Prediction of Γ-L and L-X crossovers in Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>Sb alloys by empirical pseudopotential method

Using empirical pseudopotential method Γ-L crossover is found for the Ga_0.74Al_0.26Sb. The conduction band minimum is observed to switch at the (0.87, 0, 0) point for Ga_0.51Al_0.49Sb which shifts to the X point for Ga_0.21Al_0.79Sb and remains at X leading finally to indirect band gap in AlSb. Band structure calculations for a large number of alloys are performed and bowing parameters b_X and b_L are proposed for the E_X and E_L respectively. Our findings may serve as directive to select the materials in a range of composition to examine the bowing parameters and thereby effective mass experimentally for the Ga_xAl_1-xSb alloys.     

Study of solid-state reactions in Sm(Co,Fe, Cu,Zr)z 2:17-type alloys by means of in situ electrical resistivity measurements

In the present work, solid-state reactions in Sm₂(Co, Fe, Cu, Zr)₁₇-type alloys have been investigated by means of in situ electrical resistivity measurements. Changes in the electrical resistivity of a Sm(Co_0.74Fe_0.1Cu_0.12Zr_0.04)_8.5 alloy after solid solution treatment at 1190 °C, quenching to room temperature, and during isothermal ageing at temperatures between 400 and 900 °C, have indicated microstructural/phase changes occurring at temperatures below those commonly used for the development of high coercivity in Sm(Co, Fe, Cu, Zr)_z-type materials. Subsequent crystallographic and magnetic transition measurements have shown a high degree of correlation with respect to the changes observed in the electrical resistivity during isothermal ageing.     

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr_0.29Ni_0.16C_0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni₃Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.     

Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

A phenomenological explanation for the buckling of surface alloys

A phenomenological model, in which the interactions between the nearest-neighbor (NN) atoms are described as bondings but not hard sphere contacts, is proposed to explain the unexpected reduced buckling in surface alloy systems. In the model, the forces acting on an adsorbate atom from its NN substrate atoms in different layers may be either repulsive or attractive, depending on whether the bond between the adsorbate atom and its NN substrate atoms is compressed or stretched. It is found that the forces on the adsorbate atom from its NN substrate atoms in the sub-surface layer play a more important role for the buckling of surface alloy than those from its NN substrate atoms in the surface layer do. The bucklings expected by the model are significantly smaller than those predicted by the simple hard sphere model and are in good agreement with the experiments when the equilibrium bond length of the NN adsorbate-substrate atom pairs is taken as the sum of the corresponding metal radii.     

Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo

Experimental trials of autogenous deep penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 5.0 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding velocity and defocusing distance on the morphology, welding depth and width as well as quality of the welded seam were investigated. Results show that full keyhole welding is not formed on both K418 and 42CrMo side, simultaneously, due to the relatively low output power. Partial fusion is observed on the welded seam near 42CrMo side because of the large disparity of thermal–physical and high-temperature mechanical properties of these two materials. The microhardness of the laser-welded joint was also examined and analyzed. It is suggested that applying negative defocusing in the range of Raylei length can increase the welding depth and improve the coupling efficiency of the laser materials interaction.     

The Effect of Platinum on Diffusion Kinetics in β‐NiAl: Implications for Thermal Barrier Coating Lifetimes

Platinum is added to thermal barrier coatings (TBCs) as it is observed empirically to extend their lifetime, but the mechanism by which Pt acts is unknown. Since Pt has been proposed to alter diffusivities in NiAl, a key component of TBCs, we use first‐principles quantum mechanics calculations to investigate atomic level diffusion mechanisms. Here, we examine the effect of Pt on five previously proposed mechanisms for Ni diffusion in NiAl: next‐nearest‐neighbor jumps, the triple defect mechanism, and three variants of the six jump cycle. We predict that Pt increases the rate of Ni diffusion by stabilizing point defects and defect clusters that are diffusion intermediates. Previously, we predicted the triple defect mechanism to be a dominant Ni diffusion mechanism; it simultaneously results in long‐range Al diffusion in the opposite direction. Since Pt increases the rate of Ni diffusion, it also increases Al diffusion in NiAl, which may be key to extending the coating lifetime.     

高碘酸钾法测钢铁及合金中锰的改进建议

试样经酸溶解后,以磷酸-高氯酸混合酸(4 1)或(9 1)氧化至冒烟.在硫酸、磷酸介质中,用高碘酸钾将锰氧化到七价,测其吸光度.     

Quantum hall effect observations in commercial MOSFETs and effects of thermoelectric power

Summary We have performed quantum Hall effect measurements on commercial low-mobilityn-channel MOSFETs. The channel resistancevs. electron concentration has the expected oscillating behaviour and gives the quantized values for the Hall resistance, at least in the region of higher mobility. One class of the tested samples gave a surprising behaviour, both in QHE and in field effect mobility measurements: this is thought to be due to uncontrolled differences in the diffusion zones at the interface between the channel and the contacts. In particular, we attribute the deformation of the QHE curves to the onset of a thermoelectromotive force which is present in the electron gas even if the sample is isothermal. A clear evidence for thermoelectric effects is given by measurements made on a high-quality Hall geometry MOSFET.

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Riassunto Abbiamo eseguito misure di effetto Hall quantistico su MOSFET commerciali di bassa mobilità a canalen. Le curve di resistenza di canale in funzione della tensione di gate presentano l'atteso andamento oscillante e danno i valori quantizzati, almeno nelle regioni di maggiore mobilità. Una categoria di campioni ha mostrato andamenti sorprendenti, sia in misure di QHE che in misure di mobilità per effetto di campo: riteniamo che ciò sia dovuto a differenze incontrollabili nelle regioni di diffusione alle interfacce tra i contatti e il canale. In particolare, attribuiamo la deformazione delle curve di QHE all'instaurarsi di forze termoelettromotrici, che sono presenti nel gas elettronico nonostante i campioni siano isotermi. Misure eseguite su un MOSFET a geometria Hall di alta qualità forniscono una chiara evidenza dell'esistenza di effetti termoelettrici.

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Резюме Проводятся измерения квантового эффекта Холла на коммерческих MOSFET с малой подвижностью вn-канале. Зависмость сопротивления канала от концентрации электронов обнаруживает осцилляторное поведение и дает квантованные значения сопротивления Холла, по крайней мере, в области высокой подвижности. Один класс исследованных образцов обнаруживает удивительное поведение при измерении квантового эффекта Холла и при измерении зависимости подвижности от поля. Такое поведение, по-видимому, обусловлено неконтролируемыми различиями в зонах диффузии между каналом и контактами. В частности, мы приписываем деформацию кривых квантового эффекта Холла возникновению термоэлектродвижущей силы, которая присутствует в электронном газе, даже в изотермических образцах. Приводятся подтверждения для термоэлектрических эффектов, проводя измерения эффекта Холла в MOSFET с высокой подвижностью.

     

Self-consistent Monte Carlo simulations of the electron and ion distributions of inhomogeneous liquid alkali metals. II. Longitudinal and transverse density distributions in the liquid-vapor interface of binary metallic alloys

We present the results of Monte Carlo simulations of the liquid-vapor interface of sodium-cesium alloys. The longitudinal density profile of each alloy shows that the liquid-vapor interface consists of a well-defined monolayer of cesium sitting on top of a slab of the bulk alloy. Underneath the monolayer there is a slight excess of sodium. A comparison with a van der Waals analog of one of the alloys shows that the presence of the well-defined monolayer of cesium on the outside of the liquid-vapor interface is a feature peculiar to metallic mixtures. The transverse pair correlation functions of the cesium monolayer are insensitive to the composition of the bulk of the slab.