Age-hardening Behavior Research Articles

Investigation of age-hardening behaviour of Al alloys via feature screening-assisted machine learning

Age hardening stands as a crucial strengthening process for aluminium (Al) alloys. However, determining the optimal ageing conditions has traditionally relied on resource-intensive trial-and-error methods. To streamline the design of Al alloys, this study introduces a novel feature screening-assisted machine learning (ML) approach to explore age-hardening behaviour across varying compositions and heat treatment parameters, focusing on yield tensile strength (YTS), ultimate tensile strength (UTS), and elongation (EL). A comprehensive pool of features, incorporating alloy composition and fundamental elemental properties, was generated to provide metallurgical insights for ML predictions. Subsequently, feature screening methodologies, including correlation analysis, feature elimination, and multi-objective genetic algorithm (MOGA), were employed to identify key features from the vast feature pool, balancing model complexity and prediction accuracy. Employing support vector regression models trained on these optimized feature sets, we demonstrate enhanced prediction accuracy for strength properties while maintaining model simplicity for EL, with minimal impact on accuracy. In addition, experimental results further validate the method's ability to reproduce ageing curves across all three mechanical properties for both commercialized and newly designed Al alloys.

Rapid age-Hardening Behavior Induced By Dislocation-Induced Precipitates In Zeg220 Magnesium Alloy

Rapid age-Hardening Behavior Induced By Dislocation-Induced Precipitates In Zeg220 Magnesium Alloy

Effect of dispersoid formation on age hardening and precipitation behavior of an Al-Si-Mg(-Cu) alloy with Mn and Cr addition

In the present study, Mn and Cr elements were added to the alloy to induce the formation of the α-AlFeMnCrSi dispersoids during solution treatment. The effect of the dispersoids on subsequent age-hardening response and precipitation behavior was investigated. The results indicate that there is a partially coherent structure between the dispersoid and the α-Al matrix, but the coherence is weak. The precipitation of dispersoids resulted in accelerated age-hardening response and precipitation kinetics, but weakened peak-aged hardening effect. With the aging time increasing from 0 to 16 h, a new precipitation sequence was proposed in Mn+Cr modified Al-Si-Mg(-Cu) alloy: Supersaturated solid solution (SSSS) → GP zones (under-aged) → β'' + β' (peak-aged) → Si + β'' + β' + Mg2Si + Q + α (over-aged). Among them, the extra precipitates were heterogeneous nucleated on the dispersoids, which depended on the orientation relationship between the dispersoid and the precipitation, the interfacial energy of the dispersoid/matrix/precipitation, and the local solute element concentration. Heterogeneous precipitation of coarse particles on dispersoids resulted in the formation of precipitation-free zones (PFZ), which was mainly responsible for the weakened peak-age hardening effect. Moreover, compared to the base alloy, finer and denser precipitates were obtained in the matrix of the modified alloy, due to the increased density of dislocations induced by the dispersoids and the higher Mg supersaturation.

The effect of melt thermal-rate treatment on precipitation hardening and mechanical properties of Al–Si–Mg alloys

In this study, we investigate the age-hardening behavior and mechanical properties of direct chill (DC)-cast Al–10Si-0.35 Mg alloys subjected to melt thermal rate treatment (TRT). Our findings reveal that TRT refines the microstructure and precipitation morphology. Grain analysis indicates the prevalence of twin dendritic grains in both TRT and non-TRT (NTRT) alloys during casting, with the TRT billet exhibiting smaller grains. The microstructural analysis demonstrates that TRT reduces the size of secondary dendrite arm spacing and eutectic phases while ensuring a uniform distribution of intermetallic particles. The precipitation mechanism in both alloys remains consistent, with the TRT alloy exhibiting a higher density of fine precipitates. Additionally, we observe a novel coprecipitation mechanism of Si and β″ phase, attributed to a lower misfit between (003)β″ and (011)Si compared to that between (003)β″ and (0 2‾ 0)Al. Furthermore, the TRT alloy displays slower precipitation kinetics due to homogeneous solute distribution, resulting in higher activation energy for precipitation nucleation. Consequently, a single age-hardening peak is observed in TRT alloys during aging. TRT alloys exhibit higher hardness and tensile strength after artificial aging at 190 and 210 °C, while maintaining comparable elongation to NTRT alloys. Theoretical calculations of precipitation strengthening also show higher values for TRT alloys, consistent with experimental results. Thus, our findings comprehend the underlying mechanisms of melt treatments crucial for improving precipitation hardening behavior and mechanical properties of Al–Si–Mg alloys.

Enhanced T5 age-hardening behavior of Al–Si–Mg casting alloy with prolonged melt holding and high quench temperature

The effects of the quench temperature (cooling rate) and melt-holding time at high temperatures on the precipitation kinetics and nanostructure of the Al–Si–Mg alloy were investigated by measuring the hardness and electrical conductivity, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. The results showed that a high quenching temperature (high cooling rate) and long melt holding simultaneously increased the hardness and electrical conductivity of the Al–Si–Mg alloy. They also accelerated the overall precipitation kinetics; however, the effect was more evident when the duration of high-temperature melt holding increased. Nanostructural analysis of the peak-aged conditions revealed that the accelerated precipitation kinetics of the alloy with a high quenching temperature were primarily related to the high amount of Si solutes, which prevented the formation of coarse Si precipitates. Conversely, the high precipitation kinetics caused by longer melt-holding treatments depend on the increased amount of solutes and dissolution of clusters and could disturb the formation of strengthening β″ precipitates. These results indicate that the mechanical properties of the T5-treated Al–Si–Mg alloy can be significantly enhanced by increasing the quenching temperature and providing sufficient melt holding at high temperatures.

Influence of Mg and Cu on precipitation behaviors and mechanical properties of Al–Si alloys

Al–8Si-0.5 Mg, Al–8Si–2Cu, and Al–8Si–2Cu-0.5 Mg alloys have been designed in this study, aiming to reveal the influence of Mg and/or Cu additions on microstructures and mechanical properties of Al–Si alloys. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Kampman-Wagner Numerical (KWN) model have been used to investigate age-hardening behaviors as well as their relation to mechanical properties. Microstructure analysis suggest that β'', θ′ and QP precipitates have formed in peak-aged Al–8Si-0.5 Mg, Al–8Si–2Cu and Al–8Si–2Cu-0.5 Mg alloys, respectively. Notably, the Al–8Si–2Cu-0.5 Mg alloy exhibits faster precipitation kinetics, larger age-hardening response and remarkable thermal stability compared to the other two alloys. Theoretical calculations indicate that the nucleation of QP precipitates is easier than β'' and θ′ precipitates, but the slower diffusivity of Cu limits the growth rate of QP precipitates during the growth stage. Furthermore, the contribution of β'', θ′ and QP precipitates to yield strength of peak-aged Al–Si alloys has been quantified, which shows a significant variation. β'' precipitates in the Al–8Si-0.5 Mg alloy exhibit the highest strengthening effect (∼186.4 MPa), followed by QP precipitates in Al–8Si–2Cu-0.5 Mg (∼160.2 MPa) and θ′ precipitates in the Al–8Si–2Cu alloy (∼13.4 MPa). This work provides valuable insights into the influence of Mg and/or Cu additions on the aging response and mechanical properties of Al–Si alloys, offering a reference for further enhancing the performance of Al–Si alloys.

The influence of natural aging on the precipitation behavior of the low-alloy content Al-Zn-Mg aluminum alloys during subsequent artificial aging and related mechanisms

The age hardening and precipitation behavior of the low-alloy content and widely used Al-Zn-Mg alloy during artificial aging was studied adopting methods of Vickers hardness testing and transmission electron microscopy. To meet with practical application of the production line conditions, the influence of natural aging (NA, at room temperature) on the precipitation behavior during subsequent artificial aging (AA, at 130 °C∼150 °C) and related mechanisms are discussed based on the classical nucleation theory. The results show that precipitates like solute clusters and GP zones will form during the NA process, which have a significant influence on the subsequent precipitation behavior during AA. These NA precipitates will dissolve at the initial stage of AA resulting in abundant localized supersaturated regions which are the favorable nucleation sites for η′ phase. This positive effect is highly dependent on the temperature adopted during the AA process and the duration of NA. Higher AA temperatures result in a more noticeable increase in hardness compared to the samples without NA but require a longer duration of AA to achieve a relatively high and stable peak-aging hardness. However, an excessive long NA period can lead to a significant drop in hardness in the over-aging status. In the low-alloy content Al-Zn-Mg alloys, the actual nucleation rates of precipitation during the early stages of AA vary significantly with the AA temperature, which serves as the underlying cause of the observed NA effect. These results provide rather an important guidance for the production process of the 7003alloy and other aluminum alloys with similar chemical composition.

M23C6-assisted formation mechanism of grain boundary cellular structure and its role on the brittle fracture in an austenitic Fe-Mn-Al-C lightweight alloy

The austenitic Fe-Mn-Al-C lightweight alloy is limited using its full-hardened state due to its brittleness after long-term aging treatment. This study investigates the formation mechanism of grain boundary precipitates and their effect on crack initiation and propagation. The alloy showed typical age-hardening behavior but exhibited intergranular fracture after a critical aging time. TEM study revealed that the amount of Mn segregation increased and Al decreased at the grain boundary at the early aging stage. M23C6 is first formed at the grain boundary, and α-ferrite is formed at the coherent M23C6/γ interface. Finally, the grain boundary cellular structure of α-ferrite+κ-carbide is formed by the continuous formation of α + κ and growing into their incoherent interfaces with γ. As this cellular structure decorates the grain boundaries, the fracture type changed from ductile to intergranular brittle fracture. The grain boundary cracks initiated at the coarse grain boundary κ-carbide and propagated through the {100} planes of α and κ. The lowest theoretical cohesive strengths in the {100} planes of both phases are suggested as the origin of the crack initiation and propagation.

Effects of cooling rate in homogenization on microstructure, hot deformation resistance and subsequent age-hardening behavior of an Al–Mg–Si alloy

In this paper, the microstructure evolution in an Al–Mg–Si alloy during the soaking and cooling of homogenization was investigated. Moreover, the effects of the cooling rate on the hot deformation and subsequent age-hardening behavior were studied. During the soaking, the eutectic Mg2Si is dissolved, the solute segregation is eliminated, and the Fe-bearing phase is refined. During subsequent cooling, no precipitate is formed in the water-quenched sample, while β'' and β-Mg2Si are the predominant phases formed in the air- and furnace-quenched samples, respectively. The experiments agree with the simulated continuous cooling transformation (CCT) curves. The cooling rate in homogenization has a significant effect on the flow stress when the compression is performed at 450 °C. The air-quenched sample exhibits the highest flow stress due to the formation of nanoscale β-Mg2Si particles. The effect of the cooling rate diminishes as the compression temperature increases. At a compression temperature of 500 °C, the water-quenched sample exhibits a slightly higher flow stress due to the precipitation of micrometer-scale β-Mg2Si particles and the Mg and Si solutes retained in the matrix. The compressed samples have nearly similar thermal effects and peak aging hardness increments after compression at 500 °C, indicating that the age-hardening potential of the alloys is equivalent. The same types of precipitates are formed in all compressed samples at peak aging, including the GP zone, β'', β′ and B′, where β'' is the main strengthening phase. The relationship between the precipitation behavior and the increments of hardness, electrical conductivity (EC) was analyzed.

Prediction of age-hardening behaviour of LM4 and its composites using artificial neural networks

This research work highlights the prediction of hardness behaviour of age-hardened LM4 and its composites fabricated using a two-stage stir casting method with TiB2 and Si3N4. MATLAB - Artificial Neural Networks is used to predict the age-hardening behaviour of LM4 and its composites. Experiments (hardness and tensile tests) are conducted to collect data for training an ANN model as well as to investigate the effect of reinforcements and age-hardening treatment on LM4 and its composites. The results show that with an increment in the reinforcement wt%, there is an enhancement in hardness and ultimate tensile strength (UTS) values within the monolithic composites. As-cast hybrid composites display a 37 to 54% improvement in hardness compared to as-cast LM4. Heat-treated samples, specifically those treated with peak aging with MSHT and 100 °C aging, perform better than as-cast samples and other heat-treated samples in terms of UTS and hardness. Compared to as-cast LM4, MSHT, and 100 °C aged samples display an 85 to 202% increment in VHN. Hybrid composites perform better in terms of hardness, while composites with 3 wt% of TiB2 (L3TB) perform better in terms of UTS, peak aged (MSHT and 100 °C aging) L3TB display 68% increment in UTS when compared to as-cast LM4. ANN model is developed and trained with five inputs (wt% of TiB2, wt% of Si3N4, type of solutionizing, aging temperature, and aging time) and one output (VHN) using different algorithms and a different number of hidden neurons to predict the age hardening behaviour of composites. Among them, Lavenberg-Marquardt (LM) training algorithm with normalized data and 30 hidden neurons performs well and shows a least average error of 1.588364. The confirmation test confirms that the trained ANN model can predict the output with an average %error of 0.14 using unseen data.

Age-hardening Behavior in <i>γ</i>′-phase Precipitation-hardening Ni-based Superalloy

Dislocations are often introduced in Ni-based superalloys to impart sufficient strength at both room temperature and high temperatures prior to their use in automobile exhaust gaskets. However, the interaction between the representative γ′ (Ni3(Al, Ti))-phase precipitates and dislocations in high temperature remains unclear. Therefore, this study examined the effect of cold rolling on age-hardening behavior and microstructure evolution, focusing on the formation of γ′-phase Ni3Ti during aging at 700°C for up to 400 h after 60% cold rolling of solution-treated specimens. During the early stage of aging, at 0.03 h, the hardness rapidly increased from 401 HV to 496 HV. Age-hardening continued until 3 h and reached its peak of 536 HV, followed by gradual decrease with aging time. 3D atom probe investigation revealed that the γ′-phase was confirmed after 0.3 h of aging. However, the composition-modulated structure speculated to be caused by spinodal decomposition was observed in the 0.03 h aged specimen. The change in strength with aging time was considered by calculating the contribution of each strengthening mechanism. In the initial stage of aging (0–3 h), dislocation and solid-solution strengthening dominated along with spinodal strengthening. Strengthening by spinodal decomposition in the 0.03 h aged specimen is presumptively accelerated by the introduced dislocations, which is followed by further precipitation strengthening caused by γ′-phase precipitates. In the later stage of aging (3–400 h), precipitation strengthening became dominant and reached its peak at 20 h aging, while dislocation strengthening decreased with aging time.

Enhanced mechanical properties and homogeneous T5 age-hardening behavior of Al-Si-Cu-Mg casting alloys

In this study, the effects of melt-holding temperatures on the mechanical properties and T5 age-hardening behavior of Al-Si-Cu-Mg alloys were investigated. An enhanced yield strength, tensile strength, and elongation were observed simultaneously as the melt-holding temperature was increased from 720 °C to 820 °C. The enhanced mechanical property of the T5-treated Al-Si-Cu-Mg alloy with high melt-holding temperature was attributed to several microstructural changes, including a refinement of the microstructure and an acceleration of the precipitation kinetics caused by enhanced second-phase dissolution. The high melt-holding temperature was particularly effective in changing the precipitation kinetics. This was demonstrated by the formation of a single peak upon homogenization of the solute distribution, replacing the double hardness peaks found in the Al-Si-Cu-Mg alloy with a low melt-holding temperature. The homogeneous solute distribution not only increased the number of Si- and β-type precipitates in the alloy with a high melt-holding temperature but also refined their size. In addition, the high melt-holding temperature accelerated the segregation behavior of Cu on the Si precipitates, which is critical for the refinement of precipitates.

The effect of Cu additions and prestraining on the age-hardening behavior of AA6016 sheets

In this study, we investigated the effect of Cu additions (0.1 and 0.3 wt%) on the age-hardening behavior of AA6016 Al–Mg–Si alloys with and without 3 % prestraining by hardness testing and transmission electron microscopy analysis. Our results showed that higher Cu addition slightly increased the peak hardness, but significantly shortened the time to reach peak aging in both alloys with and without prestraining. Prestraining can improve the peak hardness by making the higher Cu addition more pronounced. In alloys without prestraining, the higher Cu addition slowed the hardness reduction in the overaged state, and the 0.3Cu alloy consistently exhibited finer precipitates with higher number density than the 0.1Cu alloy throughout the aging process. In contrast, higher Cu addition in alloys with 3 % prestraining accelerated the hardness decrease in the overaged state and led to a coarser microstructure after 24 h aging. This is primarily because Cu atoms can slow down the reduction in dislocation density during aging, whereas the higher dislocation density promoted the growth of precipitates, resulting in accelerated coarsening of precipitates as the aging time proceeded. Moreover, dislocations accelerated the formation of precursors of the Q′ phase, which was more pronounced in the 0.3Cu alloy.

Age-hardening behaviors of the weld metals of 22% Cr and 25% Cr duplex stainless steels at 400 °C

Duplex stainless steel (DSS) alloys are attractive materials for pressure components in nuclear power plants as they offer a good combination of high strength, corrosion resistance, and weldability. However, DSS suffers from embrittlement and hardening upon exposure to temperatures of 250–500 °C. Most previous studies addressed only solution-treated materials, and limited research has been conducted on weld metals. In this study, the relationship between the nanostructural evolution and hardening of 22% Cr and 25% Cr DSS weld metals (fabricated with grade-2209 and 2594 welding materials, respectively) during aging at 400 °C was investigated. The results revealed that the degree of hardening of ferrite after aging at 400 °C for 1000 h was greater in the 22% Cr DSS weld metal than in the 25% Cr DSS weld metal. The ferrite phases of both samples underwent two different phase decomposition processes during aging, (1) phase separation (α → Fe-rich α + Cr-rich α’) and (2) solute clustering to form G-phase precursors, both of which were identified within the body-centered cubic structure of ferrite in the weld metals. Phase separation was less pronounced, whereas formation of G-phase precursors was more pronounced in the 22% Cr DSS weld metal than in the 25% Cr DSS. These results indicate that the extent of formation of G-phase precursors has a significant influence on predicting the degree of hardening of ferrite. The enhanced formation of G-phase precursors in the 22% Cr DSS weld metal was elucidated using thermodynamic simulations; the formation of the G-phase precursor was predominantly facilitated by the higher Mn fraction in the 22% Cr DSS weld metal. The findings of the present study are expected to aid the selection and/or further improvement of the compositional design of DSS welding materials, thereby increasing the long-term safety of DSS welded components in nuclear applications.

Improvement in the age-hardening response of Mg–7Sn alloy by compressive stress-assisted aging

Stress aging is an effective method to improve age-hardening behavior of Mg alloys. In this paper, the stress aging was applied to improve poor age-hardening response of Mg–7Sn alloy. The age-hardening behavior, microstructure evolution and mechanical properties of Mg–7Sn alloy with an external compressive stress were investigated. The stress aging significantly altered the size, morphology, amounts and distribution of Mg2Sn phase. During the stress aging, micro/nano scale Mg2Sn particles precipitated on the basal plane of the stress aged alloy and their volume fraction remarkably increased as compared with the isothermal aging alloy. Interestingly, small amounts of Mg2Sn phases precipitated on the non-basal planes. The stress aging significantly reduced the peak aging time (96 h) and increased peak aging hardness (68.4 HV). The tensile tests showed that the stress peak-aged alloy at 80 MPa exhibited the highest tensile strengths (σ0.2 = 140 MPa and σb = 217 MPa), which were mainly attributed to the precipitation strengthening of Mg2Sn phase.

Age-hardening behavior, corrosion mechanisms, and passive film structure of nanocrystalline Al-V supersaturated solid solution

• Synthesis of a nanocrystalline Al-5at.%V alloy with high solid solubility of V by high-energy ball milling • Study on the age hardening behavior of the ball milled Al-V alloy • Corrosion behavior as a function of age hardening time and temperature • Study of the passive film and pits using transmission electron microscope • Microstructural features causing passive film breakdown and subsequent pit growth or repassivation The effect of age-hardening on microstructure, hardness, and corrosion of an Al-5at.%V alloy, produced using high-energy ball milling and subsequent cold compaction, has been investigated. The alloy exhibited a grain size below 100 nm and extremely high solid solubility of V in Al (3.1 at.%). The age-hardening was carried out at 150, 200, and 250 ℃. The peak-aged condition of 150 ℃ demonstrated the highest hardness—transpired from grain refinement, precipitation, and solid solution hardening. The corrosion resistance of the Al-5at.%V alloy was studied as a function of aging conditions. The peak-aged condition retained the corrosion resistance while it deteriorated in the over-aged condition. Nonetheless, the corrosion resistance of the ball-milled Al alloys in all the aging conditions was superior to that of pure Al. The passive film structure and origin of corrosion were studied using scanning/transmission electron microscopy (S/TEM). The high corrosion resistance of the alloy was attributed to the V enrichment at the film/metal interface and deposition of V on the cathodic phases, which suppresses the dissolution of Al within the pit and therefore promotes repassivation in the early stages of corrosion.

Influence of Sn and Zn on age-hardening behavior of Mg-6Al magnesium alloy

ABSTRACT In the present work, a thermally stable as-cast Mg-6wt% Al-3wt% Sn (AT63) and a commercial AZ series Mg-6wt% Al-3wt% Zn (AZ63) magnesium alloy were developed to study age-hardening behavior. Age-hardening behavior was compared at temperatures between 200°C and 350°C for a prolonged period. The former alloy AT63 has two precipitates, Mg17Al12 and Mg2Sn, showing a poor ageing profile, whereas the latter has only Mg17Al12 precipitate and significant age hardening. Scanning electron microscopy (SEM) was used to observe precipitate formation in the aged samples of both alloys under the same annealing conditions. It was concluded that adding Sn to Mg-6Al alloy slows down the formation of Mg17Al12 and consequently reduces the ageing effect, though it aids in forming a new precipitate Mg2Sn.

Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg−5Sm−0.6Zn−0.5Zr (wt.%) alloy have been systematically investigated by means of transmission electron microscopy (TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 °C for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β0′ precipitate (orthorhombic, a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ” precipitate (hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ” precipitate is more thermostable than prismatic β0′ precipitate in the present alloy. β0′ precipitates gradually coarsened and were even likely to transform into β1 phase (face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.

Investigation on the age-hardening behavior of Al–Si–Cu–Mg cast alloys by mechanical testing, microstructural characterization, and a CNGT based precipitation model

Abstract In the present contribution, the age-hardening behavior of two commercial Al–Si–Cu–Mg alloys was studied during heat treatments at 200 °C and 240 °C. Size and volume fraction of precipitates were characterized by means of transmission electron microscopy, small-angle X-ray scattering, and small angle neutron scattering. Additionally, tensile tests were performed to evaluate the mechanical behavior. Finally, a multi-component multi-phase classical nucleation and growth theory precipitation model was developed to predict the evolution of Mg- and Cu-containing precipitates and their hardening effect. The simulations were in good agreement with experimental findings.

Age hardening of Al-7Si-0.5Mg alloy: Role of Si size and distribution

The age-hardening behavior of the Al-7Si-0.5Mg alloy with different size and distribution of Si phase was investigated. The wedge-shaped mold was used to prepare specimens with different characteristics of Si. The specimen with fine and homogeneous Si showed accelerated age-hardening kinetics and higher peak hardness than the one with coarse and clustered Si due to the higher dislocation density resulted from the mismatch in the thermal contraction of Si and Al. The overlapping of the plastically-affected zone of the adjacent Si phase in the specimen with large and clustered Si led to a reduced volume fraction of the plastically-affected zone, lower dislocation density and lower age-hardening kinetics. The relationship between the radius of Si and the critical inter-particle spacing that minimize the interaction of adjacent Si to maximize the dislocation density and accelerate age-hardening kinetics was established.