Age-hardenable Alloys Research Articles

Reshaping precipitate structure and morphology in an Al-Zn-Mg-Cu alloy through dislocation-precipitate interactions

Mechanical properties of age-hardenable alloys are significantly influenced by the intricate interplay between precipitates and mobile dislocations. This study examined how dislocations interact with nanosized precipitates at the atomic scale in an under-aged Al-Zn-Mg-Cu alloy. Results revealed that precipitates hinder dislocation movement but are also sheared by them during deformation. Furthermore, the sheared precipitates experienced structural collapse and deviated from their usual evolutionary path, forming ordered but non-periodic topologically close-packed structures and altered morphologies. The discoveries provide insights into dislocation-precipitate interactions and strain-induced structural changes, which could inspire strategies to modulate precipitates with tailored microstructures and thereby improved alloy properties.

On the order–disorder transformation within a main hardening precipitate in Al–Mg–Si alloys

ABSTRACT The earliest stages of solid-state precipitation in age hardenable alloys, before the first ordered phase formation, are notoriously elusive, yet often of central importance to material properties quantification. The order–disorder precipitate transformations are currently accepted to proceed by long-range chemical ordering on a substructure extending through the particle, with the Al–Mg–Si alloy Guinier-Preston (GP) zone → β″ transformation representing the most well-documented example. It is shown here that the structurally distinct β″ phase arises directly from solute diffusion-controlled precipitation within a precipitate, providing an alternative pathway for the transformation of one precipitate to a more thermodynamically stable precipitate. The emergence of dispersed β″ nuclei in the otherwise fcc-based particle is revealed by structural and chemical signs of a significant spatial solute variation. Furthermore, upon reduced aging temperature, heavily structurally flawed final particles result from increasingly challenging nucleus coalescence. The highly localised origin of this process suggests that early-stage precipitate transformation mechanisms be revisited for other age hardenable alloys.

Influence of the material properties on the clinching process and the resulting load-bearing capacity of the joint

This study focuses on the phenomenological change in material strength caused by a specific heat treatment and the subsequent analysis of the influence on the clinching process and the resulting joint properties. For this purpose, three series of tests were performed. In the first series of tests, the influence of heat treatment up to 340 °C on the mechanical properties of an age-hardenable AlMgSi alloy was investigated. Holding time and temperature were varied and the material strength was evaluated by tensile and hardness tests. Two strength-increasing and two strength-reducing heat treatment parameters were identified. In the second series of tests, selected heat treatment parameters were applied to a larger number of specimens and the joint strength was investigated by shear and head tensile tests. In the shear tensile test, mainly the properties of the punch-side material have an influence on the resulting joint strength. A change in strength of the die-side material can be neglected. In contrast, the properties of both sheets are important in the head tensile test. The strength of the joint will only increase if the strength of both sheets is increased. In general, a strength increasing heat treatment resulted in higher joint strength. In the third series of tests, the factor of punch displacement was considered, which was demonstrated to directly influence the formation of the clinched joint geometry.

Influence of Zinc Chloride Exposure on Microstructure and Mechanical Behavior of Age-Hardened AZ91 Magnesium Alloy.

The AZ91 magnesium alloy was subjected to a complex treatment involving age hardening (supersaturation and artificial aging) and simultaneous surface layer modification. The specimens were supersaturated in contact with a mixture containing varying concentrations of zinc chloride, followed by cooling either in air or water. After supersaturation, the specimens were subjected to artificial aging and then air-cooled. This process resulted in the formation of a surface layer made of zinc-rich phases. The thickness and microstructure of the surface layer were influenced by the process parameters, namely, the zinc chloride content in the mixture and the cooling rate during supersaturation. The treated specimens exhibited favorable tensile strength and greater elongation compared to the as-cast AZ91 alloy, with values comparable to those of the alloy subjected to standard T6 tempering. No cracking of the layer was observed under moderate deformation, though greater deformation resulted in the formation of cracks, primarily in the areas containing the Mg5Al2Zn2 intermetallic phase. The produced layer demonstrated strong metallurgical bonding to the AZ91 substrate.

Influence of aging temperature on the microstructure and mechanical properties of a Cu-2Be alloy

As a contribution to existing knowledge, this work studied the evolution of the hardness, elastic modulus and microstructure of an age-hardenable Cu-2Be alloy when subjected to aging treatments at different times, in order to obtain the hardness curves at all stages, and for different aging temperatures in the range of 320–400 °C. In addition, the effect of performing a prior heat treatment consisting of 2 h at 225 °C on the mechanical behavior of the material was analyzed. The elastic modulus and hardness were measured using IET technique and Vickers hardness, respectively. The best mechanical properties (higher hardness values and high elastic modulus) were obtained for treatments at aging temperatures of 320 and 350 °C, with aging times of 40 and 5 h, respectively. The maximum hardness decreases with temperature and increases by about 25 HV by performing a prior thermal treatment at 225 °C. At the maximum hardness peaks, the elastic modulus exhibits similar values, while the asymptotic value increases slightly with temperature. SAXS measurements were used to study the size and shape of the nanoprecipitates formed after aging treatments at different temperatures and times. At the hardness peaks, precipitates have ellipsoid shapes and their size increases with the temperature from 18 to 30 nm. After the hardness peak at 400 °C, the hardness and modulus decrease slightly, while the precipitate size increases to 58 nm.

Selective laser melting of Ti modified age-hardenable Al–Cu alloy: Microstructure and mechanical properties evolution along the building direction

In this work, the differences in the microstructure and mechanical properties of the Ti modified Al–Cu alloy along the selective laser melting (SLM) build direction are investigated. The results showed the mechanical properties decrease gradually from bottom to top along the building direction, the hardness decreases from 121HV at the bottom to 108HV at the top, the tensile strength also decreases from 375.8 MPa to 359.2 MPa, while the ductility is not improved. Transmission electron microscopy observation confirms that numerous θ′ phase is formed in the bottom, while rarely in the top, indicating that the difference of mechanical properties is caused by the θ′ phase, which is formed during SLM due to the substrate heating. Additionally, the top sample requires longer time to reach peak hardness during post-heat treatment at 200 °C. Therefore, appropriate substrate temperature and post-heat treatment are necessary for age-hardenable alloys to achieve consistent mechanical properties along the building direction.

Regulation of dislocation density and precipitates to maximize the mechanical strength and electrical conductivity in continuously extruded Cu–Ni–Si alloys

Without homogenization or special micro-alloying treatment, the tensile strength and electrical conductivity of a commercial Cu–Ni–Si alloy can achieve 813 MPa and 51.2% IACS, respectively. The excellent combination properties were achieved by regulating the thermomechanical treatment to obtain a critical status of precipitates and dislocation densities. Although a short time pre-aging process can increase the mechanical strength, the electrical conductivity was not dramatically enhanced due to the limited precipitation process. A long-time pre-aging process or a too-high density of dislocations is not favorable to the nucleation of new precipitates, which, instead, causes the coarsening of pre-precipitates, resulting in a limited increase in strength. A critical dislocation density can be determined to boost the nucleation sites of precipitates to simultaneously maximize the mechanical and electrical properties of Cu–Ni–Si alloys. The obtained mechanical and electrical properties can meet the requirements for very large-scale lead frames. Thus, our investigation not only provides a new route for manufacturing high-performance Cu–Ni–Si alloys but also new insights into the role of dislocations in enhancing the mechanical properties of age-hardening alloys.

Crystal Plasticity Finite Element Analyses on the Formability of AA6061 Aluminum Alloy with Different Ageing Treatments

Different ageing treatments have been developed to achieve targeted properties in aluminum alloys through altering microstructures. However, there is a lack of understanding regarding the effect of ageing treatments on the formability of these alloys. In this study, we employed crystal plasticity finite element (CPFE) modeling, in conjunction with the Marciniak-Kuczynski (M-K) approach, to investigate the effects of ageing treatments on the mechanical properties and formability of AA6061 aluminum alloy. The as-received sheet was in the T6 heat treatment state, which was subjected to artificial ageing and pre-ageing, respectively, to achieve two age-hardened alloys with modified precipitation states. The microstructures and crystallographic textures of the three alloys were measured using the electron backscattering diffraction (EBSD) technique, and uniaxial tensile tests were performed along the rolling direction (RD), transverse direction (TD), and diagonal direction (DD, 45° to the RD) for each alloy. The forming limit curve (FLC) of the as-received alloy was determined using the Nakazima test. The dependence of mechanical strength, tensile ductility, and work-hardening behavior on the ageing treatments was clarified. Then, the tensile test results were utilized to calibrate the modeling parameters used in the CPFE model, whereas the FLC predictability of the developed model was validated with the experimental one. In the formability analysis, the effects of the ageing treatment on the FLC exhibit a notable dependency on loading paths, and the pre-aged alloy exhibits better formability than the other two at the plane strain tension state, thanks to its high work-hardening levels. In addition, the deformed textures along the different loading paths and the effects of the initial texture on the FLC are also discussed.

Tailoring hierarchical microstructures and nanoprecipitates in additive-manufactured Al-Zn-Mg-Cu-Nb alloys for simultaneously enhancing strength and ductility

Additive manufacturing provides an efficient way of producing metallic components with complex geometries. Their microstructure is substantially different to those from conventional processing, creating opportunities for manipulating the final microstructure and properties via heat treatment. Here, we demonstrate that as-built heterostructures in an Al-Zn-Mg-Cu-Nb alloy produced during the solidification of molten pools provide a driving force and additional Zener pinning sources for recrystallization. This creates a bimodal grain structure after solution treatment, causing additional hetero-deformation-induced strengthening and hardening. Coarse grains are found to promote work hardening and blunt the propagate of cracks during deformation, increasing ductility. Together with precipitation strengthening from a high number density nanoprecipitates, the simultaneous improvement of strength and ductility in a highly alloyed Al-Zn-Mg-Cu-Nb alloy is achieved. These results provide a simple strategy for the development of additively manufactured age-hardening alloys with improved strength and ductility for high performance structural applications.

Quantifying the energy of vacancy mediated diffusion of solute Cu, Mg, Ag and Zn atoms in an aluminum Matrix: An insight from ab initio calculations

In this work, a consistent and accurate set of energy barrier values for the migration process, assisted by a vacancy, of atoms of different chemical elements belonging to the two most relevant families of age-hardenable Al-Cu and Al-Zn-based alloys is given. These values were calculated using the Nudged Elastic Band method within the Density Functional Theory framework for a vacancy diffusing in simple configurations of solute atoms, specifically the main alloying elements Cu and Zn involving a single solute atom or more complex configurations, such as pairs of elements of the same or different chemical species. In both cases, in our calculations, the presence of Mg and/or Ag solute atoms is also considered. The results obtained are analyzed in terms of the energetic interaction among vacancies and the different solute atoms. The advantage of having a set of vacancy diffusion energy barrier values calculated under the same criteria is that it can be used as reliable input parameters for future kinetic Monte Carlo simulations to study the precipitation kinetics in a wide type of age-hardenable alloys.

Kinetic Monte Carlo simulations of solute clustering during quenching and aging of Al–Mg–Zn alloys

The physical mechanisms behind cluster formation during quenching and aging of age-hardening metallic alloys are poorly understood based on classical nucleation and growth theories, especially in multicomponent alloys with supersaturated vacancies and fast-diffusing solute atoms. Here, solute clustering in an Al–Mg–Zn alloy during quenching immediately after high-temperature (800 K) solution treatment is modeled with a newly developed kinetic Monte Carlo (kMC) framework. This includes accurate surrogate models trained using first-principles-calculated data to predict vacancy migration energetics on the fly as functions of local lattice occupations. First, kMC simulations at constant aging temperatures revealed that temporal evolution of the number, sizes, and compositions of solute clusters change with aging temperature. Such changes are consistent with our analyses based on classical nucleation theory (CNT) and Monte Carlo simulations. Second, the kMC algorithm was revised for simulating quenching processes by iteratively updating the kMC temperature based on simulated cooling rate profiles. These quenching simulations show that rapid solute clustering mainly occurs from ∼600 to ∼400 K during cooling. Below ∼400 K, the clustering is suppressed to a steady state because vacancies are trapped by stable clusters with sizes of ∼2 nm and high magnitudes of formation energies. The configurations of these steady-state clusters closely resemble the solute clusters we observed experimentally in 7XXX series Al–Mg–Zn-based alloy samples immediately after quenching. A two-stage vacancy trapping mechanism revealed in our simulations can provide physical guidance to tune precipitation kinetics during natural and artificial aging.

Localized Corrosion Resistance of Nickel Alloy 718 in Chloride-Containing Environments

Resistant to aggressive reducing environments and combining excellent mechanical properties, the age-hardenable Alloy 718 is the most broadly used nickel alloy in the oil and gas industry. Nevertheless, its localized corrosion resistance in oxidizing halide-containing environments has yet to be thoroughly investigated, with conflicting results discussed in the literature. In this regard, Alloy 718 has a relatively low pitting resistance equivalent, limiting in practice its use, where localized corrosion is expected, particularly in seawater applications. This work quantified the localized corrosion resistance of the alloy in chloride-containing environments. The potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) technique was used to determine the crevice corrosion repassivation potential (ER,CREV) in deaerated natural seawater at different temperatures for two Alloy 718 grades (i.e., aerospace, as per ASTM B670, and oil and gas, as per API 6ACRA, grades). Additionally, PD polarization testing was conducted for the API 6ACRA grade in acidified solutions with varying chloride concentrations to simulate pit/crevice-like conditions. In the aerospace heat treatment condition, Alloy 718 suffered crevice corrosion at temperatures as low as 10°C, while the API 6ACRA grade remained unaffected at the same testing temperature. Stable crevice corrosion occurred at 20°C and 50°C for both grades.

Regulating microstructure and improving precipitation hardening response of fine-grained Mg-RE-Ag hot-extruded alloy by extreme short-time heat treatment

The difficulty in exerting beneficial effects of fine grains, high number density nano-precipitates as well as weakened texture simultaneously is currently the bottleneck limiting the development of high performance (high strength, high plasticity, and isotropic) age-hardenable Mg-RE alloys. The key difficulty is that the generation of excessive dynamic precipitates is always accompanied by the formation of fully dynamic recrystallized ultrafine grains during thermal deformation, causing a significant depletion of solutes within the entire matrix and finally seriously weakening the subsequent aging nano-precipitation. In this work, the process of extreme short-time (2 min) heat treatment (ESHT) followed by peak aging treatment is applied to an ultrafine-grained weak-textured Mg-RE-Ag as-extruded alloy with numerous dynamic precipitates. The precipitation hardening response is dramatically increased (ΔHV = 37.2 HV) and the yield strength is increased by nearly 50 MPa. This is attributed to the fact that concentration of main solutes in matrix is greatly increased within only 2 min of heat treatment promoting the precipitation of nano-precipitates (prismatic β′ and basal γ′′) in the absence of excessive grain growth. The large difference between the initial and equilibrium solute concentrations, the extremely short distance required for solute diffusion within sub-microscale grains, and the effective pinning of grain boundaries by excess dynamic precipitates are responsible for the unusual microevolution during the ESHT. In addition, the formation of precipitation-free zones (PFZs) is attributed to ‘uphill’ diffusion during aging, and novel γ′′-PFZs and β′-PRZs are found. The ESHT followed by peak aging treatment provides a new way to simultaneously exert the beneficial effects of fine grains and high number density nano-precipitates.

Improving microstructure and deformation surface quality of novel Al-Mg-Zn-Zr alloy by varying homogenization regimes

Al-Mg-Zn-based alloys are newly developed high-performance age-hardenable 5xxx series alloys. However, they suffer from poor surface quality due to the Lüders bands and the Portevin-Le-Chatelier (PLC) effect during plastic deformation, and the elongation (EL) is impaired by the large constituent particles. In this work, various homogenization regimes combined with minor addition of Zr were employed to manipulate the microstructural evolution of Al-7Mg-2Zn-0.15Zr alloy and thus the deformation characteristics. The results demonstrate that a two-step homogenization treatment of 400 °C/12 h + 465 °C/12 h can sufficiently dissolve the constituent phase and eliminate the micro-segregation, as well as achieve a fine and uniform distribution of Al3Zr dispersoids, which results in the enhanced yield strength (YS) of 201 MPa and ultimate strength (UTS) of 376 MPa after rolling and annealing. Most importantly, it has been revealed that the dispersed Al3Zr formed via two-step homogenization is favorable to impede the movement of dislocations, which extends the waiting time, increases the stress amplitude, and thus diminishes the number of PLC bands. Moreover, the dislocation network induced by dispersed Al3Zr can further inhibit the movement of active dislocations, which inhibits the formation of Lüders bands. This study provides a simple and effective homogenization regime to optimize the mechanical response and surface quality of alloys with high-Mg contents.

ALUMINUM OXIDE IN VACUUM CONDENSATES BASED ON COPPER

Strengthening of copper and copper alloys with aluminum oxide particles is an important technological technique. Composite materials based on copper strengthened with aluminum oxide particles have a significant operating temperature range and better mechanical properties at elevated temperatures than age-hardening alloys. However, at moderate operating temperatures, the use of age-hardening alloys remains more economically justified. The main direction in the development of composites strengthened with oxide particles is the dispersion of the strengthening phase to sizes of several or one nanometer, which will bring their mechanical properties to the level of age-hardening alloys. However, dispersion leads to a significant increase not only in strength, but also in electrical resistance. The adverse effect on the electrical conductivity of dispersed particles will decrease with a decrease in the volume fraction of oxide, bringing the electrical conductivity closer to the level of copper. Based on an analysis of the materials of scientific research devoted to the production of dispersion-strengthened Cu–Al2O3 composites, it was concluded that the most significant progress in the dispersion of oxide particles with uniform distribution in the copper matrix of the composite was achieved using the method of electron beam evaporation and simultaneous deposition (condensation) of vapors of components in vacuum (EB–PVD). Structural studies of the morphology of aluminum oxide particles in dispersion-strengthened copper composites were carried out using methods such as: X-ray fluorescence analysis, transmission electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). The average size of aluminum oxide particles in the studied dispersion-strengthened composites was in the range from 1.8 to 3 nm. In the work, the dependence of the specific electrical resistance of composites (ρ) on the oxide content was investigated. It was found that a decrease in the size of oxide particles leads to an increase in the electrical resistance of the dispersion-strengthened composite. The studies of the method of electron beam evaporation and subsequent condensation in vacuum, presented in the work, confirm the possibility of further dispersion of aluminum oxide particles with simultaneous narrowing of the distribution histogram. This opens up prospects for further improvement of vacuum dispersion-strengthened copper-based composites.

Investigation of the Formability of AA6010 in an Integrated Forming and Hardening Process Aiming to Reduce the Energy Consumption in High Volume Production of Automotive Components

Hot deformation and in-die quenching of aluminum components for the automotive industry is a cost and energy efficient technique that has been developed and thoroughly evaluated in recent years. The performance of this process is considered higher when compared to traditional cold metal forming due to shorter process times, low-cost machinery, and a high level of structural integrity in fabricated parts. The work presented in this paper provides several approaches for the formability of age hardenable 6xxx alloy sheets when forming at different temperatures. Warm tensile testing and formability cup testing were carried out to investigate the alloy formability at different temperatures. The results indicate that the formability of candidate alloys is not significantly affected by deformation temperatures or conditions, which provides great freedom when designing an automated production process with high productivity and minimal environmental impact. The candidate alloy can be deep drawn without severe thinning at the whole temperature range, from room temperature (RT) to solutionizing temperature.

Strength-ductility synergy in a wrought AZ80 magnesium alloy by microstructure engineering

The present investigation is conducted on microstructure engineering of a Mg–8Al-0.5Zn (AZ80) alloy by different processing routes including conventional aging, cold rolling followed by aging, and aging followed by warm and hot rolling in order to find a strength and ductility synergy. Aging at 175 °C followed by hot rolling at 200 °C is suggested to obtain remarkable mechanical properties with a strength of ∼450 MPa together with an acceptable total elongation of ∼8%. This procedure decorates a bimodal microstructure containing a low-volume fraction of discontinuous precipitates at the grain boundaries and a high-volume fraction of fine continuous precipitates in grain interiors. Microstructural analyses and a microstructure-based strength calculation confirm that the recommended thermomechanical treatment leads to bimodal structure and grain boundaries strengthening together with increasing precipitation strengthening significantly by reducing the size of CP and increasing CP/DP. Achieving superior mechanical properties without adding alloying elements for strengthening is the main advantage of the proposed method. It is suggested that aging followed by hot rolling plays a pivotal role in engineering the microstructure to improve the mechanical properties of AZ80 alloy which is applicable to other age-hardenable magnesium alloys.

Improving the Mechanical Properties of an Age-Hardenable Magnesium Alloy via Regulating Precipitate Scale and Building Dislocation Configuration

Improving the Mechanical Properties of an Age-Hardenable Magnesium Alloy via Regulating Precipitate Scale and Building Dislocation Configuration

Dislocation density-mediated creep ageing behavior of an Al-Cu-Li alloy

Creep aging is well-known to be a time-dependent, coupled process of deformation and precipitation strengthening for age-hardening alloys. Its existing mechanisms are mainly attributed to those interactions between atomic diffusion and dislocation motion. However, an understanding of the relationship between dislocation density and a special multistage creep behavior, i.e., double steady creep feature, is still far limited. Here we investigate the effect of various dislocation density levels on such an abnormal multistage creep of an Al-Cu-Li alloy. We find that the increased dislocation densities enable an apparent time decrease (from 6.2 h to 0.8 h) of their first steady Ⅱ-stage. The yield strength of post-aged samples increases from 425.0 MPa to 580.0 MPa while the corresponding elongation decreases from 12.3% to 7.3% for the creep-aged samples #1 to #4. Microstructural results also reveal that a great difference in dislocation configuration, tailored by various density levels, results in varying creep processes of the Ⅱ-stage. This stage is closely related to the nucleation and early growth of T1 precipitates. Their number densities (maximum: 2.9 × 1019 m–3) and the average length (maximum: 21.3 nm) of T1 precipitates are much smaller than those of the stable peak-aged T1 phases, suggesting that creep Ⅱ-stage of all three creep-aged samples is dominant by the nucleation and initial growth of those T1 precipitates. This study provides valuable insights into the dislocation density-mediated creep deformation of an Al-Cu-Li alloy.

Formation mechanism of [formula omitted] phase in age-hardenable magnesium-rare earth alloys: Insight from in-situ and ex-situ observations with HAADF-STEM

The formation mechanism of β1 phase has been a controversy in age-hardenable magnesium-rare earth alloys for over two decades. Its nucleation and growth were not clearly illustrated so far due to the lack of direct experimental evidence. In the present work, we unveil the formation mechanism of β1 phase based on the in-situ and ex-situ observations using high-angle annular dark-field scanning transmission electron microscopy. The direct evidences reveal that β1 phase is transformed from βF′ phase by short-range solute diffusion, atomic shuffle coupled with shear mechanism. The βF′ precipitate firstly converts to intermediate precipitate via short-range solute diffusion in its zig-zag array on (101¯0)α plane, and then transforms into β1 precipitate via atomic shuffle in the modified βF′ lattice and shear of (101¯0)α zig-zag array by ∼5° with respect to [112¯0]α orientation. These findings shed light on the formation mechanism of β1 phase, and hopefully address the long-standing controversy.