Formation Of Guinier-Preston Zones Research Articles

Early precipitate morphologies in Mg-Nd-(Zr) alloys

Aging of MgNd alloys involves the formation of Guinier-Preston zones followed by β‴ precipitation that is associated with peak hardness. Using atom probe tomography and high-angle annular dark-field scanning transmission electron microscopy, the three-dimensional morphologies of the coherent Guinier-Preston zones and β‴ precipitates were quantified. The Guinier-Preston zones form equiaxed plates while the β‴ precipitates are elongated along the [0001]Mg direction thereby minimizing elastic strains. The β‴ precipitates exhibit additional faceting along the [0001]Mg direction and along the 11̅01Mg and 11̅02Mgdirections when viewed along the 112̅0Mg zone axes, which can be affected by strain field interactions from nearby precipitates.

Effect of a plastic deformation on the Guinier-Preston zones growth in Al-5at%Ag alloy

The formation of the Guinier-Preston zones in aluminium alloys is closely linked with the excess vacancies. A plastic deformation exerts an influence on the Guinier-Preston zones precipitation. As well as in the non deformed alloy and in the deformed alloy, the growth kinetics obeys to the JMAK law of the growth controlled by the solute atoms diffusion. However, during the GP zones growth, two stages are observed. During the first one, the plastic deformation slows down the Guinier-Preston zones formation and during the second stage, the plastic deformation accelerates their formation. The plastic deformation promotes the formation of the metastable γ’ phase.

Strong and ductile heat-treatable Mg–Sn–Zn–Al wrought alloys

A newly developed Mg–Sn–Zn–Al based alloy shows substantial strengthening by artificial aging. A Na-doped Mg–5.4Sn–4.2Zn–2.0Al–0.2Mn–0.1Na (TZAM5420−0.1Na) (wt.%) alloy exhibited a significant increase in yield strength from 243 to 347MPa by a T6 treatment due to the uniform dispersion of nanoscale precipitates by aging. The trace addition of Na causes the formation of Sn–Na co-clusters in the early stage of aging, which provides heterogeneous nucleation sites for Mg2Sn precipitates. However, Na strongly segregates at grain boundaries and this degrades the ductility significantly. To overcome this problem, we developed a Na-free Mg–6.6Sn–5.9Zn–2.0Al–0.2Mn (TZAM6620) alloy, in which nano-scale MgZn2 precipitates are uniformly dispersed by double aging. Pre-aging caused the formation of Zn-rich Guinier Preston zones, which acted as heterogeneous nucleation sites for the MgZn2 precipitates. The double-aged TZAM6620 alloy exhibited a very high yield strength of 370MPa with large elongation of 14%.

Nanoscopic mechanism of Cu precipitation at small-angle tilt boundaries in Si

We investigate copper (Cu) precipitation at small-angle tilt boundaries on (220) in Czochralski-grown p-type silicon (Si) ingots using transmission electron microscopy, atom probe tomography, and ab initio calculations. In the initial stage of precipitation, Cu atoms agglomerate along the boundaries, forming coherent layers (less than about 2 nm thick) of ${\mathrm{Cu}}_{3}\mathrm{Si}$ with a body-centered-cubic structure in a metastable state ($a=0.285$ nm). As the layers thicken, they become semicoherent with misfit dislocations on the (220) interphase boundaries, reducing coherency strains. Subsequently, the metastable layers convert into incoherent polyhedrons of orthorhombic ${\ensuremath{\eta}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Cu}}_{3}\mathrm{Si}$ in the equilibrium state, forming interphase boundaries on {112} in Si. These results are similar to the Cu precipitation processes found in metallic alloys: the formation of Guinier-Preston zones followed by a conversion into the equilibrium $\ensuremath{\theta}$ phase.

Thermodynamic Aspects of Homogeneous Nucleation Enhanced by Icosahedral Short Range Order in Liquid Fcc-Type Alloys

We have recently shown that minute solute element additions to liquid metallic alloys can strongly influence the nucleation of the fcc phase and act as a grain refinement method. Electron back-scattered diffraction observations revealed a concomitant increase in the percentage of nearest neighbor (nn) grains that are in a twin relationship. Furthermore, multiple-twinned (MT) nn grain configurations with a fivefold symmetry around a common h110i direction have been identified, an occurrence that can be explained when the symmetry of the icosahedron is accounted for. It was then conjectured that a new nucleation mechanism occurs in two steps: first, the formation of small icosahedral quasicrystals in the melt, followed by heteroepitaxy of the fcc phase on facets of these quasicrystals. In the present contribution, based on thermodynamics arguments, it is proposed that the first step occurs by spinodal decomposition of the liquid, in a manner similar to Guinier–Preston zones formation in solid state precipitation, while the second step is a transformation of these quasicrystal precursors into MT-fcc nanocrystals once the driving force for this transformation is sufficient to overcome the fcc-liquid interfacial energy and the elastic strains associated with MT-fcc nanoparticles. This explanation sets up guidelines for finding solute elements and composition ranges that favor this grain refinement mechanism.

Electrical resistivity anomaly: A consequence of nanometric particles in a metal matrix

The additament of an impurity to a metal matrix generates a shift in the local electron density of states due to a change in the local electric potential so that an electron or electron beam will be scattered upon interaction with the ensuing potential, causing its wave function measured at a distance far removed from the source to show a quantum phase change. This phenomenon, reflected in Friedel's law, permits to relate the quantum phase shifts with both the density of states at the Fermi level and the transport properties of the system. This work calculates the electric resistivity of nanometric clusters in an aluminiumsupercell, emulating the formation of Guinier-Preston zones during pre-precipitation in an Al-Zn alloy. The density of states is evaluated each time a new atom of solute is added to the Al supercell using the ab initio method of the Density Functional Theory or DFT in its GGA and LDA approximations via the Dmol^{3} package of Materials Studio Software. A resistivity anomaly is obtained when, in concordance with literature reports for diluted AlZn alloys, the supercell contains 25 atoms, thus confirming that the electric resistivity anomaly during cluster formation is a consequence of both density of states variations and quantum transitions at the Fermi Level for a nanometric size in particular. The results are contrasted with those obtained via the transport equation.

Small-angle scattering from GP zones in Al-Cu alloy

It is well known that Guinier Preston (GP) zones form in Al-Cu alloys upon solutionizing and artificial aging, which are extensively used in commercial practice. It is well established that GP zones are discshaped precipitates, i.e. disks of clusters of copper atoms in the FCC aluminium matrix. These disks have coherency strain fields in aluminium that give the alloy its high yield strength. The formation of GP zones in the supersaturated aluminium matrix is thought to be heterogeneous nucleation and growth. Some authors have believed that the formation of GP zones is by spinodal decomposition of the supersaturated Al-Cu solid solution. The main objective of the present work is to test whether spinodal decomposition is responsible for the formation of GP zones in Al-Cu alloy. The experimental alloy AA2219 was selected for its high copper content (Al-6%Cu-0·2%Zr). After solutionizing and artificial aging, the aging curve was plotted and small-angle scattering experiments were carried on the powdered samples as a function of time during artificial aging. Small-angle scattering data were analysed, and evidence has been obtained for the occurrence of spinodal decomposition as the mechanism responsible in the early stages of formation of GP zones.

Study of the formation kinetics of Metastable phases in quenched Al-Mg-Si Alloys

The results of the experimental study of the formation kinetics of metastable phases during decomposition of supersaturated solid solutions of quenched Al-Mg-Si alloys are presented. The process has been studied by measuring the electrical conductivity at low temperatures (18–85°C) and by measuring the Young’s modulus using the acoustic method in the temperature range 120–220°C. The method of measuring the Young’s modulus is characterized by a high precision and has made it possible to distinguish between the successive stages of the decomposition due to the formation of Guinier-Preston zones, particles of the pre-β″/β″ and β′-phases. The effective activation energies have been calculated using the obtained data on the characteristic durations of the stages of the process at different temperatures. It has been shown that the activation energy of the formation and evolution of particles in the β″-phase is considerably lower than the activation energy of diffusion of alloying element atoms at equilibrium conditions, which is caused by the effect of long-lived quenching vacancies. This energy is close to the activation energy of migration of the ν + Mg complex and, according to the obtained results, is equal to 0.58 eV.

Aging processes in precipitation-hardening composite materials based on a D16 aluminum alloy

Aging of composite materials (CMs) based on an aluminum D16 alloy and reinforced by Al3Ti intermetallic inclusions (0–10 vol %) having formed upon an in situ reaction and by SiC particles (0–30 vol %) ≤3 or 28 μm in size is studied. Oxide ceramic nanoparticles (0.1 wt %) are used to modify the structure of the CMs. The structures of the CMs before and after aging are analyzed by optical microscopy and scanning electron microscopy on a microscope equipped with an X-ray energy dispersive spectrometer. The hardness of the CMs is measured. The overall hardening of aged CMs is shown to result from a competition between the hardening effects induced by the formation of Guinier-Preston zones and the precipitation of the high-temperature θ and S phases. These effects are controlled by the dislocation density in the matrix.

Effect of heat-treatment conditions on the structure and physicomechanical and chemical properties of a Ni-Cr-Cu-Ti maraging steel

The effects of quenching (from 950°C or from 950 and 850°C) and the aging conditions on the structure, properties, and delayed fracture (DF) of 03Kh11N10M2DT maraging steel has been studied by dilatometry, X-ray diffraction, and fracture tests. The DF-crack growth rate is maximal after aging at 400°C irrespective of the quenching conditions, and the corrosion rate is maximal after aging at 350–400°C in the case of single quenching and at 350°C after double quenching. The kinetics and mechanism of the early stages of the decomposition of a supersaturated α solid solution are investigated by electrical-resistance measurements and transmission electron microscopy. In the state after single quenching, aging occurs in two stages at all isothermal heat treatments; in the state after double quenching, aging occurs in one stage at a time exponent n = 0.2 in the Johnson-Mehl equation. Upon aging at 400°C, the intermediate ordered Fe3(Ni,Ti) phase with a complex cubic lattice precipitates, and the intermetallic compound Ni3Ti precipitates upon subsequent aging. Moreover, copper-rich e-phase precipitates form only in the case of single quenching. The substantial increase in the crack growth rate during DF with n < 0.2 is likely to be caused by the formation of Guinier-Preston zones enriched in nickel and titanium.

Low temperature post-precipitation after precipitation of β′ and β phases in Al–12 wt.% Mg alloy

TEM observation confirms the presence of Guinier–Preston (GP) zones in close association with particles of β-phase after natural ageing at room temperature of Al–12 wt.% Mg alloy which has been preliminary homogenised, quenched and aged at 250 °C. Each time the sample is sufficiently matured at ambient temperature, after a first pre-precipitation (formation of GP zones) or precipitation (formation of β′ or β phases), new zones form without the presence of the quenched-in vacancies. Since quenched-in vacancies, obtained after a rapid quench in water, have greatly increased the rate of β′ and β phases formation and have reached their equilibrium concentration, the GP zones formation occurs without their presence. These results have been obtained preliminary by dilatometric and calorimetric analysis, which confirm their importance and their complimentarily in the study of the ageing processes in different alloys due to their sensitivity to various precipitation stages.

The use of DSC in describing the structure evolution of an AlZnMg alloy

The potential of DSC in suggesting modifications of thermal treatments of Al alloys to increase mechanical properties is described. Significant results can be obtained from calorimetric evolution after a series of annealings, even without a direct observation of the microstructure. The role of a reference baseline is discussed. The Guinier-Preston (GP) zones formation, dissolution or transformation is followed, and their relevance for the microhardness increase is shown, for an AlZnMg alloy of technical interest. Multi-stage thermal treatments have been confirmed to be beneficial. A secondary precipitation occurs at room temperature after annealing at temperatures at which primary precipitation is almost complete.

Electronic structure, microstructure, and crystal structure of the precipitation-hardened alloyCu98Be1.8Co0.2

Precipitation hardening of copper alloys results in improved elastic properties but is accompanied by reduction in electrical conductivity. We study the electronic structure, microstructure, and crystal structure of precipitation-hardened Cu:Be:Co (98:1.8:0.2 weight %) alloy to look for coupled changes accompanying the precipitation hardening. X-ray diffraction is used to study the strain in the Cu matrix upon Guinier-Preston zone formation and the subsequent precipitation. Using x-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM), we compare the Cu matrix and Co beryllides of well-characterized as-obtained and precipitation hardened alloys. SEM confirms the evolution of the microstructure typical of Guinier-Preston zone formation and precipitation. The binding energies and line shapes of Cu 2p, Co 2p, and Be 1s core levels are investigated using XPS. In spite of the Co beryllides migrating to the grain boundaries as an entity, XPS indicates that the Be atoms get oxidized upon migration, while the Co atoms remain metallic. The Cu 2p core levels shift 0.3 eV to higher binding energy in the as-obtained and partially hardened alloys. In addition, a line shape change observed only in the partially hardened alloy is attributed to strain in the Cu matrix upon Guinier-Preston zone formation. In contrast, for the fully hardened alloy, the binding energy and line shape reverts back to that of pure copper. But for the chemical potential shift, the valence band spectral features exhibit negligible changes in spectral shape upon hardening. The results are consistent with a change in the chemical potential due to metastable Co beryllides and increasing strain in the initial stages of hardening due to Guinier-Preston zones. In the fully hardened alloy, the observed reduction of the chemical potential shift is related to precipitation and a corresponding readjustment of the Fermi energy.

Microstructural evolution in the heat-affected zone of a friction stir weld

The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.

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.

Effects of short mullite fibres on aging response of Al–4.5Cu based metal matrix composite

Short mullite fibre reinforced Al–4.5Cu composite and its monolithic alloy have been produced by squeeze casting. The age hardening behaviour at various aging temperatures, aging precipitation characteristics and micromorphologies of the composite and the base alloy have been investigated by means of hardness measurement (HB), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), respectively. It is shown that the aged hardness of the reinforced composite is always higher than that of the unreinforced base alloy during the whole aging procedure and at the various temperatures, indicating that short mullite fibres can reinforce Al–4.5Cu binary alloy. The aging response of 3Al2O3.2SiO2f/Al–4.5Cu composite is accelerated considerably by short mullite fibres, compared with Al–4.5Cu alloy, i.e. the precipitation of both θ″ and θ′ phases is apparently accelerated in the composite, based on DSC analysis and TEM examination. But Guinier–Preston zone formation is heavily suppressed in the reinforced composite owing to the high density of dislocations in the near vicinity of the fibre/matrix interface.

Influence of Cd, Sn and Ge on the pre-precipitation phase of the Al-Zn-Mg-Cd(Sn) and Cu-Be-Cd(Ge) solid solutions

The influence of Cd, Sn and Ge on the kinetics of the Guinier-Preston (GP) zones formation has been studied for the Al-7 %Zn-3 % Mg-0,05 % Cd(0,05 %Sn) and Cu-2 % Be-0,3 %Cd(0,3 % Ge) solid solutions. The volumic fraction of the formed GP zones is determined by a method based on microhardness measurements. Traces addition of Cd and Sn cause, respectively, a delay in the GP zones formation and an elimination of these zones, in Al-Zn-Mg-Cd(Sn). For the Cu-Be-Cd(Ge) alloys, the added Cd atoms accelerate the GP zones formation, while Ge atoms at a concentration of 0.3 % have no influence.

Characterization of plastic instability steps occurring in depth-sensing indentation tests

Plastic instabilities in solid solution Al–Mg and supersaturated Al–Zn–Mg–(Cu) alloys were investigated by depth-sensing microhardness tests. Experimental results show that in the case of Al–Mg alloys the characteristics of the plastic instability depend only on the solute concentration. In the case of age-hardenable Al–Zn–Mg–(Cu) alloys, however, although plastic instabilities were observed during indentation following the quenching, the development of the instability steps is strongly influenced by Guinier–Preston zone formation in the early stage of natural aging. After a certain time, t i, of aging the steps disappear completely. The value of t i obtained for the Cu-containing alloy was found to be considerably longer than that obtained for the ternary Al–Zn–Mg alloy. This fact – together with the smaller initial rate of hardening in the Cu-containing alloy – indicates that the addition of Cu retards the early part of the decomposition process at room temperature in this alloy system.

Influence of Guinier-Preston Zone Formation on Plastic Instabilities in Depth Sensing Indentation Tests

Guinier-Preston zone formation and its effect on plastic instabilities in supersaturated Al-Cu and Al-Zn-Mg-(Cu) alloys were investigated by tensile tests and depth sensing microhardness measurements. It has been shown that plastic instabilities occur in depth sensing microhardness tests performed on supersaturated solid solutions immediately after quenching. In these measurements the load-depth (F-h) indentation curves are not smoothly changing functions, but contain characteristic steps indicating the oscillation of dynamic microhardness. Experimental results show that the occurrence of the instability steps depends on the composition of the alloy. Furthermore, the formation of GP zones suppresses the instabilities, therefore the plastic instabilities disappear at a certain state of decomposition. In this paper some characteristics of the dynamic Vickers indentation instabilities and the effect of GP zone formation on these parameters are investigated. An interpretation is given for the occurrence of instability steps from the aspect that this phenomenon is a form of Portevin-Le Châtelier type plastic instabilities which is associated with a negative strain rate sensitivity. Results of indentation tests are compared with those of constant loading rate tensile tests.

First-principles prediction of equilibrium precipitate shapes in Al-Cu alloys

A first-principles thermodynamic approach for studying coherent precipitation in size-mismatched alloys is presented and applied to the well studied (but still controversial) problem of Guinier-Preston (GP) zone formation in Al-Cu alloys. The exceptionally soft anharmonic elastic response of Cu along {100} is crucial in determining the shape of clusters of Cu atoms embedded in the Al matrix which are plate-like {100} structures. First-principles calculations involving both strain and interfacial energies yield two types of discshaped precipitate, namely GP1 and GP2, which are single Cu monolayers and two monolayers separated by three A1 layers respectively, in agreement with many (but not all) measurements. Competition between the thermodynamic driving force (favouring GP2) and interfacial energy around the perimeter of the zones (favouring GP1) leads to a size-dependent transition in the equilibrium precipitate shape from GP1 to GP2 at about 150 A. This theoretical approach (generally applicable to metals, ceramics and semiconductors) should aid in understanding the coherent precipitate shapes in a variety of alloy materials.