Multistage Aging Research Articles

Clustering behaviour of pre-aged Al-Mg-Si alloy during subsequent natural ageing and paint baking

The clustering behaviour of a pre-aged Al-Mg-Si alloy during natural ageing (NA) and paint baking (PB) was systematically investigated by hardness measurements and atom probe tomography (APT). The results show that pre-ageing (PA) can effectively retard the negative effect of NA and enhance the paint-baking response even after long-term NA (up to one year). The pre-aged alloy shows a good resistance against clustering within a short NA period of one week, and the capability is enhanced with increasing pre-ageing time. However, the clustering becomes significant after prolonged NA, especially for one year. The increasing number density of clusters containing 10–30 solute atoms contributes most to the hardness increase after prolonged NA. The mechanism about two synergistic processes of ‘vacancy trap’ and ‘vacancy escape’ is introduced to explain the phenomenon. The alloy with a prior PA above 2 h exhibits a good PB response even after one-year NA. Such performance is attributed to large-size precipitates (GP zones or pre-β″ precipitates) formed during PA, which can serve as nuclei for β″ due to a similar composition. Moreover, a strength model has been established to predict the different strengthening effect and evaluate contributions from clusters, GP zones and pre-β″/β″ precipitates during multi-stage ageing in the Al-Mg-Si alloy.

Effects of sub-solvus ageing on the tensile and creep properties of a new cast nickel-based superalloy

For nickel-based superalloys with medium volume-fraction γʹ phase (20 %–40 %), dual or multi-stage aging treatments are usually conducted to generate a microstructure containing the multimodal distribution of γʹ for a balance of strength and plasticity. In the present study, the microstructure and high-temperature properties of a novel cast nickel-based superalloy K4800 were investigated after being subjected to three heat treatments (HT) procedures, namely HT1: 1180 °C/4 h + 1090 °C/2 h + 800 °C/16 h, HT2: 1180 °C/4 h + 1060 °C/2 h + 800 °C/16 h and HT3: 1180 °C/4 h + 800 °C/16 h. It was found that the sub-solvus aging treatments at 1090 and 1060 °C precipitated sub-micron-sized (∼300 nm) primary γʹ phase which enhanced the ductility during 800 °C tensile (the total elongation of T1, T2, and T3 samples were 6.75 %, 7.3 %, and 3.25 %, respectively) without evidently impairing the strength. After careful microstructure observation and deformation mechanism analysis, the enhancement of elongation was rationalized that the precipitation of the sub-micron-sized primary γʹ phase decreased the volume-fraction and size of the nanometer-sized γʹ phase which was precipitated at 800 °C, and simultaneously, promoted the dislocation movement by suppressing the non-planar slip. However, an excessive amount of the sub-micron-sized primary γʹ phase led to a faster ripening process of the nanometer-sized γʹ during creep, which decreased the creep life at 800 °C/430 MPa (T1: 125 h, T2: 199 h, and T3: 198 h). Based on this, we monitored the number density of nanometer-sized γʹ phase coexisting with different amounts of large γʹ during creep. An area fraction less than 7 % of the sub-micron-sized γʹ phase was considered to have little detrimental effect on the creep life of K4800 alloy, which corresponded to a sub-solvus temperature range about 1080–1090 °C.

Microstructural and mechanical properties of additive manufactured 2319 aluminum alloy annular-shaped components: As-built and multistage heat treatment

A novel multistage aging heat-treatment method was applied to investigate the microstructural evolution and properties of an additive-manufactured 2319 aluminum alloy. The microstructure and mechanical properties were analyzed, and the results revealed that the as-built annular-shaped components were characterized by an alternating distribution of thin equiaxed crystals, dendrites, and coarse columnar crystals. The microstructure occurs recrystallization and the average grain size decreased after the two-stage aging treatment. The maximum texture strength in the selected area increased after the two-stage aging treatment, and the angle grain boundaries were evenly distributed. Solid solution plus aging could significantly improve ultimate tensile strength and hardness. The tensile fracture morphology was typical of ductile fracture. Two-stage aging exhibited a higher corrosion resistance.

Microstructure and mechanical properties of selective laser melted Al-Zn-Mg-Si-Sc-Zr alloy with high hot-cracking resistance

A novel additive manufacturing dedicated Al-Zn-Mg-Si-Sc-Zr alloy is designed in this work, and its formability during selective laser melting (SLM) is investigated. Adding 3 % Si to the Al-Zn-Mg alloy leads to a refined microstructure and significant enhancement of hot-cracking resistance. By means of SEM, TEM and EBSD analysis some aspects of the alloy microstructure in as-built and thermal-treated states are discussed. Unlike traditional Al-Zn-Mg alloys, the main strengthening phase in the Al-Zn-Mg-Si-Sc-Zr alloy is Mg2Si, which forms a cellular structure and contributes to the favorable mechanical properties of the as-printed alloy. The as-built alloy exhibits a hardness of 138 HV, ultimate tensile strength (UTS) of 383 MPa, and elongation of 9.5 %. After solution and aging treatment, the UTS decreases to approximately 341–349 MPa, while the elongation increases to 17.6 %. However, with direct multistage aging treatment, the UTS can be increased to 409 MPa, albeit with a reduced elongation of 0.6–1.6 %. The variations in mechanical properties are attributed to the morphology of the Mg2Si phase and its coherent relationship with the Al matrix under different heat treatment processes.

Evaluation of the Composition, Thermal and Mechanical Behavior, and Color Changes of Artificially and Naturally Aged Polymers for the Conservation of Stained Glass Windows.

Investigations of historical conservation materials on historical stained glass windows of the Naumburg Cathedral in Germany offered an opportunity for the study of polymers, naturally aged in a non-controlled environment. This allowed the conservation history of the cathedral to be traced and expanded by valuable insights. The historical materials were characterized through the use of spectroscopy (FTIR, Raman), thermal analysis, PY-GC/MS, and SEC on taken samples. The analyses show that acrylate resins were predominantly used for conservation. The lamination material from the 1940s is particularly noteworthy. Epoxy resins were also identified in isolated cases. Artificial aging was used to investigate the influence of environmental influences on the properties of the identified materials. Through a multi-stage aging program, influences of UV radiation, high temperatures and high humidity can be considered in isolation. Piaflex F20, Epilox, Paraloid B72 as a modern material and combinations of Paraloid B72/diisobutyl phthalate and PMA/diisobutyl phthalate were investigated. The parameters yellowing, FTIR spectra, Raman spectra, molecular mass and conformation, glass transition temperature, thermal behavior, and adhesive strength on glass were determined. The effects of the environmental parameters on the investigated materials are differentiated. UV and extreme temperatures tend to show a stronger influence than humidity. The comparison of the artificially aged samples with the naturally aged samples from the cathedral shows that the latter were less aged. Recommendations for the conservation of the historical stained glass windows were derived from the results of the investigation.

Strengthening effect of prefabrication (10–12) tensile twinning on AZ80+0.4%Ce magnesium alloy and inhibition mechanism of discontinuous precipitation

This paper provided an effective method to further improve the mechanical properties of the AZ80+0.4%Ce magnesium alloy wheel spoke. The effect of high strength and ductility was obtained with a yield strength of 295.36 MPa, an elongation of 10%, by the combination of pre-deformation (7% deformation) and two-stage aging treatment (120 °C/9 h + 175 °C/24 h). The evolution of the microstructure and properties of the alloy was explored under the coupling conditions of different pre-deformation degrees and multi-stage aging. The results show that, pre-deformation introduced a large number of (101¯2) tensile twinning and dislocations, which greatly promoted the probability of continuous precipitates (CPs) appearing. On the contrary, the discontinuous precipitates (DPs) were limited by the vertical and horizontal twin structure. As a result, the pre-nucleation method of two-stage aging increased the proportion of CPs by 34%-38%. Owing to the DPs was effectively suppressed, the alloy's yield strength has been greatly improved. Besides, under multi-stage aging, the twin boundaries induce protruding nucleation to form static recrystallization by hindering the migration of dislocations, and the matrix swallows the twins, then the texture gradually tilts from the two poles to the basal plane. As an important supplement, the grain refinement and oblique texture promoted the improvement of the yield strength of the component.

Effect of multi-stage aging on the precipitation strengthening and mechanical properties for an Al-Mg-Si-Ag alloy

Multi-step aging and microalloying are the methods that can effectively facilitate the precipitation strengthening and enhance the performance of Al-Mg-Si alloys. The effect of multi-step aging on the precipitation and properties for an Ag-microalloying Al-Mg-Si alloy is systematically investigated in this article. Controlling temperatures of the primary and secondary aging steps are essential for promoting the effectiveness of the multi-stage aging. The optimal multi-stage aging treatment is identified as: 100 °C × 2 h + 180 °C × 2 h, which gives a peak hardness of 142.4 HV (13.7 HV higher than that of the single-stage aging) for this alloy. It is demonstrated that the primary aging at 100 °C can promote the formation of high density of Cluster(2), and the secondary aging at 180 °C accelerates the transformation of Cluster(2) to β″ caused by the good combination of thermodynamics and kinetics of the transformation. The incorporation of Ag atoms can further promote the nucleation of the β” phase. • Multi-stage aging can improve the precipitation strengthening and mechanical performances of Al-Mg-Si-Ag alloys. • Controlling clusters formed in primary aging is key for enhancing the precipitation of β″ phases during secondary aging. • The optimal multi-stage aging treatment is 100 °C × 2 h + 180 °C × 2 h, which gives a peak hardness of 142.43 HV.

Influence of Cu distribution on the coercivity of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during microwave aging process

In this study, we investigated the magnetic properties, microstructures, and Cu distribution on the mechanism of coercivity enhancement of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during two-step and multistage microwave aging treatments. The intrinsic coercivity, Hcj, and maximum energy product, (BH)max, increase from 15.9 to 24.4 kOe and from 24.3 to 26.5 MGOe, respectively, upon transitioning from two-step to multistage aging. The aged alloys possess a cellular microstructure comprising rhombohedral 2:17 R cell phase and hexagonal 1:5 cell boundary phase after a short microwave aging time of 3 h. The multistage aged alloy exhibits a larger cell size, thicker cell walls, and higher Cu concentration in the 1:5 phase than the two-step aged sample. Elemental mapping reveals Cu atom enrichment at the cell boundary centers and at the 2:17 R/1:5 triple junction of the cell boundary phase, but depletion at the cell boundary edges during multistage aging. First-principles calculations indicate that Cu atoms occupy the Co(2c) sites of a SmCo5 unit cell and weaken not only the strong Co(2c)–Co(3g) direct exchange-coupling interaction, but also the exchange-coupling interaction between Sm 4f and Co 3d electrons, giving rise to low 1:5 phase exchange constant values. Furthermore, the enrichment of Cu atoms at the center of the cell wall and at the 2:17 R/1:5 triple junction of the cell boundary phase produces a gradual increase in the domain wall energy difference as a function of the edge-to-center distance in the cell boundary phase. This enrichment enhances the domain wall pinning strength and increases coercivity. A high concentration and optimal distribution of Cu are critical for achieving a high Hcj. The results of this study provide theoretical guidance to future studies on the mechanism controlling the coercivity of Sm2Co17-based permanent magnets.

Microstructure analysis of 7050 aluminum alloy processed by multistage aging treatments

In the current study differential scanning calorimetry (DSC) and transmission Electron Microscopy (TEM) analyzes were performed in order to investigate the bimodal microstructure feature by taking into account both density and morphology of precipitates, which are transformed from the retrogression and reaging (RRA) along with interrupted aging (T6I4) conditions within the 7050 aluminum alloy. Results showed that RRA treatment is conducive to a higher density of fine precipitates distributed homogeneously in the Al matrix, with precipitates essentially being shear-resistant η’ in view of a smaller density of shearable GP zone and incoherent η precipitates present. Conversely, the T6I4-65 treatment generated a smaller density of less fine shear-resistant precipitates, but with a higher density of shearable GP zone and incoherent η precipitates. Thus, the RRA treatment might promote a beneficial effect on correlation between yield strength and ductility owing to higher balance of combination mechanisms between shearable GP zones and shear-resistant η’ precipitates with movement dislocations as opposed to T6I4-65 treatment within the 7050 aluminum alloy. Both RRA and T6I4-65 treatments culminated in a slight decrease in either potential or pitting corrosion in comparison to the T7451 treatment.

Multistage strain aging phenomenon of low-carbon steels with rolling pre-strain

In this study, a multistage strain aging method that used rolling pre-strain (compression) was developed to study the effects of temperature, and inter-pass time on static strain aging behavior of low carbon steel. An increase in hardness and strength caused by work hardening due to the forming process and aging at every stage of aging that is calculated separately. To comparing the effects of multistage aging against typical strain aging, the samples were rolled and subjected to the aging process both exist in typical one-stage aging that setting a 20% rolling pre-strain and in multiple stages pre-strain by setting a 5% rolling ratio in four stages. The mechanical properties of aged samples are compared with each other to study the effects of time and temperatures on strength and hardness. To evaluate the effects of pre-strain type (tensile or compression) on mechanical properties, one sample was subjected to tensile and compared with rolling pre-strain samples in similar conditions. The results show that in multistage static strain aged samples with rolling pre-strain, the peak of the strain stress curve occurs in 10–15 min rather than samples that aged after tensile pre-strain. Multistage strain aging increased samples’ strength by 10% and reduced their plasticity by 5%. To evaluate the effects of temperature on the time, we propose the maximum hardness and strength, the aging temperature of the samples was increased to 250 °C and it was observed that the maximum hardness and strength time is reduced by 10 min.

Control of hysteretic properties in powder alloys based on the Fe–Cr–Co system

Hysteresis alloys based on the Fe–Cr–Co system are of scientific and practical interest, primarily due to their high manufacturability, high level and temperature stability of magnetic properties, which provide the required hysteresis magnet performance including residual magnetic induction, coercive force, and loop squareness ratio. The research was aimed to control and stabilize the Fe–Cr–Co ridge alloy magnetic properties using reageing. The 22Kh15K4MS hard magnetic powder alloy was investigated after quenching and multistage aging. Billets were obtained by cold pressing at a pressure of 600 MPa and subsequent sintering in vacuum. The samples obtained by sintering in the α phase in the presence of the liquid phase formed during contact melting had a porosity of up to 1 %. The concentration heterogeneity of chromium and cobalt distribution was 0.06–0.08. The alloy magnetic structure parameters were determined by electron microscopy. The relationship between the magnetic structure formation kinetics during aging and the level of magnetic properties was established. After aging, the fine structure of the 22Kh15K4MS alloy was represented by elongated α1 phase sections in the α2 phase matrix. The average particle sizes of the α1 phase were »124 nm in length and »44 nm in width after the first stage of aging, and they remained the same after final aging. It was shown that it is possible to control magnetic properties by reaging without repeated quenching. A slight change in the size and morphology of magnetic phase particles was observed during aging. The influence of the number of reaging cycles on the stability of magnetic properties over time was determined.

Comparison finite element analysis on duralium strength against multistage artificial aging process

Purpose: To analyze and estimate the strength of duralium rivets which had been treated by using multistage artificial aging compared with duralium that had not been treated. This processwas necessary to be conducted in riveting process effectively. Duralium has been widely used in aerospace industry, one of duralium usage in aerospace industry is aircraft fittings such as rivet. Riveting is one of method that used for joining airframe structural components. During riveting process, the load transfer causing stress that led to the fatigue. Riveting process also causes deformation on the rivet and sheet metal. Deformation that occurs on the rivet will affect the performance of rivet structure. Thus, duralium rivet was analyzed its total deformation, shear stress, and its equivalent stress Von Misses. Design/methodology/approach: that used in this study was finite element analysis. Geometry of rivet that used in this study was drawn by using Autodesk Inventor Professional 2018. While total deformation, shear stress and equivalent stress Von Mises on duralium rivets were found out by using ANSYS Workbench 18.1. Findings: Comparison result was obtained between duralium rivet with and without treatment of multistage artificial aging. The result shown that total deformation, shear stress and equivalent stress Von Mises which obtained by duralium rivet with multistage artificial aging had the lower value than duralium rivet without multistage artificial aging. Duralium rivet with multistage artificial aging could be used as aircraft fitting which had the higher strength. Research limitations/implications: Direct experiment on duralium rivet had not been done yet, this study only did simulation based on data that obtained form previous research that had been conducted by the researcher. Practical implications: Duralium rivet with multistage artificial aging had lower value on total deformation, shear stress, and equivalent stress Von Misses, thus duralium rivet with multistage aritificial aging had a higher strength.

Three-dimensional Phase-field simulation of γ″ precipitation kinetics in Inconel 625 during heat treatment

Metastable γ″ (D022-Ni3Nb) particles are strengthening precipitates for commercial Inconel 625 Ni-based superalloy. Understanding their morphological evolution is critical for evaluating its hardening effects and guiding heat treatments to improve its yield strength. Here we present a phase-field model to model and analyze the nucleation and growth kinetics of metastable γ″ in Inconel 625 during isothermal and non-isothermal aging conditions with thermodynamic properties, diffusion coefficients, and misfit strain data from both the literature and calibration to experimental results. We implemented the classical nucleation theory to introduce local nucleation taking into account local supersaturation. The simulated mean particle length and aspect ratio are in agreement with experimental data at 600 °C and 650 °C during isothermal aging. Utilizing the phase-field simulation results as input parameters, the coherency strengthening effect of γ″ as a function of aging temperature and time is predicted. A multistage aging strategy to optimize the γ″ strengthening effect and to reduce aging times using the developed phase-field model and coherency strengthening model is suggested.

Multistage aging treatment influenced precipitate characteristics improve mechanical and corrosion properties in powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy subjected to T6, RRA, T76 and T74 aging treatments were investigated. The TEM and DSC results reveal that the dominant intragranular precipitates under those conditions are GP (I, II) + η' phase, GP (II) + η' phase, η' phase and η' + η phases, respectively. The quantitative characteristics (size, interspacing and volume fraction) show that the small volume fraction of intragranular precipitates limits the yield strength of the T6 alloy, while the enlarging of interspacing caused by the coarsening of intragranular precipitates is the main reason for the reduction of yield strength of over-aged alloys. The continuously distributed grain boundary precipitates (GBPs) with low Cu content in the T6 alloy are responsible for higher corrosion susceptibility, while the separated and coarsened GBPs with high Cu content formed in the deepening aging process contribute to the better corrosion resistance. At the same time, the drop in corrosion potential of Al matrix also reduces the corrosion driving force of GBPs with the aging degree deepening. The optimal comprehensive performance of the 7055 Al alloy can be obtained after RRA treatment, namely the highest tensile strength of 739.7 MPa and the acceptable corrosion resistance comparable to that of the T76 state.

Influence of aging treatments on the strength and localized corrosion resistance of aged Al–Zn–Mg–Cu alloy

The effect of multi-stage aging heat treatments on the hardness, tensile strength, electrical conductance, and exfoliation corrosion (EXCO) resistance of the Al–Zn–Mg–Cu alloy has been studied through the Vickers hardness, tensile, conductivity, and EXCO testing combined with microstructure characterization using a transmission electron microscope. Experimental results demonstrated that multi-stage aging conditions could effectively enhance the localized corrosion resistance through a lower strength loss compared to T6 aging condition (120 °C/24 h). The designed four-step aging (FSA) treatments could improve the EXCO properties to PB degree along with the ultimate tensile strength and yield strength to 373 MPa and 324 MPa, respectively. The development trend of localized corrosion resistances has been further verified using electrochemical impedance spectroscopy and polarization dynamic curves. Microstructures results indicated that dense precipitates in matrix, discontinuous coarse precipitates on grain boundaries, and a proper precipitation free zone play an important role in balancing the strength and corrosion properties of the FSA aged alloy.

Effect of multi-stage aging treatments on the precipitation and mechanical properties of Al-Zn-Mg alloys

The mechanical properties of Al-Zn-Mg alloys via multi-stage aging treatments were studied to explore the strength-toughness trade-off. The microstructures of aged alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that large numbers of GP zones and some part of η′ phases were formed during the low-temperature (90 °C) aging at the third-stage aging, which consumed considerable solid solubility. This prevents fast growth and coarsening of precipitates at the fourth-stage aging (150 °C). Finally, the mixed precipitation state of mainly η′ phases and some η phases formed. The grain boundary phase grew insignificantly, which is beneficial to decrease the strength gap between intragranular and grain-boundary. The intragranular equilibrium η phase is beneficial to pin the movement of dislocation, which avoids fracture caused by accumulation of dislocations around the large second-phase and on the grain boundaries. Although the strength of materials decreased slightly, the impact toughness of materials significantly increased due to the positive effect of the intragranular equilibrium η phase. Compared with the conventional retrogression and re-aging (RRA) treatment (120 °C/24 h + 185 °C/105min + 120 °C/12 h), the designed four-stage ageing treatment (FSA) (120 °C/24 h + 185 °C/105min + 90 °C/12 h + 150 °C/12 h) resulted in the decrease of tensile strength and yield strength by only 3% and 4%, respectively, but contributed to the increases of elongation and impact toughness by 15% and 17%, respectively.

Effect of aging treatment on corrosion resistance of Inconel 718 fabricated by selective laser melting

The microstructure and corrosion resistance of Inconel 718 alloy made by selective laser melting (SLM) by aging heat treatment (AT) were studied. After the solution treatment at 1050°C, the aging treatment temperature ranges from 600°C to 800°C. Scanning electron microscopy showed that γ” was precipitated out of the γ austenite matrix, with σ decrease in delta phase. The morphology, distribution and formation mechanism of precipitates in these microstructures were analyzed and discussed. Open circuit voltage curve (OCPT), alternating current impedance spectrum (IMP) and TAFEL curve (TAFEL) were tested by electrochemical workstation. Electrochemical corrosion results and corrosion resistance of samples after four different heat treatments were analyzed, and the best aging heat treatment mode and multistage aging treatment were discussed.

Effect of Pre-Aging, Over-Aging and Re-Aging on Exfoliation Corrosion and Electrochemical Corrosion Behavior of Al-Zn-Mg-Cu Alloys

By means of TEM, hardness, conductivity, tensile strength test, fracture toughness test, polarization curve and EIS, the Al-Zn-Mg-Cu alloys treated by a new multi-stage aging system, i.e. pre-aging, over-aging and re-aging (120°C/24h + 160°C/8h + 120°C/24h), were characterized. It is found that compared with the Al-Zn-Mg-Cu alloys treated by T76 (120°C/24h + 160°C/8h), the new multi-stage aging treatment can improve the tensile strength, fracture toughness, hardness and conductivity of the alloys at the same time. This is mainly due to the pre-aging, over-aging and re-aging process of super high strength aluminum alloys. Compared with the two-stage over aging process, the formation of multi-stage multi-phase precipitation structure can improve the strength, toughness and corrosion resistance of the alloys at the same time. The polarization curve is consistent with the conclusion. Therefore, we conducted this study to test how the comprehensive properties of the alloy can be improved.

Influence of quench rate on multi-stage ageing of AA6014 alloy

The influence of quench rate after solution heat treatment on the microstructure in the as-quenched state and subsequent ageing kinetics of alloy AA6014 was investigated by means of transmission electron microscopy, positron annihilation lifetime spectroscopy and hardness measurements. Various ageing temperatures and stages were taken into consideration. Consistent with previous studies, we found that solute and vacancy supersaturation decrease during slow quenching due to precipitation and annihilation, respectively. Additionally, we observed cluster formation during cooling below 200 °C. As for the influence on ageing behaviour we observe different behaviour for high and low ageing temperature: Artificial ageing is more affected than pre-ageing and natural secondary ageing. The detrimental effect of natural ageing on paint-bake hardening also depends on the quench rate. Possible interpretations are associated with cluster formation during natural ageing and also during quenching. The influence of pre-ageing at different temperatures on subsequent ageing kinetics is similar for slower industrial-type quenching and for fast quenching, thus allowing to apply the findings from idealised quenching conditions to situations closer to real application.

Effects of multi-stage aging treatments on the precipitation behavior and properties of 7136 aluminum alloy

The solutionized 7136 Al alloy was heat treated by one-step aging, two-step aging, retrogression and re-aging (RRA) and RRA plus further aging treatment, respectively. The influences of different aging treatments on the precipitation behavior, electrical conductivity and tensile properties were investigated. The results showed that one-step aging treatment forms continuous grain boundary precipitates (GBPs), coherent Guinier-Preston (GP) zone and semi-coherent η’ grain interior precipitates (GIPs). By two-step aging treatment, the GIPs are coarsened and partly transformed into incoherent η phase, which result in a reduction of strength but an enhancement of ductility and electrical conductivity. RRA treatment makes the GBPs to be discontinuous and forms precipitates free zone (PFZ) near grain boundary. The RRA alloy shows similar strength but higher electrical conductivity in comparison with one-step aging. When the RRA alloy further aged at 160 °C, the strength keeps almost constant until 10 h, while the electrical conductivity is increased with increasing aging time. The PFZ is widened and the GIPs become coarser by further aging at high temperature. By contrast, the PFZ is eliminated, accompanying with the formation of GP(II) as further aged at low temperature of 70 °C, therefore a higher strength and lower electrical conductivity are obtained.