Abstract:Cyclic deformation behavior and micromechanisms in strain controlled low cycle fatigue of a near β-Ti alloy Ti-3Zr-2Sn-3Mo-25Nb (TLM) with the different microstructures, which are solution treatment and aging treatment conditions, were comparatively investigated at room temperature. The results showed that the cyclic hardening/softening behavior of TLM alloy was significantly correlated with strain amplitude and microstructure.SThe low cycle fatigue life of the alloy at the solution treatment was higher, and the elastic modulus increased from constant to sharp with the increase of cycle number and total strain amplitude, while the elastic modulus of the aged alloy had a little change during the cycle deformation.SThe microstructure characteristics showed that deformation mechanism of the solid solution alloy was mainly dependent on the formation and their interaction of dislocation slip, the deformation-induced α martensites and the unique "Z" type β twin, while the aging alloy was mainly dependent on dislocation slip.

Abstract:Electric pulse treatment experiments were carried out on lamellar structure TA15 alloy subsequent to uniaxial compression deformation. Then, the morphologies of lamellar structure, dislocation structure and interfacial structure were characterized, and the microstructure evolution mechanisms of pre-deformed TA15 alloy with lamellar structure under the action of electric pulses were analyzed in depth. Results show that electric pulse promotes the atom diffusion and defect reactions in TA15 alloy, and introduces the mechanisms of boundary migration, termination migration and Ostwald ripening, indicating the occurrence of electricity-induced static globularization. With the increase of electric current density and energization time, the globularization rate of lamellar structure increases and the average grain size of α lamella first increases then decreases. Interlamellar shearing is regarded as the main globularization mechanism for the α lamella with severe strain localization, while boundary splitting is found to be the main globularization mechanism for the β matrix with relatively smaller deformation. The additional interfacial strengthening brought by the globularized α phase leads to a maximum micro-hardness improvement of 26.41%.

Abstract:The friction and wear properties of TC4 alloy surface were investigated by using ball/plane contact form of TRB3 friction testing machine. Using different upper samples which were TC4 ball, GCr15 steel ball and Si3N4 ceramic ball studied the friction scratches and wear characteristics of TC4 alloy under different normal forces. The friction scratch morphology was measured by 3D laser confocal microscope, and the energy wear models which could effectively predict the wear process were established to explore the change mechanism of contact state. The results indicated that the friction scratches of TC4 alloy whose length and width increased linearly with the increase of the load and depth fluctuates like a "sawtooth" were affected by the dislocation wall of indenter front and the adhesion particles of indenter offside, because the contact state of ball/plane changed to micro-plane/plane contact state with the enhance of the hardness of the upper sample. The cutting to plasticity ratio (fcp) which distributed to both sides of fcp =0.5 indicated the damage of TC4 alloy by indenter that were interacted by micro-ploughing and micro-cutting interacting to explain protective effect of process hardening phenomenon on the surface. In the meantime, the linear relationship between scratch hardness and surface roughness could predict the damage during material deformation.

Abstract:The thermal compression test of TC6 titanium alloy under low strain rate and large deformation was carried out by Thermecmastor Z100kn thermal simulation machine. The true stress-strain curves were obtained when the deformation temperature was 900 ~ 945℃, the strain rate was 0.0001 ~ 0.1s-1 and the deformation degree was 70%.The traditional friction correction model and the improved friction correction model were used to correct the true stress-strain curve, and the strain-compensated Arrhenius model based on the friction correction was established. The results show that the modified friction correction model can better characterize the true stress-strain dynamic response of materials under large deformation. The strain-compensated Arrhenius model modified by friction model has higher linear correlation, smaller mean absolute error and higher prediction accuracy.

Abstract:A novel deformable Ti-43Al-3.5Mn-0.5W (at.%, the same below) alloy was designed and fabricated. The microstructure characterization, room and elevated temperature tensile mechanical properties, anti-oxidation resistance, and deformation capacity were investigated. The results indicate that compared with Ti-42Al-5Mn, the developed alloy has better strength, high temperature anti-oxidation resistance and thermal deformation ability. The α2 and βo phases of the novel alloy have lower Mn content, which reduces the precipitation tendency of Mn-rich Laves phase at near service temperature. The evolution of the phases in the alloy can be generalized as follows: β→β+α→β+α+γ→β+βo+α+α2+γ→βo+α2+γ, with the Tγ-solv≈1250℃, Tβ≈1360℃. The microstructure of the alloy after heat treatment is lamellar structure and a large number of βo and γ mixed phases at the lamellar interface. Through two-step heat treatment, the high temperature strength and stability of the forged alloy are improved to a certain extent, which is mainly attributed to the increase of lamellar structure content and the refinement size of lamellar colonies. At 800℃, the yield strength, tensile strength, and elongation of the forged alloy treated with 1260℃/30min/AC+800℃/3h/FC were 320 MPa, 555 MPa, and 16%, respectively

Abstract:There are great differences in fatigue cumulative damage and strength degradation of TC4 (Ti-6Al-4V) titanium alloy under different working conditions. In order to fully characterize the influence of load parameters, based on Chaboche damage model and improved multiaxial fatigue damage criterion, a new strength degradation model is proposed to predict the multiaxial high cycle fatigue (HCF) life and evaluate the strength degradation of TC4 titanium alloy. Firstly, the proportional and non-proportional multiaxial fatigue experiments of TC4 titanium alloy under five kinds of loading paths are carried out. The nonlinear multiaxial fatigue damage computation and life prediction of TC4 titanium alloy are conducted via union of the critical plane method and Chaboche damage criterion with the improved damage control parameters. Secondly, a cumulative damage based modified strength degradation model is further established, and it is proved that higher accuracy can be obtained using the model under different load conditions. The experimental results show that, due to considering the influence of load parameters, the accuracy of the prediction results of fatigue life and strength degradation of TC4 titanium alloy proposed in this paper is much higher than other prediction models.

Abstract:It is generally believed that the "β fleck" in TB6 titanium alloy will reduce the mechanical properties of the component, and the main cause of the β fleck in TB6 alloy is Fe element segregation. In this paper, the differences of microstructure, chemical composition and hardness between β fleck and normal area were analyzed by OM, SEM, EDS and EPMA, and the effect of β fleck on the tensile deformation behavior of TB6 alloy was studied. The results show that the content of primary α phase in β fleck is less than 5%, and the β grain size is coarse up to 0.58mm, which is more than 60 times of normal grain size. The vickers hardness of β fleck was slightly higher than that of normal area. The results of composition analysis showed that there was no significant difference of V and Fe between in β fleck and normal area, but there were fluctuations and uneven distribution of Fe content within the microregions . Fe content dispersion in β fleck was higher than that in normal area. The difference of V and Fe elements in microregions is the main reason for the difference in the phase transition temperature of TB6 alloy during subsequent heating and deformation. The preconverted β grains grow rapidly due to the lack of grain boundary α phase pinning. In the process of in situ SEM tensile, the crack starts at the grain boundary of the coarse β grain, grain boundary and intra-crystal slip is the main deformation mode of β fleck structure. The fracture mode of TB6 alloy contains of β fleck is intergranular and transgranular mixed fracture mode.

Abstract:Developing novel materials for nuclear reactors is a crucial research task. Due to the harsh conditions in the reactors, core materials must possess exceptional high-temperature properties, including high strength, ductility, corrosion resistance, and irradiation resistance. Additionally, the low neutron absorption cross-section and the neutron activation are also important considerations. It is widely acknowledged that the choice of core material for a typical space nuclear reactor is primarily determined by the operation temperature. Generally, with increasing the designed operation temperature of reactor, 316 stainless steel, nickel-based superalloys, oxide-dispersion-strengthened (ODS) steel, refractory metals, and SiC ceramics are preferred in order. Besides, the high entropy alloys attract extensive attention for nuclear applications. This review summaries the evolution of mechanical properties of different material systems during irradiation process, providing guidance for the further research in irradiation resistance.

Abstract:Ti and its alloys have become one of the most widely used biomedical metal materials due to their lightweight, low elastic modulus, excellent biocompatibility, and superb osseointegration. However, their low plasticity, inferior corrosion resistance, and poor wear resistance restrict the development and applications of Ti and its alloys. Severe plastic deformation is considered as one of the most effective methods for grain refinement of metal materials. Additionally, equal channel angular pressing (ECAP) is the commonly used preparation method for bulk ultrafine-grain (UFG)/nanocrystalline metal materials. Through ECAP deformation, UFG Ti and its alloys with superior comprehensive properties can be prepared. In this research, the progress of ECAP preparation for biomedical UFG Ti and its alloys was reviewed. The effects of ECAP deformation on the microstructure, mechanical properties, corrosion resistance, and wear resistance of Ti and its alloys were discussed. The deformation and grain refinement mechanisms were analyzed. The development direction of further optimization in the properties of Ti and its alloys through ECAP coupled with traditional plastic working methods and post-deformation heat treatment was proposed.

Abstract:Laser remelting enables the solidified layers to melt and solidify rapidly again, thus could improve the relative density and surface quality. It is a type of surface modification technologies, and has been widely used in traditional manufacturing processes. Recent researches showed that laser remelting could be used to refine the microstructure, reduce the porosity and improve surface quality of SLM processed metals and alloys. Laser remelting technology can also improve the mechanical properties such as hardness and ductility. This paper made a thorough literature survey and summarized how laser remelting could affect the porosities, microstructures, surface qualities and residual stress of SLM processed metals and alloys. Furthermore, the working parameters of laser remelting and their influences on the porosities, microstructures, surface qualities and residual stress are introduced.

Abstract:Refractory metals are widely used in aerospace, equipment manufacturing, nuclear industry and biomedical fields due to their excellent comprehensive properties. However, due to the characteristics of high melting point and high ductile-brittle transition temperature, there are still some problems such as difficult manufacturing, long production cycle and high equipment requirements, which limit its application and development. Laser additive manufacturing (LMD) is one of the emerging digital manufacturing technologies in recent years, which provides a new development idea for manufacturing and processing refractory metals. In this paper, the hot fields of laser additive manufacturing for refractory metals in recent years were introduced, including tungsten and tungsten heavy alloys (WHAs), pure molybdenum and molybdenum-silicon-boron alloys (Mo-Si-B), niobium-silicon and niobium-titanium alloys and porous tantalum, and the existing problems were summarized. Finally, the future development direction of laser additive manufacturing for refractory metals was prospected.

Abstract:Multicomponent lanthanum based hexaborides which formed by solid solution of heterometallic elements into the lattice of lanthanum hexaboride (LaB6) to replace La atoms, have great application prospects in electronics and optics due to their low work function and adjustable light absorption, and attract an increasing attention in recent years. However, multicomponent lanthanum based hexaborides have great varieties and complex change because of the various kinds and countless combinations of heterometallic elements used to replace La atoms. Herein, research progress on crystal structure, preparation technology and photoelectric performance of multicomponent lanthanum based hexaboride solid solutions are reviewed in this paper. Meanwhile, current challenges faced by multicomponent lanthanum based hexaboride solid solutions are analyzed. Lastly, future development directions of theory, technology and application of multicomponent lanthanum based hexaboride solid solutions are prospected.

Abstract:Currently, bonding the catalyst to the substrate by using adhesive is the mainstream way to prepare electrodes for hydrogen precipitation in electrolytic water, but the adhesive itself has high resistance and low bonding strength, and is easy to fall off. Metal Pt is currently recognized as the best metal electrode for hydrogen precipitation, but its expensive price limits the industrial use on a large scale. A self-supporting titanium-copper laminate electrode was constructed by combining titanium, which has low overpotential for gas precipitation, good stability and high strength, with copper, which has excellent electrical conductivity, through explosion welding technology. Electrochemical tests have shown that the laminated electrode exhibits good electrolytic hydrogen precipitation (HER) performance and conductivity and excellent stability in 1 mol/L KOH solution with a potential fluctuation of only 0.05 V at a current density of 10 mA-cm-2 and a stability test time of 20,000 s. The uneven and porous surface morphology produced during operation has been used to expose more of the electrodes. The experimental results of hydrogen precipitation measurement of electrolytic water can also verify the superior hydrogen precipitation performance of titanium-copper laminated electrodes in practice. The surface morphology exposed more active sites and electrochemically active surface area, so that the HER performance of the laminated electrode will not decrease and will be slightly improved during long-time operation, which provides an effective and cost-reducing direction for electrode material preparation and has a broad engineering application prospect.

Abstract:The anisotropy of Poisson’s ratio of single crystal superalloy is essential to understand its mechanical behavior, e.g. calculating the contact stress between blade tenon and turbine disc. However, it’s difficult to determine the Poisson’s ratio of single crystal superalloy of every orientation. In this paper, one simple experimental method is employed to measure the stiffness constants and then the Poisson’s ratio of different orientation is calculated. The slabs of a third generation single crystal superalloy in two orientations á001?á100? and á011?á110? are prepared by seeding technique in Bridgman method. The Young’s modulus and shear modulus of the first specimen and the shear modulus of the second specimen are measured by resonance method from room temperature to 1100 ℃. The three stiffness constants C11, C12 and C44 of this superalloy are calculated from the measured moduli. The Poisson’s ratio in any orientation can be calculated based on the stiffness constants. Further, the 3D distribution map of maximum and minimum of Poisson’s ratio of every primary orientation can be drawn, so the distribution feature of Poisson’s ratio in 3D space can be acquired conveniently. When the primary orientations are along á001? and á111?, the Poisson’s ratio in plane is isotropy with secondary orientation. When the primary orientation is along á011?, the Poisson’s ratio demonstrates significant anisotropy with secondary orientation, the Poisson’s ratio reach minimum with secondary orientation á110? with negative value, while maximum is obtained in secondary orientation á100?.

Abstract:SiC nanowires with excellent high temperature strength, high thermal conductivity, high wear resistance, and high corrosion resistance were adopted as additive into the Ni-Cr-P filler metal. The microstructures and properties of filler metal/brazed joints were studied. Results show that the microstructure of filler metal is composed of Ni(Cr) solid solution, Ni₃P intermetallic compound phase, and Ni(Cr)+Ni₃P eutectic structure. A small amount of SiC nanowire can refine the matrix microstructure and improve the shear strength of brazed joints by 29.6%. The addition of SiC nanowire can increase the melting temperature of filler metal by about 4 °C and significantly enhance the wettability of filler metals on steel substrate by 12.5%. However, excessive addition of SiC nanowire can significantly coarsen the matrix microstructure, reduce the wettability of filler metal, and decrease the shear strength of brazed joints. Among the Ni-Cr-P filler metals with different SiC contents, Ni-Cr-P-0.1SiC filler metal/brazed joint shows the obvious superiority.

Abstract:The microstructure evolution, grain boundary character distribution, strain distribution, and texture evolution of Ni-based superalloy during cold rolling and subsequent recrystallization annealing treatments were studied by electron back-scattered diffraction technique. Results show that when the cold deformation degree is small (ε≤45%), the grains are elongated along the rolling direction into a flat shape and distributed evenly in the matrix. The strain is mainly concentrated near the grain boundary and the twin boundary (TB), and the high-angle grain boundaries (HAGBs) and TBs are gradually transformed into sub-grain boundaries (Sub-GBs) and low-angle grain boundaries (LAGBs). Meanwhile, the Goss texture {110}<001>, Brass-R texture {111}<112>, Twinned-Copper texture {552}<115>, and Copper texture {112}<111> appear. When the rolling reduction exceeds 70%, the grain shape gradually changes from flat to fibrous, the deformation uniformity of the grains gradually becomes better, the strain distribution becomes uniform, and LAGBs begin to dominate. In addition, the texture types do not change, but the texture intensity increases. After the annealing at 1120 °C for 15 min, the length fraction of annealing twins is increased with increasing the rolling reduction. Besides, the deformation textures are transformed into the recrystallization textures, the texture types are increased, but the texture intensity weakens. Furthermore, the Copper texture {112}<111> is continuously transformed into the Twinned-Copper texture {552}<115> when the proportion of annealing twins increases. Additionally, the {124}<211> texture is generated in the as-annealed alloy after rolling reduction of 30%–80%.

Abstract:Pb-Co anodes with different Co contents (0.5wt%, 1wt%, and 2wt%) were prepared by powder metallurgy, and the traditional Pb-Ca-Sn anode was used as the reference for comparison. The electrochemical behavior of the anodes in the electrolyte containing 160 g/L H₂SO₄ and 500 mg/L Cl^- ions was evaluated by electrochemical measurements. The phase composition, surface morphologies, and element distribution of the anodic oxide layers after galvanostatic polarization for 72 h were investigated. With increasing the Co content, the anodic potential, charge transfer resistance, and oxygen evolution overpotential of the Pb-Co anodes are decreased gradually. After galvanostatic polarization for 72 h, the oxygen evolution overpotential of Pb-2wt% Co anode is lower than that of Pb-Ca-Sn anode by 101 mV. In addition, due to the presence of Cl^- ions, the charge transfer is improved, the oxygen evolution reaction is inhibited, and the oxide layer is deteriorated.

Abstract:The corrosion resistance of 2A97-T3 and 2A97-T6 Al-Cu-Li alloys was studied by the electrochemical method through potentiodynamic polarization curves. The typical third generation 2060-T8, 2099-T83, and 2024-T4 alloys were used as reference for comparison. Through the analysis of electrochemical parameters and corrosion morphology, the results reveal that the corrosion resistance of the alloys in NaCl solution of different concentrations is as follows: 2A97-T3>2A97-T6>2024-T4>2060-T8>2099-T83. With increasing the concentration of NaCl solution, the corrosion potential (E_Corr) of all alloys is decreased. Moreover, the surface pitting and intergranular corrosion become severe. The T₁ phase greatly increases with more uniform distribution in 2A97-T6 alloy, which is obtained from the 2A97-T3 alloy after the solid solution coupled with double-stage artificial aging treatment. Consequently, the heat treatment process reduces the corrosion potential of 2A97-T6 Al alloy, which leads to the slightly weaker corrosion resistance of 2A97-T6 Al alloy than that of 2A97-T3 alloy. The disintegration of the intergranular θ phase induces the exfoliation corrosion morphology of 2A97-T3 alloy, and the pitting morphology of 2A97-T6 alloy is caused by the dissolution of intragranular T₁ phase.

Abstract:Al-Mn-Mg-Sc-Zr alloy was prepared by selective laser melting (SLM) technique. The microstructure of Al-Mn-Mg-Sc-Zr alloy before and after heat treatment was characterized by X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, and transmission electron microscope, and the hardness of the alloy was tested. Results show that the microstructure of as-deposited alloy is mainly composed of α-Al, Al₆Mn, and primary Al₃Sc phases. The SLMed molten pool presents the fish-scale shape, and a large number of fine equiaxed crystals are formed near the fusion line of molten pool with average grain size of about 0.57 μm. The center of the molten pool is composed of columnar crystals with average width of about 0.48 μm. The rodlike Al₆Mn phase is mainly distributed along the grain boundary, and a small amount of granular primary Al₃Sc phase exists inside the grain, which indicates that the primary Al₃Sc can be used as heterogeneous nuclear particle to refine the α-Al matrix. After heat treatment, the size of equiaxed crystals and the width of columnar crystals in the molten pool are increased, and a large number of fine and dispersed secondary Al₃Sc particles are precipitated in the structure. The hardness test results show that the hardness of the alloy is increased firstly and then decreased with prolong the aging durations at different temperatures. Compared with that of the as-deposited alloy, the hardness of the alloy after peak aging (325 °C/180 min) heat treatment increases by about 30%, reaching 1813.0 MPa.

Abstract:The etching of low energy argon (Ar) ion beam of 0–1000 eV on Be was investigated. Different surface polishing methods were compared. Results show that the high-quality Be surface can be obtained by Ar ion beam etching. With the etching proceeding, the Be surface quality is gradually improved, and the surface roughness becomes stable, reaching 0.63 μm after etching at 600 eV and 100 mA for 6 h. White light interferometer (WLI) and focused ion beam (FIB) measurement methods were compared. Results show that FIB measurement method is more suitable for measurement of Be etching thickness. The experiment results and theoretical calculations suggest that the Be sputtering process is similar to the ionization process of Be by Ar ion bombardment. The influence law of Ar ion energy on sputtering yield of Be can be obtained with the first ionization energy as the sputtering threshold, and the variation of Be etching rate with the product of Ar ion beam energy and sputtering yield is obtained, providing foundation for engineering application of Be etching.

Abstract:The Al₃Zr/A356 aluminum matrix composites (AMCs) with 3wt% reinforcing phase were prepared by in-situ reaction method. Through X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, and microhardness tests, the microstructures and corrosion resistance of the welded joints after tungsten inert gas (TIG) welding with different welding parameters were investigated. Results show that when the welding current is 140 A, the formation of weld seam is optimal, and no welding defects, such as pores or cracks, appear. The fine Al₃Ti reinforcing particles are generated during the welding process, presenting the spherical and short rod shapes, and they are dispersed in the matrix. The hardness of the welded joint is higher than that of the base metal, and the strengthening effect of the reinforcing particles becomes obvious. With prolonging the immersion duration in 3.5wt% NaCl solution, the pitting degree of weld seam is aggravated, and the pitting mainly occurs around the grain boundaries and strengthening phases. The micro-area electrochemical experiment results show that when the welding current is 140 A, the corrosion potential fluctuation is small, the corrosion tendency is low, and the corrosion resistance is optimal.

Abstract:To improve the processing and practical performance during the manufacture of complex thin-walled parts, the mechanical properties and microstructure revolution of complex thin-walled parts of 6061 aluminum alloy prepared by ellipse bidirectional vibration incremental forming were investigated. The traditional incremental forming, vertical unidirectional ultrasonic vibration incremental forming, and ellipse bidirectional ultrasonic vibration incremental forming methods were compared. To verify the enhancement effect on forming quality and mechanical properties of complex thin-walled parts of aluminum alloy, the microhardness and residual stress of as-prepared parts were analyzed, and their morphologies were observed. The microhardness test results show that the ellipse ultrasonic incremental forming process can soften the material and improve its plasticity and toughness, thereby promoting the formability of thin-walled parts of aluminum alloy. A great number of dimples appearing on the fracture surface further confirms this conclusion. The microstructure characteristic on the surface of as-prepared parts shows that the ellipse bidirectional vibration incremental forming method can significantly improve the surface quality. In addition, the ellipse bidirectional ultrasonic vibration incremental forming method can form a residual compressive stress layer on the surface of 6061 aluminum alloy, which improves the fatigue resistance performance of complex thin-walled parts.

Abstract:The adsorption and dissociation behavior of O₂ molecules on U-Mo alloy surface was studied based on the first-principles simulation. One U atom at the highly symmetrical adsorption sites of the top layer was replaced by one Mo atom, and four U atoms at the top layer were replaced by four Mo atoms, resulting in the fact that γ-U(100)/Mo and γ-U(100)/4Mo slabs were established on the basis of the γ-U(100) slab with five layers. The configuration parameters, adsorption energy, Bader charge, electronic structure, and surface work function were calculated under different adsorption configurations. Results show that the O₂ molecules are chemically absorbed on the γ-U(100)/Mo and γ-U(100)/4Mo surfaces with the adsorption energy of -12.552 and -8.661 eV, respectively. The most stable adsorption configuration is the hollow horizontal adsorption configuration. The O₂ molecule adsorption on U-Mo alloy surface can be divided into dissociated adsorption and undissociated adsorption, which jointly contribute to the stable adsorption behavior. In addition, the dissociated adsorption is more stable than the undissociated adsorption. The Bader charge results show that during the oxygen adsorption, the charge transfer mainly occurs at the atoms of the top two layers of adsorption surface. The electronic structure results show that the slight overlapping hybridization occurs in O 2s with U 6p orbitals. Meanwhile, the strong overlapping hybridization occurs in O 2p with U 6d, Mo 5s, Mo 4p, and Mo 4d orbitals. This research clarifies the O₂ molecule adsorption mechanism on U-Mo alloy surface and provides theoretical basis for the oxidation corrosion mechanism of U-Mo alloy surface.

Abstract:The quasi-static and dynamic mechanical properties of TWIP steel were investigated by using Zwick/Roell Z100 universal testing machine and Hopkinson tensile bar. The strain hardening and strain strengthening in Johnson-Cook dynamic constitutive model were modified based on mechanical test results. The microstructure of TWIP steel before and after tensile deformation were analyzed by XRD, SEM and EBSD. The findings demonstrate that: When subjected to quasi-static loads, TWIP steel exhibits negative strain rate sensitivity and positive strain rate sensitivity when subjected to dynamic loads. Twinning induced plasticity is the primary deformation mechanism of TWIP steel during tensile process, and slip also plays a significant role. The initial strain and volume fraction of deformation twins in TWIP steel under dynamic loading are smaller than those under quasi-static loading. TWIP steel has exceptional strength and plasticity because deformation twins and the grain refinement brought on by twin contact, implying that it has a broad application in the fields of impact resistance and anti-explosion.

Abstract:The oxidation behavior of NbC particle-reinforced cobalt-based wear-resistant alloy in the air from 950 to 1050℃ was investigated. The alloy belonged to the oxidation-resistant level at 950°C, the sub-oxidation-resistant level at 1000°C, and the non-oxidation-resistant level at 1050℃. The oxidation kinetic curves basically complied with the parabolic law. A mixed oxide layer consisting of CoCr2O4, CoNb2O6, Cr2O3, and Al2O3 was established on the alloy surface. The preferential in-situ oxidation behavior of the blocky NbC phase in the matrix resulted in the loose porous oxide layer and a non-uniform thickness. After oxidation of 1050℃/100h, a Cr depletion zone was formed in the matrix below the oxide layer, leading to the inability to re-form a continuous and protective oxide film after oxide peeling.

Abstract:Abstract: Al-Li alloy is a new alloy material used in aviation manufacturing. The riveting technology of Al-Li alloy is an important research direction of modern aircraft manufacturing. The distribution of residual stress after riveting is affected by the amount of interference in the hole of Al-Li alloy sheets. The automatic dirlling and riveting equipment was used to rivet Al-Li alloy sheets of different thickness for the purpose of researching the relationship between riveting process and residual stress distribution. The automatic riveting process of Al-Li alloy was simulated by ABAQUS/Explicit, the interference amount generated in Al-Li alloy wall under different riveting force, rivet length, the thickness of sheets and rivet material was analyzed by experiment and simulation. Then the residual stress distribution characteristics along the thickness direction of Al-Li alloy sheet under various conditions was deduced. The resules indicate that the interference produced in the hole wall of Al-Li alloy sheets and the residual stress have the highly consistent tendency under various technological conditions. The specific trends are as follows, in which the interference and residual stress increase with the argument of the riveting force. However, they will decrease with the increase of the rivet length. As the total thickness of the sheet is increasing, the average interference amount and the average residual stress increase in the beginning and then decrease. When the total thickness of the sheet is 4.2mm, the interference and residual stress arrive at the peak value. In addition, the average interference amount produced by 7050-T73 aluminum alloy rivets is reduced by 6%~12% compared to the 2117-T4 aluminum alloy rivets. And the residual stress is reduced by 8%~12%.

Abstract:In order to improve the quality of explosive welding of 5083Al and 304 stainless steel and improve the thermal insulation effect, in this paper, 1060Al, TA1, and Ni were used as interlayer materials to prepare a five-layer explosive composite plate with thermal conductivity gradient. In order to eliminate the residual stress after explosive welding and reduce defects such as adiabatic shear bands and micro-cracks, the five-layer explosive composite plate was annealed by 550℃/60min, and analyzed by means of SEM, EBSD, and the universal testing machine. The effect of annealing on its microstructure evolution and mechanical properties was studied. The results showed that the four welding interfaces of the five-layer explosive composite plate were all corrugated, and there were defects such as micro-cracks, holes, adiabatic shear bands, and vortex areas at the interfaces. After annealing, the four welding interfaces were recrystallized to different degrees, and the defects such as microcracks and adiabatic shear bands were effectively improved; On the basis of the original TiNi3 melting layer, the interface added a new TiNi melting layer and a new Ti2Ni melting layer. The tensile and shear strengths of the interface had decreased, but they were still far higher than the requirements of the corresponding national standards; the pull-off samples were fractured and separated at the 5083Al/1060Al interface.

Abstract:In order to explore the corrosion behavior of Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge (mass fraction, %) alloy in 400 ℃ super-heated steam with different oxygen contents, Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge alloy specimens were put into static autoclave and dynamic autoclave for super-heated steam corrosion tests under deaeration, 300 ppb dissolved oxygen (DO) and 1000 ppb DO environment at 400 °C/10.3 MPa, respectively. The scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscope were used to characterize the microstructure of alloys and oxide films, as well as the valence state of the alloying element. Results show that compared with the deaeration environment, the average corrosion rate of Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge alloy in 300 ppb DO and 1000 ppb DO environments increases by 23.5% and 29.4%, respectively, indicating that DO can accelerate the corrosion of the alloys, and the higher the DO content, the more obvious the corrosion acceleration effect. DO not only promotes the oxidation of Fe, Cr, Sn and Ge in the oxide film, but also accelerates the reaction process of Zr→ZrOx→ZrO2 in the transition layer at the O/M interface. Based on the above two aspects, a mechanism of DO accelerating the corrosion of zirconium alloys at 400 °C/10.3 MPa is proposed: The increase in DO content in the corrosive environment leads to an increase in the concentration of O2- and OH- participating in the reaction in the oxide film. On one hand, this promotes the oxidation of alloying elements, leading to the increase of defects in number and local stress in the oxide film; on the other hand, this also accelerates the evolution of Zr→ZrOx→ZrO2 at the O/M interface, resulting in higher stress in the oxide film and providing less time for stress release during oxidation. The roles of the two aspects both promote the generation of pores and cracks, and accelerate the diffusion of O2- and OH-, thus accelerate the corrosion of zirconium alloys.

Abstract:The residual thermal stress (RTS) distributes heterogeneous and has complex interactions with the applied loadings, which significantly influences the mechanical properties of cemented carbides. The traditional experimental and simulation methods can only analyze the stress distribution statistically, and they cannot be applied in the study on the microstructure scale to get the detailed stress distribution. Using the finite element method, this study investigated the RTS in cemented carbides and the stress distribution under different loading conditions. Based on the stress analysis, it proposed a method to strengthen cemented carbides by tailoring their microstructures. The new approach presented in this study applies to stress analysis and microstructure tailoring for a broad range of multiphase composites.

Abstract:In the arc additive manufacturing of aluminum alloy, different frequencies (30 KHz, 40 KHz, 50 KHz) of ultrasonic stirring were applied in the molten pool using a stirring needle moving synchronously with the arc, and the results showed that 1) the organization of the additive layer showed grain refinement, a decrease in the number of columnar crystals, and an increase in the number of equiaxed crystals compared with that without ultrasonic stirring; and the grain refinement became more obvious as the ultrasonic frequency increased; 2) the porosity of the additive layer ultrasonic stirring tended to decrease with increasing ultrasonic frequency, 0.73%, 0.64%, and 0.59%, respectively, but increased compared to the specimen without ultrasonic stirring (0.2%); 3) the tensile strength of the accretion layer was 266.57 MPa (without ultrasonic), 278 MPa, 282 MPa, and 299 MPa, respectively, and the elongation was 30.67% (without ultrasonic) 31.54%, 35.53%, 41.86%; with the increase of ultrasonic stirring frequency, the strength and plasticity of the reinforced layer were increased.

Abstract:Inconel 625 base metal and weld metal were subjected to 75% Na2SO4-25% NaCl mixed molten salt hot corrosion experiments at 750°C. The physical phase, surface morphology, corrosion weight loss rate and corrosion mechanism of corrosion products were studied and analyzed. The results show that the organization of Inconel 625 joints is austenitic equiaxial crystal at the base material and dendrite at the weld. The weight loss curves of both base metal and weld metal follow parabolic laws with weight loss rate constants of 3.43 and 4.18, respectively. The weld metal is less resistant to corrosion compared to the base material due to the precipitation of Nb-rich second phase, and corrosion exhibits "inhomogeneity". Inconel 625 in both states has the same corrosion layer structure, the outer layer of NiO particles and lamellar NiCr2O4, the middle layer of dense Cr2O3 oxide layer, and the inner layer of "honeycomb" Ni2S3 sulfide layer of the three-layer structure. The main corrosion mechanism of Inconel 625 alloy in 75% Na2SO4-25% NaCl mixed salt at 750°C is: "sulfidation-oxidation" corrosion.

Abstract:The Cu-10wt.%WC composite powder was prepared by reduction, and the Cu-10wt.%WC composite material was obtained by hot-pressing sintering, then hot rolled. Scanning electron microscope, X-ray diffractometer, transmission electron microscope and tensile test were used to study the effect and mechanism of hot rolling on the microstructure and properties of Cu-10wt.%WC composites. The results show that, after hot rolling, the WC particle size does not change but rearranges along the rolling direction in composites. The grain size of Cu decreases, and when the copper matrix recrystallizes, it grows preferentially along the (220) crystal plane, which reduces the misfit of the interface with the WC particles. After rolling, the tensile strength of the composites increased from 426MPa to 492MPa, the hardness increased from 149.7 HV0.2 to 166.8 HV0.2, and the electrical and thermal conductivity remained unchanged. Hot rolling can adjust the distribution of dislocations by rearranging WC particles, improve the bonding strength of Cu grains interface with WC particles, and strengthen the grain refinement, which improves the comprehensive properties of composites.

Abstract:The mixed powders of Ti-48Al-2Nb-2Cr alloy with multilayer graphene oxide, SiC particles and BN nanosheets were sintered by spark plasma sintering (SPS) at 1300 ℃, and the different reinforcement on microstructure evolution and properties of TiAl composites were investigated. The results showed that the microstructure of composites was significantly changed with the addition of different reinforcement. The micro-nano Ti₂AlC in-situ precipitated at the interface of α₂ lath and γ lath of TiAl matrix after adding graphene. While a microstructure composed of networked core-shell (CS) structured cells can be formed in TiAl matrix with the addition of SiC particles and BN nanoplatelets. Moreover, Ti₂AlC can be in situ precipitated and CS structure can be formed by adding graphene and BN nanoplatelets simultaneously. The compressive properties and wear properties of TiAl composites were improved significantly, which showed optimal performance by adding graphene and BN nanoplatelets simultaneously. TiAl composites provided a new idea for alloy designation and broken through the performance limitation of TiAl alloy.

Abstract:Al_0.1CoCrFeNiTi_0.1 high-entropy alloy with FCC/L1₂ two-phase coherence was prepared by vacuum electromagnetic suspension melting (VMIS) method combined with specific heat treatment process and the quasi-static uniaxial constant temperature compression test of the system is carried out in a wide temperature (298K-973K).Observation of the microstructure and morphology of high-entropy alloys by SEM and XRD. At the same time, the lattice parameters and thermodynamic parameters of the FCC solid solution phase and the L1₂ phase nanoprecipitation phase were calculated according to the thermodynamic and crystallographic theories, and the reasons for their formation were explained In addition, the size, spacing and volume fraction of the L1₂ phase within the crystal and at the grain boundary were automatically counted by Image j image processing software. The results show that the ratios of L1₂ phase to FCC phase lattice constant and interplanar spacing are about 1.008 and 0.605, respectively. The average atomic radius of the high-entropy alloy ;the average mixing entropy ;

Abstract:In this paper, cold metal transfer + pulse welding (CMT+P) method was used to weld 7A52 aluminum alloy, and the effects of different heat treatment processes on the microstructure and mechanical properties of welded joints were studied. The results show that the microstructure dominated by dendrite or equiaxed dendrite in the weld zone has transformed into equiaxed crystal after heat treatment, but the grain structure and the size of the second phase of AA(Artificial Aging) sample are coarser, and there is segregation at the grain boundary. The amount of precipitates in T6(Solid Solution + Artificial Aging) weld zone is more than AA, and the size of η′(MgZn2) strengthening phase and the width of precipitation free zone is decreased in the heat affected zone. The joint strength of AA and T6 samples reached 315 MPa and 356 MPa respectively. AA has limited improvement on the mechanical properties of the joint, while T6 has increased by about 14.10% compared with AW. The micro fracture of AA sample is mainly dimple transgranular fracture, while the number of dimples in the fracture of T6 sample is significantly reduced and replaced by an uncharacteristic platform. At this time, the joint is mainly transgranular + intergranular ductile brittle mixed fracture.

Abstract:Zr-0.3Nb-xCr (x=0.2, 0.5, 1.0 wt.%) alloys were prepared and corroded in 400 ℃/10.3 MPa superheated steam in an autoclave, and the microstructures of the alloy matrix and oxide film were characterized using SEM and TEM, in order to investigate the effect of Cr on the microstructure and corrosion behavior of Zr-0.3Nb alloy. It was found that the second phase particles in the Zr-0.3Nb-xCr alloys were face centered cubic and hexagonal close-packed ZrCr₂ phase with a size in a range of 10~100 nm. By increasing the Cr content, the size of second phase particles was nearly unchanged, but their number was increased. Appropriate Cr addition to the Zr-0.3Nb alloy could promote the growth of columnar crystals in the oxide film and delay the transformation from columnar crystals to equiaxed crystals, thereby improving the corrosion resistance of the Zr-0.3Nb alloy. The corrosion resistance was relatively better when 0.5 wt.% Cr was added to the Zr-0.3Nb alloy. This may be due to the compact oxide film and the presence of a sub-oxide layer at the oxide/metal interface, which could delay the microstructure evolution of the oxide film and thus improve the corrosion resistance of the Zr-0.3Nb-0.5Cr alloy.