On the precipitation of magnesium silicide in irradiated aluminium–magnesium alloys

Thermal neutron irradiation of aluminium or its alloys causes the production of silicon by transmutation. In aluminium–magnesium alloys, the transmutation-produced silicon reacts with magnesium and forms small precipitates. The precipitation in irradiated Al–Mg alloys is similar to the early stage of aging in thermally treated Al–Mg–Si alloys. This study evidences the simultaneous generation of two crystallographically different precipitate types. On the basis of electron diffraction patterns, unit cell parameters are derived and compared with structures found in thermally aged alloys. One of the two precipitate types has an Mg₂Si composition, while the other is an Al–Si–Mg intermetallic compound with high aluminium and silicon but low magnesium content. The formation of magnesium poor precipitates is important since it indicates that the threshold neutron fluence for grain boundary precipitation of silicon may be much higher than estimated in the past.     

Effect of scandium and titanium–boron on grain refinement and hot cracking of aluminium alloy 7108

Abstract

The effect of scandium and titanium–boron (Tibor) additions on the solidification behaviour of castings and welds of aluminium alloy 7108 has been investigated. A circular patch test was adopted to evaluate the effects of these elements on the hot cracking suscepti bility of welds made on cast coupons treated with different grain refiner additions. It was observed that grain size, as well as cracking susceptibility, decreased with increasing amounts of scandium and that hot cracking was completely eliminated at scandium additions above 0·25 wt-%. A more pronounced grain refining effect in welds was observed with Tibor and, in addition, no hot cracking was observed with Tibor additions as low as 0·02 wt-%Ti (0·004 wt-%B). Castings, however, were more effectively grain refined with scandium, achieving a finer grain size than with Tibor.     

Investigation of the weldability of an aluminium–lithium alloy

Investigations were carried out to study the effect of the chemical composition of filler material in welding an alloy of the Al–Mg–Li system on the hot crack formation resistance and the mechanical properties of welded joints. The effect of metallurgical factors on the formation of defects in the fusion zone was investigated using the spot weldability testpiece. The chemical composition of 1420 alloy was optimized and this greatly reduced the number of defects. The investigations were carried out in the framework of the research project 10.8. Technology of fusion welding of new constructional materials (‘Strategic directions of development of materials and technologies of processing these materials for the period up to 2030’) [1 Kablov EN. The strategic directions of development of materials and technologies of processing these materials for the period to 2030. Aviatsionnye materialy i tekhnologii. 2012;5:7–17. [Google Scholar]].     

Effects of δ phase and cold drawing ratio on the LCF properties of alloy 718 wire

The effects of the amount and distribution of δ particles on the low cycle fatigue (LCF) properties of alloy 718 wire were investigated. The amount and distribution of δ particles were controlled by cold drawing followed by a variety of agings. As the cold drawing ratio and aging time at 1116K increased, the well developed granular δ particles increased in amount and their distribution at grain/twin boundaries became more uniform. Regardless of the aging conditions, the LCF life increased as the cold drawing ratio increased. The granular particles precipitated along the grain boundary also improved the LCF life of alloy 718 wire since they inhibited crack propagation. After Merrick heat treatment, 50% of the cold drawn wire displayed lower 698K tensile and yield strength than 30% of the cold drawn wire. This was because the higher strain induced by the cold drawing prior to the first aging at 1116K appeared to promote the precipitation of the δ phase during aging, which has no influence on the strength of the material but has same stoichiometry with the γ phase as Ni₃Nb and, as a result, the higher strain precipitated a smaller quantity of γ particles with subsequent aging, which is a major hardening phase of the alloy. Cold drawing also lowered the precipitation temperature of the δ phase.     

On the negative Poisson’s ratio of an orthorhombic alloy

The existence of the counterintuitive property for which a material laterally expands when stretched, is described with reference to the orthorhombic CuAlNi alloy. In particular, it is shown that there is a set of planes for which Poisson’s ratio is always negative.     

Surface plasmon resonance and electrical properties of RF: magnetron sputtered carbon–nickel composite films at different annealing temperatures

In this work, the optical absorption spectra of carbon–nickel films annealed at different temperatures(300–1000 °C) with a special emphasis on the surface plasmon resonance(SPR) were investigated. The films were grown on quartz substrates by radio-frequency(RF)magnetron co-sputtering at room temperature with a deposition time of 600 s. The optical absorption peaks due to the SPR of Ni particle are observed in the wavelength range of 300–330 nm. With annealing temperature increasing up to 500 °C due to the increase in Ni particle size, the intensity of the SPR peaks increases, but weakens with annealing temperature increasing over 500 °C. The Ni nanoparticle size, the dielectric function of carbon matrix(ε_m) and the plasma frequency of the free electrons(ω_p) at500 °C have the maximum values of 21.63 nm, 0.471 and5.26 9 10~(15)s~(-1), respectively. The absorption peak shows a redshift trend up to 500 °C and then turn to blueshift with annealing temperature increasing over 500 °C. These observations are in a good agreement with the electrical measurements in temperature range of 15–520 K and the Maxwell–Garnett(M–G) effective medium theory(EMT).     

Growth kinetics of the diffusion zone at the interface of the explosion-welded nickel–aluminium composite

The phase composition of molten metal in explosion-welded AD1 + NP2 + AD1 laminated composite is investigated and the transformation of the composite during heat treatment is studied. The equations linking the growth kinetics of the diffusion zone with the energy consumption in explosion welding are derived.     

Evolution of microstructure and mechanical properties of steel in the course of pressing–drawing

The combined continuous pressing–drawing process is proposed after a comprehensive analysis of available plastic structure-forming techniques taking into account the promising trends in their development. This combination of severe plastic deformation in equal-channel step die and drawing allows one to obtain a wire of desired size and shape in the cross section with an ultrafine-grained structure after a few deformation cycles. It also enables initial workpieces of any length to be processed and, therefore, allows one to obtain finished products up to several tens of meters in length. The aim of this study is to investigate the effect of new combined pressing–drawing technique of plastic deformation on the structure and mechanical properties of the steel. These studies have shown that the proposed deformation technique has a significant advantage of the techniques currently used to manufacture a steel wire.     

The structure and properties of deposited alloys of the nickel–aluminium system

Twin-arc equipment using nickel and aluminium electrode wires is proposed for depositing intermetallic alloys of the nickel–aluminium system on steel. The processes of twin-arc surfacing and the properties of the deposited alloys of the nickel–aluminium system are investigated.     

Analysis of the electrochemical behaviors of WC–Co alloy for micro ECM

Micro electrochemical machining (ECM) of tungsten carbide with cobalt binder (WC–Co) was studied using ultrashort pulses. In ECM, the machining characteristics were investigated according to machining conditions such as electrolyte, workpiece potential, and applied voltage pulse. Using a mixture of sulfuric acid and nitric acid, microstructures with a sharp edge and good surface quality were machined on tungsten carbide alloy. The potentials of workpiece electrode and tool electrode were determined by considering the machining rate, machining stability, and surface quality of products. With the negative potential of the workpiece electrode, oxide formation was successfully prevented and shape with good surface quality in the range from R_a 0.069 μm to 0.075 μm were obtained by electrochemical machining. Moreover, the performance of ECM, which includes machining gap, tapering, surface roughness, and machining time, without tool wear was compared with that of electrical discharge machining (EDM). Microstructures of WC–Co with a sharp edge and good surface quality were obtained by electrochemical milling and electrochemical drilling. Micro electrochemical turning was also introduced to fabricate micro shafts.     

Mechanisms of Guinier–Preston zone hardening in the athermal limit

The interaction of dislocations with Guinier–Preston (GP) zones is a process that contributes to the yield strength of many underaged precipitate-strengthened alloys. Here we use atomistic modeling to investigate this process in an Al–Cu alloy using a newly developed interatomic potential. The study focuses on edge dislocation interactions in the athermal limit. The critical shear stress and the mechanism by which dislocations overcome GP zones is found to vary significantly depending upon GP zone size, orientation and offset from the dislocation glide plane. Dislocation cutting, looping, leading partial cutting with trailing partial looping, diffusionless climb and defect nucleation at the dislocation–GP zone contact point are all observed. In the majority of cases dislocation looping is the controlling mechanism, challenging the applicability of traditional continuum dislocation cutting models to the underaged Al–Cu system at 0 K.     

Evaluation of the heat flow from plasma to the end of an aluminium wire–anode

Investigations were carried out to develop a method of determining the density of the heat flow from plasma to the surface of a wire–anode depending on the wire feed rate, the thickness of the molten metal film at the end surface and the angle between the axis of the wire and the molten metal surface. It is taken into account that the heat flow, arriving from plasma to the end of the wire, is used to change the enthalpy of the metal not only in the solid but also liquid state. It is shown that the mean mass temperature of heating the metal film is 2/3 of the change of temperature in the height of the film. In the conditions investigated in these experiments, the superheating of the metal film above the melting point was 283 K. The surface temperature of molten aluminium was considerably lower than the boiling point so that the evaporation of metal was ignored when calculating heating of the wire.     

Simulation of the three-dimensional morphology of solidification porosity in an aluminium–silicon alloy

A novel extension of the cellular automata technique for microstructural modelling is presented, allowing simulation of the evolution of the complex three-dimensional morphology of porosity during the solidification of an aluminium–silicon alloy. The complex morphology arises due to the restriction of the growth of the pores by the developing solid phase. The model predicts the average properties of the porosity formed, together with the distribution in size and morphology.The model is used to determine the influence of a variety of applied conditions (e.g. thermal history, pressure, hydrogen content) and material properties (nucleation behaviour, alloy composition) upon the pore morphology, as characterized by the average and extreme dimensions. The relative magnitude of the effect of each parameter and the interactions between parameters upon the porosity are statistically analysed. The simulated pore size shows the largest sensitivity to applied pressure, hydrogen content and solidification time, together with interactions between solidification time and pressure. These results are in good agreement with previously reported experimental behaviour.     

Effect of aluminium on the passivation of zinc–aluminium alloys in artificial seawater at 80 °C

The effect of Al (0.15, 0.3 and 1.0 wt.%) on the passivation of Zn–Al alloys in artificial seawater at 80 °C is investigated by electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is found that the presence of Al in Zn–Al alloys can retard passivation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements show that Al increases the current density but decreases the corrosion resistance of passive films, respectively. Mott-Schottky analysis reveals that Al increases the electrical conductivity and the capacitance of the films. Passivation of Zn–Al alloys occurs in artificial seawater when the immersion time is between 120 and 288 h, due to the presence of various Zn and Al protective compounds at the surfaces. Depassivation occurs when the immersion time is between 288 and 720 h, probably due to the decrease of solution pH and the Cl⁻ penetration mechanism.     

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x ≈ 0.75 for Cr_1?xAl_xN and x ≈ 0.625 for Cr_1?x?yAl_xY_yN with y = 0.125 are consistent with experimentally obtained values of x = 0.69 for Cr_1?xAl_xN and x = 0.68 and 0.61 for Cr_1?x?yAl_xY_yN with y = 0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.     

On some limitations of the Johnson–Mehl–Avrami–Kolmogorov equation

Some limitations of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation used widely for describing kinetics of phase transformation are demonstrated using probabilistic analysis and Monte Carlo simulations. The JMAK equation predicts correctly the real transformed fraction only if the number of the growing nuclei in the controlled volume is large. If the number of growing nuclei is small, the transformed fraction predicted by the JMAK equation deviates significantly from the real transformed fraction, no matter how large the volume of the controlled volume is. As an alternative, another equation should be applied, which for any number of the growing nuclei predicts correctly the true amount of transformed fraction.     

Special features of the formation of the microstructure and chemical heterogeneity in welded joints in alloys of the Al–Zn–Mg–Cu system alloyed with scandium

Special features of the formation of the microstructure and the development of chemical heterogeneity in the weld metal and heat-affected zone (HAZ) in arc welding of a complex alloyed alloy of the Al–Zn–Mg–Cu system are discussed. It is shown that the effect of welding heating results in melting of the phase components of the alloy in the HAZ with the formation of structures of the eutectic origin in the form of long interlayers at the boundaries of recrystallized grains, causing brittleness of the metal. Alloying the alloy with scandium reduces the intensity of the processes of recrystallization and the extent of liquation at the grain boundaries and also localizes the melting of the grains without the formation of thick eutectic interlayers.     

Effect of backing runs on the properties of welded joints in aluminium–lithium alloys

The effect of the parameters backing runs and the composition of filler material on the properties of a welded joint and the heat-affected zone is investigated. It is shown that the porosity of the weld metal in the areas with backing runs depends strongly on the welding conditions, and when using Sv-1557 wire, the strength and plasticity are higher than those of the Sv-AMg6 wire.