Showing papers on "Aluminium alloy published in 1994"

Mechanisms-based creep constitutive equations for an aluminium alloy

Abstract: A number of mechanisms-based constitutive equations were assessed in an effort to describe the creep behaviour of an aluminium alloy at 150°C. It was found that a sinh function of stress, r...

Serrated flow in aluminium base alloys

Abstract: The most well known manifestation of dynamic strain aging is serrated plastic flow during mechanical testing. Serrated flow was first observed in aluminium alloys by Portevin and Le Chatelier after whom the Portevin-Le Chatelier effect is named. The work presented here concentrates on a review of the diversity of occurrence of serrated flow in aluminium base alloys. Binary alloys are discussed initially and the effect of individual alloy additions on serrated plastic flow is examined. This is followed by a discussion on the manifestations of dynamic strain aging in commercial aluminium alloys subdivided into separate classes distinguished primarily by the major alloying element. Relatively recent reports of serrated flow in aluminium-lithium alloys and aluminium alloy composites are also considered. Finally, the effect of precipitation processes on serrated flow phenomena in aluminium alloys is specifically discussed.

Mixed-mode I/II fracture behaviour of an aluminium alloy

Abstract: Mixed-mode fracture in aluminium 7075-T6 has been investigated by means of experimental and numerical considerations. Efforts were focused on incipient crack propagation where fractographical examinations revealed the presence of two different types of growth. In the near mode II range initiation occurred by straight, macroscopically stable, shear separation. For the remaining part, crack growth initiation took place in a similar way as in pure mode I where extensive shearing was observed to occur in a zig-zag pattern. The shift in initiation behaviour seemed to result in an increase of fracture toughness. In the numerical study where crack growth was assumed to occur as a consequence of expired work-hardening capacity, the location and orientation of incipient shear band development was investigated in a comparative manner, which indeed showed some resemblance to the experimentally observed features.

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

Abstract: NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.

Factors affecting the damping capacity of cast aluminium-matrix composites

Abstract: The damping capacity of stir-cast aluminium-matrix composites containing graphite and silicon carbide particles, were studied using a cantilever beam specimen and an HP 5423A Structural Dynamics Analyser. Damping data were determined in the first mode of vibration. Aluminium-matrix composites containing 5–10 vol % graphite particles and 10 vol % silicon carbide particles were prepared by the stir-casting technique and die cast to obtain standard samples (6 mm×25 mm100 mm). Graphite particles were found to be more effective in enhancing the damping capacity of composites compared to silicon carbide particles. The damping capacity of composites increased with the volume percentage of graphite within the range studied. However, no notable improvements in damping capacity were observed by dispersion of silicon carbide in aluminium alloy. The results have been analysed in terms of the effect of size, shape, nature and volume fraction of particles on the damping capacity of the aluminium matrix particulate composites and compared with the damping capacity data available in the literature. The effects of frequency, strain amplitude, temperature and processing on damping capacity of the aluminium matrix composites are reviewed.

Room-Temperature strength and deformation of Tib2-reinforced near-γ titanium aluminides

Abstract: A series of TiB2-reinforced near-γ titanium aluminide (Ti-Al) matrix composites have been produced in investment-cast form and characterized with respect to microstructure and tensile deformation. The Ti-Al matrices of the composites examined are based upon the binary composition Ti-47 Al (at. pct), with varying proportions (2 to 6 cumulative percent) of manganese, vanadium, chromium, and niobium. TiB2 has been introduced into the microstructuresvia XD* processing at levels of 7 and 12 vol pct and compared to unreinforced (0 vol pct TiB2), base variants. The influences of heat-treatment temperature and time have also been studied for each composition and reinforcement variant. The addition of dispersed TiB2 leads to a fine, stable, and homogeneous as-cast matrix microstructure. The measured TiB2 size within the composites examined ranged from 1.4 to 2.6 µm. Increasing the volume fraction of TiB2 leads to increased elastic moduli, increased ambient temperature tensile strengths, and in general, increased strain-hardening response. In some instances, the overall ductility of the alloy increases with the addition of TiB2 reinforcement. The flow stresses of both the monolithic and composite variants exhibit conventional power-law plasticity. The results indicate that the strengthening and the flow behavior in these composites are derived from both indirect and direct sources. Strengthening contributions are indirectly derived from the microstructural changes within the matrix of the composite that evolve due to the presence of the reinforcement during its evolution and development, for example, due to grain refinement and reinforcement-derived interstitial solid-solution strengthening. Direct contributions to strength are those that can be specifically attributed to the presence of the reinforcement during deformation,e.g., through the interaction of dislocations with the reinforcing particles. When the estimates of the indirect contributions are isolated and arithmetically removed from the magnitude of the total observed strength of the composite, the increase in flow stress correlates in all instances with the inverse square root of the planar interparticle spacing for all alloy compositions, heat treatments, and levels of strain examined.

Corrosion behaviour of aluminium-based metal matrix composites

Abstract: The corrosion characteristics, in 3.5 wt% NaCl solution, of aluminium alloy composites containing a range of reinforcements have been investigated using potentiostatic measurements and simple immersion tests. Complementary microstructural studies carried out on corroded surfaces and sections through corroded material have identified a number of preferential corrosion sites; these include the fiber/matrix interface, especially where it contains chemical reaction products resulting from composite fabrication, as well as second phases and pores in the metal matrix. The effect on corrosion behaviour of the different reinforcements, with particular reference to their chemistry and geometry, is discussed, as is the influence of composite manufacturing route.

Oxygen solubility and a criterion for the transition from internal to external oxidation of ternary alloys

Abstract: Smith's model is expanded in order to derive expressions to quantitatively describe the oxygen-solubility behavior in ternary alloys as a function of alloy composition. Multicomponent-diffusion theory is used to establish a criterion for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least-noble metal in a ternary alloy. The oxygen-solubility model and the criterion are applied to the oxidation of Ni−Cr−Al alloys in 76 torr of oxygen at 1100 and 1200°C, predicting the minimum Al concentrations required to form a protective Al2O3 scale. It shows that sufficient Cr additions would significantly reduce the oxygen solubility and also alter the oxygen distribution in the ternary alloys, avoiding the oxygen supersaturation necessary for the onset of internal oxidation. These two factors make it easier to establish the protective Al2O3 scale.

Synthesis of amorphous and metastable Ti40Al60 alloys by mechanical alloying of elemental powders

Abstract: Ti40Al60 amorphous and metastable alloys have been prepared by mechanical alloying (MA), under controlled milling conditions in a planetary mill. Three different quantities of kinetic energy at the collision instant have been achieved by using balls of different size, φb = 5, 8 and 12 mm, keeping constant all other device parameters. Assuming the collision between the balls and the vial walls to be inelastic, during the early stage of alloying, the amount of energy transferred to the trapped powder could be estimated. The experimental results show that the milling with balls of diameter φb = 5 or 8 mm leads to a solid-state amorphization of the Ti40Al60 mixture, through the attainment of a supersaturated solid solution of aluminium into α-titanium. Otherwise, the milling causes the nucleation of the A1-fcc disordered form of the TiAl intermetallic compound. The end products of MA-induced solid-state reaction (SSR) have been ascribed to the different temperature reached by the powder during each collision and to the reaction time scale for the formation of the amorphous phase, δt a, and for the nucleation of the non-equilibrium intermetallic compound, δt d. Differential scanning calorimetry has indicated that the crystallization of amorphous samples follows a two-step reaction. At a temperature T c≈400 °C, the amorphous phase crystallizes into the A1 -fcc. TiAl phase having a measured heat of crystallization of 6.2 kJ(g at)−1. Upon further heating, the system undergoes A1 → L1o reordering transition with an enthalpy release of about 3.2 kJ (g at)−1.

Microstructural characterization and adherence of α-Al2O3 oxide scales on Fe-Cr-Al and Fe-Cr-Al-Y alloys

Abstract: Several features of the microstructure and the adherence of alumina scales formed on Fe−Cr−Al and Fe−Cr−Al−Y single- and polycrystalline alloys after oxidation at 1000°C were examined. The convolutions of the scale and especially of the scale/alloy interface are thought to be the major reason of poor spallation resistance of scales on the yttrium-free alloy. The flat oxide scale on the even interface of the yttrium-doped alloy, on the contrary, exhibits excellent adherence upon cooling. Interfacial cavities observed on the Fe−Cr−Al alloy result from the scale undulation under compressive growth stresses. The shape and the number of cavities depend on the initial surface orientation and most probably reflect a balance between the interfacial energy of convoluted substrate in contact with the oxide layer and the energy of separated surfaces.

Structure and properties of functionally gradient aluminium alloy 2124/SiC composites

Abstract: Functionally gradient materials exhibit a progressive change in composition, structure, and properties as a function of position within the material. Functionally gradient materials and conventional metal matrix composites have been manufactured from gas atomised aluminium alloy 2124 powders (matrix) and particulate silicon carbide (reinforcement) by mechanical mixing followed by hot pressing at 500°C at a pressure of 1·5 GN m−2. In this work the structure and mechanical properties of model functionally gradient materials consisting of two discrete layers are compared with those of composites of homogeneous composition. A careful microstructural investigation has revealed a uniform distribution of the reinforcing SiC and defect free interfaces between adjacent layers of the functionally gradient materials. The strengths of the functionally gradient materials and composites have been determined via three point bend tests and the fracture toughness properties assessed using single edge notch bend ty...

Aluminium alloy brazing sheet

Abstract: Aluminium brazing alloy composition is (in wt.%): Mn 0.7 - 1.5, Cu 0.5 - 1.0, Fe not more than 0.4, Si not more than 0.15, Mg up to 0.8, V and/or Cr up to 0.3, Ti up to 0.1, others up to 0.05 each, 0.15 total, balance A1 of at least commercial purity. Improved properties include: post-brazed strength and sag resistance; corrosion resistance; ability to withstand interannealing and some homogenisation.

Strengthening potential of the cubic ? precipitate in Al-Cu-Mg-Si alloys

Abstract: An interesting cubic-shaped precipitate was observed in the matrix of a squeeze-cast Al-4.3 wt % Cu-2.0 wt % Mg/SiC composite which was heat-treated to a T7 condition. Although this phase had been observed by a few investigators in the past, it had never been examined in detail until now. This cubic phase generally had an edge length ranging from 30 to 50 nm and existed in volume fractions as high as 3.8%. Theoretical strengthening models predicted this phase to have good potential for precipitate strengthening. In addition, the cubic phase exhibited a low rate of coarsening at temperatures as high as 250 °C; apparently due to its low interfacial energy. Consequently, this cubic precipitate shows potential for increasing the useful temperature range of aluminium alloys and composites and could be of great importance to the aerospace and automotive industries.

Effect of Hydrostatic Pressure on Passivating Power of Corrosion Layers Formed on 6061 T6 Aluminium Alloy in Sea Water

Abstract: The effect of an increase in hydrostatic pressure (from 1 to 300 atm) on the composition of the corrosion layer formed on 6061 T6 aluminium alloy in sea water was studied using electrochemical (polarisation potentiodynamic curves and impedance measurements) and surface analysis (X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy) techniques. Increasing the hydrostatic pressure caused an increase in the Mg/Al ratio and in the ratio of non-hydrated to hydrated aluminium oxides, so that a more compact film with lower selfrepairing power was formed. Thus, the corrosion current density decreased while the pit nucleation probability increased.

High strain rate superplasticity of AlN particulate reinforced aluminium alloy composites

Abstract: Ceramic whisker or particulate reinforced aluminium alloy composites have a great potential for automobile engineering components, aerospace structures, semi-conductor packaging and so on, because of the composites ability to exhibit a high specific elastic modulus and specific tensile strength, excellent wear resistance and heat resistance, low thermal expansion and good dimensional stability A serious problem involving practical application of ceramic whisker or particulate reinforced aluminium alloy composites is due to the low tensile ductility, fracture toughness at room temperature and, also, their hardness qualities that make it difficult to deform by conventional forming processing and machining by ordinary tools It has been found, however, that aluminium alloy composites reinforced by SiC or Si[sub 3]N[sub 4] whiskers or particulates produce superplasticity at a high strain rate of about 01s[sup [minus]1] Superplastic deformation mechanisms of the ceramic whisker or particulate reinforced aluminium alloy composites are fine grain boundary sliding, interfacial sliding at a liquid phase and dynamic recrystallization An AlN particulate reinforced aluminium alloy composite exhibits a high elastic modulus and a high thermal conductivity, and their thermal expansion is similar to silicon in that the AlN particulate reinforced aluminum alloy composite is expected to apply to semi-conductor packaging in the aerospacemore » structure In addition, if the composite could produce superplasticity at high strain rates, the market of aerospace application for superplastic composites could be expanded The purpose of this study is to make clear if an AlN particulate reinforced aluminium alloy composite can produce superplasticity at high strain rate and the superplastic characteristics« less

The interface in Al2O3 particulate-reinforced aluminium alloy composite and its role on the tensile properties

Abstract: The interface characterization of the aluminium alloy reinforced with Al2O3 particulates ((Al2O3)p/AI composite) was performed using X-ray diffractometry and energy dispersive X-ray spectroscopy. A layer of MgAl2O4 single crystals was observed at the (Al2O3)p/Al interface in the as-received extruded composites. Such MgAl2O4 crystals formed at the surface of (Al2O3)p are believed to grow by consuming a certain amount of (Al2O3)p. Upon loading, interfacial debonding was observed to occur at the boundary between MgAl2O4 and the aluminium alloy, or along the MgAl2O4 layer itself. These experimental observations are correlated with the tensile properties of such composites.

Interface contribution to the SiC-titanium and SiC-aluminium tensile strength prediction

Abstract: Fragmentation tests of single SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix have been analysed in an effort to obtain the interface contribution in terms that could be incorporated into a tensile fracture model for unidirectional composites. Depending on the matrix, two regimes of interfacial stress transfer can be distinguished within the whole range of tested temperatures. For the SCS2/5083 system, plastic deformation of the alloy limits the stress transfer, and the interface contribution thus finds its expression in the shear stress of the matrix. for the SCS6/Ti-6Al-4V system, friction is the leading process and the interface contribution strongly depends on the stress state around the fibre. Assuming a temperature dependent compressive radial stress up to 925‡C, an effective transfer shear stress may be easily calculated for unidirectional SCS6/Ti-6Al-4V composites.

Coarsening of precipitates and dispersoids in aluminium alloy matrices: a consolidation of the available experimental data

Abstract: Experimental data on the coarsening of precipitates and dispersoids in aluminium-based matrices are reviewed. Available data are tabulated as K=(r 3−r 0 3 )/t where r 0 is the initial particle radius and r is its value after time t at temperature T, and then plotted as log (KT) against 1/T for consolidation and assessment. The considerable body of data for δ′-A3Li in Li-containing alloys is well represented by K=(K 0/T) exp (−Q/RT) with K 0=(1.3 −0.5 +3.0 ) × 10−13m3Ks−1 and Q=115±4kJ mol−1. The relatively limited data for θ′ and θ″ in Cucontaining alloys are representable by the same relationship with K 0∼4 × 10−8 and — 4 × 10−10 m3 Ks−1, respectively, and Q — 140 kJ mol−1. Available data for coarsening of L12 − Al3(Zr, V) and related phases in Zr-containing alloys and of Al12Fe3Si and related phases in Al-Fe based alloys indicate (i) rates of coarsening at 375 to 475 °C (0.7 to 0.8Tm) five to eight orders of magnitude less than would be expected for δ′, θ′ and θ″ in this temperature range, and (ii) high activation energies of ∼300 and 180 kJ mol−1, respectively.

Effect of Alloying Elements on Anodic Polarization Properties of Titanium Alloys in Acid Solutions

Abstract: The effect of Zr, Sn, Nb, Ta, Pd, N and O on the corrosion resistance of titanium alloys for medical implants was investigated by measuring the anodic polarization curves at 310 K in 5 mass% H 2 SO 4 and 5%HCl solutions deaerated by high-purity N 2 gas bubbling. The critical current density for passivation (I c ) and passive current density at 0.6 V versus SCE of the titanium alloys markedly decreased with increasing Zr, Nb, Ta, and Pd contents in the range from 5 to 20, 4 to 8, 2 to 4 and 0 to 0.2%, respectively, although Sn had an adverse effect in the range from 10 to 20 mass%. The effect of N and O on I, was undetected. The log (I c ) value decreased linearly with increasing bond order (B o ) value representing the covalent bond strength between Ti and the alloying elements