Role of alloying and heat treatment on microstructure and mechanical properties of cast Al-Li alloys:A review

Due to the prominent advantages of low density,high elastic modulus,high specific strength and specific stiffness,cast Al-Li alloys are suitable metallic materials for manufacturing complex large-sized components and are ideal structural materials for aerospace,defense and military industries.On the basis of the microstructural characteristics of cast Al-Li alloys,exploring the role of alloying and micro-alloying can stabilize their dominant position and further expand their application scope.In this review,the development progress of cast Al-Li alloys was summarized comprehensively.According to the latest research highlights,the influence of alloying and heat treatment on the microstructure and mechanical properties was systematically analyzed.The potential methods to improve the alloy performance were concluded.In response to the practical engineering requirements of cast Al-Li alloys,the scientific challenges and future research directions were discussed and prospected.

Predict the evolution of mechanical property of Al-Li alloys in a marine environment

The ocean is one of the essential fields of national defense in the future,and more and more attention is paid to the lightweight research of Marine equipment and materials.This study it is to develop a Machine learning(ML)-based prediction method to study the evolution of the mechanical properties of Al-Li alloys in the marine environment.We obtained the mechanical properties of Al-Li alloy samples under uniaxial tensile deformation at different exposure times through Marine exposure experiments.We obtained the strain evolution by digital image correlation(DIC).The strain field images are voxelized using 2D-Convolutional Neural Networks(CNN)autoencoders as input data for Long Short-Term Memory(LSTM)neural networks.Then,the output data of LSTM neural networks combined with corrosion features were input into the Back Propagation(BP)neural network to predict the mechanical properties of Al-Li alloys.The main conclusions are as follows:1.The variation law of mechanical properties of2297-T8 in the Marine atmosphere is revealed.With the increase in outdoor exposure test time,the tensile elastic model of 2297-T8 changes slowly,within 10%,and the tensile yield stress changes significantly,with a maximum attenuation of 23.6%.2.The prediction model can predict the strain evolution and mechanical response simultaneously with an error of less than 5%.3.This study shows that a CNN/LSTM system based on machine learning can be built to capture the corrosion characteristics of Marine exposure experiments.The results show that the relationship between corrosion characteristics and mechanical response can be predicted without considering the microstructure evolution of metal materials.

Role of micro-Ca/In alloying in tailoring the microstructural characteristics and discharge performance of dilute Mg-Bi-Sn-based alloys as anodes for Mg-air batteries

The influence of micro-Ca/In alloying on the microstructural charac teristics,electrochemical behaviors and discharge properties of extruded dilute Mg-0.5Bi-0.5Sn-based(wt.%)alloys as anodes for Mg-air batteries are evaluated.The grain size and texture intensity of the Mg-Bi-Sn-based alloys are significantly decreased after the Ca/In alloying,particularly for the In-containing alloy.Note that,in addition to nanoscale Mg_(3)Bi_(2)phase,a new microscale Mg_(2)Bi_(2)Ca phase forms in the Ca-containing alloy.The electrochemical test results demonstrate that Ca/In micro-alloying can enhance the electrochemical activity.Using In to alloy the Mg-Bi-Sn-based alloy is effective in restricting the cathodic hydrogen evolution(CHE)kinetics,leading to a low self-corrosion rate,while severe CHE occurred after Ca alloying.The micro-alloying of Ca/In to Mg-Bi-Sn-based alloy strongly deteriorates the compactness of discharge products film and mitigates the"chunk effect"(CE),hence the cell voltage,anodic efficiency as well as discharge capacity are greatly improved.The In-containing alloy exhibits outstanding discharge performance under the combined effect of the modified microstructure and discharge products,thus making it a potential anode material for primary Mg-air battery.

EFFECTS OF IMPURITIES AND RARE EARTH ADDITION ON THE TOUGHENING LEVELS OF Al-Li BASED ALLOYS 2090 AND 1420

The effects of the impurities Fe, Si, Na, K and a rare earth addition, Ce, on the intrinsic and extrinsic toughening levels have been investigated for the Al-Li based alloys 2090 and 1420. 29K reduction in the toughening level for the alloy 2090 with impurities 0.42% Fe+Si or 0.0132% Na+K i4 identified to be caused by the impurities.An improvement on the fracture toughness can be made by adding 0.05%-0.25% Ce to the alloy 2090. The reason behind this is that Ce microalloying can not only enhance both the intrinsic toughening level and the extrinsic toughening level but also suppress the embrittling behavior of the impurities. However, 0.06%-0.15% Ce microalloying fails to bring about any beneficial effect to the toughening level for the alloy 1420.

Effect of Impurities and Cerium on Stress Concentration Sensitivity of Al-Li Based Alloys

A notch sensitivity factor was derived in order to evaluate the stress concentration sensitivity of Al Li based alloys. The factor values for the Al Li alloy sheets containing various contents of impurities and cerium addition were evaluated by determining the mechanical properties. It is found that the impurities Fe, Si, Na and K significantly enhance the stress concentration sensitivity of the alloys 2090 and 8090, whereas cerium addition reduces the stress concentration sensitivity to a certain degree for the high strength alloys. However, an excess amount of cerium addition in the high ductility alloy 1420 can significantly increase the stress concentration sensitivity. As compared with conventional aluminum alloys, the Al Li based alloys generally show high stress concentration sensitivity. Therefore, a special attention must be paid to this problem in the practical application of Al Li based alloys.

PHYSICAL METALLURGY OF MULTISTAGE RSP Al-Li BASED ALLOYS

Bascd on serires authors study ofAl—Li alloys in recent years,the formation theory of powders,characteristics,powder compaction,the microstructures and the mechanical behaviors of RSP Al-Li based alloys analyzed and discussed in this paper.

Hot corrosion behavior of Ni-xCr-6.8Al based alloys

The hot corrosion behaviors of as-cast and preoxidized Ni-xCr-6.8Al based alloys in the mixture of Na2SO4＋25% NaCl at 873 K were studied. The results show that the mass loss of Ni-xCr-6.8Al based alloys decreases with the increase of Cr content. Preoxidation improves the resistance to corrosion regardless of the concentration of Cr. The kinetics of as-cast Ni-12Cr-6.8Al and Ni-16Cr-6.8Al based alloys fits the parabolic law well, while that of the as-cast Ni-20Cr-6.8Al based alloy fits the power law. The kinetics of all the preoxidized samples obey the logarithmic law. The mechanism of the as-cast alloys can be well explained by the acid-base melting model. The behavior of the preoxidized alloys is found to be mainly determined by the properties of the oxide layer formed during the preoxidation to a large extend.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti-Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing （ECAP） at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti-Mo-Nb-V-Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Hot corrosion behavior of Ni–16Cr–xAl based alloys in mixture of Na_2SO_4-NaCl at 600 ℃

The hot corrosion behaviors of Ni–16Cr–xAl（x=4.5%, 6.8%, 9.0%, mass fraction） based alloys in Na2SO4–25% NaCl molten salts at 600 °C were investigated. The effects of pre-oxidation and Al content on the resistance to hot corrosion were examined. The hot corrosion resistance of Ni–16Cr–xAl based alloy with Al addition from 4.5% to 9.0% increases with increasing Al content. The alloy with Al content of 9.0% shows the highest hot corrosion resistance among the examined alloys because more β–NiAl phases are obtained to sustain the Al2O3 scale repaired during hot corrosion. Pre-oxidized specimens have a superior hot corrosion resistance compared with the as-cast specimens, due to a protective oxide scale formed after pre-treatment.

Effect of Re element on oxidation resistance of Ni_3Al-Mo based alloys at 1150 ℃

The isothermal oxidation behaviors of three kinds of Ni3Al-Mo based alloys at 1150 ℃ were studied.The morphology,structure and element distribution of oxide scales of the alloys were researched by scanning electron microscopy,X-ray diffraction and electron probe microanalysis.The research results show that there are three oxide layers,an outer layer of NiO,an intermediate layer mainly composed of NiO,NiAl2O4 and a small amount of NiMoO4,and an inner layer of NiAl2O4 and Al2O3.Re element was mainly distributed in the intermediate layer,which plays a role as ＂diffusion barrier＂ in the process of oxidation,and effectively reduces the diffusion rate of Al and Mo elements outward and diffusion rate of O element inward.As a result,a Al-rich oxide layer formed in the inner layer inhibits the growth of oxide layer and improves the oxidation resistance of the alloy.

Effect of the interaction between the chemical and the magnetic ordering on the phase equilibria of iron base alloys

It is well known that the magnetic properties such as the Curie temperature Tmag <sub>C and the mean magnetic moment β of ordered compounds have different values from those of the disordered solutions. For instance, both Tmag c and β of the Ni3Pt （L12） and NiPt （L10） and Tmag <sub>c of the CoPt （L10） and CoPt3 （L12） ordered compounds are strongly depressed due to the ordering compared with those of the metastable disordered Ni-Pt and Co-Pt alloys. On the other hand, the γ’-FeNi3 （L12） and the α’-FeCo （B2） ordered compounds have higher Tmag <sub>c and β values comparing with the disordered solution phases, γ （A1） and α （A2）, respectively. In consequence, the stability of the ordered phase is depressed or enhanced due to the interaction between the chemical and magnetic ordering caused by the decrease or increase of Tmag <sub>c and β values. The purpose of this study is to investigate the effect of the interaction between the chemical and the magnetic ordering on the phase equilibria in the Fe-X（X=Al, Co, Ni, Rh, Si） binary systems.The Gibbs energy of the α（A2）, γ（A1） and liquid phases is described by a sub-regular solution approximation. The ordering contribution to the Gibbs energy ,ΔGorder <sub>m, and deviations of magnetic properties, ΔTmag <sub>c and Δβ, of the ordered compounds, FeAl （B2）, Fe3Al （D03）, FeCo （B2）, FeRh （B2）, FeSi （B2）, Fe3Si （D03） and FeNi3 （L12） is introduced by the split compound energy formalism. Effect of the interaction between the chemical ordering, B2, D03 and L12 and the magnetic ordering on the phase equilibria will be discussed according to the calculated phase diagrams of the Fe-X binary systems.

Crystal Nucleation and Growth of Al-based Alloys Produced by Electrolysis

The nucleation and growth of grains in a series of Al-based alloys produced by electrolysis are observed under SEM. The atomic Ti/AI ratios of the nuclei and the distribution of Ti at certain points are analyzed by point EDS. The particles in different atomic Ti/AI ratios might act as the nuclei of α-Al. At the early stage of growth, the spherical Ti-enriched regions might form around these particles within very limited temperature ranges in which the reactions such as the peritectic reactions etc occur. At the latter stage of growth, the dendrites freely develop in the radial orientations, and the concentration of Ti decreases linearly along the dendrite arm and becomes negligible in the region near the periphery of the dendrite. It is believed that the nucleation is closely related with the number and dispersion of primary spherical areas in the melts, and the segregation of Ti leads to the free growth of dendrite, which is necessary for the formation of equiaxial grains.

MICROSTRUCTURES OF TiAl BASED ALLOYS PREPARED BY HOT PRESSING ELEMENTAL POWDERS

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Tensile properties and fracture behavior of Ti_2AlNb based alloys at room temperature

The tensile mechanical properties and fracture behaviors of Ti 22Al 20Nb 7Ta alloys were studied at room temperature. Three typical microstructures of Ti 2AlNb based alloys were obtained by combination of thermal mechanical processing and heat treat ment. They are: 1) lath mixture of O + B 2 with remaining β grain boundaries and α 2 phase; 2) equiaxed O phase in B 2 matrix; 3) fine lath mixture of O + B 2 without remaining β grain boundaries. It is shown that the microstructure obviously affects the tensile properties of Ti 2AlNb based alloys. The microstructure of fine lath mixture of O + B 2 without remaining β grain boundaries has good combination of yield stress and ductility, while the microstructure with lath mixture of O + B 2 with remaining β grain boundaries and α 2 phase has low yield stress and elongation. The fracture mode was also controlled by the microstructure of Ti 2AlNb based alloys. By means of SEM, it was found that the dominated fracture mode of microstructure with lath mixture of O + B 2 with remained β grain boundary and α 2 phase was intergranular, and the fracture mode of the other two microstructures was mainly transgranular.

Effect of Nb additions on microstructure and properties of γ-TiAl based alloys fabricated by selective laser melting

The γ-TiAl based Ti.Al.Mn.Nb alloys with different Nb additions were fabricated by selective laser melting (SLM) on the TC4 substrate. The effects of Nb content on microstructure and properties of the alloys were investigated. The results reveal that the alloys consist of γ-TiAl phase with tetragonal lattice structure and α2-Ti3Al phase with hcp lattice structure, and show a sequential structure change from near full dendrite to near lamellar structure with the increase of Nb addition. Owing to the higher Nb content in γ-TiAl phase and the formation of near lamellar structure, the alloy with 7.0 at.% Nb addition has the best combination of properties among the studied alloys, namely, not only a high hardness of HV 2000, a high strength of 1390 MPa and a plastic deformation of about 24.5%, but also good tribological properties and high-temperature oxidation resistance.

Urinary levels of nickel and chromium associated with dental restoration by nickel-chromium based alloys

This paper aims to investigate if the dental restoration of nickel-chromium based alloy(Ni-Cr) leads to the enhanced excretions of Ni and Cr in urine.Seven hundred and ninety-five patients in a dental hospital had single or multiple Ni-Cr alloy restoration recently and 198 controls were recruited to collect information on dental restoration by questionnaire and clinical examination.Urinary concentrations of Ni and Cr from each subject were measure by graphite furnace atomic absorption spectrometry.Compared to the control group,the urinary level of Ni was significantly higher in the patient group of < 1 month of the restoration duration,among which higher Ni excretions were found in those with either a higher number of teeth replaced by dental alloys or a higher index of metal crown not covered with the porcelain.Urinary levels of Cr were significantly higher in the three patient groups of <1,1 to <3 and 3 to <6 months,especially in those with a higher metal crown exposure index.Linear curve estimations showed better relationships between urinary Ni and Cr in patients within 6-month groups.Our data suggested significant increased excretions of urinary Ni and Cr after dental restoration.Potential short- and long-term effects of Ni-Cr alloy restoration need to be investigated.

DEVELOPMENT OF MoS_2-CONTAINING Ni-Cr BASED ALLOYS AND THEIR HIGH-TEMPERATURE TRIBOLOGICAL PROPERTIES

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Microstructure and texture evolution of Ti-Nb-Si based alloys for biomedical applications

Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. In as-quenched sample, relatively random distribution of pole figure was detected without showing a specific texture component. In as-cold rolled sample, however, it is found well-developed several texture components consisting of rotated cube, α-fiber and γ-fiber texture components which are frequently observed in bcc-structured metals and alloys were found. Therefore, texture components developed in the present alloys are closely related to the deformation of β-phase even though small amount of α″ phase co-exist in the microstructure. In recrystallized sample, α-fiber texture component is weakly detected while the other texture components, rotated cube and γ-fiber components, appears to be relatively unchanged. No additional texture components were detected besides those texture components observed in the cold rolled samples.

MICROSTRUCTURAL STABILITY OF LAMELLAR TiAl BASED ALLOYS AT HIGH TEMPERATURES

The microstructural stability of lamellar TiAl base alloys at high temperatures was studied by conventional and high resolution transmission electron microscopy. The influence of substructures on the thermal stability of lamellar structure was emphasized. These substructures produced by thermal mechanical treatments include the interfacial dislocations and ledges, the subgrain boundaries, the impinged T(Q) twins and misorientated lamellar interfaces. The microstructural change of three kinds of lamellar TiAl base alloys containing differents type and densities of substructures were compared during exposure at 800~1 000 ℃. It was found that the existence of such substructures could accelerate the degeneration of lamellar structure, leading to the rapid necking and break up of α 2 plates, the coarsening of γ plates, and the formation of new γ grains. As a result, the lamellar structure with substructures started to degenerate after thermal exposure at 800℃ for 4.5 h. While only slight coarsening was observed at the colony boundaries in the lamellar structure without substructures even after exposure at 900 ℃ for 7 d.

Characterization of localized corrosion pathways in 2195-T8 Al-Li alloys exposed to acidic solution

The corrosion properties of aluminum-lithium(Al-Li) alloys, which are potential materials used to construct for tanks of liquid rockets or missiles, are essential for safe propellant storage and transport. In order to manifest the corrosion resistance of the 2195 Al-Li alloy in practical propellant tanks filled with N2O4, the alloy was soaked in 30% nitric acid solution, an accelerating corrosion environment, to test its corrosion behavior. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM)were used to characterize microstructure and corrosion morphology of the alloy. Focused ion beam(FIB),combined with SEM, was used to demonstrate localized corrosion features and the propagation of corrosion pathways beneath the alloy surface. It was found that the corrosion network was formed with most intergranular corrosion and sparse intragranular corrosion. Additionally, the distribution and number of intermetallic particles influenced the localized corrosion degree and the direction of corrosion pathways. Aggregated particles made corrosion pathways continuously and caused more severe corrosion. The results from this work were valid and useful to corrosion prevention and protection for storage safety on propellant tanks in N_(2)O_(4).