Binary Al-Li Alloys Research Articles

Modeling and simulation for phase coarsening: a comparison with experiment.

The phase coarsening of precipitates is modeled in the framework of Debye-Hückel theory. The interactions observed among a population of precipitates dispersed throughout a matrix can be described by diffusion screening. The relationship between the maximum particle radius and the volume fraction of the phases is established, and the rate of coarsening is related to the volume fraction and the self-similar particle size distribution. We simulated the dynamics of late-stage phase separation using multiparticle diffusion methods. Experimental measurements on the rates of coarsening of delta(') ( Al3 Li) precipitates in binary Al-Li alloys are compared with our results using modeling and simulation. The theoretically predicted particle size distributions and the maximum radius expected for particles in the microstructure agree well with recent experimental results.

Mechanism of Stress Corrosion Cracking of Al–Li Alloys

G.V. Akimov's concepts of the corrosion–electrochemical properties of aluminum alloys containing lithium are developed. It is found that binary Al–Li alloys are insusceptible to stress corrosion cracking, even though their dissolution rate under normal conditions can increase by up to 30 times because of the selective dissolution of lithium. The interaction of dislocations with phases formed upon heat treatments is demonstrated to play a determining role in the stress corrosion cracking of all the basic aluminum–lithium alloys, namely Al–Li, Al–Li–Cu, Al–Li–Cu–Mg, and Al–Li–Mg alloys. The stress corrosion cracking of both binary aluminum–lithium alloys and alloys which are in addition alloyed with copper and magnesium has mainly a dislocation–electrochemical mechanism. The effect of electrochemical factors is well represented by the difference in the magnitude between the pitting initiation potential and the repassivation potential.

Flow stability of binary Al–Li alloys

The constitutive stress–strain behavior of extruded Al–1 wt.% Li and Al–2 wt.% Li has been characterized in the temperature range 573–823 K between strain rates of 5×10 −4 and 1 s −1. These results together with dynamic materials modeling (DMM) were utilized to establish strain rate–strain-temperature processing maps. Stable regions, defined on the basis of two mechanistic stability criteria, 0< m<1 and δ log(m)/δ log( ε ̄ ̇ )<0 , and two thermodynamic criteria, s>1 and δ log(s)/δ log( ε ̄ ̇ )<0 , were found to be associated with dynamic recovery (DRC) at low temperatures and dynamic recrystallization (DRX) at higher temperatures. Unstable flow was observed at both low temperatures (573–623 K)/high strain rates (10 −1 s −1) and at high temperatures (773–823 K)/low strain rates (10 −3–5×10 −4 s −1), these instabilities being associated, respectively, with macroscopic shear band formation and dynamic grain growth.

Environment-Assisted Cracking of Al 2195 Alloy

Aluminum-Lithium alloys can be susceptible to environment-assisted cracking (EAC) depending on their composition and aging temper. Our previous studies showed that binary Al-Li alloys are in general resistant to EAC whereas ternary alloys such as Al-2.2Li-2.9Cu and Al-3Li-ICu are susceptible. The degree of embrittlement of the latter alloys was found to correlate very well with (i) the precipitation characteristics of the T 1 (Al 2 CuLi) phase and in particular its volume fraction present at the grain boundaries and (ii) the slip behavior that is primary determined by the matrix precipitates. In the present study, the EAC behavior of Al 2195 alloy (Al-lLi-4Cu) was studied by conducting alternate immersion and dynamic charging experiments of round cylindrical tensile specimens. The role of the alloy microstructure was assessed by investigating the peak aged and two overage tempers. In the first series of experiments, specimens were pre-exposed in 3.5% NaCI solution under alternate immersion for various periods of time and then tensile tested under slow strain rate (7.2×10 -6 s -1 ). In the dynamic charging experiments, the specimens were cathodically charged under potensiostatic control at -1500 mV (SCE) while they were tensile tested under slow strain rate. Fractographic examinations were conducted by using SEM to correlate mechanical behavior and fracture surface morphology. TEM studies were carried out to determine microstructural characteristics of the various tempers. The EAC behavior of Al 2195 is discussed in view of the previous results and the new experimental findings.

Nucleation, Growth and Coarsening of Spherical Precipitates in Aluminum Alloys

A model, compatible with a thermodynamic routine previously described [1], simulates homogeneous nucleation, growth and coarsening of spherical precipitates in aluminum alloys. This kinetic subroutine considers size distribution for each type of precipitate present in the matrix. Local equilibrium is assumed at the matrix-precipitate interface; the corresponding solubility product is corrected by the curvature effect. The equilibrium concentration of each solute element at the matrix-precipitate interface is solved by an iterative method so that the flux of each element is compatible with the solubility product and with the stoichiometry of the considered precipitate. Nucleation and growth of the different precipitates are solved numerically. Coarsening of precipitates is embedded in the equations via the curvature correction of the solubility product. For simple cases, i.e. presence of only one type of precipitate in the microstructure, the coarsening rate can be solved analytically. In that case, the numerical results are the same as the analytical equation results. An example of application will be presented for a binary Al-Li alloy.

Serrated flow in aluminium alloys containing lithium

Serrated-flow characteristics have been studied in binary Al Li-alloys with different compositions. Load-elongation curves seem divided into two sections of which the first is weakly, the second heavily, serrated. The initial weak serrations, which diminish with ageing, arise probably from the interaction of dislocations with lithium atoms in solid solution. It is proposed that the larger serrations which appear later on the load-elongation curve (if the elongation to necking is large enough to allow them to develop), and which increase in size with progressive ageing, are connected with shearable δ′-precipitates. The effects of zirconium and magnesium additions to Al Li-alloys on serrations have also been investigated. These elements do not change the basic mechanisms of serrated flow. However, the apparent activation energies as calculated for the alloy containing magnesium from the onset strains do not make much sense, because two elements contribute now to serrated flow, and the situation becomes complex.

A positron study on the microstructural evolution of Al-Li based alloys in the early stages of plastic deformation

The formation of voids by coalescence of microvoids initiated at precipitates has been proposed to explain the fracture mechanisms in alloys containing a large number of second phase particles whereas in binary Al-Li alloys with shearable particles the brittleness could be linked with the grain boundary fracture. Most of the microstructure studies of Al-Li alloys have been performed by deforming to fracture; however, little is known about the processes and mechanisms involved in the early stages of plastic deformation. Butler et al. have studied a quaternary Al-Li alloy and have found that there is a critical effective strain to cause voiding, which is about 0.06 and 0.1% for the aged and for the solution treated material respectively. It is very well established that positrons are very sensitive to vacancy-like defects. With the aim of clarifying the behavior of Al-Li based alloys in the very early stages of deformation, and detecting the eventual formation of microvoids, the authors have studied the response of the positron lifetime parameters to the degrees of deformation in age-hardenable Al-Li based alloys plastically deformed under tensile stress.

Simultaneous SAS and 100 Experiments on Phase Decomposition and Reversion in Al–Li Binary Alloys

Phase decomposition and reversion processes in Al–Li binary alloys have been studied by synchrotron-radiation small-angle/100 scattering experiments. The microstructure and its evolution obtained from small-angle scattering (SAS) and 100 profiles during phase decomposition and reversion are discussed. For the coarsening and reversion processes where a well defined interface between the δ′ precipitates and the matrix can be expected, the information obtained from the SAS and 100 profiles was essentially the same. On the other hand, the structural information they convey can be different in the early stage of phase decomposition. The interpretation of the SAS and 100 intensities by means of an extension of the two-phase model has been examined.

About yield stress of AlLi alloys at 0.5 to 295 K

The dependence of the yield stress of binary Al-Li alloys on the strain temperature has been studied in the range 0.5 to 295 K. It has been shown that as a result of ageing it becomes very weak. The temperature dependence obtained is regarded as a result of the superposition of interaction mechanisms of dislocations with impurity atoms and coherent, ordered δ'-precipitates.

Mechanism of serrated flow in binary Al-Li alloys

The work on serrated flow in Al-Li alloys has given rise to a controversy--whether serrations in these alloys are caused by lithium atoms in solid solution or by {delta}{prime}(Al{sub 3}Li)-precipitates. This controversy calls for further work to clarify the mechanism of serrated flow in the Al-Li alloys. Kumar and McShane have shown that in an Al-2.5Li-2Mg-0.14Zr alloy, non-shearable {delta}{prime}-precipitates, which are obtained in the under-aged and peak-aged conditions, might directly initiate serrated flow. However, the latter result was ambiguous because of the presence of other alloying elements, and the need to work on a binary Al-Li alloy was emphasized. The present work discusses the results from the binary Al-Li alloys.

Embrittlement in Recrystallised and Unrecrystallised Al-Li Binary Alloys

Embrittlement in Recrystallised and Unrecrystallised Al-Li Binary Alloys

Zener Relaxation in Al-Li Binary Alloys

Internal friction (I.F.) and modulus measurements have been carried out as a function of temperature in order to study the Li mobility in an Al - 8.1 at% Li - 0.03 at% Zr alloy. The results show that the sequence of δ'and δ precipitation can be followed through the evolution of the internal friction spectra. Two peaks appear in the internal friction spectrum. The low temperature peak Pz (450 K), has associated a dynamic modulus defect, an activation energy of 1.2 eV and the width of a Debye peak. The analysis of the behaviour of this relaxation allows us to identify the Pz peak with the Zener relaxation process. The relaxation parameters lead us to determine the activation energy and the diffusion coefficient of Li in Aluminium.

Inverse behaviour of the onset strain of serrated flow

The tests on Al-Mg and Al-Li-Mg-Zr alloys demonstrate that the cause of the inverse behavior, i.e., the onset strain of serrated flow increasing with temperature, above a certain temperature is not the precipitation during the tensile test of the specimen. It also appears that the shearable {delta}{prime}-precipitates may directly initiate serrated flow. However, more work on simple binary Al-Li alloy is needed to clearly establish the mechanism of serrated flow in the Al-Li alloys.

δ' nucleation and growth on low-angle dislocation boundaries in Al-Li

A transmission electron microscopy (TEM) investigation indicates that, at small undercooling, δ' (Al 3 Li) precipitates are restricted almost entirely within the strain fiield of dislocations in the early stages of aging and preferentially nucleate on edge dislocations and low-angle dislocation boundaries. The study is conducted on binary Al-Li alloys which are cold rolled to a 50% reduction in area. Two types of dislocation boundaries, i.e. simple tilt boundaries and a planar network of dislocations, are characterized by two-beam analysis. The role of the simple tilt boundaries on competitive growth kinetics of δ' precipitates is quantitatively examined. The experimental results support the related theoretical prediction on the effect of the simple tilt boundary stress field on solute diffusion. Concerning the dislocation network, δ' nucleates on dislocation node configurations where the reacted dislocation has a large edge component and at the site of the network plane where the dilatation stress is compressive. This is confirmed by calculation of the dilatation stress field of the dislocation nodes which also further clarifies and is consistent with the experimental observations

Positron Lifetime Study in Al-Li Alloys

Light alloys such as Al-Li binary alloys are very important from the technological point of view due to their mechanical properties. The Al-rich side of the Al-Li binary alloy for Li concentrations between 5% and 25% atomic contains at room temperature a two-phase region formed by the α terminal solid solution and the ordered metastable δ'-phase Al 3 Li. We present a systematic positron lifetime study for Li content below and above 5 at% (the appearance of δ'-phase) in the Al-rich side of the binary alloy to investigate the influence of Li content and also the influence of Zr addition. We also present the study of the formation and dissolution of δ' precipitates, the recovery behaviour of an electron irradiated alloy, aging studies to monitor Li losses, and the Li influence on the nucleation of defects in quenched alloys. The technique has proved to be very sensitive to Li-rich zones in the α solid solution and to the growth and dissolution of δ' precipitates due to the high positron affinity for Li in an Al matrix

δ Precipitation in an AlLiCuMgZr alloy

AlLi based [delta] phase has an NaTl structure (i.e., a diamond cubic) with a = 0.637nm and is an equilibrium phase in the binary Al-Li system. In heat treated binary Al-Li alloys of appropriate compositions, [delta] phase can format grain boundaries as well as within the grains. In commercially heat treated Al-Li-Cu alloys of 2090 specification, the grain boundary precipitate [delta] of the binary Al-Li system is replaced by a combination of T[sub 2](Al[sub 6]CuLi[sub 3]), R(Al[sub 5]CuLi[sub 3]) and T[sub 1](Al[sub 2]CuLi) phases. In similarly treated Al-Li-Cu-Mg alloys of 8090 specification, the copper rich T[sub 2] phase, present in the form of Al[sub 6]CuLi[sub 3[minus]x]Mg[sub x], is known to be the major coarse g.b. precipitate. The presence of an Al-Li-Cu-Mg based C phase at the grain boundaries of the commercially heat treated 8090 alloys has also been documented. No detailed study has yet been carried out to verify whether the [delta] phase can be present at the grain boundaries of the commercially heat treated 8090 alloys. Given the correlations between the g.b. phase morphology, g.b. phase chemistry, and the stress corrosion cracking resistance of these alloys, it is important that the g.b. precipitates be examined and identified. In thismore » paper results using TEM are presented to show that the [delta] phase can be present in varying amounts at the grain boundaries in an 8090 alloy when heat treated in the temperature range of 170--350 C. An examination is also made of the [delta] precipitation within the grain to establish that the T[sub 2]/[alpha]-Al interface is the dominant nucleation site for the noncoherent [delta] phase.« less

Effects of Li concentration and a Mg addition on serrated flow in AlLi alloys

Serrated flow phenomena have been reported in a variety of precipitation-strengthened aluminum alloys. In the particular case of precipitation-strengthened Al-Li alloys, serrated flow effects of similar character have been reported in binary Al-Li alloys and in commercial-type Al-Li alloys containing multiple alloying elements. Observations of serrated flow in binary Al-Li alloys indicate that the presence of Li alone is sufficient to produce serrated flow. Aging time has been used to probe the mechanisms that cause serrated flow in individual Al-Li alloys, and several investigators have noted that serrated flow disappears when Al-Li alloys are aged to peak strength or overaged. Much of the available experimental evidence supports dislocation-[delta][prime] interactions as the cause of serrated flow in Al-Li alloys, rather than dislocation-solute atom interactions to which serrated flow phenomena are traditionally attributed. Additional support for this conclusion could be provided by comparison of stress-strain curves for a solid solution Al-Li binary alloy of the same composition as the matrix phase of a precipitation-strengthened Al-Li binary alloy. The purpose of the present paper is to show stress--strain curves for Al-1.38Li, Al-1.80Li and Al-1.39Li-1.0Mg alloys, and to interpret the results in terms of the interactions proposed to account for serrated flow in Al-Limore » alloys.« less

Structure change of δ' precipitates during heating at a constant rate in Al-Li binary alloys

Structure change during heating preaged Al-Li binary alloy samples at a constant rate has been investigated by means of in-situ synchrotron radiation small-angle scattering (SR-SAXS) in order to discuss the differential scanning calorimetry (DSC) results. It ws found that the dissolution and the coarsening of δ . precipitates often occurred simultaneously, which affect the shape of the specific heat

The solidification behavior of 8090 Al-Li alloy

In this work, the solidification and segregation behaviors of 8090 Al-Li alloy have been investigated with differential thermal analysis (DTA) and the metallographic-electron microprobe method. The results show that 8090 Al-Li alloy has a much more complex solidification path than Al-Li binary alloy due to the addition of many alloying elements and the presence of impure elements. Solidification begins at about 635 °C with the reaction of L → α-Al + L′, and this reaction goes on to termination. The alloying element Cu and impure elements Fe and Si have a strong segregation tendency. During solidification, Cu segregates to the interdendrite and finally forms α-Al + T2 eutectic. As a result, the solidification temperature range is greatly extended. Iron and Si form the insoluble constituents Al7Cu2Fe, AlLiSi,etc., although their concentrations in the alloy are quite low. With the increase of Fe content, there is a eutectic reaction of α-Al/Al3Fe at about 595 °C. The formation of insoluble constituents is influenced by both concentrations of impure elements in the alloy and the cooling rate.

Ａｌ‐Ｌｉ合金の鋳造割れ性

Effects of Li content and grain-refiner addition on the hot-tearing characteristics of Al-Li binary word alloys (0-3mass%) and the commercial Al-Li alloys (2090, 2091, 8090 and 2219+1.5mass%Li) were investigated by using hindered type hot-tearing test (I-beam test). High strength alloys, 2024 and 7075 were also investigated to compare with the commercial Al-Li alloys. In Al-Li binary alloys, the tearing tendency was decreased with increasing Li content, but was not improved by the grain-refiner addition, and the efficiency of grain refinement was less than in the pure aluminum. In the commercial Al-Li alloys, the tearing tendency was same as 2024 and 7075 alloys in the case of no additioning grain-refiner, but was not so improved by the grain-refiner addition, while in 2024 and 7075 alloys remarkably improved, and the efficiency of grain refinement was less than in 2024 and 7075 alloys.