Precipitates In Aluminum Alloys Research Articles

Scanning precession electron diffraction data analysis approaches for phase mapping of precipitates in aluminium alloys

Mapping the spatial distribution of crystal phases with nm-scale spatial resolution is an important characterisation task in studies of multi-phase materials. One popular approach is to use scanning precession electron diffraction which enables semi-automatic phase mapping at the nanoscale by collecting a single precession electron diffraction pattern at every probe position over regions spanning up to a few micrometers. For a successful phase mapping each diffraction pattern must be correctly identified. In this work four different approaches for phase mapping of embedded precipitates in an Al-Cu-Li alloy are compared on a sample containing three distinct crystal phases. These approaches are based on: non-negative matrix factorisation, vector matching, template matching and artificial neural networks. To evaluate the success of each approach a ground truth phase map was manually created from virtual images based on characteristic phase morphologies and compared with the deduced phase maps. The percentage accuracy of all methods when compared to the ground truth was satisfactory, with all approaches obtaining scores above 98%. The optimal method depends on the specific task at hand. Non-negative matrix factorisation is suitable with limited prior data knowledge but performs best with few unique diffraction patterns and requires substantial post-processing. It has the advantage of reducing the dimensionality of the dataset and handles weak diffracted intensities well given that they occur repeatedly. The current vector matching implementation is fast, simple, based only on the Bragg spot geometry and requires few parameters. It does however demand that each Bragg spot is accurately detected in each pattern and the current implementation is limited to zone axis patterns. Template matching handles a large range of orientations, including off-axis patterns. However, achieving successful and reliable results often require thorough data pre-processing and do require adequate diffraction simulations. For artificial neural networks a substantial setup effort is demanded but once trained it excels for routine tasks, offering fast predictions. The implemented codes and the data used are available open-source. These resources and the detailed assessment of the methods will allow others to make informed decisions when selecting a data analysis approach for 4D-STEM phase mapping tasks on other material systems.

Interfacial segregation and structure of precipitates in aluminium alloys

Interfacial segregation and structure of precipitates in aluminium alloys

Interactions between plastic deformation and precipitation in Aluminium alloys: A crystal plasticity model

This work presents a crystal plasticity model for dynamic precipitation in aluminium alloys. It takes into account both the influence of an evolving precipitate distribution on the critical stress for dislocation glide, and the accelerating effect of deformation on precipitation kinetics. The effect of precipitates on deformation behaviour is integrated into the crystal plasticity constitutive law. The effect of deformation on precipitation kinetics is modelled spatio-temporally using a multi-class precipitation kinetic model (KWN) incorporating the effect of deformation through accelerated solute diffusion caused by the production of excess vacancies. The model is applied to growth and coarsening of shearable precipitates in pre-aged AA7075 alloy under deformation at 150°C, which corresponds to an industrially relevant production. This paper first explores the influence of dynamic precipitation on the tensile behaviour, showing that dynamic precipitation might be responsible for a gain in uniform elongation and tensile stress of respectively 2% strain and 50 MPa for the case at hand. The influence of dynamic precipitation on the development of plastic strain heterogeneities is discussed. The model demonstrates how spatial heterogeneities in precipitate distribution may develop during deformation, and how these heterogeneities correlate with the development of strain heterogeneities. The precipitate distributions obtained under static or dynamic ageing are predicted and compared with each other, and the influence of texture is discussed.

Direct ageing experiments on nanometre-scale aluminium alloy samples

Studies of the kinetics of precipitation in aluminium alloys help to understand the role of quenched-in excess vacancies in the ageing process. Investigations of ageing in nanometre-sized samples are valuable because excess vacancies are believed to anneal out quickly while the alloy is still supersaturated with solute atoms. We prepare samples suitable for atom probe tomography (APT) with a sharp tip of some tens of nm radius and verify that they can be solutionised and quenched, after which they are still suitable for APT. The investigated Al-Zn alloy is known to show extremely strong clustering after quenching of the bulk material but we find that this clustering is completely suppressed in solutionised and quenched samples. Precipitation calculations explain this by a fast loss of vacancies, within seconds, during and after quenching. As such calculations also suggest that at elevated temperatures clustering could occur in the thermal equilibrium, we carry out exploratory ageing experiments on APT samples at 100°C and 135°C. No clusters are found in such samples, which could be related to experimental restrictions (Zn losses) or unfavourable nucleation conditions in nm-sized samples.

Interactions between Plastic Deformation and Precipitation in Aluminium Alloys: A Crystal Plasticity Model

Interactions between Plastic Deformation and Precipitation in Aluminium Alloys: A Crystal Plasticity Model

First-principles calculation for displacive phase transition of atomic-scale precipitates in aluminum alloys

In the precipitation of β″ phase in Al-Mg-Si and Guinier-Preston-Bagaryatsky (GPB) zone in Al-Cu-Mg alloys, columns of atoms shifting phenomenon was reported as an important step for the displacive phase transition. We build an atomic crystal model and use first-principles calculation to simulate the displacive phase transitions in the aluminum alloys. We show that the model could be successfully applied in explaining experimental results of the phase transitions of early-stage precipitation in both Al-Mg-Si and Al-Cu-Mg alloys. Then we use the model to predict the promising ternary aluminum alloys that may have similar displacive phase transitions at early-stage of aging. The first-principles method and the database of displacive phase transition identified here enrich the theory of phase transitions in atomic-scale materials and help design novel aluminum alloys with early-stage aging hardening.

Direct observation of the dynamic evolution of precipitates in aluminium alloy 7021 at high strain rates via high energy synchrotron X-rays

An improved understanding of the phenomenon of dynamic precipitation is important to accurately model and simulate many industrial manufacturing processes with high strength Al-alloys. Dynamic ageing in 7xxx Al-alloys can occur as a result of both the strain and heat. Small angle X-ray scattering (SAXS) is an advanced technique that allows the precipitation processes to be studied in situ, but to date this has only been possible at lower than industrially relevant strain rates (e.g. < 10−3). In this contribution, we demonstrate the potential of in-situ SAXS studies of metallic alloys at higher strain rates (10−2) than previously, using a high energy synchrotron X-ray. The time resolved SAXS information has been used to evaluate dynamic precipitate evolution models and has demonstrated that at high strain rates a new regime must be considered which includes the more significant effect of vacancy annihilation, leading to a clear strain rate, rather than just strain, kinetic dependence.

Study of First Principles of Mg-Si Binary Systems as Precipitates from T6 Heat Treatment of AL-MG-SI ALLOYS

&lt;p&gt;One of the purposes adding the two main alloying elements Si and Mg in small quantities (≈ 1%) on Aluminum is to form strengthening precipitates in aluminium alloy, which typically contribute with a fivefold strength increase from pure aluminum. In the last decades, the investigation has begun to understand the structure and formation of the different phases during heat treatment. An important precipitate in Al-Mg-Si Alloy is β" and β. when the Precipitate β is created after β" is that the strength suddenly drops and the alloy is said to be overaged. By means of a first-principles supercell approach and density functional theory calculations, this research studied precipitate energy and interface energy of precipitate β" and β. Results show that precipitate β have precipitate energy higher then β" but have interface energy lowers then β". And each low energy interface was found possesses interface with the Al matrix is fully incoherent. The final result of calculation and simulation values for the interfacial energies, precipitate energies and strain energies that can be used in future predictions of the characteristic precipitate.&lt;/p&gt;

In situ investigation of precipitation in aluminium alloys via thermal diffusivity from laser flash analysis

In this work, a commercially available laser flash analysis (LFA) device is used for in situ precipitation monitoring in aluminium alloys by following thermal diffusivity for the first time. The LFA measurement methods and data processing are adapted to allow continuous heating experiments over a wide range of heating rates (0.001–1 K s−1). Methods for LFA temperature calibration and thermal lag correction are suggested. Results of continuous heating of Al Mn0.5Mg0.5 aluminium alloy from the as-cast state are compared to in situ differential scanning calorimetry (DSC) and ex situ transmission electron microscopy. It is shown comparing in situ LFA, and DSC substantially improves the interpretation of superimposed reactions, in particular, the precipitation and dissolution of Mn-containing dispersoids and Mg–Si-containing secondary particles.

Grain refinement and age precipitation in aluminum alloys using ARB process

Grain refinement and age precipitation in aluminum alloys using ARB process

On the miniaturised sacrificial protection achieved by surface precipitation in aluminium alloys

Metal rich primers comprising of zinc (Zn) or magnesium (Mg) particles are applied onto certain aluminium (Al) alloys to sacrificially protect the alloys. An alternative method to sacrificially protect Al alloys is to induce the formation of a homogeneous distribution of anodic surface precipitates upon them. In this contribution, surface precipitation is exploited to impart sacrificial corrosion protection to Al alloys. Metallurgical thermal treatments were performed to form anodic surface precipitates (MgZn2, Al2MgCu and Mg2Si) on two commercial Al alloys: AA7050 and AA6061. Corrosion tests demonstrate that these miniature surface precipitates formed upon Al alloys can impart sacrificial cathodic protection.

On the electrostatic potential assisted nucleation and growth of precipitates in Al-Cu alloy

The effect of electrostatic potential (EP), specifically without current and Joule heating effect, on the precipitation in aluminum alloy during artificial aging (180°C) was investigated via microhardness testing, transmission electron microscopy (TEM), positron annihilation spectroscopy (PAS), and measurements of vacancy formation enthalpy and vacancy migration enthalpy. The results elaborated that EP promoted the nucleation of solute clusters/GP zones in the early stage during aging. After extended aging, highly dense fine precipitates were retained in Al matrix leading to an improved strength. EP-assisted artificial aging provides a new pathway to tailor the precipitation behavior in Al alloys.

Raziskave oborin v aluminijevih zlitinah z visokoresolucijsko elektronsko mikroskopijo in računalniško simulacijo

Raziskave oborin v aluminijevih zlitinah z visokoresolucijsko elektronsko mikroskopijo in računalniško simulacijo

Morphology Development and Kinetics of Plate or Rod Shaped Precipitates in Aluminum Alloys

Abstract Microstructure quantitatively analysis is significant for building the relationship between microstructure and properties, especially the ageing precipitation kinetics in aluminum alloys. In the present paper, a core parameter (aspect ratio) was introduced to describe the morphology change of precipitation. Aspect ratios of T 1 -plate precipitates in Al-Cu-Li-Zr alloys, S-plate precipitates in Al-Cu-Mg alloys and β -rod precipitates in Al-Mg-Si alloys were quantitatively analyzed. Results indicate that aspect ratios of precipitates increase at the early stage of ageing, then reach the peak and finally decrease slowly. The soft-impingement theory and HHC theory were introduced to model the ageing kinetics. Thermodynamics and kinetics parameters of different precipitates were also calculated in the model. Simulated results of different precipitates agree well with the experimental results.

Atomic-resolution chemical mapping of ordered precipitates in Al alloys using energy-dispersive X-ray spectroscopy

Scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) is a common technique for chemical mapping in thin samples. Obtaining high-resolution elemental maps in the STEM is jointly dependent on stepping the sharply focused electron probe in a precise raster, on collecting a significant number of characteristic X-rays over time, and on avoiding damage to the sample. In this work, 80kV aberration-corrected STEM-EDS mapping was performed on ordered precipitates in aluminium alloys. Probe and sample instability problems are handled by acquiring series of annular dark-field (ADF) images and simultaneous EDS volumes, which are aligned and non-rigidly registered after acquisition. The summed EDS volumes yield elemental maps of Al, Mg, Si, and Cu, with sufficient resolution and signal-to-noise ratio to determine the elemental species of each atomic column in a periodic structure, and in some cases the species of single atomic columns. Within the uncertainty of the technique, S and β” phases were found to have pure elemental atomic columns with compositions Al2CuMg and Al2Mg5Si4, respectively. The Q’ phase showed some variation in chemistry across a single precipitate, although the majority of unit cells had a composition Al6Mg6Si7.2Cu2.

Characterization of Precipitates in Aluminium Alloy 6013 after Cold-Rolling and Annealing

Characterization of Precipitates in Aluminium Alloy 6013 after Cold-Rolling and Annealing

Material design and development: From classical thermodynamics to CALPHAD and ICME approaches

This paper presents an overview and examples of material design and development using (1) classical thermodynamics; (2) CALPHAD (calculation of phase diagrams) modeling; and (3) Integrated Computational Materials Engineering (ICME) approaches. Although the examples are given in lightweight aluminum and magnesium alloys for structural applications, the fundamental methodology and modeling principles are applicable to all materials and engineering applications. The examples in this paper have demonstrated the effectiveness and limitations of classical thermodynamics in solving specific problems (such as nucleation during solidification and solid-state precipitation in aluminum alloys). Computational thermodynamics and CALPHAD modeling, when combined with critical experimental validation, have been used to guide the selection and design of new magnesium alloys for elevated-temperature applications. The future of material design and development will be based on a holistic ICME approach. However, key challenges exist in many aspects of ICME framework, such as the lack of diffusion/mobility databases for many materials systems, limitation of current microstructural modeling capability and integration tools for simulation codes of different length scales.

Complementarity of Atom Probe, Small Angle Scattering and Differential Scanning Calorimetry for the Study of Precipitation in Aluminium Alloys

Two examples of precipitation studies (in Al-Li-Cu and Al-Li-Mg alloys) are shown to demonstrate the complementarity of atom probe tomography, small-angle-scattering and differential scanning calorimetry for precipitation studies. It will be used to unravel an unexpected two-step precipitation behavior of T1 in Al-Li-Cu and to ascertain precipitates size in Al-Li-Mg. through a model free comparison between atom probe and SAXS.

Quasi in situ analytical TEM to investigate electrochemically induced microstructural changes in alloys: AA2024-T3 as an example

A quasi in situ method using a specially developed holder is presented to follow electrochemically induced microstructural changes on a nanometre scale using analytical transmission electron microscopy (TEM). By following a specific specimen area after repeated exposure to oxygen bubbled through aqueous HCl of pH=3, de-alloying of a grain-boundary precipitate of ∼200nm was observed by monitoring the changes in the EELS copper L2,3-edge and EFTEM elemental maps. These results, supporting the existing models of de-alloying of copper rich precipitates in aluminium alloys, demonstrate the effectiveness of the quasi in situ specimen holder for analytical TEM studies of corrosion.

Quantitative Analysis of Precipitate Compositions in an Al-Li-Mg-Cu Alloy Using Atom Probe Tomography

The composition of  precipitates in aluminium alloy 8090 has been analysed using a 3 dimensional atom probe with fast data acquisition rates. The effects of experimental conditions for the quantitative atom probe analysis have been examined in detail. The results show that i) lithium is prone to preferential DC field evaporation at temperatures &gt; 25K and with a pulse fraction &lt; 20%; ii) the lithium concentration of  precipitates can vary from precipitate to precipitate, ranging from 19.1 to 25.3 at.%, and iii) the stoichiometric composition of the  phase can be obtained provided that the probing temperature is  25K and pulse fraction is  20%.