THERMODYNAMICS OF MARTENSITIC TRANSFORMATION IN β-Cu BASE ALLOYS

An approach for the thermodynamics of the thermoelastic martensitic transformation in β-Cu base alloys is suggested.Driving forces for martensitic transformation,T_0 and M_s tempera- tures in Cu-Zn,Cu-Al and Cu-Zn-Al alloys were calculated and the calculated M_s are in good agreement with the experimental ones.Ordering of the parent phase lowers M_s(T_0)in Cu-Zn and Cu-Zn-Al alloys,but raises M_s(T_0)in Cu-Al alloys.Two methods for the es- timation of the critical driving force for the thermoelastic martensitic transformation are also introduced.

Effect of Al_(3)Zr particles on hot-compression behavior and processing map for Al-Cu-Li based alloys at elevated temperatures

The influence of Al3Zr particles on the hot compression behavior and processing map(PM)of Al-Cu-Li based alloys under isothermal plane-strain compression in the temperature range of 400-500℃and at the strain rates of 0.01-10 s^-1 was investigated.The corresponding microstructure was analyzed using optical microscopy(OM),electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results showed that dynamic recovery(DRV)played a greater role than dynamic recrystallization(DRX)in dynamic softening.At low temperatures,the Al3Zr particles were the significant barriers to inhibit DRV and DRX grain growth.When the temperature reached 500℃,the Al3Zr particles readily spread along grain boundaries just like a necklace due to the dissolution of Al3Zr particles and rapid diffusion of Zr through grain boundary,resulting in generating the macroscopic cracks and forming an instability domain at 490-500℃,0.01 s^-1 in the PM.

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.

High damping capacities of Mg-Cu based alloys

The dynamic mechanical analyzer(DMA)was applied to investigate the damping properties of Mg-Cu based alloys.The results show that the as-cast hypoeutectic Mg-Cu binary alloys exhibit ultra-high damping capacities,while the eutectic Mg-Cu alloy exhibits low damping capacity.The strain amplitude dependent damping performance reveals that the dislocation damping mainly dominates in Mg-Cu alloys.Furthermore,the influence of eutectic phase on damping mechanisms of Mg-Cu binary alloys was discussed in detail and the effect of Si addition on the damping of Mg-1%Cu based alloy was also reported.Two damping peaks are observed on the temperature dependent spectrum of Mg-Cu based alloys.One is located at room temperature,which is dislocation related peak;and the other is located at moderate temperature,which is caused by the grain boundary sliding.

Effect of pressure on microstructures and mechanical properties of Al-Cu-based alloy prepared by squeeze casting

A new high-strength aluminum alloy with better fluidity than that of ZL205A was developed. The effect of applied pressure during squeeze casting on microstructures and properties of the alloy was studied. The results show that the fluidity of the alloy is 16% and 21% higher than that of ZL205A at the pouring temperature of 993 K and 1 013 K, respectively. Compared with permanent-mold casting, mechanical properties of the alloy prepared by squeeze casting are much higher. The tensile strength and elongation of the alloy are 520 MPa and 7.9% in squeeze casting under an applied pressure of 75 MPa, followed by solution treatment at 763 K for 1 h and at 773 K for 8 h, quenching in water at normal temperature and aging at 463 K for 5 h. The improvement of mechanical properties is attributed to the remarkable decreasing of the secondary dendrite arm spacing(SDAS) and eliminating of micro-porosity in the alloy caused by applied pressure.

Influence of Si and Ge Microalloying Additions and Plastic Deformation on Precipitation Processes in Al-Cu Based Alloys

The combined effect of micro-alloying with Si and Ge and/or plastic deformation prior to ageing at 160°C on age hardening has been studied in an Al-2 at% Cu alloy. The results obtained indicate that the hardness response is faster and the peak hardness is higher when plastic deformation and micro-alloying are applied together than performing each procedure individually. Different amounts of deformation, ranging from 0% to 30% have been utilized. An optimum deformation degree for the hardening response has been established around 8% for the Al-Cu-Si-Ge alloy. Characterization by transmission electron microscopy (TEM) shows that the peak hardness is due to a complex microstructure that contained θ" disc shaped precipitates, rod-shaped Si-Ge precipitates and θ' plates that were heterogeneously nucleated on the Si-Ge particles. Pre-deformation has been found to stimulate the growth of the θ' plates due to enhanced diffusion along dislocation cores. Increasing deformation reduces the influence of the Si-Ge precipitates on heterogeneous nucleation, leading to reduced peak hardness and faster over-ageing.

Properties of Ti-Based Hydrogen Storage Alloy

An efficient and safe hydrogen storage method is one of the important links for the large-scale development of hydrogen in the future. Because of its low price and simple design, Ti-based hydrogen storage alloys are considered to be suitable for practical applications. In this paper, we review the latest research on Ti-based hydrogen storage alloys. Firstly, the machine learning and density functional theory are introduced to provide theoretical guidance for the optimization of Ti-based hydrogen storage alloys. Then, in order to improve the hydrogen storage performance, we briefly introduce the research of AB type and AB2 type Ti-based alloys, focusing on doping elements and adaptive after treatment. Finally, suggestions for the future research and development of Ti-based hydrogen storage alloys are proposed. .

Microstructure of cast γ-TiAl based alloy solidified from β phase region

The microstructures and phase transformation of Ti-43Al-4Nb alloy in as-cast and heat-treated states were investigated by using optical microscopy, scanning and transmission electron microscopy as well as differential scanning calorimetry. The results show that a fine microstructure of the as-cast alloy can be obtained by solidifying through the β phase. γ grains can nucleate directly from the β phase. The coexistence of β phase and γ phase along primary α grain boundaries contributes to the decrease in the grain size of the as-cast alloy. The phase transformation sequence during solidification of the Ti-43Al-4Nb alloy is suggested as L→L＋β→β→α＋β→α＋βr→α＋γ＋βr→lamellae（α2＋γ）＋γ＋βr. The microstructure of the alloy after heat treatment at 1 250 ℃ for 16 h exhibits a certain coarsening compared with that of the as-cast state. The remnant β phase can be removed by the heat treatment process due to the diffusion of Nb and the non-equilibrium state of β phase.

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.

Friction and wear behaviors of Nb-Ti-Si-Cr based ultrahigh temperature alloy and its Zr-Y jointly modified silicide coatings

Zr-Y jointly modified silicide coatings were prepared on an Nb-Ti-Si-Cr based ultrahigh temperature alloy by pack cementation process. The wear behaviors of both the base alloy and coatings were comparatively studied at room temperature and 800 ℃ using SiC balls as the counterpart. The Zr-Y jointly modified silicide coating is mainly composed of a thick （Nb,X）Si2 outer layer and a thin （Ti,Nb）5Si4 inner layer. The coatings possess much higher microhardness than the base alloy. The wear rates of both the base alloy and coatings increase with increasing the sliding loads. However, the coatings have much lower wear rates than the base alloy under the same sliding conditions. The coatings have superior anti-friction property, and can provide effective protection for the base alloy at both room temperature and 800 ℃ in air.

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 gelcasting conditions on quality of porous Al-Cu alloy

The porous A1-Cu alloy was prepared by the gelcasting process. And the effects of gelcasting conditions, such as monomer, the volume ratio of cross-linker and monomer, dispersant and redox initiating system on the height, gelling time and the quality of green body were investigated. （It was found that the dispersant and monomer played significant roles in the height and quality of green bodies, respectively.） The optimal conditions were 10% monomer, 2% cross-linker, 0.2% initiator （volume fraction）, and 1.2 g dispersant, in which the green body exhibited the best quality. The mechanisms of process conditions in eliminating the cracks and forming the pores of in the five stages were proposed. Mercury porosimetry provided a description of pore diameter ranging from 10 to 10000 nm and open porosity of 38.78 %.

Microstructure and microhardness of directionally solidified and heat-treated Nb-Ti-Si based ultrahigh temperature alloy

Directionally solidified （DS） specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at （1 500 ℃, 50 h） and （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, respectively. The results show that the microstructures become uniform, the long and big primary （Nb,X）sSi3 （X represents Ti and Hf elements） plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the （Nb,X）sSi3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution （Nbss） in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, which is 72.8 % higher than that under DS condition.

Effect of high temperature treatments on microstructure of Nb-Ti-Cr-Si based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy,coupons were homogenized at 1 200-1 500 °C for 24 h,and then aged at 1 000 °C for 24 h.The results show that the heat-treated alloy is composed of Nb solid solution（Nbss）,（Nb,X）5Si3 and Cr2Nb phases.With the increase of heat-treatment temperature,previous Nbss dendrites transformed into equiaxed grains,and petal-like Nbss/（Nb,X）5Si3 eutectic colonies gradually changed into small（Nb,X）5Si3 particles distributed in Nbss matrix.A drastic change occurred in the morphology of the Laves phase after homogenizing treatment.Previously coarse Cr2Nb blocks dissolved during homogenizing at temperature above 1 300 °C,and then much finer and crowded Cr2Nb flakes precipitated in the Nbss matrix in cooling.Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti,Hf and Al in Nbss,（Nb,X）5Si3 and Cr2Nb phases reduced.

Effect of electropulsing treatment on corrosion behavior of nickel base corrosion-resistant alloy

Electropulsing treatment（EPT） was performed on a nickel base corrosion resistant alloy during aging.The effect of EPT on the microstructure and corrosion resistance of the alloy and the mechanisms were investigated.The results show that the intergranular corrosion resistance can be improved substantially without the degradation of mechanical properties of the alloy by EPT.The EPT has an effect of enhancing the interface diffusion rate of the alloying element,which is higher than the body diffusion rate.And thus discontinuous precipitation of M23C6 type carbides appears at the grain boundary in the alloy by EPT,which decreases the depletion extent of the alloying elements at the grain boundary substantially.As a result,the intergranular corrosion resistance of the alloy can be improved by the EPT without any degradation of mechanical properties.

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.

Tribological behavior of different films on Ti-6Al-4V alloy prepared by plasma-based ion implantation

The tribological behaviors of TiN coating and TiN+TiC+Ti(C, N)/diamond like carbon (DLC), TiN/DLC, TiC/DLC multilayers on Ti 6Al 4V alloy prepared by plasma based ion implantation (PBII) were compared. Under the test conditions of counterbody AISI 52100, load 1 N and speed 0.05 m/s, the tribological properties of the alloy are improved by these films in the order of TiN, TiC/DLC, TiN/DLC and TiN+TiC+Ti(C,N)/DLC. Tribological behavior is affected by the conditions of surface modification and triboexperiments. The appearance of “peaks” in the wear dynamic resistance profiles may be due or correspond to the process of formation and breaking apart of transition films. The breakthrough of the DLC coated samples may start from partially wearing out, and end with joining piece dilamination. There are transition films on all counterbodies AISI 52100. When AISI 52100 counterbody is changed to Ti 6Al 4V, the wear of most modified samples is changed from only disc to both disc and ball abrasive dominated.