Precipitate-free Zones Research Articles

Visiting the microstructures, mechanical properties and corrosion resistance of hot-rolled Al-Cu(-Mg) alloys with and without T6 treatment

It is important to optimize the microstructures of Al-Cu based alloys for expanding their practical applications. In this work, the effects of Mg additions and T6 treatment on the microstructures, mechanical properties and corrosion resistance of hot-rolled Al-Cu alloy were investigated. The hot-rolled Al-4Cu, Al-4Cu-0.25Mg and Al-4Cu-0.5Mg (wt.%) alloys with and without T6 treatment were prepared. Experimental results indicated that the grain size of the as-rolled samples was decreased by adding Mg because of the introduction of S′-Al2CuMg phases, and the T6 treatment also obviously reduced the grain size with the help of static recrystallization regardless of alloy composition. However, the grain size of the Mg-free sample was smaller than that of the Mg-containing samples after T6 treatment, which might be resulted from the more nucleation sites provided by stored energy. The combination of Mg additions and T6 treatment could significantly enhance the mechanical properties with the help of increased second phases and decreased precipitate free zone widths, and the yield strength, ultimate tensile strength and fracture elongation were 333.5MPa, 397.1MPa and 24.5% for the T6-treated Al-4Cu-0.5Mg alloy. In addition, the corrosion resistance in 3.5wt.% NaCl solution was obviously enhanced by adding Mg, which was attributed to the narrow PFZs. Differently, the amounts of second phases were increased after T6 treatment, which induced more PFZs and damaged the corrosion resistance. As a result, the hot-rolled Al-4Cu-0.5Mg alloy with corrosion potential of -0.667V and corrosion current density of 2.668×10-7 A/cm2 exhibited the highest corrosion resistance. It is believed that this work can offer referenced information to design and process the high-performance Al-Cu based alloys in various service conditions.

Effect of Precipitation-Free Zone on Fatigue Properties in Al-7.02Mg-1.98Zn Alloys: Crystal Plasticity Finite Element Analysis.

Age-strengthened aluminum alloys, as important lightweight structural materials, have significantly lower fatigue properties compared to non-age-strengthened aluminum alloys. In this study, the polycrystalline models containing precipitation-free zones (PFZ) were constructed by secondary development of the traditional polycrystalline model by modifying the mesh file. Polycrystalline finite element simulations of peak age-treated Al-7.02Mg-1.98Zn alloys were carried out with this model. The results demonstrate that the PFZ's presence markedly reduces the alloy's yield strength and a substantial stress concentration occurs adjacent to the PFZ, generating significant compressive stresses at the PFZ. Under cyclic loading, the maximum strain energy dissipation in the model containing the PFZ far exceeds that observed in the conventional polycrystalline model, and the strain energy dissipation observed in the PFZ is significantly higher than that at other locations. This indicates that the PFZ is the main region for fatigue crack initiation. In addition, the introduction of a rotation factor to simulate the inhomogeneous rotation within the grain reveals that the additional stress concentration in the PFZ introduced by the aluminum alloy-forming process further increases the fatigue crack initiation driving force.

Effect of grain boundary precipitation on corrosion behaviour of nonisothermally aged Al-Zn-Mg-Cu alloys

The corrosion properties of five nonisothermally aged (NIA) Al-Zn-Mg-Cu alloys with different heating rates are studied and compared with those of the T6-treated alloys by electrochemical experiments and weight loss test. The results show that reducing the heating rate results in an increase in the corrosion resistance of the alloy. The aging treatment at a heating rate of 10℃/h results in an alloy with an excellent corrosion resistance and a low corrosion current density. Additionally, the hardness of the alloy is similar to that of the alloy treated with T6. Coarse precipitates are obtained by 10℃/h aging treatment, with the grain boundaries (GBs) transferring to intermittent phases with narrow precipitation-free zones (PFZs). Therefore, this approach effectively prevents anodic dissolution and improves the corrosion resistance of the alloy. These findings suggest that NIA can enhance the mechanical properties and corrosion resistance of an alloy. Additionally, the 10°C/h aging process reduces the processing time by approximately 50 %, compared to that of the T6 treatment, offering a cost-effective alternative. These results provide valuable insights into the processing of large aluminium components.

Influence of post-weld rolling and artificial aging on microstructure and mechanical properties of friction stir welded 2195-T4 Al-Li alloy joints

Fiction stir welding (FSW) of a naturally aged (T4) 2195 Al-Li alloy was conducted to study the effect of post-weld artificial aging (AA) and rolling+artificial aging (R+AA) on microstructure and mechanical properties of the FSW joints. Grain features were analyzed by electron back scattered diffraction (EBSD) technology, and precipitates were characterized using transmission electron microscopy (TEM) along [011]Al and [001]Al zone axes. Mechanical properties of the joints were evaluated through micro-hardness and tensile testing. The results indicate that grain development, including grain orientation, grain boundary components and dislocation density, varies within different regions of the As-welded joints due to complicated thermal-mechanical history. The alloying elements mostly exist as Guinier-Preston zones in the base material (BM) with a few formations of δ'/β', and slightly precipitate into T1 in the heat affected zone (HAZ), form σ in the thermal-mechanical affected zone (TMAZ), as well as develop T1 and S' in the nugget zone (NZ) after FSW. The lowest hardness zone (LHZ) exhibits elevated precipitation levels, generating coarse T1 and grain boundary precipitates with evident σ and precipitate free zone. The overall precipitation level of the As-welded joints remains minor, resulting in a slight decrease in hardness at the LHZ. Post-weld AA promotes many T1 formations, enhancing the joint strength and hardness. As a result, the ultimate tensile strength (UTS) of the joints increases from 440 MPa to 506.5 MPa, while the elongation decreases from 13.5% to 3.4%. Furthermore, hardness profiles transform from a low-amplitude wave curve to a high-amplitude W-shaped curve, generating a significant LHZ due to the minimal precipitation of T1. Pre-rolling deformation can enhance dislocation density and low angle grain boundaries (LAGBs), promoting the nucleation of high-density, fine T1 during artificial aging, and especially improves precipitation ability of the LHZ, leading to an elevated hardness increment in the region after AA. Compared to the Joint-AA, both strength and elongation of the Joint-R+AA increase, obtaining 530.5 MPa and 3.9%, respectively.

Unravelling the dominant influence of microsegregation on creep rupture behaviour of additively manufactured inconel 718

This investigation focuses on unravelling the dominant influence of microsegregation and microstructure altered due to heat treatment cycle variation on creep rupture behaviour of additively manufactured Inconel 718 (AM-IN718). Two microstructural variants differing in the fraction of recrystallized grains while δ-phase being absent, were produced. A typical heat treatment (HT) cycle includes the stress-relieving of the as-built specimens at 980 °C, followed by solution treatment at 1080 °C (STA1080, a partially recrystallized microstructural variant) or 1150 °C (STA1150, a fully recrystallized microstructural variant), and double ageing (soaking at 720 °C for 8h and subsequent furnace cooling, followed by 8h at 620 °C and air cooling).Detailed microstructural characterization of two microstructural variants through correlative microscopy revealed a prevalent existence of Nb-rich precipitate-free zones (PFZ) in STA1080 than in STA1150. Creep characterization of the two microstructural variants in the temperature range of 625–675 °C and at 500–750 MPa demonstrated superior creep resistance in STA1150. The correlation of kinetic analysis and comprehensive post-deformation microstructural characterization suggests grain boundary cavitation as the main damage/softening mechanism and the reason for the difference in creep rupture behaviour between the two microstructural variants. The long-term exposure heat treatment methodology demonstrates that PFZs are the major influencing factor responsible for microsegregation-dependent creep rupture behaviour. Interestingly, the presence of the δ phase within PFZs appeared to retard cavity coalescence and failure during creep, despite its usual detrimental role in creep resistance.

Healing the high-temperature-retrogression-caused wide precipitation-free zones in Al-Zn-Mg-Cu alloy via strain-aging induced precipitates

Retrogression and re-aging (RRA) treatments have been demonstrated to effectively increase the corrosion resistance of high-strength Al-Zn-Mg-Cu alloys. However, RRA-treated Al-Zn-Mg-Cu alloys still suffer from poor ductility and intergranular corrosion because the wide precipitation-free zones (PFZs) caused by high-temperature retrogression treatment cannot be eliminated during the following re-aging process. Herein, inspired by the fact that retrogression dissolves part of the existing precipitates to obtain supersaturation in the matrix and dislocations as heterogeneous nucleation sites for precipitation are more prone to accumulate into the soft PFZs, a strategy of introducing pre-deformation after retrogression to trigger the secondary precipitation in the retrogression-caused PFZs during the subsequent re-aging is developed to heal this inherent weakness. Furthermore, the dislocations aggregation around the precipitated phases within grains accelerate the transformation of their morphology from platelet-like to circle-like after re-aging. In particular, the precipitates in the dislocation-dense band structures become coarser than those in the out-of-band regions. The synergy of the secondary precipitation at retrogression-caused PFZs and the circle-like and band-distributed precipitates within grains weakens the strength and electrochemical difference between the grain interior and the grain boundary. Compared with the traditional RRA treatment, this innovative technique further enhances the corrosion resistance and ductility of the fully recrystallized Al-8.99Zn-2.12Mg-1.98Cu-0.092Hf-0.12Zr alloy without sacrificing strength. This finding offers practical and straightforward implications for overcoming the current limitation in RRA-treated Al-Zn-Mg-Cu alloys and has the potential to expand to other aging-hardening alloys.

Investigation of Precipitation Behavior of a Novel Ni-Fe-Based Superalloy during High-Temperature Aging Treatment.

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ'-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ' remained spherical, and the size kept increasing with aging time according to the Lifshitz-Slyozov-Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ'; as γ' gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ' increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ' achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ' precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Effect of age hardening precipitates on the corrosion performance of laser Powder-Directed energy deposited CuNi2SiCr

This study explores Laser Powder − Direct Energy Deposition (LP-DED) processing of CuNi2SiCr and the effect of heat treatment on corrosion behavior. The findings pave the way to increasing the life of the components and the possibility of refabrication upon failure. LP-DED manufactured CuNi2SiCr was subjected to solution treatment followed by age-hardening at 500℃ for 1,3,5 and 7 h. The microstructure analysis showed the formation of Cr3Ni precipitates due to a higher cooling rate in the LP-DED process. Upon aging, Ni3Si, Ni2Si, and CrSi2 precipitates evolved. Due to the Orowan phenomenon, microhardness increases with the aging time as the number of precipitates along the grain boundary increases with the aging time. The 5-hour aged sample exhibited the best corrosion resistance due to precipitation coherency in the matrix and the medium-sized precipitates with uniform precipitation-free zones (PFZ) in the grain boundary.

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.

Investigation on the microstructural evolution and corrosion behaviors of Zn-microalloyed Al-Cu-Li alloys

Zn microalloying made great contributions to microstructures evolution and performances enhancement. The effects of various contents of Zn on the precipitate microstructure, tensile property and corrosion behavior of Al-3.2Cu-1.5Li alloys with medium Cu/Li mass ratio were examined by tensile testing and corrosion measurement. Experimental results demonstrated that the tensile strength of the Zn microalloyed samples dropped slightly as the Zn content increased due to the number density of precipitates with minor size variation showed a downward trend within the grains. Corrosion testing revealed that the corrosion type of Zn microalloyed samples was primarily localized intragranular corrosion, slight intergranular corrosion and local pitting corrosion was also partially involved. The emergence of Zn element in grain boundary (GB) coarse (T1) phases and the suppressed precipitation of intragranular phases in the alloys with higher content Zn were found to reduce corrosion sensitivity with the increase of Zn content, which was ascribed to the decreased potential difference between the GB and adjacent matrix, and between the GB precipitates and precipitation free zone (PFZ).

Effect of Cu content and quenching rate on the intergranular corrosion behavior of Al-Zn-Mg-Cu alloys

The effect of Cu content and quenching rate on the intergranular corrosion behavior of Al-Zn-Mg-Cu alloys has been investigated. The study reveals that at the same quenching rate, the intergranular corrosion susceptibility increases with the elevation of Cu content. Meanwhile, for alloys containing the same Cu content, the intergranular corrosion susceptibility increases as the quenching rate decreases. Remarkably, at high quenching rates, primarily pitting corrosion occurs. However, once the quenching rate falls below a specific threshold, the corrosion tends to propagate along grain boundaries. The critical quenching rate required for this transition increases with the increase of Cu content. Both the corrosion initiation and propagation are related to the Volta potential difference between grain boundary precipitates and the matrix (VPDG), which escalates correspondingly with the elevation of Cu content or the reduction of quenching rate. The intergranular corrosion behavior has been discussed through a comprehensive analysis encompassing VPDG, the features of grain boundary precipitates and precipitate-free zones.

Quench sensitivity and transformation kinetics of high vacuum die casting AlSi10MgMn alloy

AlSi10MgMn alloy high vacuum die castings are being widely used in automotives. T6 heat treatment can increase the casting mechanical properties. In this study, the time-temperature-transformation (TTT) and time-temperature-property (TTP) curves of high vacuum die casting (HVDC) AlSi10MgMn alloy were measured by interrupted quenching method, and the quench sensitivity range of the alloy was obtained. Based on the results, the quench factor analysis (QFA) method was used to predict the mechanical properties of alloy at different cooling rates. The microstructure transformation of the alloy during isothermal process was analyzed by phase transformation kinetics. The results show that the quench sensitivity range is 295–445°C with the nose temperature of 370°C. The phase transformation rate of the alloy at 460°C is higher than that at 280°C based on the results of phase transformation kinetics. The rod-shaped β-Mg2Si phases precipitate during the isothermal process and grow up with the extension of holding time, which Reduce the density of the fine age-induced precipitates β '' and increase the size of precipitate free zones (PFZ) after the followed aging treatment. The critical cooling rate of the alloy should be higher than 7.7°C /s to obtain good comprehensive mechanical properties.

Characterization of Fe-Cr alloys irradiated by neutrons at intermediate temperature

A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho National Labs (INL), USA, up to a dose of ∼6.7 dpa at a temperature of ∼456 °C. Transmission electron microscopy (TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were characterized using a TEM. It was found that the size and number density of these defects varied widely over the different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to 9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ∼7e20 /m3 to 6e22 /m3 for the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.%Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15 at.% Cr and increased slightly at 18 at.% Cr to ∼22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm), estimated to be about 1e23/m3. The number density of large voids increased from ∼0 at 3 % Cr to a peak of ∼6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉 type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a mixture of ½ 〈111〉 and [100] dislocation loops.

Comparative study of intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for BM and HAZ in a novel Fe-Ni-based alloy joint

Intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for the base metal (BM) and heat affected zone (HAZ) in a novel cost-effective Fe-Ni-based alloy joint were investigated comparatively. It was found that the BM tended to initiate acicular nano-precipitates perpendicular to grain boundaries, while their formation was suppressed at the HAZ. After creep at 650 °C under 300 MPa, nano-precipitates composed of γ´-Ni3(Al, Ti), M23C6 and σ phase at BM would expand along the grain boundaries (GBs) with significant GB migration. Creep cavities were more prone to initiate at the precipitate-free zone and the acicular nano-precipitate roots due to the precipitation strengthening reduction and local strength mismatch. Furthermore, grain boundary embrittlement induced by acicular precipitates and the presence of recrystallized grains at the crack tip also promoted the crack propagation. In contrast, the welding thermal cycle contributed to the M23C6 redistribution at the GBs, which decreased element diffusion rate and dragging grain boundary movement, thereby exhibiting better creep crack resistance at the HAZ. The finding provided profound implications for the comprehensive understanding of acicular nano-precipitate evolution in the newly designed Fe-Ni-based alloy joint under service condition.

Precipitate-mediated fatigue crack propagation behavior of Al–Zn–Mg–Cu alloy tailored by non-isothermal aging

The fatigue crack propagation behavior of non-isothermally aged Al–Zn–Mg–Cu alloy was examined at different aging stages. It has been revealed that non-isothermal aging (NIA) improved the fatigue crack resistance by adjusting the thermodynamics, and thus modified the precipitation configuration. Enhanced crack propagation threshold and decreased crack propagation rate were achieved when NIA was applied and carried out thoroughly. Fine and dispersed GP zones existing in underaged alloy significantly intensified the slip localization, which effectively mitigated the stress concentration near the crack tip and showed an approximately 8.2 MPa·m1/2 crack propagation threshold. Smaller width of the precipitate-free zone enabled the slip bands to propagate through the grain boundaries. However, relatively low strength and intense slip localization led the fatigue crack to propagate along the crystallographic slip bands, responsible for the higher crack propagation rate. For minorly over-aged alloys, coarsening and elevated number density of intragranular precipitate effectively hindered the movement of dislocation around, where strain concentration was frequently and preferentially induced, causing the degradation of resistance to fatigue crack propagation.

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.

Unraveling the plastic deformation, recrystallization, and oxidation behavior of Waspaloy during thermal fatigue crack propagation

Thermal fatigue is one typical failure mode for engine components involving cyclic thermal stresses/strains. However, the microstructure evolution and its microscopic interaction with thermal fatigue cracking in Ni-based superalloys featuring intragranular γ′ and intergranular carbides have yet to be clarified. In this study, cyclic temperature variations ranging from 25 °C to 700 °C were conducted on Waspaloy to unravel the synergistic damage mechanisms including stress distribution, plastic deformation, recrystallization, oxidation, and crack propagation. The thermal fatigue cracks are found to predominantly propagate along grain boundaries with a gradually descending rate from 3.5 μm/cycle to 1.5 μm/cycle. Compared with the macroscopic thermal stress up to ∼800 MPa, the local thermal stress induced by different sizes of M23C6 carbides marginally affects the crack propagation. Intensive slip bands and sparse deformation twins in the deformed matrix are observed, indicating the plastic deformation is mainly achieved by dislocation slipping and supplemented by twinning. Trans-granular cracks parallel to {111} planes form occasionally when the propagation directions of intergranular cracks are almost parallel to the {111} slip bands. Besides, dislocations promote the oxidation damage of the crack surface through pipe diffusion of solutes, and the γ′-precipitate free zones (PFZs) and recrystallized areas are produced accordingly. The quantitation analysis of the PFZ thickness provides a reference for predicting crack initiation time during thermal fatigue.

Significant reduction of anisotropy in stress relaxation aging and mechanical properties improvement for 2195 Al-Cu-Li alloy subjected to plastic loading

Although the plastic loading can enhance creep deformation and yield strength, the anisotropic Stress Relaxation Aging (SRA) behavior and mechanism under plastic loading remain unclear, which presents a significant challenge in accurately shaping aluminum alloy panels. In this study, the SRA behavior of 2195-T4 Al-Cu-Li alloys were thoroughly studied under initial loading stresses within the elastic (210/250 MPa) and plastic (380/420 MPa) ranges at 180 ℃ by stress relaxation and tensile tests as well as microstructure characterization. The findings reveal that compared with those under elastic loadings, in-plane anisotropy (IPA) values of the stress relaxation amount, yield strength and fracture elongation under plastic loadings are reduced by 60%-80%, 70%-90% and 72%-89%, respectively. Similarly, IPA values of precipitate size in grains and Precipitation-Free Zones (PFZ) width at grain boundaries under plastic loading decrease by 31.4% and 94.4% respectively. These results indicate plastic loading significantly weakens the anisotropic SRA behavior, owing to numerous uniformly distributed fine T1 phases and small IPA values of both T1 precipitates size and PFZ width in various loading directions. Compared with those of elastic loading-aged alloys, yield strength of plastic loading-aged alloys shows high strength-ductility because of the combined effect of closely dispersed fine T1 precipitates, narrowed PFZ and numerous sheared and rotated T1 phases at different locations during tensile process. The uniformly distributed larger Kernel Average Misorientation (KAM) and Schmidt factor values of the plastic loading-aged alloy, as well as the cross-slip generated, also help to enhance the strength and ductility of the alloy.

Co-, Ni- and Fe-rich grain-boundary phases enhance creep resistance in θ′-strengthened Al-Cu alloys

Microstructural evolution and creep response were investigated in the cast Al-5.0Cu-0.3Mn-0.2Zr (wt.%) alloy with and without addition of slow-diffusing, intermetallic-forming elements Fe, Ni, or Co. Baseline Al-5.0Cu-0.3Mn-0.2Zr alloy exhibits high creep resistance at 300 °C, up to ∼75 MPa, which is attributed to high-aspect-ratio, intragranular θ′-Al2Cu precipitates that effectively suppress dislocation climb. However, θ′-Al2Cu precipitate-free zones form along grain boundaries upon heat treatment, whose extent is amplified during subsequent creep. Such weak regions experience dislocation creep, leading to strain localization and acceleration of grain-boundary sliding. Adding Ni and Co, individually or in combination, leads to the formation of grain-boundary precipitates (Al9Co2, Al3Ni2) which are resistant to coarsening, thus suppressing the formation of θ′-Al2Cu precipitate-free zones. This microstructure provides high creep resistance at stresses up to 75–80 MPa, with strain rates much lower than in unmodified Al-5.0Cu-0.3Mn-0.2Zr. Adding Fe, which results in extensive decoration of grain boundaries with coarsening-resistant Al7Cu2Fe, and then increasing the Cu content to compensate for the Cu loss to this new phase, leads to a new Al-7.4Cu-1.6Fe-0.3Mn-0.2Zr alloy with creep resistance at 300 °C that surpasses known cast aluminum alloys. Adding Fe to improve the creep resistance of Al-Cu alloys is both cost-effective and sustainable. Our findings offer guidelines applicable to various alloy systems on controlling the evolution of precipitate-free zones and its ensuing effects on creep deformation.

Influence of microstructure characteristics on the fatigue properties of 7075 aluminum alloy

Grain size and aging state are the two major microstructural factors affecting the mechanical properties of Al alloys. In this study, fatigue tests were conducted on 7075 Al alloy with varying grain sizes and aging states. The results indicate that both grain refinement and over aging (OA) treatment are beneficial for enhancing fatigue performance, with the 7075-Min alloy exhibiting the highest fatigue strength of 265.8MPa in the OA state. For grain size, a quantitative positive linear relationship was found between the reciprocal of the square root of the average grain size and the fatigue strength coefficient. Grain refinement optimizes the fatigue performance by increasing the fatigue strength coefficient. Compared to the under aging (UA) state, OA treatment promotes the growth and coarsening of precipitates at grain boundaries (GBs) and within the interior of grains. This change in precipitate morphology effectively facilitates dislocation cross slip, reducing fatigue damage caused by dislocation blocking. The impact of the precipitation-free zone (PFZ) on fatigue performance is negligible compared to that of the precipitates. This study clarifies the relationship between microstructure and fatigue performance, providing valuable insights for microstructure regulation to optimize the fatigue performance of Al alloys.