Study of ageing in Al–Mg–Si alloys by positron annihilation spectroscopy

In many common Al–Mg–Si alloys (6000 series) intermediate storage at or near ‘room temperature’ after solutionising leads to pronounced changes of the precipitation kinetics during the ensuing artificial ageing step at ≈180 °C. This is not only an annoyance in production, but also a challenge for researchers. We studied the kinetics of natural ‘room temperature’ ageing (NA) in Al–Mg–Si alloys by means of various different techniques, namely electrical resistivity and hardness measurement, thermoanalysis and positron lifetime and Doppler broadening (DB) spectroscopy to identify the stages in which the negative effect of NA on artificial ageing might appear. Positron lifetime measurements were carried out in a fast mode, allowing us to measure average lifetimes in below 1 min. DB measurements were carried out with a single detector and a ⁶⁸Ge positron source by employing high momentum analysis. The various measurements show that NA is much more complex than anticipated and at least four different stages can be distinguished. The nature of these stages cannot be given with certainty, but a possible sequence includes vacancy diffusion to individual solute atoms, nucleation of solute clusters, Mg agglomeration to clusters and coarsening or ordering of such clusters. Positron lifetime measurements after more complex ageing treatments involving storage at 0 °C, 20 °C and 180 °C have also been carried out and help to understand the mechanisms involved.     

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

The effect of Cu on precipitation in Al–Mg–Si alloys

TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg?+?Si?=?1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a?=?b?=?4.05?Å, c?=?4.05?Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a?=?10.32?Å, b?=?8.1?Å, c?=?4.05?Å, γ?=?101°.     

Radiation-stimulated diffusion in Al–Si alloys

A di-vacancy low-temperature diffusion is proposed to explain diffusion-controlled processes in Al–Si alloys responsible for neutron-induced silicon precipitation. Ab initio calculations of potential barriers for Si atom hopping in aluminium lattice showed that in the case of di-vacancy diffusion, they are small compared with that of mono-vacancy diffusion. The low temperature diffusivity of mono-vacancies is too small to account for the measured Si diffusivities in aluminium. The dependencies of radiation-stimulated diffusion on the neutron flux and on the temperature are obtained and can be used for the experimental verification of the developed model.     

Early stages of precipitation and microstructure control in Mg–rare earth alloys

Hardening precipitation frequently occurs in Mg–rare earth (RE) alloys after heat treatment in the 150–200°C range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg–RE alloys (Mg–Y–Gd and Mg–Y–Nd). Two types of structures may be involved in the precipitation sequence: a DO₁₉ phase and the so-called orthorhombic β′ phase. The structural relationship between DO₁₉ and β′ phases has been established in underaged and overaged states from the observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO₁₉ or the β′ phase will grow in further ageing. The Mg–Y–Gd and Mg–Y–Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the β′ phase, the Mg–Y–Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.     

Aberration-corrected HAADF-STEM investigations of precipitate structures in Al–Mg–Si alloys with low Cu additions

Precipitates in a lean Al–Mg–Si alloy with low Cu addition (~0.10 wt.%) were investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Most precipitates were found to be disordered on the generally ordered network of Si atomic columns which is common for the metastable precipitate structures. Fragments of known metastable precipitates in the Al–Mg–Si–(Cu) alloy system are found in the disordered precipitates. It was revealed that the disordered precipitates arise as a consequence of coexistence of the Si-network. Cu atomic columns are observed to either in-between the Si-network or replacing a Si-network column. In both cases, Cu is the center in a three-fold rotational symmetry on the Si-network. Parts of unit cells of Q′ phase were observed in the ends of a string-type precipitates known to extend along dislocation lines. It is suggested that the string-types form by a growth as extension of the B′/Q′ precipitates initially nucleated along dislocation lines. Alternating Mg and Si columns form a well-ordered interface structure in the disordered Q′ precipitate. It is identical to the interface of the Q′ parts in the string-type precipitate.     

Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.     

Electron irradiation-enhanced core/shell organization of Al(Cr,Fe, Mn)Si dispersoids in Al–Mg–Si alloys

The age hardening 6061-T6 aluminium alloy has been chosen as structural material for the core vessel of the material testing Jules Horowitz nuclear reactor. The alloy contains incoherent Al(Cr, Fe, Mn)Si dispersoids whose characterization by energy-filtered transmission electron microscopy (EFTEM) analysis shows a core/shell organization tendency where the core is (Mn, Fe) rich, and the shell is Cr rich. The present work studies the stability of this organization under irradiation. TEM characterization on the same particles, before and after 1 MeV electron irradiation, reveals that the core/shell organization is enhanced after irradiation. It is proposed that the high level of point defects, created by irradiation, ensures a radiation-enhanced diffusion process favourable to the unmixing forces between (Fe, Mn) and Cr. Shell formation may result in the low-energy interface segregation of Cr atoms within the (Fe, Mn) system combined with the unmixing of Cr, Fe and Mn components.     

Solute-second phase interaction for Mg,Ag and Zn in Al–Li alloys

ABSTRACT

Early experiments have shown the promises of alloying with Mg?+?Ag (or Mg?+?Zn) on the performance of Al–Li alloys. To better understand the interaction between solutes and second phases in Al–Li alloys, Mg, Ag and Zn segregation to Al/δ′ interface as well as their substitution in δ′ bulk were investigated at the atomic level using first principles modelling and calculations. Energetics results and local charge analyses revealed that Mg, Ag and Zn can segregate to Al/δ′ interface by different preference, but have no significant influence on the interface adhesion. Ag and Zn can also dissolve into δ′ bulk, and enhance the local metallic bonding with nearest-neighboring Al atoms. Based on these results, a multi-fold benefit mechanism was suggested for the combined alloying with Mg?+?Ag (or Mg?+?Zn) in Al–Li alloys.     

Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

Atomistic details of precipitates in lean Al–Mg–Si alloys with trace additions of Ag and Ge studied by HAADF-STEM and DFT

Abstract

Bonding energies and volume misfits for alloying elements and vacancies in multicomponent Al–Mg–Si alloys have been calculated using density functional theory (DFT). A detailed atomic scale analysis has been done for characteristic precipitate structures, using high-angle annular dark-field scanning transmission electron microscopy. Two new stacking configurations of the important strengthening phase β′′ were discovered in the Ge-added alloy. All three stacking variations were found to be energetically favourable to form from DFT calculations. The second stacking configuration, β₂′′, contains vacated columns in its unit cell, consequently requiring less solute to create the same volume fraction of precipitate needles. DFT suggests a lower formation enthalpy per atom for β₂′′ when Si is exchanged with Ge. In the alloy containing Ag additions, a new Q’/C-like local configuration containing Ag instead of Cu was discovered, also this phase was deemed energetically favourable from DFT.     

Interdiffusion and thermotransport in Ni–Al liquid alloys

In this paper, we present extensive self-consistent results of molecular dynamics (MD) simulations of diffusion and thermotransport properties of Ni–Al liquid alloys. We develop a new formalism that allows easy connection between results of the MD simulations and the real experiments. In addition, this formalism can be extended to the case of ternary and higher component liquid alloys. We focus on the temperature and composition dependence of the self-diffusion coefficients, interdiffusion coefficients, thermodynamic factor, Manning factor and the reduced heat of transport. The two latter quantities both represent measures of the off-diagonal Onsager phenomenological coefficients. The Manning factor and the reduced heat of transport can be related to experimentally obtainable quantities provided the thermodynamic factor is available. The simulation results for the reduced heat of transport show that for all compositions, in the presence of a temperature gradient, Ni tends to migrate to the cold end. This is in agreement with an available experimental study for a Ni_21.5Al_78.5 melt (only qualitative result is available so far).     

Fundamental understanding of Na-induced high temperature embrittlement in Al–Mg alloys

Sodium is an undesirable impurity in aluminium–magnesium alloys. In trace amounts it leads to high temperature embrittlement (HTE), due to intergranular fracture, which results in edge cracking during hot rolling. In the present work, the results of a thermodynamic investigation to elucidate the mechanism are presented. Correlations between HTE, phase formation, temperature and composition in Al–Mg alloys were determined. It is suggested that: (i) HTE is related to the formation of an intergranular Na-rich liquid phase, which significantly weakens the strength of grain boundaries; (ii) for a given Mg content, there exists a maximum Na content above which HTE cannot be avoided; and (iii) for a given alloy, a proper hot-rolling temperature should be chosen with respect to Na and Mg contents to suppress HTE. The HTE sensitive zone and a hot-rolling safe zone of Al–Mg–Na alloys are defined as functions of processing temperature and alloy composition. The tendency of HTE formation was evaluated based on thermodynamic simulations of phase fraction of the intergranular Na-rich liquid phase.     

Thermal and microstructural investigation of Cu–Al–Mn–Mg shape memory alloys

There are many studies to improve the properties of Cu–Al–Mn shape memory alloys, such as high transformation temperatures, ductility and workability. Most of them have been performed by adding a quaternary component to the alloy. In this study, the effect of trace Mg addition on transformation temperatures and microstructures of three different quaternary Cu–Al–Mn–Mg alloys has been investigated using thermal analysis, optical microscopy and XRD techniques. The transformation temperatures are within the range of 120–180 °C, and they have not changed significantly on decreasing the Mn content, replacing with Mg. The fine precipitates have been observed in the alloys with the Mg content up to 1.64 at%. Calculated entropy change and XRD analysis reveal that the alloys with high Al content have mainly 18R-type structure which could be responsible for good ductility and workability.     

63Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

⁶³Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al₂CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α?+?S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al₂CuMg. Analysis of the hyperfine structure of the ⁶³Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al₂CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α?+?θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al₂Cu. A hyperfine-structured ⁶³Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7?at.%?Cu–0.33?at.%?Mg alloys containing 0.1 and 0.18?at.%?Ag, but not in the same Al–Cu–Mg alloy without Ag.     

Wetting behavior of Al–Si–Mg alloys on Si3N4/Si substrates: optimization of processing parameters

The wetting behavior of Al–Si–Mg alloys on Si₃N₄/Si substrates has been investigated using the sessile drop technique. Based on a Taguchi experiment design, the effect of the following processing parameters on the contact angle (θ) and surface tension (σ_LV) was studied: processing time and temperature, atmosphere (Ar and N₂), substrate surface condition (with and without a silicon wafer), as well as the Mg and Si contents in the aluminium alloy. In nitrogen, non-wetting conditions prevail during the isothermal events while in argon a remarkable non-wetting to wetting transition leads to contact angles θ as low as 11±3° and a liquid surface tension σ_LV of 33± 10×10^-5 kJ/m². According to the multiple analysis of variance (Manova), the optimum conditions for minimizing the values of θ and σ_LV are as follows: temperature of 1100 °C, processing time of 90 min, argon atmosphere, no use of a silicon wafer, and the use of the Al-18% Mg-1% Si alloy. A verification test conducted under the optimized conditions resulted in a contact angle of θ=9±3° and a surface tension of σ_LV=29± 9×10^-5 kJ/m², both indicative of excellent wetting. PACS 68.03.Cd; 81.05.Bx; 68.08.Bc; 05.70.-a; 61.10.Nz     

Texture evolution in Mg–Mn and Mg–Mn–Ce alloys and the effect of as-cast grain size

Texture evolution in rolled (400°C) and annealed (450°C) Mg–1% Mn-based alloys containing different levels of Ce was examined. It was found that Ce refines the as-cast and rolled/annealed grain structure. The overall intensity of the basal texture decreases with Ce additions in both the rolled and subsequently annealed condition. A strong relationship was found between maximum intensity (M) of rolling and annealing textures and the as-cast effective grain size, d?′, i.e. M?∝?e^{0.4 d ?′} (R ^2?=?0.8), which was attributed to the role of plastic compatibility stresses which lead to grain boundary deformation altering the balance of deformation modes. Contributions from Ce solute effects and lattice parameter changes are also discussed.