Stress dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in bcc metals

The recently formulated constrained nudged elastic band method with atomic relaxations (NEB + r) (Gröger R, Vitek V. Model Simul Mater Sci Eng 2012;20:035019) is used to investigate the dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in body-centered cubic metals on non-glide stresses. These are the shear stresses parallel to the slip direction acting in the planes of the 〈1 1 1〉 zone different from the slip plane, and the shear stresses perpendicular to the slip direction. Both these shear stresses modify the structure of the dislocation core and thus alter both the Peierls barrier and the related Peierls stress. Understanding of this effect of loading is crucial for the development of mesoscopic models of thermally activated dislocation motion via formation and propagation of pairs of kinks. The Peierls stresses and related choices of the glide planes determined from the Peierls barriers agree with the results of molecular statics calculations (Gröger R, Bailey AG, Vitek V. Acta Mater 2008;56:5401), which demonstrates that the NEB + r method is a reliable tool for determining the variation in the Peierls barrier with the applied stress. However, such calculations are very time consuming, and it is shown here that an approximate approach of determining the stress dependence of the Peierls barrier (proposed in Gröger R, Vitek V. Acta Mater 2008;56:5426) can be used, combined with test calculations employing the NEB + r method.     

Effects of two-step annealing on properties of Cd1-xZnxTe single crystals

The Cd1-xZnxTe（CZT） single crystals were annealed by a two-step method including a vapor-environment step and a liquid-environment step in sequencel The effects of annealing on the properties of CZT were analyzed in detail. IR transmission measurement results show that IR transmission of CZT is improved dramatically after annealing. X-ray rocking curves indicate that the annealing treatment ameliorates crystal quality obviously, which is ascribed to the release of residual stress and the reduction of point defects. Photoluminescence（PL） spectra reveal that the full width at half maximum（FWHM） of the donor-bound exciton （D^0, X） peak is reduced obviously, and the free exciton emission is weakened after annealing. Meanwhile, the intensity of the donor-acceptor pair（DAP） peak decreases to a great degree, which implies that the impurities are removed from CZT wafers. In addition, the deep defect-related emission band Dcomplex disappears after annealing, which mean that Cd vacancies are well-compensated. The results confirm that the two-step annealing is an effective approach to improve the qualities of CZT single crystals.     

Effect of microstructures on superplasticity of A1-11%Si alloy

Three kinds of A1-11%Si （mass fraction） alloy samples with different processes were produced to investigate the effect of microstructures on its superplasticity. Among them, the as-ECAP sample pressed 16 passes has ultrafine grains （300 nm） and the finest secondary particles. The ECAP-T6 sample, with ECAP 16 passes followed by T6 treatment, has fine secondary particles （3 μm） but the largest grains （8 μm）. Contrarily, the T6-ECAP sample, with T6 treatment followed by ECAP 16 passes, has ultrafine grains and the large secondary particles （7 μm）. The tensile testing results show that the as-ECAP sample exhibits superplasticity at high strain rate of 5.75×10^-1 s^-1 due to its fine secondary particles and ultrafine grains. The ECAP-T6 sample, however, does not exhibit superplasticity at the same high strain rate of 5.75×10^-1 s^-1 because it has relatively large secondary particles and large grains. Remarkably, the T6-ECAP sample does not have superplasticity even at the lower strain rate of 1.15×10^-1 s^-1, attributing to its comparatively large secondary particles. When most secondary particles are larger than 7 μm, the high strain rate superplasticity could not be obtained even if this sample has ultrafine grains.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Synthesis of CaNi1−xPdx (0.1≤x≤1) alloys and hydrogenation properties of CaPd

An evaluation of the substitution of a third element for the constituent elements (Ca, Ni) in the compound was conducted in order to obtain information about stabilization of the CaNi compound which does not exist as a stable phase. Prior to the experimental study of the substitution, the candidates for the third element were investigated using Miedema’s semi-empirical model, from the standpoint of formation enthalpy, and as a result, Pd was chosen as the element to stabilize the compound. The samples with the nominal compositions of CaNi_1−xPd_x (0.1≤x≤1) were prepared by sintering and high frequency heating, and the constituent phases and hydrogenation properties were examined. New ternary phases such as Ca₁(Ni, Pd)₁ were not obtained under the present conditions; however, some pieces of new information such as the substitution of Ni for Pd in CaPd₂ and reversible hydrogenation of CaPd at 373 K were obtained through the studies.     

Positron Annihilation Study of High-Temperature Oxidation Behavior of Zr–1Nb Alloy

The oxidation behavior of Zr–1Nb alloys exposed at 873 and 973 K in air was investigated by positron annihilation lifetime spectroscopy together with mass gain, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mass-gain results showed that during the oxidation process, a transition of the oxidation rate occurred. The transition times of the specimens oxidized at 873 and 973 K were 30 and 6 h, respectively. In the pre-transition stage, the mass-gain curves obeyed the subparabolic law (n?=?2.3), while at the post-transition stage, the mass-gain curves obeyed the linear law. The positron lifetime measurements indicated that in pre-transition stage, the formed oxide scale mainly consisted of a compact layer that only contained small-size vacancy defects. The accumulation of these vacancy defects together with the high compressive stress might cause the breakaway of the oxide layer. During the post-transition stage, the thickness of the porous oxide layer with more and larger-size defects such as voids and pores increased rapidly as increasing the oxidation time. These large-size defects, together with the cracks produced during the transition from protective to breakaway-type oxide, increased the oxygen absorption rate and accelerated the diffusion of oxygen. The formation of cracks in the porous layer was confirmed by SEM examinations.     

Galvanic corrosion of A1 alloys — II. Effect of solution composition

Galvanic corrosion of the A1 alloys 1100, 2024, 2219, 6061 and 7075 coupled to Cu, stainless steel 304L, Ti-6A1-4V, 4130 steel or zinc has been studied in 3·5% MaCl, tapwater and distilled water using electrochemical and weight loss data. In 3·5% NaCl the galvanic effect decreases in the order Cu > 4130 steel > SS304L > Ti-6A1-4V for A1 alloys coupled to one of these metals, while in tapwater and distilled water the ranking is Cu > SS304L ～ Ti-6A1-4V > 4130 steel. Zinc, although being the anode in all galvanic couples, can sometimes accelerate corrosion rates of A1 alloys. Dissolution rates of A1 alloys coupled to a given dissimilar material are higher in 3·5% NaCl than in tapwater and distilled water where they are found to be comparable. In assessing galvanic corrosion behaviour of a given A1 alloy as a function of environment, one has to consider the effect of the dissimilar metal. The dissolution rate of A1 6061 is, for example, higher in tapwater with Cu as cathode than in 3·5% NaCl with SS304L or Ti-6A1-4V as cathode.     

Influence of surface modification on isothermal oxidation behavior of EB-PVD NiA1 coating

The isothermal oxidation behaviors of the as-deposited NiAl coating on the nickel-based superalloy by electron beam physical vapour deposition（EB-PVD） and the NiAl coating after surface modifications of grinding and polishing were investigated. The as-deposited coating shows the least mass gain, the initially formed θ-Al2O3 scale spalls after only 1h, and the succeeding scale formed is coarse and discontinuous and thus can not be used as protective coatings. Among the two surface-modified coatings, the ground coating results in the highest oxide growth rate, which is consistent with the SEM results where the scale spalls heavily and many voids appear between the scale and the NiAl coating. The scale spallation and void formation mechanisms during isothermal oxidation test ofEB-PVD NiAl coating were also discussed.     

Mechanical alloying and characteristics of spark plasma sintering of Ti-A1 composite powders

The mechanical alloying process of Ti-Al composite powders were carried out by use of high energy ball-milling machine. Structure variations of powder mixtures during mechanical alloying and characteristic of spark plasma sintering were investigated. The results show that during milling, TiAl, Ti3Al and Ti2Al phase intermetallic compounds are formed, simultaneously with powder refinement for the （TiH2-45Al-0.2Si-SNb） and （TiH2-45Al-0.2Si-7Nb） mixtures. The particle sizes of two powder mixtures are less than 300 nm after milling for 30 h. Sintering process of the milled powder can be completed in a short time by spark plasma sintering, and the sintering microstructure is composed of fine and homogeneous TiAl and Ti3Al phase.     

Effect of Sr on forming properties of A1-Mg-Si based alloy sheets

The effects of Sr element on the forming properties of the AI-Mg-Si based alloy sheets were studied by tensile test, metallograph, DSC, XRD, SEM and TEM. The results show that the tensile strength of aluminum alloy sheet added 0.033%（mass fraetion）Sr increases comparing with that of free Sr. Simultaneously, the forming properties of sheets evidently increase, the elongation hardenability （n） and plastic strain ratio （r） and Erichsen number increase 27.8%, 11.1%, 10.8% and 12%, respectively, and the forming limit diagram increases evidently, too. The analysis shows that Sr is surface active element, which can refine grains of alloys, promote precipitation, reduce activation energy of β＂ phase, and lead the formation of α-（A18Fe2Si） phase instead of β-（AlsFeSi） phase. As a result, the forming properties of the alloy sheet increase.     

Preparation of Single Crystals of Co_xZn_(1-x)S and Co_xZn_(1-x)Se—New Materials of Dilute Magnetic Semiconductors

In this paper, we report the growth of single crystals of Co_x Zn_(1-x)S and Co_x Zn_(1-x)Se (0相似文献   

Preparation of LPE GaInAsSb Epilayers and Its Photodiodes for Detection of 1．8～2．1μm

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The onset and blocking of <0 1 1> slip in NiAl

NiAl single crystals that are well annealed and oriented along the [0 0 1] orientation to suppress <1 0 0> dislocations have been used to study the onset of slip. Under a constant stress, slip is initiated abruptly with increasing temperature. The slip vector is identified to be <0 1 1> by slip line and transmission electron microscopy (TEM) observations. There is a rate-independent slip initiation temperature that decreases approximately linearly with stress. TEM observations show that the <0 1 1> slip is blocked by dislocation locking along both of the two <1 1 1> directions in the {0 1 1} glide plane. The locking is explained by the non-planar splitting of the screw component along the three-fold line directions. The blocking of <0 1 1> slip by dislocation locking long two line directions is in contrast to <1 1 1> dislocations in NiAl that are locked only along the screw orientation, and to <1 0 0> dislocations that are not locked at all. This property explains why <0 1 1> slip is more difficult than <1 1 1> and <0 0 1> slips, the general absence of <0 1 1> dislocations in annealed crystals, and the need for these dislocations to be nucleated in [0 0 1]-oriented NiAl. Possible mechanisms for the onset of <0 1 1> slip in [0 0 1] NiAl are discussed and compared.     

Solidification microstructures and mechanism of grain refinement of electrolytic low titanium A1 alloys

The solidification microstructures and the mechanism of grain refinement of electrolytic low titanium Al alloys were investigated by means of the wedge-shaped sample, the directional solidification and the rapid solidification ribbon. The results show that the coarse columnar grains formed in pure Al are transformed into the equiaxed grains in electrolytic low titanium Al alloys. The grain refinement is resulted from the constitutional supercooling caused by Ti and heterogeneous nucleation of Al3Ti particles. Under the condition of normal cooling rate, the grains are refined by the increment of constitutional supercooling when the content of titanium is less than 0.2%. With the increment of content of titanium, the grains are mainly refined by heterogeneous nucleation of AI3Ti particles. The grain size is decreased with the increment of cooling rate. When the cooling rate is larger than 10^5 ℃/s, the grain size is decreased to 0.1-10μm, the grain refinement is resulted from the larger cooling velocities mainly. After directional solidification, the equiaxed grains can be formed and the Ti element is distributed at the center of the grains.     

Effect of spray forming on evolution of microstructure and mechanical behavior of Mg-9Al-1Zn alloy

The cylindrical billet of Mg-9Al-1Zn alloy was produced by spray forming and hot extruded at 698-723 K with an extrusion ratio of 22：1, The microstructural evolution was investigated systematically. The mechanisms of grain refinement, solubility extension of Al and Zn elements in Mg matrix, and the precipitation of the second phase β-Mg17Al12 as well as their effect on the mechanical properties of the Mg-9Al-1Zn alloy were analyzed and discussed. The results show that the spray formed Mg-9Al-1Zn alloy has homogeneous equiaxed fine grains with the average grain size of 17 μm, of which the average grain size is refined further to 5 μm due to dynamic recrystallization during hot extrusion process. The evolution of size, proportion and distribution offl-Mg17Al12 phase was also observed and analyzed. The mechanical properties for the extruded rods are improved remarkably at ambient temperature.     

Effect of calcination temperature on characteristics of LiNi1/3Co1/3Mn1/3O2 cathode for lithium ion batteries

LiNi1/3Co1/3Mn1/3O2 was synthesized by sol-gel method and effect of calcination temperature on characteristics of LiNi1/3Co1/3Mn1/3O2 cathode was investigated. The structure and characteristics of LiNi1/3Co1/3Mn1/3O2 were determined by XRD, SEM and electrochemical measurements. The results show that the compound LiNi1/3Co1/3Mn1/3O2 has layered structure with hexagonal lattice. With the increase of calcination temperature, the basicity of the material decreases, and the size of primary particle rises. The LiNi1/3Co1/3Mn1/3O2 calcined at 900 ℃ for 12 h shows excellent electrochemical performances with large reversible specific capacity of 157.5 mA-h/g in the voltage range of 2.75-4.30 V and good capacity retention of 94.03% after 20 charge/discharge cycles. Capacity of LiNi1/3Co1/3Mn1/3O2 increases with enhancement of charge voltage limit, and specific discharge capacities of 179.4 mA.h/g, 203.1 mA.h/g are observed when the charge voltages limit are fixed at 4.50 V and 4.70 V, respectively.     

Mathematical modellling of spreading of current in arc welding. Part 1 ‐ effect of current supply position and geometrical parameters of sheets

The article describes the results of investigation into the possibilities of controlling the geometrical dimensions of weld metal in pulsed-arc consumable electrode welding and welding with pulsed feed of the consumable electrode of structural steels with controlled parameters.     

Onset of plasticity in InxGa1−xAs multilayers

Coherently strained In_xGa_1?xAs multilayers with zero net strain have been investigated in order to determine the influence of coherency strains on the yield stress. The deformation behaviour has been studied using both ex- and in situ nanoindentation and transmission electron microscopy. Nanoindentation showed that the pressure required for the onset of yield decreased with increasing coherency strain, consistent with previous work, while the hardness remained constant. Transmission electron microscopy revealed that deformation was more prevalent in one layer. Using these observations, a straightforward analysis has been developed in which the yield pressure of the multilayer is related to the onset of flow in the weaker layer determined by both its intrinsic yield pressure and coherency strains. This gives good agreement with the experimental observations and is consistent with the observations of the effects of internal stresses in films elsewhere in the literature.