Width Of Precipitate Free Zone Research Articles

Probing the micro-mechanism of precipitate-strengthened alloys with precipitate free zone: An experimental and theoretical study

Precipitate free zone (PFZ) consistently forms near the grain boundaries (GBs) in precipitate-strengthened alloys, significantly weakening the materials because of their intrinsic softness compared to the bulk. However, the influence of PFZ near GBs on deformation mechanism remains largely unrevealed. Here, we systematically investigate the effects of PFZ on the macroscopic mechanical behavior and the microstructure deformation mechanism of the modelled precipitate-strengthened Al-Zn-Mg-Cu alloy, using a combination approach of experiments, molecular dynamics (MD) simulations, and theoretical modeling. Four Al-Zn-Mg-Cu alloys with highly different PFZ widths are prepared by tailoring the quenching media and applying the new deformation heat- treatment process proposed by us. Experimental characterizations demonstrate that severe dislocation accumulation occurs at the interface between PFZ and bulk. Meanwhile, MD simulations further reveal that PFZ is prone to plastic deformation during tensile process, contributing to the softening of materials. The PFZ exhibits significant strain concentration, leading to the preferential formation of dislocations within PFZ rather than at GBs. It is found that the level of strain concentration and the degree of dislocation accumulation are not sensitive to the PFZ width. Based on these mechanisms, a PFZ-dependent strength model is developed to quantitatively evaluate the influence of PFZ on tensile strength by considering dynamic strengthening of PFZ. It is predicted that an increase in PFZ width greatly reduces the tensile strength, with a 21 % reduction observed when PFZ width reaches 268 nm, emphasizing the important impact of PFZ width on materials strength.

Effect of aging treatment on microstructure and corrosion properties of Al-Cu-Mn-Si-Mg-Er-Zr alloy

The microstructure, mechanical properties and intergranular corrosion resistance of Al-Cu-Mn-Si-Mg-Er-Zr alloy with squeeze casting in the peak aging state at different temperatures were studied. The alloy was aged at 165 °C, 175 °C and 185 °C, and reached peak aging at 18 h, 12 h and 8 h respectively, among them, the alloy treated with 175 °C/12 h aging has the highest hardness, reaching 149 HV, and the yield strength is also the highest, which is 387 MPa. It is because the Q phase precipitated during the aging process of the alloy provides a heterogeneous nucleation site for the θ′ phase, the average size of the θ′ phase is 73.38 nm. The intergranular corrosion depth of the alloy after aging treatment at 175 °C/12 h was the deepest, reaching 284.91μm. At this time, the width of precipitation-free zone(PFZ) is the widest in the grain boundary microstructure, which is 92.21 nm, and the grain boundary precipitation phase is continuously distributed.

Localized corrosion characteristics of Al-Mg-Si aluminum alloy with aging time and pre-delayed aging condition

The effects of single aging, delayed aging and pre-delayed aging treatment on the corrosion resistance of Al-Mg-Si alloy were studied by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), three-dimensional X-ray tomography (3D-XRT), confocal laser scanning microscopy (CLSM) and electrochemical measurements. The surface potentials of Al(MnFe)Si, Mg2Si, and Si-rich remnant phases are determined, identifying Al(MnFe)Si phase as the crucial cathodic site for corrosion initiation, and the increase in surface potential implies an accelerated corrosion rate after immersion. The intergranular corrosion susceptibility of the alloy is predominantly influenced by the microstructure caused by aging conditions. The factors affecting corrosion resistance are summarized as (i) adjustment of electrochemical reaction activity by controlling the area fraction of Al(MnFe)Si phase, (ii) the spacing of grain boundary precipitates (GBPs), and (iii) the width of the precipitation-free zone (PFZ). The delayed aging (DA) sample exhibits inferior corrosion resistance due to the continuous GBPs and extended sub-grain boundaries corrosion propagation paths. The pre-delayed aging treatment successfully mitigates the corrosion degree of DA by widening the GBPs spacing and narrowing the PFZ width. This suggests that pre-aging treatment is a potent strategy for enhancing corrosion resistance without compromising the alloy's strength.

Effects of pre-deformation on microstructure, mechanical properties and corrosion performance of an Al-Cu-Mg-Ag alloys

The influence of pre-deformation on the microstructure, mechanical properties, and corrosion behavior of AA2029-T8 was investigated. The pre-deformation may potentially impede the formation of Mg-Ag clusters during the early stage of aging, altered the normal precipitation sequence of the alloy, and significantly increased dislocation density that facilitated non-uniform nucleation of S phase at dislocations, resulting in finer dispersion of precipitation. Moreover, it led to a decrease in Ω phase volume fraction and comparable aspect ratio(diameter/thickness). With increasing deformation, aging response was accelerated, leading to shorter time to peak strength but reduced plasticity. Additionally, the width of precipitate-free zone (PFZ) remained relatively unchanged with increasing pre-deformation; however, average size of grain boundary precipitates (GBPs) decreased while becoming more discontinuous. This resulted in narrowing intergranular corrosion (IGC) channels and decreased maximum corrosion depth as well as a transition from IGC to pitting corrosion behavior. These findings suggest that the mechanical properties and corrosion resistance of the alloy can be improved by increasing pre-deformation levels.

Tailoring precipitate distribution in 2024 aluminum alloy for improving strength and corrosion resistance

For the traditional peak-aged (PA) AA2024 alloy, the formation of large S-phase precipitates within the grains, wide precipitate-free zones (PFZs) near the grain boundaries (GBs), and continuous distribution of grain boundary precipitates (GBPs) can be observed. As a result, the PA alloy exhibits relatively high strength but poor corrosion resistance. However, with the application of cyclic plasticity treatment, high-density 1–2 nm clusters form within the matrix, and no PFZs form near GBs. In this study, this treatment yields the optimal balance between strength–elongation characteristics and corrosion resistance. By combining cyclic plasticity and ageing heat treatment with different heating rates, the nanoscale clusters play a crucial role as heterogeneous nucleation sites, resulting in the formation of finer and higher number density of S precipitates within the matrix. Additionally, the presence of these clusters reduces the formation of GBPs and minimizes the width of PFZs. Consequently, compared to the traditional PA sample, this approach achieves a significantly higher yield strength (increased by 46 %) and ultimate tensile strength (increased by 18 %), along with superior corrosion resistance. Although the influence of ageing heat treatment with different rates on mechanical properties is not significant, it notably affects the formation of GBPs and corrosion resistance. Specifically, a slower heating rate leads to an increase in the spacing between adjacent GBPs, resulting in improved corrosion resistance. In summary, cyclic strengthening, as a novel method for alloy strengthening, when combined with ageing heat treatment, modulates the distribution of S precipitates within the matrix and GBs. This optimization maximizes the effects of precipitation strengthening and breaks the inverse relationship between strength and corrosion resistance.

Achieving high performance by optimized heat treatment in a spray formed Al–Zn–Mg–Cu alloy

High strength Al–Zn–Mg–Cu alloys are widely used for structure components in engineering applications, such as aerospace, transportation, owing to a superior combination of high strength to density ratio and excellent mechanical properties. In present work, the effect of solution treatment and aging treatment on precipitates and mechanical property is investigated in a spray formed (SFed) Al–Zn–Mg–Cu alloy. The SFed Al alloy after optimized solution treatment shows fine grains and high fraction of substructures, which increase the kinetics for precipitation and refine precipitates. It is found that the aging temperature mainly affects the size and phase type of matrix precipitates, size of grain boundary precipitates and width of precipitate free zone. It confirms that both the strength and ductility are controlled by precipitates, which are strongly dependent on aging temperature. Specifically, the sample aged at 120 °C for 16 h (120 °C@16h) behaves highest yield strength (661 MPa) caused by the fine precipitates, while the sample aged with 100 °C@24h behaves highest ultimate tensile strength (745 MPa), uniform elongation (9.1 %) and total elongation (12.5 %). In addition, the strain hardening behavior of SFed Al–Zn–Mg–Cu alloy is discussed by Kocks-Mecking model and an empirical rule.

Exploring the dependence of creep behavior on the grain size of a peak-aged Mg-10.5Y alloy

In this work, the effects of grain size on the compressive creep behavior of a peak-aged Mg-10.5 wt%Y alloy were studied. Compressive creep tests were conducted at 523 K using three group samples, each having an average grain size of d = 25 µm, d = 50 µm and d = 90 µm along the rolling direction. An inverse Hall-Petch relationship for mechanical strength could be used to evaluate the dependence of creep behavior on the grain size, and the creep resistance was continuously deteriorated with decreasing grain size. The dominant creep mechanisms were pyramidal <c+a> slip, cross-slip and prismatic slip in the d = 25 µm sample whilst pyramidal <c+a> slip, cross-slip and basal slip controlled the creep process in both the d = 50 µm and d = 90 µm samples. In addition, precipitate-free zones (PFZs) decorated randomly the grain boundaries during creep. Both the PFZs width and PFZs amounts were increased as the average grain size decreased, and the softening effects from the frequent activated prismatic slip and profuse PFZs induced the worst creep resistance in the d = 25 µm sample. Thus, the creep resistance of the fine-grained samples was always unsatisfactory even though grain boundary sliding was absent at creep duration. It hopes that this work can enrich the understanding of creep theories and offer important reference to the microstructure design of heat-resistant Mg-RE alloys.

Distinguished roles of static aging and strain aging in the microstructure and creep resistance of Mg-4Y-3.5Nd alloy

This work concerns the distinguished roles of static aging and strain aging in the creep resistance of a hot-rolled Mg-4Y-3.5Nd alloy. The solution-treated sample is named AS while the peak-aged sample obtained from static aging at 220 °C is named AA. The strain aging (creep loading) was performed for both AS and AA samples at 220, 250 and 280 °C, respectively. The results showed that the creep resistance of both samples was closely related to the width of precipitate-free zones (PFZs). Under low stress, the dislocation cross-slip was effectively delayed by the precipitates and the existing PFZs widened slowly in the AA sample, leading to its stronger creep resistance compared to the AS sample. Inversely, under high stress, pyramidal 〈c+a〉 slip was more frequently activated, which could not be delayed by the coarsened precipitates. Consequently, the widening rate of PFZs became fast and the creep resistance became weaker in the AA sample. From the above-mentioned results, this work provides a novel guide for using Mg alloys with rare-earth addition. At the temperature range of 220–280 °C, static aging is positive for creep resistance under low stress, while directly performing strain aging without static aging is recommended for creep resistance under high stress.

Effect of Grain Structure and Quenching Rate on the Susceptibility to Exfoliation Corrosion in 7085 Alloy.

The influence of grain structure and quenching rates on the exfoliation corrosion (EXCO) susceptibility of 7085 alloy was studied using immersion tests, optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The results show that as the cooling rate decreases from 1048 °C/min to 129 °C/min; the size of grain boundary precipitates (GBPs); the width of precipitate-free zones (PFZ); and the content of Zn, Mg, and Cu in GBPs rise, leading to an increase in EXCO depth and consequently higher EXCO susceptibility. Meanwhile, there is a linear relationship between the average corrosion depth and the logarithm of the cooling rate. Corrosion cracks initiate at the grain boundaries (GBs) and primarily propagate along the HAGBs. In the bar grain (BG) sample at lower cooling rates, crack propagation along the sub-grain boundaries (SGBs) was observed. Compared to equiaxed grain (EG) samples, the elongated grain samples exhibit larger GBPs, a wider PFZ, and minor compositional differences in the GBPs, resulting in higher EXCO susceptibility.

Effects of Rapid Quenching on Grain Boundary Microstructure and Mechanical Properties of an Al-Mg-Si-Cu Alloy.

Precipitate free zones (PFZs) near grain boundaries generally soften alloys. The quenching rate after solution treatment is an important factor influencing the width of PFZs in Al-Mg-Si-Cu alloy. This study explored the effects of high quenching rates on the grain boundary microstructures and mechanical properties of an Al-Mg-Si-Cu alloy. Samples of various thickness were quenched in water at room temperature and in ethylene glycol at -40 °C, respectively. The results showed that the rapidly quenched samples at -40 °C exhibited better comprehensive mechanical properties than the water-quenched samples. Transmission electron microscopy studies revealed the rapidly quenched samples had wider PFZs, shorter intragranular precipitates, and larger grain boundary precipitates (GBPs) than water-quenched samples. It is proposed that when the quenching rate exceeds the critical cooling rate, e.g., in water quenching or rapid quenching, the formation of PFZs is controlled by the solute depletion mechanism rather than the vacancy depletion mechanism. The nucleation and growth of GBPs thus lead to the depletion of solute atoms, resulting in wider PFZs rather than thinner PFZs according to previous knowledge. This research provides valuable insights into the application of rapid quenching technology for modifying alloys' microstructures and properties.

The effect of trace level copper content on intergranular corrosion of extruded AA6082-T6 alloys

The effect of trace level Cu content on intergranular corrosion in an acidified salt solution has been systematically studied in four AA6082-T6 extrusions containing 0.001, 0.02, 0.03 and 0.05 wt% Cu. Although these Cu concentrations are smaller than those expected to directly contribute to intergranular corrosion (IGC) in aged AA6082 alloys, IGC is observed in the present experiment even in alloy with the lowest Cu content. Whereas the extent of IGC is negligibly small in the alloy containing 0.001 wt% Cu, it increases with increasing Cu content. Profound IGC is observed in the alloy containing 0.05 wt% Cu. Microstructural characterization reveals Cu segregations along some grain boundaries in each alloy. The different alloys demonstrate differences in the size and number density of precipitates as well as in the width of precipitate free zones.

Effects of creep age forming time on microstructure and high cyclic fatigue of 7075 aluminum alloy

AbstractIn this paper, the hardness, ultimate tensile strength, yield strength, elongation E100, S−N curves, and fatigue performance of 7075 aluminum alloy were obtained after aged at 170 °C for different times (10 h, 15 h, and 20 h). Additionally, the microstructure and fatigue fracture of the alloy were observed. The investigation results show that as the forming time increased, the hardness, ultimate tensile strength, and yield strength decreased, the elongation first decreased, then increased, and the fatigue limit increased. As the forming time increased, the metastable phase gradually transformed into a steady phase and coarsened, the width of precipitate free zone increased, and the width of the fatigue strip decreased. After creep age forming for 20 h, the precipitate free zone was the widest, approximately 100 nm.

Effect of Zr addition on the microstructure evolution and mechanical properties of extruded Al-Cu-Li-Mn alloys

The effects of Zr addition on microstructure evolution and mechanical properties of extruded Al-Cu-Li-Mn alloy were investigated during heat treatment processing. The recrystallization and grain growth were profoundly inhibited in solid solution-treated (SST) Al-Cu-Li-Mn-Zr alloy (Zr-containing), and massive substructures were retained, which were attributed to the higher Zener pinning pressure provided by Al3Zr dispersoids. But, complete recrystallization and considerable grain growth occurred in SST Al-Cu-Li-Mn alloy (Zr-free). Besides, the deformation textures containing Brass and S components were predominant in Zr-containing alloy, which contributed to a higher average Talyor factor (M) value. The higher strength behavior in SST Zr-containing alloy mainly originated from the extra Al3Zr dispersoids apart from the higher M value and dislocation density. The higher dislocation density and Al3Zr dispersoids in Zr-containing alloy promoted the precipitation of T1 phases and narrowed the width of precipitate-free zones (PFZs). Compared with Zr-free alloy, the enhanced combination of good strength and ductility shown in T6-aged Zr-containing alloy was mostly due to the higher T1 phase density and M value, higher dislocation density, and extra Al3Zr dispersoids.

Effect of trace Sc on high-temperature creep performance of spray-deposited Al-Zn-Mg-Cu alloy

In this work, the effect of trace Sc on the mechanical property and high-temperature creep performance of the spray-deposited Al-Zn-Mg-Cu alloy was systematically investigated. The experimental results showed that the microstructure of the deposited and extruded alloy was greatly refined by the addition of Sc. For the Sc-containing alloy, precipitates within grains and the width of precipitate free zone (PFZ) are similar with those of the Sc-free alloy, whereas the area fraction of the precipitates at the grain boundaries is dramatically decreased. Both the mechanical property and high-temperature creep performance can be simultaneously improved by the addition of Sc. After 0.2 wt% Sc was added, the ultimate tensile strength of 831 MPa and elongation of 10% can be obtained, while the steady-state creep rates reached 1.1 × 10−6/h and 1.8 × 10−5/h at 80 °C and 120 °C, respectively. Based on the microstructure observation, amount of fine and dispersed Al3(Sc, Zr) secondary phase particles were generated by the addition of Sc. It indicated that the Sc microalloying can not only bring about additional strengthening effect, but also inhibited the dislocation migration and coarsening of precipitates at high temperature, leading to the enhanced mechanical and creep resistance performance.

Through variable temperature retrogression to enhance mechanical properties and corrosion resistance of extruded 7055 aluminum alloy

A variable temperature retrogression and re-aging (VT-RRA) was exploited to extruded 7055 aluminum alloy, which can make the comprehensive performance of the alloy exceeds that of traditional constant temperature retrogression RRA (CT-RRA). In the variable temperature retrogression stage (VTRT-stage), the 7055 alloy undergone pre-aged at 120 °C/24 h is heated to 200 °C at a rapid rate of 8 °C/min, then cooled to 120 °C at three different rates: 10 °C/h, 15 °C/h and 20 °C/h. GP zones and small-sized η′ phases dissolve back into the matrix, while the η′ phases with larger size are inclined to coarse in the VTRT-stage. As the cooling rate increases, the average size of precipitates in the finished alloy undergoing re-aging at 120 °C/24 h is smaller (9.6→6.9→6.7 nm), which can effectively prevent excessive degradation of mechanical properties. In the VTRT-stage, the grain boundary precipitates (GBPs) change from continuous to discontinuous, forming the precipitate free zone (PFZ), and the larger the cooling rate, the less the contents of Zn, Mg and Cu elements in the GBPs, the narrower the PFZ (101.0→77.5→51.2 nm), the lower the electrical conductivity (41.9→39.3→36.7 %IACS), and the corrosion resistance achieves good results at around 15 °C/h. Uncoarsened precipitates with higher proportion (41.6%) of η′ phase and less of η phase (2.1%) can be obtained through VT-RRA with cooling rate of 15 °C/h, making the mechanical properties exceed those of traditional CT-RRA. And the appropriate width of PFZ and the discontinuous distribution of GBPs are the reasons why VT-RRA alloy has better corrosion resistance than CT-RRA.

Synchronous improvement of mechanical properties and stress corrosion resistance by stress-aging coupled with natural aging pre-treatment in an Al-Zn-Mg alloy with high recrystallization fraction

Enhancing the strength of Al-Zn-Mg alloys is critical to the weight-lightening of structural components in the application of high-speed trains and aerospace industries, while high stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg alloys (especially the alloy with high recrystallization fraction) with high strength makes it difficult. In this study, the influence of tensile stress-aging coupled with natural aging pre-treatment on the mechanical properties and SCC resistance of Al-Zn-Mg alloy with high recrystallization fraction has been investigated. The results show that tensile stress-aging at 160 ℃ can inhibit the dissolution of clusters/Guinier-Preston (GP) zones formed during long-term natural aging pre-treatment, which increases the number density of matrix precipitates (MPts), narrow the width of precipitate free zone (PFZ), and dramatically improve the mechanical properties of the experimental Al-Zn-Mg alloy. Meanwhile, the precipitation of the high density of MPts within the matrix will assume a large number of solute atoms during artificial aging, which will reduce the supplement of solute atoms to grain boundaries. As a result, the volume of anodic active grain boundary precipitates (GBPs) and the content of free solute atoms at grain boundaries are reduced, which reduces the possibility of initial nucleation and propagation of SCC crack. The coupled treatment method proposed in this study proves efficient in resolving the contradiction between the strength and SCC resistance in Al-Zn-Mg alloy.

Effects of ultrasonic vibration on the microstructure and corrosion properties of the 7046 Al alloy

Low-Cu Al–Zn–Mg series 7046 alloys were prepared using ultrasonic vibration casting and conventional casting methods. The grain sizes, grain boundary precipitates, and precipitate-free zones of the alloys were studied, and the polarization curves and electrochemical impedance spectra were measured to reveal the effects of ultrasonic vibration on the alloy microstructure and corrosion properties. The results showed that ultrasonic vibration refines and homogenizes the microstructure, promotes the discontinuous distribution of grain boundary precipitates, and increases the precipitate-free zone width. Consequently, the corrosion resistance is enhanced, and the corrosion rate is decreased.

Precipitation behavior and quenching sensitivity of 2196 Al–Li alloy extrusion profile

Al–Li alloy usually has quenching sensitivity, affecting the mechanical properties of its forming components. This paper investigated the effects of extrusion parameter and quenching cooling rate on precipitation of quench-induced phases, aging phases and quenching sensitivity of extruded 2196 Al–Li alloy profiles. The results showed that a band structure constituted by coarse quench-induced phases including T1 (Al2CuLi) and fine aging phases including σ (Al5Cu6Mg2), θ' (Al2Cu) and T1 was formed as the quenching cooling rate was lower than that of air-quenched. With the increase of quenching cooling rate, the precipitation of quench-induced T1 phases was suppressed, and the size and density of other quench-induced phases, the width of precipitation free zone as well as the quenching sensitivity were significantly reduced. The grains of the profiles in extrusion and solution state both exhibited a fibrous structure, with a strong <112>//ED and <111>//ED orientation. The profile extruded at 370 °C × 1.5 mm/s exhibited abnormal coarse grains and gained a higher proportion of recrystallization after solution. The higher proportion of recrystallization and high angle grain boundaries would increase the hardness quenching sensitivity of 2196 Al–Li alloy. By comparison, extrusion parameters have a stronger impact on the strength quenching sensitivity than on the hardness quenching sensitivity. The effect of extrusion parameters on quenching sensitivity was less than that of quenching cooling rate. The elongation of the profiles increased with the decrease of quenching cooling rate at low extrusion temperature or speed, while it exhibited an opposite trend at high extrusion temperature or speed.

Effect of preaging temperature on microstructure evolution, mechanical and corrosion behavior of RRA‐treated high‐zinc 7068 alloy

AbstractMechanical and corrosion behavior of high‐strength, high‐zinc (&gt;7 wt%) containing 7068 aluminum alloy is investigated after employing different retrogression and reaging (RRA) treatments. The effect of preaging conditions on the distribution of copper, zinc, and magnesium, the volume fraction of η′ phase, and the width of precipitate free zones (PFZ) have been investigated. Microstructural and compositional features characterized by scanning electron microscope‐energy dispersive spectroscopy, scanning transmission electron microscope, and differential scanning calorimetry are correlated with hardness and corrosion performance. The localized attack is manifested in the dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. A combination of two opposite effects, that is, the presence of nobler, high‐copper grain boundary precipitates and microgalvanic effect of PFZ along with the distribution of alloying elements, that is, Cu, Zn, and Mg govern the electrochemical behavior of RRA‐treated 7068 alloy. Optimum preaging and RRA conditions are identified for this high‐zinc 7xxx series alloy.

Sc-containing hierarchical phase structures to improve the mechanical and corrosion resistant properties of Al-Mg-Si alloy

Although the effect of Sc addition on the nano-phase transformation of precipitates in Al-Mg-Si alloys has been evaluated in the literature, the properties and related explanation based on the multi-scale microstructures are less available yet. In this paper, the hierarchical microstructures and properties of peak-aging Al–0.5 Mg–0.4Si (wt.%) alloy (AlMgSi) and AlMgSi alloy microalloying with 0.4 wt% Sc (AlMgSiSc) were investigated by atomically-resolved transmission electron microscopy and atom probe tomography. The results show that a certain number of Sc solutes exist in the Al matrix, forming a solid solution strengthening matrix with reduced corrosion susceptibility. In addition, the strengthening and toughening of AlMgSiSc are associated with the formation of primary Al3Sc dispersoids, which cause dispersion strengthening and fine-grain strengthening. Moreover, the precipitate free zones (PFZs) containing Sc solutes were observed in the AlMgSiSc, leading to the excellent resistance to intergranular corrosion and elongation due to the smaller width and higher corrosion potentials of PFZ compared with AlMgSi. The Sc solutes can significantly promote the precipitation of GP zones and solute clusters. Further study reveals that smaller-scale precipitates, GP zones, and solute clusters have enrichment of Sc as well, thereby bringing out remarkable precipitation hardening to AlMgSiSc.