Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure. This revised version was published online in November 2006 with corrections to the Cover Date.     

The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy

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. Potential candidates are precipitation hardenable and belong to the Al-Li-Cu family. The intrinsic microstructural features have a pronounced influence on the mechanical response of these alloys. In this work, the mechanisms responsible for the formation of precipitate-free zones along grain boundaries in precipitation-strengthened lithium-containing aluminium alloys were examined. The influence of grain morphology and the nature and type of precipitate coverage at the grain boundary in controlling the formation of these zones was analysed. The presence and influence of these zones along the grain boundaries on mechanical properties was studied for an Al-4.5Cu-1.21 Li alloy. It was found that while strength is comparable with existing high-strength alloys, the ductility decreases due to the presence of precipitate-free zones. The degradation in ductility is attributed to the particular mode of plastic deformation of this alloy, and to the restriction of plastic deformation in narrow planar zones along the grain boundaries. Fracture occurs when a critical local strain is reached in these zones. The overall consequences of precipitate-free zones along grain boundaries on mechanical properties are discussed in the light of competing effects involving the nature of matrix-strengthening precipitates, grain-boundary particles and deformation characteristics. Previous address: Materials Modification Inc., Falls Church, Virginia 22044, USA Previous address: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.     

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.     

Mechanisms governing cyclic fracture in an Al–Cu–Li alloy

Abstract

The fracture behaviour of a peak-aged, partially recrystallized Al–4·5Cu–1·21Li–0·51Mn–0·20Cd alloy has been investigated as a function of strain amplitude, stress intensity, and environment. It was found that the failure was predominantly intergranular separation, regardless of the environment, stress intensity, or strain amplitude, and that the fracture behaviour was influenced mostly by intrinsic microstructural features, rather than the nature of the environment. The shearable nature of matrix strengthening precipitates, large recrystallized grains, and precipitate-free zones along the high-angle grain boundaries aid in localizing the deformation, resulting in low-energy intergranular fracture. The iron- and silicon-rich intermetallic precipitates in the alloy promote void nucleation following fracture of the particle. A model is proposed which suggests the need for high stresses and strains for the initiation and spontaneous growth and coalescence of microvoids. The mechanisms of fracture behaviour of the alloy are discussed in terms of several concurrent processes involving strength of the material, intrinsic microstructural effects, deformation behaviour, state of stress, and strain.

MST/497     

Strengthening mechanisms in whisker-reinforced aluminium composites

Silicon carbide whisker reinforced aluminium alloys were mechanically tested in the as-cast and heat treated conditions; the microstructures were examined by transmission electron microscopy. The mechanical properties were strongly dependent upon the nature of the matrix alloying elements, the heat treatment conditions and the processing routes. Al-Cu and Al 6061 reinforced alloys were very responsive to heat treatment while Al-Mg and Al-Si reinforced alloys were not. The microstructures and mechanical properties were analysed in an attempt to determine the operative strengthening mechanisms and deformation processes. Three distinct types of matrix microstructure were observed, distinguished by the presence or absence of subgrains and/or precipitates. Using these observations, the composite properties could be quite well modelled using dislocation theories, indicating that the matrix microstructure dominates the mechanical properties.     

Mechanical properties of Al–Li alloys Part 1 Fracture toughness and microstructure

Abstract

Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 8090, 8091, and 2091 are examined as a function of plate orientation, with specific emphasis on the role of microstructure. In Part 1, results on the uniaxial tensile and plane strain fracture toughness properties are presented and the behaviour is discussed in terms of the role of the matrix and grain boundary precipitates, associated precipitate free zones (PFZs), and the occurrence of short-transverse delamination. It is seen that in general peak aged microstructures show an excellent combination of strength and toughness (L–T, T–L), equal to or exceeding that shown by traditional 2000 and 7000 series high strength aluminium alloys. The superior toughness of peak aged compared with naturally aged microstructures seems to be associated with widespread matrix precipitation of platelike precipitates (T₁ in Al–Li–Cu alloys and S in Al–Li–Cu–Mg alloys), β′-dispersoids and second phase particles which promote ductile (void coalescence) fracture, and with secondary cracking (through thickness delamination) caused by poor short transverse properties. By contrast, the deterioration in fracture toughness with overaging is primarily attributed to extensive grain boundary precipitation and corresponding formation of PFZs, similar to traditional aluminium alloys. All alloys show highly textured, predominantly unrecrystallised grain structures that render the properties to be strongly orientation dependent; specifically, fracture toughness values for the short-transverse orientations (S–L, S–T) are typically 50% lower than in the longitudinal and transverse orientations.

MST/926a     

Influence of heat treatment on the tensile properties and fracture behaviour of an aluminium alloy-ceramic particle composite

The tensile deformation and fracture behaviour of aluminium alloy 2124 reinforced with different amounts of silicon carbide particulates was studied, in the as-extruded and heat-treated conditions, with the objective of investigating the influence of heat treatment and composite microstructural effects on tensile properties and quasi-static fracture behaviour. Results indicate that for a given microstructural condition, the elastic modulus and strength of the metal-matrix composite increased with reinforcement content in the metal matrix. For a given volume fraction of reinforcement, the heat-treated composite exhibited significantly improved modulus and strength-ductility relationships over the as-extruded counterpart. The increased strength of the Al-SiC composite is attributed to the competing and synergistic influence of strengthening precipitates in the matrix metal, residual stresses generated due to intrinsic differences in thermal expansion coefficients between components of the composite and strengthening from constrained plastic flow and triaxiality in the ductile matrix due to the presence of brittle reinforcement. Fracture on a microscopic scale is initiated by cracking of the individual or clusters of SiC particles present in the microstructure. Particle cracking was dominant for the as-extruded composite microstructure. For both the as-extruded and heat-treated conditions, particle cracking increased with reinforcement content in the matrix. Final fracture of the composite resulted from crack propagation through the matrix between clusters. Although these composites exhibited limited ductility on a macroscopic scale, on a microscopic scale the fracture mechanism revealed features reminiscent of ductile failure.     

Tensile deformation and fracture behavior of a ductile phase reinforced dispersion strengthened copper composite

Niobium particle reinforced aluminum oxide (Al₂O₃) dispersion strengthened copper composite is an attractive and emerging engineered material for applications requiring high strength, high thermal and electrical conductivities and resistance to softening at elevated temperatures. In this paper, the microstructure, tensile deformation and fracture behavior of the composite is examined. The strength of the material decreases with an increase in temperature with a concomitant improvement in ductility. The composite microstructure maintains a high value of yield strength/ultimate tensile strength ratio. The factors contributing to increased strength and the intrinsic mechanisms governing fracture characteristics of the composite are examined in light of intrinsic microstructural effects, nature of loading and deformation characteristics of the matrix.     

SiC晶须增强铝合金复合材料的强化效果研究

考察了几种不同基体和状态的SiC晶须增强铝合金复合材料在拉伸变形过程中流变应力的变化情况,发现复合材料的强化效果与基体强度有关,混合法则(ROM)对复合材料抗拉强度的预测与实验值偏差较大,基体强度高的复合材料在拉伸变形过程中出现低应力屈服现象,复合材料的加工硬化能力明显高于基体,文中对出现这些现象的原因进行了探讨.     

On precipitates in Fe–Ni base alloys used for USC boilers

This paper overviews the effects of some precipitates in Fe–Ni base boilers materials. The volume fraction of these precipitates is usually below 10% for maintaining the microstructure stability and manufacture ability. They appear in a number of morphologies, but all of them improve the creep strength when they exist in small size. For the alloys serviced below 650°C, the strengthening precipitates disperse within the matrix and maintain their diameter below 100 nm during long time exposure. For the alloys serviced above 650°C, the strengthening precipitates are normally larger than 100 nm in equivalent diameter. They may experience long growth duration and thus keep their size below 500 nm. Meanwhile, the precipitates may improve creep strength when they are dispersed over the grain boundaries.     

Microstructural characterization of two lithium-containing aluminium alloys

The microstructures of two lithium-containing aluminium alloys have been investigated. The two alloys were an Al-Li-Mn alloy, heat treated to provide an under-aged, peak-aged and an over-aged condition, and a commercial Al-Cu-Li alloy, 2020, heat treated and aged to contain ordered precipitate structures. It was observed that both materials were recrystallized with fairly large grains. The Al-Li-Mn material had a high volume fraction of Al₆Mn dispersoids and the Al-Cu-Li alloy had a substantial volume fraction of coarse intermetallic particles and intermediate size disperoids. The major strengthening precipitates were identified from brightfield and dark-field images and selected-area diffraction patterns taken in the transmission electron microscope. Precipitate-free zones were found to be present in both the Al-Li-Mn and Al-Cu-Li alloys. The results of this study suggest that the peak-aged Al-Cu-Li alloy and the under-aged and peak-aged Al-Li-Mn alloys enhance deformation to occur primarily by planar slip, and the larger particle size and interparticle spacing of the over-aged Al-Li-Mn promotes a combination of planar slip and Orowan looping.     

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

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.     

Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd—Ag dental casting alloys

The fatigue limits and fracture characteristics for three Pd–Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91, Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress amplitude (R-ratio) of −1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits for these alloys are attributed to their complex microstructures and perhaps to casting defects.     

Dependence of fracture toughness on multiscale second phase particles in high strength Al alloys

Abstract

An improved model is described to predict variations in fracture toughness of high strength aluminium alloys with volume fraction, size, and characteristics of the contained multiscale second phases, i.e. ellipse shaped constituents, sphere shaped dispersoids, and disc shaped precipitates, in an integrated manner. Results show that predictions are in broad agreement with values measured experimentally for an aged Al-Cu-Mg alloy. Furthermore, the model was employed torelate the anisotropic fracture toughness ofalloy plate to its orientation. A diagramis presented to illustrate the relationship between yield strength and fracture toughness of the aged alloy.     

Evaluation of the tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys

The tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys produced by Nuclear Metals, Inc., is determined as a function of temperature. The wrought alloy and the casting alloy are both shown to have a very high stiffness to density ratio compared with common structural materials. The wrought alloy was found to have superior fatigue strength, tensile strength and ductility relative to the casting alloy; it also maintained a greater fraction of its tensile strength as a function of temperature. The stiffness of the materials can be readily explained using standard composite theory, where the material is treated as a discontinuous beryllium-reinforced aluminium matrix composite. The strength of the casting alloy is controlled to a large extent by the strength of its aluminium alloy matrix. In contrast, strengthening increments from both dislocation-based mechanisms and load transfer appear to be operative for the wrought material. Fractographic analysis of tensile specimens showed that preferential failure of the aluminium regions or the beryllium/aluminium interfacial regions occurs under certain circumstances. Fracture analysis of fatigue samples revealed no obvious fracture initiation sites and no evidence of limited/controlled crack growth regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the Applicability of the Schwalbe's Model to the Fracture Toughness Calculation in 7075 Aluminum Alloys

Four 7075-T651 aluminum alloys have been tested in tension in order to assess the applicability of the Schwalbe's model to the fracture toughness calculation. Standard K _IC tests were performed on compact tension samples at room temperature, and the results compared with those from the Schwalbe's model which takes into account several mechanical properties derived from a conventional tensile test applied on round unnotched tensile samples, and the average dimple size of the corresponding fracture surfaces. The values of K _IC calculated through the Schwalbe's model, correlate qualitatively well with those from the standard technique.Fracture toughness deterioration is accompanied by a loss of the true fracture strain, strain hardening capacity and average dimple size. On the other hand, the higher the Zn/Mg ratio, the volume fraction of precipitates and the yield strength, the lower the fracture toughness. All these effects are originated in the presence of matrix precipitates. Therefore, the reduction in K _IC can be explained in terms of the matrix response to the applied stress field as a function of the differences in volume fraction of the strengthening precipitates.The round tension samples corresponding to the four materials, failed in a predominantly ductile transgranular fashion, which facilitates the application of the Schwalbe's model based in the characteristic dimples, developed in this mode of fracture, as a microstructural element size.     

Annealing behaviour of dilute aluminium alloys following hot deformation

Abstract

The microstructure and texture of three dilute aluminium alloys after hot deformation and annealing was assessed; In particular, the influence of deformation temperature, strain rate, and strain on the annealed texture was examined, as well as the effect of alloy composition. The microstructures of the commercially pure materials studied (Al, Al+1%Mn and Al+1%Mg) varied in the volume fraction of coarse intermetallic particles, the type of dispersoid present, and the level and type of solute in solid solution. Furthermore, the initial stages of recovery and recrystallisation were studied in detail for one of the alloys (commercially pure Al). It was found that the main recrystallisation texture component was the cube and its strength, as well as the recrystallised grain size, depended strongly on the deformation strain. The deformation strain rate and temperature, and the alloy composition also strongly influenced the grain size and cube texture strength. These results are discussed in the context of current theories for cube nucleation within cube bands in the hot deformed microstructure. The present work was carried out as part of a wider research programme, partially supported by the European Union (Brite/Euram funded), to develop micromechanical models to describe the evolution of microstructure and texture during hot deformation and annealing of aluminium alloys.

MST/3376     

非连续增强铝合金复合材料的力学性能

用粉末冶金法制备了ＳｉＣｐ／铝合金复合材料，并对其进行了力学性能测试和断裂特性分析；综述了用不同工艺生产的非连续增强ＭＭＣ的性能及影响因素；试图说明增强体／基体界面结合力是铝合金复合材料性能的控制因素；指出寻求适当的界面结合力是复合材料设计中的一个重要内容。     

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.     

Small-angle scattering from GP zones in Al-Cu alloy

It is well known that Guinier Preston (GP) zones form in Al-Cu alloys upon solutionizing and artificial aging, which are extensively used in commercial practice. It is well established that GP zones are discshaped precipitates, i.e. disks of clusters of copper atoms in the FCC aluminium matrix. These disks have coherency strain fields in aluminium that give the alloy its high yield strength. The formation of GP zones in the supersaturated aluminium matrix is thought to be heterogeneous nucleation and growth. Some authors have believed that the formation of GP zones is by spinodal decomposition of the supersaturated Al-Cu solid solution. The main objective of the present work is to test whether spinodal decomposition is responsible for the formation of GP zones in Al-Cu alloy. The experimental alloy AA2219 was selected for its high copper content (Al-6%Cu-0·2%Zr). After solutionizing and artificial aging, the aging curve was plotted and small-angle scattering experiments were carried on the powdered samples as a function of time during artificial aging. Small-angle scattering data were analysed, and evidence has been obtained for the occurrence of spinodal decomposition as the mechanism responsible in the early stages of formation of GP zones.