Synergistic effect of Zr and Mo on precipitation and high-temperature properties of Al-Si-Cu-Mg alloys

This study focuses on finding a solution to the sharp decline in mechanical properties of Al-Si-Cu-Mg alloys due to rapid coarsening of traditional intermediate phases at high temperature.A new type of modified al oy,to be used in automobile engines at high temperatures,was prepared by adding Zr and Mo into Al-Si-Cu-Mg alloy.The synergistic effects of Zr and Mo on the microstructure evolution and high-temperature mechanical properties were studied.Results show that the addition of Zr and Mo generates a series of intermetallic phases dispersed in the alloy.They can improve the strength of the alloy by hindering dislocation movement and crack propagation.In addition,some nano-strengthened phases show coherent interfaces with the matrix and improve grain refinement.The addition of Mo greatly improves the heat resistance of the alloy.The extremely low diffusivity of Mo enables it to improve the thermal stability of the intermetallic phases,inhibit precipitation during aging,reduce the size of the precipitates,and improve the heat resistance of the alloy.

Enhanced thermal stability and mechanical properties of high-temperature resistant Al-Cu alloy with Zr and Mn micro-alloying

The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying additions of Zr and Mn elements have presented a significant role in stabilizing the main strengthening metastableθ′precipitates at a temperature as high as 573 K.Simultaneously,the HT tensile test demonstrated that ACMZ alloy retained their strength of(88.6±8.8)MPa,which was much higher than that of AC alloy((32.5±0.8)MPa)after the thermal exposure at 573 K for 200 h.Finally,the underlying mechanisms of strength and ductility enhancement mechanism of the ACMZ alloy at HT were discussed in detail.

Influencing factors and mechanism of high-temperature oxidation of high-entropy alloys: A review

High-temperature oxidation is a common failure in high-temperature environments,which widely occur in aircraft engines and aerospace thrusters;as a result,the development of anti-high-temperature oxidation materials has been pursued.Ni-based alloys are a common high-temperature material;however,they are too expensive.High-entropy alloys are alternatives for the anti-oxidation property at high temperatures because of their special structure and properties.The recent achievements of high-temperature oxidation are reviewed in this paper.The high-temperature oxidation environment,temperature,phase structure,alloy elements,and preparation methods of high-entropy alloys are summarized.The reason why high-entropy alloys have anti-oxidation ability at high temperatures is illuminated.Current research,material selection,and application prospects of high-temperature oxidation are introduced.

Effect of RE addition on solidification process and high-temperature strength of Al-12%Si-4%Cu-1.6%Mn heat-resistant alloy

The effect of RE addition on solidification process and high-temperature strength of Al-12%Si-4%Cu-1.6%Mn(in wt.%)heat-resistant alloy was investigated by microstructure observation and tensile test.A great number of fine needle-like RE-rich phases are observed in the alloys with RE addition. Solutionizing treatment does not change their morphologies and sizes, indicating that they have good thermal stability. The addition of RE totally alters the solidification process of eutectic CruAl2 phase, from network-like phase in the form of segregation at the final eutectic grain boundaries to discretely blocky phase growing on the hair-filamentous RE-rich needles. In the alloys with Ce addition, blocky CuAl2, particulate Al15Mn3Si2 and needle-like RE-rich needle phases grow together, but they did not occur in the alloy with only La addition. The addition of RE does not considerably improve the strength of the alloy at high temperatures. The formation of RE-rich phases also does not significantly alter the originating and propagating of micro-cracks in the alloy during tensile test.

Effect of heat treatment on microstructure and mechanical properties of Ti-containing low alloy martensitic wear-resistant steel

Effects of quenching temperature and cooling conditions(water cooling and 10%NaCl cooling)on microstructure and mechanical properties of a 0.2%Ti low alloy martensitic wear-resistant steel used for die casting ejector plate were investigated.The results show that lath martensite can be obtained after austenitizing in the range of 860-980℃and then water cooling.With an increase in austenitizing temperature,the precipitate content gradually decreases.The precipitates are mainly composed of TiC and Ti4C2S2,and their total content is between 1.15wt.%and 1.64wt.%.The precipitate phase concentration by water-cooling is higher than that by10%NaCl cooling due to the lower cooling rate of water cooling.As the austeniting temperature increases,the hardness and tensile strength of both water cooled and 10%NaCl cooled steels firstly increase and then decrease.The experimental steel exhibits the best comprehensive mechanical properties after being austenitized at 900℃,cooled by 10%NaCl,and then tempered at 200℃.Its hardness,ultimate tensile strength,and wear rate reach551.4 HBW,1,438.2 MPa,and 0.48×10^(-2)mg·m^(-1),respectively.

Microstructure and high-temperature mechanical properties of near net shaped Ti−45Al−7Nb−0.3W alloy by hot isostatic pressing process

Near net shaped Ti−45Al−7Nb−0.3W alloy(at.%)parts were manufactured by hot isostatic pressing(HIP).The microstructure and high-temperature mechanical properties of the alloy were investigated by X-ray diffractometry(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results show that at a temperature of 700℃,the peak yield stress(YS)and ultimate tensile stress(UTS)of alloy are 534 and 575 MPa,respectively,and the alloy shows satisfactory comprehensive mechanical properties at 850℃.The alloy exhibits superplastic characteristics at 1000℃ with an initial strain rate of 5×10^−5 s^−1.When the tensile temperature is below 750℃,the deformation mechanisms are dislocation movements and mechanical twinning.Increasing the tensile temperature above 800℃,grain boundary sliding and grain rotation occur more frequently due to the accumulation of dislocations at grain boundary.

On the intergranular fracture behavior of high-temperature plastic deformation of 1420 Al-Li alloy

The tensile deformation hot simulation test of as-cast 1420 Al-Li alloy was performed on Gleeble-1500 Thermal Simulator in the deformation temperature range from 350 to 450 ℃ and the strain rate range from 0.01 to l0.0s-1.The tensile fracture behavior of the 1420 Al-Li alloy at high temperature was studied experimently. The results show that the tensile fracture mode of the 1420 Al-Li alloy at high temperature is changed from typical transgranular ductile fracture to intergranular brittle fracture with the increase of the deformation temperature and the strain rate. It is made out that the precipitation of LiH is the fundamental reason for the intergranular brittle fracture of the 1420 Al-Li alloy at high temperature. The mechanism of hydrogen embrittlement of the 1420 Al-Li alloy at high temperature was discussed, and it was proposed that the hydrogen embrittlement at high temperature is an integrated function of the dynamic and the static force, which enrichs the theories of hydrogen embrittlemen t.

Simple and scalable synthesis of super-repellent multilayer nanocomposite coating on Mg alloy with mechanochemical robustness,high-temperature endurance and electric protection

Multi-functionalization is the future development direction for protective coatings on metal surface,but has not yet been explored a lot.The effective integration of multiple functions into one material remains a huge challenge.Herein,a superhydrophobic multilayer coating integrated with multidimensional organic-inorganic components is designed on magnesium alloy via one-step plasma-induced thermal field assisted crosslinking deposition(PTCD)processing followed by after-thermal modification.Hard porous MgO ceramic layer and polytetrafluoroethylene(PTFE)nano-particles work as the bottom layer skeleton and filler components separately,forming an organic-inorganic multilayer structure,in which organic nano-particles can be crosslinked and cured to form a compact polymer-like outer layer with hierarchical surface textures.Remarkably,the chemical robustness after prolonged exposure to aqua regia,strong base and simulated seawater solution profits from polymer-like nanocomposite layer uniformly and compactly across the film bulk.Moreover,the self-similar multilayer structure coating endows it attractive functions of strong mechanical robustness(>100th cyclic rotary abrasion),stable and ultra-low friction coefficient(about 0.084),high-temperature endurance,and robust self-cleaning.The organic-inorganic multilayer coating also exhibits high insulating property with breakdown voltage of 1351.8±42.4 V,dielectric strength of 21.4±0.7 V/μm and resistivity of 3.2×10^(10)Ω·cm.The excellent multifunction benefits from ceramic bottom skeleton,the assembly and deposition of multidimensional nano-particles,and the synergistic effect of organic inorganic components.This study paves the way for designing next generation protective coating on magnesium alloy with great potential for multifunctional applications.

AN INVESTIGATION OF HIGH-TEMPERATURE DEFORMATION STRENGTHENING AND TOUGHENING MECHANISM OF TITANIUM ALLOY

The high-temperature deformation strengthening and toughening mechanisms of titanium alloys have been investigated in this paper. The materials processed by this method produce a new tri-modal microstrvcture, which consists of 10-20% equiaxed alpha, streaky alpha and transformed beta matrix. It is found that the higher ductility of tri-modal microstructure is attributed to the equiaxed alpha's coopemtive slip and coordinated deformation with the transformed beta matrix. The streaky alpha phases not only increase the strength and creep properties, but also increase the fracture toughness. Propagating along grain boundaries between two neighboring streaky alpha phases, cracks in tri-modal microstructure make a more tortuous way, and then the materials show a higher fracture toughness. This new method is applicable to α, near α,α+β and near β titanium alloys.

Research on microstructure and high-temperature friction and wear properties of a cobalt-based alloy for hot extrusion die

The hot extrusion die is a key tool for determining the surface quality and dimensional accuracy of extruded products.Because its service process is subject to high temperature,high pressure,and wear,it must be resistant to these conditions.In this paper,the high-temperature friction and wear properties of a cobalt(Co)-based alloy were investigated and compared with those of a titanium carbide(TiC)cemented material.The results show that the high-temperature wear performance of the Co-based alloy is better than that of the TiC cemented material,and that Co-based materials have the potential for replacing TiC cemented materials as hot-extrusion-die materials.Due to the high density and good combination of the matrix and carbide,the carbides do not easily peel off from the matrix during the wear process.Due to the higher impact toughness of the Co-based alloys,microcracks that can cause worn-surface peeling are not easily generated.As a result,the high-temperature wear performance of Co-based alloys is found to be better than that of TiC cemented materials.

Effect of Ti-Al content on microstructure and mechanical properties of C_f/Al and TiAl joint by laser ignited self-propagating high-temperature synthesis

Cf/Al composites and TiAl alloys were joined by laser ignited self-propagating high-temperature synthesis（SHS） with Ni-Al-Ti interlayer. The effect of Ti-Al content on interfacial microstructure and mechanical properties of the joints was investigated. Localized melt of the substrates occurred in the joints. γ-Ni0.35Al0.30Ti0.35, NiA l3 and Ni2Al3 reaction layers formed adjacent to the substrates. Joint flaws, such as pores and cracks, made the joint density decrease and worked as the fracture source, which led to the sharp decline of joint strength. Additive Ti-Al increased joint density and strengthened the interlayer adhesion to Cf/Al. The joint flaws could be controlled by changing the Ti-Al content. When the Ti-Al content was 0.1, the joint was free of cracks with high density and reached the maximum shear strength of 24.12 MPa.

MICROSTRUCTURES OF A NOVEL HIGH STRENGTH AND WEAR-RESISTING ZINC ALLOY

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High-temperature Tensile Behavior of Laser Welded Ti-22Al-25Nb Joints at Different Temperatures

The high-temperature tensile behavior of laser welded Ti-22Al-25Nb (at%) joints was investigated at 500,650,800,and 1 000 ℃.The temperatures for tensile tests were selected according to the phase transformation sequence of Ti2AlNb-based alloys.At temperatures lower than the B2+O phase field (500 ℃) and higher than the B2+O phase field (1 000 ℃),the joints fracture in the base metal in ductile fracture mode.By contrast,the joints exhibit obvious high-temperature brittleness in the B2+O phase field (650 °C and 800 ℃).Heat treatments were conducted with respect to the thermal history of tensile specimens.Intergranular microcracks along the grain boundary of B2 phase are found in the fusion zone after the heat treatments at 650 ℃ and 800 ℃.The high-temperature brittleness at 650 ℃ and 800 ℃ is attributed to the B2→O transformation along the grain boundary.The stress concentration caused by the volume change of B2→O transformation also contributes to the high-temperature brittleness of laser welded Ti-22Al-25Nb joints.

Oxidation behavior and improvement in nonflammability of LPSO-type Mg–Zn–Y–Sr alloy

Mg_(97)Zn_(1)Y_(2)alloys with high ignition temperatures were developed by adding Sr.The addition of Sr resulted in the formation of a uniform and thin Y_(2)O_(3)film.Mg–Zn–Y alloys containing at least 0.25 at.%Sr exhibited ignition temperatures of 1270–1320 K.As a result of EDS measurement,Sr was found to be concentrated in the Y_(2)O_(3)film.In addition,a mixed film of MgO and Sr O formed on the outer layer in the 1.5 at.%Sr-containing Mg_(97)Zn_(1)Y_(2)alloy.These findings suggest that the uniform and thin Y_(2)O_(3)film that maintains high soundness at high temperatures was formed owing to valence control and the formation of a protective outer oxide film.

Laser powder bed fusion of a Ni3Al-based intermetallic alloy with tailored microstructure and superior mechanical performance

Ni3Al-based alloys are excellent candidates for the structural materials used for turbine engines due to their excellent high-temperature properties.This study aims at laser powder bed fusion and post-hot isostatic pressing(HIP)treatment of Ni3Al-based IC^(-2)21 M alloy with a highγ0 volume fraction.The as-built samples exhibits unavoidable solidification cracking and ductility dip cracking,and the laser parameter optimization can reduce the crack density to 1.34 mm/mm^(2).Transmission electron microscope(TEM)analysis reveals ultra-fine nanoscaleγ0 phases in the as-built samples due to the high cooling rate during rapid solidification.After HIP treatment,a fully dense structure without cracking defects is achieved,which exhibits an equiaxed structure with grain size~120-180μm and irregularly shapedγ0 precipitates~1-3μm with a prominently high fraction of 86%.The room-temperature tensile test of as-built samples shows a high ultimate tensile strength(σUTS)of 1039.7 MPa and low fracture elongation of 6.4%.After HIP treatment,a significant improvement in ductility(15.7%)and a slight loss of strength(σUTS of 831.7 MPa)are obtained by eliminating the crack defects.Both the as-built and HIP samples exhibit retained highσUTS values of 589.8 MPa and 786.2 MPa,respectively,at 900C.The HIP samples exhibita slight decrease in ductility to~12.9%,indicating excellent high-temperature mechanical performance.Moreover,the abnormal increase in strength and decrease in ductility suggest the critical role of a highγ0 fraction in cracking formation.The intrinsic heat treatment during repeating thermal cycles can induce brittleness and trigger cracking initiation in the heat-affected zone with notable deteriorating ductility.The results indicate that the combination of LPBF and HIP can effectively reduce the crack density and enhance the mechanical properties of Ni_(3)Al-based alloy,making it a promising material for high-temperature applications.

Precipitation behavior of 14H LPSO structure in single 18R phase Mg-Y-Zn alloy during annealing at 773K

The microstructural evolution of a 18R single phase （S 18） alloy during annealing at 773 K for 100 h was investigated in order to reveal the formation mechanism of 14H phase. The results showed that the as-cast S 18 alloy was composed of 18R phase （its volume fraction exceeds 93%）, W particles and α-Mg phase. The 18R phase in S18 alloy was thermally stable and was not transformed into 14H long period stacking ordered （LPSO） phase during annealing. However, 14H lamellas formed within tiny α-Mg slices, and their average size and volume fraction increased with prolonging annealing time. Moreover, the 14H phase is nucleated within α-Mg independently on the basis of basal stacking faults （SFs）. The broadening growth of 14H lamellas is an interface-controlled process which involves ledges on basal planes, while the lengthening growth is a diffusion-controlled process and is associated with diffusion of solute atoms. The formation mechanism of 14H phase in this alloy could be explained as α-Mg＇→α-Mg＋14H.

Effect of Ce,Co,B on formation of LaCo_(13)-structure phase in La(Fe,Si)_(13) alloys

The effect of Ce, Co, and B on the formation of 1：13 phase in La（Fe, Si）13 alloys was investigated by XRD, SEM and EDS. The results show that Co can improve the formation of 1：13 phase in as-cast LaFe11.6-xCoxSi1.4 alloys, but in as-cast and annealed LaFe11.6Si1.4-xCox alloys, it will hamper the formation of 1：13 phase and help the formation of a-Fe（Co, Si） solid solution. Ce2Fel7 phases will form when x reaches a certain value in as-cast and annealed La1-xCexFe11.5Si1.5 alloys. B can improve the formation of 1：13 phase accompanied with Fe2B phase in as-cast LaFe11.6-xBxSi1.4 alloys. B improves the formation of a-Fe solid solution in LaFe11.6Si1.4-xBx alloys, and there is almost only a-Fe in as-cast and annealed LaFe11.6Si0.9B0.5 alloy. In all, the introduction of Co, B, and Ce cannot eliminate the a-Fe phases in corresponding alloys prepared by the high-temperature and short-time annealing process.

Formation mechanism of high-temperature oxidation film on WE43 magnesium alloy and its effect on corrosion performance

The oxidation behavior of WE43 magnesium alloy in dry air at three temperatures(225,440 and 525℃)and the corresponding corrosion performance of samples attached to oxide film in 3.5 wt%NaCl solution was investigated.The results show that the oxide films formed at all three temperatures are a complex MgO·RE_(2)O_(3)·ZrO film with different compositions.The film formed at 225℃is flat and dense,whose components are 3.2MgO·1.8RE_(2)O_(3)·1ZrO.The oxidation ridges begin to form,and then gradually grow into nodular oxides and form a loose and porous oxide layer as the temperature increases to 525℃.The oxide films formed at all three temperatures improve the corrosion resistance of the alloy due to the MgO·RE_(2)O_(3)·ZrO,with the protective properties of oxide films following the order of 225℃>440℃>525℃,because the dense MgO·RE_(2)O_(3)·ZrO film formed at 225℃can provide better protection to the substrate than the loose oxide film formed at higher temperatures.

Creep rupture life prediction of high-temperature titanium alloy using cross-material transfer learning

High-temperature titanium alloys are the key materials for the components in aerospace and their service life depends largely on creep deformation-induced failure.However,the prediction of creep rupture life remains a challenge due to the lack of available data with well-characterized target property.Here,we proposed two cross-materials transfer learning(TL)strategies to improve the prediction of creep rupture life of high-temperature titanium alloys.Both strategies effectively utilized the knowledge or information encoded in the large dataset(753 samples)of Fe-base,Ni-base,and Co-base superalloys to enhance the surrogate model for small dataset(88 samples)of high-temperature titanium alloys.The first strategy transferred the parameters of the convolutional neural network while the second strategy fused the two datasets.The performances of the TL models were demonstrated on different test datasets with varying sizes outside the training dataset.Our TL models improved the predictions greatly compared to the mod-els obtained by straightly applying five commonly employed algorithms on high-temperature titanium alloys.This work may stimulate the use of TL-based models to accurately predict the service properties of structural materials where the available data is small and sparse.

Effect of Ag on High-Temperature Oxidation Behavior of Mg-6.5Gd-5.6Y-0.1Nd-0.01Ce-0.4Zr Alloy

In this paper,the isothermal oxidation experiments were used to study the effect of Ag on the high-temperature oxidation behavior of Mg-6.5Gd-5.6Y-0.1Nd-0.01Ce-0.4Zr(wt%)alloy oxidized at 350℃,400℃ and 450℃ for 120 h.The results show that the oxidation weight gain of the alloy mainly occurs in the early oxidation stage(0-20 h).This reason attributes to the lack of protective oxide film and the rapid inward diffusion of oxygen through the macroscopic defects of the incomplete oxide film.When dense oxide films such as Y_(2)O_(3),Gd_(2)O_(3),and ZrO2 form,they hinder the inward transport of oxygen ions and improve the high-temperature oxidation resistance of the alloy.In addition,the role of the Ag element at three temperatures is different.The addition of Ag mainly promotes the formation of eutectic phases such as Mg3Gd,Mg24Y5,and Ag2Gd,which reduces the content of Gd and Y elements in the alloy matrix,resulting in a decrease in the diffusion rate of Gd and Y elements during the oxidation process at 350℃ and 400℃,and weakens the oxidation resistance of Ag-containing alloys.However,in the oxidation experiment at 450℃,a large amount of eutectic phase is solid dissolved into the matrix,reducing the difference in element content.At this time,it is detected that the Ag element promoted the outward diffusion of Gd and Y elements,accelerating the formation of the oxide film.The oxidation resistance of Ag-containing alloys is improved.