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A new methodology for the characterization of solute clusters leading to compositional fluctuations is presented and discussed. The methodology makes use of contrast variation arising from a combination of small-angle scattering using neutrons and X-rays, and adapts a model for solute correlation to extract the chemistry and length scale of clustered states after quench and after natural ageing. In three subsets of the Al-Cu system, Cu-rich clusters are reported for all cases. The presence of Mg strongly enhances Cu clustering in the naturally aged state and results in more than double the number of clusters in the complex Al-Cu-Li-Mg system. The results are compared with those obtained using atom probe tomography. [ABSTRACT FROM AUTHOR]

We apply a combinatorial approach to study the influence of Mg concentration on the precipitation kinetics in an Al–Cu–Li alloy using a diffusion couple made by linear friction welding. The precipitation kinetics is monitored in the composition gradient material using simultaneous space and time-resolved in-situ small-angle X-ray scattering measurements during ageing, and the strengthening of the precipitates is evaluated by micro-hardness profiles. This data provides an evaluation of the amount of Mg necessary to promote precipitation of the T 1 –Al 2 CuLi phase. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we report a quantitative study, using small-angle X-ray scattering, of the precipitation kinetics during ramp heating and isothermal ageing in an AA7075 aluminium alloy processed by high-pressure torsion. The precipitation behaviour has been compared with that of the same material processed in a conventional manner and observations are supplemented by transmission electron microscopy for precipitate and grain size characterization using automated crystal orientation mapping. After severe plastic deformation and natural ageing, the material is shown to contain a high density of GP zones. During ageing, the precipitate size distribution becomes bimodal, with small precipitates behaving similarly to those of the conventionally processed material and large ones associated with the crystalline defects and reaching large sizes at considerably lower temperatures and shorter times as compared to the conventionally processed material. [Copyright &y& Elsevier]

Abstract: State-of-the art atom probe tomography (APT) combined with transmission electron microscopy (TEM) were used to investigate the microstructure at different stages of the ageing process of an alloy of composition (at%) Al–1.68%Cu–4.62%Li–0.33%Mg–0.1%Ag. These alloys were shown to exhibit a complex microstructure of T1 plates and several metastable phases, including θ′ and S. We will highlight the early stages of clustering, precipitate interactions and possible solute segregation at the matrix/precipitate interfaces and detail the chemical composition of the different phases. [Copyright &y& Elsevier]

Abstract The aging response of two Al-3Mg alloys with Cu addition < 1 wt% has been tracked under simulated automotive paint bake conditions (∼ 20 min, 160 and 200 °C) to quantify the processes controlling hardening. The decomposition of the solid solution, observed by atom probe tomography, has been interpreted using a novel pair correlation function approach and incorporated into a model for prediction of precipitation hardening. It is shown that the hardening is controlled by clusters/Guinier-Preston-Bagaryatsky (GPB) zones similarly to what has been previously observed in much higher Cu containing 2XXX-series alloys. Interestingly, it is shown that very small additions of Cu (< 0.1 at%) can be used to catalyze a high number density of strengthening particles owing to the high enrichment in Mg compared to particles found in more conventional high Cu/low Mg alloys. This allows hardening during the first hour of aging that is as high as that obtainable in these high Cu alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Room temperature solute clustering in aluminium alloys, or natural ageing, despite its industrial relevance, is still subject to debate, mostly due to its experimentally challenging nature. To better understand the complex multi-constituents’ interactions at play, we have studied ternary and quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and Al-Cu-Li-Mg. We used a recently introduced correlative technique using small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the chemically resolved kinetics of room temperature clustering in these alloys, which we completed with DSC and micro-hardness measurements. The comparison of the clustering behaviours of each subsystem allowed us to highlight the paramount role of Mg as a trigger for diffusion and clustering. Indeed, while a strong natural ageing was observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very slight addition of Mg (0.4%) to this system, however, drastically changed the situation to a rapid formation of essentially Cu-rich hardening clusters, Mg only joining them later in the reaction. This diffusion enabling effect of Mg is discussed in terms of diffusion mechanism and complex interactions with the quenched-in vacancies. [ABSTRACT FROM AUTHOR]

The effect of Mg content on the natural ageing of Al-Cu-Li-(Mg) alloys has been investigated on a compositionally graded material made by linear friction welding, allowing to probe Mg contents between 0 and 0.4 at.%. High throughput time- and space-resolved characterization of natural ageing kinetics has been achieved using small-angle X-ray scattering, supplemented by differential scanning calorimetry. Natural ageing results mainly in the formation of Cu-rich clusters whose characteristics strongly depend on the presence of Mg. Larger precipitates of 2 nm size also form to a lesser extent above a Mg concentration of 0.1 at.%. [ABSTRACT FROM AUTHOR]

The use of high strength Al-Zn-Mg-Cu alloys in automotive components is restricted by their low formability. Warm forming in the under-aged state improves formability, but induces a strain-dependent microstructure evolution due to dynamic precipitation. We present a new methodology to quantitatively evaluate the dynamic precipitation strain dependence which is applicable to strain rates relevant to forming. The plastic strain is varied using a tapered tensile sample and the precipitation state is measured in terms of size and volume fraction using spatially resolved small-angle X-ray scattering, showing its evolution during straining is dependent on the imposed strain level and rate. [ABSTRACT FROM AUTHOR]

Abstract: The plastic behaviour of two recently developed Al–Cu–Li alloys with different Cu/Li ratios, AA2198 and AA2196, was studied as a function of precipitation state by tensile tests, strain hardening rate analysis, Bauschinger tests, slip line imaging and atomic scale characterization of precipitate shearing by advanced electron microscopy. For early ageing times where δ′ and/or solute clusters are present, plasticity occurs heterogeneously, the strain hardening rate is high, and the Bauschinger effect is moderate. When T1 precipitates are present, plasticity becomes more homogeneous, and the strain hardening rate shows a slower decrease compared with other aluminium alloys containing shearable precipitates. In addition, these ageing conditions show a high reversibility of plastic strain, and it is observed that precipitates are sheared only by single steps. The possibility of a specific shearing mechanism of the T1 phase is discussed in light of these results. [Copyright &y& Elsevier]

Abstract: The impact of the Cu/Li ratio on the sequence and kinetics of solid-state precipitation is studied for two recently developed Al–Cu–Li–Mg–Ag alloys: AA2198 and AA2196. A quantitative evaluation of the alloy microstructure is carried out using small-angle synchrotron X-ray and neutron scattering, conventional and aberration-corrected (scanning) transmission electron microscopy and differential scanning calorimetry. It is shown that small modifications to alloy chemistry profoundly change the phases formed during natural ageing: Cu-rich clusters in the Li-lean alloy and the δ′(Al3Li) phase in the Li-rich alloy. Despite this difference in early ageing, the peak aged microstructures of the two alloys are similar, dominated by high-aspect-ratio T1 plates with a thickness of 1.3nm and a diameter up to 50nm. However, the incubation time for nucleation and saturation size after growth of the precipitates is found to be significantly different between the two alloys. Small amounts of other phases from the binary Al–Cu sequence are also observed. Mechanisms of formation of the T1 phase are discussed in view of these experimental results. [Copyright &y& Elsevier]

Abstract: The kinetics of the microstructural evolution during ageing at 200°C of an Al–2.5%Cu–1.5%Mg (wt.%) alloy is evaluated using nuclear magnetic resonance and in situ small-angle X-ray scattering. This alloy is known to exhibit “rapid hardening” upon artificial ageing, where 50–70% of the total age hardening increment is reached within minutes at elevated temperatures. It is shown that formation of small solute-rich entities which correspond to Cu–Mg clusters or GPB zones occurs within seconds at 200°C. These entities have radius ∼0.5nm, volume fraction ∼2% and Cu concentration ∼30 at.%. Within the stage of constant yield strength (rapid hardening plateau) at this temperature, they are shown to be the dominant constituent of the precipitate microstructure and to be extremely stable in terms of size and volume fraction. The S phase is observed to nucleate within the hardening plateau, but remains a minor constituent until the end of this stage. Based on the quantitative measurement of their size and volume fraction, the obstacle strength of these solute-rich clusters is calculated to be about one-tenth of the Orowan strength. The energy required to shear these clusters (∼0.5eV) is shown to be compatible with existing atomistic calculations of similar objects. [Copyright &y& Elsevier]

Abstract: The structure of T1 precipitates which formed under ageing in Al–Li–Cu alloys has been studied using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging. Complementary information on the precipitate composition has been obtained on the same system by small-angle X-ray scattering (SAXS). According to HAADF-STEM imaging, the currently accepted structures based on the stacking of {111} type planes are ruled out. It is shown that the T1 precipitate structure is consistent with the one derived from X-ray diffraction study of the bulk T1–Al2LiCu phase. However, for this very thin plate shape precipitates (thickness less than one cell), deviations to the Al2LiCu stoichiometry may occur. A structural analysis based on local environment and packing rules classical of complex phases is proposed to make the connection between the atomic structure and precipitation features like nucleation and interface with the matrix. [Copyright &y& Elsevier]

Increasing demand for a reduction in fuel emissions in passenger vehicles has generated the need for lighter weight materials to be used in automobile manufacture for body-in-white applications. Aluminium alloys in the 6000-series, containing Mg and Si are ideal candidates for these applications but lack the formability found in commonly used steels, providing a need to more fully understand the factors influencing the formability of these alloys at high strains. Conventionally, a high strain rate sensitivity (SRS) is tied to increased formability because it retards the increase in the local strain rate found in the diffuse neck interior. However, most experimental work neglects that the regions exterior to the neck will undergo a local decrease in the strain rate which causes a corresponding material softening. Observations of an asymmetry between up-change and down-change SRS of these alloys in the natural aged condition show that different mechanisms are controlling the SRS depending on the direction of rate change. Following a characterization of the state of clustering by differential scanning calorimetry, continuous tensile and precision strain rate sensitivity testing results are presented, elucidating the differences between the up-change and down-change SRS tests. It is shown that these differences are due to the activation of different thermal obstacles during the two directions of rate changes. The role of a change in Si content on the mechanical properties is explored and its suspected role on the asymmetric SRS is discussed. Image 1 [ABSTRACT FROM AUTHOR]

Abstract: The precipitation behaviour in an Al–5wt.% Mg–1.8wt.% Li alloy has been systematically investigated for both isothermal and non-isothermal heat treatments, using atom probe tomography, transmission electron microscopy and in situ small-angle X-ray scattering. In the investigated temperature range, the δ′ phase is shown to form, with a Mg content similar to that of the alloy, and a Li content of 18.5at.%. The quantitative data on isothermal and non-isothermal kinetics from the in situ experiments are compared to the outcome of a precipitation class model adapted to the case of high volume fractions. The model parameters suggest that the main effect of Mg on the precipitation of the δ′ phase in Al is to lower the solubility of Li in the α-Al solid solution, with little effect on the interfacial energy of the precipitates or on the Li diffusivity in Al. [Copyright &y& Elsevier]

[Display omitted] The softening and strengthening contributions in pre-deformed and aged Al–Mg–Cu alloys containing 3 wt.%Mg and 0.5 wt.%Cu are evaluated by a combination of microscopy, mechanical testing and modelling. A refined phenomenological model for the work hardening response, accounting for the separate effects of recovery and precipitation, is shown to be suitable for an unambiguous determination of the precipitation hardening contribution in these alloys. Significantly, it is found that the mechanical response of these alloys is not strongly impacted by Cu content (in the low Cu content regime), pre-deformation level or aging temperature meaning that the alloys are robust with respect to variations in composition. This is interesting from the perspective of alloy design concepts based on 'recycling friendly' compositions in applications that include paint-baking. [ABSTRACT FROM AUTHOR]