Abstract:In order to study the optimal combination of Ti-6Al-4V (TC4) alloy with silicon carbide (SiC) ceramics and ultra-high molecular weight polyethylene (UHMWPE) in composite armor, the penetration resistance and macro-damage of the complex material with composite structure of SiC/UHMWPE/TC4 (I) and SiC/TC4/UHMWPE (II) were analyzed, and the micro-damage of TC4 alloy was also discussed. Results show that the bullet hole edges of TC4 alloy in composite structure I are relatively smooth without cracks, and there are a few adiabatic shear bands (ASBs) propagated along the straight lines. The bullet hole edges of TC4 alloy in composite structure II are rough, and there are cracks and spalling damage. There are many curved and bifurcated ASBs in composite structure II. The penetration process of TC4 alloy in composite structure II includes the pit opening stage, stable penetration stage, and perforation stage. The adiabatic shear behavior of TC4 alloy in composite structure II is more complicated than that in composite structure Ⅰ, resulting in more energy consumption. In addition, the tensile failure caused by UHMWPE in the composite structure II is also one of characteristics of the high energy consumption failure mode. Therefore, the SiC/TC4/UHMWPE composite structure can efficiently exert the energy consumption mechanisms of TC4 alloy and UHMWPE, and the anti-penetration performance of the complex armor with this composite structure is better than that with the composite structure Ⅰ.

Abstract:Four TiAl alloys with different Mo contents were designed, and the microstructure and mechanical properties of these Mo-TiAl alloys were studied by scanning electron microscope, nanoindentation, and hot compression simulation methods. Results show that with increasing the Mo content, the content of γ phase is gradually decreased, while that of β phase is gradually increased. The Mo element mainly exists in the form of β phase in the TiAl alloy. During the hot isostatic pressing process, the Mo element is diffused from the γ and α₂ phases to the β phase. The nanoindentation hardness of Mo-TiAl alloy reaches the maximum when the Mo content is 1.59at%, and it is negatively correlated with the interlamellar space. As the content of Mo element increases, the flow stress of Mo-TiAl alloys decreases, and the TiAl alloys with 2.11at% and 3.94at% Mo addtion have poor plasticity due to the Al element segregation.

Abstract:The effects of ytterbium (Yb) on the thermal behavior, wettability, and microstructures of Ag30-Cu-Zn-Sn (BAg30) filler metals were investigated. The shear strength and fracture morphology of brazed joints were analyzed to optimize the filler composition. Results reveal that the addition of 1wt% Yb in BAg30 filler metal can significantly decrease the temperature difference between solidus temperature and liquidus temperature, improve the wettability of filler on the steel substrate, and refine the microstructures. However, the excessive addition of Yb (more than 1wt%) can degrade the properties and coarsen the microstructure. According to the shear strength tests, the BAg30-1wt% Yb brazed joint shows obvious superiority in shear strength and fracture morphology: its shear strength is higher than that of the original BAg30 brazed joint, and the fracture mechanism is ductile fracture, presenting the homogeneous and fine dimples.

Abstract:HfC-TaC was employed to modify the C/SiC-ZrC composites and to improve their oxidation resistance. The composites were fabricated by the combined process of precursor infiltration and pyrolysis method and chemical vapor deposition method, and the oxidation resistance was evaluated by static oxidation tests (1600 °C/5 h and 1600 °C/20 h). According to the test results of flexural strength and mass loss, it is found that HfC-TaC can effectively improve the anti-oxidation behavior of C/SiC-ZrC composites without obviously decreasing their mechanical properties. By virtue of X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry, the crystal structure and microstructure of composites were analyzed. Results show that the superior anti-oxidation behavior of C/SiC-ZrC composites after HfC-TaC modification can be ascribed to the uniform element distribution and well-integrated components in the matrix.

Abstract:The microstructure evolution and variation of mechanical properties of nickel-based U720Li superalloy were investigated during long-term aging (3000 h) at 680, 700, and 730 °C. The changes in size and morphology of γ′ phase were determined. Results show that the coarsening behavior of γ′ phase is controlled by the diffusion process. The tensile and creep properties of U720Li superalloy remain stable during the long-term aging at 680 and 700 °C. However, the yield strength gradually decreases and the ductility increases after long-term aging at 730 °C for 500 h. The fatigue and creep properties of U720Li superalloy decreases sharply after long-term aging at 730 °C, which is mainly related to the morphology evolution of γ′ phase.

Abstract:The Ti-6Al-4V titanium alloy with YbB₆ addition was fabricated by spark plasma sintering (SPS), and the effects of YbB₆ addition on microstructure and mechanical properties of Ti-6Al-4V titanium alloy were investigated. The results show that with increasing the YbB₆ content, the microstructure of the composites is clearly changed, and the grains are obviously refined. The in-situ formation of TiB whiskers and Yb₂O₃ particles is beneficial to the improvement of mechanical properties of the composites. Furthermore, with the addition of 0.6wt% YbB₆, the relative density, microhardness, yield strength, ultimate tensile strength, and elongation of the sintered alloys are 99.43%, 4030 MPa, 903 MPa, 1148 MPa, and 3.3%, respectively. Compared to those of Ti-6Al-4V alloy, the aforementioned properties of the sintered alloys with 0.6wt% YbB₆ are increased by 0.37%, 13.8%, 38.07%, and 17.14%, respectively. The strengthening mechanism is mainly attributed to the microstructure transformation, grain refinement, and dispersion strengthening. With increasing the YbB₆ content, the fracture mode is the combination of ductile fracture and brittle fracture.

Abstract:The amorphous carbon coating was synthesized on the TA2 substrate surface by direct current balanced magnetron sputtering. The corrosion resistance and conductivity of TA2 substrate and amorphous carbon coating were analyzed by electrochemical and surface contact resistance tests in proton exchange membrane fuel cell (PEMFC) with different F^- contents. The results show that the corrosion resistance and conductivity of amorphous carbon-coated TA2 alloy are better than those of bare TA2 substrate. When the F^- content is increased from 1×10^-6 mol/L to 1×10^-3 mol/L, the defect density of TA2 substrate and amorphous carbon coating is increased, indicating the decreased corrosion resistance of both materials. The amorphous carbon coating at potential of 0.6 V demonstrates the corrosion current density of 0.68 μA/cm² in the F^- environment with high content (1×10^-3 mol/L), still providing a good protection for TA2 substrate. Moreover, the amorphous carbon coating can also improve the surface conductivity of the TA2 substrate: the interface contact resistance decreases from 76.40 mΩ·cm² (TA2 substrate) to 6.52 mΩ·cm².

Abstract:The microstructures, mechanical properties, biodegradation behavior, and cytocompatibility of the as-cast Mg-2Ga and Mg-5Ga alloys were investigated. The microstructure characteristics were studied by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The results show that the Mg₅Ga₂ precipitates with globular and strip-like shapes are formed along the grain boundaries in Mg matrix after Ga addition. The grains of Mg-Ga alloys are obviously refined by Ga addition. The tensile tests reveal that the mechanical properties of Mg-Ga alloys are enhanced by Ga addition via solution strengthening, grain boundary strengthening, and precipitation strengthening. The electrochemistry and immersion corrosion tests in simulated body fluid at 37 °C reveal that the improved corrosion behavior is attributed to the enhanced surface stability caused by Ga addition of low content. In addition, the indirect contact assay suggests that both Mg-2Ga and Mg-5Ga alloys exhibit favorable cytocompatibility with no cytotoxicity for the mouse fibroblast (L929) cells.

Abstract:The effect of surface etching by H₂C₂O₄ and H₂SO₄ on Ti substrates on the electrochemical properties and surface morphologies of titanium oxide anode was investigated through different etching methods. The scanning electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy were used to analyze the specimen structure. The electrocatalytic activity and the electrochemical stability of the specimens were also evaluated by the electrochemical workstation and accelerated lifetime tests, respectively. Results show that the dual acid etching can achieve denser and more homogeneous surface with better catalytic stability. In addition, the influence mechanism of pretreatment on the lifetime of Ti anodes was discussed. The catalytic activity and stability of IrO₂-Ta₂O₅/Ti anodes are strongly dependent on the sequence of acid etching and the surface structure of anodes, and thereby the relationship between the pretreatment methods and the anode performance is established. The dual acid etching can achieve a Ti surface with moderate roughness, therefore improving the coating adhesion. The titanium hydride formed through the dual acid-treatment is transformed into the rutile with barely changed surface morphology, which is conducive to the electrons transport. Therefore, the coating adhesion is enhanced and the accelerated lifetime is prolonged.

Abstract:The Ni coating of 10 μm in thickness was firstly deposited on the cylinder surface of pure tungsten by electrochemical deposition, and then the W/steel joint cylinders were prepared by the hot isostatic pressing (HIP) diffusion bonding process for nuclear component application. The HIP bonding parameters were set as 900 °C/100 MPa/1 h. The structure and composition analyses show that the metallurgical bonds are achieved with a tensile strength of about 236 MPa. However, the W/steel joint fractures at W substrate near the bonding interface due to the residual stress concentration. The Cu addition was used as the soft intermediate layer to release the residual stress by creep or yield mechanism, thereby improving the tensile strength of W/steel joint to about 312 MPa. The adhesive force of coating and the hardness distribution in the bonding interfaces were also discussed.

Abstract:Due to different influences of cooling rate on the solidification structure of ZL205A alloy under different temperature gradients, the ZL205A alloy with stepped structure of different wall thicknesses was used, and the specimens of different thicknesses were prepared under different solidification conditions. The effects of cooling rate and temperature gradient on solidification structure and tensile strength and elongation of alloys after T5 treatment were explored. In addition, the correlation effect of the phase transformation law with the structure and performance was explored. Results show that the temperature gradient caused by different wall thicknesses have a certain impact on the microstructure and morphology of the alloys. Reducing the temperature gradient can significantly improve the mechanical properties of the alloys. After T5 treatment, the tensile strength of the alloys achieves 506 MPa.

Abstract:Based on the orthogonal experiment results, the significance of process parameters of near-β forging+solution and aging was analyzed. The effect of process parameters on the microstructure of TA15 alloy was discussed, and the proper process parameters were obtained to achieve the tri-modal microstructure with excellent properties. Results show that the deformation temperature, solution temperature, and solution duration are the most important process parameters, which affects mostly the volume fraction and diameter of the equiaxed α_p phase, the volume fraction of the lamellar α_s phase, and the thickness of the lamellar α_s phase, respectively. The optimal processing parameters are 970 °C/0.1 s^-1/deformation degree of 60%/water quenching+930 °C/1.5 h/air cooling+550 °C/5 h/air cooling.

Abstract:In order to improve the corrosion resistance of pure titanium in Ringer's simulated body fluid and simulated oral saliva environment, the equal channel angular pressing (ECAP) technique was used to modify the commercial pure titanium prepared by selective laser melting (SLM). The microstructures of SLM pure titanium and SLM+ECAP pure titanium were detected by transmission electron microscope and electron back-scattered diffractometer, and their corrosion resistance was tested by the three-electrode system. Results show that compared with SLM pure titanium, SLM+ECAP pure titanium has smaller grain size, more grain boundaries, higher dislocation density, and modest preferred orientation of the pole diagram with increased pole density. In Ringer's simulated body fluid and simulated oral saliva environment, SLM+ECAP pure titanium has lower self-corrosion current density, higher polarization resistance, and larger impedance radius than SLM pure titanium does. The equivalent circuit fitting of alternating current impedance spectrum was conducted by ZSimpWin software, and the fitting results are in good agreement with the experiment data. The corrosion resistance of SLM+ECAP pure titanium is better than that of SLM pure titanium.

Abstract:The initiation condition for deformation mechanism of extrusion AZ60 Mg alloy during tensile deformation along different directions at room temperature was investigated through experiments and visco-plastic self-consistent (VPSC) modeling, and the relationship between the deformation mechanism with the flow curves, texture evolutions, and microstructure of ZK60 Mg alloys was also analyzed. By adjusting the parameters in VPSC model, the crystal plastic mechanics model with slip coupled with twin was established. The differences in texture evolution during tension along different directions were compared, and the influence of deformation mechanism on the yield asymmetry was analyzed. The experiment and simulation results show that due to the initiation of {102} twin, most grains are rotated at a large angle (about 90°) when the tensile direction is perpendicular to the extrusion direction, namely PED. The prismatic <a> slip is the main deformation mode resulting in the yield asymmetry of ZK60 alloy during tensile deformation along different directions. When the ZK60 alloys are under tensile deformation along extrusion direction (ED), due to the preferential orientation, the {101} twin is difficult to initiate and the higher yield strength can be obtained. The yield stress under tension along the direction with an angle of 45° to ED is higher than along PED. As the tensile stress gradually increases, the prismatic <a> slip is gradually initiated, resulting in the higher stress curves along PED than those along the direction with an angle of 45° to ED.

Abstract:The molecular dynamics simulations were used to study the effect of grain size and twin density on the plastic deformation of nano-polycrystalline aluminum alloy. The results show that the dislocation density after relaxation is crucial to the microstructure evolution and the inverse Hall-Petch relation of the nano-polycrystalline Al. The staggered tetrahedrons and complex staggered structures are formed in the fine grains, which is attributed to the restriction of grain size. Thus, the auxiliary deformation of grain boundary is activated. The Shockley partial dislocations nucleate and multiply at the grain boundaries when the twin boundary spacing (TBS) is relatively large. However, with decreasing the TBS, the twin boundary becomes the source of the Shockley partial dislocations. A large number of partial dislocation nucleations at the twin boundary will cause the twin boundary to migrate or even disappear. The deformed nano-twins can also be observed during the plastic deformation process. This research provides theoretical basis for the development of advanced nano-polycrystalline Al alloy with adjustable mechanical properties.

Abstract:A multi-scale coupling method was used to predict the texture of TC18 titanium alloy bar. Firstly, the macroscopic finite element method was used to simulate the multi-pass forging process of TC18 titanium alloy bar under the conditions close to the actual ones, and the characteristics of inhomogeneous distribution of effective strain and shear stress σ_XY at the center and edge of the alloy bar during forging were obtained. Then, the multi-scale coupling method of macroscopic finite element model and mesoscopic visco-plastic self-consistent (VPSC) model was used to simulate the texture evolution at the center and edge of alloy bar during forging. The results show that in the center of the alloy bar, the texture transformation from {110}<112> to {111}<110> and from {110}<110> to {111}<110> occurs. The forging process is composed of mutual transformation between {111} and {110} textures. The transition texture shows the similar characteristics of shear texture in the pole figure. After analysis, it is confirmed that the transition texture is formed by the interaction of hexagonal forging and mutual transformation between {110} and {111} textures, which is not the shear texture. The {100} and {111} textures formed at edge were derived from the deformation process. Through comparison, it is found that the hexagonal forging can hardly produce the unfavorable {100} texture, and it is conducive to the weakening and elimination of {100} texture. The tensile test results show that the mechanical properties of the hexagonal forging specimen can meet the standard requirements.

Abstract:Based on the equivalency method (the impact of all alloying elements on the thermo-physical properties can be expressed through the equivalent impact of a reference element), Zn was regarded as a reference element, and the parameters required for equivalency calculation were obtained through the numerical fitting of the Al-rich liquidus line in the binary phase diagram of 7XXX series aluminum alloys. The sum of the equivalent concentration of other elements and the actual concentration of the reference element was used to calculate the liquidus temperature and latent heat of materials. The calculation results are in good agreement with the measured data by differential scanning calorimetric (DSC) apparatus. Compared with Jmatpro software, the equivalency method shows better accuracy.

Abstract:The representative C35M alloy among FeCrAl alloys was selected as the research object, and a small fuel rod model was established. Based on the user material (UMAT) subroutine, the radiation creep model of the C35M alloy was embedded in the subroutine. The thermomechanical coupling behavior of C35M alloy under neutron irradiation was calculated by the finite element software ABAQUS. Using Zr-2 alloy as a comparison, the evolution of the distribution of the temperature field, stress field, displacement field, and gap distance of the cladding over time of the alloys was analyzed. Results show that the temperature field and the stress field of the two alloys are basically the same. The temperature distribution is mainly affected by the coolant, while the stress field is related to the temperature and creep rate. During the simulation, the Zr-2 alloy mainly grows through irradiation, while C35M alloy shows the irradiation creep and has a little thermal expansion deformation. The gap closure rate of Zr-2 alloy is much faster than that of C35M alloy, which indicates that C35M alloy can prolong the accident response time. However, in order to adapt to the complex environment in the reactor, the material still needs to be optimized to improve its strength and creep rate.

Abstract:In this paper, the acoustic emission response of single crystal γ-TiAl alloy during nano-cutting process is studied by molecular dynamics method. The mechanism of crack formation in the cutting process of single crystal γ-TiAl alloy is described at the atomic scale. It was found that periodic shear bands are formed in the shear zone with the continuous increase of cutting force at the beginning of cutting. At the same time, under the combined action of high pressure stress and elastic stress wave, the formation of the amorphous atomic band in the grain boundary blocks the continuous emission of the shear band, so that the stress in the main shear zone can not be released through the shear band in time,and result in local stress concentration, which leads to the initiation and propagation of cracks. By analyzing the acoustic emission signal collected, it was found that the compressive stress contributes to the decrease of acoustic emission power in cutting process. In the time domain, the acoustic emission response characteristics of lattice vibration, shear band and crack initiation and propagation in nano-cutting process are described by analyzing the microdefect evolution and AE power-frequency comparison. At the same time, the power and frequency characteristics of damage are obtained by clustering analysis.

Abstract:Foundry integral forming technology is helpful to fabricate large size and complex structure ceramic/metal composite components, which has important theoretical significance and application value. In this paper, based on the finite element method, the flow field of SiC ceramic /K4169 alloy composite castings during filling process, the thermal interaction between ceramic and metal, the generation and distribution characteristics of thermal stress and residual stress in the solidification process were investigated. The results show that unstable flow occurs at the inner gate during the filling process. The surface temperature of the ceramic rises sharply after contacting the metal liquid, and a temperature gradient of 20-30℃/mm is generated inside the ceramic. The casting thermal stress decreases first and then increases. And the residual stress is concentrated on the interface between the ceramic and the metal, and the greater stress is on the ceramic side. Futhermore, the peak value of the residual stress is 2-3mm away from the interface.

Abstract:The heterogeneous precipitation behavior of η phase at E/Al interfaces in 7475 aluminum alloy has been studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) electron tomography. It is found that the nucleation location, the nucleation rate, the growth rate and morphology are greatly different when η phase are formed at E particles with different morphology and volume or the same E particle. The decisive factors that affect the nucleation and growth of η phase are the interface energy and elastic strain energy of E phase. The growth of η phase belongs to theSdiffusion-controlled transformation. Solute atoms are transferred by the interphase interface diffusion at a high speed.

Abstract:In this paper, the high-temperature tensile tests were carried out on the millimeter-level coarse-grained new Ni-Cr-Co-based alloy with different γ′ phase sizes at 600 °C and 5 × 10_{^{? 4}}s_{^{? 1}}, and the effects of γ′ phase sizes on the high-temperature tensile behavior and sawtooth rheological effect of the alloy were studied by OM, SEM, and TEM. The results show that the size of γ′ phase has a significant effect on the mechanical properties of the alloy. With increasing of the size of γ′ phase, the material’s strength increases at first and then decreases. The main deformation mechanism of the alloy changes from dislocation cutting through γ′ phase to dislocation by passing γ′ phase. When the size of γ′ phase continues to increase, it is difficult for the dislocation movement to be blocked to by pass the γ′ phase. The solute atoms can pin the movable dislocation. When the stress increases to a certain extent, the dislocation is depinning. Repeated pinning and depinning, that is to say, the dynamic strain aging leads to the sawtooth flow phenomenon in the deformation process of the alloy, which can be weakened by adjusting the size of γ′ phase. When the average size of γ′ phase is 57.18 nm, the serrated flow effect is weak, the critical strain is the largest, and the mechanical properties became higher. Therefore, the optimal size of the γ′ phase is 57.18 nm.

Abstract:Abstract: The isothermal compression tests of hydrogenated Ti6Al4V alloy at deformation temperature of 750 ℃, 800 ℃, 850 ℃, 900 ℃, 950 ℃ and 1000 ℃ and strain rate of 1 s-1 were carried out on a Gleeble-1500 thermal simulator. The results show that the flow stress of Ti6Al4V alloy decreases first and then increases with the increase of hydrogen content. When the deformation temperature is 750℃, 800℃ and 850℃, the flow stress of alloy at hydrogenation content of 0.31wt% is the lowest. When the deformation temperature is 900℃, 950℃ and 1000℃, the hydrogen content corresponding to the minimum flow stress is 0.17wt%, 0.1wt% and as received, respectively. The high temperature deformation constitutive model of hydrogenated Ti6Al4V alloy was established based on the self-consistent model. The model reflected the effect of hydrogen on the flow stress of Ti6Al4V alloy by adjusting the strengthening effect of hydrogen on β phase and reducing the transition temperature of β phase. Compared with the experimental results, it is shown that the model can predict the variation of flow stress with hydrogenation content and deformation temperature well.

Abstract:600℃ high temperature titanium alloy samples were fabricated on forged Ti-6246 alloy by laser additive manufacturing with different linear energy density (AM-Ti150). The microstructure characteristics and mechanical properties of the deposited layer and interface bonding zone were studied by SEM, XRD, TEM and tensile test, and the formation mechanism was discussed. The results show that the microstructure of AM-Ti150 alloy is basketweave structure composed of α" martensite. In the range of linear energy density in this paper, with the increase of linear energy density, the defects decrease, the density increases, the width of martensite lamellar increases, and the tensile strength and elongation at room temperature and high temperature increase. When the linear energy density is 90 J?mm-1, the density of AM-Ti150 alloy is 99.67%, the tensile strength and elongation at room temperature are respectively 1075 MPa and 4.7%, the tensile strength and elongation at high temperature are respectively 808.7 MPa and 14.3%. Under the influence of heat source, the non-uniform structure of interface bonding zone’s Ti-6246 alloy is formed from top to bottom, and the ghost structure is formed due to the insufficient diffusion of Al and Mo elements in the upper part. With the increase of linear energy density, the diffusion of Al and Mo elements are gradually sufficient, and the size of the ghost structure decreases.

Abstract:A new process of laser-induction hybrid quenching was adopted in this paper, which combined the laser and electromagnetic induction heat source to improve the depth and uniformity of hardened layer. The COMSOL Multiphysics 5.5 software was used to analyze the evolution process of temperature field in hybrid quenching process of 42CrMo steel. The model was verified by experiments and the simulated depth of hardened layer was in good agreement with the experimental one. The surface temperature and depth of hardened layer by the hybrid quenching, single laser quenching and single induction quenching were compared in the model, and the effects of different scanning speed and laser spot size on the depth of hardened layer were analyzed. The results show that the hybrid quenching can effectively improve the surface quenching temperature of the workpiece, increase the width and depth of the hardened layer, and make up for the shortage of the single laser quenching power. The optimal scanning speed and laser spot size of the hybrid quenching were predicted by the model. Compared with two single quenching，the change trend of grain size and microstructure in depth direction of the hybrid quenching is similar to that of the laser quenching and the average hardness of hardened layer is larger.

Abstract:In this paper, Al-7%Si-0.3%Mg-x%Sc (X=0, 0.1, 0.2, 0.3, 0.5 and 0.8) casting alloys were prepared by vacuum induction melting furnace. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were used to characterize the microstructure of the experimental alloy, and the refinement and modification mechanism of the experimental alloy was discussed. The results show that the main phases of the experimental alloy include α-Al, eutectic Si, Al₃Sc, AlSc₂Si₂ and iron-rich phases (β-AlFeSi and π-AlSiMgFe). The microstructure of the experimental alloy was refined by adding Sc element. With the increase of Sc content, the dendritic spacing of α-Al decreases and the size of eutectic Si decreases. When the content of Sc is 0.3%, the effect on refining of the experimental alloy is the best. A large number of fine Al₃Sc particles are produced in the alloy containing Sc. Al₃Sc particles are coherent with α-Al matrix, and the lattice mismatch between them is 1.0%. Therefore, Al₃Sc can be used as an effective heteronucleation particle of α-Al to fine α-Al After the addition of Sc, the eutectic Si will deteriorate and get refined. This is because the rare earth Sc elements existing in the eutectic Si can be adsorbed on the eutectic Si{111} dense surface as impurity elements, thus promoting the formation of high-density twins for the eutectic Si. In addition, AlSc₂Si₂ formed in the alloy can consume part of Si element, which reduces the amount and size of eutectic Si.

Abstract:The fracture process of human bone under impacting is accompanied by the absorption of energy. The design of porous implant should consider compressive behavious and energy absorption characteristics of the whole structure. A series of lattice structures with different unit cell size and relative density is established by topology optimization designing and laser additive manufacturing fabricating, and the suface quality, fracture and deformation regularity and energy absorption performance of lattice structure are investigated by melt pool monitoring, unidirectional compression test and finite element simulation. The results show that the structure parameters of lattice structure are affected by the temperature field of molten pool and the supporting force of powder layer. The compressive behavious of lattice structures follows the law of elasticity and brittleness foam, the crush band forms at an angle of 45o with the fabricating direction. The fracture mechanism of lattice structure is ductile fracture, and the crack propagation direction is distributed along the internal micro-pores. The energy absorption capacity is directly proportional to the relative density and inversely proportional to unit cell size. The energy absorption efficiency is inversely proportional to the relative density and proportional to the cell size.

Abstract:The experiments of high pressure torsion(HPT) subjected to tungsten were conducted under 1 turn, 2 turns, 5 turns and 10 turns at 550 ℃. The microstructures of sintered and HPT-processed tungsten were characterized by electron backscatter diffusion(EBSD) and transmission electronic microscopy(TEM). Results show that the grains of sintered tungsten were refined and the propotion of high angle grain boundary increased. Meanwhile, the dislocations were moving towards the grainboundary and rearranged orderly during HPT process. The grain boundary energy of sintered and HPT-processed tungsten were calculated by modified dislocation model. It was found that the grain boundary energy kept increasing and it was main from the excess dislocation energy. Also, the non-equilibrium grain boundary was observed. It is only formed in nano-materials that is prepared by severe plastic deformation. In high resolution TEM,it shows the features of thick grain boundary thickness and moire fringes caused by crystal plane interference.

Abstract:Sintering process has great influence on the performance of porous materials. In order to obtain the best mechanical properties and required parts shape, it is necessary to study the thermal deformation evolution of sintering. Based on coupled thermo-mechanical SVOS visco-plastic constitutive model the macro deformation of porous materials was calculated about the sintering process of stainless steel porous materials. The parameters of sintering stress, shear viscosity and volume modulus were fitted by using fourth-order Gauss functions base on free sintering and bending experiments. The relative density, linear shrinkage rate changing with the sintering time and temperature were discussed. The results show that the simulated process is consistent with the experiments that validate the rationality of the model. The difference of linear contraction curves under different heating rate of material is very little. Therefore, heating rate has small influence on sintering process. When the heating rate is slow or fast, computations are not easy to converge, and the calculation time is longer. The sintering simulations of the green parts prepared by dry-press process shows that the initial density distribution inhomogeneity causes eventually density distribution inhomogeneity, while the range of density is not big.

Abstract:In this paper, the mechanical response and fracture mechanism of 2219 aluminum alloy with different heat treatment conditions under dynamic loading were studied by means of split Hopkinson tension bar experiment. The results show that: The tensile strength and strain rate sensitivity of 2219-T6 and 2219-T4 increase with the increase of strain rate, while that of 2219-O increases first and then decreases. And the strain rate sensitivity and strain hardening rate of 2219 aluminum alloy decrease with the increase of strain; Under the dynamic loading conditions, the material forms the partial dissociation morphology and the morphology of the second phase particle break under the higher stress concentration degree, and at higher strain rate (≥650 s-1), with the increase of strain rate, the stress concentration degree becomes higher, and the proportion of dissociation morphology becomes larger. At the same time due to the material can still undergo a certain amount of deformation at the same time of crack propagation, the material as a whole has a higher forming limit under the higher strain rate (≥650 s-1), it also leads to the formation of fracture morphology with low plasticity under the action of higher work hardening.

Abstract:The study aims to investigate the heat treatment process of light high-entropy alloy AlZnMgCuMn and its effect on microstructure, mechanical properties and corrosion resistance. The microstructure of this alloy was characterized by XRD, SEM and EDS. The mechanical properties were evaluated by electronic universal testing machine and Vickers hardness tester. The corrosion resistance and mechanism were analyzed by potentiodynamic polarization test, impedance spectrum test, CLSM and AFM. The results show that AlZnMgCuMn alloy is composed of Al-Mn quasicrystalline phase and HCP phase, and the effect of heat treatment on the morphology, distribution and volume fraction of two phases is small. The compression fracture strength, fracture strain and Vickers hardness of as-cast AlZnMgCuMn are 415 MPa, 4.4% and 461.2 HV0.5, respectively. The Ecorr and Icorr of as-cast alloy are -726.344 mV and 1.882 μA/cm2. After electrochemical corrosion, obvious corrosion pittings are formed on the surface of alloy, the depth of which was about 12 μm. The dendritic Al-Mn phase has a lower potential as an anode and is preferred to be corroded. As the corrosion intensifies, the corrosion microdomains are connected with each other and develop into a large corrosion area.

Abstract:In order to improve the comprehensive mechanical properties of additive components, the heat treatment process of high strength wrought 2A50 aluminum alloy was studied. The effects of different heat treatment parameters on the microstructure and mechanical properties of additive samples were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness tester and tensile test. The results show that the as deposited samples have obvious columnar dendrite structure. After heat treatment, the coarse columnar dendrites fracture, and the grains begin to spheroidize and form a uniformly distributed massive second phase at the grain boundary. Under the optimized heat treatment conditions (540 ℃ × 1 h+150 ℃ × 16 h), combined with the solid solution strengthening of solute elements and the precipitation strengthening of the second phase, the average values of yield strength, tensile strength and microhardness of the additive samples increased from 90.7 HV, 85 MPa and 207 MPa in the as deposited state to 137.2 HV, 245 MPa and 321 MPa after heat treatment, respectively.

Abstract:In this study, a variety of energy fields were applied to GH4169 alloy, and transmission electron microscope/scanning electron microscope were used to characterize the microstructure of the alloy, so as to explore the evolution law and action mechanism of microstructure under the action of multi-field coupling. The results show that, compared with the conventional temperature field/stress field, a large amount of granular δ phase is precipitated in the grain boundary of GH4169 alloy under the coupling action of temperature field/stress field/pulse current energy field, and the size of γ "phase in the grain increases. The reason is that the introduction of pulse energy field significantly enhances the dislocation motion and the diffusion ability of atoms, which is conducive to the growth of precipitated phase.

Abstract:To explore the size effect on the yielding behavior of ultrathin superalloy sheets with different thicknesses and grain sizes, the experimental yield loci were obtained via uniaxial tensile and biaxial tensile tests under different loading ratios. The results indicated that the yield locus of ultrathin superalloy sheet shrink inward entirely with the increase of grain size, and the shape of yield locus is also changed from ellipse to square, indicating the grian size dependence of yield locus. In addition, the four typical macroscopic yield criteria were selected for comparative analysis, and results showed that the Yld2000-2d yield criterion keeps the best precision for describing the yielding behavior of ultrathin superalloy sheet. However, the prediction accuracy is degraded with the increase of grain size.

Abstract:Dynamic recrystallization (DRX) affects the microstructure, texture evolution and final mechanical properties of zirconium alloys during hot rolling. In this study, hot compression tests are performed at temperatures between 550 ℃and 700℃ with strain rates between 0.01s_^-1 and 10s_^-1 to investigate dynamic recrystallization behavior of Zr-1.0Sn-1.0Nb-0.1Fe. Critical strains for initiation of dynamic recrystallization and peak strains are identified through the analysis of work hardening rate from measured stress-strain results. Dynamic recrystallization is identified by the softening in the flow stress during plastic deformation and quantified as the difference between a calculated dynamic recovery curve and the measured stress-strain curve. Dynamic recrystallization is modeled using calculated critical strain, peak strain, Zener-Hollomon (Z) parameter, and volume fraction of dynamic recrystallization. Finally, the dynamic recrystallization model of Zr-1.0Sn-1.0Nb-0.1Fe was verified by flow stress curves and volume fraction of dynamic recrystallization.

Abstract:To optimize the process of hot rolling for nickel-section austenitic stainless steel with large deformation, the isothermal hot compression tests of 1Cr14Mn10Ni1.5 stainless steel were carried out on Gleeble-3500 thermal simulation system at temperatures of 950 - 1250℃, strain rates of 0.01-5.0 s-1, and strains of 0.36, 0.69 and 0.92. The 3D hot processing maps were established based on the strain effect. The thermal activation energies under three strains were calculated by using the Arrhenius type constitutive equation. The evolution behavior of the hot processing map under the strain effect was analyzed by combining it with the microstructure. The results show that when the true strain increases from 0.36 to 0.69 and 0.92, the thermal activation energy Q decreases respectively from 501kJ/mol to 427kJ/mol and 424.86kJ/mol, indicating that the rate of dislocation multiplication and generation is lower than the rate of dislocation movement and annihilation in the strain range of 0.36-0.69. The hot processing map shows that the peak and valley regions change with the increasing strain, mainly in the direction of low temperature and high speed, which is caused by the increase of the total energy of strain input. There are three peak regions in the hot processing map of the experimental steel, only 0.69 strain, under the conditions of 1175-1225℃, 1.0-5.0 s-1 can reach the maximum processing efficiency of 38%, which is related to the temperature rise at high speed. As the strain increases to 0.69 and 0.92, the instability region extends first and then shrinks. The stress-strain curves and microstructure show that the softening mechanism of the high efficiency region is dynamic recrystallization(DRX), while the unstable region is characterized by discontinuous dynamic recrystallization(DDRX) and dynamic recovery(DRV).

Abstract:A stepwise acid dissolution method to extract non-metallic inclusions accurately in nickel-based superalloy powders for quantitative characterization is proposed in this work. The superalloy powder is firstly dissolved by the mixed concentrated acid with the optimal ratio of HCl:HNO3=6:1, and the acid soluble products and insoluble metal elements are then completely dissolved by using the H2O2-oxalic acid system and dilute acid. The quantity, morphology, composition, and size distribution of the acquired non-metallic inclusions are analyzed by SEM, EDS and other characterization methods. The results indicate that this extraction method has the advantages of simplicity, short time, high separation efficiency, high recovery rate, and less influence on the inclusions. It is suitable for extracting a wide range of inclusion particle size. In addition, the extraction effects of the stepwise acid dissolution method and the water elutriation method are compared. It is found that both methods have similar extraction effects for the inclusions with a particle size greater than 50μm, however, the stepwise acid dissolution method exhibits significantly improved extraction effects for the inclusions less than 50μm.

Abstract:The effects of semi-solid (quenching or air cooling) and hot extrusion on the microstructure and mechanical properties of Mg-3.88Zn-1.56Gd (wt.) alloy were studied. The results show that after semi-solid quenching, the second phase presents nano eutectic lamellar structure in the alloy, while after semi-solid air cooling, the second phase presents trigeminal island, block and micron lamellar structure in the alloy. After secondary extrusion, the semi-solid + extruded alloy presents typical bimodal structure due to incomplete recrystallization. Compared with the non semi-solid alloy, the semi-solid + extruded alloy exhibit higher strength. The main reason for the performance enhancement of the alloy is due to the formation of bimodal structure. The fine grains provide the grain boundary strengthening, while the coarse grains without recrystallization produce the strengthening effect by inhibiting the base plane slip. The second important reason is the refinement of quasicrystal phase and its dispersion in the matrix.

Abstract:The porous structure of triply periodic minimal surfaces (TPMS) has been studied extensively, but the porous structure of deformed TPMS is less studied, and the porous structure of deformed TPMS has potential advantages in the mechanical properties in a certain direction. The parametric design method of the porous structure of the Gyroid cell is studied, and the normal and deformed porous titanium alloy samples of the Gyroid cell with the porosity of 60% and 75% are prepared by the selective laser melting molding technology (SLM). The morphological characteristics of the sample were observed by Micro-CT, and the internal connectivity was good, and no obvious structural fracture and pore blockage were found. The Instron electronic universal material testing machine was used to carry out the mechanical compression test. The results showed that the compressive strength of the porous structure of the deformed Gyroid unit with a porosity of 60% increased by 49.3% compared with the porous structure of the normal Gyroid unit, and the elastic modulus increased by 63.5%; When the porosity is 75%, the compressive strength increases by 40.5% and the elastic modulus increases by 70.5%. The research results show that under the same porosity, the deformed Gyroid unit structure of the long axis in the compression direction has better mechanical properties.

Abstract:The undercooling behavior of 3 single crystal（SC）superalloys DD5, DD6 and WZ30 was investigated under the industrial process condition. During isothermal heating and cooling processes the liquidus temperature TL and the critical nucleation temperature TN of the alloys were measured. As the undercoolability the average critical nucleation undercooling ?TN = TL-TN was respectively determined to be 33, 36 and 42K for the 3 alloys. In the samples of alloy DD5 and DD6 with relatively low ?TN value, very fine dendritic microstructure was formed during isothermal cooling processes. In the sample of alloy WZ30 with relatively high nucleation undercooling, the solidification structure became a mixture of SC dendrite frame and grain clusters with random orientations. It appears that, the most random oriented grains were equiaxed crystal formed directly from the interdendritic residual melt, instead of dendrite arm fragment owing to remelting.

Abstract:The mechanical property tests of near β-type Ti55531 titanium alloy after solution (820 _^oC/2 h) plus aging (580~640 _^oC/6~10 h) treatments were carried out on INSTRON-5948R micro tensile machine. The effects of solution plus aging treatment on microstructural evolution and mechanical properties of Ti55531 alloy were studied to obtain a better combination of ultimate tensile strength and ductility. Furthermore, the fracture mechanism of the alloy during tensile test was discussed. The results show that the secondary lamellar α phase is more sensitive to the change of aging parameters than the primary α phase. and the thickness of lamellar α phase is linearly positively correlated with aging temperature or time. Compared with aging time, the coarsening rate of the secondary lamellar α phase is less sensitive to aging temperature, and the coarsening rate of the secondary lamellar α phase with aging temperature and aging time is about 1 nm/oC and 8 nm/h, respectively. After solution and aging treatment, the mechanical properties of Ti55531 alloy are significantly improved, which reach the best comprehensive mechanical properties with solution treatment at 800 _^oC for 2 h plus aging at 640 _^oC for 8 h. Under this condition, the ultimate strength is 1144 MPa, the elongation is 8.16%, and the strength-ductility product exceeds 9.3 GPa .%. The tensile fracture modes of Ti55531 alloy are ductile and brittle mixed fracture, and mainly ductile fracture, including intergranular cracking and microvoid coalescence mechanisms.

Abstract:In order to effectively improve the comprehensive high temperature mechanical properties of GH3230 superalloy, the samples of GH3230 was formed by selective laser melting technology, and the solid solution treatment was carried out according to the optimized heat treatment system. The microstructure of the alloy before and after solution treatment was analyzed, the high temperature tensile mechanical properties of the alloy were tested, the influence of the morphology and distribution of precipitated carbides on the high temperature tensile mechanical properties was studied and the mechanism of high temperature tensile fracture was discussed. The results show that the microstructure of GH3230 superalloy is composed of single γ solid solution columnar crystals with the same growth direction as the material stacking direction. After solution treatment, M₆C type fine carbide particles with chain distribution are precipitated along the grain boundary of γ solid solution columnar grains, and M₆C type ultrafine carbide particles with dispersion distribution are precipitated inside the columnar grains. The columnar grains become coarser, and the grain orientation difference decreases, showing a trend of equiaxed grain transformation. The degree of anisotropy of high temperature tensile mechanical properties is weakened. Because the microstructure is still columnar grain with directional solidification characteristics, the mechanical properties of high temperature tensile in different directions are still different. Both longitudinal and transverse tensile fracture mechanisms are intergranular ductile fracture.

Abstract:Silicon tetraboride (SiB4) is one of the indispensable functional filler of current high-emissivity thermal protection coating, which protects hypersonic aircrafts with speed of Mach number 5 and above from extremely high temperature environment. With the increase of reusable launch vehicles’ (RLV) speed, nowadays the demand of coating service safety and reliability has also been put forward for higher requirements. It is very of significance to understand the oxidation model and kinetic of SiB4, hence, so that can make further improvements of coating properties. The oxidation kinetics of SiB4 powders with particle size of 40 μm in air flow at temperature up to 1300 ℃ was investigated using non-isothermal analysis at 5 ℃/min, 10 ℃/min and 20 ℃/min, and the Flynn-Wall-Ozawa (FWO) method also been utilized to calculate the activation energy Ea, the preexponential factor A and the oxidation kinetic model function. The results show that starting temperature of oxidation reaction of SiB4 powder was about 650 ℃, and TG curves had three distinguishable stages in the temperature ranges of < 650℃, 650-1000℃ and 1000-1300 ℃ accompanying a trend of mass constant → mass gaining → mass constant. There were two reasons, which were the protecting of SiO2-B2O3 glass and the competitive effect between volatilization of B2O3 and mass gaining for oxidation of SiB4, respectively, for the mass constant of SiB4 samples at high temperature stage. And there was an obviously connection between the heating rates and SiB4 oxidation that the faster temperature rises, the more obvious the exothermic effect. The activation energy of 40 μm silicon tetraboride was 239.14 kJ/mol and the preexponential factor was 2.1901×104 K/s. The conversion function of oxidation reaction of SiB4 was G(α)=ln[-ln(1-α)]^1.7574.

Abstract:NiCrAlY+YSZ thermal barrier coatings (TBCs) were deposited on HA188 alloy using air plasma spraying technique. The cyclic oxidation behavior of TBCs was studied at 1100 oC, 1120 oC and 1150 oC high temperature. The results indicated that TBCs lifetime significantly decreased with the increasing of thermal cyclic temperature. The failure of TBCs was attributed to the formation and propagation of crack in YSZ layer near the YSZ/NiCrAlY interface. XRD analysis indicated that the phase composition of YSZ top coat had no change during all thermal cyclic process, and kept the same metastable tetragonal-prime (t"-ZrO2) structure. It was proposed that thermally grown oxide (TGO) growth following a parabolic law had an important effect on the formation and propagation of crack in YSZ.

Abstract:CoCrNi medium entropy alloy powder was prepared by mechanical alloying, in combination with discharge plasma sintering or high vacuum sintering, the bulk was prepared. The effects of milling time and annealing on the morphology, particle size and phase structure of CoCrNi medium entropy alloy powder were studied, The microstructure and mechanical properties of the alloy blocks prepared by different sintering methods were studied. The results show that: with the extension of milling time, the particle size of each elemental powder decreases and gradually merges. After milling for 25 h, the raw powder is mainly FCC solid solution structure, with a small amount of BCC phase; In the subsequent sintering process, a small amount of BCC phase changes, and only FCC phase structure exists in the structure. The elastic modulus of the annealed sample is 6.57 GPa, which is 1.55 times of that of the vacuum sintered sample. The yield strength of the annealed sample is 279.28 MPa, which is equivalent to that of the vacuum sintered sample. The elongation of the annealed sample is 35.97%, which is significantly higher than that of the direct vacuum sintered sample; The results showed that SPS sintered bulk alloy has a yield strength of 793.72MPa, a plastic strain of 61.08%, and a Vickers hardness of 399HV. Compared with the other two sintering methods, SPS has more potential in realizing rapid low temperature sintering of HEAs.

Abstract:Tungsten, with excellent properties, has become one of the candidate materials for plasma facing materials in nuclear fusion reactor. During the operation of nuclear fusion reactor, the transmutation element Re generated by neutron irradiation of tungsten, which will continue to produce and accumulate in tungsten to form the transmutation product tungsten-rhenium alloys. Therefore, the thermodynamic parameters and thermal loading resistance of tungsten plasma facing materials have changed, which will affect the service properties of plasma facing materials, and even related to the stable operation of the reactor. At present, the generation of fusion high-energy neutrons under laboratory conditions is limited. Therefore, the research on the transmutation product tungsten-rhenium alloys is mainly based on the tungsten-rhenium alloys prepared in the laboratory. In this paper, the major preparation processes and thermal loading behavior of tungsten-rhenium alloys at present are summarized, and the existing scientific problems in the thermal loading behavior of tungsten-rhenium alloys are analyzed, which provides reference for the application of tungsten as plasma facing materials in future nuclear fusion reactor.

Abstract:As one of the widely used methods of material joiningSin manufacturing industry, brazing is widely used in the fields of medical treatment, power electronics and automobile, etc. Among many joining methods, the brazing is the most effective and promising way of the joining of ceramic/metal dissimilar materials. In this paper, the investigations on the joining of ceramic/metal dissimilar materials in recent 20 years are reviewed in detail. Firstly, the research statusSof ceramic/metal brazing joint is summarized. Secondly, the research progress of ceramic/metal dissimilar brazing joint is reviewed in detail from four kinds of ceramic materials, including oxide ceramics, carbide ceramics, nitride ceramics and ceramic matrix composites, as well as five kinds of brazing methods including active metal brazing, air reaction brazing, contact reaction brazing, glass brazing and ultrasonic assisted brazing. And then the applications of the ceramic/metal dissimilar brazing joint in medical treatment, power electronics and automobile are introduced. At last, the limitations in the research and development of ceramic/metal dissimilar brazing technology are pointed out, and then the developmentStendency of ceramic/metal dissimilar brazing technology is expected. This aim of this paper is to provide the theoretical basis and technical support for the investigations of ceramic/metal dissimilar brazing technology based on the summary of relevant researches.

Abstract:The metal additive manufacturing technology has the intrinsic characteristics of "micro-area super-metallurgy" and "rapid cooling solidification . Realizing the dynamic monitoring and control of the evolution of defects, stress and organization in the additive manufacturing process is currently the difficulty and hotspot of the international frontier research in this field. In this paper, starting from the key common problems of metal additive manufacturing in the fields of aeronautical space and automobile, the research progress of metal additive manufacturing, based on two methods of synchrotron radiation and neutron diffraction, showed in the following aspects: metallurgical dynamics in metal additive manufacturing and in-situ analysis of its internal defects, superconventional solidification of liquid metal, microstructure and phase transition process, formation and evolution of internal stress. Subsequently, the deficiencies of the research progress were explained, and the future development of metal additive manufacturing was prospected.

Abstract:Detonation experiments were carried out with three precursor gases (CH4-2O2、H2-0.5O2 、C2H2-2.5O2). The detonation characteristics of premixed gas were characterized by the measured detonation pressure, velocity and cell size. Based on the measured detonation parameters, a detonation duct with length of 2m, inner diameter of 80mm, wall thickness of 8mm and ignition energy of 40J is designed independently. Nano titanium dioxide was synthesized by gas-liquid detonation with hydrogen, oxygen and titanium tetrachloride as mixed precursors in the independently designed detonation duct. The products were characterized by TEM. The results show that the spherical or quasi spherical particles of the obtained nano titanium dioxide are about 20-150 nm.

Abstract:Electron beam welding was carried out on TC4/TC17 dissimilar titanium alloy, and tensile tests were carried out at 400℃. The results show that there were significant changes in heat-affected zone and weld microstructure compared with base material with the action of welding thermal cycle. The microstructure of heat-affected zone on TC4 side was martensite. The heat-affected zone of TC17 side was metastable β phase, and coarse columnar crystals were observed at the weld. The hardness of TC17 base material was higher than that of TC4 where the hardness of weld center was the highest. The tensile strength of TC17 base metal was the best at 400℃，meanwhile the tensile strength of welded joint was similar to that of TC4 base metal. The fracture surface of TC4/TCI7 electron beam welded joint was characterised by large size, few and shallow tough nests, which were oval in shape and showed good plasticity.