Showing papers on "Aluminium alloy published in 1996"

Measurement of the stress in oxide scales formed by oxidation of alumina-forming alloys

Abstract: The alumina scales on a variety of high-temperature alloys are found to fluoresce when illuminated with light having a frequency greater than 18,000 cm−1. The fluorescence exhibits two narrow lines characteristic of chromium-doped alpha-aluminum oxide. The frequency shift of the two lines from the room-temperature, stress-free values of 14,402 cm−1 (1.786 eV) and 14432 cm−1 (1.789 eV) provides a noncontact measure of the stress in the alumina scales using the piezospectroscopic effect. In addition, the broadening of the lines is a measure of the stress gradient in the scale. The physical basis for the fluorescence technique is described together with its implementation for highspatial-resolution (∼2 μm) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100°C on single-crystal NiAl, polycrystalline Ni3Al, two Fe−Ni−Cr−Al alloys, and two Ni−Al base superalloys are presented.

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Abstract: Twenty-four ternary element additions were made to a binary TiAl alloy (Ti−34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al2O3 barrier being formed. This is probably due to the doping of those elements in TiO2. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al2O3 layer as the second layer from the outer surface and the relatively massive precipitation of Al2O3 in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.

Microstructure of a pressure die cast magnesium—4wt.% aluminium alloy modified with rare earth additions

Abstract: Addition of cerium-rich mixtures of rare earth (RE) elements to aluminium-containing magnesium pressure die cast alloys is known to improve the creep properties at elevated temperatures. In the present investigation, a detailed description of the microstructure of a magnesium-4 wt.% aluminium alloy containing 1.4 wt.% of a cerium-rich mixture of RE elements is presented. Particle types occurring and their distribution in the microstructure, as well as the distribution of elements in solid solution, are described. Heat treatment is carried out to verify if solid state precipitation occurs. Some qualitative arguments for the beneficial effect of RE elements on the creep properties are presented.

Tensile strength, ductility and fracture of magnesium-silicon alloys

Abstract: Tensile tests were performed between 293–573 K in order to investigate the mechanical properties of cast and extruded Mg-Si alloys. For the cast materials, Mg-high Si (⩾ 10 wt%) alloys showed lower values of the highest tensile strength at temperatures up to 373 K, as compared to pure Mg and Mg-low Si (<10 wt%) alloys, whereas the strength at 573 K increased with increasing Si content. The addition of aluminum and zinc to the alloys was effective in increasing the strength. The fact that the Mg-high Si alloys showed lower strength than the Mg-low Si alloys was because a high volume of Mg2Si embrittled the Mg-Si alloys. Microstructural investigations revealed that the particles of Mg2Si were coarse for the cast materials and fracture of the particles was caused by deformation. The mechanical properties of the cast materials were improved by hot extrusion. Microstructural refinement by hot extrusion was responsible for the improvement of the mechanical properties.

The effect of stress triaxiality and strain-rate on the fracture characteristics of ductile metals

Abstract: Notched tensile tests have been carried out on three common metals (pure iron, mild steel and aluminium alloy BS1474) over a wide range of strain-rates (10−3 to 104 s−1) and the strain-to-failure measured. The ductility of all three materials was found to be strongly dependent on the level of stress triaxiality in the specimen, this dependency being greatest for the ferrous materials and least for the aluminium alloy. No significant effect of strain-rate could be ascertained from the experimental results provided fracture remained fully ductile. However, for mild steel, a transition to a brittle fracture mode was observed for a given level of stress triaxiality as the strain-rate was increased. Numerical simulations of the experiments have been used to derive constants of a semi-empirical fracture model from the measured results. This model was found to give reasonable predictions of fracture over the range of conditions investigated.

Oxidized coloured aluminium pigments, process for their production and their use

Abstract: The invention concerns oxidised aluminium or aluminium alloy pigments having a content of metallic aluminium of not more than 90% by weight with respect to the total weight, wherein the pigments are colored, flake-like, shiny and non-agglomerated, a process for the production thereof and use thereof as special-effect pigments and as base pigments for further colouring with organic or inorganic color pigments.

Localized corrosion of laser surface melted 2024-T351 aluminium alloy

Abstract: Laser surface melting of 2024-T351 aluminium alloy with a CO2 laser operating at 2 kW with a spot size of 1.5 mm and a substrate traverse rate of 20 mm s−1 produced a relatively thin surface-melted layer with a refined microstructure and a modified distribution of the alloying elements. The laser treatment changed both the anodic polarization behaviour and the form of localized corrosion in deaerated 3% NaCl solution. Immersion tests in the same solution under the condition of natural aeration showed that for the as-received alloy both intergranular and pitting corrosion occurred with pits distributed mainly along the rolling direction while for the laser surface melted material only pitting corrosion was present with pits distributed uniformly. This difference in corrosion behaviour as a result of laser surface melting is attributed to changes in the distribution and composition of the second-phase particles present in the alloy.

Ceramic composites with a ductile Ni3Al binder phase

Abstract: Composites of B-doped ductile Ni3Al alloys with both non-oxide (WC, TiC) and oxide (Al2O3) ceramic powders were produced by hot-pressing The Ni3Al alloys wet the non-oxide ceramic powders well and form a semi-continuous intergranular phase However, the Ni3Al alloys do not wet the oxide powders well and tend to form discrete “islands” of the metallic phase Mechanical property testing showed the flexural strength is retained to temperatures of at least 800 °C The fracture toughness and hardness were found to be equal to or higher than comparable Co-based hardmetal systems Initial corrosion tests showed excellent resistance to acid solutions

Microstructure, tensile properties and fracture behaviour of Al2O3 particulate-reinforced aluminium alloy metal matrix composites

Abstract: The tensile deformation and fracture behaviour of aluminium alloy 2014 discontinuously-reinforced with particulates of Al2O3 was studied with the primary objective of understanding the influence of reinforcement content on composite microstructure, tensile properties and quasi-static fracture behaviour. Results reveal that elastic modulus and strength of the metal-matrix composite increased with reinforcement content in the metal matrix. With increase in test temperature the elastic modulus showed a marginal decrease while the ductility exhibited significant improvement. The improved strength of the Al-Al2O3 composite is ascribed to the concurrent and mutually interactive influences of residual stresses generated due to intrinsic differences in thermal expansion coefficients between constituents of the composite, constrained plastic flow and triaxiality in the soft and ductile aluminium alloy matrix due to the presence of hard and brittle particulate reinforcements. Fracture on a microscopic scale initiated by cracking of the individual or agglomerates of Al2O3 particulates in the metal matrix and decohesion at the matrix-particle interfaces. Failure through cracking and decohesion at the interfaces increased with reinforcement content in the matrix. The kinetics of the fracture process is discussed in terms of applied far-field stress and intrinsic composite microstructural effects.

Recrystallisation and texture in hot deformed aluminium alloy 7010 thick plates

Abstract: The recrystallisation behaviour and its effect on the crystallographic texture during solution heat treatment of thick aluminium alloy 7010 plates has been studied both on commercially hot rolled p...

Effect of Cr, Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

Abstract: The oxidation behavior of Ni3Al+2.90 wt.% Cr, Ni3Al+3.35 wt% Co, and Ni3Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni3Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni3Al alloys was the existence of small α-Al2O3 particles in the middle of the α-Al2O3 scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.

Effect of carbon content on mechanical properties of electroslag remelted Fe3Al based intermetallic alloys

Abstract: The effect of carbon content (0.013 to 0.50 wt%C) on structure and mechanical properties of electroslag remelted iron aluminides based on Fe3Al containing approximately 16 wt% (28 at%) aluminium alloy has been studied. At low carbon contents (0.031 wt% or less) the ESR ingots suffer from hydrogen embrittlement which results in poor mechanical properties. At high (0.074 wt% or more) carbon contents the hardness and strength of ESR ingots increase with increasing carbon content without significant change in ductility. This is attributed to the solid solution strengthening arising from interstitial carbon present in the Fe3Al matrix as well as to the increased amount of Fe3AlC (perovskite) precipitation. The mechanical properties of these high carbon ESR cast alloys are comparable or superior to those reported for high purity wrought alloys with similar aluminium contents. The machinability of ESR ingots improves with increase in carbon content. This is attributed to the presence of uniformly distributed Fe3AlC precipitates in the ingots which allow formation of small and uniform size chips during machining.

Method of producing a slide surface on a light metal alloy

Abstract: In internal combustion engines with an engine block made of a magnesium alloy or aluminium alloy the piston bearing surface can be formed by a grey cast iron bush. According to the invention a layer of between 10 and 70 % molybdenum powder and between 90 and 30 % steel powder is applied directly to the light metal alloy by thermal spraying, in particular plasma spraying, to form a slide layer. The coating is highly wear-resistant and adheres well to the light metal alloy. The invention is used in reciprocating piston engines with engine blocks made of a light metal alloy.

Solid-state reactions in the Al-Fe system induced by ball milling of elemental powders

Abstract: Elemental aluminium and iron powders have been mechanically alloyed in the atomic Al∶Fe ratios of 1∶1 and 1∶3. The structural evolution of the samples was followed by X-ray diffraction and differential calorimetry. Extended milling caused the formation of an almost completely disordered b c c solid solution instead of the equilibrium B2 and DO3 compounds. Upon heating to 700 °C the formation of the B2 phase was observed in the equiatomic samples and no significant variation of the long-range state of order in the iron-rich samples. Upon heating of samples pre-milled for short or intermediate times, at about 400 °C, the nucleation and growth of Al5Fe2 was observed which, upon further heating to 700 °C, transformed to the B2 phase or to the b c c solid solution, depending on the sample composition. These results suggest that the solid-state reactions proceed through diffusion of iron atoms in the aluminium layers, and that for an iron concentration in these layers below about 50 at%, the Al(Fe) solid solution could be in a metastable phase.

Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Abstract: Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed, protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti−24Al−11Nb, Ti−22Al−27Nb, and Ti−20Al−22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO2 scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO2. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi2. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500–1000°C for the Ti−Al−Nb alloys, or at 500–875°C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.

Processing and properties of SiC particulate reinforced Al6.2Zn2.5Mg1.7Cu alloy (7010) matrix composites prepared by mechanical alloying

Abstract: SiC particulate reinforced aluminium alloy (7010) matrix composites have been processed by mechanical alloying route starting from elemental powders. Proper post-consolidation by hot pressing or hot extrusion results in a fully dense composite compact that retains the fine microstructure developed during mechanical alloying. Addition of SiC was found to result in lower strengths at room temperature but higher strength at temperatures above 200°C. There was nearly a 64% increase in the yield strength at 350°C, brought about by addition of 20 wt.% SiC. The beneficial effects of SiC additions on mechanical strength are best realized at elevated temperatures. Fine precipitates of various intermetallics (CuAl2, Mg2Zn11, etc.) are found to be present in the solutionised and aged samples in addition to fine dispersoids of Al2O3, MgAl2O4, etc. inherited from the as-milled powders and/or formed during subsequent degassing and consolidation.

TiB2 particulate ceramic reinforced Al-alloy metal-matrix composites

Abstract: Two methods of producing a ceramic reinforced Al-alloy metal-matrix composite are described. The first one comprises the steps of dispersing a ceramic phase (of titanium diboride) in a liquid aluminium or aluminium alloy, mixing the ceramic phase with a cryolite or other fluoride flux powder and melting the mixture together with the aluminium or aluminium alloy phase at a temperature of between 700° and 1000 °C. In the second method, the fluoride flux is reduced in situ by either molten aluminium or its alloying elements (Mg, Ca) to yield TiB2 crystallites of different size and size distribution that can be predetermined by fixing the flux and alloy composition and the processing temperature.