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.     

A study of work hardening in austenitic FeMnC and FeMnAlC alloys

Two austenitic FeMnAlC alloys with aluminium contents of 0 and 2.7 wt% were strained in tension between 193 and 823 K. Serrated stress-strain curves, inverse strain-rate dependence of flow stress, and high work hardening exhibited in particular temperature ranges for both alloys were characteristic of dynamic strain aging. The apparent activation energy for the onset of serration increased from 14.4 to 22.3 kcal/mol due to the addition of 2.7 wt% Al. It was found that the high work-hardening rate cannot be attributed to strain-induced deformation twinning when serrated stress-strain curves occurred. From the evidence of the present study and the known effect of aluminium on the diffusivity and activity of carbon in austenitic high-manganese steel, it is suggested that dynamic strain aging is the major cause of work hardening within the intermediate temperature range from 298 to 493 K for 0 wt% Al and 393 to 593 K for 2.7 wt% Al in the present austenitic FeMnAlC alloys.     

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.     

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.     

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.     

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.     

Stress affected bainitic transformation in a FeCSiMn alloy

Isothermal transformation experiments are reported in which the formation of bainitic ferrite occurs under the influences of stresses below the yield strength of the austenite. The response of the transformation was monitored by simultaneously measuring the longitudinal and radial transformation strains. This enabled the dilatational and deviatoric strain components to be deconvoluted from the total transformation strain. The data have been analysed by comparison with a theoretical model for the stress-assisted growth of bainite. The results confirm that the microstructure readily responds to stresses well below the yield strength of the parent phase. Furthermore, those crystallographic variants which are favoured by the stress grow first in the sequence of transformation. Experiments where the stress just exceeds the yield strength are also reported.     

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.     

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.     

Solidification paths of TiTaAl alloys

Microstructure evolution during solidification and high temperature phase equilibria were investigated for TiTaAl alloys in the vicinity of the 50 at.%Al isoconcentrate. Examination of dendrite morphologies and segregation profiles were used to deduce the phase sequencing during solidification and the boundaries of the relevant liquidi surfaces. In situ high-temperature X-ray diffraction and isothermal annealing experiments were conducted to determine the phases present at elevated temperatures and coupled with extensive characterization by transmission electron microscopy to elucidate the solid state transformations during cooling of the solidification microstructure. For approximately equiatomic TiAl, the primary phase selected from the liquid was α for the leaner Ta concentrations (< 7% Ta), as in the binary alloys of equivalent Al content, but changed to β with increasing Ta levels (> 10 %). With increasing Al and Ta the α liquidus penetrates deeply into the ternary. The interdendritic segregate was always γ. Dendrites of the β-forming alloys were heavily segregated leading to different microconstituents in the core and bulk dendrite regions during post-solidification cooling. In alloys with < 15 % Ta, (α₂ + γ_t) lath formed in the dendrite bulk due to the decomposition of α, with σ precipitating in the core (> 10% Ta). With increasing Ta levels the lath is gradually replaced by polycrystalline γ which grows into the dendrite bulk, and the core decomposes into a lamellar (γ + σ) microstructure from the decomposition of σ. The γ segregate does not transform further.     

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.