Peak Aging Time Research Articles

Influence of quenching temperature on peak aging time and hardness of Al-Mg-Si-Cu alloys strengthened by nano-sized precipitates

The quenching temperature plays a rather important role in the subsequent artificial aging behavior and resulting hardness of the Al-Mg-Si (or Al-Mg-Si-Cu) alloys, but related reports are limited. This paper presents a detailed investigation about the influence of the quenching temperature (ranging from 490 °C to 570 °C) on the precipitate microstructure, precipitation rate and aging hardness of a typical Al-Mg-Si-Cu alloy 6061. Methods such as TEM microscopy, real-time electrical resistivity monitoring and Vickers hardness testing were employed. The TEM results show that the main precipitate in peak aged status is needle shaped β″ with sizes of several nanometers in cross section and ten to twenty nanometers in length. It gradually grows up in size and transforms to Q′ phase in the over aged status. Hardness and resistivity measurement results show that the hardness increases while the resistivity decreases as the artificial aging proceeds. With higher quenching temperature, the size of the β″ phase gets finer and its total amount becomes more. Its precipitation kinetics also turns out to be much higher. The time for reaching the peak aging hardness prolongs from about 2–10 h as quenching temperature increases from 490 °C to 570 °C. But these effects becomes not so obvious when quenching temperature exceeds 550 °C, pointing out that it is the critical quenching temperature for this kind of alloy. Related mechanisms are explained from the aspects of super saturation solid solution level, quenched-in vacancy concentration, precipitate nucleation, growth rate etc. These results establish a significant heat-treatment guideline for mass production of these alloys.

Processing, microstructure and ageing behavior of in-situ submicron TiB2 particles reinforced AZ91 Mg matrix composites

In this paper, in-situ submicron TiB2 particles (2.5 wt.%) reinforced AZ91 matrix composites are processed by the master alloy method combined with an optimized SHS technique. The effect of TiB2 particles on the microstructure, ageing behavior and mechanical properties of Mg matrix composites is investigated. The results reveal that only TiB2 phase is in-situ formed in the Al-TiB2 master alloy and exhibits the size distribution of 100 nm–700 nm. Compared with the unreinforced AZ91 alloy, the grain size and morphology of Mg17Al12 phase in as-cast composites are much refined. The ageing process of composites is accelerated and the peak-aged time of composites decreases from 42 h to 22 h significantly with the introduction of TiB2 particles. Large numbers of fine Mg17Al12 in composites preferentially precipitate at grain boundaries and near TiB2 particles regions. In addition, the hardness, yield strength, ultimate tensile strength and elongation of as-cast composites are improved by 16.1%, 53.0%, 26.3% and 25.6%, respectively. After T6 heat treatment, the strength of composites is increased further due to large amounts of submicron Mg17Al12 phase, and the age hardening efficiency of composites is higher than that of AZ91 alloy.

Combined Effects of Pre-deformation and Pre-aging on the Mechanical Properties of Al-Cu-Mg Alloy with Sc and Zr Addition

The combined effects of pre-deformation and pre-aging on the mechanical properties of Al-Cu-Mg alloy with Sc and Zr addition were investigated. It is revealed that the introduction of pre-deformation can enhance the peak-aging strength, as well as tensile and yield strength, effectively due to the formation of finer and more dispersive precipitation. Pre-aging process before pre-deformation can increase the elongation while maintaining higher strength with a discontinuous distribution of precipitates at grain boundary. The precipitates of bean-like Al3(Sc, Zr) particles further strengthen the alloy via pinning the dislocations which are formed during pre-deformation process and hindering the dislocation motion. Furthermore, pre-deformation and pre-aging accelerate the kinetics of precipitation due to preferential sites provided by the dislocation and the increase of GPB zones’ size and distribution. The synergism of pre-deformation and pre-aging achieves a combination of better mechanical properties and shorter peak-aging time.

Influence of aging treatment on corrosion behavior and mechanism of Mg–Y alloys

This paper studied the influence of aging treatment on the corrosion behavior and mechanism of Mg—Y alloys with different Y content through corrosion mass loss test, electrochemical test and corrosion morphologies observation. Results show that the peak-aging times of Mg–(0.25, 2.5, 5, 8 and 15) Y alloys at 250 °C were 4, 6, 10, 12 and 16 h. The aging treatment reduced the corrosion resistance of Mg–Y alloys, and the corrosion resistance of Mg–Y alloys became worse with increasing of the aging time. The change magnitude of the open circuit potentials for Mg–(0.25, 2.5)Y alloys was greater than that of Mg–(5, 8 and 15)–Y alloys. The polarization curves of Mg (0.25, 2.5, 5, 8 and 15) Y alloys had the similar shape after aging treatment, and the slopes of the anodic branch were greater than those of the cathodic branches. After aging treatment, the corrosion modes of Mg–0.25Y and Mg–(2.5, 5, 8 and 15) Y alloys were uniform corrosion and pitting corrosion with small local deep corrosion.

Grain-Refined AZ92 Alloy with Superior Strength and Ductility

Grain-refined AZ92 (GR-AZ92) alloy with superior tensile properties is developed by adding 1 wt% Zn and a very small amount of SiC (0.17 wt%) to commercial AZ91 alloy for enhancing the solid-solution strengthening effect and refining the crystal grains, respectively. The homogenized GR-AZ92 alloy with an average grain size of 91 μm exhibits a tensile yield strength (TYS) of 125 MPa, ultimate tensile strength (UTS) of 281 MPa, and elongation of 12.1%, which are significantly higher than those of AZ91 alloy with a grain size of 420 μm (TYS of 94 MPa, UTS of 192 MPa, and elongation of 7.0%). The peak-aging time of GR-AZ92 alloy (8 h) is significantly shorter than that of AZ91 alloy (32 h) owing to a larger amount of grain boundaries in the former, which serve as nucleation sites of Mg17Al12 precipitates. A short-aging treatment for less than 1 h of the GR-AZ92 alloy causes an effective improvement in its strength without a significant reduction in its ductility. The 30-min-aged GR-AZ92 alloy has an excellent combination of strength and ductility, with a TYS of 142 MPa, UTS of 304 MPa, and elongation of 8.0%.

Effects of Pre-Strain on the Aging Behavior of Al 7075 Alloy for Hot-Stamping Capability

The present study investigates the significance of pre-strain on the T6 aging behavior of an Al 7075 alloy for evaluating the applicability of hot stamping. In practice, the alloy was pre-strained up to 15% during solution heat treatment at 480 °C prior to quenching, and artificial aging was conducted at 120 °C. The peak aging time and precipitation behavior were compared with the alloy with pre-straining at room temperature after quenching but immediately before the artificial aging. The results showed that increasing amounts of pre-strain tend to reduce the aging time up to 50% for achieving peak hardness, which is consistent with the alloy at the T6 condition. There is a limitation for the maximum attainable amount of pre-strain of 10% for the homogeneous distribution of strain when the alloy is strained at room temperature (RT) due to the low formability. The pre-strained alloy as hot stamping exhibited lowering of the peak reaction temperatures for dissolution and formation of Guinier–Preston (GP)-Zones and precipitated with increasing amounts of pre-strain towards 15% through the differential scanning calorimetry analysis, thereby confirming the shortening of the peak aging time. The present study confirms the excellent potential of the hot-stamping process to extend the capability of an Al 7075 alloy.

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of β-Mg17Al12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.

Enhanced aging kinetic of Al3BC/6061 Al composites and its micro-mechanism

In this work, the aging behavior of Al3BC/6061 Al composites was investigated and the impacts of Al3BC on the aging kinetic as well as its micro-mechanism were unveiled. It is found that the aging kinetic of the composites is considerably accelerated and the peak aging time at 175 °C comes down from 14 to 6 h. Both positive and negative impacts due to the incorporation of Al3BC on the aging kinetic of the composites were discussed. The high density dislocations can either act as the heterogeneous nucleation substrate or play as the diffusion path to expedite precipitation process, while the low vacancy concentration and solute segregation on the interface suppress the formation of GP zone and reduce the chemical driving force for precipitation, respectively. Obviously, the positive effects caused by the dislocation contribute more in the current work. Microstructure characterizations verify that needle-like β″ phase and lath Q′ phase precipitate in the matrix and are impacted by the Al3BC differently. In the area free of Al3BC particle, they distributed uniformly in the matrix. However, in the vicinity of Al3BC particles, the precipitation of β″ is inhibited for the low vacancy concentration and segregation of Mg, Si elements on the Al3BC/Al interface, while the precipitation of Q′ phase is accelerated for the preferred nucleation of Q′ phase on the dislocation.

Microstructure Evolution and Mechanical Properties of 2219 Al Alloy During Aging Treatment

Hardness and tensile properties of 2219 Al alloys were tested at various temperature (150, 165, 175 °C) and subjected to T6 temper heat treatment to identify the peak aging time at various temperature. Microstructure evolution and precipitate behavior were analyzed with transmission electron microscope (TEM), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). It is found that the peak aging time is 24 h at 150 °C and does not vary down to 165 °C. When the aging temperature rise to 175 °C, the peak aging time down to 12 h. Considering the strength and elongation, the optimum aging treatment is at 165 °C for 24 h after the solution treatment at 535 °C for 1.5 h. Compared with that of only water-quenched sample, after aged at 165 °C for 24 h, the tensile strength of the 2219 Al alloy increases from 324.5 to 411.8 MPa, yield strength from 168 to 310.8 MPa, respectively. The improvement in the mechanical performance is mainly attributed to the precipitation strengthening of the GP zones, θ″ and θ’ phases.

Effects of uniaxial creep ageing on the mechanical properties and micro precipitates of Al-Li-S4 alloy

In the present work, effects of creep ageing (CA) on the mechanical properties and micro precipitates of Al-Li-S4 alloy was investigated. Compared with single stress-free ageing (SFA), experimental results show that CA has significantly improved the mechanical properties of Al-Li-S4 alloy, including hardness, strength and elongation. Meanwhile, the peak-ageing time was also prolonged under CA treatment. When specimens were continuously placed under an exterior stress of 240MPa after CA treatment at 153°C for 25h, more T1(Al2CuLi) nano-precipitates developed and further contributed improvement in mechanical properties. It is mainly attributed to the exterior stress that promoted to form T1 nano-precipitates. Moreover, CA enabled to restrict both the coarsening of precipitates at grain boundaries and the formation of precipitation free zone. In addition, fractographic examination of SFA-treated and CA-treated Al-Li-S4 tensile specimens were carried out, respectively. Through controlling the formation process of fine T1 nano-precipitates during CA treatment, it can facilitate the development of novel microstructures in Al-Li-S4 alloys with certain promising mechanical properties.

Ageing behavior of as-cast SiCp/AZ91 Mg matrix composites

SiC particles exhibited necklace-type distribution in SiCp/AZ91 Mg matrix composites fabricated by stir casting, and thus, the SiC particles could be divided into segregated particles region and particle free region. The effects of this particle distribution on the discontinuous precipitation of Mg17Al12 phase during ageing were investigated. The high density dislocations were induced due to the sharp mismatch between the thermal expansion coefficients of SiC and AZ91 alloy. The overlapped mismatch among the segregated particles causes higher density dislocations. And as a result, the discontinuous precipitations as the ageing products started born mainly near particles, especially near segregated particles. However, few Mg17A112 precipitated in the matrix within the particle segregation. The high-density dislocations and the numerous SiC/Mg interfaces made Mg17A112 phases much easier and earlier to precipitate in the composite. Thus, the SiCp addition reduced the peak ageing time from 38h (for AZ91 alloy) to 28h (for the composite). The ultimate tensile strength of the composite was enhanced by 46% after peak-ageing. SiC particles could also improve the age hardening efficiency of the composite because the high-density dislocations, the fine grain sizes and the numerous SiC/Mg interfaces cause the finer and more uniform precipitation of Mg17A112 Above all, the addition of SiC particles is effectively to improve the mechanical properties of magnesium matrix composite materials.

Effect of Cu content on precipitation and age-hardening behavior in Al-Mg-Si-xCu alloys

The effect of varying the Cu content of Al-Mg-Si-xCu (x = 0.5, 1.0, 2.5 and 4.5, in wt.%) alloys on precipitation and their age-hardening behavior was investigated through hardness measurements, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The results indicated that aging at 165 °C, the peak aging hardness increased from 109 to 218 HV with the Cu content increasing from 0.5 to 4.5 wt%. Similarly, there was an increase in the peak aging time with Cu content. However, in over-aging stage, the hardness of 4.5Cu alloy decreased more rapidly than that of 0.5, 1.0 and 2.5Cu alloys. Furthermore, an increase in Cu content brought about an apparent change in the main precipitate of the peak-aged samples from needle-shaped β″ phase in 0.5Cu alloy to: needle-shaped β″ phase and granular Q′ phase in 1.0Cu alloy, granular Q′ phase and θ′ platelets in 2.5Cu alloy, and θ′ platelets with a small amount of Q′ phase in 4.5Cu alloy.

Role of silicon in the precipitation kinetics of dilute Al-Sc-Er-Zr alloys

The precipitate nanostructure and the strength of an Al-0.055Sc-0.005Er-0.02Zr at% alloy with Si additions, in the range 0–0.18at%, were investigated utilizing micro-hardness, electrical conductivity, scanning electron microscopy and atom-probe tomography techniques. Si-containing alloys are cost-effective due to the existence of Si in commercial purity Al. In all studied alloys, homogenization for at least 0.5h at 640°C is needed to eliminate Al3Er primary precipitates. Alloys containing the higher Si concentrations achieve higher microhardness by increasing the heterogeneous nucleation current of (Al, Si)3 (Sc, Zr, Er) precipitates. The alloy containing 0.18at% Si achieves an 60% improvement in peak-microhardness compared to the Si-free alloy, during isothermal aging at 400°C. Silicon additions reduce the peak-aging time in the temperature range 300–400°C, indicating that the Er and Sc diffusion kinetics are accelerated. Silicon also enhance the Zr diffusion kinetics, accelerating precipitate growth during aging at 300°C and precipitate coarsening at 400°C. Addition of Si modifies the concentration profiles within the nanoprecipitates, enhancing the chemical homogeneity of Sc and Er in their cores, rather than forming Er-enriched-cores/Sc-enriched-shells that we have observed in prior research. Finally, the microhardness of the alloys, containing 0.12 and 0.18at% Si, only diminishes slightly from the peak values after isothermal aging at 375°C for about 2000h, suggesting that the studied alloys can be practically utilized at this operating temperature.

Influence of Heat Treatment on Microstructure and Mechanical Properties of Rare Earth-Rich Magnesium Alloy

The experimental activity was aimed at optimizing the T6 heat treatment conditions of the innovative rare earth-rich magnesium alloy EV31A (Elektron 21®). The investigated alloy (Mg–Nd2.8–Gd1.5–Zr0.5–Zn0.2) contains Nd and Gd in proper amounts to maximize both castability and high-temperature performance. The effect of treatment parameters on microstructure, hardness and tensile properties was evaluated. Mg12NdxGd(1−x) ternary eutectic, Zr clusters and Mg/Nd–Gd intermetallic compounds were found in the as-cast alloy. A remarkable microstructural evolution, involving Mg/Nd–Gd compounds dissolution, was observed in the aged alloy; the precipitation sequence was identified as βII → βI → β. DTA analyses confirmed that 793 K (520 °C/968 °F) is the optimum solutionizing temperature in order to avoid incipient melting; on the contrary, the solutionizing time, should be reduced with respect to the standard conditions without loss in the final alloy hardness and tensile properties. The effect of aging parameters was also investigated at fixed solutionizing and quenching conditions; different aging temperatures [463, 473, 483 K (190, 200, 210 °C/374, 392, 410 °F)] and aging times (up to 48 h) were studied. The aging curves at 463 K (190 °C/374 °F) and 473 K (200 °C/392 °F) showed a similar trend, with a large peak-aging plateau; on the contrary, the peak-aging time was significantly reduced at 483 K (210 °C/410 °F). Tensile tests, performed on the most promising heat treatment conditions, did not show remarkable differences in terms of yield, ultimate tensile strength and elongation to failure with respect to the standard heat treatments conditions. This study therefore suggests that standard solutionizing and aging times can be considerably shortened, ensuring excellent mechanical properties, compared to standard T6 heat treatment, leading to commercial implications with regard to operational costs and energy saving.

Effects of ageing time on the mechanical and conductivity properties for various round bar diameters of AA 2219 Al alloy

In the present study, AA 2219 alloys of 120mm round bar are forged into various round bars (25, 50, 75mm) and subjected to T6 temper heat treatment to identify the peak ageing time for various round bars. The electrical conductivity of the round bars is also measured and correlated with the mechanical properties. It is found that the peak ageing time is 23h and does not vary up to 75mm round bar. It increases to 25h for the 120mm round bar. The yield and tensile strength of the alloy are found to be in the range of 288–304MPa and 410–428MPa for the peak ageing condition. The hardness and conductivity vary in the range of 121–128 BHN and 30–30.96% IACS respectively for the peak ageing condition. Although a near linear relation is found to exist between the strength/hardness and the electrical conductivity values for the selected heat treatment parameters (solution temperature: 535°C, ageing temperature: 191°C and the peak ageing time: 23h), the slope of the curve is different for different round bars. Microstructure characterization studies show the precipitate coarsening with increasing ageing time. Further, precipitate segregation is not found in the microstructure for any cases of round bar diameter and ageing time combinations.

Determination on the Effect of Tin Content on Microstructure, Hardness, Optimum Aging Temperature and Aging Time for Spinodal Bronze Alloys Cast in Metal Mold

A study was conducted to determine the effect of tin content on microstructure, hardness, optimum aging temperature and aging time of spinodal bronze alloys cast in metal mold. Copper, nickel and tin with appropriate compositions were melted in an electric furnace under argon atmosphere and were cast into metal molds. The cast specimens were solutionized at 825 °C for 10 h and were aged at 250, 300, 350, 400 and 450 °C for 1–5 h. The solutionized and aged specimens were subjected for microstructural evaluation and hardness testing. It was observed that the grain boundary precipitates form upon over-aging. The peak hardness increases with increase in tin content from 4 to 12 wt% in solutionized and aged condition. The peak aging time was found to remain a constant with increase in Sn content. The optimum aging temperature and aging time were also determined for spinodal bronze alloys cast in metal mold.

Effect of Nano-Scale and Micro-Scale Yttria Reinforcement on Powder Forged AA-7075 Composites

The present investigation deals with the development of AA-7075 metal matrix composites reinforced with nano yttria particles (0.1 to 3 vol.%) and micron yttria particles (1 to 15 vol.%) by powder forging. Matrix powders (AA-7075) and reinforcement powders (yttria) were blended, cold compacted, sintered under pure nitrogen, and finally hot forged in a closed floating die. The hot forged samples were artificially age hardened at 121 °C for various time durations to determine the peak aging time. The mechanical properties in the peak-aged condition as well as density and microstructure were determined and correlated with the reinforcement size and content. The nano composites exhibited a well-densified structure as well as better hardness and tensile/compressive strength as compared to micro-scale composites. The mechanical properties in nano-scale composites peaked at 0.5 vol.% yttria addition while for micro-scale composites these properties peaked at 5 vol.% yttria addition.

Effect of natural ageing and pre-straining on the hardening behaviour and microstructural response during artificial ageing of an Al–Mg–Si–Cu alloy

Al–0.42Mg–0.5Si–1.0Cu alloy was prepared by hot extrusion, followed by quenching and ageing treatment. The keeping and pre-straining immediately after quenching were performed before ageing, to investigate natural ageing and pre-straining effects on the age-hardening and microstructural response of the alloy, hardness measurement, TEM, and HRTEM characterisation were used. The results indicated that, room-temperature keeping led to a loss of peak ageing hardness for subsequent artificially aged alloy. The peak ageing hardness decreased and the peak ageing time increased with increasing keeping time. The formation of clusters/GP zones induced by natural ageing decreased the densities of β″ and Q′ phases, which contributed to the decrease of peak-ageing strengthening. Pre-straining before ageing significantly increased the peak ageing hardness and shortened the peak ageing time of the alloy. Furthermore, it significantly mitigated the effects of natural ageing on the hardening behaviour of the alloy during subsequent artificial ageing by restraining the formation of clusters/GP zones and accelerating the nucleation of β″ and Q′ phases in the early stage of subsequent artificial ageing. In the over-ageing stage, dislocations induced by pre-straining facilitated the coarsening of precipitates and consequently led to an apparent decrease of hardness.

Determination of the Effect of Nickel Content on Hardness, Optimum Aging Temperature and Aging Time for Spinodal Bronze Alloys Cast in Metal Mould

An investigation was conducted to find the optimum aging temperature and aging time for Cu-Sn-Ni spinodal bronze alloys with varying nickel wt% cast in metal mould. The specimens were subjected to solutionisation and were aged at 250°C, 300°C, 350°C, 400°C and 450°C to induce spinodal decomposition. The microstructural observation reveals that the dendritic structure of the as-cast alloy was completely eliminated after solutionisation and grain boundary precipitates were formed with increase in aging time. The peak hardness and the peak aging time increases with increase in nickel content. This implies that nickel contributes to spinodal decomposition process.

An investigation on the aging responses and corrosion behaviour of A356/SiC composites by neural network: The effect of cold working ratio

In the present study, an artificial neural network model has been used for predicting the corrosion behaviour, aging and hardness responses of aluminium-based metal matrix composites reinforced with silicon carbide particle. Hyperbolic tangent sigmoid and linear activation functions are employed as the most appropriate activation function for hidden and output layers, respectively. The developed artificial neural network model is used to predict the corrosion current density, peak aging time and peak hardness of the composites. Feed forward back propagation neural network has been trained by Levenberg Marquardt algorithm. The regression correlation coefficients ( R2) between the predicted and the experimental values of the corrosion current densities are found as 0.99986, 0.99629 and 0.99671 for the training, testing and validation datasets, respectively. Also, some case studies have been predicted by artificial neural network model. Test results indicate that the proposed network can be used efficiently for the prediction of the polarization response, peak aging time and peak hardness of the composites for different SiC volume fractions and deformation ratio without using any experimental data.