Precipitate-free Zones Research Articles

Microstructure, mechanical properties and residual stress of high vacuum die casting AlSi10MgMn alloys with different spray quenching

Quenching cooling rate in solution treatment is the decisive parameter for solute supersaturation and mechanical properties after following aging of wrought and casting Al alloys. In this study, the microstructure quantitative characteristics, mechanical properties and residual stress of high vacuum die casting (HVDC) AlSi10MgMn alloys with different cooling rates during spray quenching were comprehensive studied, which were compared with the conditions of air and wind cooling as well as water quenching. The results showed that, with the decrease of quenching cooling rate, the cross-section diameters of the nano-precipitates β" increased, the precipitates number density decreased in the aged alloys, and the precipitate free zone (PFZ) at grain boundary widened. In addition, β' and β phases appeared in the samples quenched by air and wind. As the cooling rate dropped, the ultimate tensile strength (UTS) and yield strength (YS) of as-aged alloys decreased whereas the elongation (EL) increased, and the quenching residual stress declined. A critical quenching cooling rate over 30 °C/s followed with aging treatment was found for strengthening the AlSi10MgMn alloys and reducing quenching residual stress. The results are important for controlling mechanical properties and residual stress in the alloy practical application.

Insight into the microstructure configuration and strengthening mechanisms of a cast in-situ Al3Zr reinforced Al–Li matrix composite

In this work, a cast Al3Zr/Al–2Li composite was fabricated using the in-situ method, which shows improvements of ∼32 % in strength and ∼12 % in stiffness compared with the Al–2Li matrix alloy. The refined grains, microscale D023-Al3Zr particles, nanoscale Al3(Zr, Li) phases, finer Al3Li precipitates, and narrowed Al3Li precipitate-free zones (PFZ) were observed in the composite. The interface between the reinforcement and matrix is well-combined. It was indicated that Orowan strengthening of Al3(Zr, Li) and coefficient of thermal expansion (CTE) mismatch strengthening of D023-Al3Zr were two main strengthening contributors. The fracture of the composite was transformed from intergranular to transgranular mode due to the narrowed PFZ and the stress concentration effect of Al3Zr.

Study on Quench Sensitivity during Isothermal Treatment of 7A65 Aluminum Alloy.

The quenching sensitivity of 7A65 aluminum alloy was investigated using interrupted quenching experiments. The time-temperature transformation (TTT) and time-temperature performance (TTP) curves of the alloy were determined. The results indicate that the nose temperature is about 320 °C and the quenching sensitivity temperature range is from 240 °C to 360 °C. During the isothermal treatment, the supersaturated solid solution resolves to the equilibrium phase of η (MgZn2), and the precipitation rate is the largest at about 320 °C. Through transmission electron microscope (TEM) observation and X-ray diffraction (XRD) tests, it was found that with the extension of the isothermal holding time, the originally dispersed η' phase gradually decreases until disappear, and the number of η phase increases and gradually grows up at the grain boundary or around the Al3Zr particles. The rod-like η phase at the grain boundary is distributed from discontinuous distribution to chain-like continuous distribution, and the precipitation free zone (PFZ) is gradually generated and widened as the holding time is extended. At the nose temperature, the driving force of nucleation is high, and the diffusion rate is fast, which promotes the precipitation and growth of η phases. The coarse η phase weakens the mechanical properties. According to the results, it is recommended to increase the cooling rate at the sensitivity temperature range to reduce the precipitation of the η phase and decrease the quenching cooling rate from solution temperature to 360 °C to reduce residual stresses in the components.

Regulating microstructure and improving precipitation hardening response of fine-grained Mg-RE-Ag hot-extruded alloy by extreme short-time heat treatment

The difficulty in exerting beneficial effects of fine grains, high number density nano-precipitates as well as weakened texture simultaneously is currently the bottleneck limiting the development of high performance (high strength, high plasticity, and isotropic) age-hardenable Mg-RE alloys. The key difficulty is that the generation of excessive dynamic precipitates is always accompanied by the formation of fully dynamic recrystallized ultrafine grains during thermal deformation, causing a significant depletion of solutes within the entire matrix and finally seriously weakening the subsequent aging nano-precipitation. In this work, the process of extreme short-time (2 min) heat treatment (ESHT) followed by peak aging treatment is applied to an ultrafine-grained weak-textured Mg-RE-Ag as-extruded alloy with numerous dynamic precipitates. The precipitation hardening response is dramatically increased (ΔHV = 37.2 HV) and the yield strength is increased by nearly 50 MPa. This is attributed to the fact that concentration of main solutes in matrix is greatly increased within only 2 min of heat treatment promoting the precipitation of nano-precipitates (prismatic β′ and basal γ′′) in the absence of excessive grain growth. The large difference between the initial and equilibrium solute concentrations, the extremely short distance required for solute diffusion within sub-microscale grains, and the effective pinning of grain boundaries by excess dynamic precipitates are responsible for the unusual microevolution during the ESHT. In addition, the formation of precipitation-free zones (PFZs) is attributed to ‘uphill’ diffusion during aging, and novel γ′′-PFZs and β′-PRZs are found. The ESHT followed by peak aging treatment provides a new way to simultaneously exert the beneficial effects of fine grains and high number density nano-precipitates.

Effect of Ni on microstructure and mechanical properties of Al-Cu-Mn alloy

The effects of Ni content on the microstructure and mechanical properties of Al-Cu-Mn alloys were systematically studied at both room temperature and high temperature. The results demonstrate that Ni effectively refines the grains. The primary Ni-containing phases in the 0.3% Ni alloy are Al3CuNi and Al7Cu4Ni phases. When the Ni content exceeds 0.3%, the dominant Ni-containing phase transforms into the Al7Cu4Ni phase. The increase in Ni content leads to the accumulation of Ni-containing intermetallic compounds at grain boundaries. Simultaneously, a significant amount of effective Cu concentration is consumed, reducing the presence of the strengthening Al2Cu phase in the alloy. Consequently, this causes a decrease in the alloy’s mechanical properties at room temperature. At a 0.3% Ni content, the alloy enriches the best high-temperature performance, revealing a tensile strength of 143 MPa at 300 °C and 94.8 MPa at 350 °C, which is 14.72% and 7 MPa higher than that of the base alloy. Due to the high thermal stability of the formed Ni-containing intermetallic compound, the grain boundary slip is effectively hindered under high-temperature conditions, thereby enhancing the high-temperature strength of the alloy. Additionally, the introduction of Ni at 300 °C effectively inhibits the growth of precipitate-free zones (PFZs). With increasing Ni content, the alloy undergoes the transition from ductile fracture to brittle fracture at room temperature. At 300 °C, the fracture evolution mode of the alloy gradually shifts from ductile fracture to ductile-transgranular mixed fracture and finally to transgranular fracture. At 350 °C, there is a gradual shift from the mixed intergranular-transgranular fracture mode to intergranular fracture.

Cyclic and Isothermal Oxidation Behavior of a Directionally Solidified Ni-Al-Ti-Co-Cr Superalloy

Nickel based superalloys are widely used in such applications as gas turbine blades due to their excellent mechanical properties at elevated temperatures. Oxidation is one of the life limiting factors at such temperatures since it accelerates failure mechanisms, e.g. fatigue and creep. In this project, oxidation behavior of Ni-3Al-4.8Ti-9.5Co-14Cr (wt.%) superalloy was investigated under various testing conditions. The alloy was investment-cast with preferential crystal growth direction [100]. Isothermal oxidation tests were carried out at 800°C for 150h. Cyclic oxidation tests were then conducted between 100-800°C for 150 one-hour cycles. An additional one-hour cycle was tested to replicate real-life working condition of the alloy, which consisted of heating the samples up to 800°C followed by slow cooling to 400°C. During the testing, samples were periodically taken from the furnace and weighted with an accuracy of 10-4 g to calculate the weight gain and to analyze the microstructure of formed oxide. Scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) and X-ray diffraction were used to study the microstructure of oxidation products. The results reveal that the highest amount of weight gain is obtained in isothermal testing, while the lowest in cyclic. Diffusion of Ti and Ni to the free surface results in the early-stage formation of needle-shaped oxide phases followed by severe oxidation of Cr and Al. The layer of chromium oxide is continuous while aluminum oxide is blocky at the deepest position from the free surface. Cracks form at the interface of carbide particles and gamma phase near the free surface of cyclic oxidation samples due to concentration gradient of Ni and Al. Precipitate-free zone are observed near the free surface of all the samples at the end of the tests, but internal oxidation does not occur significantly in all the samples. It is found that temperature fluctuation increases oxide spallation, which causes crack formation. At the same time, in isothermal testing, dense, crack-free layer of oxide form on the surface of superalloy.

Controllable precipitation behavior near grain boundaries enabled by artificial strain concentration in age-hardening aluminum alloys

Regulating the precipitation behavior in the vicinity of grain boundary (GB) is crucial for improving the mechanical property and corrosion resistance of age-hardening aluminum alloy. In this work, we innovatively propose a strain-concentration controlled strategy to realize the controllable fabrication of the thinner precipitate-free zone (PFZ), associated with the finer grain boundary precipitates in Al–Zn–Mg–Cu alloy. Specifically, the primary PFZ with certain solute atoms is introduced by short pre-aging treatment, followed by the subsequent cyclic deformation at room temperature to induce the localized plastic deformation in the primary PFZ. Compared with the traditional aging, the Al–Zn–Mg–Cu alloy treated by the current process manifests superior combination of impact toughness and corrosion properties, as well as the slightly improved unidirectional tensile properties. Furthermore, the atomic-scale mechanism of the unique precipitation behavior near GB is revealed using the molecular dynamics simulation. A large number of dislocations are generated in PFZ, which acts as the favorable precipitation sites and contribute to the secondary precipitates in the primary PFZ. The present work provides a novel design route and an atomic-scale understanding for controlling the precipitation behavior in the vicinity of GB, and it has the potential to direct the optimization of mechanical performance and corrosion resistance in a board spectrum of aging-hardening alloys.

Change in Mechanical Properties of Laser Powder Bed Fused AlSi7Mg Alloy during Long-Term Exposure at Warm Operating Temperatures.

Al-Si-Mg alloys are most commonly used to produce parts by laser powder bed fusion for several industrial applications. A lot of papers have already focused on the effects induced by conventional heat treatments on the microstructure and mechanical properties of AlSi10Mg alloys, rather than on AlSi7Mg. Nobody has investigated thermal stability during long-term direct and artificial aging heat treatments of AlSi7Mg. This study investigates the changes in mechanical properties induced by long-term exposure (512 h) at 150 and 175 °C (the operating temperature of AlSi7Mg) after (i) the laser powder bed fusion process performed on a pre-heated build platform (150 °C), and (ii) heat treatments to the solution at 505 °C per 0.5 and 4 h. Thermal stability was evaluated through both Vickers microhardness measurements to obtain the aging profiles, and tensile tests to evaluate the mechanical properties in specific conditions. An optical microscope was used to investigate the microstructure. It was found that aging at 175 °C confers the same effects induced by a secondary aging heat treatment on as-built samples and, simultaneously, the worst effects on the solution heat treated AlSi7Mg alloy after long-term exposure. The AlSi7Mg DA at both 150 °C and 175 °C showed the same Vickers microhardness (~95 HV0.5), UTS (~300 MPa), and YS (~200 MPa) values for the longest exposure times because the fine and cellular α-Al matrix confers higher stiffness and strength despite the over-aged conditions. On the other hand, the coarsening effects that affected the precipitates during aging at 175 °C, as well as the formation of the precipitate-free zones along the grain boundaries, justified the highest detrimental effects induced on the SHTed samples.

Atomic-level characterization of η-twins with non-homogeneous nucleation on the E(Al18Mg3Cr2) phase in 7075 alloy: TEM observations and DFT calculations

In this work, an uncommon type of η-twin with non-homogeneous nucleation on the surface of the E-phase is characterized, and two defects and growth processes of the twin are analyzed. According to the transmission electron microscopy image analysis, heterogeneous nucleation of the η phase at the dominant site of the E phase can lead to the formation of such η-twin only when a rather strict site-direction relationship is satisfied as well as when it is at the interface adjacent to the E phase. The formation of twin defects is related to the E phase but ultimately determined by the nature of the η phase, and the crystallographic relationship between the defect structure and the E phase is self-consistent. Combined with the analytical results from STEM, first-principles calculations show that the solute segregation at the defects reduces the total energy of the system, which favors the transition of the η phase of the heterogeneous nucleation and the formation of twins, and that solute Al is more favorable than Mg. In addition, the elemental distribution and effects in the precipitation-free zone were analyzed by EDS mapping.

Atomic Structure of Hardening Precipitates in Al-Mg-Si Alloys: Influence of Minor Additions of Cu and Zn.

Shifting toward sustainability and low carbon emission necessitates recycling. Aluminum alloys can be recycled from postconsumer scrap with approximately 5% of the energy needed to produce the same amount of primary alloys. However, the presence of certain alloying elements, such as copper and zinc, as impurities in recycled Al-Mg-Si alloys is difficult to avoid. This work has investigated the influence of tiny concentrations of Cu (0.05 wt %) and Zn (0.06 wt %), individually and in combination, on the precipitate crystal structures in Al-Mg-Si alloys in peak aged and overaged conditions. To assess whether such concentrations can affect the hardening precipitate structures, atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography were adopted. The results indicate that low levels of Cu or Zn have a significant influence. Both elements showed a relatively high tendency to incorporate into precipitate structures, where Cu occupies specific atomic sites, creating its own local atomic configurations. However, Zn exhibited distinct behavior through the formation of extended local areas with 2-fold symmetry and mirror planes, not previously observed in precipitates in Al-Mg-Si alloys. Incorporation of Cu and/or Zn will influence the precipitates' electrochemical potential relative to matrix- and precipitate-free zones and thus the corrosion resistance. Furthermore, the presence of Cu/Zn structures (e.g., β'Cu, Q'/C) enhances the thermal stability of these precipitates and, accordingly, the mechanical properties of the material. The results obtained from this work are highly relevant to the topic of recycling of aluminum alloys, where accumulation of certain alloying elements is almost unavoidable; thus, tight compositional control might be critical to avoid quality degradation.

High-temperature ratcheting and low-cycle fatigue failure of a friction stir welding Al-Zn-Mg-Cu alloy

High-temperature ratcheting fatigue behavior of a friction stir welding (FSW) Al-Zn-Mg-Cu alloy was investigated. The substructured grains in the weld nugget zone (WNZ) undergo continuous dynamic recrystallization (CDRX) during ratcheting fatigue. The WNZ re-precipitates intragranular η′ and η particles and forms the precipitate free zone (PFZ) in the vicinity of the grain boundaries. At high mean stress or stress amplitude, ratcheting failure with obvious necking takes place due to large ratcheting strain. At low stress levels, intergranular fatigue microcracks initiate from grain boundaries of the WNZ due to the local strain incompatibility between the PFZ and grain interiors. The fatigue microcracks coalesce along grain boundaries and propagate via typical striation formation mechanisms. Low-cycle fatigue failure occurs at the WNZ instead of the low hardness zone (LHZ).

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.

Study on multi-field composite strengthening, surface integrity, and microstructural evolution mechanism of 7075-T6 aluminum alloy

Multi-energy field composite reinforcement technology has become a critical technique for improving the service life of key components in aviation engines. This paper aims to investigate the strengthening mechanism of high-speed cutting and solid particle-entrained waterjet peening (HSC-WJP) composite reinforcement on 7075-T6 aluminum alloy. Samples subjected to single solid particle-entrained waterjet peening (WJP) reinforcement were selected as the control group. The surface quality and microstructure evolution of 7075-T6 aluminum alloy after composite reinforcement were examined using SEM, EDS, XRD, TEM, and HRTEM techniques. The research results indicate that, under the same jet parameters, HSC-WJP composite reinforcement is superior to single WJP reinforcement in terms of surface roughness, except when the jet pressure is below 25 MPa and the track spacing is 0.53. The surfaces of the 7075-T6 aluminum alloy workpieces after reinforcement are mainly characterized by the presence of pits and micro-pores. Surface roughness shows a positive correlation with jet pressure and nozzle traverse speed. After HSC-WJP composite reinforcement, the surface roughness decreases by 0.132 μm compared to single WJP reinforcement, and the size of surface pits is reduced by 4–20 μm. Surface roughness decreases first, then increases, and then decreases again with increasing target distance, while it increases first and then decreases with increasing path spacing. A significant precipitation-free zone (PFZ) is present in the 7075-T6 aluminum alloy after reinforcement, and its width is positively correlated with the nozzle traverse speed. When the nozzle traverse speed is 120 mm/min, the average size of the PFZ at grain boundaries after single WJP reinforcement is 12–20 nm, while it decreases by approximately 3 nm after HSC-WJP composite reinforcement. The main precipitate phase in the reinforced aluminum matrix is η′ phase, and under the same jet parameters, the HSC-WJP grain size is smaller, approximately 25–34 nm, and exhibits significant dislocation walls and dislocation tangles, with a higher dislocation density compared to single WJP reinforcement.

Effect of forging temperature on the microstructure, subsequent aging precipitation behavior, and mechanical properties of Mg-Gd-Y-Zr-Ag alloy

The deformation mechanism, microstructure evolution, and precipitation behavior of a Mg-8.9Gd-1.8Y-0.5Zr-0.2Ag (wt.%) alloy multi-directionally forged at three different temperatures were investigated. As the forging temperature increases, the particle-stimulated nucleation (PSN) effect diminishes as the number of dynamic precipitates decreases, pyramidal slip is activated, grain boundary migration accelerates, and continuous dynamic recrystallization (CDRX) dominates. The microstructures varied greatly, although fine-grained structures were formed at all different forging temperatures. Competitive precipitation between dynamic precipitate growth, dislocation-induced precipitation, and homogeneous precipitation was observed after aging treatment. Among them, the medium temperature (748 K) forged and aged alloy exhibits the best mechanical performance, with an ultimate tensile strength of 436 MPa, and elongation of 16.3%. The calculation indicates that the mixed precipitation structure containing the β′ precipitate band provides a 35% higher strengthening contribution than the typical homogeneously distributed precipitates. The formation of precipitation-free zones (PFZs) ensures that aging will not cause a dramatic decrease in ductility, which provides a reference for the industrial preparation of high-performance wrought Mg-Gd series alloys.

Influence of heat treatment on the microstructure and creep response of additively manufactured cobalt superalloy Mar M509

ABSTRACT This work explores the influence of various post-build heat treatment procedures on the microstructural characteristics and creep response of laser powder bed fusion (L-PBF) Mar M509 superalloy. Besides MC-type carbides formed primarily during the L-PBF process, the obtained results indicate the influence of heat treatment on the nucleation of M6C and M7C3 carbides due to elemental segregation. Short-term creep-rupture tests have been conducted at 900°C/110MPa on eight specimens, each subjected to a specifically individual heat treatment procedure. Rupture times are found to be sensitive to the heat treatment applied and vary from ~178 to ~641 hours. Detailed microscopic characterisation of ruptured specimens highlights the synergistic influence of oxidation and M7C3/M6C precipitate-free zone formation on macroscopic intergranular failure facilitated by grain boundary sliding along favourably oriented small grains. Besides notable grain growth, the presence of a serrated grain boundary structure with relatively less MC-type carbide coarsening is found to offer the best creep resistance among the eight L-PBF specimens. Further improvement of the elevated temperature performance of the alloy is seen to require a focus on the L-PBF process conditions to optimise the morphology of MC carbides, as they form.

Effect of non-isothermal creep aging on the microstructure, mechanical properties and stress corrosion cracking resistance of 7075 alloy

The effect of non-isothermal creep aging (NICA) on the microstructure, mechanical properties, and stress corrosion cracking (SCC) resistance of 7075 alloy was investigated. The results showed that the tensile strength of the alloy increased to 565 MPa when the alloy was heated to 210 °C (CH210) and reached 580 MPa when it was subsequently cooled to 120 °C (CC120). Simultaneously, the SCC susceptibility of rtf increased from 50.8 % to 98.4 %. As compared with traditional creep aging process [1], a large strength increment with excellent stress corrosion resistance have been obtained by NICA. The microstructure revealed that a lot of dislocations have been introduced by creep during the heating stage which could improve the precipitates volume fraction and accelerate the diffusion of solutes; while during the cooling stage, η′ was greatly refined, and GPI and GPⅡ were re-precipitated from the matrix due to the decreased solid solubility and increased critical radius R*; both of them are responsible for the continuous strength increase during NICA. Moreover, the width of the precipitate free zone (PFZ) was narrowed from 46.1 nm (CH210) to 28.6 nm (CC120). The microchemical analysis reveals that solutes were more homogenously distributed in grain boundary precipitates (GB-ppts), matrix precipitates, the PFZ, and the matrix with the help of creep. The narrower PFZ and homogeneous solute distribution are responsible for improving the SCC susceptibility in the CC120 alloy.

The evolution of precipitates and mechanical properties in rapidly solidified 2195 Al–Cu–Li alloy during thermal exposure

The thermal stability of Al–Cu–Li alloy is critical to the quality and reliability of aerospace structural components. In this work, the microstructure and properties of rapidly solidified Al–Cu–Li alloy (T84) thermally exposed at 150 °C, 175 °C, 200 °C and 225 °C for 500 h were systematically studied. The results show that the T84 alloy exhibits excellent thermal stability with increased strength after thermally exposed at 150 °C, which is attributed to the increased number density of the T1 phase -Al2CuLi. With the further increase of thermal exposure temperature, the size of the T1 phase increases, the number density decreases significantly, and coarse precipitates such as T2 phase-Al6CuLi3 are formed. Meanwhile, the grain boundary precipitates coarsen and the precipitation free zones widen. The strengthening mechanism of the T1 phase transforms from shear mechanism to by-passing mechanism, and its contribution to yield strength decreases dramatically. In addition, the calculated activation energy indicates that coarsening of the T1 phase is controlled by the diffusion of Cu atoms.

Influence of stabilization heat treatment temperature on microstructure and stress rupture properties of Inconel 706 superalloy

Although the stabilizing heat treatment between solution treatment and aging treatment in Inconel 706 alloy is important to improve its stress rupture life, the influence of stabilization heat treatment temperature on the microstructure and stress rupture properties has not been well understood. In this study, heat treatments consisting of three stabilization temperatures (800 °C, 845 °C, and 880 °C) were performed in Inconel 706 alloy. The results showed that the stress rupture properties were improved with the increase of stabilization temperature. When compared with stabilization at 800 °C, the alloy stabilized at 880 °C exhibited a 96% increase in stress rupture life and a 110% increase in elongation. Furthermore, increasing the stabilization temperature led to the increased amount of η phase at grain boundaries. The η phase appeared in granular, rod-shaped or needle-shaped morphology. The microstructure in the grain interior exhibited compact γ'/γ" coprecipitates in the samples stabilized at 800 °C and 845 °C, while that showed noncompact γ'/γ" coprecipitates in the sample stabilized at 880 °C. The formation of 2.34 vol% η phase was found to be beneficial for the stress rupture properties. The improvement in stress rupture properties was ascribed to the presence of η phase and the associated precipitate-free zones at grain boundaries.

Synergistic reinforcement of in situ ZrB2 particles and Sc on the corrosion behaviour of 7N01 alloy

In this study, the effects of in situ ZrB2 particles and rare earth element Sc on the corrosion behavior of 7N01 alloy were investigated separately, with the characteristics of their synergistic enhancement being elucidated. The introduction of Sc into the ZrB2/7N01 composites led to an enhancement in both the distribution and morphology of the in situ particles, with the size of ZrB2 particles being significantly reduced and large particle clusters almost disappearing compared to the situation without Sc addition. A decrease in the grain size of the matrix alloy 7N01 from 170 μm to 33 μm was also observed when the Sc addition was 0.5 wt%. The low corrosion susceptibility observed after the addition of Sc was attributed to the refinement of grains and the inhibition of the formation of precipitate-free zones. The addition of ZrB2 improved corrosion resistance by promoting the precipitation of Al3Sc precipitates. Importantly, it was found that the 7N01 alloy with 3 wt% ZrB2 and 0.5 wt% Sc exhibited high mechanical properties and admirable corrosion resistance, which makes it a promising material for various applications. In summary, this study highlighted the individual and combined effects of in situ ZrB2 particles and rare earth element Sc on the corrosion behavior of 7N01 alloy, with valuable insights being provided for the development of advanced structural composite materials.

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.