Pre-aged Alloy Research Articles

Realizing strength-ductility combination of Ni-12Mo-7Cr-2Nb based superalloy via nano-sized MC carbides at stacking faults and grain boundaries

Developing corrosion-resistant alloys with the outstanding strength-ductility combination at elevated temperatures are highly desirable for molten salt reactor (MSR) applications. In this study, by appropriate preaging process, the yield strength and elongation of one newly developed Ni-12Mo-7Cr-2Nb based superalloy at high temperatures respectively increase by ∼50 % and ∼100 % as compared with the solid-solution state. This preaged alloy exhibits the excellent strength and overcomes the intermediate temperature embrittlement problem as compared with other alloys. Microstructure observations indicate that the strengthening and ductility improvement are related to the formation of nano-sized MC carbides. After preaging process, the extrinsic stacking faults are formed along {111} close-packed planes and decorated with ultra-dense MC carbide particles, thus are called as stacking faults precipitates (SFPs). SFPs contribute to the precipitation strengthening by the complex interaction with dislocations. Surprisingly, the SFPs exhibits the better strengthening effect than the uniformly dispersed MC carbides with the same volume fraction and size. Furthermore, it was found that the “pseudo-continuous” intergranular MC carbide particles are beneficial to the high-temperature ductility, although the similar intergranular precipitates were widely regarded as harmful in other studies. Using crystal plasticity finite element method, it was proved that the strain concentration at the triple junctions would be obviously alleviated because the intergranular MC carbide particles can suppress grain boundary sliding. If tested at 750 °C, the intergranular MC carbide particles would trigger the particle-stimulated nucleation and lead to the local dynamic recrystallization at grain boundaries and crack tips. These small recrystallized grains can suppress crack propagation and bring about the improvement in ductility in the aged alloy.

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.

Synergistic mechanism of hetero-structured Al–Mg–Si–Cu–Zn–Fe–Mn alloys with greatly enhanced strength, ductility and formability

Al–Mg–Si alloy sheets applied in automobile industry are required to possess good formability, high strength and ductility. Herein, our results show that a heterogeneous cellular microstructure, i.e., soft domains embedded in hard domains, can be readily attained via a short-flow processing route with concomitant improvement in the strength (YS: 177–184 MPa, UTS: 317–328 MPa), elongation (25.7%–27.3 %) and formability (r‾= 0.771, Δr = 0.047) of pre-aged alloy. The distribution of recrystallization grains, texture and coupled soft/hard domains in the Al–Mg–Si–Cu–Zn–Fe–Mn alloys and the effect on mechanical properties have been investigated. Moreover, the formation and synergistic action mechanisms of coupled soft/hard domains were put forward in this work.

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.

Effect of pre-aging on microstructure, activation energy, texture evolution of Mg–9Gd–4Y–2Zn–0.5Zr alloy during compression deformation

This paper illustrated the effects of pre-aging on the microstructure, texture evolution, and deformation behavior of Mg–9Gd–4Y–2Zn–0.5Zr alloy during hot compression. Dynamic recrystallization (DRX) behavior, texture evolution, stress-strain curve, and activation energy of the solid solution and pre-aged alloy were evaluated. The results showed that the second-phase particles of the pre-aged samples exhibit combination of two types particles structure, which is composed of coarse dynamic precipitation particles and fine nanoparticles. The combination of these two types particles showed the best grain refinement effect by accelerating the DRX process, as the coarse dynamic precipitation particles promoted DRX through the PSN mechanism and the fine granular phase pinned dislocations and inhibited grain growth. As a result, the pre-aged samples had a higher DRX grain volume fractions and smaller average grain sizes than the solid solution alloys. In addition, the pre-aged sample had higher deformation activation energy (Q = 294.8 kJ/mol) than solid solution sample (Q = 223 kJ/mol) because β-phase pin dislocations formed during deformation restrained the movement of grain boundaries. The pre-aged alloys also showed a more concentrated texture than the solid solution alloys, with the c-axis of the coarse grains distributed from the compression direction (CD) at 30° inclination to the normal direction (ND). The c-axis distribution of the coarse grains was more scattered in the solid solution alloys. A hot processing map was constructed for the pre-aged alloy and used for evaluation. The optimal deformation parameters for the pre-aged Mg–9Gd–4Y–2Zn–0.5Zr alloy were between 0.001 and 0.1 s−1 at 400 °C–480 °C. These significant observations not only provide theoretical guidance but also practical application for setting plastic forming deformation parameters in industrial production.

Warm Aging of Pre-aged AA6013 Sheet and Its Relevance to Room Temperature and Warm Forming Applications—Experimental and Modeling Analyses

AbstractWarm forming has been shown to be an effective way to increase the formability of various aluminum sheet alloys. If applied to precipitation hardenable alloys, aging and/or coarsening may occur during heating and/or forming processes. As such, there is a potential to leverage the warm forming process to artificially age precipitation hardenable alloys. In this study, the aging characteristics of a pre-aged Al-Mg-Si-Cu alloy (designated AA6013-PA and comprising a 4 h, 100 °C pre-aging cycle following solutionization) were examined using: (i) room temperature tensile and (ii) bendability experiments, as well as (iii) elevated temperature tensile and formability experiments in the temperature range of 220-280 °C. In the pre-aged condition, the alloy exhibited very high work hardening, elongation and bendability. It was found that T6-like properties can be obtained with short duration secondary aging of the AA6013-PA starting temper (410 s at 235 °C), followed by a paint bake cycle (30 min at 177 °C). In such heavily aged conditions, the bendability is found to be moderate and consistent with a T6 temper. Bendability of the pre-aged alloy is shown to be enhanced by reducing the extent of aging, which may be beneficial for applications requiring energy absorption during crash, for example. In the pre-aged condition, AA6013 displayed a 55% improvement in room temperature formability, relative to the T6 condition, for near plane-strain loading using a Nakazima punch, although neither temper exhibited formability improvements through warm forming. The aging kinetics of AA6013-PA were determined from mechanical test data and used to calibrate an aging kinetics model. A strong obstacle model was used to relate precipitation state to yield strength, for artificial aging in the range 220-280 °C. The incremental strength increase due to exposure to an automotive paint cure cycle was also modeled and found to yield accurate results. Warm deformation, as well as room temperature deformation, were shown to enhance the paint bake response of the alloy, reducing the time to the peak aged condition by up to 14% for artificial aging at 177 °C.

Microstructure analysis, constitutive relationship, and processing map of novel pre-aged Mg-Zn-Gd-Er alloy with different deformation ranges

Abstract Pre-aged Mg-6Zn-1Gd-1Er alloy is a novel rapid aging hardening Mg alloy, and studying its hot deformation behavior has an important role in promoting the development of lightweight alloy materials. To study this, the flow stress curves of pre-aged Mg-6Zn-1Gd-1Er alloy at 180–380 °C and 10−3−10 s−1 were obtained by isothermal compression tests. The constitutive equations of the medium-high temperature deformation (MHTD) and the low-temperature deformation (LTD) were established, and their activation energies were 155.78 kJ mol−1 and 178.00 kJ mol−1, respectively. Based on the constitutive equation analysis, the glide and climb of dislocations and the cross-slip of dislocations was the deformation mechanism during MHTD and LTD, respectively. In order to determine the appropriate hot processing parameters, the hot processing map of the pre-aged Mg-6Zn-1Gd-1Er alloy under 0.2–0.8 strain was constructed based on the dynamic material model. The hot processing maps indicate that this pre-aged alloy at low temperature (180–230 °C) and high strain rates (1–10 s−1) mainly occurs flow instability, and the optimal hot processing window appears at a 330–380 °C and 10−3 to 10−2 s−1 range. Furthermore, the deformation mechanism of the stable domain with high power dissipation efficiency in the hot processing map was continuous dynamic recrystallization, discontinuous dynamic recrystallization, and particle-stimulated nucleation mechanisms.

Synergy of homogeneous/heterogeneous nucleation in Al-Mg-xSi-Cu-Zn alloys with rapid age-hardening response

The low age-hardening response dilemma of aluminum alloys is an urgent challenge that needs to be overcome to save costs and improve comprehensive mechanical properties for industrial applications. In this study, we have proposed a novel low-cost strategy to improve the age-hardening response of advanced Al alloys by the synergy of homogeneous/heterogeneous nucleation. The results reveal that nano-precipitates can preferentially nucleate and grow along dislocations due to the rapid diffusion rate of solute atoms assisted by dislocations. Meanwhile, the critical nucleation size of the solute clusters in the pre-aged alloy can be decreased by pre-deformation during the isothermal aging process due to the higher energy for diffusion and higher total strain energy in the matrix. Furthermore, the interaction force between Mg and Zn atoms in the matrix of no dislocation can be enhanced. Accordingly, many fine solute clusters with a high Zn/Mg ratio can also precipitate inside the grain. As a result, remarkable bake-hardening response and excellent strain hardening properties are obtained in the pre-deformed alloys after bake-hardening treatment based on the synergy of homogeneous/heterogeneous nucleation, and the related mechanism has been proposed.

The effect of different preaging conditions on mechanical properties and clustering behavior of AA6016 sheets for automotive outer panels

Preaging (PA) is an important process to enhance the performance of automotive aluminum outer panels through regulating solute clustering. In this work, we investigated the effect of PA temperatures in the range of 60°C-225 °C on the hardness before and after paint baking (PB) with different holding times. Tensile properties of partial alloys were checked to help explore the optimum PA condition. Before PB, the PA hardness of alloys preaged in the low-temperature regime (60°C-100 °C) was relatively stable with increasing PA time, but it increased significantly in the high-temperature regime (120°C-200 °C). After PB, the hardness of alloys preaged at temperatures up to 80 °C was low. Alloys preaged at 100 °C for 4 h could reach the same PB hardness as the solution-treated (ST) alloy. However, after reaching saturation of the hardness of preaged alloys in the low-temperature regime, extending the PA time did not induce a higher PB hardness. By contrast, the hardness decreased after long-term PA. The saturation PB hardness of the alloy further increased when preaged at a higher PA temperature and reached a maximum at 170 °C. The optimum PA condition for this alloy is 150 °C for 10 min as it produced better elongation before PB and the highest BHR after PB. The related mechanisms, including clustering and precipitation behaviors, have been discussed in detail.

Effect of retrogression treatment with different heating rates on microstructure, strength and corrosion behaviors of 7050 alloy

In order to achieve the combination of mechanical and corrosion properties for 7050 alloy, a novel retrogression and re-aging regime were proposed in this study. During the retrogression, the pre-aged alloys were heated from room temperature to 190 °C with different retrogression heating rates, and then were cooled quickly for following re-aging treatment. The size of precipitates and the width of precipitation free zones decreased with the increasing retrogression heating rate, the mechanical properties of 7050 alloy were improved, while the corrosion resistance was reduced. When the retrogression heating rate was controlled at 3 °C/min, a good combination of strength and corrosion resistance of 7050 alloys could be obtained. The excellent properties were attributed to the homogeneous distribution of intragranular precipitates (η’) and the coarsening of intergranular precipitates (η) after the novel retrogression and re-aging regime.

Improvement in Strength of High Concentration Corson Alloy with the Heterogeneous-Nano Structure

Effects of pre-aging of Corson alloys with high concentrations of Ni and Si (Cu–4.2 mass%Ni–0.93 mass%Si) followed by heavy cold rolling and full-aging on strengthening were systematically investigated. Especially, development of heterogeneous-nano (HN) microstructure was focused. And the relationship of mechanical/electrical properties was precisely examined. The pre-aged alloys exhibited a considerable strengthening after 90% cold rolling. This was attributed to the increase in the volume fraction of deformation twin domains in the HN microstructure after cold rolling as well as the precipitation strengthening. The sample prepared by a thermo-mechanical process via pre-aging at 723 K, subsequent cold rolling and full-aging at 673 K for 600 s exhibited the best strength/conductivity balance with an ultimate tensile strength (UTS) of 1096 MPa and electrical conductivity (E) of 29% IACS. When examined the effects of solid-solution (SS) temperature on the microstructure and properties, SS at 1223 K derived finer grain size than at 1323 K and the achieved best balance of UTS and E with 1061 MPa and 33% IACS. Therefore, grain refinement prior to pre-aging had no significant effect on strengthening but contributed to modification of electrical conductivity.

Modelling Dynamic Precipitation in Pre-Aged Aluminium Alloys Under Warm Forming Conditions

Modelling Dynamic Precipitation in Pre-Aged Aluminium Alloys Under Warm Forming Conditions

Investigation of formability, microstructures and post-forming mechanical properties of heat-treatable aluminum alloys subjected to pre-aged hardening warm forming

Currently, the hot forming of aluminum alloys has garnered significant attention for overcoming poor formability at room temperature (RT). However, the current hot forming technique is facing problems such as time-consuming procedures and high economic cost. To improve forming efficiency, we herein propose a novel forming technique for heat-treatable aluminum alloys, termed pre-aged hardening warm forming (PHF) process. An Al–Zn–Mg–Cu alloy is used to investigate the flow behavior, drawing formability, and post-forming mechanical properties during the aforementioned process. Uniaxial tensile tests and Erichsen tests are conducted to evaluate the plastic flow behavior and drawability, respectively. Stamping tests are performed on a cap beam component to verify the engineering feasibility of the forming technique. Optical microscopy (OM) and transmission electron microscopy (TEM) are performed to characterize the microstructure evolution during the PHF process. The tensile test results show that the pre-aged alloys exhibit true fracture elongation similar to the fracture elongation of the O-temper alloy at RT, and that the Erichsen values of the pre-aged alloys are 5%–16% higher than that of the O-temper alloy at 200 °C. The results of the stamping tests for the cap beam component indicate that the optimal final strength of the stamped component achieves σ/σ0.2 = 566 MPa/500 MPa, exceeding the strength of the AA7075-T6 alloy. OM observations show that the stamping process does not significantly affect the grain sizes of the pre-aged alloys. TEM results show that numerous thermostable GPⅡ zones exist in the pre-aged alloys, and that the stamping procedure transforms the GPⅡ zones to dispersed η′ phases and results in the entanglement of numerous dislocations. The dual effects of phase transformation and plastic deformation during the PHF process contribute to the desirable mechanical properties of the stamped components.

Ageing Behaviour of Al-Mg-Si Alloys After Cryogenic and Room Temperature Deformation.

The aim of this study is to investigate the effects of cryogenic and room temperature pre-deformation on subsequent artificial ageing of Al–Mg–Si alloys. Naturally aged and pre-aged samples were strained to 5%, 10% and 20% at RT (25 °C) and under liquid nitrogen, and artificially aged at 185 °C. Pre-deformation generally increases ageing kinetics for both the naturally aged and pre-aged alloys, which increase in proportion to the degree of pre-deformation, and which are slightly more pronounced for the cryogenic condition. The peak strength is constant, except for when a low degree of pre-deformation is used, in which case it is slightly reduced. Cryogenically deformed samples show an increased strength and hardness, compared to samples pre-deformed at RT, when subjected to an equal magnitude of strain. This difference is reduced during artificial ageing. Synchrotron measurements reveal that this behaviour can be linked to the greater dislocation density, which is not completely recovered even after prolonged ageing at 185 °C.

Enhancing the bake-hardening responses of a pre-aged Al-Mg-Si alloy by trace Sn additions

Effects of additions of trace Sn on the bake-hardening responses of a pre-aged Al-0.85Mg-0.85Si (in wt%) alloy were investigated through mechanical tests, differential scanning calorimetry, electrical resistivity and transmission electron microscopy. Results indicate that trace Sn additions reduced the number density of pre-aging clusters by inhibiting the formation of unstable counterpart during pre-aging treatment, leading to low strength and high supersaturation of solute atoms. In a subsequent paint-bake treatment, the presence of highly supersaturated solute atoms and high concentrated free vacancies moderated the activation energy barrier of βʺ phase and thus kinetically accelerated the formation of βʺ. Consequently, the trace Sn additions enhanced the bake-hardening responses of the pre-aged alloys significantly. The Sn-containing pre-aged Al-Mg-Si alloys with low strength and great bake-hardening responses hold promising potential for automotive body skin applications.

Microstructure and Properties of a Cu-Ni-Sn Alloy Treated by Two-Stage Thermomechanical Processing

The effects of two-stage thermomechanical processing on the microstructure and properties of Cu-15Ni-8Sn-1.0Zn-0.5Al-0.2Si alloy have been investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, and mechanical and electrical property testing. Spinodal decomposition and β-Ni3Sn precipitates with L12 ordering structure appeared in the preaged alloy. The crystal orientation relationships between the copper matrix and β-Ni3Sn precipitates were (200)Cu||(100)β, [001]Cu||[001]β and ( $$ \bar{2}20 $$ )Cu||( $$ \bar{1}10 $$ )β, [112]Cu||[112]β. The high strength of the studied alloy can mainly be attributed to the combined effects of precipitation strengthening and substructure strengthening. The interaction between the nascent nanoparticles that form during pre-aging and the dislocation configurations that form during cold rolling promotes precipitation in the matrix and suppresses formation of cellular precipitates and coarse precipitates at grain boundaries, improving the comprehensive properties of the alloy. The peak-aged alloy treated with two-stage thermomechanical processing showed hardness of 387 HV, electrical conductivity of 8.5%IACS, tensile strength of 1176 MPa, yield strength of 1106 MPa, elongation of 3.86%, and strength–ductility product of 4539 MPa%.

Effect of Natural Aging on Mechanical Response of the Artificially Aged EN AW 6063 Aluminium Alloy

The effect of natural pre-aging time (from 0.1 to 10000 h) on mechanical response during subsequent artificial aging of EN AW 6063 aluminium alloy at 170°C was investigated using Vickers microhardness measurements, tensile test analysis and transmission electron microscopy characterization. The microhardness and tensile strength of EN AW 6063 alloy increased slightly with natural aging time. Afterward, the artificial ageing from 18 to 20 hours induced the maximum increasing of hardness and strength for variously naturally pre-aged states of alloy. But, it was found that when pre-aging time was prolonged from 0.1 h to 10000 h, the mechanical response of artificial aging applied for the pre-aged alloy states was slightly improved. It was suggested, that as pre-aging time was increased, the size of β'-phase particles formed in solid solution of pre-aged alloy state during artificial aging was decreased and their amount was increased.

Improvement of mechanical properties of a cryorolled Al-Mg-Si alloy through warm rolling and aging

The Al-Mg-Si alloys with and without pre-aging treatment were subjected to cryorolling, warm rolling and subsequent aging. Mechanical behavior and microstructural evolution were investigated by tensile tests, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Tensile results showed that the strength and ductility of the cryorolled alloy were simultaneously enhanced by warm rolling and aging. After final aging, the pre-aged alloy exhibited better combination of strength and ductility than the alloy without pre-aging. Microstructural results demonstrated that pre-aging treatment was beneficial for the formation of strengthening phases during warm rolling and final aging, which accounted for the improved strength. The presence of finely dispersed nano-precipitates as well as a decrease in dislocation density played an important role in good ductility.

Combined effect of pre-aging and Ag/Cu addition on the natural aging and bake hardening in Al-Mg-Si alloys

The combined effect of pre-aging and Ag/Cu addition on natural aging and bake hardening in Al-Mg-Si alloys were studied by Vickers microhardness measurement, tensile test, atom probe tomography and transmission electron microscopy. The results showed that Ag and/or Cu addition could further enhance the positive effect of pre-aging on bake hardening for Al-Mg-Si alloys. Although the pre-aged alloys with individual Ag/Cu addition had higher paint bake hardness, the hardness in T4P temper was relatively high due to effectively forming of Cluster(2) during pre-aging. While the pre-aged Ag-Cu-added alloy had low T4P temper hardness and higher bake hardening response. The strong interaction energies between Ag/Cu atoms and Mg atoms were responsible for the facilitated clustering and precipitation processes. After paint bake treatment, most of Ag atoms accumulated preferentially at β″/α-Al matrix interface, while Cu atoms entered into the interior of β″resulting in disordered structure within the precipitate.

Enhancement of Mechanical Properties of Extruded Mg–9Al–1Zn–1MM–0.7CaO–0.3Mn Alloy Through Pre-aging Treatment

The effect of pre-aging treatment before extrusion has been investigated in Mg–9.0Al–1.0Zn–1MM–0.7CaO–0.3Mn alloy. The as-cast microstructure consists of α-Mg dendrite with secondary solidification phase particles, (Mg, Al)2Ca, β-Mg17Al12 and Al11RE3 at the inter-dendritic region. After extrusion, β-Mg17Al12 precipitates are present, but higher density and more homogeneous distribution in pre-aged alloy. In addition, μm-scale banded bulk β-Mg17Al12 particles are generated during extrusion. Al11RE3 particles are broken into small particles, and are aligned along the extrusion direction. (Mg, Al)2Ca particles are only slightly elongated along the extrusion direction, providing stronger particle stimulated nucleation (PSN) effect by severe deformation during extrusion. The mechanical properties can be significantly enhanced by introducing pre-aging treatment, i.e. β-Mg17Al12 precipitates provide grain refining and strengthening effects and (Mg, Al)2Ca particles provide PSN effect.