Transformation defects on θ′ precipitates in Al–Cu alloys

Growth defects in the form of transformation disconnections (TDs) are considered for the precipitation of θ′ in an Al–Cu αsolid solution. The minimum sized TDs are predicted to be 1–2?nm in height, in agreement with experimental observations. One type of TD is glissile and the other (more prevalent) type is sessile, requiring climb for motion. The presence of a fault plane on one side of the sessile TD does not impede its formation. The results are in accord with symmetry theory and with the topological model for precipitation.     

Formation of Cu precipitates and vacancy clusters in neutron-irradiated Fe–Cu alloys

Two Fe–Cu binary model alloys, Fe–0.3Cu and Fe–0.6Cu, were irradiated with fission neutrons at doses ranging from 4?×?10^?6 to 0.16 dpa (displacements per atom) at ～573 K to investigate the formation of Cu precipitates and microstructural evolution. The Cu content only affected the formation of Cu precipitates and microvoids at low doses. In Fe–0.3Cu, the formation of microvoids and Cu precipitates initiated at doses of 1.2?×?10^?4 and 4?×?10^?5 dpa, respectively. On the other hand, the formation of microvoids started at a dose of 4?×?10^?4 dpa in Fe–0.6Cu, and Cu precipitates were formed even after irradiation to 4?×?10^?6 dpa. On further irradiation, the difference in the formation of Cu precipitates and microvoids was small. Microvoids grew with increasing irradiation dose up to 3?×?10^?3 dpa in both alloys. Prominent aggregation of Cu atoms occurred upon irradiation from 3?×?10^?3 to 1.6?×?10^?2 dpa and the microvoids shrank. The Cu precipitates no longer grew, and microvoids nucleated and grew in the matrix above a dose of 1.6?×?10^?2 dpa in both alloys. The present studies clearly reveal the relationships between the formation and growth of Cu precipitates and microvoids with irradiation dose.     

Effects of Si addition on the microstructure evolution of Al–Cu–Mg alloys in the α + S + T phase field

The precipitation behaviour and age-hardening response of Al–1.5Cu–4.0Mg (wt.%) alloys microalloyed with Si have been investigated by means of hardness measurement, TEM and HRTEM. Compared to the ternary alloy, the quaternary alloys exhibit a higher hardness. It is found that the underaged microstructure in the Al–1.5Cu–4.0?Mg alloy contains some fine precipitates, which are identified as the T phase by FFT spectra. The peak-aged microstructures of the ternary alloy is dominated by the T phase, while the peak-aged microstructures of the Si-containing alloys are dominated by the S phase. The volume fraction of the S phase is found to increase as more Si is added.     

Enhancement of hardening and thermal resistance of Mg–Sn-based alloys by addition of Cu and Al

Mg–Sn-based alloys are considered as a promising precipitation-hardening system for applications at elevated temperatures, but the hardening effect is not satisfactory owing to sluggish nucleation and rapid coarsening of the major Mg₂Sn lath precipitates. In this study, Cu and Al are added to a Mg–6Sn–1Mn base alloy. The age-hardening response and the microstructures of these modified alloys have been investigated and are compared to that of the base alloy. The additional elements are found to bring several beneficial effects to the alloys for applications at elevated temperatures. Firstly, a eutectic structure consisting of strong intermetallic phases, i.e. Mg₂Cu in the Mg–6Sn–1Mn–2Cu alloy and Al_0.93Cu_1.07Mg in the Mg–6Sn–1Mn–2Cu–2Al alloy, remains stable along the grain boundaries after solution and ageing heat treatments. Secondly, the precipitate density has been increased significantly and the precipitate size has been refined remarkably during ageing at 200?°C. Moreover, the growth of the precipitates is inhibited remarkably during the over-ageing period. Therefore, the age-hardening response and over-ageing resistance are notably improved.     

Coexistence of adjacent vacancy-ordered and eutectic phases in Al–Cu–Ni alloys

ABSTRACT

Copper-mould-cast Al–Cu–Ni alloys show adjacent coexistence of in situ grown ordered and eutectic phases. A bimodal microstructure of α–Al and eutectic α-Al+θ-Al₂Cu phases with length-scale hierarchy evolves during solidification. Microstructural analysis through Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) shows the presence of Vacancy-ordered phases (VOPs) with different morphologies in two different compositions.     

Effect of Si on the formation and stability of the icosahedral quasicrystalline phase in Al–Fe–Cr–Ti alloys

The formation and stability of the quasicrystalline icosahedral (i) phase in melt-spun Al_{93– x} Fe₃Cr₂Ti₂Si _x (x?=?0–5) ribbons are reported. Samples were characterized by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Primitive (P-type) ordered i phase particles were found to be homogeneously distributed in an fcc α-Al matrix in the as-melt-spun ribbons. The size of the i phase particles decreased and their thermal stability increased with increasing substitution of Al by Si. The i phase had a decomposition temperature of approximately 480°C in an Al₉₃Fe₃Cr₂Ti₂ ribbon whereas that in an Al₉₂Fe₃Cr₂Ti₂Si₁ ribbon was approximately 500°C. The i phase particles are resistant to coarsening prior to decomposition into crystalline phases. The presence of a small quantity of Si (up to 1.0?at.%) is beneficial to both the thermal stability and the hardness of nanoquasicrystalline Al–Fe–Cr–Ti alloys.     

Chemistry and morphology of β′ precipitates in an aged Mg-Nd-Y-Zr alloy

While the Mg–Y–Nd system is used for industrial applications, the details of the precipitation sequence and exact role of each alloying element during ageing have not been fully quantified. Focusing on WE43, a Mg–Y–Nd alloy containing Zr, the chemistry of β′ precipitates and matrix during isothermal ageing at 250 °C is investigated using atom probe tomography. Precipitate morphologies and compositions are compared with previous electron microscopy observations, and the roles of the alloying elements are discussed.     

A first-principles study of the formation of θ′′ phase in Al–Cu alloys

The θ′′-Al₃Cu phase plays an important role in the precipitation process of Al–Cu alloys. This phase has a sandwich structure—every two {200}_Cu layers are separated by three {200}_Al layers. To analyse the formation mechanism of this structure, the elastic strain energy of the {200}_Cu and {200}_Al layers, and the chemical bonding energy that reflects the interaction between the electrons in Cu and neighbouring Al atoms are calculated and analysed by first-principles calculations, projected density of states and Bader analysis. Our computation results reveal that this sandwich structure is energetically preferred in the competition of elastic strain and chemical bonding energies. To minimise the elastic strain energy of {200}_Al and {200}_Cu layers, the {200}_Cu layers prefer being apart from each other, whereas the chemical bonding energy favours the opposite arrangement because the intermetallic bond between Al and Cu atoms may form through p-d hybridization.     

Orthorhombic approximant phases and their relation to quasicrystals in mechanically alloyed Al–Cu–Fe–Si alloys

The possibility of synthesis of quasicrystalline and associated approximant phases during mechanical alloying and subsequent heat treatment of Al₆₅Cu₂₀Fe₁₀Si₅ alloy powders has been investigated. It is shown that, in addition to the stable ordered quasicrystal and 1/1 cubic approximant, two more phases having orthorhombic symmetry coexist. The a and c parameters of both these phases are similar to the lattice parameters of the cubic approximant, whereas their b parameters are, respectively, three and four times larger than the c parameters. It is suggested that these phases evolve on account of local symmetry breaking of the cubic approximant phase, possibly due to ordering.     

Effect of Mg content on Portevin–Le Chatelier band strain in Al–Mg sheet alloys

Experimental measurements of stress drops and band strains in type-B Portevin–Le Chatelier (PLC) bands were carried out for 5xxx series Al–Mg sheets with Mg content between 1.8 and 4.5?wt%. While the stress drops increase with global strain, the band strain values in all the samples follow a common linear relationship with global strain. The results indicate that the type-B PLC band strain is independent of solute content at given strain rate.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

Formation and analysis of amorphous and nanocrystalline phases in Al–Cu–Mg alloy under friction stir processing

Homogeneous amorphous and nanocrystalline phases formed in the nugget zone of a friction stir-processed Al–Cu–Mg alloy have been studied. X-ray diffraction analysis indicated a diffuse scattering peak with characteristics of an amorphous phase existed in the range 15°–29°. Further, TEM analysis proved the existence of an amorphous structure. Friction stir processing provides special physical conditions, such as high temperature, high hydrostatic pressure and large shear stress, which could induce the amorphization of the alloy.     

Reply to the comments on “Room-temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield-strength development”

We present transmission electron microscopy observations and a crystallographic analysis that show that the only significant report on the observation of {112} pseudotwins in B2 structures by Goo et al. (1985, Acta metall., 3, 1725) is a misidentification of the {114} true twins.     

Influence of Cd and Ge on the kinetics of Guinier–Preston zone formation in Cu–Be alloys

The influence of Cd and Ge on the kinetics of Guinier–Preston zone formation has been studied for Cu–2?wt%?Be–0.3?wt%?Cd and Cu–2?wt% Be–0.3wt%?Ge solid solutions. The relative volume fractions of these Guinier–Preston (GP) zones are determined for different times of ageing at 473?K by a method based on microhardness measurements. Cd atoms in Cu–Be–Cd accelerate GP zone formation. This is due to the supersaturation of Cd atoms in Cu and their strong binding energy with vacancies. In contrast, the Ge atoms do not influence GP zone formation in Cu–Be–Ge. The binding energies between the two solute atoms and a vacancy, and the concentration of the quenched-in vacancies bound to the solute atoms, are calculated using the Hasiguti formula and the Kimura equation.     

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.     

Nanostructure and related mechanical properties of an Al–Mg–Si alloy processed by severe plastic deformation

Microstructural features and mechanical properties of an Al–Mg–Si alloy processed by high-pressure torsion (HPT) have been investigated using transmission electron microscopy, X-ray diffraction, three-dimensional atom probe, tensile tests and micro-hardness measurements. It is shown that HPT processing of the Al–Mg–Si alloy leads to a much stronger grain size refinement than of pure aluminium (down to 100 nm). Moreover, massive segregation of alloying elements along grain boundaries is observed. This nanostructure exhibits a yield stress even two times higher than that after a standard T6 heat treatment of the coarse-grained alloy.     

Relations between quasicrystals and crystalline phases in Al─Li─Cu─Mg alloys: A new class of approximant structures

Abstract

From an electron-diffraction study the crystalline c-phase, observed together with the quasicrystals in an Al─Li─Cu─Mg alloy, has been analysed as exhibiting an almost perfect icosahedral symmetry. Orientation relationships with quasicrystals are determined and serve to define a new class of approximant structures, including, beyond the c-phase, the Ti₂ Ni and Ti₂ Fe structures.     

Characterisation of magnesium oxide and its interface with α-Mg in Mg–Al-based alloys

Magnesium oxide (MgO) films and particles have been collected by pressurised filtration of a Mg-8.6wt%Al-0.67wt%Zn (AZ91D) alloy melt. The morphology of the oxides and their interfaces with the α-Mg phase were investigated by high-resolution transmission electron microscopy. It was found that the oxide films consisted of large numbers of sub-micrometre-sized MgO particles, and that melt shearing can effectively break up the oxide films and disperse the oxide particles. For the first time, orientation relationships (ORs) of OR I: [1?1?1]_MgO～2° from (0?0?0?1)_α-Mg and (0?1?1)_MgO?//[2?1?1?0)_α-Mg; and OR II: (1?1?1)_MgO//(1?1?0?1)_α-Mg and [0?1?1]_MgO//[1?2?1?1]_α-Mg, were observed between the MgO particles and the α-Mg matrix. The calculated Bramfitt planar disregistries were 5.5% and 2.5% for the two ORs, respectively, indicating good lattice matching between MgO and α-Mg at the interface. With the evidence of grain refinement effect observed in the sheared AZ91D magnesium alloy, the possibility of MgO particles to act as potent nucleants for heterogeneous nucleation of α-Mg grains is discussed in terms of the crystallographic criterion.     

Effect of thermoelectric magnetic force on the array of dendrites during directional solidification of Al–Cu alloys in a high magnetic field

By eliminating the effect of the magnetic moment arising from the magnetic crystalline anisotropy, the effect of the thermoelectric magnetic force on the array of dendrites during directional solidification of Al–35?wt%Cu and Al-40?wt%Cu alloys in a high magnetic field has been investigated experimentally. The results indicate that the dendrite array is essentially destroyed, a result that could have general significance for understanding the processes involved in the solidification of alloys in a magnetic field.     

Application of the Bons–Azuma method and determination of grain growth mechanism in rolled Ti–Zr alloys

Zr-containing Ti alloys have widely been developed owing to the infinite solid solubility of Zr in Ti and its avirulence, leading respectively to high strength and good biocompatibility. It is known that the Zr addition gives rise to grain refinement when rolled Ti–Zr alloys are annealed; nevertheless, the governing mechanism by which Zr addition in Ti can reduce grain size is not fully understood. In this study, the grain growth behaviour of rolled Zr-free and Zr-containing (Ti–10Zr, wt.%) alloys is analysed using analytical transmission electron microscopy and the classical and Bons–Azuma methods by evaluating the grain growth exponent. Irrespective of the evaluation technique and Zr content, the grain growth exponent is found to be close to ~0.3, indicating the occurrence of normal grain growth in the Zr-free alloy and solute drag mechanism in the Zr-containing alloy. It is found that the grain size and grain growth rate are significantly reduced by Zr segregation near grain boundaries, resulting from the solute drag mechanism.