Abstract:The chemical compatibility between Li₄SiO₄ pebbles and ODS steel is important for the safe operation of the fusion reactor. In the present paper, the transformation of microstructure and component for contact interface between ODS steel and pebbles after 300 h heat preservation in an argon atmosphere at 500, 600, and 700 °C was investigated. The results show that serious element interdiffusion and reaction can be observed at the interface between Li₄SiO₄ pebbles and ODS steel at temperatures of 600~700 °C. For the surface of Li₄SiO₄ pebbles, a thin reaction layer appears due to the diffusion of Fe and Cr from ODS steel, which also causes the increase of density and decrease of crush load from 51 N (500 °C) to 32 N (700 °C). XRD patterns show that the new phase of LiCrO₂ and LiFeO₂ appears on the surface of ODS steel, which suggests that the Li and O atoms in Li₄SiO₄ pebbles can diffuse into ODS, and react with Fe and Cr elements to form corrosion layers at high temperature. The corrosion layers can be divided into two oxide sub-layers at 700 °C. The outermost layer is a mixture of LiFeO₂ and LiCrO₂, and the next layer is mainly LiFeO₂. For the surface of ODS steel, the oxygen diffusion coefficient is 2.2×10^-14 cm²/s at 700 °C for 300 h. It suggests that the ODS steel as a blanket structure material steel needs a resisted corrosion coating in the design of a blanket in the future.

Abstract:The fully-coated TiB₂-SiC coating was prepared by supersonic atmospheric plasma spraying (SAPS). The oxidation performance of TiB₂-SiC coating at 400 and 800 °C was studied and the oxidation mechanism was investigated. The corrosion resistance of TiB₂-SiC coating to aluminum melting salt at 900 °C was studied, and the anti-corrosion mechanism of molten salt was discussed. The results show that the TiB₂-SiC coating prepared by SAPS has good anti-oxidation performance. The oxidation rate constant at 400 °C is 1.92×10^-5 mg²·cm^-4·s^-1, and that at 800 °C is 1.82×10^-4 mg²·cm^-4·s^-1. The TiB₂-SiC coating prepared by SAPS has good resistance to molten salt corrosion at 900 °C. TiB₂-SiC coating maintains a dense structure after molten salt corrosion, and cracking and peeling of the coating do not occur.

Abstract:ZrB₂-ZrC-SiC coating with micro-submicron reinforced structure was prepared on the C/C composites by vacuum infiltration and reactive melt infiltration. The micro-submicron reinforced structure consisted of micron-sized SiC as the skeleton and submicron-sized ultra-high temperature ceramic particles (UHTCs) as the filler. The ablation resistance of the composites was tested by plasma flame. Results show that during ablation, the micro-submicron reinforced structure composed of concentrated SiC particles and UHTCs is prone to form larger defects under the erosion of ablative airflow. The defects induce ablation gaps that are connected with the surrounding cracks to form larger pits, which cause part of the coating to fall off, and the failure of the whole coating system.

Abstract:In order to improve the oxidation resistance and wear properties of AISI 304, a TiC-MoSi₂ complex phase enhanced composite coating was designed and in-situ prepared on AISI 304 substrate by plasma transferred arc (PTA) cladding technique. Microstructure of the composite coating before and after oxidation was analyzed. The hardness distribution of the coating was tested. The oxidation kinetics curves of the coating were measured and fitted. The oxidation mechanism of the coating was discussed. Results show that typical microstructure of the composite coating consists of TiC-MoSi₂ complex phases, primary TiC dendrites and γ-(Ni, Fe)/NiSi₂ eutectics. The TiC-MoSi₂ complex phases and TiC dendrites as reinforcing phases are uniformly distributed on the γ-(Ni, Fe)/NiSi₂ eutectic matrix. Due to the strengthening effect of the TiC-MoSi₂ complex phases and the binding and supporting action of the ultrafine γ-(Ni,Fe)/NiSi₂ eutectic matrix, the composite coating exhibits high and uniform hardness distribution, good strength and toughness. Thanks to its unique microstructure, the composite coating shows good oxidation resistance.

Abstract:Copper-based coatings were prepared on the surface of Q235 steel by fiber laser under different powers. Microstructure, phase composition, corrosion resistance of the coatings were revealed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and electrochemical workstation, respectively. Results show that the coatings are well bonded to the substrate. The coating microstructure consists of cellular, coarse dendrites, fine dendrites and columnar grain structure from the bottom to the top. The coatings mainly contain two phases including Cu(Ni,Fe) and Ni(Cr,Mo,Fe) solid solution. The coating with two layers prepared under 2000 W exhibits optimal corrosion resistance due to its highest corrosion potential and lowest corrosion current density.

Abstract:Agglomerated YSZ (Y₂O₃ partially stabilized ZrO₂) particles with 0wt%, 1wt%, 2wt% and 3wt% of whiskers (named as 0#,1#, 2# and 3# powders, respectively) were prepared by spray granulation technology. Four groups of coatings (0#, 1#, 2# and 3# coating) were prepared by APS (atmospheric plasma spraying). The dispersion and quantitative characterization of SiC whiskers, morphology of agglomerated particles and microstructure of the coatings were studied by SEM (scanning electron microscopy) and light-optical microscopy, and the forming process of the coating containing SiC whiskers was analyzed. The result shows that as the stirring time increases to 5 h, the whiskers show better dispersion and the corresponding area percentage of whiskers of the specimen is 11.03%. The particles containing whiskers are mainly “droplet shaped” and “spindle shaped”, and the percentage of these two streamlined particles in 1#, 2# and 3# powders is 16.5%, 22.7% and 39.3%, respectively. Due to the obstructing effect of non-horizontal state whiskers on impacting and spreading process of unmelted raw powders, the porosity increases as the whiskers content increases. The porosity of 0# coating is 3.89%, while the porosity of 1#, 2# and 3# coatings is 3.15, 4.17 and 7.52 times larger than that of 0# coating, respectively.

Abstract:The solid lubrication films with high hardness and low friction coefficient should be developed to satisfy the severe working conditions of mechanical rotating parts of spacecraft. The pure WS₂ film, Ti doped WS₂ composite film (Ti/WS₂) and La-Ti doped WS₂ composite film (La-Ti/WS₂) were prepared by unbalanced magnetron sputtering. The micro morphology, composition, hardness and tribological performance of the films were examined. Results show that compared with pure WS₂ films and Ti/WS₂ composite films, the microstructure of La-Ti/WS₂ composite film becomes more compact. Meanwhile, the hardness and elastic modulus of La-Ti/WS₂ composite film also increase significantly. Furthermore, the friction coefficient of La-Ti/WS₂ composite film decreases, and the hardness/elastic modulus (H/E) ratio of La-Ti/WS₂ composite film increases, which suggests that the wear rate of La-Ti/WS₂ composite film is reduced. It is indicated that La doping contributes to the formation of stable transfer film on the friction contact surface, and thus improves the wear resistance and carrying capacity of La-Ti/WS₂ composite film.

Abstract:The structure and properties of nanocrystalline TiN films deposited by direct current magnetron sputtering (dcMS), high power pulsed magnetron sputtering (HPPMS) and modulated pulsed power magnetron sputtering (MPPMS) were compared. Results show that columnar structure with a few gaps is obtained through dcMS because of low ionization rate and low kinetic energy of sputtered species, which results in poor mechanical properties; the deposition rate is 51 nm/min. The TiN film deposited by HPPMS exhibits dense structure and smooth surface, which is because HPPMS can improve ionization rate of sputtered species under the conditions of high peak target power and low duty cycle. The mechanical properties are improved, but the average deposition rate is relatively low, only 25 nm/min. MPPMS has the capability to modulate peak target power and duty cycle to achieve high ionization degree and deposition rate. Thus, the TiN film deposited by MPPMS shows dense columnar structure, smooth surface, superior mechanical properties and enhanced deposition rate of 45 nm/min.

Abstract:Different textures are formed in different thickness zones of CR340 tailor rolled blank (TRB) during rolling, including {111}<01> and {141}<22> textures in the thin zone, {225}<10> and {211}<01> textures in the transition zone, and {876}<5> and {411}<01> textures in the thick zone. The polycrystalline plastic finite element model in different thickness zones was established based on the results of EBSD test to study the influence of grain structure in different thickness zones on the activation of slip system and stress-strain distribution under uniaxial tension. The results show that the {111}<01> texture of thin zone and {876} <5> texture of thick zone are conducive to slip system activation, and 8 and 9 sets of slip systems are activated, respectively, which weaken stress concentration of equal thickness zone in the deformation process, so equal thickness zone has good plastic deformation behavior. However, the grain slip system of {225}<10> and {211}<01> textures in transition zone is less activated, and the number of activation is just 6 and 7, respectively, which lead to the high stress concentration, and thus the plastic deformation behavior is poor. The TRB presents obvious non-uniformity in plastic deformation because of the difference in texture of each thickness region, and the fracture position of the TRB appears in the transition zone with poor plastic deformation during uniaxial tension.

Abstract:A first-principles calculation was applied to study the adsorption behavior of CO₂ on δ-Pu(100) surface using a slab model within the framework of density functional theory. Results demonstrate that CO₂ molecules are adsorbed on δ-Pu(100) surface at C-terminated bent state by multi-bond binding of C-Pu and O-Pu. The adsorption type belongs to strong chemical adsorption and the most stable adsorption configuration is H₁-C₄O₄ with the adsorption energy of -6.430 eV. The adsorption stability order is hollow site>bridge site>top site. CO₂ molecule mainly interacts with Pu surface atoms, while the interaction with other three Pu atoms is weak. The transfer of more electrons to the CO₂ 2π_u orbital is beneficial to the bending and activation of C-O bonds. Moreover, the chemical bonding between Pu atoms and CO₂ molecule is mainly ionic state and the reaction mechanism is that the C 2s, C 2p, O 2s and O 2p orbitals of CO₂ molecule hybrid with Pu 6p, Pu 6d and Pu 5f orbitals, resulting in a new bond structure. The work function of the H₁-C₄O₄ site changes the least, indicating that other electrons readily escape from the metal surface and the required energy is the smallest.

Abstract:A novel local hot gas forming (LHGF) process with non-uniform temperature field was proposed to form AZ31 magnesium alloy long axis bellow. Hot uniaxial tensile tests at temperatures ranging from 573 K to 673 K and strain rates ranging from 0.001 s^-1 to 0.1 s^-1 were carried out to study the hot deformation behavior of AZ31 magnesium alloy. Forming apparatuses with independent heating and cooling facilities were developed to achieve the non-uniform temperature field. Local hot gas forming tests of single wave bellow were performed to study the effects of temperature and gas pressure on the forming process and to determine the proper processing windows. A long axis bellow with 5 waves was finally formed to validate the novel process. Results show that the maximum temperature in the cooling zone can be kept below 50 °C and the temperature in the heating zone can be accurately con-trolled with fluctuation less than ±5 °C. A qualified AZ31 magnesium alloy long axis bellow with 5 waves is formed with small dies at 623 K under a constant gas pressure of 14 MPa. The average grain size at the wave crest is refined from 21.8 μm in the initial to 16.56 μm after forming due to the dynamic recrystallization during the forming.

Abstract:The close-packed hexagonal structure pure magnesium has few independent slip systems which result in poor plasticity. In the present paper, the pure magnesium was deformed by equal channel angular pressing (ECAP). Results show that due to grain refinement and weakening of base texture, plasticity of pure magnesium is significantly improved. The strength of pure magnesium decreases after ECAP deformation mainly because the influence of base texture weakening is greater than that of grain refinement. Moreover, the corrosion resistance of pure magnesium is significantly enhanced after shear stress deformation because of the increase of self-corrosion potential and corrosion current density. The corrosion mechanism of pure magnesium may change from local corrosion to uniform corrosion, which can reduce the corrosion falling and ensure the integrity of samples during immersion in the standard simulated body fluid.

Abstract:A novel porthole extrusion process combined with corner extrusion principle for AZ91 pipe was presented. This new extrusion process can not only extend the length of the welding chamber, but also increase the rigidity of the latch needle, thereby ensuring the dimensional accuracy of pipe. Meanwhile, this new extrusion method can increase the deform degree and improve the dynamic recrystallization of the pre-welded metal, which can improve the welding quality and pipe quality. Then, the metal flowing characteristic, distribution feature of effective strain and mean stress in the corner porthole chamber were revealed; the results show that the effective strain of the separated metal becomes larger after flowing through the corner of the welding chamber, which can promote the quality of the welding seam. And the mean stress in the welding chamber is larger than 240 MPa, which satisfies the welding pressure condition. The influence rules of extrusion speed, die angle and billet preheating temperature on the mean stress in the welding chamber were also discussed; the result shows that the higher extrusion speed and the higher preheating temperature of the billet can enhance the mean stress in the welding chamber, which can improve the welding quality. The larger die angle can lead to higher mean pressure.

Abstract:The influence of B and Y on the microstructure, microsegregation, and tensile behaviour of the Ti45Al8Nb0.2W0.25Cr (at%) alloy was investigated. The β-stabilizer elements in the high-Nb TiAl alloy promote the formation of the γ phase in the microsegregation region and lead to the formation of large blocky microsegregation areas. The large blocky microsegregation regions with low specific surface areas reduce the nucleation rate of cavities and cracks at the interfaces of the microsegregation, which are harmful to colony boundary strengthening and decrease tensile resistance. The addition of B and Y affords an obvious refinement in the lamellar colony, renders an increasing opportunity for cavity nucleation at the colony boundary, and thus improves the tensile resistance. The tensile mechanisms of the alloys before and after (B, Y) addition were also compared and analysed.

Abstract:Crack-free TiAl joints were welded by linear friction welding with online stress relieving process. Microstructural examination demonstrates that the joint has a clearly identified weld zone which consists mainly of equiaxed γ grains with a few α₂ phase. Lamellar grains in thermo-mechanically affected zones near the weld margin are oriented along with the flow direction of the plasticized material. The microhardness of the joint presents a global increase from the base metal to the weld interface by about 1700 MPa; the tensile strength of the joints ranges from 683 MPa to 717 MPa at ambient temperature, which is comparable to that of the base metal. The refinement strengthening contributes to enhancing the joint strength.

Abstract:Homogenization treatment for Al-Zn-Mg-Cu alloy with Ho addition was investigated by scanning electron microscope (SEM), optical microscope (OM), and differential scanning calorimeter (DSC). Electrical conductivity and micro-hardness were tested during different homogenization processes. The results show that there are four kinds of second phases in as-cast alloy: T(AlZnMgCu), Al₇Cu₂Fe, Al₈Cu₄Ho and S (Al₂CuMg), which cause severe micro-segregation in microstructure. After homogenization treatment at 475 °C for 20 h, T phase is completely dissolved back into matrix and no S phase is observed, with Al₇Cu₂Fe and Al₈Cu₄Ho remaining. Micro-hardness and electrical conductivity are associated with vibration of T phase, dissolution of which can increase micro-hardness and decrease electrical conductivity. Besides, precipitation of Al₃Ho also contributes to the increase in both micro-hardness and electrical conductivity during homogenization at 475 °C for 5~20 h. Finally, through homogenization kinetic analysis, the appropriate homogenization parameter is determined as 470~475 °C/20~25 h.

Abstract:The coefficient of friction (COF) of a novel bionic trabecular structure that was manufactured through 3D printing with Ti-6Al-4V power was investigated. Cortical and spongy bones were used as counterparts. The contact angle results show that the bionic structure has better surface wettability than the homogeneous structure at the early-stage. However, the two structures have highly hydrophilic surface. For homogeneous trabeculae, the average values of COF range from 0.71 to 0.82, which are significantly lower than the values of 0.82~0.99 for the bionic trabeculae. The static COF values range from 1.52 to 1.96 for the bionic trabeculae and 0.99 to 1.26 for the homogeneous trabeculae. The maximum static COF value of 1.96 is acquired at 10 N for the cortical bone against the bionic trabeculae. The results indicate that the bionic trabecular structure has a higher COF than the homogeneous trabeculae and other components. Hence, the novel bionic trabecular structure can provide sufficient friction at the bone-implant interface, thus achieving primary stability.

Abstract:Fe_0.5MnNi_1.5CrNb_x (x=0, 0.05, 0.1, molar ratio) high-entropy alloys were prepared by vacuum induction melting. The effect of Nb content on the microstructure and mechanical properties of the new alloy was analyzed. The results show that the Nb-free alloy has a single-phase fcc structure, and its tensile strength and elongation to failure (i.e., ductility) are 519 MPa and 47%, respectively. With the addition of Nb (x=0.05), the (200) texture and Fe₂Nb Laves phase appear. The ductility of alloy increases to 55%, while the tensile strength increases to 570 MPa. When the molar ratio of Nb increases to 0.1, the texture diminishes, whereas the volume fraction of the Fe₂Nb phase is increased. The resultant tensile strength and ductility are 650 MPa and 45%, respectively.

Abstract:In order to study the low-cycle fatigue behavior of a new powder superalloy A3, fatigue tests with total strain amplitudes from 0.6% to 1.4% at 700℃ were conducted. Fatigue test results were analyzed including cyclic stress-strain responses, Massing characteristics and strain-life curves. The fatigue life data were fitted by Manson-Coffin equation, Ostergren model and three-parameter energy method. The accuracy of fatigue life prediction obtained by Manson-Coffin equation is optimum. The micro-mechanisms were studied through scanning electron microscopy (SEM). As the strain amplitude gets larger, the number of fatigue sources increases, and the location of the fatigue source changes from the core of the sample to the surface, and the area of the crack propagation decreases while the ductile fracture zone increases. The observation of these three typical areas of fatigue fracture shows that light white river-like lines were found around the fatigue origin in the crack initiation zone which direct to crack propagation. Secondary cracks were found in the crack propagation zone. Dimples and cleavage fracture characteristics were found in the ductile fracture zone. The fatigue fracture mode of A3 alloy is mainly trans-granular.

Abstract:In order to study the effect of steady magnetic field on Fe element distribution in laser cladding Inconel 718 coatings. A three-phase mixed solidification model based on volume average method was established. The steady magnetic field was coupled into the laser cladding process to simulate the preparation of Inconel 718 coatings on 316L stainless steel. The 2D transient model, combined with heat and mass transfer, fluid flow and Induced Lorentz force, was used to simulate the distribution of element in the laser cladding coatings and compared with the experimental results. The results show that the Fe element content in the cladding layer is uniform when no magnetic field is applied; However, the Fe element content in the cladding layer is uneven when the steady magnetic field is applied, the Fe element content in the top of the cladding layer is lower than that in the bottom of the cladding layer. The simulation results are in good agreement with the experimental results, which proves the reliability of the calculation model. It also shows that the steady magnetic field can inhibit the flow of the molten pool, affect the heat and mass transfer of the moving molten pool, and ultimately change the element distribution in the laser cladding coating.

Abstract:An experimental set-up was designed for measuring the self-field losses of Bi2223/Ag HTS tapes using a transport current method. Ti/Al was introduced into U-5.5Mo alloy by induction melting, and a new alloy U-Mo-Ti/Al is obtained in this way, its microstructures and properties are controlled by quenching and heat treatment of aging. The relationship between the composition, distribution and properties of precipitates was analyzed, the effect of Ti/Al alloy elements on the mechanical properties of U-5.5Mo alloy was investigated as well. It is found that added trace element Ti has obvious effect of solution strengthening on the alloy of U-5.5Mo. The mechanical properties of U-Mo-Ti ternary alloy can be greatly improved by low temperature aging. It can promote the formation of stable multi-component intermetallic compounds rich in titanium and aluminum in U-Mo-Ti-Al quaternary alloy under high temperature by adding trace element Al, multitudes of irregular continuous distribution of the trace element in the matrix and grain boundary have significant precipitation strengthening effect on the alloy, as a result, the strengthen of U-Mo-Ti-Al quaternary alloy is greatly improved while the plasticity almost decreases to zero.

Abstract:In order to realize the characterization and regional division of cyclic strain field at fatigue crack tip, digital image correlation (DIC) method and Irwin model were used to study the crack growth law, experimental division method of cyclic strain field and the evolution law of cyclic strain loops of commercial pure titanium TA2 compact tensile specimen under continuous fatigue loads. Firstly, the fatigue crack growth law of TA2 compact tensile specimen was obtained by the continuous fatigue experiment. On this basis, combining DIC method and Irwin model, the experimental division method of cyclic strain field at fatigue crack tip was established, and the division of cyclic plastic zone, monotonic plastic zone and elastic zone was realized. On the other hand, the hysteresis loops in different regions were obtained by DIC method, and the differences of strain loops in different regions were discussed. Thus the effectiveness of the division method was demonstrated, and the evolution law of strain field at crack tip from elastic zone, monotonic plastic zone to cyclic plastic zone during crack growth was revealed. The research work in this paper provides an experimental method to characterize the cyclic strain field at crack tip for the study of fatigue crack growth behavior, which can meet the needs of the experimental study of fatigue crack growth.

Abstract:To study Al on the properties of ferritic heat resistant steel phase change and the influence law, a new type of high aluminium ferritic heat resistant steel was prepared by adding Al element and adjusting additive Ni on the basis of T92 alloy composition. The relationship of ferritic heat resistant steel phase change temperature under different Al contents and heat treatment processes and quenching organization grain size, hardness and oxidation resistance was studied. The results show that the equilibrium phase transition point of the experimental steel increases with the increase of Al content, and the Ac1 and Ac3 temperature of the experimental steel becomes higher with the austenite grain size becoming larger. When the total amount of Al and Ni elements added in steel is 2.4 wt %, the austenite after quenching grain size is the smallest and the hardness is the largest. Adding 3 wt % Al to steel at 650 °C is more resistant to high temperature oxidation than adding 1 wt % Al. At 750 °C, the modified additive Ni plays a certain role. The larger the Al-Ni ratio, the stronger the oxidation resistance of the steel.

Abstract:The effects of normal load (60 N, 40 N and 20 N) on the tangential fretting wear properties of the TC21 titanium alloy and plasma electrolytic oxidation (PEO) coatings were investigated under displacement amplitudes of 50 μm and 150 μm with a ball-on-plate contact configuration. The results showed that with increase of the displacement amplitude and decrease of the normal load, the fretting wear regime changed from partial slip regime to slip regime. In partial slip regime, the widths of the wear tracks along the fretting direction decreased with the decreasing normal load. Although there was no significant material loss, the extent of micro-cracks initiation and propagation in the micro-slip zone of the TC21 titanium alloy deepened with the decreasing normal load. However, the fretting only played a smooth effect on the PEO coatings. In slip regime, the widths of the wear tracks along the fretting direction increased with the decreasing normal load. There existed local wear in both materials, and the wear degree increased with the increasing amplitude and decreasing normal load. The maximum wear depth of the PEO coatings was shallower than that of the TC21 titanium alloy, which proved that they could improve the fretting wear resistance.

Abstract:Selective laser melting (SLM) technology can realize the direct fabrication of products with complex shape, high dimensional accuracy, and excellent mechanical properties. However, improper selection of fabrication parameters of SLM will introduce defects in the products. For the problem of defects, this investigation studied the influence of the two main fabrication parameters, laser power and scanning speed, on the type, size and quantity of internal defects in fabricated titanium alloy, and explored the evolution of defects with fabrication parameters. The results shown that there are mainly irregular shape and regular spherical shape defects in the titanium alloy fabricated by SLM. The area with low laser power (≤130 W) and high scanning speed (≥900 mm/s) is mainly irregular shape defects. Insufficient energy is the main reason for formation this type defect. The area with high laser power (≥190 W), low scanning speed (≤600 mm/s) is mainly regular spherical shape defects. The gasification of alloying elements caused by excessive energy is the main reason for formation this type defect. With the increasing of laser energy density, the SLM fabricated titanium alloy processing map was drawn according to the evolution law of defects. The evolution of defects presents three stages for the area of irregular shape defects gradually decreasing, micro-scale irregular defects to micro-scale regular spherical, and regular spherical defects gradually grow up.

Abstract:The effect of solution treatment on the microstructure of Mg-Zn-Gf-Y-Zr alloy was studied by OM, SEM and TEM, and the corrosion resistance and mechanical properties of the alloy were tested. The results show that the solution treatment can effectively improve the inhomogeneity structure of the as-cast alloy. The grain size of the alloy increases with the increase of solution treatment temperature in the range of 460-510 ℃, and the second phase dissolves and tends to be spherical. When the solution temperature is greater than 490 ℃, a small amount of Zn₂Zr₃ phase is precipitated and the precipitation phase tends to increase and coarsening with the increase of temperature. The T4-490 sample exhibits uniform microstructure, good corrosion resistance and mechanical properties. The corrosion rate of the weight loss test is 0.472±0.048mm /a, and the tensile strength, yield strength and elongation were 196.2±3.5 MPa, 111.1±6.4 MPa and 18.9±1.3%, respectively. The tensile properties of T4-490 sample after immersed in SBF decreased sharply within 1-7 days, and decreased more slowly within 7-14 days. With the extension of soaking time, the fracture form changes from quasi cleavage fracture to brittle fracture.

Abstract:To overcome AgCuZnSn filler metal with high tin content is difficult to form for its high brittleness, a composite flux-cored silver filler metal with CuSn alloy powder core was designed based on the concept of flux-cored wire. The effects of 30wt%~70wt%Cu60Sn40 alloy powder on the wettability, microstructure, interfacial microhardness and tensile strength of silver filler metal, copper/Q235 steel brazing joint were studied. The results show that AgCuZnSn filler metal with high tin content is in-situ synthesis by CuSn alloy powder in core and BAg30CuZnSn of outer layer during brazing process, and uniform composition with good joint structure was obtained. With the increase of the content of alloy powder in the core, the wetting area of composite flux-cored silver solder on copper plate and Q235 steel plate is promoted constantly, the microhardness of copper/Q235 steel brazing joint is rised continuously, and the tensile strength is increased first and then decreased. When the content of CuSn alloy powder in the powder core is 30wt%, the maximum tensile strength (198.91MPa) of Cu/Q235 steel brazing joint is obtained, which is enhanced by 23.4% compared with the core silver brazing filler metal without alloy powder added.

Abstract:In this work, U720Li castings with different W contents (1.02 wt.%, 0.52 wt.% and 0 wt.%) were prepared by vacuum induction melting and subsequently homogenized. The effect of W on hot deformation behavior and fracture characteristics of as-homogenized U720Li alloy was studied emphatically. The 1000 ℃ tensile test results of homogenized and furnace cooled alloys revealed that with the reduction of W content the deformation resistance decreased gradually, and when the W content was reduced to zero the tensile plasticity was greatly improved and the fracture mode was shifted from intergranular fracture to a mixed-mode of intergranular and transgranular fracture. For both furnace-cooled and air-cooled alloys, with the reduction of W addition the grain-interior microhardness decreased gradually, the size of precipitation strengthening phase γ′ increased and the volume fraction of γ′ decreased markedly. The main reason for the influence of W on deformation resistance and thermal plasticity is that the decrease of W not only reduced the solid solution strengthening, but also modified the characteristic of γ′ which promoting the occurrence of dynamic recrystallization. In addition, the distribution of W in U720Li alloy has been revealed in this paper, i.e., W is mainly enriched in the γ matrix, and the mechanism by which W influencing the precipitation of γ′ was discussed accordingly in the text.

Abstract:In order to solve the problem that Ti6Al4V (TC4) alloy is prone to bacterial infection and release of harmful ions as an orthopedic hard tissue implant during long-term service in the human body, a sol-gel method was used to prepare nano-zinc oxide (ZnO) sol with gradient dynamic heating treatment, then the nano-ZnO coating and ZnO/PDA (polydopamine)composite coating were successfully formed on the surface of TC4. The phase structure, micro morphology, coating adhesion, wettability and corrosion performance of the two coatings in simulated body fluids were systematically analyzed. The result shows that the ZnO/PDA composite coating prepared on the TC4 substrate has a more uniform distribution of nano-ZnO particles than that of a single ZnO coating. The chelating effect of PDA on metal ions promotes the bonding force between the substrate and ZnO through the Ti-O-Zn bond. Simultaneously, the introduction of PDA further improves the wettability of the ZnO coating, reduces its surface roughness, and improves the corrosion resistance of TC4.

Abstract:In order to improve the friction and wear behavior and high-temperature oxidation resistance of titanium alloy, AlCoCrFeMoVTi high-entropy alloy (HEA) were designed and AlCoCrFeMoVTi HEA coatings were prepared by laser cladding technique on Ti6AI4V titanium alloy (TC4) surface. The phase composition and microstructure of AlCoCrFeMoVTi HEA coating were analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The microhardness of coatings was measured by HDX-1000 Vickers hardness tester. The wear resistance and high-temperature oxidation resistance of the coating were tested by UMT-3 friction tester and GSL-1400X tubular resistance furnace. The results show that the AlCoCrFeMoVTi HEA coating is mainly composed of face centered cubic (FCC) structures and body centered cubic (BCC) structures binary eutectic phase. The maximum microhardness of the coating was 1099 HV0.2, which was 3.29 times that of TC4 titanium alloy substrate. The friction coefficient of the coating was 0.31, and the wear volume was 1.79×10-4 mm3, respectively 59.62% and 12.01% of the matrix. After oxidation at 800℃ for 50 h, the oxidation weight of the coating was 1.49 mg, which was only 16.37% of the matrix. Laser cladding AlCoCrFeMoVTi HEA coating can significantly improve the friction and wear behavior and high-temperature oxidation resistance of Ti6Al4V titanium alloy.

Abstract:The novel gas-sensing material of Graphene-loaded PdCl₂ was prepared successfully by the facile method without the use of any additive or organic active agent. The morphology of the as-prepared electrode material was consisted of two different morphologies: thin sheets and small particles with the size of several nanometers. In order to investigate its gas properties, it was used to fabricate a whole solid-state planar electrochemical carbon monoxide sensor. The results showed that the sensor has a practical-use level: response time, 10 s; detect of limit, 100 ppb; sensitivity, 63.34 mV/ppm.

Abstract:In this work, research on the surface morphology, sub-surface damage and tool force in the repeated nanofabrication of monocrystalline nickel was conducted via molecular dynamics. And the conclusion of the theoretical analysis was properly verified by the nano-scratch experiment The paper traced the coordinates of a series of atoms during processing, revealing the formation of grooves on the surface and the reasons for the changes during repeated processing. Through CAN analysis and DXA technology, it was concluded that in monocrystalline nickel nano-processing, repeated processing once will improve the sub-surface quality of the workpiece, but there was basically no difference between repeated processing twice. At the same time, the tangential force and the normal force of the tool in the first machining were obviously larger, and the force of the tool was basically the same in the subsequent two repeated machining. The surface roughness of the single crystal nickel after the nano-scratch experiment was detected by an atomic force microscope, and it was found that repeated processing once would appropriately improve the surface quality of the processed workpiece.

Abstract:Ni-Cr-Cu ternary alloys electrode materials were prepared by activation reaction sintering method with tinplate, Ni powder and Cr powder as raw materials. The phase, constitute morphology and element component of the electrode were characterized by XRD, SEM, EDS. The electrocatalytic hydrogen evolution performance of alloy electrode materials was investigated by cyclic voltammetry curve, linear polarization curve, potentiodynamic polarization curve, electrochemical impedance spectrum. The results show that the Ni-Cr-Cu ternary alloy film electrode has excellent hydrogen evolution performance, and the content of Cr can improve the hydrogen evolution catalytic activity. At room temperature, Ni30wt%Cr-Cu ternary alloy electrode material is 6mol/L. The catalytic activity of hydrogen evolution in KOH solution is the strongest, the hydrogen evolution overpotential is only -0.13 V (vs SCE), the potential is -0.145 V (vs SCE) as the exchange current density of 10mA/cm2, and the open circuit potential (η) of the electrode material is from 0.05 to 0.02 V after 36000 s, which only increased by 0.03 V, indicating that the hydrogen evolution electrode has good chemical stability.

Abstract:W skeleton with high strength (347.8±10.6 Mpa(1450℃), 407.4±14.2 Mpa(1500℃), 543.9±8.7 Mpa(1550℃) and hierarchical pores structure (3-10 m\200-500 nm) was prepared by SPS at different sintering temperatures (1450℃, 1500℃, 1550℃) and at axial constant pressure (40Mpa) for 5 minutes by using the particle size distribution of 15-106 m porous spherical agglomerate W powder as raw material. The densification behavior, microstructure changes and mechanical properties of SPS sintering process were systematically studied by XRD、SEM and universal testing machine. The results show that SPS can produce W skeleton have developed sintered neck and hierarchical pores structure with no significant change in phase before and after sintering and porosity of 30%-40%. The sintering densification process of W skeleton with hierarchical pores structure by SPS can be divided into four stages: the first stage is the pressure increase stage, the relative density of sintered sample increases rapidly with the increase of pressure; the second stage is a typical particle rearrangement stage; the third stage is a typical sintering metaphase, the relative density of sintered sample increases with the increase of temperature, when temperature higher than 1000℃; the fourth stage is the end of sintering, and the densification degree is further improved due to high temperature creep inside the sintered body. The densification mechanism of the porous spherical W powder was determined as Pure diffusion densification by using the high temperature creep model, and further determined volume diffusion as the main mechanism and grain boundary diffusion as the auxiliary mechanism by the neck length equation.

Abstract:The austenitic stainless steel samples containing residual δ ferrite were solution treated at different temperatures. The SEM, EBSD, TEM and hardness tester were applied to investigate the microstructure, texture, hardness and precipitate phase characterization of the experimental samples. The results show that there are three phases of austenite, ferrite and sigma in austenitic stainless steel after solution treatment at 900~1100℃ for 30min. M23C6 and Chi phase are not found. The sigma phase forms in the range of 900~1000℃, which also enhances the hardness of the matrix. Sigma phase is mainly formed by decomposition of residual δ ferrite. The {001}<110> and {001}<100> oriented δ ferrite preferentially transforms to sigma phase. With the increase of temperature, the content of sigma decreases, the average grain size of austenite increases, and the hardness decreases gradually. After 1050℃, sigma phase is completely dissolved into austenite, and the average grain size of austenite grows significantly. This results into the hardness decreases rapidly. Meanwhile,{001}<110> and {001}<100> textures of δ ferrite were re-enhanced。After solution treatment at 1100℃, the content of residual δ ferrite decreased to 0.2%.

Abstract:The LaMgAl₁₁O₁₉ was prepared by air plasma spraying technology to investigate the corrosion behavior of 38CaO-5MgO-8AlO_1.5-49SiO₂ (CMAS) on high temperature. The microstructure, element distribution and phase composition of LaMgAl₁₁O₁₉ coating before and after CMAS corrosion were investigated. The results show that CaAl₂Si₂O₈ and MgAl₂O₄ can be observed in LaMgAl₁₁O₁₉ coating after corroded at 1250 ℃ for 24 h. Besides, according to the thickness of corroded zone in LaMgAl₁₁O₁₉ coating, the apparent activation energy in the temperature range of 1050~1250 ℃ is 94.1 kJ/mol.

Abstract:In this paper, according to the structural characteristics of 140mm thick hollowed-out load-bearing TC4 titanium alloy beam, a double-side and double-pass electron beam welding (EBW) scheme with unequal penetration was proposed. The welding test was carried out and the microstructure and properties of the obtained joint were analyzed. It is found that the microstructure of the weld zone is obviously non-uniform in the direction of thickness, and the overlapping zone at the root of the two welds is the most noteworthy part. There were a lot of needle α′ phase and scattered β phase in the overlapping area of the root of the two welds, while the needle phase was short and the interlacing phenomenon of α′ phase was obvious. The needle α′ phase andα phase distributed in the original β grain boundary were observed in other weld areas of the joint, but the laths were longer and arranged in a more obvious direction. The microhardness of the weld in the overlap zone of the root of the two welds was about 10% higher than that of the other weld zones in the joint (341HV~346HV). At room temperature, the impact energy of the weld in the overlap zone at the root of the two welds was about 11% lower than that of the other weld zones in the joint (about 47J). The area where the roots of the two welds overlap was the area where the joint had the lowest tensile strength. With the aid of finite element simulation, it was found that the cooling rate around 900℃ of the overlapping area of the root of the two welds was higher than that of the other areas of the welds, which was an important reason for the difference in the microstructure and performance between the overlapping area and the other areas of the welds. It was noted that all joints were broken in the base material area during tensile tests, and it was believed that the inhomogeneity of microstructure and properties of large-thickness forgings in the direction of thickness resulted in the inhomogeneity of tensile strength of joints. The results show that it is feasible to use double-side and double-pass unequal penetration EBW welding scheme to fabricate thick hollowed-out load-bearing TC4 titanium alloy beam. Meanwhile, some beneficial measures such as improving the uniformity of microstructure and property of the base material and strictly controlling the interval time between two passes of welding are recommended

Abstract:The effects of annealing processes, aging processes and cyclic strain on shape memory behaviors of Ti-50.8Ni-0.1Zr alloy were investigated by tensile test, optical microscopy and TEM. The alloys annealed at 350~400 ℃ and 600~700 ℃ present superelasticity (SE), the alloys annealed at 450~550 ℃ present shape memory effect (SME), the alloys aged at 300 ℃ for 1~50 h and at 400 ℃ for 1 h present SE, and the alloys aged at 400 ℃ for 5~50 h and 500 ℃ for 1~50 h present SME. With increasing annealing temperature, the platform stress (σM) of the stress-strain curve in the alloy decreases firstly and then increases, and the minimum 200 MPa obtained in the alloy annealed at 500 ℃; the residual strain (εR) increases firstly and then decreases, and the maximum 2.64% obtained in the alloy annealed at 500 ℃. With increasing aging time, the σM decreases, and the εR is small in the alloy aged at 300 ℃; the σM decreases, and the εR increases firstly and then tends to stable in the alloy aged at 400 ℃ and 500 ℃. With increasing stress-strain cycle number, the superelasticity type alloy transforms from incompletely nonlinear superelasticity to completely nonlinear superelasticity, and the σM and energy dissipation (△W) decrease firstly and then tend to stable; the σM and △W decrease firstly and then tend to stable in shape memory effect type alloy.

Abstract:In order to obtain fine-grained TiAl alloy and effectively reduce internal defects caused by traditional casting，in this paper, the Ti-44Al-2Cr-4Nb-0.2W-0.2B alloy was prepared by vacuum hot pressing sintering process, and the effect of sintering temperature on the microstructure and mechanical properties of TiAl alloy was studied. The results show that according to Ti and Al element powder reaction synthesis mechanism, the alloys sintered at three temperatures (1150 ℃, 1240 ℃, 1300 ℃) are mainly composed of γ and α2 phases after XRD inspection. With the increase of sintering temperature, the content of γ phase increases, and that of α2 phase decreases. Combined with SEM observation, it is found that different typical structures of TiAl alloy can be obtained by changing the sintering temperature. The sintered alloy at 1150 ℃ with a near-gamma structure, at 1240 ℃ with a dual-state structure, and at 1300 ℃ with a near-lamellar structure, the microstructure is become more uniform with the increasing sintering temperature. With EDS analysis, the density of the alloy increases gradually and the Nb element in the matrix phase is diffusied more homogeneously with the increasing sintering temperature. When the sintering temperature rises to 1300 ℃, the density of the alloy reaches to 4.419 g/cm3. Through mechanical property testing, the TiAl alloy sintered at 1240 ℃ shows better comprehensive mechanical properties with a microhardness of 527 HV at room temperature and good compressive strength at high temperatures.

Abstract:Supported noble metal nanocatalyst is an effective way to improve the utilization rate of precious metal catalyst for the economic cost reduction, and is also a new potential surfacing enhanced Raman spectroscopy (SERS) substrate material. In this paper, reduced graphene oxide (rGO) reinforced polyacrylonitrile/polyaniline fibers (PAN/PANI/rGO) were first fabricated by electrospinning technology. Au nanoparticles were then grown on the surface of as-prepared PAN/PANI/rGO fibers by in-situ reduction method. The structure and morphology of the final formed PAN/PANI/rGO/Au fibers were characterized by SEM, FTIR, XRD, XPS, Raman and UV-Vis spectroscopy. The catalytic reduction of 4-nitrophenol (4-NP) in the presence of NaBH₄ in water was used to evaluate the catalytic and in-situ SERS detection properties of the synthesized PAN/PANI/rGO/Au fibers. Compared with PAN/PANI/ Au and PAN/PANI/GO/Au fibers, the results show that the prepared PAN/PANI/rGO/Au fibers has the best catalytic activity, in-situ SERS detection ability and cycle performance.

Abstract:Al_xTiCrMnCu (x=0, 0.25, 0.5, 0.75, 1.0) high entropy alloys were prepared by laser deposition. The XRD、SEM and corrosion electrochemical testing methods were used to investigate the effect of Al on the microstructure and properties of Al_xTiCrMnCu alloys. The results show that, with the increase of Al addition, phases of Al_xTiCrMnCu alloys change from simple FCC and HCP1 solid-solutions to single HCP2 solid-solution. The microhardness of Al_xTiCrMnCu alloys increases gradually with the increase of Al addition. The corrosion resistance of Al_xTiCrMnCu alloys in 3.5% NaCl solution increases firstly and then decreases with the increase of Al addition. The optimum corrosion resistance can be obtained when x=0.25.

Abstract:_{:At present, the production methods of Ti6Al4V alloy powder mainly include atomization method, mechanical alloy method, and hydrodehydrogenation method, but they are still based on the Kroll method to produce sponge titanium. Molten salt electrolytic method and metal reduction method are still in the laboratory research stage. According to the thermodynamic characteristics of step-by-step reduction of high valent metal oxides and the thermodynamic transmutation of different equilibrium phases in the reduction process, a new idea of preparing Ti6Al4V alloy powder by multi-stage reduction of metal oxides (TiO2, V2O5) as raw materials were put forward. Firstly, the Gibbs free energy change of TiO2-V2O5-Mg-Ca system is calculated. The results show that it is thermodynamically feasible to obtain Ti6Al4V alloy powder by magnesium thermal self propagating reaction and calcium thermal deep reduction reaction. Then it is verified byexperiments. After acid leaching of the product of magnesium thermal self-propagating reaction, MgO can be removed to obtain a porous Ti-Al-V-O precursor with an oxygen content of 15.84wt.%.Low-oxygen Ti6Al4V alloy powder(Al: 6.2wt.%, V: 3.64wt.%, O: 0.24wt.%, Mg: 0.01wt.%, Ca: < 0.01wt.%) can be obtained by adding metal Ca in the later stage for deep deoxidation。}

Abstract:The thermal compression tests of Ti80 alloy with bimodal microstructure was conducted at the deformation temperature of 860-980℃ and the strain rate of 0.01~1s^(-1)on the Gleeble-1500 thermal-mechanical simulator. The flow stress and dynamic recrystallization behavior of the alloy during hot deformation were investigated, the critical strain of dynamic recrystallization was determined with the help of the work hardening rate. The results show that dynamic recrystallization is an important softening mechanism during hot deformation of Ti80 alloy, and the critical strain of dynamic recrystallization decreases with increasing temperature and decreasing strain rate. Based on the Z parameter and the improved Avrami equation, the critical strain and kinetics model for dynamic recrystallization of Ti80 alloy were constructed.

Abstract:High entropy alloys have been attracting more and more attention in recent years due to performance superior to that of conventional alloys, while the high temperature oxidation limites the application development. Compositionally complex makes the high temperature oxidation process quite different between high entropy alloys and a pure metal. Dynamics of different oxidation periods differ a lot. During the early oxidation stage, several elements undergo oxidation reactions, the spieces of oxide and the structure of scales change with time, and the oxidation products will not become unchanging until the stable oxidation stage. This paper will dissect the detailed mechanism of each oxidation stage during the initial selective oxidation, transition oxidation and stable oxidation, and summarise corresponding approaches to improve antioxidation, hence providing theoretical reference for material design and performance control of high entropy alloys.

Abstract:Flexible hard coatings with high density, high surface integrity and smooth surface morphology exhibit simultaneously high hardness, high toughness and high crack resistance which represent a new class of high-performance coatings. However, their depositions are a hard work. Deep oscillation magnetron sputtering (DOMS) is a novel high-power impulse magnetron sputtering, which become a hotspot in tribological hard coatings around world. Using a series of modulated micro-pulse of voltage oscillation, DOMS can achieve virtually arc-free high-power discharge to generate highly ionized target species to obtain high dense plasma with low ion energy and high ion flux. These features allow to prepare high-performance nanostructured coating, which can be modulated by composition, structure and properties using a series of micro-pulse under optimized deposition conditions. In this paper, we summarize characteristics of flexible hard nanostructured coatings and research progress in recent years of flexible hard nanostructured coatings deposited by DOMS technology.

Abstract:After the Fukushima nuclear accident in Japan, the world-wide research is focused on the accident tolerant fuel (ATF) program, aimed to enhance the stability and safety of nuclear reactor under normal or off-normal conditions. In the short-term-plan of ATF program, coating technology is considered as a revolutionary approach. Synthesizing a protective coating on the surface of Zr based cladding would improve oxidation resistance obviously with minimal effect on structure of the existing fuel system and properties of the existing nuclear Zr based materials. In this study, a review of coating development status based on Zr alloy is offered, including the coating types, properties and application prospects，also the characteristics of synthesization technology. Based on these analyses, it’s believed that Cr coating could be considered as the best candidate ATF material for industrial application at present, due to its good corrosion resistance at high temperature steam, especially higher than 1200℃. At the same time, it is a key problem to explore a Cr coating synthesization technology suitable for engineering application, with none structure defects, more density and higher interface performance.

Abstract:Metal additive manufacturing technology is an advanced manufacturing technology developed from the early 1990s, which can realize the non-die, fast, fully dense near-net forming of complex high-performance structural metal parts. W-Ni-Fe heavy alloys are widely used in defense industry and national economy due to their properties of high density, high strength and high plasticity. In recent years, the additive manufacturing of W-Ni-Fe heavy alloys has attracted extensive attention. This paper summarizes the research progress of additive manufacturing of W-Ni-Fe heavy alloys from domestic and foreign research institutions, which use selective laser melting (SLM), laser melting deposition (LMD), electron beam selective melting (EBSM) and binder jet printing (BJP) to prepare W-Ni-Fe heavy alloys. The forming process, microstructure and mechanical properties of the additive manufactured parts are analyzed. The research trends are finally speculated.

Abstract:Electrical explosion spraying is a new method of thermal spraying, which uses high voltage to pulse discharge the spraying material, and the instantaneous high current heats and explodes it. High temperature particles are produced and sprayed to the surface of substrate with shock wave to form coating. It is characterized by high particle velocity and small equipment size, which is suitable for spraying on the inner wall of the cavity. In this paper, the development status of electrical explosion spraying method is reviewed, which has experienced free, directional and constrained electric explosion spraying. The free spraying electrode is directly contacted, which causes serious burning damage and limits the pipe diameter. It is only used for the inner wall of small diameter pipe/hole. Ceramic materials are commonly used in the confinement chamber of directional spraying, which are easy to break or ablate during explosive impact. These problems can be solved by using ablation material to make the confinement cavity and introducing current into the gas discharge in constrained spraying. Secondly, the characteristics of the coating prepared by this method are described, that is, the coating is metallurgical bonded with the substrate. The compact coating with ultra-fine grain and nano-crystalline structure is formed, which has good wear resistance, corrosion resistance and high hardness. Then the relationship between the process parameters and the coating is given. The energy density and spraying distance are the main factors affecting the coating performance. Finally, the problems to be studied in the future are analyzed, and the development trend of electric explosive spraying method is prospected.

Abstract:Under the reactor neutron irradiation, the point defects and their evolution in zirconium alloys will affect its mechanical properties. At present, the mechanism of the effect of irradiation defects on the mechanical properties of Zr-Nb binary system is still unclear. Therefore, based on the density functional theory, the effects of point defects and Nb-doping on the mechanical properties of α-Zr are studied using first principles method. The results show that vacancy increases the shear modulus while Nb-doping decreases the shear modulus of α-Zr. It is found that the introduction of point defects will weaken the anisotropy of α-Zr structure by calculating the three-dimensional Young"s modulus of α-Zr with different configurations of point defects, so it can be predicted that single point defects are helpful to alleviate the degradation of irradiation properties of zirconium alloys.

Abstract:5 wt% Al and 5 wt% Al-5Ti-1B (ATB) master alloy (grain refiner) were added into pure V to form V-5Al and V-5ATB alloy ingots by arc melting. Optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness tester were used to analyze the microstructure and hardness variation of V-Al alloy before and after grain refinement. The results show that the grain size of V-5ATB alloy decreases obviously due to the precipitation of long needle-like second phases TiB. Compared with V-5Al alloy, the hardness of V-5TB alloy increases due to strengthening effect of solid solution, fine grain and hard TiB phase.