Hot corrosion failure in the first stage nozzle of a gas turbine engine

First-stage nozzles of gas turbines, which are the first elements after the combustion chamber, encounter hot gases from the combustion process and have the task of directing the fluid path and increasing the velocity of combustion products. This paper reports on the incidence and failure of the first-stage nozzles of a gas turbine in September 2013 at a seaside pump-house located in the South-West of Iran. The nozzle was made of nickel-based superalloy Nimonic105. Due to nozzle failure, the turbine was damaged severely. The cause of nozzle failure was investigated. The results of visual inspection, XRD analysis of deposits on the blade airfoil, SEM images and EDAX analysis showed the characteristics of hot corrosion. Finite-element analysis (FEM) revealed that the cause of blade trailing edge failure was thermal stress leading to thermal fatigue, which accelerated nozzle blade failure in addition to the hot corrosion.     

A comparative study of laser-arc double-sided welding and double-sided arc welding of 6 mm 5A06 aluminium alloy

In order to study the interactions between the two heat sources in both laser-arc double-sided welding (LADSW) and double-sided arc welding (DSAW), some welding characteristics including weld configuration, energy efficiency, weld microstructure and mechanical properties of the both processes were contrastively investigated. The results show that the weld cross-section of LADSW within the proper welding parameter takes on the combination of typical weld profiles of gas tungsten arc welding and laser welding, while the DSAW takes on a quasi-symmetrical shape. The energy efficiency of LADSW is higher than DSAW, probably due to the higher heat transfer efficiency in laser welding and stronger effect of laser on the arc. The weld microstructures of the both processes characterized by scanning electron microscope mainly consist of α and β phase, whereas the grain size and second-phase particle size vary a great deal for the different heat input. The tensile strength of LADSW is 91.7% of base metal, compared with that of 82.3% of DSAW, and the elongation is also higher than DSAW. The fracture micromorphology of LADSW indicates a more typical dimple fracture than that of DSAW. It is considered that the better mechanical properties of LADSW are attributed to the finer grain size.     

Effects of a a-Si:H layer on reducing the dark current of 1310 nm metal-germanium-metal photodetectors

Two approaches of hydrogenated-amorphous-silicon (a-Si:H), as Schottky-barrier height (SBH) enhancement and passivation layers, were investigated to suppress dark current of 1310 nm metal-germanium-metal photodetectors (MGM-PDs). Observations show that when a-Si:H is inserted between metal and Ge, the dark current is effectively reduced due to SBH enhancement, but similarly lowers photocurrent resulting from the blocking of a-Si:H. In contrast with a-Si:H acting as a passivation layer a very high photo-to-dark current ratio of 6530 is achieved with a high responsivity of 0.72 A/W, attributing to the defect centers on the Ge surface which are passivated. Such a result suggests that the a-Si:H passivation layer is a good candidate in fabricating high-quality 1310 nm MGM-PDs.     

Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si

The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi₂ and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metal/Si interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted.     

The adsorption properties of PdZnx alloy on Pd(1 0 0): Preparation and characterization

The formation of PdZn_x alloy on Pd(1 0 0) and its characteristics were investigated by various methods, such as photoelectron, auger-electron, electron energy loss, thermal desorption spectroscopic methods and work-function measurement. The alloy was produced by the decomposition of diethyl zinc on Pd(1 0 0). The alloy surface reacts with O₂ and ZnO_x is formed. The reactivity of alloy to hydrogen is similar to that of K/Pd. The stability of adsorbed CO is lower than on clean Pd(1 0 0).     

Effects of oxygen pressure on the growth of pulsed laser deposited ZnO films on Si(0 0 1)

ZnO thin films were prepared on Si(0 0 1) substrates using a pulsed laser deposition (PLD) technique and then their growth and properties were investigated particularly as a function of ambient O₂ pressure during film growth. It was found that the microstructure, crystallinity, orientation and optical properties of the films grown are strongly dependent on the O₂ pressures used. Completely c-axis oriented ZnO films are grown in a low O₂ pressure regime (5×10⁻⁴-5×10⁻² Torr), whereas a randomly oriented film with a much lower crystallinity and a rougher grained-surface is grown at an O₂ pressure of 5×10⁻¹ Torr. This deterioration in film quality may be associated with the kinetics of atomic arrangements during deposition. Our results suggest that ambient O₂ pressure is an important processing parameter and should be optimized in a narrow regime in order to grow a ZnO film of good properties in PLD process.     

Electrical parameter evaluation of a 1 MW HTS motor via analysis and experiments

A 1 MW class HTS (high-temperature superconducting) synchronous motor has been developed. Design concerns of the developed motor are focused on smaller machine size and higher efficiency than conventional motors or generators with the same rating simultaneously reducing expensive Bi-2223 HTS wire which is used for superconducting field coil carrying the operating current around 30 K (−243 °C). Influence of an important parameter, synchronous reactance, has been analyzed on the machine performances such as voltage variation and output power during motor and generator operation. The developed motor was also analyzed by three-dimensional electromagnetic FEM (finite element method) to get magnetic field distribution, inductance, electromagnetic stress and so forth.This motor is aimed to be utilized for industrial application such as large motors operating in large plants. The HTS field coil of the developed motor is cooled by way of Neon thermosiphon mechanism and the stator (armature) coil is cooled by water through hollow copper conductor. This paper also describes evaluation of some electrical parameters from performance test results which were obtained at steady state in generator and motor mode of our HTS machine.     

Effect of long time service exposure on microstructure and mechanical properties of gas turbine vanes made of IN939 alloy

In the present study, the effects of long-term service exposure have been investigated on microstructure and mechanical properties of gas turbine vanes made of IN939 superalloy. The major microstructural changes for the investigated service-exposed vanes include the formation of continuous grain boundary carbides and the transformation (degeneration) of MC carbides located at the grain boundaries. The brittle σ phase, which is predicted to be stable on the basis of thermodynamic calculations, is not observed in the microstructure of service-exposed vanes. The microstructural changes during service lead to a loss in room temperature ductility as well as in creep properties of the alloy.     

Double subband occupation of the two-dimensional electron gas in InxAl1 − xN/AlN/GaN/AlN heterostructures with a low indium content (0.064 ≤ x ≤ 0.140) barrier

We present a carrier transport study on low indium content (0.064 ≤ x ≤ 0.140) In_xAl_1 − xN/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33-300 K) and a magnetic field (0-1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger-Poisson equations.     

Au8 cluster adsorption on Si(100):2 × 1 asymmetric surface studied by a first principles calculation

In this work, we investigate the adsorption of Au₈ cluster onto H-terminated Si(100):2 × 1 asymmetric surface by density functional theory within local density and generalized gradient approximations. We study the site-dependent shape of Au₈ cluster, adsorption energies, band structures and the corresponding charge distribution. We show that the electronic properties of the cluster and substrate complex change with the location of the cluster on the surface.     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

Deformed microstructure evolution in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s

Deformed microstructure in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s⁻¹ were investigated through optical microscope, scanning electron microscope and transmission electron microscope. The results show that four deformed zones around the crater can be classified based on the different deformed microstructure, including ultrafine grain zone, ultrafine grain and deformation twin zone, high and low density deformation twin zones. The dislocation slipping, deformation twins and ultrafine grains are the dominant components in the four deformed zones, and the evolution of deformed microstructure is speculated based on the deformed microstructure observed in four zones. Slipping and twinning play a critical role for the formation of the dynamic recrystallized grains, and twinning-induced rotational dynamic recrystallization mechanism is thought to be the main mechanism for the formation of ultrafine grains. The microhardness and dynamic compressive strength in different deformed zones were measured, and the high microhardness and yield strength in ultrafine grain zone should be attributed to the strain hardening and grain refining.     

Influence of aluminum ions implanted on oxidation behavior of ZIRLO alloy at 500 °C

The beneficial effect of aluminum ion implantation on the oxidation behavior of ZIRLO alloy at 500 °C has been studied. ZIRLO alloy specimens were implanted with aluminum ions with fluence range from 1×10¹⁶ to 1×10¹⁸ ion/cm², using a MEVVA source at an extraction voltage of 40 kV at maximum temperature of 380 °C. The weight gain curves were measured after being oxidized in the air at 500 °C for 120 min, which showed that a significant improvement was achieved in the oxidation behavior of ZIRLO alloy implanted with aluminum compared with that of the virgin ZIRLO alloy. It has been obviously found that when the fluence is 1×10¹⁸ ion/cm², the oxidization of the implanted ZIRLO alloy is reduced into 30% of the virgin ZIRLO alloy.     

Effect of torsional oscillations on the stress-strain behavior of Al-5 wt% Mg alloy

The microstructure and mechanical properties of a precipitation hardenable Al-5 wt% Mg alloy subjected to low frequency torsional oscillations and aging treatments are reported in the present work. The stress-strain response was examined at different frequencies (1.53-3.3 Hz) and at constant shear strain amplitude (φ = 5.69 × 10⁻⁴). An excessive in the softening was observed by increasing both frequency of the applied oscillations and deformation temperature. The change in the hardening and softening parameters of stress-strain relations was explained in view of spinodal decomposition in Al-Mg systems. The mean value of activation energy was found to be equal to that quoted for precipitate-dislocation intersection.     

Effects of growth atmosphere and homo-buffer layer on properties of ZnO films prepared on Si(1 1 1) by PLD

ZnO films were synthesized on Si(1 1 1) substrates by pulsed laser deposition (PLD) under four different growth conditions. The structural and optical properties of the samples were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and photoluminescence (PL) measurement. It is found that when ZnO film is directly prepared on Si, oxygen atmosphere can significantly enhance the near-band-edge (NBE) emission and decrease the deep-level (DL) emission, but cause a polycrystalline film. By introducing a homo-buffer layer fabricated at 500 °C in vacuum, epitaxial ZnO film with three-dimensional (3D) growth mode is achieved instead of the polycrystalline film. In particular, the epitaxial film with the buffer layer shows more intensive NBE emission and narrower full-width at half maximum (FWHM) of 98 meV than the film without the buffer layer. The experimental results suggest that both oxygen atmosphere and buffer layer are quite efficient during PLD to grow high-quality ZnO/Si heteroepitaxial films suitable for applications in optoelectronic devices.     

Microstructure and texture studies on twin-roll cast AZ31 (Mg-3 wt.%Al-1 wt.%Zn) alloy and the effect of thermomechanical processing

The microstructure and texture of the twin-roll cast (TRC) AZ31 (Mg-3 wt.%Al-1 wt.%Zn) sheet, with a thickness of 6 mm, have been investigated. The TRC AZ31 exhibits a dendritic microstructure with columnar and equiaxed grains. These contain Al-Mn and Mg-Al-Zn second-phase particles that are approximately 1 μm in size. This heterogeneous structure is attributed to the effect of the cooling rate, which varies from 325 °C/s on the surface to ∼150 °C/s in the mid-thickness of the sheet. No surface segregation, but a certain degree of macrosegregation is observed in the mid-thickness which persists after annealing and rolling. Recrystallization at 420 °C leads to a bimodal grain-size distribution, while a fine-grain structure is obtained after rolling and annealing. The TRC AZ31 sheet exhibits basal textures in the (i) as-received, (ii) rolled and (iii) rolled-annealed conditions. However, post-annealing of the TRC AZ31 at 420 °C produces a relatively random texture that has not been previously observed in the conventional AZ31 sheet. The texture randomization is attributed to the particle-stimulated nucleation of new grains in the TRC structure. The preliminary evaluation of mechanical properties indicates that such annealing treatment slightly increases the ultimate tensile strength (UTS), but significantly improves elongation.     

Effects of partial crystallinity and quenched-in defects on corrosion of a rapidly solidified Ti-Cu alloy

Rapid solidification by planar flow casting has been found to have introduced deficiencies, viz. partial crystallinity, air pockets and compositional difference in the ribbons of rapidly solidified Tiin42.9Cuin57.1 alloy. In order to investigate the effects of these deficiencies on the corrosion of rapidly solidified Tiin42.9Cuin57.1 alloy ribbons, electrochemical behaviour of alloy ribbons has been investigated in the acidic chloride environments at room temperature by taking into consideration each side of the alloy ribbon separately. The alloy displayed passivity followed by pitting corrosion. In the as- solidified condition, air pockets appear to be the most detrimental defect from the viewpoint of corrosion resistance of the alloy ribbons.     

Initial growth stage of InTaO4 films deposited on MgO(0 0 1) substrates together with assisting O2-ion-beam by Ar-ion-beam sputtering of Ta target partly covered with In sheets

Thin-film growth process of InTaO₄ on MgO substrates held at 700 °C is analyzed on the basis of the thickness dependence of the structure determined by X-ray diffraction analysis and the composition determined by X-ray photoemission spectroscopy. Monoclinic InTaO₄ thin film grows at thicknesses above a critical value, while the amorphous phase grows at thicknesses below the critical thickness because the amount of In becomes insufficient to form the respective monoclinic structure. The In atoms are re-evaporated from the MgO surface at the initial growth stage. The growing films possibly comprise amorphous TaO_x. The In atoms start to be incorporated into the growing films due to the reduction of re-evaporation of In atoms. This leads to the start of the interaction with Ta and O atoms resulting in the formation of InTaO₄. The critical thickness to form a crystalline InTaO₄ film is controlled by adjusting the incoming flux of In atoms.     

Study on laser‐tungsten inert gas hybrid welding of dissimilar Mg alloy and steel with Ni as interlayer

Dissimilar Mg alloy and Q235 steel lap joints are produced by Laser‐ tungsten inert gas (TIG) hybrid welding with Ni as an interlayer. Fe and Ni are joined together in the form of solid solution, while Mg alloy and Ni foil are joined together by intermetallic compound Mg₂Ni. During tensile testing, the joints fail at the interface between Ni foil and Mg alloy. The shear strength of the Mg/Steel joints with Ni as interlayer is 170 MPa, which is higher than that without interlayer 120MPa.