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.     

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.     

Study of the new Frank-Kasper phases in AlLiCuMg alloys

A new group of Frank-Kasper phases has been observed in AlLiCuMg alloys using high resolution electron microscopy (HREM). The Y phase, which has a face-centre cubic structure with a = 2.0 nm, occurs in grain boundaries during precipitation heat treatment of supersaturated solid solution. Considerable amount of intrinsic faults, extrinsic faults and microtwins were observed in the Y phase. A new domain denominated as D intergrowth with Y phase along {111} planes has been found. By combination of Y phase and D domain, new domains can be constructed.     

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.     

Ageing kinetics of a silicon carbide reinforced AlZnMgCu alloy

The ageing kinetics of a composite of an AlZnMgCu powder alloy (“CW67”) combined with a varied volume fraction of particulate silicon carbide were investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). DSC revealed that the maximum rate of precipitation of the metastable η′ phase was substantially lower for CW67/SiC/20p than for the unreinforced alloy or CW67/SiC/10p. TEM of isothermally aged material revealed differences between the unreinforced alloy and composites in respect of precipitate size and morphology. We conclude that SiC additions, by dint of additional dislocations generated during quenching, can affect the ageing of CW67 either by accelerating the nucleation of precipitates or by accelerating precipitate growth. The ageing rate in CW67/SiC/20p was increased by accelerating both the nucleation of precipitates and growth, whereas the ageing in CW67/SiC/10p was enhanced by accelerating precipitate growth only.     

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.     

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.     

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.     

The fracture and shear band formation in an AlCuLiMgZr alloy

The tensile properties of an Al3.2Cu1.6Li1.1Mg0.3Zr alloy with different cold rolling before peak aging were studied. The strength of the material reaches a maximum with a moderate reduction in thickness by rolling. For heavily cold rolled and aged alloy, when tensile tested in the transverse direction, both the strength and elongation are higher than in the longitudinal direction. When tested in the longitudinal direction, two sets of shear bands formed in the specimen and wedge type tensile fractures were observed to occur along the shear bands irrespective of the change in width/thickness ratio of the specimens. When tested in the transverse direction, the fracture was intergranular “woody” type, and no shear bands were observed. An explanation based on the assumption of the {110} 〈001〉 texture development in the alloy with the rolling operation was given to rationalize satisfactorily all the observations in this study.     

Investigation of thermoelastic martensitic transformation in a CuZnAl alloy

Thermoelastic martensitic transformation in a Cu-29%Zn-3%Al alloy was investigated experimentally and theoretically. The phase structures and morphological changes occurring during transformation were studied using optical microscopy and high voltage TEM (1000–1200 kV), both equipped with combination heating and cooling stages. A “single crystal pure shear” experiment was designed to measure the relationship between M_s and shear stress, and from these data changes of enthalpy and entropy of the transformation were calculated through the Clausius-Clapeyron equation. The changes of enthalpy and entropy were also obtained by using calorimetric measurement. Both results were in good agreement. The kinetic behavior of the transformation in polycrystalline alloy as a function of applied stress was followed using electric resistance measurement. The slope of the transformation rate was constant over the range of 20–70% transformed, which corresponds physically to transformation occurring by plate growth in the unpartitioned parent phase or equivalently, the interphase boundary “moving freely.” A phenomenological theory was suggested to describe the constant slope portion of the transformation. Here, phase boundary motion is related to thermal hysteresis and quantitatively described the effect of applied stress on the transformation behavior.     

Precipitate stability in AlCuMgAg alloys aged at high temperatures

A study has been made of the thermal stability of the Ω phase in AlCuMgAg alloys aged at high temperatures (200 to 350°C). This phase, which precipitates as thin plates on the {111}_α planes, has been shown to be replaced by the equilibrium precipitate θ (Al₂Cu) after prolonged ageing (e.g. 2400 h at 250°C). Measurements have been made of the thickening behaviour of the Ω plates and the various orientations and morphologies of the θ phase have been characterised. Whilst there is some evidence for the direct allotropic transformation of Ω to θ, it is concluded that a gradual dissolution/re-precipitation mechanism dominates the changes to microstructure at these high temperatures. Although magnesium and silver are known to segregate to the Ω phase, they were not detected in association with θ. Rather they were found to partition to sites of the S phase (Al₂CuMg) which forms as a minor precipitate under these ageing conditions.