Abstract:TC10 titanium alloy was annealed by different processes. The relationship between microstructure and impact properties of the alloy after different annealing processes was studied by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM) and impact property test. The results showed that: After single annealing treatment, the microstructure is composed of primary α phase (αp) and secondary α phase (αs). It is confirmed that no α" phase and α" phase are precipitated. After double annealing treatment, αp phase is almost unchanged, and αs phase is transformed into coarse lamellar αs phase and fine lamellar αs phase. The impact property of the alloy after single annealing is higher than that after double annealing, and the impact property after heating in two-phase region is higher than that in single-phase region. In the two annealing processes, when the heating temperature is two-phase region, the fracture morphology is mainly composed of equiaxed dimples. When the heating temperature is single-phase region, the fracture morphology is dominated by rock-like morphology. And there are shallow small dimples distributed on the surface.

Abstract:The XRD, SEM, EBSD were used to analyze the microstructure characteristics in different zone of welded joints in the welding state and post-welding heat treatment state, and the fracture toughness and fatigue crack propagation properties of welded joints were researched. The results show that the weld zone is main consisted of recrystallization organization. The equiaxed prior α_Pphase transform the rodlike structure in thermo-mechanically affected zone. The microstructure of the heat affected zone is basically the same base metal. The acicular martensite α" phases with cross angle of approximately 60° are formed within part original β grains in thermo-mechanical affected zone and thermal affected zone. After post-weld heat treatment, the residual metastable β phase was decomposed and a large number of discontinuous microstructure were formed between lamellar α_S phase. A large number of parallel or cross distributed flake α phases and complex phase interface structure within the α grains in weld zone can effectively hinder the crack propagation and change the crack propagation path, and fracture toughness and fatigue crack propagation resistance of welded joints are improved.

Abstract:In order to study the effect of different deformation parameters of thermal processing on the microstructure of TC21 titanium alloy during plastic forming, this paper uses the Gleeble-3500 thermal simulation testing machine to conduct isothermal constant strain rate thermal compression experiments, and studies the TC21 titanium alloy under different deformation conditions. The thermal deformation behavior of TC21 titanium alloy; and based on the microstructure evolution of TC21 titanium alloy during thermal compression, through the derivation of dislocation density model, recrystallization nucleation and grain growth model of TC21 titanium alloy, a cellular automatic Based on the cellular automata model, the dynamic recrystallization behavior of TC21 titanium alloy during deformation in the β single-phase region was simulated and verified. The results show that the flow stress of the alloy increases with the decrease of temperature and the increase of strain rate; according to the analysis of cellular automata simulation results, the dynamic recrystallization volume fraction and deformation temperature of the alloy in the β single-phase region proportional to the strain rate and inversely proportional to the strain rate.

Abstract:The microstructure and texture of FSP pure titanium in nitrogen atmosphere were investigated. The results showed that there were equiaxed grains in the stir zone. The average grain size of dynamic recrystallized grains near AS side and RS side was 2.29 μm. The combined effects of rising temperature and friction stir in the near surface and central zone, the average grain size grew to 6.48 μm and 8.64 μm respectively. The thermal-mechanical affect zone presented a gradient distribution. The grains near the stir zone had been seriously broken and refined. In the zone far away from the stir zone, the deformation of the original grains was slightly, and the deformation mechanism was dominated by deformation twinning. The width of the thermal-mechanical affect zone at AS side, bottom and RS side was 900 μm,700 μm and 480 μm respectively. The mainly deformation twins in the thermal-mechanical affect zone were {10-12} tensile twins. The structure of stirring pin was an important factor affecting the texture evolution of FSP pure titanium. Moreover, the c-axises of the grains in the stir zone were distributed close to the PD direction. Due to the combined effect of grain refinement and work hardening, the hardnesses in the stir zone were overall higher than the initial material, and the maximum hardness reached to 223.7HV0.5.

Abstract:The rolled piece is assumed to be an ideal isotropic material in the traditional cold-rolled strip theory, and the yield stress in the RD-TD plane is often used for rolling calculation, while the anisotropy of the strip is rarely considered. For this reason, the yield behavior of pure titanium strips was studied based on uniaxial tensile test and crystal viscoplastic self-consistent model (VPSC), and the yield trajectory with RD tensile yield stress as reference was constructed using equal plastic work and yield criterion. The influence of the yield stress difference in each direction on the characteristics of rolling deformation zone was investigated through the cold rolling process simulation. The research results show that the yield stress of the basal bimodal textured pure titanium strip is the largest in ND, the second in TD, and the smallest in RD, and the anisotropy of the yield stress leads to a large error between the traditional theoretical results and the actual results. For the convenience of application, the Fleck rolling model in the traditional cold-rolled strip theory was modified based on the Hill48 anisotropic yield stress with the RD tensile yield stress as a reference, and the calculated values were in good agreement with the theoretical values.

Abstract:Titanium alloys are prevalently applied in aerospace, ship, and medical fields due to their excellent properties such as high special strength, corrosion resistance, and biocompatibility. Additive manufacturing technology provides a revolutionary way for processing the integrate and light-weight parts with short cycles. However, the coarse columnar grains produced in the forming process lead to the anisotropy of the components, which limits the full play of the properties for the alloys. To control the size and morphology of the grains is the hotspot in recent years. This study briefly describes the feature and formation mechanism of typical grain microstructures in general additive manufacturing methods, expounds grain-control methods in parameters optimization, novel processing methods, micro-alloying/new alloy composition design, subsequent heat treatment, external field assistance, and the combination of these methods. The grain regulation mechanisms are summarized and the control effects are evaluated. Most of the control methods can obtain full-equiaxed grains and weaken the anisotropy of mechanical properties in the room-temperature tensile test. Subsequent treatments and mechanical properties evaluation of the obtained equiaxed microstructure alloys are expected to achieve the target properties and engineering application through further research.

Abstract:To study the mechanism of structure damage and the law of performance weakening of TiAl alloy turbine under pressure, an experimental method is designed that compresses and then stretches the TiAl alloy turbine. The slips and microcracks on the surface and inside of the compressed turbine journal are analyzed by scanning electron microscope (SEM), and the tensile fracture morphology is observed. The experimental results show that the residual tensile strengths of the compressed TiAl turbines gradually decrease as the pressure on the turbine. When the pressure reaches 610 MPa, the residual tensile strength is only 86 MPa, and the strength loss rate is as high as 70%. During the compression process of TiAl alloy, the deformation damage characteristics, which are mainly inter-lamellar cracks and supplemented by trans- lamellar cracks, are formed. Lamellar cracks in the direction of maximum shear stress at 45 ° to the compression axis are the main form of compression damage found in the TiAl alloy. The tensile fracture of TiAl alloy turbine after compression damage occurred at the thin journal near the casting riser side of turbine. The microcracks in the lamellar structure after compression deformation continue to expand under the subsequent tensile stress until the ligament bridge is penetrated, and the small cracks merge into large cracks, resulting in a mixed fracture morphology along the layer and through the layer on the fracture surface.

Abstract:The ultrafine grained (UFG) 1050 aluminum alloy was prepared by equal channel angular pressing at cryogenic temperature, namely cryoECAP process. The tensile behavior and microstructures of UFG 1050 aluminum alloy after annealing at 90–210 °C for 4 h without and with high magnetic field of 12 T were investigated by tensile tests, transmission electron microscope, and electron backscattered diffraction analyses. After cryoECAP and annealing treatments, the 1050 aluminum alloy has ultrafine grains with 0.7–1.28 μm in size, the ratio of ultimate tensile strength to yield strength is less than 1.24, and the uniform elongation is less than 2.3%. With increasing the annealing temperature from 90 °C to 210 °C, the yield-drop phenomenon becomes more obvious due to the decrease in mobile dislocations to maintain the applied strain rate during tensile deformation. The uniform elongation decreases from 1.55% to 0.55%, the dislocation density reduces from 5.6×10¹⁴ m^-2 to 4.2×10¹³ m^-2, and the fraction of high-angle grain boundaries (HABs) increases from 63.8% to 70.8%. These phenomena cause the higher annihilation rate of dislocations, thereby leading to the degradation of strain hardening effect. During annealing under high magnetic field at 90–210 °C, the low fraction of HABs (61.7%–66.2%) can provide a slower annihilation rate of dislocations, therefore resulting in the higher uniform elongation (0.64%–1.60%) and slower decrease in the flow stress after the yield peak.

Abstract:In order to improve the uniformity of microstructure and comprehensive mechanical properties of 7034 aluminum alloy, this paper carried out the optimization design of the severe plastic deformation process and solution and aging treatment based on the reciprocating extrusion, characterized the mechanical properties of the material, the size distribution of grain and secondary phase and the quantification of dislocation configuration evolution, established the relationship between plastic deformation and heat treatment and the microstructure and mechanical properties of 7034 aluminum alloy. The results show that, the average grain size is refined from 59μm to 9μm, and the standard deviation of the average grain size decreases from 3.05 to 0.8 after three passes reciprocating extrusion; Compared with the two-stage solution and aging treatment, the MgZn₂ phase after single-stage solution and aging treatment is smaller in size, higher in density, and mostly semi-coherent η" phase, and the dislocations pile-up becomes in sub-grain. The interaction between the two is stronger and the strengthening effect is better. The optimal heat treatment system is single-stage solution + single-stage aging. under this condition, the tensile strength and elongation reach 747MPa and 4.3% respectively, which are better than the two-stage solution + two-stage aging system.

Abstract:In order to investigate the effect of two kinds of mandrel direct hole expansion strengthening methods, solid mandrel and split mandrel, on the fatigue properties of 7050 aluminum alloy. A three-dimensional finite element simulation model of 7050 aluminum alloy of mandrel direct hole expansion strengthening was established, direct hole expansion strengthening test of mandrel were carried out, hole wall stress and fatigue properties of no expansion strengthening and mandrel direct expansion strengthening specimens were compared and analyzed. The results show that, the difference between the maximum residual compressive stress of the hole wall before and after the ream machining of the mandrel direct hole expansion strengthening specimen is less than 20 MPa. The residual compressive stress formed in the hole wall of the specimen after the direct hole expansion strengthening of the mandrel can offset the tensile stress generated in the process of partial loading. The median fatigue life of the solid mandrel hole expansion strengthening specimen is 1.46 times that of the no expansion strengthening specimen, and the median fatigue life of the split mandrel hole expansion strengthening specimen is 1.52 times that of the no expansion strengthening specimen. The mandrel direct hole expansion process reduces the number of fatigue sources, reduces the fatigue striation spacing in the fatigue crack propagation zone, and improves the fatigue life of hole structures.

Abstract:In this paper, the Al-Si-Mg-Cr alloy was prepared by vacuum electromagnetic induction melting furnace. Thermo-calc software was used for thermodynamic simulation, and SEM, EDS and other test methods were used to characterize the microstructure of Al-Si-Mg-Cr alloy under different heat treatment conditions tissue and test its mechanical properties. The corrosion properties of the experimental alloys were tested by weight loss method and electrochemical method. The results show that the main phases of the experimental alloy include primary α-Al, (α-Al+Si) eutectic, Al13Cr4Si4, β-Al (Cr, Fe) Si, and Fe-rich phases (β-Al5FeSi and π-AlSiMgFe). After heat treatment, the experimental alloy structure was refined, the eutectic area is reduced, the eutectic Si was spheroidized, and the Al13Cr4Si4,β-Al(Cr, Fe)Si phases were dispersed and distributed in the alloy. The corrosion test results show that the main corrosion mode of the experimental alloy is intergranular corrosion. After heat treatment, the eutectic area of the experimental alloy decreases, which leads to the narrowing of the corrosion channel and improves the corrosion resistance of the alloy. When the heat treatment process is 535℃ 6h+160 26h, the micromechanical properties and corrosion resistance of the experimental alloy are the best.

Abstract:In this paper, through tests such as constant stress creep tensile, room temperature tensile and slow strain rate tensile stress corrosion performance tests, combined with OM, SEM, TEM and EBSD microstructure observation and analysis to explore the effects of deformation and fine-grained two different grain structures on stress Effects of aging treatment on precipitation behavior and properties of 2195 Al-Li alloy. The results show that compared with the fine-grained sheet, the time for the deformed sheet to reach the peak hardness is shortened from 18h to 4h, the peak hardness is increased from 165.3HV to 228HV, and the tensile strength is increased from 584.6MPa to 641.9MPa. By calculating the contribution values of grain refinement strengthening, dislocation strengthening and precipitation strengthening to the improvement of alloy strength, it is found that the improvement of mechanical properties of deformed sheet is mainly due to the contribution of dislocation strengthening. At the same time, compared with the fine-grained sheet, the ISSRT value of the deformed sheet was reduced from 7.6% to 4.8%, and the stress corrosion susceptibility was reduced. The proportion of large-angle grain boundaries of the deformed sheet is reduced from 64.6% to 41.1% of the fine-grained sheet, the grain boundary precipitation phase distribution is more discrete, and the precipitation-free zone is hardly observed, which is the main reason for obtaining relatively excellent stress corrosion resistance. reason.

Abstract:Based on the distribution law of temperature field in the electron beam cladding process simulated by ANSYS software, the melting depth and melting width of the cross section of cladding layer were investigated, and then the dilution ratio of cladding layer was estimated through simulation. Afterwards, the electron beam cladding experiment was conducted to measure the actual dilution ratio. Through the comparison between simulated and experimental dilution ratios, it can be verified that the dilution ratio of electron beam cladding can be obtained by simulation. The microhardness and the wear resistance of specimen after cladding were investigated. Results show that the smaller the dilution ratio, the better the quality of the cladding layer.

Abstract:In order to eliminate the mixed grain structure of forged Ni-based superalloy parts, the heat treatment process of δ phase aging + recrystallization annealing was proposed. During aging, the δ phase can precipitate directly or indirectly by the transformation of γ″ phase. The alloy with precipitated δ phases were both annealed at high temperature and then immediately cooled by water. It was found that the aging routes and aging times greatly affect the microstructure evolution during recrystallization annealing. In the aging process, the directly-precipitated δ phase mainly distributes at grain boundaries. With the increase of aging time, some δ phases precipitate inside grains, and the main morphology is short-rod like. The distribution characteristic of the indirectly-precipitated δ phase is mainly intragranular needle like and intergranular short-rod like. The short-rod δ phase has weak interaction with dislocations during annealing and is mainly responsible for pinning grain boundaries, while the long needle-like δ phase can promote the formation of sub-grains. That is why the indirect aging is favorable for the recrystallization nucleation, and can also effectively hinder the growth of grains to avoid abnormal growth of recrystallized grains.

Abstract:During hot working, phase precipitation behavior is essential to deformed superalloys" tensile strength. This experiment tracked MC, M23C6, and γ′ precipitates of GH4738 superalloy at different stages during hot working. The behavior of precipitates in different stages of hot working: (1) The main precipitates in the superalloy billet were primary MC carbides and primary γ′. (2) After hot deformation, the primary MC carbides were partially decomposed and precipitated nano-sized intragranular secondary MC carbides and a small amount of grain boundary secondary M23C6 carbides and secondary γ′. (3) After heat treatment, the primary MC carbides were further decomposed until completely dissolved, and then the nanocrystalline secondary MC carbides were transformed into many bottle-shaped secondary M23C6 carbides. Meanwhile, γ′ was closed to complete precipitation, but γ′ the grew slightly.

Abstract:Molecular dynamics simulation was used to simulate the uniaxial tensile deformation of monocrystalline Ni and Ni₅₇Cr₁₉Co₁₉Al₅ alloy models with different cross-sectional sizes in the [100] orientation, the appropriate simulation modle size with stable plastic flow stress was determined. The tensile deformation behavior of monocrystalline Ni and its alloys of the same modle with stable flow stress were further studied. The results show that the monocrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy with smaller modle sizes are likely to form multi-layer twins or deformation twins during the tensile process because of low stacking fault energy. As the cross-sectional side length of modle is greater than 30 times of lattice constant, the flow stress, phase structures and dislocation density in the plastic flow stage tend to be stable fluctuation with the variation of strain. When the monocrystalline Ni and Ni-based alloys with same modle of stable flow stress stage are stretched, the lower the stacking fault energy is, the larger the area of the stacking faults plane during plastic deformation. During the tensile process of monocrystalline Ni and Ni-based alloys, Shockley partials play a leading role in the plastic deformation process. The formation of multi-layer twins is accompanied by dislocation exhaustion, while the formation and annihilation of deformation twins are mainly dominated by the dislocation starvation mechanism.

Abstract:The oxidation kinetics of GH3230 alloy was measured as a function of the cumulative time cyclically exposed upto 1200℃. The oxidation behaviors and the resulting composition and microstructure of the near surface layer were observed and analyzed by SEM, XRD and EPMA. It has been found that GH3230 alloy exhibits continuous oxidation weight loss during cyclic thermal exposures, and the weight loss rate decreases from the highest value to lowest level and then gradually increases with the thermal exposure time. Nevertheless, the alloy-thickness loss after 100 exposure cycles can be obviously less than the thickness-fluctuation limit of standardized plate products. The remaining oxides on the surface after heat exposure are mainly composed of Cr2O3 and MnCr2O4. The normal gas pressure produced by carbon oxidation, the increments of oxide growth stress and the thermal stress caused by over-temperature exposure lead to the cracking and spallation of the outer oxides. At mean time, internal oxidation occurs along grain boundaries significantly. While, it is worthy to note that the depth of the Ni-based solid solution layer tranformed through selective oxidation and oxidative decarbonization is always much larger than that of the internal oxidation. It indicates that the cracks initiated from the intergrannual oxides as the materials bearing tensile load will soon be blunted by in-front ductile FCC-stucture solid solution and therefore present negligible impact on the mechanical performance of GH3230 alloy.

Abstract:The ZrB₂-SiC composites with refractory metal W of different contents (1vol%, 3vol%, and 5vol%) were prepared by spark plasma sintering. The densification behavior of composites during sintering process was investigated. The influence of W additions on the microstructure evolution, phase composition, mechanical properties, and oxidation behavior of W-doped composites was studied. Results show that the W addition leads to the formation of core-shell structures in the composites, where the ZrB₂ grains are considered as the core and the in-situ formed (Zr, W)B₂ solid solution is considered as the shell, thereby effectively promoting the grain refinement and composite densification. Compared with those of the W-free composites, the Vickers hardness, flexural strength, and fracture toughness of W-doped composites are enhanced. The optimal mechanical properties can be achieved at W addition content of 3vol%: the highest hardness, strength, and toughness can be obtained for the composites. The mass gain and oxide scale thickness of composites are gradually decreased with increasing the W addition from 0vol% to 5vol%. When the W addition in composite is 5vol%, the SiC-depleted layer disappears. Finally, the influence mechanism of W addition on the performance of composites is discussed.

Abstract:Several boron nitride (BN)/aluminum oxide (Al₂O₃) composite powders were synthesized by liquid-solid phase co-mixture method. BN was modified by the freeze-thaw method and surface modification. The precursor and as-sintered BN/Al₂O₃ composite powders were obtained by changing the type of surface modification materials and the molar ratios. Additionally, the thermal conductivity of the epoxy resin (EP)-based BN/Al₂O₃ composite materials prepared by mechanical mixture was measured and analyzed. Results show that the dispersion and interfacial compatibility of BN after freeze-thaw process are obviously better than those without freeze-thaw process. The modification effect of dopamine on BN is better than that of polyethylene glycol. When the dopamine is used as the surface modification material and the molar ratio of BN to Al(NO₃)₃ is 1:1, BN powder at micro-level with uniform nano-Al₂O₃ deposition can be obtained, namely the BN/Al₂O₃ micro/nano-composite powder. The thermal conductivity coefficient of this EP-based BN/Al₂O₃ composite reaches 0.62 W·m^-1·K^-1, which is 3 times and 1.5 times higher than that of the pure EP composite and EP-based composite material prepared by pure micro-sized BN powder, respectively. The deposited nano-Al₂O₃ on BN surface can form multiple thermal conduction paths, thereby improving the thermal conductivity of the EP-based BN/Al₂O₃ composites.

Abstract:Bulk tantalum specimens of high relative density (99.93%) without obvious defects were prepared by powder bed selective electron beam melting (SEBM) process, and their microstructure as well as mechanical properties were investigated. Results show that the as-deposited tantalum specimens have strong columnar grain structures parallel to the building direction. The strong (001) texture and a large number of low-angle grain boundaries can be observed in the bulk specimens due to the extremely high cooling rate during additive manufacturing. Because of the solid solution strength of interstitial oxygen and nitrogen elements, the as-SEBMed tantalum specimens exhibit excellent room-temperature yield strength of 613.55±2.57 MPa and outstanding elongation of 30.55%±4.23%.

Abstract:The effects of single-layer and double-layer scanning strategies and energy density (246–640 J/mm³) on the microstructures and mechanical properties of Ta prepared by selective laser melting (SLM) were investigated. The microstructure of SLM-processed Ta was characterized by scanning electron microscope and electron backscatter diffractometer. The microhardness and tensile properties were also tested. Results show that the Ta microstructure is composed of columnar crystals with obviously upward growth trend and the Ta prepared by double-layer scanning has finer grain size. With increasing the input energy density, the strength, microhardness, and ductility of the as-built Ta are significantly improved. In addition, the double-layer scanning strategy can further improve the densification of Ta specimen, and even increases the ductility without strength loss. When the energy density is 640 J/mm³ (double-layer scanning), the as-built Ta exhibits optimal properties: the microhardness, ultimate tensile strength, and elongation are 2307 MPa, 527 MPa, and 11.4%, respectively.

Abstract:In order to investigate the nano-friction evolution characteristics of WC coating during the nano-scratching process, the molecular dynamics simulation model was established under different conditions (load, scratching-depth, scratching-velocity) by the large-scale atomic /molecular massively-parallel simulator. Results show that the friction force and coefficient of friction are increased with increasing the scratching depth. When the indenter scratches the specimen, the atoms are squeezed, sheared, and piled up in front of the indenter and at both sides of the groove along scratching direction. The instantaneous friction curve presents the distinct tribological characteristics during the initial and stable stages, and the dislocation, slip, interstitial, or vacancy occurs in the region under the indenter during the friction process. With increasing the scratching velocity, the strain energy of system exceeds the bonding energy caused by interatomic constraint. The atoms break the constraint and are stacked on both sides of the scratching grooves. Additionally, the surface morphology and the outer edge of accumulated atoms become rough, and defects appear in the subsurface crystal structure. This research provided the microscopic wear mechanism of WC coating at nano-scale during friction process.

Abstract:The microstructure and mechanical properties of 1060Al/TA2/CCSB explosive composite plate binding interface at different annealing temperatures were studied. The results show that the composite plate exhibits a continuous and uniform distribution of waves at its binding interface, there are swirls, melting blocks, surface microcracks and ability shear strips other defects. Take a test, cutting test, and microhardness testing on the composite plate, pulling breakage occurs at the 1060Al/TA2 interface, the shear strength and the microhardness of the 1060Al/TA2 interface are lower than the TA2/CCSB interface, and the binding interface is severe Plastic deformation, resulting in processing hardening and grain refinement of the composite plate interface, and the hardness value at the interface is highest and along the boundary to the matrix gradient distribution. The existence of a composite palette hinders the necklace process during the stretching process, and the composite plate is improved. As the annealing temperature rose to 400℃, the combined interface responded, the processing hardening effect was eliminated, so that the interface hardness was lowered; the ASB disappeared and converted into an equal shaft alpha grain, defect decreased; the melting block at the interface was dissolved, so that the composite The binding interface of microstructure is more uniform and good comprehensive performance is obtained.

Abstract:In order to further understand glass forming rules of uranium alloy systems, the characteristics of glass formation in U-Rh system were studied in this paper. A series of U-Rh alloys with 65~80 at.% U were designed in the U-rich eutectic zone, and then shaped into ingot samples by vacuum arc melting and ribbon samples by melt-spinning. These samples were surveyed to determine their phase constitutions and thermal behaviors by using X-ray diffraction and thermal analysis means. It is found that upon rapid solidification, U₇₀Rh₃₀ alloy formed partial amorphous phase, having the crystallization temperature of 642 K and the crystallization activation energy of nearly 200 kJ/mol. The coexisting phase was high temperature g-U solid solution, as the first evidence of this kind of phase forming in U-based amorphous alloys through rapid quenching, implying unique non-equilibrium solidification phase transformation of U-Rh alloys. The results indicate that U-Rh is a new amorphous alloy system, however, possessing weak glass forming ability. This is probably relevant to the capability of directly arresting high temperature g-U solid solution during rapid cooling.

Abstract:In this paper, combined with the advanced controlled rolling and cooling process represented by 2250mm hot continuous rolling production line in China, an on-line heat treatment process (DQ&P&T) is developed, which uses dense laminar flow cooling to conduct sectional quenching, isothermal distribution and low-temperature coil taking from tempering after rolling, and a comparative analysis were made with the traditional off-line quenching and tempering heat treatment process (RQ&T) to produce rare earth microalloyed ultra-high strength steel (Rm≥1200MPa). By means of optical microscope, scanning electron microscope, equipped with electron backscatter diffractometer, tensile testing machine, impact testing machine, Vickers and wet sand/rubber wheel wear testing machine, the microstructure transformation morphology and mechanical properties of the test steels under two heat treatment systems were systematically analyzed. The results show that the microstructure of the test steels at room temperature are composed of lath martensite, bainite ferrite and a small amount of residual austenite under the two heat treatment processes. Compared with off-line heat treatment (RQ&T), the lath martensite content of the steel tested by on-line heat treatment (DQ&P&T) is decreased and the bainite ferrite content is increased. The bainitic ferrite formed by isothermal transformation interposed and divided the deformed undercooled austenite, which promotes the refinement of the grain size and the increase of the proportion of large angle grain boundaries, and improves the strength, toughness and wear resistance of the material, however, the uniformity of hardness in the thickness direction of the product is inferior to that treated by RQ&T process. Relative to off-line heat treatment, the on-line heat treatment has higher efficiency and lower manufacturing cost in the production of high strength steel, and the performance of the product meets the standard requirements, which proves that online heat treatment process is feasible.

Abstract:Bi₂Sr₂CaCu₂O_x (Bi-2212) primary powder was prepared by spray pyrolysis method. The effect of heat treatment temperature on powder phase formation and properties was systematically studied. The weight loss and phase transformation of the primary powder were analyzed by TG-DSC, the phase composition of the powder was analyzed by XRD, the microstructure of the powder was observed by SEM, and the superconducting physical properties of the samples were determined by the magnetization method. It was found that at 535~570℃, the nitrate residues in the primary powder of Spray Pyrolysis gradually decomposed; at 605~640℃, the Bi-2201 phase was formed; when the temperature reached 775℃, the Bi-2212 phase began to form, With the increase of heat treatment temperature, the crystallinity of the powder continued to increase, and the content of Bi-2212 phase first increased and then decreased. 860℃ was the best phase-forming sintering temperature. In addition, through the in-depth analysis of the DSC curve and the M-T curve, it was found that the low temperature pre-decomposition at 640°C first, and then the high temperature phase-forming sintering at 860°C could further improve the quality of the final powder.

Abstract:The chloroplatinic acid and nickel chloride as raw materials, ammonium chloride as pore-forming agent, are used to synthesize PtNi alloy 3D-nanoframeworks materials by spray drying method combined with calcination reduction. The PtNi alloy 3D-nanoframeworks materials can enhance catalytic oxidation performance of methanol. The influence of ammonium chloride or no chloride on the formation of PtNi alloy 3D-nanoframeworks materials was studied, and the effects of PtNi alloy 3D-nanoframeworks with different structures on the activity and stability of catalytic methanol oxidation were investigated in detail. The results showed that the PtNi alloy prepared by adding appropriate amount of ammonium chloride has a single solid solution structure (fcc), which is composed of curved nanowires interwoven to form 3D-nanoframeworks. And the diameter of the nanowires is less than 10 nm, and the nanopore is about 10 nm. Compared with commercial Pt black catalysts and PtNi alloy nanomaterials without ammonium chloride, PtNi alloy 3D-nanoframeworks materials has higher catalytic methanol oxidation activity (611.4 mA·mg-1Pt), which is 3.58 times (170.8 mA·mg-1Pt) of commercial Pt black and 1.36 times (448.8 mA·mg-1Pt) of PtNi alloy nanomaterials without ammonium chloride, respectively. The order of stability in catalytic methanol oxidation reaction is: PtNi alloy 3D-nanoframeworks materials > PtNi alloy nanomaterials without ammonium chloride >commercial Pt black catalyst. In addition, In this paper, the method is extended to successfully prepare PtNiCoCuRuIrPd high-entropy-alloy 3D-nanoframeworks materials.

Abstract:Relative density is one of the important parameters to evaluate the performance of additive manufacturing components, but the corresponding process parameters of high relative density components need a large number of experimental studies. This paper has established a prediction model of the relative density based on the process parameters, which can effectively predict the pores caused by lack of fusion, and provide more reference for the selection and optimization of laser selective melting process parameters. The finite element analysis and numerical calculation software were used to obtain the sizes of the melting pool and simulate the splicing morphology of the melting pool with multiple layers and traces, and predict the corresponding density of the process parameters.At the same time, the fluctuation coefficient, the deflection Angle and the angle of scan vectors rotation between layers are introduced into the prediction model of melting pool to account for the influence of the size fluctuation of the melting pool, the inclination of the melting pool, and the angle of scan vectors rotation between layers on the simulation results of the molten pool. Finally, the feasibility of the prediction model has been confirmed by the selective laser melting experiments of HR-2 stainless steel. The results suggest that the predicted results of the model are in good agreement with the experimental results of the melting pool of the samples, and the deviation between the predicted results of the relative density and the experimental results is within 2%.

Abstract:Dissimilar welded joints (DWJ) of SP2215 and T92 tube were prepared by manual-gas tungsten arc welding (M-GTAW) and ERNiCr-3 filler. The microstructure, mechanical properties and tensile fracture mechanism of DWJ before and after heat treatment were studied. The results show that the weld metal (WM) is solidified into columnar dendrites with complete austenite structure, and Nb element is segregated between dendrites to form Nb-rich secondary phase particles, post welded heat treatment (PWHT) has no significant effect on the microstructure of welded seam. Island or peninsula macro-segregation is formed at both T92/WM and SP2215/WM interface, and a certain degree of carbon migration occurs across the T92/WM interface while the carbon migration across the SP2215/WM interface is not obvious, PWHT has no significant effect on macro-segregation and carbon migration; The hardness of coarse grain heat affected zone (CGHAZ) at T92 side is the highest, followed by fine grain heat affected zone (FGHAZ), while inter-critical heat affected zone (ICHAZ) has the lowest hardness, PWHT reduces the hardness of CGHAZ and FGHAZ significantly and modified the microstructure and mechanical properties of T92 HAZ; δ ferrite is formed near the fusion line in the T92 side, which is the lowest hardness area of the DWJ, PWHT has no significant effect on the hardness of δ ferrite. Both the as-welded and heat-treated DWJs are fractured at the welded seam in a ductile manner at room temperature, while fractured at the T92 side in a ductile manner at 650℃.

Abstract:The finite element simulation of wave-flat rolling and flat rolling of Ti/Al composite plate was established and experiments were carried out to analyze the effects of different rolling modes on the microstructure and mechanical properties of the composite plate.The results show that the warpage degree of the clad plate is lower than that of 35% when the reduction rate of the clad plate is 40%, the equivalent stress and strain of the wave-flat-rolled clad plate are larger than those of the flat-rolled clad plate, and no holes and cracks are found in the bonding interface.The thickness of the interfacial diffusion layer at the crest of the wave is 1.8μm, and that at the trough is 2.4μm, while the thickness of the interfacial diffusion layer of the flat-rolled clad plate is 1.6μm.The degree of recrystallization of the wave-flat-rolled clad plate Al plate is higher than that of flat-rolling, and its tensile strength, bending resistance, and hardness are also higher than those of the flat-rolled clad plate, but the elongation is lower due to work hardening.

Abstract:Searching for printable metallic materials is of great importance. In recent years, several kinds of printable alloys have been researched, such as Ti-6Al-4V, FeMnCoCrNi, stainless steels, and some refractory high-entropy alloys (RHEAs). In spite of these delightful results, the development in additive manufacturing of RHEAs is proceeding slowly. Because of the excellent behavior of RHEAs at high temperatures, more requirements are proposed for the complex shape forming. This review primarily introduced the recent studies on additive manufacturing of RHEAs. The laser beam-based, electron beam-based, and wire-based additive manufacturing techniques for fabrication of RHEAs were summarized. In addition, the opportunities and challenges of the current development of RHEAs fabricated by additive manufacturing were discussed.

Abstract:Recently, magnesium alloys attract much more attention as the biomedical metallics. Unfortunately, due to their low strength, the implantation materials of Mg alloys are prone to collapse and fracture during the in-vivo/vitro service, which seriously endangers the life-safety of patients. Rare-earth micro-alloying is an effective method to enhance the mechanical properties of degradable Mg-based alloys, which cannot only eliminate the impurities and purify the melt, but also promote the dynamic recrystallization and form the long period stacking ordered phase structure. Therefore, based on the correlations between mechanical properties and microstructure transformation of Mg alloys, the research progress on the microstructure and mechanical properties of rare-earth Mg alloys was reviewed. The essential correlations among the rare-earth elements, the secondary phases, and the mechanical properties of Mg alloys were investigated. Additionally, the strengthening and toughening mechanisms of the continuous dynamic recrystallization of medical rare-earth Mg alloys were clarified. Besides, the effect of long-period stacking ordered structure induced by rare-earth elements on the mechanical properties of Mg alloys was comprehensively summarized. Finally, the development directions of medical rare-earth Mg alloys was proposed.

Abstract:Vacuum arc deposition has become one of the indispensable techniques in the coating field, and it is widely studied and applied in the metal, decoration, hard wear resistance, and other fields. The application research of coating techniques promotes the investigation on arc source technique, which mainly focuses on the fields of long life, high reliability, and macroparticle suppression. The realization of macroparticle suppression is based on the satisfaction of long life and high reliability. With reducing the residence time of arc spots on the target surface, the macroparticle suppression can be obtained. This result can be achieved by ingenious design of permanent magnet or electromagnetism, thereby obtaining a target surface with strong transverse magnetic component. However, when the magnetic field intensity increases, the internal characteristics of target and the proportional relationship between the longitudinal magnetic field and the transverse magnetic field should be comprehensively considered. Another macroparticle suppression method is the pulsed arc technique. The pulsed arc source frequently strikes the arc, which is very different from the direct current arc source in structure design. The instantaneous current can reach thousands, even more than ten thousands amperes, therefore obtaining a high deposition rate. At the same time, the anode design leads to the directional jet of plasma, which can filter out most large particles.

Abstract:Solid oxide fuel cell (SOFC) possesses outstanding advantages of broad fuel sources and high energy conversion efficiency. It is considered one of the most promising clean energy technologies in the future. At present, the hot spot of SOFC research is to reduce the working temperature to 500 ~ 800℃ low temperature zone. This can reduce the operating cost, increase the reliability and accelerate the commercialization process of SOFC. The cathode is an important component of SOFC. It is very important to design and optimize cathode materials with high activity for oxygen reduction reaction. The electron-ion mixed conductive (MIECs) oxide with perovskite structure or layered structure derived from perovskite structure are the most studied SOFC cathode materials. First principles can compensate for the lack of experimental information. It has proved to be a powerful tool to elucidate reaction mechanism as the technique can provide electronic structure, geometrical parameters, adsorption energy and transition state information. It can provide scientific basis and theoretical guidance for rational design and development of high performance new SOFC cathode materials. This article reviewed the formation and migration of oxygen vacancies in perovskite cathode and the adsorption, the dissociation, the transmission process of oxygen on the cathode surface (including the introduction of precious metals) and summarized our previous research results. Finally, the problems of current research and the future research direction of perovskite cathode simulation are summarized and prospected.

Abstract:With the development of electronic packaging and manufacturing technology to high performance and miniaturization, 3D (three dimensional) packaging is an important development direction in the field of future electronic packaging and manufacturing technology. 3D packaging with high-density and high-reliability is the key technology for the transformation and upgrading of the IC (integrated circuit) industry. 3D packaging will reduce the size of chip bump interconnection interface to the sub-micron or nano level, making the size of the UBM (under-bump metallization) pad rapidly decreasing and may contain only a few or even a single grain. The roof-type Cu₆Sn₅ grain formed by the interfacial reactions on single crystals (001) Cu and (111) Cu shows preferred orientation, resulting in different electrical properties, strength and hardness. Therefore, the crystal orientation of the UBM will have a significant effect on the nucleation and growth process of IMCs (interfacial intermetallic compounds), and the properties of interfacial IMCs will directly affect the reliability of bumps in the micro/nano-scale interconnects. Therefore, using single crystal as UBM to study the transport of interfacial materials and the growth pattern of IMCs has important theoretical and application values. In this paper, the interfacial reaction of solder joints with single crystal Cu, Ni and Ag in recent years is comprehensively analyzed, and the formation conditions of IMC grains with special morphology on single crystal UBM, the orientation relationship between IMC and single crystal substrate, the growth kinetic process of IMC, the formation law of kirkendall voids, the crystal orientation control method of IMC on single crystal UBM, and the influence of crystal orientation on the mechanical properties and reliability of lead-free solder joints are summarized. It provides guidance for evaluating the mechanical properties and reliability of single crystal UBM.

Abstract:Li₄Ti₅O₁₂ has excellent structural stability and high safety under fast charging and discharging conditions. Therefore, it was favored by the field of new energy vehicles and energy storage. Ample researchers have focused on Li ions intercalation process of lithium titanate, Studies have shown that the discharge process is the external and all the lithium ions located at 8a position are transferred to 16c position, and the capacity of lithium titanate is limited by the number of lattice sites and reversible lithium ions involved in it. However, the lithium titanate unit cell structure suggests that the lattice sites that accommoddate lithium ions do not stop there. Li ions may replace Ti atoms occupy the octahedral 16d site, locate in tetrahedral 48f sites, reoccupy with 8a sites of the tetrahedron vacated after the phase change. This article mainly reviews, treated by chemical or electrochemical methods in the preparation process and finished product, compared with the normal LTO discharge start and end, the Li ions occupation changes and influence. Meanwhile, this paper discusses and analyzes the underlying mechanism between this change and effect.

Abstract:With the promulgation of the "Pb Restriction Laws" in various countries around the world, Sn-based lead-free solders have been widely studied to replace traditional Sn-Pb solders. However, the development of large-scale integrated circuits and advanced electronic packaging architectures in recent years has placed higher demands on the performance of lead-free solders. This review paper introduced the latest research progress on the main lead-free solder systems such as Sn-Sb, Sn-Cu, Sn-Ag, Sn-Zn, Sn-Bi, and Sn-In systems. And the effects of adding microalloying elements, rare earth elements, and nanoparticles on the microstructure, wettability, mechanical properties, corrosion resistance, and service performance of solder were also reviewed. The main development trends of high-performance lead-free solder were discussed, and innovative research concepts and methods were proposed for the development of next-generation brazing materials.

Abstract:Hydrogen separation alloys V85Ti10Y5 and V85Ti10Cu5 were prepared by high vacuum non-consumable arc melting furnace. The effects of the addition of Y and Cu elements on the hydrogen permeability, hydrogen solubility and hydrogen brittleness resistance of the alloy were investigated by SEM, TEM, XRD, hydrogen permeability test, PCT hydrogen absorption test and constant pressure slow cooling test. The results show that the as-cast V85Ti10Y5 and V85Ti10Cu5 alloys are both composed of V-matrix and the second phase, but the second phase of the former is dispersively distributed Y-rich particles, while the latter is a Cu-Ti intermetallic compound precipitated in the grain and distributed continuously along the grain boundary. The formation of Y2O3 in V85Ti10Y5 alloy and the hydrogen-repulsive action of some solid solution Cu in V85Ti10Cu5 alloy and the formation of Cu2Ti reduce the solid solution amount of Ti in V, and then reduce the concentration of hydrogen in the alloy, reduce the internal stress generated by hydrogen solution, improve the anti-hydrogen brittleness. The V85Ti10Y5 and V85Ti10Cu5 alloys showed excellent anti-hydrogen embrittlement properties without hydrogen embrittlement during slow cooling. Moreover, the hydrogen permeability at 673 K is 0.139×10-6 mol H2 m-1 s-1 Pa0.5 and 0.174×10-6 mol H2 m-1 s-1 Pa0.5, which is 5.5 and 6.9 times of Pd77Ag23. Compared with commercial palladium alloys, they all show higher permeability.