Strength differential in Al---Li alloy 8090

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10⁻³, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ_T only in the L direction and σ_C only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions.     

Characterization of surface modified Ti-6Al-7Nb alloy

In the last years different types of surface modifications were developed with the aim of improving the osteointegration ability of titanium alloys. The chemical composition, crystallographic structure and morphology of a surface layer can be modified in order to obtain a better interaction between the implant, the cells and the organic fluids. The final goal is to obtain a more efficient bone growth also in critical clinical cases. In the present paper were reported several data about the characterization of the Ti-6Al-7Nb alloy treated by two innovative surface treatments. They consist of blasting, followed by a two step chemical etching and heat treatment performed in air or in vacuum. TEM, XRD and SEM investigations were performed in order to assess the structure and morphology of the modified surfaces. The surface chemical composition was investigated by XPS ad AES analyses. The ability to interact with physiological fluids was tested by immersion of the treated materials in an acellular simulated body fluid (SBF). Metal ion concentration analyses of the fluid and SEM observations of the samples were performed after different times of soaking. The mechanical characterization involved scratch and fatigue tests.The surface of treated samples shows chemical, structural and morphological modifications. The passivation pre-treatment has influence on the surface modification. The treated samples evidenced a quite low metal ion release and interact with SBF solution, showing a moderate bioactivity. A relevant decrease in fatigue strength was observed on modified samples.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Microstructure of diffusion-bonded joints in Al-Li 8090 alloy

Joints were produced between Al-Li 8090 alloy sheet by solid state (SSDB) and transient liquid-phase (TLPDB) diffusion-bonding techniques. The bond interface was a planar, thermally stable, large-angle grain boundary in the SSDB joint. Non-planar grain boundaries in a band of coarse grains were present in the TLPDB joint. The origin of these microstructures and the measured shear strengths of the joints relative to that of the parent sheet are discussed.     

Electron-beam weld microstructures and properties of aluminium-lithium alloy 8090

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Few commercial aluminium-lithium alloys have emerged for use in the aerospace industry. One such candidate is 8090, a precipitation-hardenable Al-Li-Cu-Mg alloy. The influence of electron-beam welding on the microstructure and mechanical properties of alloy 8090 material has been evaluated through microscopical observations and mechanical tests. Microscopic observations of the electronbeam welds revealed an absence of microporosity and hot cracking, but revealed presence of microporosity in the transverse section of the weld. Mechanical tests revealed the electronbeam weld to have lower strength, elongation and joint efficiency. A change in microscopic fracture mode was observed for the welded material when compared to the unwelded counterpart. An attempt is made to rationalize the behaviour in terms of competing mechanistic effects involving the grain structure of the material, the role of matrix deformation characteristics, grain-boundary chemistry and grain-boundary failure.     

High-temperature creep of the particlehardened commercial Al-Li-Cu-Mg alloy 8090

Creep of the particle-hardened commercial Al-Li 8090 alloy has been studied at temperatures of 425 and 445 K. The measured stress sensitivity of the minimum creep rates changes abruptly at a given applied stress with stress exponents being around 4–6 at low stresses and 30–40 at high stresses. Creep activation enthalpies were determined by both temperature cycling and by comparing creep rates at two temperatures at a given applied stress, the results from both gave the same unrealistically high values. The internal stresses, _i, developed during creep were determined using the strain-transient dip test. These increased linearly with the applied stress, _a, at low stresses and were effectively constant at high stresses. The minimum creep rate was found to be a simple function of the effective stress, _a-_i, with a stress exponent of between 5 and 6, at all applied stresses. The dislocation and precipitate structure of the alloy was examined before and after creep using thin-film electron microscopy. The initial structure consisted of pancake grains with a well-developed {1 1 0}1 1 2 type texture. The grains contained well-developed sub-cells and and S precipitates. The structure developed during creep consisted of dislocation pairs, single dislocations and dislocations loops. There was evidence to suggest that slip took place on both {1 0 0} and {1 1 1} planes. The dislocation loops were most likely to have been Orowan in character and around the rodlike S precipitate, with the coherent precipitate being sheared by pairs of dislocations. The measured internal stresses result from inhomogeneity of plastic deformation. These stresses increase continuously with applied stress up to the observed macroscopic yield stress, and then become constant. The internal stresses are likely to have arisen from the Orowan loops around S and the behaviour of sub-grain boundaries. The increases in internal stress may have resulted from an increased loop density with increasing applied stress. This rate of increase is likely to slow down if S particles are sheared or fractured at high applied stresses.     

Strengthening of alloy 8090 and its fracture mechanics parameters

An investigation has been made of the tensile properties, impact-, initial fracture toughness and fracture mode of an aluminium-lithium 8090 alloy at room temperature and 77 K, depending upon the heat treatment and orientation. The peak-aged material exhibited an excellent combination of strength and toughness, equal to or exceeding that shown by the high-strength aluminium alloys of the 2000 and 7000 series. The superior strength and toughness of peak-aged plates, including that of 3% stretched material, compared to underaged material seems to be associated with the lower content of coarse insoluble precipitates, a higher density of S-precipitates in a matrix ligament (grain) which promote ductile fracture. The impact toughness of the peak-aged specimens increased at 77 K only in the L-T plate orientation, while in the T-L orientation it was somewhat lower or remained the same. The toughness increase at 77 K is discussed in terms of the role of the matrix and (sub)grain-boundary precipitates, freezing of low-melting point impurities of sodium and potassium alkaline metals at (sub)grain boundaries and the occurrence of the fine crack divider delamination toughening. The yield strength, R _o.2, increase on ageing was accompanied by a corresponding increase in initial crack divider fracture toughness, K _lc, opposite to the trends obtained for some traditional high-strength aluminium alloys. Changes of K _lc versus R _o.2 depending on orientation are discussed using models for ductile fracture toughness behaviour of aluminium alloys, based on the criterion that a critical width of the heavily strained zone at the crack tip approximates the average ligament width, d _p, i.e. the thickness of the elongated grain in the L-T and T-L plate orientations. It was also found that, for constant chemical composition and fabrication practice of the alloy, a critical plate thickness exists B 0.1 6 t _i, where _i is the initial thickness of the rolling ingot, for which the tensile strength properties in the L-T orientation are the same as that in the T-L orientation, while the plasticity (measured by elongation to failure) of the plate is a maximum. Two types of laminated cracks were observed on fracture surfaces of the specimens: large, >1 mm deep (the number of these cracks remains the same as the number of hot-rolling passes), and fine <0.4 mm (shallow laminated cracks, the number of which significantly increases with decreasing temperature, 77 K).     

Characterization of dispersed intermetallic phases in an Al-8.32wt%Fe-3.4wt%Ce alloy

A rapidly solidified Al-8.32Fe-3.4Ce (wt%) alloy was prepared by gas atomization and extrusion. The intermetallic phases present and their thermal stability, at temperatures up to 400°C, have been investigated by means of transmission electron microscopy (TEM). The metastable Al_mFe, Al₈Ce and equilibrium Al₁₃Fe₄ phases were detected in the as-extruded sample and the sample heat-exposed at 230°C, whereas the equilibrium Al₁₃Fe₄ and Al₁₃Fe₃Ce phases existed in the samples heat-exposed at temperatures above 315°C. The Al_mFe and the Al₈Ce phases were firstly observed in this alloy. The Al₁₀Fe₂Ce and Al₂₀Fe₅Ce phases, which were reported by the others in the similar alloys, do not exist in our samples. In addition, various domain structures in Al₁₃Fe₃Ce were also studied.     

Diffusion bonding of an aluminium-lithium alloy (AA8090) using aluminium-copper alloy interlayers

Diffusion bonds have been produced between sheets of an Al-Li-Cu-Mg-Zr alloy using aluminium-4% copper vapour deposited metallic interlayers. Microstructural changes occurred both in the parent alloy and in the bond interface after diffusion bonding cycles and post-bonding heat treatments were analysed. Different metallographic techniques (light microscopy, scanning and transmission electron microscopy) have been used. Diffusion bonding trials were carried out using the same alloy (AA8090), both in non-superplastic (T6) and superplastic conditions. Differences in their behaviours in relation to diffusion bonding were observed.     

Fatigue Behaviour of the Al-Li alloy 8090 compared to 2024

The high cycle fatigue and the fatigue crack propagation behaviour of the new Al-Li alloy 8090 were evaluated on 25 mm plate material and compared to the conventional high strength Al alloy 2024. The investigation covered changes of test direction, R-ratio and environment. The results revealed that for most conditions the Al-Li alloy proved to be equivalent or better than the conventional Al alloy.     

Nature of the interface between AA7072 alloy explosively clad to AA8090 aluminium alloy

Aluminium-lithium based alloy plates were explosively clad with Al-1 wt% Zn alloy sheets. Clad plates were evaluated for bond continuity, interface shape, microstructure, variation of elemental concentrations across the bond interface, and bond strength. Comparisons of selected characteristics were made with roll clad sheets developed earlier.Ultrasonic tests revealed the bond to be continuous at all locations except over 50 mm wide edges of the plates. Both straight and wavy shaped interfaces were observed, often alternating arbitrarily. Microstructures on each side of the interface were distinct and characteristic of the individual alloys bonded. No localized melting was observed in the interface regions. Elemental concentration varied sharply across the bond line in the as-clad condition, later changing to a smooth profile after heat treatment. The diffusion widths, when expressed as a percentage of the cladding thickness, were much smaller than the corresponding values of previously studied roll clad sheets.'Tensile shear strength' of the clad samples exceeded the shear strength of monolithic Al-1%Zn alloy, thus indicating good bonding. The bond strength values were marginally lower than those of roll clad sheets. These differences could, perhaps, be due to the differences in the extent of elemental diffusion across the bond interface between the two techniques.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.     

Microstructural influence on the intrinsic fatigue properties of Al---Li 8090 alloy

A study has been made to investigate the influence of microstructure on the extrinsic and intrinsic fatigue properties of the Al---Li alloy, 8090. Two types of microstructure have been produced to compare the relative fatigue properties, one with a δ′ phase dominant microstructure and the other with a S′ + δ′ microstructure. Crack closure loads measured by the crack-opening displacement method have been used to obtain intrinsic fatigue resistance of the δ′ and S′ + δ′ microstructures. Results have shown that the extrinsic fatigue resistance of the δ′ microstructure was considerably higher than that of the S′ + δ′ microstructure, especially at lower growth rate, which was mainly due to the more severe crack path tortuosity and associated high levels of crack closure. In addition, the intrinsic fatigue resistance of the δ′ microstructure was also observed to be higher than that of the S′ + δ′ microstructure, presumably due to greater slip reversibility in the δ′ microstructure.     

Critical strength criteria for DB/SPF processing of Al-Li 8090 alloy

Peel strengths are reported for solid state or transient liquid-phase diffusion-bonded (DB) joints between aluminium-lithium 8090 alloy sheets. The joints were tested under superplastic forming (SPF) conditions at 530 °C and with a progressively increasing peel angle,, in the range 0°–60°. The sheet deformed superplastically with or without peel fracture of the joints. A deformation model is proposed which predicts a critical combination of peel strength and superplastic flow stress for DB/SPF processing of the 8090 alloy and indicates peel fracture will occur when sheet thicknesses exceed 2 and 0.8 mm in solid state and transient liquid-phase diffusion-bonded joints, respectively.     

Development of mechanical properties in AA 8090 alloy produced by extrusion processing

Abstract

The effects of extrusion processing parameters on the mechanical properties of an AA 8090 alloy were monitored using a combination of hardness, tensile, andfracture toughness tests, and using light, transmission electron microscopy, and scanning electron microscopy. It was found that variations in the processing parameters affect the tensile properties to a greater extent in the as extruded condition than in the heat treated condition. In the former, the property changes occur as a result of both variation of grain structure and the solutionising effect during the process. In the latter, the tensile properties are controlled by the precipitation processes that occur, and the toughness remains essentially unaffected by changes in the processing conditions. Improved combinations of strength, ductility, and toughness are achieved when the material is subjected to suitable preaging treatments, which modify the precipitate morphology within the microstructure; the fracture surface characteristics of both tensile and fracture toughness test specimens reflect the microstructural changes.

MST/1115     

Microstructure,tensile properties and fracture behaviour of an Al-Cu-Li-Mg-Zr alloy 8090

The addition of lithium to aluminium alloys has the potential for providing a class of high strength alloys with exceptional properties suitable for aerospace applications. One such candidate is 8090, a precipitation hardenable Al-Li-Cu-Mg alloy. Detailed optical microscopical observations were used to analyse the intrinsic microstructural features of the alloy. It is shown that microstructural characteristics have a pronounced influence on tensile properties and fracture behaviour of the alloy in the peak-aged, maximum strength condition. Tensile test results indicate that the alloy has property combinations comparable with other high strength commercial aluminium alloys. The elongation and reduction in area are higher in the transverse direction of the extruded plate. A change in fracture mode was observed with direction of testing. We rationalize such behaviour based on the grain structure of the material, and the nature, distribution and morphology of the second-phase particles. An attempt is made to discuss the kinetics of the fracture process in terms of several competing mechanistic effects involving intrinsic microstructural features, deformation characteristics of the matrix, brittleness of the grain boundary precipitates and grain boundary failure. The role of stress on particle fracture is highlighted.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

Al-Li 8090 alloy overlap shear test pieces machined from 3 mm thick diffusion-bonded sheets showed two fracture zones at the bond interface. Zone 1 at the ends of the overlap showed predominantly intergranular fracture and zone 2 at the centre of the overlap showed peel-type fracture. The load appeared to be carried entirely by zone 1. Only zone 1 fracture was obtained in the base metal test piece. The fracture zones were caused by the non-planar stress distribution and by the bending moments associated with this type of test piece. The planar bond interface may accentuate the tendency in these alloys towards low ductility and toughness in the short transverse direction.     

Behavior of short fatigue cracks initiated from a notch in 8090 Al-Li alloy

The rates of growth of short fatigue cracks initiated from a notch are much greater than the rates of growth of long fatigue cracks for the same values of K. A decrease in the strength of materials caused by aging affects the behavior of long cracks. The geometric form of the notch strongly affects the behavior of short cracks. The growth rate of a short crack initiated from a sharp notch decreases and attains a minimum value at a length of 0.45 mm, which is far beyond the region of its influence. However, short cracks initiated from blunt notches exhibit slower growth in the region of stress concentration than outside this region. Strain fields induced by deformation of the tip of the notch are not the only factor inhibiting the propagation of short cracks from notches. To explain the behavior of a short crack initiated at a notch, one must take into account some other factors, in particular, crack closure.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 39–44, January – February, 1995.