Two-stage Aging Research Articles

Influence of different rolling passes and deformation amounts on the microstructure evolution and strengthening mechanism of Cu-Ni-Si alloy

Under the condition of controlling the total rolling deformation to 80.0%, the effects of multi-pass rolling, the ratio of deformation between the first and second stages in two-stage rolling, and aging parameters on the microstructural evolution and property enhancement of Cu-Ni-Si alloy were investigated. The results showed that the second-stage rolling promotes the formation of high-density dislocations, significantly enhancing dislocation strengthening. Simultaneously, the high-density dislocations facilitate the precipitation of the second phase during aging, and concurrently improving the strength and electrical conductivity of the alloy. In the two-stage rolling and aging process, a ratio of first-stage to second-stage rolling deformation less than 1 yields superior comprehensive properties. Specifically, when the first-stage rolling deformation is 35.0% and the second-stage deformation is 68.8%, the alloy achieves an optimal electrical conductivity of 49.53% IACS, hardness of 272.9 HV0.1, tensile strength of 773.0 MPa, and yield strength of 723.3 MPa. This is because a rolling ratio less than 1 results in higher dislocation density; the δ-Ni2Si precipitates have a coherent relationship with the Cu matrix, the interparticle spacing of precipitates is smaller. The average grain size is refined from 1.29 μm to 1.08 μm, significantly enhancing grain boundary strengthening. Dislocation strengthening and second-phase strengthening are the primary strengthening mechanisms of the alloy, and the calculated alloy strength agrees well with the experimental values, with errors less than 3.4%. Additionally, the Avrami equation in phase transformation kinetics was utilized to study the variation law of the volume fraction of the δ-Ni2Si phase with aging time.

Effect of heat treatment on phase transformation behavior, tensile fracture mechanism and spring application characteristics of 631 stainless steel

This study focuses on enhancing the mechanical properties of 631 stainless steel through heat treatment for spring applications. 631 stainless steel (17–7PH), can achieve increased hardness and mechanical properties, offering high strength and hardness suitable for use in aerospace, chemical, and automotive industries. Heat treatment was recommended as one of the best ways in this regard; hence, the effects of temperature and time on the microstructure, mechanical, and corrosion properties of 631 stainless steel have been studied. This research conducted a series of heat treatments on 631 stainless steel, included solid-solution treatment, CH900, RH950, and TH1050 processes to modify the temperature holding period and the degree of undercooling of phase transformations. The study explored the microstructure and mechanical properties of each heat treatment, including spring tests. The results indicate that solid-solution treatment has no significant contribution to hot-rolled 631 stainless steel, whereas two-stage aging is able to conspicuously improves material hardness and mechanical performance. Quenching in 0°C ice water (increasing the degree of undercooling) can elevate hardness to approximately HRC40. After a two-stage aging heat treatment, 631 stainless steel possesses excellent strength and corrosion resistance. Spring test results show a K value of 1.1 kg/ mm, indicating its potential application.

Improving mechanical and electrical properties of Cu-Ni-Si alloy via machine learning assisted optimization of two-stage aging processing

Recent studies have shown that synergistic precipitation of continuous precipitates (CPs) and discontinuous precipitates (DPs) is a promising method to simultaneously improve the strength and electrical conductivity of Cu-Ni-Si alloy. However, the complex relationship between precipitates and two-stage aging process presents a significant challenge for the optimization of process parameters. In this study, machine learning models were established based on orthogonal experiment to mine the relationship between two-stage aging parameters and properties of Cu-5.3Ni-1.3Si-0.12Nb alloy with preferred formation of DPs. Two-stage aging parameters of 400 °C/75 min + 400 °C/30 min were then obtained by multi-objective optimization combined with an experimental iteration strategy, resulting in a tensile strength of 875 MPa and a conductivity of 41.43 %IACS, respectively. Such an excellent comprehensive performance of the alloy is attributed to the combined precipitation of DPs and CPs (with a total volume fraction of 5.4% and a volume ratio of CPs to DPs of 6.7). This study could provide a new approach and insight for improving the comprehensive properties of the Cu-Ni-Si alloys.

Microstructure Evolution and Mechanical Properties of TiB2/Al–Cu–Mn–Cd Composite with the Adoption of Two-Stage Solution and Aging Treatment

Microstructure Evolution and Mechanical Properties of TiB2/Al–Cu–Mn–Cd Composite with the Adoption of Two-Stage Solution and Aging Treatment

Two-stage aging treatment to accelerate aging kinetics without impairing strength in B4C/7A04Al composite

Two-stage aging treatment to accelerate aging kinetics without impairing strength in B4C/7A04Al composite

Welding Heat-induced Microstructural Variations in Inconel 718 Superalloy: A Comparative Investigation between Conventionally Manufactured and 3D Printed Materials

This study focuses on the microstructural evolution of Inconel 718 superalloy, a precipitation-hardened superalloy, subjected to various manufacturing processes and subsequent thermal exposures. Conventional manufacturing methods such as casting and forging were compared with modern 3D printing techniques, notably wire-arc additive manufacturing. Subsequent treatments, including solution treatment and two-stage aging, as well as bead-on-plate welding, were performed to assess their effects on the microstructure of Inconel 718. The transformations of strengthening phases such as γ′(Ni3(Al,Ti)), γ″(Ni3Nb) and intermetallic δ(Ni3Nb) phases were observed to vary significantly under different thermal cycles, and these variations in phase transformation are anticipated to lead to a degradation in mechanical performance post-welding. Additionally, thermodynamic calculations using commercial Calphad software were utilized to investigate phase transformations in the welds, providing critical insights into how manufacturing processes and thermal exposures affect the stability and distribution of microstructural features, thereby highlighting the complexities of phase dynamics in this high-performance alloy.

Effect of Sc on the recrystallization behavior and mechanical properties of Al-Zn-Mg-Cu-Zr alloys under isothermal multidirectional compression

To better achieve grain refinement and further optimize the comprehensive mechanical properties of Al-Zn-Mg-Cu-Zr alloys, this paper investigates the effect of trace Sc on the recrystallization behavior of Al-Zn-Mg-Cu-Zr alloys under isothermal multidirectional compression (IMC) and the mechanical properties in two-stage aging state. It was discovered that the addition of Sc enhanced the tensile and yield strengths of the alloys by 3.8% and 4.3%, respectively, and simultaneously increased the elongation from 13.4% to 15.4%. The OM, EBSD and TEM techniques were used to characterize the microstructure of the alloy. The results showed that the Al3(Sc,Zr) phase produced by the addition of 0.25% Sc not only refines the grains, but also changes the recrystallization behavior of the alloy by pinning the dislocations and (sub) grain boundaries in the combination of IMC deformation. It inhibits the generation of geometric dynamic recrystallization (GDRX) grains and the nucleation of discontinuous dynamic recrystallization (DDRX) grains but promotes the formation of continuous dynamic recrystallization (CDRX) as the predominant mode of dynamic recrystallization. The alloy gains a finer grain structure by simultaneously promoting the formation and inhibiting the growth of recrystallized grains. As a result, the improvement in the overall mechanical properties of Al-Zn-Mg-Cu-Sc-Zr alloys brought about by the addition of Sc can be attributed primarily to the refinement of the initial as-cast grains, as well as fine-grain strengthening due to the inhibition of recrystallized grain growth and the Orovan strengthening of the nanoscale secondary phase Al3(Sc,Zr) itself.

Effect of heat treatment on microstructure evolution, strengthening mechanisms and mechanical properties of Zn modified Al–Mg alloys with Sc and Zr additions

In this work, a new Al–Mg–Zn alloy was designed based on thermodynamic calculations combined with the concept of cooperative strengthening and toughening. The influence of different aging processes on microstructure and mechanical properties of the designed Al–Mg–Zn alloy was studied using experimental techniques including hardness testing, room-temperature tensile testing, EBSD, and HRTEM. The results indicate that after four different aging heat treatments, fine and uniformly distributed nano T-Mg32(Al, Zn)49 phases have precipitated within the grains. Compared to single-stage aging (140 °C/36 h, 180 °C/7 h), two-stage aging (90 °C/24 h + 140 °C/18 h, 90 °C/24 h + 180 °C/5 h) resulted in a smaller size and an increased number density of precipitated phases within the grains. After the aging process of 90 °C/24 h + 140 °C/18 h, the grains exhibited the smallest precipitated phases, being of 2.77 nm in size and a number density of 1.06 × 1023 #/m³. Additionally, TEM diffraction spot analysis identified the presence of Al3(Sc, Zr) precipitates within α-Al matrix. Under the aging process of 90 °C/24 h + 140 °C/18 h, the alloy exhibited superior mechanical properties with a tensile strength of 566 MPa, yield strength of 477 MPa, and elongation of 16.5%. The primary strengthening mechanisms of the presently investigated Al–Mg–Zn alloy include precipitation strengthening, solid solution strengthening, and grain boundary strengthening. A yield strength model considering various strengthening mechanisms is used to predict the yield strength based on thermodynamic calculations and experimental data. The research findings will offer theoretical insights guiding the design of compositions and heat treatment schedule for aluminum alloys.

Simultaneous improvement of strength and toughness via two-stage aging for a secondary hardening steel

A two-stage aging, involving initial low-temperature treatment followed by high-temperature treatment, was employed to induce a high number density M2C and inhibit the occurrence of chemical ordering and formation of large ellipsoidal particles at grain boundaries. The ultimate tensile strength, yield strength and Charpy impact energy increase by 233 MPa, 191 MPa and 3.3 J, respectively.

Microstructural Evolution in a 6060 Extrudable Al-Alloy: Integrated Modeling and Experimental Validation.

Desirable properties including strength, ductility and extrudability of 6060 Al-alloys are highly dependent on processing to control the development of microstructural features. In this study, the process chain of an extrudable 6060 Al-alloy was modeled in an Integrated Computational Materials Engineering framework and validated experimentally via quantitative SEM-EDX and TEM. All critical processing stages were considered including casting, homogenization heating and holding, extrusion cooling and two-stage aging. Segregation and intermetallics formation were accurately predicted and experimentally verified in the as-cast condition. Diffusion simulations predicted the dissolution of intermetallics and completion of β-AlFeSi to α-AlFeSi transformation during homogenization, in excellent agreement with quantitative SEM-EDX characterization. Precipitation simulations predicted the development of a β″ strengthening dispersion during extrusion cooling and aging. Needle-shaped β″ precipitates were observed and analyzed with quantitative high-resolution TEM, validating predictions. Ensuing precipitation strengthening was modeled in terms of aging time, presenting good agreement with yield strength measurements. Precipitate-Free Zones and coarse, metastable β-type particles on dispersoids and grain boundaries were investigated. The proposed integrated modeling and characterization approach considers all critical processing stages and could be used to optimize processing of extrudable 6xxx Al-alloys, providing insight to mechanisms controlling microstructural evolution and resulting properties.

Enhanced toughness-thermal stability synergy and mechanisms of dual-phase Nb alloys by tuning C concentration

Nb alloy has become a promising material in aerospace due to high melting point, low density and excellent processing properties. Nevertheless, the mismatched mechanical properties and thermal stability of single-phase BCC-Nb made the aircraft unable to resist cyclic vibration loads under the action of heat/force/flow. Here, after solid solution, quenching and two-stage aging, an FCC/BCC dual-phase Nb alloy with a discontinuous grain boundary (GB) carbide network skeleton was formed. The results show that interstitial C atoms precipitate from BCC-Nb matrix resulting in volume shrinkage and form discontinuous carbide at GB, which stimulates the phase transition of BCC expansion to FCC near GB. Wherein, discontinuous GB carbides help pin the GB and dislocations, impede grain growth, and change the deformation from local to the entire network skeleton. Meanwhile, the gap provides a channel for the dislocation to pass through the GB. Additionally, the FCC provides the multi-slip system to transfer the dislocation. The results show that the yield strength, tensile strength, elongation, fracture toughness and elastic modulus of Nb alloy are increased by 79.99 %, 34.52 %, 164.22 %, 204.76 % and 1.27 %, respectively. This unique microstructure is expected to guide the design of the next generation of high-performance Nb alloys.

Effect of Pulsed Magnetic Field on Microstructure and Mechanical Properties of 7050 Al Alloy After Two-Stage Aging

Effect of Pulsed Magnetic Field on Microstructure and Mechanical Properties of 7050 Al Alloy After Two-Stage Aging

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.

Effects of Mg/Zn ratio and pre-aging on microstructure and mechanical properties of Al–Mg–Zn–Cu alloys

The alloy composition and aging process are significant factors determining the type of precipitates in Al alloys. In this work, the effects of Mg/Zn ratio and pre-aging process on the microstructure and mechanical properties of Al–Mg–Zn–Cu alloys were systematically investigated using transmission electron microscopy (TEM), hardness measurements and tensile tests. The results show that reducing the Mg/Zn ratio significantly increases the density of T′-Mg32(AlZnCu)49 precipitates, thereby enhancing and accelerating the aging response. A detailed comparison has been made between Al–Mg–Zn–Cu alloys aged by single-step and two-step artificial aging processes. With the same composition, the alloy exhibits a higher yield strength under the two-stage aging process, resulting from the higher precipitation strengthening effect (Orowan dislocation bypassing mechanism) due to the smaller size and higher density of precipitates. Furthermore, it is particularly interesting that there may be two evolution paths for GPII zones with (111)Al habit plane: one is that the GPII zones are dissolved into the matrix, and the other is that the GPII zones gradually evolve into the T′ precipitates. This work sheds some light on the precipitation sequence in Al–Mg–Zn–Cu alloys and is greatly helpful for designing age-hardening Al–Mg based alloys.

Outstanding high-temperature strength-ductility combination in inconel 718 alloy by warm rolling and simplified aging treatment

In the present study, a comparative study was conducted with the cold-rolled Inconel 718 alloy and warm-rolled Inconel 718 alloy. The influences of deformation process and aging treatment on microstructure evolution and tensile properties of Inconel 718 alloy were investigated. The results showed that some fresh recrystallized grains can be observed in the warm-rolled and subsequent aged samples, resulting in refined microstructures of the warm-rolled and aged samples. Besides, the warm rolling process can accelerate the precipitation of γ" strengthening phase. The peak hardness (∼560 H V) of the warm-rolled sample was obtained after aging at 720 °C for 2 h, which far exceeded that of cold-rolled Inconel 718 alloy even aged for 8 h (∼520 H V). According to the TEM analysis, the fraction and size of γ" particles of the warm-rolled sample after 2 h aging were about 10.3% and 5.9 nm, which was higher than the cold-rolled sample with the same aging treatment (7.6% and 2.9 nm). Moreover, most of the γ" particles in the warm-rolled sample after simplified aging was nearly spherical, rather than the disk-shaped in the traditional two-stage aging Inconel 718 alloy. Compared with cold-rolled and aged materials, the warm-rolled alloy after aging exhibited superior mechanical properties. When deformed at 650 °C, the tensile strength and elongation of warm-rolled samples after aging treatment were 1247.6 MPa and 18.7%, while the corresponding values of cold-rolled samples after aging were 1162.5 MPa and 13.6%. The excellent strength-ductility combination of Inconel 718 alloy was linked with refined grains, numerous modified precipitates, and partial recrystallized structure with great dislocation density and many dislocation cells. As a result, warm rolling may provide an effective way to simplify the traditional 18 h two-stage aging to just 2 h single-stage aging treatment, and meanwhile, to improve the mechanical properties of Inconel 718 alloy.

Linearizing Battery Degradation for Health-Aware Vehicle Energy Management

The utilization of battery energy storage systems (BESS) in vehicle-to-grid (V2G) and plug-in hybrid electric vehicles (PHEVs) benefits the realization of net-zero in the energy-transportation nexus. Since BESS represents a substantial part of vehicle total costs, the mitigation of battery degradation should be factored into energy management strategies. This paper proposes a two-stage BESS aging quantification and health-aware energy management method for reducing vehicle battery aging costs. In the first stage, a battery aging state calibration model is established by analyzing the impact of cycles with various Crates and depth of discharges based on a semi-empirical method. The model is further linearized by learning the mapping relationship between aging features and battery life loss with a linear-in-the-parameter supervised learning method. In the second stage, with the linear battery life loss quantification model, a neural hybrid optimization-based energy management method is developed for mitigating vehicle BESS aging. The battery aging cost function is formulated as a linear combination of system states, which simplifies model solving and reduces computation cost. The case studies in an aggregated EVs peak-shaving scenario and a PHEV with an engine-battery hybrid powertrain demonstrate the effectiveness of the developed method in reducing battery aging costs and improving vehicle total economy. This work provides a practical solution to hedge vehicle battery degradation costs and will further promote decarbonization in the energy-transportation nexus.

Effect of solutionizing and double ageing treatments on the microstructural characteristics and tensile properties of Inconel 718 welds

Inconel 718 joints were fabricated using the pulsating direct current gas tungsten arc welding (PDCGTAW) and laser-arc hybrid welding (LAHW) techniques using Ni-Cr-Mo-rich filler. A cyclic post-weld heat treatment (PWHT) comprising 980 °C/20 min/air cooling; followed by two-stage ageing 720 °C/8 h/furnace cooling and 620 °C/8 h/air cooling was performed on both joints. The liquation cracking was noticed on the heat-affected zone (HAZ) of the LAHW joints. Microstructural characteristics were assessed on the joints in both as-welded (ASW) and PWHT conditions. Regardless of the joints, the coarse Mo-rich segregates were noticed in the fusion zone (FZ) in the ASW conditions. On the contrary, the partial dissolution of MC, precipitation of δ-Ni3Nb needles and the coarsening of strengthening precipitates were observed on LAHW and PDCGTAW joints after subjecting to cyclic PWHT respectively. Tensile studies corroborated that there is a considerable enhancement in the tensile and 0.2% offset yield strengths of LAHW joints after being exposed to PWHT, despite the formation of HAZ liquation cracking.

Electrochemical Study on Corrosion Damage of 7B04 Aluminum Alloy/Anodic Film/Epoxy Coating System in a Simulated Marine Atmospheric Environment

This paper investigates the electrochemical characteristics of the 7B04 aluminum alloy/anodic film/epoxy coating system in a simulated marine atmosphere using electrochemical techniques and surface morphology measurements. The coating system underwent a periodic immersion test for 480 h to simulate the harsh service conditions in a marine environment. Three equivalent circuit models were proposed based on the analysis of the electrochemical failure mechanism of the coating system. Akaike information criterion and Bayesian information criterion were used to selectively choose models for different degradation stages of the coating system, revealing its electrochemical degradation characteristics. Two-stage aging kinetics and gamma models were proposed to describe the time-varying processes of electrochemical low-frequency impedance and corrosion trend, respectively. Ultimately, low-frequency impedance is identified as the most effective and rapid evaluation index for coating performance due to its strong correlation with corrosion trends. The aforementioned models and index provide a foundation for accurately identifying corrosion stages and effectively predicting the corrosion process.

The influence of electron beam welding technology on mechanical properties of GH4169 welded joints

Super-alloy GH4169 and related welded structures were widely used in the aero-engine industry region. Vacuum electron beam welding is one of the most advanced welding technologies, and the influence of the mechanical property caused by the welding technology still needs further research. Firstly, the hardness, microstructure, and tensile properties of the GH4169 electron beam welded joint were tested. After electron beam welding and two-stage aging treatment, the weld zone was strengthened and its hardness is higher than that of the base metal zone. The microstructure characterization results showed that the grain size of the weld and heat-affected zone was larger than that of the base metal. The tensile results show that the static properties of the welded joint are similar to that of the base metal, and the residual height has no obvious effect on the static strength of the welded joint. The high-temperature creep properties of GH4169 electron beam welded joints with residual height were tested. The results show that residual height will lead to a transient fracture of welded joints under relatively safe service conditions. The fatigue performance of GH4169 welded joints with and without residual height was tested respectively. The results show that the fatigue life of the welded joints without residual height was about 10 times longer than that of the welded joints with residual height under the same stress level, and for the welded joint with residual height, the residual height position will become the main fatigue crack source.

Deformation and damage mechanisms during stress rupture of Waspaloy after vacuum solution plus aging treatment

The microstructure of vacuum solution-treated Waspaloy is investigated before and after a stress rupture test. The samples with trimodal and bimodal distributions of γ′ precipitates are produced by single-stage (760 °C/16 h) and standard two-stage (845 °C/4 h + 760 °C/16 h) aging treatments, respectively. Both kinds of samples deform mainly by the propagation of dislocations in the uniformly deformed region and then by a combination of dislocation slip and deformation twinning in the necked region. The secondary and tertiary γ′ precipitates are sheared by a/2 < 110 > dislocations, while stacking faults are created in the primary γ′ precipitates by shearing of a/3 < 112 > dislocations. The finer tertiary γ′ precipitates can more effectively impede the movement of dislocations than the secondary γ′ precipitates. Thus, the single-stage aged sample exhibits a longer stress rupture life and lower strain rate than the two-stage aged sample. For the damage mechanism, the fracture in the uniform deformation stage is generally intergranular given that the deformation is mainly concentrated around the grain boundaries. A mixed mode of transgranular and intergranular fracture occurs in the necked region due to the obvious grain boundary sliding and serious deformation within the grains.