Hydride formation in an AlLiCu alloy

A hydride phase, LiAlH₄, has been identified in Al-2.0%Li-2.2%Cu alloy electrochemically charged with hydrogen. This is a hydride having the composition of LiAlH₄. The orientation relationship of the hydride and the matrix has been determined and rationalized with the O lattice theory. The thermodynamic stability of the hydride is discussed and possible formation mechanisms explained. The hydride forms from the grain boundary phase, AlLi, as the maximum amount of available Li is already present in the AlLi (δ) phase.     

Microstructure and shear band formation in rolled single crystals of AlMg alloy

The development of dislocation structures in the (211)[1̄11] single crystals of Al-3%Mg has been investigated in relation to shear band formation after rolling at room temperature and 473 K. During rolling at RT, the dislocation structures which developed with increasing strain were characterized by homogeneously distributed diffuse cells (ϵ ≲ 0.35), aligned microbands (ϵ ∼- 0.50) and shear bands (ϵ ≳ 0.50). The shear bands nucleated within the clusters of the microbands. In contrast to this, long dislocations with homogeneous distribution (ϵ ≲ 0.40) and subboundaries as well as subgrains (ϵ ≳, 0.40) were dominant microstructures at- 473 K rolling. No shear bands were obtained from these microstructures. These results were discussed in terms of the effects of rolling temperature, solute atoms and slip geometry of the AlMg alloy.     

Structure-property relationships in AlLiCuMgAgZr alloy X2095

Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al₃Li) particles and reduced subgrain boundary T₁ precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength.     

Precipitation and dissolution kinetics in AlLiCuMg alloy 8090

Resistivity and differential scanning calorimetry (DSC) techniques are used to study the kinetics of precipitation and dissolution of GPB zones and metastable phases (δ′ and S′) in AlLiCuMg alloy 8090. Three stages of the precipitation sequence during aging have been analyzed. A new analytical method is developed for the DSC technique, which requires only one heating rate to obtain the kinetic parameters, and the results show good agreement with the more conventional method of varying heating rate. The low-temperature endothermic peak in the DSC thermogram is interpreted as the dissolution of Li-bearing zones, which is supported by the hardness results. The activation energy, Q, and the growth parameter, n, determined by resistivity and DSC techniques are in good agreement with previously published data.     

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.     

Creep behavior and substructure in an AlLi alloy

The effect of stress on the creep behavior of an Al-2.1 wt% Li alloy was investigated in the solid-solution range at 773–853 K. In addition, the dislocation substructure that developed during creep was examined by means of transmission electron microscopy. The mechanical data show that under the experimental conditions used in this investigation, the creep curve exhibits an extensive normal primary stage before steady-state creep is reached; that after a stress reduction at the steady-state, the creep rate in the transient region is lower than the steady-state creep rate at the reduced stress; and that the stress exponent is ∼ 4.6. The substructural data reveal a strong tendency to form subgrains; this tendency is demonstrated by the presence of triple junctions of subboundaries at low stresses and by the observation of groups of well-developed subgrains at high stresses. The creep characteristics of Al-2.1 wt% Li are compared to those of Al and are examined in the light of suggestions and predictions regarding deformation processes in solid-solution alloys.     

Grain-boundary precipitation in an Al4.0Cu0.5Mg0.5Ag alloy

Transmission electron microscopy and electron diffraction were used to study grain-boundary precipitation in an Al-4.0Cu-0.5Mg-0.5Ag (wt%) alloy. Low-angle grain-boundaries were found to nucleate Ω precipitates on the {111}_α planes even when the {100}_α habit planes of the competinng θ′ metastable phase were closer to the grain-boundary plane. High-angle grain-boundaries, which were random in nature and had relatively large energy, nucleated Ω precipitates predominantly. A few S precipitates and a θ precipitate (G IV/V orientation) were found to co-exist with Ω in these boundaries. The proximity of the grain-boundary plane to the {111}_α plane on which grain-boundary Ω nucleated was found to be particularly important in both low and high-angle grain-boundaries, similar to results for the θ′ phase in AlCu alloys.     

Aging behavior of an Al−Li−Cu−Mg−Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain, boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ost-wald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ (Al₂CuMg orthorhombic) particles, and the dissolution of both δ and S compounds into the matrix, respectively. SADAYOSHIITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University     

Effects of changes in strain path on work-hardening in CP aluminium and an AlCuMg alloy

Effects of abrupt changes in the direction of plastic deformation on work-hardening behaviour have been investigated in two-stage stretching of sheets of AA1050 and a heat-treated 2014 aluminium alloy aged at various temperatures up to 300°C. The results show that reorganisation of dislocation distribution after a change in strain path can result in transient changes in work-hardening behaviour of two kinds. Changes of the first kind, which tend to increase the hardening rate in early stages of the second mode of deformation, are associated with reorientation of internal stresses. Changes in the second kind, which tend to cause transient reductions in hardening rate, are believed to be associated with partial dissolution of the original dislocation substructure. The relative magnitudes and strain dependencies of these two kinds of change depend on the deformation sequence and on material variables. The change in hardening rate of CP aluminium after a change in strain path is dominated by changes of the second kind which, after moderate prestrains, cause reductions in the limits of stable elongation. In similar tests on overaged conditions of the 2014 alloy the overall changes in hardening rate are dominated by changes of the first kind, so that the limit of uniform elongation is increased by a change in strain path. When dynamic ageing is active in the 2014 alloy changes of the second kind can be suppressed so that reductions in the hardening rate do not occur.     

Strain rate sensitivity and transient behaviour in an AlMgSi alloy

The effect of strain rate changes on the flow stress of an AlMgSi alloy at strains prior to the onset of the Portevin-Le Chatelier effect has been studied. Measurements of the instantaneous and steady state strain rate sensitivity of the flow stress and the flow stress transient period were carried out as a function of strain and strain rate. Negative values of the steady state strain rate sensitivity were reached prior to the onset of localised yielding, which occurred by the growth of local strain rate oscillations. The results are interpreted in terms of a constitutive flow model which takes into account time dependent changes in the local solute composition at dislocations accompanying a strain rate change.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

Microstructure and mechanical properties of a 2091 AlLi alloy—III. Quantitative analysis of portevin le chatelier instabilities and relation to toughness in AlLi,AlCuMg and AlLiCuMg (2091) alloys

The microstructure and mechanical properties of a 2091 alloy are studied and compared to simpler AlLi and CuMg alloys. For ageing times between 6 and 24 h at 150°C, the 2091 alloy exhibits a toughness drop and a simultaneous change in PLC characteristics (as evidenced by a combination of local and total strain measurements), but no significant change in microstructure, except for the size of δ′ precipitation. SEM in situ tests show that plastic instabilities are always related to extra damage. A quantitative model accounts for the toughness drop, based on plastic dissipation by PLC active bands.     

The effect of temperature on strain rate sensitivity in an AlMgSi alloy

The effect of temperature on strain rate sensitivity has been studied in an AlMgSi alloy in the range of dynamic strain ageing. Two regimes of behaviour were observed which depended on the temperature. At low temperatures (T <- 303 K) it was possible to correlate all strain rate sensitivity measurements with a single strain/strain rate/temperature parameter. In the high temperature regime the temperature and strain rate dependence of the strain rate sensitivity deviated significantly from that measured at low temperatures. The two regimes of behaviour were found to correspond with the normal and inverse Portevin-Le Chatelier effect.     

The structure of G.P. zones in an AlCuZn alloy

An Al-46 at.% Cu-2.20 at.% Zn crystal was quenched and aged at 353 K for 24 h. Anomalous dispersion was used to separate the effects of Cu and Zn in the diffuse scattering by measuring the diffuse scattering in absolute units at two different energies for each respective absorption edge. The scattering contrast between these two conditions near each edge was analyzed to get the short range order parameters for AlCu and AlZn pairs. Via computer simulation with these parameters the Cu was found to lie in small {100} plates and the Zn in short 〈110〉 strings that probably lie on planes adjacent to the Cu plates.