Grain Boundary Precipitation Research Articles

Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

Creep mechanisms are studied in θ′-Al2Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ′ precipitates within grains via the Orowan mechanism, as evidenced by (ⅰ) very high stress exponent (n∼20-25) and (ⅱ) α-Al and θ′ lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ′ precipitates. At intermediate stresses, both alloys exhibit a n∼3 regime, where α-Al and θ′ lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ′ is now inactive within the grains. Rather, dislocation motion occurs solely in θ′-precipitate-free zones (θ′-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ′-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n∼3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ′-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al2Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ′-PFZ. Thus, high creep resistance in the n∼3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ′-PFZ, which effectively share load with the grains.

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.

Microstructures and properties of wire-arc additively manufactured ultra-high strength aluminum alloy under different heat treatments

The microstructures, mechanical properties and corrosion behaviors of the ultra-high strength Al–Zn–Mg–Cu-Sc aluminum alloy fabricated by wire-arc additive manufacturing process using a self-prepared 7B55-Sc filler wire were systematically investigated under different heat treatments. The results showed that the microstructures of the as-deposited, T6, T73, and retrogression and re-aging (RRA) heat treatments were all composed of fine equiaxed grains with a size of about 6.0 μm. The grain boundary precipitates (GBPs) of the as-deposited sample were very coarse and continuously distributed along the grain boundaries, and the intragranular precipitates (IGPs) mainly consisted of a small amount of ηMg(Zn,Cu,Al)2 and η′ phases. After T6 heat treatment, the GBPs became much finer, but still continuously distributed along the grain boundaries. The IGPs mainly consisted of fine GP zones and η′ phases. After RRA heat treatment, the GBPs became coarser and discontinuously distributed along the grain boundaries. The IGPs were composed of η′ phases and ηMg(Zn,Cu,Al)2 phases. After T73 heat treatment, the GBPs became much coarser and sparsely distributed along the grain boundaries. The IGPs mainly consisted of coarser ηMg(Zn,Cu,Al)2 phases. The T6 heat-treated sample achieved the highest tensile strength of 618 MPa. While the strength of T73 heat-treated sample was sacrificed by up to 17% compared with the T6 heat-treated sample, but the corrosion resistance was the best of all heat-treated conditions. After RRA heat treatment, the strength was about 10% lower than the strength of the T6 condition, but the corrosion resistance was better than that of the T6 heat-treated sample.

Effect of variable rate non-isothermal aging on microstructure and properties of Al-Zn-Mg-Cu alloy

Variable rate non-isothermal aging (VR-NIA) treatment by changing the heating and cooling rates is proposed and applied to 7055 aluminum alloy to optimize the mechanical properties and corrosion resistance. The investigated 7055 aluminum alloy undergone two-stage solid solution is heated from 100 °C to 160 °C at a rate of 20 °C/h, then heated to 210 °C at a rate of 60 °C/h, and finally cooled to 100 °C according to the symmetrical temperature route, the hardness and tensile strength can reach 173.3 HV and 654.6 MPa, respectively, and the electrical conductivity is able to reach 38.6 %IACS. At the same time, improved corrosion resistance is obtained, the maximum depth of intergranular corrosion (IGC) is 29.4 μm, the 48 h exfoliation corrosion (EXCO) grade is EA, and the stress corrosion sensitivity index (ISSRT) is 1.8%. The results of electrochemical tests show that the open circuit potential (OCP) of VR-NIA alloy is −764.2 mV, the corrosion potential (Ecorr) is −715.3 mV, and the corrosion current density (Icorr) is 4.6 × 10−8 A/cm2. Microstructure observation shows that GPI zones, GPII zones and a small number of η’ phases are formed during the heating process of 100 → 160 °C, which enhances the mechanical properties of the alloy; in the process of 160 → 210 → 160 °C, small-sized precipitates dissolve back, and part of the large-sized η’ phases grow and transform into η phases. At the same time, the grain boundary precipitates (GBPs) change from continuous to discontinuous distribution, forming a precipitation-free zone (PFZ) with a suitable width, which greatly improves the electrical conductivity and corrosion resistance of the alloy. In the process of 160 → 100 °C, the GPI zones are re-precipitated, and more η phases are formed from the η’ phases without remarkable coarsening. Enhanced performance can be achieved in a relatively shorter time through VR-NIA, behaving with greater advantage over constant-rate non-isothermal aging.

In-situ electrochemical analytical effect of aging treatment on stress corrosion behavior of Al–Zn–Mg–Cu aluminum alloy

The stress corrosion cracking (SCC) behavior of Al–Zn–Mg–Cu alloys in different aging treatment states was dynamically studied by slow strain rate tensile (SSRT) combining in-situ electrochemical impedance spectroscopy (EIS). By analyzing the influence of microstructure and precipitation phase on stress corrosion resistance under different aging conditions, it was found that the stress corrosion process can be divided into three stages, the corresponding charge transfer resistance (Rct) shows three evolutionary patterns, with the maximum value occurred when the alloys were under the maximum stress. The regression re-aging (RRA) treatment obtained coarse and discrete grain boundary precipitates (GBPs) with higher Cu content, which improved the SCC resistance of alloys significantly.

Influence of thermal exposure on the microstructure evolution and mechanical behaviors of an Al-Cu-Li alloy

In this study, the T85-treated Al-3.78Cu-1.17Li-0.7 Mg-0.32Ag-0.25Mn-0.33Zn-0.12Zr alloy thermally exposed at a series of temperatures (100–300 °C) for 100 h, 500 h, and 1000 h was systematically investigated. The results show that the thermal stability of the alloy is more closely correlated with thermal exposure temperature than with exposure duration. Excellent thermal stability in strength is obtained below 150 °C, which can be linked to the diameter and number density of T1 (Al2CuLi) precipitates continually increasing with a thickness less than 2 nm. However, the widened precipitation-free zones (PFZs) associated with continuous grain boundary precipitates (GBPs) benefit intergranular fracture, contributing to a decrease in toughness and elongation. In the window of 175–200 °C, T1 precipitates tend to thicken and are gradually substituted by coarse θ′-Al2Cu and S-Al2CuMg with elevating temperature. Simultaneously, the continuous GBPs transverse to discrete circle-like shapes, and thus intergranular ductile fracture becomes dominant. Therefore, the related hardness and strength decrease, while the electrical conductivity and toughness are the opposite. At temperatures over 250 °C, thermal exposure can reduce the dislocation density and dissolve T1 phases to form T2-Al6Cu(Li, Mg)3 and cubic C phases, leading to a dramatic decrease in strength and toughness.

Influence of Mg Content on the Corrosion Behavior and Precipitation of Aged Al-Cu-Li-xMg Alloys

Aluminum (Al)–copper (Cu)–lithium (Li) alloys are susceptible to environmental degradation, which limits the in-service lifetime of the components. Because of their complex composition, there are seldom studies focusing on corrosion behavior influenced by one single element, which is essential to clarify the corrosion mechanism. Herein, the sole influence of Mg on the corrosion behavior of Al–Cu–Li–xMg alloys was analyzed. Results revealed that the addition of Mg can affect the nucleation and precipitation process of the T1 precipitate, resulting in a difference of potential between grain interior and grain boundary. The precipitation of the T1 precipitate was promoted in a 0.7Mg alloy while impeded in a 1.1Mg alloy, due to the competition of Cu atoms and nucleation sites between T1 and S′. Two types of corrosion behavior of Al–Cu–Li–xMg alloys appeared due to the potential difference and continuity of grain boundaries. Continuous precipitates in grain boundaries and less precipitates within the grain are likely to cause intergranular corrosion (IGC), while more precipitates within the grain will result in pitting.

Enhanced mechanical properties and corrosion resistance of 7055 aluminum alloy through variable-rate non-isothermal aging

The effect of variable-rate non-isothermal aging (VR-NIA) on the properties of 7055 aluminum alloy was investigated by hardness, electrical conductivity, tensile tests and immersion corrosion, stress corrosion and electrochemical corrosion experiments. A four-stage VR-NIA process is proposed and applied to 7055 alloy, and a good combination of mechanical properties and corrosion resistance can be obtained in a relatively short time. TEM and HRTEM images show that GPⅠ zones, GPⅡ zones and η' phases are formed during the heating process of 100→180 °C, which enhances the mechanical properties; in the process of 180→210→180 °C, GPⅠ zones, part of the small-sized GPⅡ zones and η' phases are redissolved into the matrix, while part of η' phases grows. Meanwhile, the grain boundary precipitates (GBPs) change from continuous to discontinuous, forming the proper width of the precipitate free zone (PFZ), which greatly improves the electrical conductivity and corrosion resistance. In the cooling process of 180→100 °C, "secondary precipitation" of the GPⅠ zones occurs, and the η' phases grow up, with a small part transforming into the η phases, but no remarkable coarsening occurs. During the process of VR-NIA, the content of Zn, Mg, and Cu elements in GBPs is increasing continuously.

Microstructure Evolution and Localized Corrosion Susceptibility of an Al-Zn-Mg-Cu-Zr 7xxx Alloy with Minor Cr Addition.

Microstructure optimization of Al-Zn-Mg-Cu-Zr aluminum alloys, particularly through recrystallization inhibition, for improved strength and corrosion resistance properties has been an important consideration in alloy development for aerospace applications. Addition of rare earth elements, sometimes combined with Cr, has been found to be beneficial in this regard. In this study, the role of a single addition of 0.1 wt.% Cr on microstructure evolution of an Al-Zn-Mg-Cu-Zr (7449) alloy during processing was systematically investigated by optical light microscopy, scanning electron microscopy, electron backscatter diffraction and scanning transmission electron microscopy. Susceptibility to localized corrosion after aging to T4, T6 and T76 conditions was evaluated by potentiodynamic polarization (PDP) measurements and an intergranular corrosion (IGC) test. A decrease in recrystallized fraction with 0.1 wt.% Cr addition was observed, which is attributed to the formation of Cu- and Zn-containing E (Al18Mg3Cr2) dispersoids and the larger as-cast grain size. Moreover, the depletion of alloying elements from solid solution due to the formation of the Cu- and Zn-containing E (Al18Mg3Cr2) dispersoids and η Mg(Zn,Cu,Al)2 phase at its interface affects grain-boundary precipitation. The observed decrease in localized corrosion susceptibility with minor Cr addition is correlated with the microstructure and equally discussed.

Doubled strength and ductility via maraging effect and dynamic precipitate transformation in ultrastrong medium-entropy alloy

Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.

Precipitation and evolution behaviors of grain boundary α and α side plates for ultra-high strength titanium alloy during continuous cooling

In this paper, precipitation behavior and evolution mechanism of grain boundary α (αGB) and α side plates (αSP) of ultra-high strength titanium alloy are investigated. Precipitation of αGB can change original morphology of β grain boundary into irregular shapes, and it is much more obvious at slower cooling rate. The λ, which denotes the minimum value of misorientations between 12 α variants of βnBOR and αGB, is introduced to quantitatively evaluate morphology of αGB. The β phase that maintains Burgers orientation relationship (BOR) with αGB is labeled as βBOR, conversely, it is labeled as βnBOR. The deformation degree of αGB increases with the increasing λ. However, the λ is not the only factor that determines morphology of αGB, and it is also related to the angle between habit plane and grain boundary plane. Orientation of αGB is affected by β phase on both sides. The αGB maintains BOR with βBOR and the s-BOR with βnBOR. The s-BOR is defined as (0 0 0 1) plane of αGB is not parallel to (110) plane of βnBOR, but one of 〈1 1 –2 0〉 directions of αGB is parallel or close to one of βnBOR’s 〈111〉 directions.

Review of Microstructure–Mechanical Property Relationships in Cast and Wrought Ni‐Based Superalloys with Boron, Carbon, and Zirconium Microalloying Additions

Cast and wrought Ni‐based superalloys are materials of choice for harsh high‐temperature environments of aircraft engines and gas turbines. Their compositional complexity requires sophisticated thermo‐mechanical processing. A typical microstructure consists of a polycrystalline γ‐matrix, strengthening Ni3(Al,Ti) γ′ precipitates, carbides (MC, M6C, and M23C6), borides (M2B, M3B2, and M5B3), and other inclusions. Microalloying additions of B, C, and Zr commonly improve high‐temperature strength and creep resistance, although excessive additions are detrimental. Grain boundary (GB) segregation may improve cohesion and displace embrittling impurities. Finely dispersed carbides and borides are desired to control grain size via GB pinning. However, excessive decoration of GBs may lead to failure during processing and in‐service. Hence, a systematic review on the roles of B, C, and Zr in cast and wrought Ni‐based superalloys is required. The current state of knowledge on GB segregation and precipitation is reviewed. Experimental and modeling results are compared across various processing steps. The impact of GB precipitation on mechanical properties is most well researched. Co‐precipitation in proximity to GBs interacting with local microstructure evolution and mechanical properties remains less explored. Addressing these gaps in knowledge allows a more complete understanding of processing–microstructure–properties relationships in advanced cast and wrought Ni‐based superalloys.

A novel electromagnetic shock treatment to selectively modify grain boundary and improve the corrosion resistance of aluminium alloy

A novel electromagnetic shock treatment (EST) is put forward to selectively modify grain boundary (GB) microstructures and further improve corrosion resistance of T73-Al7075 alloys. Electrochemical tests and GB microstructure characterization by TEM (HRTEM) are conducted. The results show that EST with appropriate electromagnetic pulse energy being utilized can modify GB microstructure and enhance the corrosion resistance of T73-Al7075 alloy. In EST sample, GB precipitates (GBPs) dissolve and coarsen simultaneously, and stripy/wavy GBs with nanometer scale are more commonly observed which indicates that GBs relax within nanometer scale and even promote nonlinear grain bridging. The transformation of GBPs as well as the transition of GB complexions improve the corrosion resistance of EST sample. This provides a promising method to selectively modify GB microstructure instantaneously and improve related properties of solid alloys.

Effect of Cu content on quenching sensitivity relative to exfoliation corrosion susceptibility of Al-Zn-Mg-(Cu) alloys

The effect of Cu content on quenching sensitivity relative to exfoliation corrosion susceptibility of Al-Zn-Mg-(Cu) alloys was investigated by electrochemical test, immersion test, optical microscopy, electron backscattered diffraction, scanning electron microscopy, scanning transmission electron microscopy and scanning Kelvin probe force microscopy. As quenching rate decreases from 400 °C/s to 4 °C/s, exfoliation corrosion (EXCO) susceptibility increases for Cu-containing alloys (0.5–2.6 wt%); while for the Cu-free alloy, EXCO susceptibility first increases and then decreases, reaching the lowest at the quenching rate of 70 °C/s, and then continues to increase. Quenching sensitivity of the Cu-free alloy is the lowest, and that of Cu-containing alloys is similar. The decrease of quenching rate or the increase of Cu content leads to a larger difference in the Volta potential between grain boundary precipitates (GBPs) and the matrix. Corrosion tends to propagate only along grain boundaries at high quenching rates, but along both grain boundaries and subgrain boundaries below a critical quenching rate, which increases with increasing Cu content. The corrosion behavior has been discussed primarily based on the features of GBPs and precipitates free zones, and the Volta potential difference between GBPs and the matrix. • Quenching sensitivity is the lowest for Al-Zn-Mg and similar for Al-Zn-Mg-Cu. • EXCO susceptibility of Al-Zn-Mg-Cu is higher at a lower quench rate. • EXCO susceptibility of Al-Zn-Mg is lowest at the quench rate of 70 °C/s. • Corrosion path is related to quench rate and Cu content. • Potential difference is larger at a lower quench rate or a higher Cu content.

Effect of microstructure on corrosion behaviour of Al-Cu-Li alloys

The corrosion behaviour and its relationship with microstructure, especially the distribution of precipitates at grain boundaries and within grains, of two Al-Cu-Li alloys with different Cu/Li ratio were investigated using accelerated immersion testing, advanced microscopy characterization and quasi-in-situ corrosion observation. The Al-2.23Cu-0.96Li alloy with low Cu/Li ratio exhibited excellent intergranular corrosion resistance and its corrosion modes unusually remained consistent intragranular corrosion regardless of the ageing stage. The corrosion behaviour of the counterpart Al-4.03Cu-0.91Li alloy with high Cu/Li ratio was found to follow the general evolution regulation, from intergranular corrosion to intragranular corrosion with the ageing time. Microstructure characterizations demonstrated that the coarsening and discontinuous distribution of grain-boundary precipitates played the key role in lowering the intergranular corrosion susceptibility of the two Al-Cu-Li alloys. Further quasi-in-situ corrosion observation of the Al-2.23Cu-0.96Li alloy showed that the dealloying process of the grain-boundary precipitates led to the dissolution of the precipitate-free zone, which meant that the broadening of the precipitate-free zone along the grain boundary could reduce the intergranular corrosion resistance of the alloy. Besides, the thin T 1 precipitates that were uniformly distributed within the grains did not suffer preferential dissolution, but they still affected the intragranular corrosion propagation at the nanoscale. Specifically, the corroded T 1 precipitates could become {111} Al -oriented sidewalls of the intragranular corrosion channel. • The Al-2.23Cu-0.96Li alloy kept low IGC susceptibility regardless of the ageing stage. • Intragranular T 1 precipitates do not preferentially dissolve due to dealloying. • Nanoscale corrosion behaviour was investigated through quasi-in-situ TEM experiment.

Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study

Abstract. Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming under greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote–quartz vein. The main part of the vein is deformed, producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain size reduction in the primary vein quartz. Recrystallization occurs contemporaneously with dissolution and (re)precipitation of epidote and quartz grain boundary sliding, leading to a combined process described as viscous granular flow. The combination of grain boundary sliding and dissolution locally and repeatedly produces creep cavities. These represent not only loci for nucleation of new epidote grains at the expense of dissolved ones, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability in the latter, indicate a process of chemical homogenization. The nature of the fluid that mediates deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.

Grain Boundary Precipitation Control of GCP Phase Using TCP or A2 Phase in Ni-Based Alloys

To cover the grain boundary (GB) of the Ni phase with precipitates, the GB precipitation behavior of both topologically close-packed (TCP) or A2 and geometrically close-packed (GCP) phases was investigated in two Ni–Nb–(Co, Cr) ternary systems. The Ni/TCP or A2/GCP three-phase region existed in both systems. In the Ni-Nb-Co ternary system, Nb was approximately equally partitioned into both Co7Nb2 (mC18 structure, TCP) and (Ni, Co)3Nb (D019 structure, GCP) phases. In the Ni–Nb-Cr ternary system, Nb and Cr were mainly partitioned into the Ni3Nb (D0a structure, GCP) and Cr (A2 structure) phases, respectively. In the Ni–Nb–Co ternary system, the Co7Nb2 phase grew along the GB, whereas the (Ni, Co)3Nb phase grew toward the grain interior (GI). However, the growth of the Ni3Nb phase toward the GI was suppressed in the Ni–Nb–Cr ternary system. The suppression of growth of the GCP phase and covering the GB using both the TCP or A2 and GCP phases might be possible in a system where the precipitation of the GCP phase nucleating on the GB prior to the TCP or A2 phase increases supersaturation for precipitation of the TCP or A2 phase.

Gigacycle fatigue of the turbocharger gear wheel

The goal of the study is to elucidate the reasons for early fracture of the gear wheel teeth of a Cameron TA9000 turbocharger (1820 kW) after an operational load up to 1. 3 x 109 cycles. The chemical composi­tion and the microstracture of the tooth metal were studied using the methods of metallography, microhardness and optical microscopy. The microrelief of fracture surfaces of operational fractures was studied using electron scanning microscopy. Analysis of chemical composition proved the steel grade of the tooth metal (DIN 31CrMoV9) declared by the manufacturer. Visual analysis of the fragments under study re­vealed numerous cracks present on the tooth contact surfaces. The fatigue fracture origins detected on the fracture surfaces are typical of high cycle and gigacycle fatigue fracture. In the latter case, the detected fracture looks like a "fish eye" exhibiting an area of?? structural heterogeneity with inclusions and pores in the center. The fracture probably developed from the first tooth fragment to the fifth one being accom­ panied by an increase in the number of fatigue fracture origins known to be attributed to an increase in the stress amplitude. Metallographic study showed the presence of a subsurface hardened layer with a thickness of 120 - 200 pm with a defect-containing structure associated with grain-boundary precipitates (presumably, carbides (Fe, Cr)3C), which can result from violation of the modes of heat treatment of the gear wheel. Formation of brittle intergranular cracks on the contact surface and their subsequent develop­ment in the entire depth of the subsurface hardened layer appeared to be the reason for a decrease in the strength and bearing capacity of the gear teeth.

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.

Mechanical Properties and Fracture Behavior of Laser Powder-Bed-Fused GH3536 Superalloy

Heat treatment (HT) is an important approach to tune the structure and mechanical properties of as-printed or hot-isostatic-pressed (HIPed) additive manufacturing materials. Due to the carbide precipitates extensively existing after HT with air cooling, this paper studies the microstructure and mechanical behavior of laser powder-bed-fused (L-PBFed) GH3536 superalloy with laminar carbide precipitates at grain boundaries. By comparing with air-cooling samples and water-quenched samples, the results revealed that air cooling often introduced precipitates at grain boundaries, which impede the plastic deformation and are prone to lead to severe transgranular cracks on the fracture surface, contributing to a higher strain-hardening rate but lower ductility of HTed sample. Water quench can largely eliminate the grain-boundary precipitates, contributing to an optimized ductility even with smaller grain size. This work provides more details on the precipitate-deformation relation after HT.