Barrington Muddle

Combined electron beam imaging and ab initio modeling of T1 precipitates in Al–Li–Cu alloys

Among the many considerable challenges faced in developing a rational basis for advanced alloy design, establishing accurate atomistic models is one of the most fundamental. Here we demonstrate how advanced imaging techniques in a double-aberration-corrected transmission electron microscope, combined with ab initio modeling, have been used to determine the atomic structure of embedded 1 nm thick T1 precipitates in precipitation-hardened Al–Li–Cu aerospace alloys. The results provide an accurate determination of the controversial T1 structure, and demonstrate how next-generation techniques permit the characterization of embedded nanostructures in alloys and other nanostructured materials.

The bonding electron density in aluminum

A combination of microscopy and first-principle calculations is used to study the bonding charge density in aluminum. Aluminum is considered to approach an “ideal” metal or free electron gas. The valence electrons move freely, as if unaffected by the presence of the metal ions. Therefore, the electron redistribution due to chemical bonding is subtle and has proven extremely difficult to determine. Experimental measurements and ab initio calculations have yielded substantially different results. We applied quantitative convergent-beam electron diffraction to aluminum to provide an experimental determination of the bonding electron distribution. Calculation of the electron distribution based on density functional theory is shown to be in close agreement. Our results yield an accurate quantitative correlation between the anisotropic elastic properties of aluminum and the bonding electron and electrostatic potential distributions.

Ductile-to-brittle fracture transitions in 8090 Al–Li alloys

Abstract Variables influencing transitions from ductile transgranular fracture to brittle intergranular fracture in a very underaged 8090 Al–Li–Cu–Mg–Zr alloy plate, in which there were no grain-boundary precipitates or precipitate-free zones, have been studied. For the short-transverse crack-plane orientation, ductile-to-brittle transition temperatures increased with increasing ageing times, and were lower for more strongly textured material. Slip was more uniformly distributed at lower temperatures, suggesting that localised planar slip was not a factor contributing to embrittlement. The activation energy for embrittlement was 84±5 kJ/mol for most conditions, which is consistent with embrittlement due to lithium diffusion to grain boundaries and subsequent segregation. There were also notable similarities with segregation-induced fracture in other alloys. The ductile-to-brittle fracture transitions are possibly associated with changes in atomic structure at grain boundaries below a critical temperature that is dependent on the degree of lithium enrichment at grain boundaries and the grain-boundary misorientation.

Assisted nucleation of θ′ phase in Al–Cu–Sn: the modified crystallography of tin precipitates

The formation of particles of elemental tin in association with the nucleation of the precipitate phase θ′ in an Al–1.7u2009at.%u2009Cu–0.01u2009at.%u2009Sn alloy has been investigated by high resolution transmission electron microscopy. Analysis of lattice images has demonstrated that these tin particles associated with θ′ platelets formed during short-term ageing (typically 3u2009min at 200°C) exhibit a crystallographic form that is distinctly different from that previously reported in such ternary alloys and also from that observed in the binary alloy Al–0.01u2009at.%u2009Sn. Those tin particles that could be clearly identified as β-Sn phase shared a crystallographic relationship of the form (001)Sn‖(001)Al‖(001)θ′, [110]Sn‖[100]Alu2009‖u2009[100]θ′ with the α-Al and θ′ phases with which they were simultaneously in contact. The observations imply that, while tin may assist nucleation of platelets of θ′, the formation of θ′ equally influences the crystallography of β-Sn that forms in association with θ′. Following prolonged ageing (typically 50u2009h at 200°C), the β-Sn particles in contact with θ′ plates had coarsened and diminished in number, and most now exhibited an orientation relationship with α-Al matrix similar to that observed in the binary alloy Al–0.01u2009at.%u2009Sn (i.e. (100)Sn ) and to that previously observed in the later stages of ageing of the ternary alloy Al–1.7u2009u2009at.%u2009u2009Cu–0.01u2009at.%u2009Sn.

Mechanisms of brittle intergranular fracture in Al-Li alloys and comparison with other alloys

Abstract Evidence that brittle intergranular fracture in Al-Li alloys is associated with segregation of lithium, and maybe the formation of two-dimensional grain-boundary phases, is briefly reviewed. Particular attention is drawn to similarities between embrittlement of Al-Li alloys and segregation-induced embrittlement in other materials such as steels. High-resolution fractographic studies of Al-Li alloys to elucidate the atomic mechanisms of brittle intergranular fracture are then described. Brittle intergranular fracture surfaces were often featureless when examined by scanning electron microscopy, but transmission electron microscopy of replicas often revealed very shallow dimples, indicating that crack growth occurred by a very localized microvoid-coalescence process. The observations are explained in terms of decreased cohesion across grain boundaries due to lithium enrichment and grain-boundary structural changes. Low cohesion may facilitate void nucleation by decohesion (in the absence of grain-boundary precipitates in very underaged alloys) and may promote void growth by facilitating the emission of dislocations from crack tips. The applicability of the proposed mechanism to brittle intergranular fracture in other materials is also briefly discussed.

Atom probe analysis of early-stage strengthening behaviour in an Al-Mg-Si-Cu alloy.

The strengthening of an Al-Mg-Si-Cu alloy during natural ageing and subsequent short artificial ageing was investigated using three-dimensional atom probe (3DAP) analysis and tensile testing. The contingency table and Markov chain analyses confirmed that non-random arrangements of atoms already exist after a natural ageing time of only 3.5h. Extensive use of particle analysis tools in the IVAS and PoSAP software packages revealed that whilst the commonly used minimum aggregate size (N(min)) of 10 is a reasonable choice, much more useful information about the system can be gained by additionally employing a wide range of larger and smaller N(min) values. In particular, it was found that the density and volume fraction of solute aggregates increased with increasing natural ageing time in the T4 condition. After a 0.5h artificial ageing treatment at 170 °C (designated as T6), the size, volume fraction and Mg/Si ratio of the aggregates were all found to decrease with increasing prior natural ageing time. These findings are used to discuss the detrimental effect of natural ageing, where the T6 strength has been observed to decrease rapidly with increasing prior natural ageing time before stabilising after several hours of natural ageing.

Precipitate assemblies formed on dislocation loops in aluminium-silver alloys

A detailed study of the precipitation of the ( ) phase in undeformed aluminium-silver alloys has been conducted. Several previously unreported features were observed, including the formation of discrete three-dimensional assemblies composed of five–seven precipitates on faulted dislocation loops. The precipitate assemblies adopted a morphology approximating a tetrahedral bipyramid. The bounding defect of the stacking fault was found to control both the nucleation of additional precipitates, which was responsible for the formation of the assembly structure, and also the growth characteristics of individual precipitates with these assemblies.

Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Abstract Precipitation of the γ ′(AlAg 2 ) phase was investigated in Al–Ag(–Cu) alloys using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of Ag to stacking faults, followed by thickening in steps corresponding to single unit cell height ledges. In conjunction with gradual segregation of Ag and Al into ordered layers, this yielded γ ′ phase platelets with a thickness of either 2 or 3xa0×xa0the AlAg 2 c -lattice parameter. Plates with a thickness of 2 c (AlAg 2 ) could not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable Ag segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.

A Pseudoboehmite-Silane Hybrid Coating for Enhanced Corrosion Protection of AA2024-T3

This work reports the performance of pseudoboehmite (PB)-silane hybrid coatings for the corrosion protection of AA2024-T3. A survey of the coating pretreatment and treatment steps was undertaken to reveal the electrochemical impact of such treatments on AA2024-T3 and its constituents, with testing also done on model microstructural analogs including Al-4Cu, Al2CuMg , and Al7Cu2Fe . The hybrid coating performs very well in immersion and salt-spray conditions. The protective effect of the PB pretreatment is not additive but synergistic. PB surface layers could be integrated as a precursor to subsequent coatings owing to the functionality they can impart to the surface.

Differential quantitative analysis of background structure in energy-filtered convergent-beam electron diffraction patterns

Measurements of electronic structure in solids by quantitative convergent-beamelectrondiffraction(QCBED)willnotreachtheirultimateaccuracyorprecisionuntil the contribution of the background to the reflections in energy-filteredCBED patterns is fully accounted for. Apart from the well known diffusebackground that arises from thermal diffuse scattering of electrons, there is acomponent that has a much higher angular frequency. The present work reportsexperimental evidence that this component mimics the angular distribution oftheelastically scatteredelectrons within each reflection. A differentialapproachto QCBED is suggested as a means of quantitatively accounting for thebackground in energy-filtered CBED data.1. IntroductionThe high accuracy and precision of electronic structuremeasurements in inorganic solids with a high degree of crystalperfection by quantitative convergent-beam electron diffrac-tion (QCBED) is now well established (Zuo et al.,1988;BirdSZuo,1993;Deiningeretal.,1994;Holmestadetal.,1995;PengZSaundersetal.,1995,1996,1999;Zuo et al.,1997,1999;TsudaTStreltsovet al.,2001, 2003; Tsuda et al.,2002;Jianget al.,2003;Ogataet al.,2004; Friis, Madsen et al.,2003;Friis,Jianget al.,2003;Jiangetal.,2004;Friiset al.,2004,2005;Nakashima,2005,2007).Suchmeasurements are precise enough to allow meaningfulcomparisons of experimentally measured electronic structurewith different ab initio theoretical models (Zuo et al.,1997,1999; Saunders et al.,1999;Friis,Madsenet al.,2003;Jiangetal.,2003,2004;Friiset al.,2004,2005;Nakashima,2005),including those derived from density functional theory andperiodic Hartree–Fock, Dirac–Fock and linear combination ofatomic orbitals calculations.QCBED originated in 1940 (MacGillavry, 1940) withsporadic application until the late 1980s (Goodman L Voss et al.,1980)whenthedevelopmentofenergy-filtering optics for transmission electron microscopesresulted in a strong revival of the technique (Zuo et al.,1988,1997, 1999; Bird & Saunders, 1992; Zuo, 1993; Holmestad etal.,1995;PengZSaunderset al.,1995,1996,1999;Tsuda & Tanaka, 1999; Streltsov et al.,2001,2003;Tsudaet al.,2002; Jiang et al.,2003;Ogataet al.,2004;Friis,Madsenet al.,2003; Friis, Jiang et al.,2003;Jianget al.,2004;Friiset al.,2004,2005; Nakashima, 2005). The ability to exclude almost all ofthe inelastic signal by energy filtering, coupled with highdynamic range digital signal detection (via CCDs) and thecontinuous expansion in computing power, allowed unprece-dented analysis of experimental CBED data via patternmatching based on elastic scattering theory. As a result, therefinement of Fourier coefficients of the crystal potential(structure amplitudes or structure factors) during the pattern-matching process became sufficiently precise and accurate tobecomparabletothemostpreciseandaccuratemeasurementsever made by X-ray diffraction (Dawson, 1967; Kato, 1969;Hart & Milne, 1970).Even so, QCBED has not reached its full potential becauseenergy filtering does not remove all components of a CBEDpattern that are unaccounted for by elastic scattering theory.Thermaldiffuse scattering (TDS) ofelectrons results in energylossesoflessthan0.1 eV,wellbelowthe resolution ofthemostmodern energy filters and also below the spread in energiesfrom the latest monochromated electron sources. Argumentsagainst applying calculations incorporating a full treatment ofTDS because of their nearly prohibitive cost in computingpower and time (compared with purely elastic scatteringcalculations) will eventually fade with the advent of newsupercomputing technologies such as graphics processingunits. A growing number of CBED calculations incorporateTDS (Rossouw et al.,1990;Loaneet al.,1991;Wang,1992;Muller et al.,2001;Omotoet al.,2002;Dwyer,2003,2005;Dwyer & Etheridge, 2003), with Omoto et al. (2002) illus-tratinghowTDScalculationscanbeincluded(non-iteratively)in QCBED. However, there is very little information in theliterature about the structure of the background associatedwith each disc in a CBED pattern. Most calculations deal onlywiththetotalsignalinthepattern,withoutseparatingtheTDSand elastic components. Omoto et al. (2002) examined thebackground due to TDS experimentally and theoretically inthe absence of the CBED pattern, but without comment as to