Formation Of Guinier-Preston Zones Research Articles

Strengthening mechanism of T8-aged Al−Cu−Li alloy with increased pre-deformation

The microstructure evolution and mechanical properties of a T8-aged Al−Cu−Li alloy with increased pre-deformation (0−15%) were investigated, revealing the microstructure−strength relationship and the intrinsic strengthening mechanism. The results show that increasing the pre-deformation levels remarkably improves the strength of the alloy but deteriorates its ductility. Dislocations introduced by pre-deformation effectively suppress the formation of Guinier-Preston (GP) zones and provide more nucleation sites for T1 precipitates. This leads to more intensive and finer T1 precipitates in the samples with higher pre-deformation levels. Simultaneously, the enhanced precipitation of T1 precipitates and inhibited formation of GP zones cause the decreases in number and sizes of θ′ precipitates. The quantitative descriptions of the strength contributions from different strengthening mechanisms reveal that strengthening contributions from T1 and θ′ precipitates decrease with increasing pre-deformation. The reduced diameters of T1 precipitates are primarily responsible for their weakened strengthening effects. Therefore, the improved strength of the T8-aged Al−Cu−Li alloy is mainly attributed to the stronger strain hardening from the increased pre-deformation levels.

Atomistic modeling of nucleation kinetics of Guinier–Preston zones in Al–Cu alloys: Two formation scenarios and prediction of the time-temperature-transformation diagram

Metastable precipitates of solute atoms, known as Guinier–Preston (GP) zones, play a significant role in the age hardening of Al–Cu alloys. In this study, we utilized the classical nucleation theory (CNT) to obtain time-temperature-transformation (TTT) diagrams for steady-state nucleation over a wide temperature range. The CNT model, which is based on atomistic simulations using artificial neural network potentials, was constructed to predict the nucleation kinetics of GP zones in the Al–Cu system with solute concentrations ranging from 1.3 to 2.5 at% (3.0 to 5.7 wt%). The nose temperature at which the steady-state nucleation time for GP zone formation is the shortest was calculated. The nose temperature increased and the minimum nucleation time decreased as the solute concentration increased. These predictions are comparable to previous theoretical results and are in good qualitative agreement with experimental observations. Furthermore, two formation scenarios of double-layer GP (GP2) zones, considering synchronous and asynchronous attachments of solute atoms to clusters, were compared in terms of nucleation efficiency. This provides new insights into the nucleation pathways of the GP2 zone in Al–Cu alloys.

Achieving high strength-ductility synergy via Guinier-Preston zone formation in a new Mg–Zn–Al–Ca–Mn–Ce sheet alloy

ZAXME11100 (Mg–1Zn–1Al-0.5Ca-0.4Mn-0.2Ce, wt.%) is a recently developed sheet alloy with excellent formability at room temperature. The yield strength of this alloy is significantly improved by a short 1-h aging treatment at 210 °C (T6), from 159 MPa in solution-treated state (T4) to 270 MPa in the T6 condition, with a slight reduction in tensile elongation (from 31 % in T4 to 26 % in T6). In this work, precipitation microstructure was characterized to explain the aging hardening and high strength-ductility synergy of the new alloy. A high density (8 × 1023/m3) of ordered monolayer Guinier-Preston (GP) zones enriched in Al, Ca and Zn uniformly form on the Mg basal planes after T6 treatment. Grain boundary segregation of Al, Ca and Zn is also observed. Deformation microstructure of T4 and T6 samples tested to various strain levels was examined. At small strain levels, basal and non-basal <a> dislocation slip dominates the deformation process, and cross slip of <a> dislocations between basal and non-basal planes is observed in T6 samples. The GP zones are gradually destroyed by plastic deformation. Based on those results, the strengthening contribution of ordered GP zones was modeled to be 116 MPa. Therefore, formation of GP zones is an effective way to achieve high strength-ductility synergy in wrought Mg alloys.

Effects of Sn addition on precipitation of a pre-naturally aged Al–Zn–Mg alloy during artificial aging

The effect of Sn addition and pre-natural aging on the artificial aging behavior of Al–Zn–Mg alloy was investigated using both experimental approaches and first principle study. The result shows that Sn retards hardness increase at the initial stage of artificial aging by inhibiting the formation of Guinier-Preston zones (I). Atom probe tomography and density functional theory have revealed that the high binding energy of Mg-Sn disturbs Mg-Zn clustering and delays the formation of Guinier-Preston zones (I). However, the retardation effect of Sn on the artificial aging was found to be suppressed when the pre-natural aging was applied to the Al–Zn–Mg–Sn alloy: hardness at the initial stage increased rapidly compared to the Al–Zn–Mg alloy with pre-natural aging. The reason for the rapid increase of hardness in the Al–Zn–Mg–Sn alloy with pre-natural aging is that Guinier-Preston zones (II) was directly formed from vacancy-rich clusters in the Al–Zn–Mg–Sn alloy in contrast to the Al–Zn–Mg alloy with pre-natural aging. This means Mg-Sn and Zn-rich cluster formed during pre-natural aging can accelerate precipitation kinetic by providing nucleation site for Guinier-Preston zones (II) during artificial aging.

Effects of Sn addition on precipitation of Al–Zn–Mg alloy at early stage of natural aging

The effect of Sn addition on the natural aging behavior of Al–Zn–Mg alloy was investigated using atom probe tomography (APT) experiments and density functional theory (DFT) calculations. The results demonstrate that Sn retards the natural aging behavior by facilitating the preferential formation of Mg–Sn clusters, which suppress the formation of Guinier-Preston zones (I) during the earliest aging stage. Quantitative solute analysis conducted by APT has suggested increasing amount of Mg–Sn clusters in Sn-contained alloy due to their high binding energy, supported by DFT calculations on the Sn.

Microstructure characteristics and mechanical properties of Cu/Al laminated metal composites fabricated by electropulsing assisted ultrasonic additive manufacturing

In this paper, electropulsing (EP) technique was employed to enhance the capability of ultrasonic additive manufacturing (UAM) of metals. The microstructure and mechanical properties of Cu/Al laminated metal composites (LMCs) fabricated by EP-UAM and preheating (PH) assisted UAM were comparatively investigated. Results show that by introducing EP into UAM, the synergistic effect of ultrasonic shear and EP is produced, which promotes the plastic flow of Al near the Cu/Al interface, offsetting the negative influence of no-preheating. Compared to the PH-UAM, EP-UAM induces atomic diffusion and formation of a tiny of Cu-Al intermetallic compounds more effectively, which is beneficial to the interfacial bonding strength. The Cu/Al LMCs fabricated by EP-UAM exhibited an obviously higher tensile strength and total elongation than those by PH-UAM. Such improvements in mechanical properties are mainly ascribed to (ⅰ) the higher strength retained for Cu and Al layers, (ⅱ) the cooperative deformation of Cu/Al and (ⅲ) back stress strengthening due to the formation of Cu atoms Guinier-Preston (GP) zones within Al matrix.

In situ laser-ultrasonic monitoring of elastic parameters during natural aging in an Al-Zn-Mg-Cu alloy (AA7075) sheet

Solution heat treatment increases the formability of 7xxx series aluminum alloys by dissolution of nanoscale hardening phases. However, subsequent natural aging at room temperature partly reverses this process within a few hours, posing a logistical challenge to industrial operations as formability decreases and springback increases. Hence, non-destructive monitoring of mechanical properties may help facilitate forming of parts from 7xxx series alloys, but no suitable non-destructive direct measurement technique has been established so far. Here we demonstrate an in situ laser ultrasonic method based on the generation and detection of plate resonances to continuously monitor small changes (<1%) of Young’s modulus, shear modulus, and Poisson’s ratio during natural aging of an age-hardenable aluminum alloy (AA7075) sheet. Additionally, we followed plastic mechanical properties by tensile testing and we intermittently measured electrical conductivity and performed differential scanning calorimetry to assess precipitation during natural aging. We find qualitative correlation between Guinier-Preston zone formation and an increase in elastic moduli and material strength.

Early-stage clustering and precipitation behavior in the age-hardened Al–Mg–Zn(-Cu) alloys

T-Mg32(AlZnCu)49 phase-strengthened Al–Mg–Zn–Cu alloys have shown a promising age-hardening response. However, the age-hardening mechanism underlying the different heat treatments is not clear during the early-stage precipitation. Additionally, the lack of information on the early-stage precipitation impedes the development of this series alloys. In this study, the formation of the Guinier-Preston (GP) zone and T′-phase of the novel Al–Mg–Zn(-Cu) alloys during the early-stage precipitation has been investigated using transmission electron microscopy and atom probe tomography analysis. Investigation revealed that the Cu content and thermal treatment had significant effects on the microstructure evolution of the alloys. GP zones were observed to act as sites for the formation of the T′-phase. The higher number density of Cu-incorporated GP zones led to higher strength and bake-hardening response after pre-aging. Occasionally, some GPI zones were observed to dissolve into the Al matrix without further transforming into T′ precipitates during natural aging (NA). Pre-aging prior to NA facilitated stable GP zone formation and mitigated the negative effect of NA. Furthermore, the precipitates were grown to an appreciable size after single-step aging at 180 °C, which adversely affected the mechanical properties of the novel alloys. The present findings provide an efficient guideline for designing the T-phase in the crossover Al–Mg–Zn-(Cu) alloys.

Effect of micro-alloyed Ce on the microstructure evolution and mechanical properties of rolled Mg–0.6Al–0.5Mn–0.2Ca alloy sheets

In this study, the effect of minor Ce addition (0, 0.1, and, 0.3 wt.%) on the microstructure evolution and mechanical properties of rolled Mg–0.6Al–0.5Mn–0.2Ca (AMX100) alloy sheets is systematically investigated under different annealing processes. With 0.1 wt.% Ce addition, partial Al8Mn5 phases turn into Al8Mn4Ce. With 0.3 wt.% Ce addition, the number of the Al8Mn4Ce further increases and the Al–Ce phases are observed. The single-stage annealed alloys exhibit similar uniform fine-grained structures (∼6 μm) regardless of the Ce content, while the grain sizes of the two-stage annealed alloys increase obviously (∼10.9–11.9 μm). Especially, the abnormal grain growth occurs in the two-stage annealed AMX100-0.3Ce alloy. After the aging treatment, the yield strength (YS) of the two-stage annealed alloys is increased by ∼25–45 MPa, which is greatly higher than that of the single-stage annealed alloys (∼1–8 MPa). Note that the two-stage annealed AMX100–0.1Ce alloy displays a significant age-hardening response with the ultimate tensile strength (UTS) of ∼272 MPa and the elongation to failure (EF) of ∼14%, which is attributed to the formation of Al and Ca enriched Guinier-Preston (G.P.) zones. This work sheds light on the design and fabrication of low-cost dilute magnesium alloys with admirable age-hardening ability.

Influence of neutron irradiation and ageing on behavior of SAV-1 reactor alloy

This study observed the effect of neutron irradiation and ageing on the microstructure, hardness, and corrosion resistance of SAV-1 (Al–Mg–Si) alloy. The investigated material was irradiated with neutrons to fluences of 1021–1026 n/m2 in the WWR-K research reactor and kept in dry storage. Long-term irradiation led to an increase in hardness of the alloy and a deterioration of pitting corrosion resistance. Post-irradiation ageing for 1 h at 100–300 °C resulted in a decrease in microhardness of the irradiated SAV-1. The effect of post-irradiation ageing on pitting corrosion was made clear through the formation of Guinier-Preston zones and secondary precipitates in the Al matrix. Ageing at 250 °C corresponded to the development of stable microstructure and the highest corrosion resistance for the irradiated samples. Mg2Si, Si, and needle-shaped β″ precipitates were formed in SAV-1 alloy that was irradiated with low fluences. β″ and clusters of rod-shaped B-type precipitates were observed in highly irradiated samples. The precipitates were similar to those seen in non-irradiated pseudo-binary Al–Mg2Si alloys with Si excess.

Cold spray deposition of solution heat treated, artificially aged and naturally aged Al 7075 powder

The deformation behaviour of particles and substrate in cold spray is dictated by the intrinsic properties of both materials, and their bonding directly affects the resulting properties of the deposited coating. The processing through heat treatment of aluminium alloy powders has only recently been developed for both cold spray and additive manufacturing, hence the necessity to evaluate and further understand the evolution of their properties. In this study, an Al 7075 gas-atomised powder was solution heat treated, quenched and subsequently aged. The powder in its as-quenched, T4 (natural ageing at room temperature for 21 days) and T6 (artificial ageing at 120 C for 24 h) condition was characterised through differential scanning calorimetry and scanning electron microscopy to evaluate the precipitation kinetics. Powders were cold sprayed using heated N2 at 500 C and 6.0 MPa and the resulting deposits were evaluated using tubular coating tensile and pull-off bond strength tests. The precipitation development in the gas atomised powder was found to be similar to the bulk alloys, with development of η′ precipitates during artificial ageing and Guinier-Preston zone formation and development during natural ageing. A relationship between the deposition efficiency of the powders and the coating properties was discovered and explained through an adapted densification mechanism based on powder tamping.

Temperature-dependent nucleation kinetics of Guinier-Preston zones in Al–Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation theory approach

Guinier-Preston (GP) zones, which form in Al–Cu alloys, exhibit characteristic patterns of Cu-rich, disk-shaped atomic clusters along the {100} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al–Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al–Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al–Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al–Cu system.

Multi-scale Cluster Dynamics modelling of Guinier–Preston zone formation in binary Al–Cu alloys

We developed a multi-scale model for the prediction of precipitation kinetics of binary alloys and present its application to the formation of Guinier–Preston zones (GPZ) in Al-rich binary Al–Cu alloys. The approach is covering three length scales: (i) Starting from density functional theory (DFT) calculations a Cluster Expansion (CE) for the binary Al–Cu system on the fcc lattice is constructed. Based on the obtained CE (ii) a lattice Monte Carlo (MC) sampling technique, known as the overlapping distribution method, is used to calculate cluster free energies of precipitate clusters. Finally (iii) a meso-scale Cluster Dynamics (CD) model is constructed by taking the cluster free energies as input. The result is a fast, parameter free method that is able to predict the evolution of precipitate size distributions. To validate the model kinetic MC simulations on the precipitation of GPZ are conducted. The results agree well with those obtained by our model. When comparing our results to experimental data a problem of time scaling arises, which is discussed in the results section.

Cluster Dynamics Modelling of Guinier-Preston Zone Formation in Binary Al-Cu Alloys Derived from Cluster Expansion Energetics

We developed a multi-scale model for the precipitation of Guinier-Preston zones (GPZ) in Al-rich binary Al-Cu alloys. The approach is covering three length scales: (i) Starting from density functional theory (DFT) calculations a Cluster Expansion (CE) for the binary Al-Cu system on the fcc lattice is constructed. Based on the obtained CE (ii) a lattice Monte Carlo (MC) sampling technique, known as the overlapping distribution method, is used to calculate cluster free energies of Cu-clusters. Finally (iii) a meso-scale Cluster Dynamics (CD) model is constructed by taking the cluster free energies as input. The model is able to predict the evolution of size distributions of GPZ for different alloy compositions and ageing temperatures. To validate the model, kinetic MC simulations on the precipitation of GPZ are conducted. The results agree well with those obtained by our model.

Precipitation kinetics of GP zones, metastable η′ phase and equilibrium η phase in Al−5.46wt.%Zn−1.67wt.%Mg alloy

Abstract Thermal analysis is one of the most used techniques for analyzing the behavior of aluminum alloys in order to analyze the precipitation of Guinier−Preston (GP) zones and different phases formed. In the present work, the behavior of the Al−5.46wt.%Zn− 1.67wt.%Mg alloy was studied. The mechanism and kinetics of precipitation of the GP, the metastable phase η′ and the equilibrium phase η were investigated using DSC carried out between room temperature and 480 °C at heating rates of 5, 10, 15 and 20 °C/min. The apparent activation energies, calculated by DSC from isothermal calculation method using JMAK model, for GP, η′ and η were 56, 79 and 96 kJ/mol, respectively, and those calculated by non-isothermal calculation method using Kissinger methods were 57, 82 and 99 kJ/mol, respectively. The values of Avrami parameter, n, from isothermal calculation, during the precipitation of the GP, η′ and η were 1.103, 1.9075 and 1.92, respectively, and those calculated by non-isothermal were 0.86, 2.30 and 2.24, respectively. The results show that GP zones formation is governed by the migration of Zn and Mg atoms while the precipitation of the η′ metastable phase and the η stable phase is governed by both the migration and the diffusion of the solute atoms.

Atomistic simulations of early stage clusters in Al[sbnd]Mg alloys

The Cluster Expansion formalism based on Density Functional Theory (DFT) simulation data has been applied for AlMg alloys with high accuracy (∼1 meV/atom). The atomistic simulations are used to model the AlMg phase diagram, phase boundaries and the initial solute clustering at different compositions and temperatures. The obtained free energies of formation for the FCC, HCP and γ-phase are in accordance with the experimental phase diagram. The calculations demonstrate the formation of Guinier-Preston (GP) zones of Al3Mg (L12 phase) within the Al matrix under varying conditions. The computed transition temperatures where the ordered structures dissolve are approximately 50K higher than experimental data. The free energy barriers associated with the formation of GP-zones increase as the solute (Mg) concentrations are reduced and the temperature is increased.

Atomistic origins of Guinier-Preston zone formation and morphology in Al-Cu and Al-Ag alloys from first principles

Using first-principles computations we provide comprehensive energetics at zero Kelvin of Guinier-Preston zone (GPZ) formation and topologies of GPZs in two classical Al binaries: Al-Cu and Al-Ag. Without a priori geometric and morphological assumptions, we conclusively show that GPZs in Al-Cu prefer a planar arrangement of {100} layers while GPZs in Al-Ag consist of clusters with {111} facets. We expand the model to 34 additional elements from the fourth and the fifth rows of the periodic table. The morphologies can be entirely related to directional nature and hybridization of Al 3s and 3p orbitals in the presence of secondary elements.

The precipitation behavior and mechanical properties of cast Al-4.5Cu-3.5Zn-0.5Mg alloy

The precipitation behavior of a cast Al-4.5Cu-3.5Zn-0.5Mg alloy was comprehensively analyzed using differential scanning calorimetry (DSC), Vickers hardness measurement, electrical conductivity measurement, and transmission electron microscopy (TEM). The results revealed that three exothermic reactions occur during the heating process of DSC, giving rise to exothermic peaks at around 130, 250, and 280 °C due to the formation of Guinier-Preston (GP) zones and the precipitation of Ω phase and θ′ phase, respectively. TEM observations indicated that the predominant phase was the Ω phase, with only small amounts of the θ′ phase precipitating at 280 °C. STEM analysis showed that elements Zn and Mg segregate at the Ω–α interface. The precipitation of the Ω phase was remarkably promoted by the addition of Zn. The tensile strength and elongation heated up to 280 °C with the heating rate of 5 °C/min were 403 MPa and 9.2%, respectively. It was thus concluded that the precipitation sequence of the cast alloy was supersaturated solid solution (SSS)→GP zone→Ω phase→θ′ phase. The cast alloy possesses high strength and high toughness due to the predominance of the Ω phase and the lack of substantial θ′ phase.

Solid solution decomposition and Guinier-Preston zone formation in Al-Cu alloys: A kinetic theory with anisotropic interactions

Using methods of statistical kinetic theory parametrized with first-principles interatomic interactions that include chemical and strain contributions, we investigated the kinetics of decomposition and microstructure formation in Al-Cu alloys as a function of temperature and alloy concentration. We show that the decomposition of the solid solution forming platelets of copper, known as Guinier-Preston (GP) zones, includes several stages and that the transition from GP1 to GP2 zones is determined mainly by kinetic factors. With increasing temperature, the model predicts a gradual transition from platelet-like precipitates to equiaxial ones and at intermediate temperatures both precipitate morphologies may coexist.

The mechanism of two-step increase in hardness of precipitation hardened CuCoNiBe alloys and characterization of precipitates

The precipitation mechanism and microstructure of aged CuCoNiBe alloys have been studied. Two-step increase in the hardness of the material was ascertained and each increase was attributed to the formation of Guinier-Preston (G.P.) zones and precipitation of beryllides utilizing differential thermal analysis (DTA). Composing of different phases capable of precipitation hardening, characterization of the precipitates end up with ternary CoNiBex phase. Detailed FE-SEM analysis revealed that the secondary beryllides responsible for the hardening of the material precipitate inside the primary beryllides which were formed during solidification. Morphology of the precipitates aged at 460 °C for 2.5,4 and 24 h was also examined. It was concluded that the precipitates having rod-like morphology for aging 4 h and disk-shaped morphology for aging 2.5 and 24 h.