. Analysis of the temperature dependence of the resonance field and intensity reveals the evolution of the inter-layer coupling as a function of temperature. Our study suggests that ferromagnetic resonance provides a useful method for investigating the inter-layer coupling in bilayered manganites.

The influences of composition and Mn-Ga/Al disordering on the magnetic and elastic properties of the Ni(2)Mn(Al(1-x)Ga(x)) Hensler alloy are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The transition temperature from the completely disordered B2 phase to the ordered L2(1) phase, estimated by the use of Bragg-Williams-Gorsky approximation, is in excellent agreement with experiments. The site-occupancy in the partially disordered alloy is determined by comparing the free energies of the alloys with different site-occupation configurations. It is found that Mn atoms generally prefer to exchange with Al atoms for the alloy with low degree of disorder and low Ga concentration. With increasing degree of disorder and Ga concentration, Mn may exchange with Ga as well. The total magnetic moment of the completely ordered alloy exhibits a minimum around x = 0.7 due to the competition between the increasing magnetic moments of Mn atoms and the decreasing moments of Ni atoms with increasing x. For the partially disordered alloy, both total and local magnetic moments of Mn and Ni decrease linearly with increasing degree of disorder, and Ga atoms show significant magnetism. The shear moduli C of the alloys are calculated with respect to both x and the degree of disorder. The results show that C decreases with increasing x and degree of disorder. The dependence on composition and on the degree of disorder of the martensitic transition temperatures is discussed in terms of the calculated C. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The previous compact hydrogen maser with sapphire microwave cavity in the Beijing Institute of Radio Metrology and Measurement is not suitable for a space application in navigation systems since a bit large volume and weight. In order to reduce the total size of hydrogen maser atomic clocks further, in this work, the authors have performed the detailed analysis and some theoretical and experimental result in optimizing parameters in a TE011 mode sapphire-loaded cavity resonator more suitable for space borne hydrogen maser. The minimization of the total cavity volume and maximization of the quality factor are both sought finally. Methods of theoretical calculations, finite element simulation, and related experiments have been performed in the process of designing a sapphire loaded cavity. Based on the analysis, a miniaturized sapphire microwave cavity with the total volume of 3.04 dm3, the quality factor 67500, and frequency-temperature coefficient - 59.7 kHz/□ is developed. The experimental results are completely consistent with modeled values. In addition, the theoretical calculation result shows that the product of the z-component of the magnetic energy filling factor in the bulb region and the cavity TE 011 mode Q-factor is 4.08E4 at 50 □ in the designed miniaturized sapphire cavity. In addition, two sapphire loaded cavities in the NICT, Japan and in IRCOM, France are detailedly compared with that of our designs.

We demonstrate reversible movement of 1/2[1 (1) over bar0](110) dislocation loops generated from nanodisturbances in a beta-titanium alloy. High resolution transmission electron microscope observations during an in situ tensile test found three reversible deformation mechanisms, nanodisturbances, dislocation loops and martensitic transformation, that are triggered in turn with increasing applied stress. All three mechanisms contribute to the nonlinear elasticity of the alloy. The experiments also revealed the evolution of the dislocation loops to disclination dipoles that cause severe local lattice rotations.

The effect of Zr on the martensitic transformation (MT) behavior and mechanical properties of (Ti(0.5-x)Zr(x))Ni(0.5) alloys is investigated by calculating the elastic constants and elastic moduli in the B2 phase as a function of x for 0

A first-principles plane-wave pseudopotential method based on density functional theory is used to investigate the concentrations of point defects and their interaction in TiNi. The calculations show that, in the thermal equilibrium state, TiNi develops antisite-type point defect configurations. In accordance with the sequence of the stability of the B2, B19, and B19(') phases, the concentrations of the point defects in stoichiometric TiNi at the identical temperature decrease from B2 to B19 to B19('). At high temperature, there are considerable amounts of vacancies in Ti-rich compounds. The calculated interaction energies show that the nearest-neighboring Ni antisite and Ti antisite are attractive to each other, whereas the second-nearest-neighboring Ni antisites repel each other and Ti antisites mutually attract, which indicates that a Ti-rich domain may appear in stoichiometric TiNi, and also explains the high solubility of excess Ni and low solubility of excess Ti in TiNi. The electronic structure mechanisms behind the interactions between the point defects are discussed based on charge density calculations. It was shown that the attractive interaction between Ti antisites is due to the accumulation of electron density between the Ti antisites, whereas the repulsion between Ni antisites is due to the depletion of electron density between them.

We report a detailed study of steplike magnetization and resistivity jumps in a bilayered (La0.5Nd0.5)(1.2)Sr1.8Mn2O7 single crystal. The sample exhibits very sharp metamagnetic transitions at low temperature when the magnetic field is applied either in an ab plane or along the c axis, which causes huge magnetization steps. The critical field depends on the cooling magnetic field as well as the sweep rate of the magnetic field. Meanwhile, the evolution with time of the magnetization exhibits a spontaneous step when both the temperature and magnetic field are constant. Similar steplike behaviors are also observed in resistivity. These results suggest that a martensiticlike transformation could happen in bilayered manganites.