Abstract:The β grain growth behavior of a novel high strength TB18 titanium alloy was investigated through the variation of β grain size in TB18 titanium alloy under the conditions of different temperatures and holding times. Results show that the heat treatment temperature and holding time have significant effects on the β grain growth of TB18 titanium alloys. The β grain size is increased with increasing the solution temperature and holding time. Besides, the coarsening temperature of TB18 alloys is 920 °C. The grain growth exponents and grain growth activation energy were calculated by Beck equation and Arrhenius equation, respectively. The grain growth exponent n is 0.13~0.26 for the TB18 alloys. The grain growth activation energy of the TB18 alloy is 34.27~60.58 kJ/mol.

Abstract:A novel approach was developed to select the optimal duration for solid solution heat treatment of Ni-based single-crystal (SX) superalloys based on the statistical analysis of γ′ phase size in the second-generation SX superalloys. The microstructure uniformity ω was introduced through the statistical analysis. The ω curves can be divided into three stages according to the statistical analysis: the significant change area, transition area, and steady state area. The transition area shows the optimal duration range for solution heat treatment of Ni-based SX superalloys. This approach was verified by measuring the γ′ rafting rate and testing the creep properties of alloys. Meanwhile, the accuracy of this approach was also validated through the fourth-generation Ni-based SX superalloys. This approach can effectively optimize the heat treatment and provide guidance for the design and engineering applications of SX superalloys.

Abstract:The microstructure and mechanical properties of Mg-Zn-Y-Nd-Zr alloys were studied in this research. Results show that the added Nd element partially replaces the Y element in the W phase (Mg₃Zn₃Y₂), forming a new secondary phase Mg₃Zn₃(Y, Nd)₂. A typical bimodal structure consisting of fine equiaxed recrystallized grains and coarse elongated unrecrystallized grains can be observed in the alloy after thermal extrusion. The Nd addition promotes the dynamic recrystallization during hot extrusion. With increasing the Nd content, the dynamic recrystallization ratio is increased, and the overall texture strength of the as-extruded alloy is weakened. The addition of Nd element refines the grains and improves the mechanical properties of Mg-Zn-Y-Zr alloy. When 0.5wt% Nd is added, the mechanical properties of as-extruded alloy show a good combination of high strength and high plasticity: the yield strength is 362 MPa, the ultimate tensile strength is 404 MPa, and the elongation is 10.2%. After aging treatment, the tensile strength of the alloy is further improved, and the ultimate tensile strength of the alloy after peak aging treatment reaches 421 MPa. The high strength of the Mg-Zn-Y-Nd-Zr alloy is mainly attributed to ultrafine recrystallized grains and precipitation strengthening.

Abstract:A novel high-temperature IN690 alloy was prepared via a hot extrusion process. The Gleeble-3500 thermal simulation test machine was used to perform isometric compression tests to study the effects of different temperatures, strain rates, and deformations on the dynamic recrystallization (DRX) of IN690 alloy. Using metallographic microscopy and electron backscatter diffraction (EBSD), the metallographic structure, grain orientation, grain boundary distribution and grain orientation difference of the IN690 alloy before and after thermal deformation were systematically analyzed. The true stress-true strain curve obtained using the test showed that as the temperature decreased or strain rate increased, the IN690 alloy flow stress increased. The major softening mechanisms during the IN690 alloy deformation were the dynamic recovery (DRV) and DRX; The proportions of high-angle grain boundary increased with an increase in the true strain or a decrease in the strain rate due to the the DRX nucleation for a large true strain or low strain rate.

Abstract:La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3 master alloys were prepared by medium frequency induction furnace, then annealed, saturatedly hydrogenated, and finally crushed into powders. The multiple components of La_0.8Ce_0.2Fe_11.7-xMn_xSi_1.3H_1.8 (x=0.23, 0.26, 0.29, 0.32, wt%) powders with the Curie temperature (T_C) interval of 5 K were mix-bonded by epoxy resin to extend the full width at half maximum of magnetic entropy of alloy. The magnetic properties of the mix-bonded specimens were measured by VersaLab and adiabatic temperature change direct test device. The maximal magnetic entropy change of the mix-bonded specimens is decreased, whereas the full width at half maximum of magnetic entropy and the relative cooling power are increased, compared with those of the single-component-bonded specimens. The maximal relative cooling power is 139.2 J/kg for the four-component-bonded specimen.

Abstract:The (Zr_0.55Cu_0.3Al_0.1Ni_0.05)_100-xEr_x (x=0, 0.5, 1, 2, 3, 4, 5) Zr-based bulk alloy with diameter of 3 mm was prepared by water-cooled copper crucible smelting and copper mold suction casting using zirconium sponge as raw material. By comparing the microstructure and mechanical properties of sponge Zr-based alloy with different Er contents, the influence of Er on the amorphous forming ability, thermal stability, and mechanical properties of Zr-based bulk alloy was studied. Results show that when the zirconium sponge is used as the raw material, the amorphous formation ability and mechanical properties of sponge Zr-based alloy are obviously reduced. The sponge Zr-based amorphous alloy cannot be prepared when x=0. After adding Er element, the amorphous forming ability and mechanical properties of the sponge Zr-based alloy with the amorphous structure are improved obviously. The optimal mechanical properties of the sponge Zr-based alloy are achieved when x=2: the compression strength σ_bc is 2142.5 MPa and the plastic strain ε_p is 10.01% at room temperature. Compared with the amorphous (Zr_0.55Cu_0.3Al_0.1Ni_0.05)₉₈Er₂ alloy with the diameter of 3 mm prepared from high-purity Zr, the compressive strength and plasticity at room temperature of the sponge Zr-based alloy recovers by 97.63% and 69.95%, respectively. The Er addition is beneficial to improve the amorphous forming ability and mechanical properties of sponge Zr-based alloy, which also provides a new approach for low-cost preparation of Zr-based amorphous alloy.

Abstract:The high-temperature oxidation experiments were conducted on the micro-arc oxidation film on 2A12 aluminum alloy surface. Results show that the high-temperature oxidation resistance of the micro-arc oxidation film is decreased with increasing the temperature. However, the oxidation index is above 2, indicating that the micro-arc oxidation film has a protective effect for the 2A12 aluminum alloy and can effectively prevent the oxygen diffusion during high-temperature oxidation. The air-cooled thermal shock resistance property of the micro-arc oxidation film layer is better than its water-cooled thermal shock resistance property. After 60 thermal shock cycles, the film falls off at the corners of alloy after water-cooled thermal shock, while only cracks appear on the surface of alloy after air-cooled thermal shock without film shedding. The failure of micro-arc oxidation film after thermal shock is mainly caused by the difference in coefficients of thermal expansion between film and substrate and the formation of hydroxides and oxides through the reactions of film with H₂O and oxygen, respectively. The CeO₂ inside the film can reduce the pore size and the impact of thermal shock on the film at the initial reaction stage. However, with increasing the thermal shock cycles, the subcarbonate and hydroxide are generated, leading to the failure of micro-arc oxidation film.

Abstract:The arc erosion behavior of CuCr25 cathode material in oxygen, argon, and carbon dioxide atmospheres under voltage of 9 kV was studied. The arc duration and arc energy are decreased with changing the atmosphere in the order of oxygen, argon, and carbon dioxide, whereas the variation trend of breakdown strength is opposite. The erosion morphologies of molten pools, bulges, holes, splashes, and cracks formed on the CuCr25 material surface under the high temperature and high energy of the arc were analyzed by scanning electron microscope and three-dimensional laser scanning confocal microscope. The results indicate that the most severe erosion occurs in oxygen atmosphere and the slightest erosion occurs in carbon dioxide atmosphere, which can be attributed to the differences in arc energy. According to the results of X-ray photoelectron spectroscopy analysis, the oxidation reactions in oxygen and carbon dioxide atmospheres produce CuO, Cr₂O₃, and CrO₃, while no new compounds are formed in argon atmosphere.

Abstract:CoCrFeNi high-entropy alloy (HEA) was fabricated by mechanical alloying (MA) and vacuum hot pressing sintering (VHPS). The phase and microstructure of alloys were characterized by X-ray diffraction, scanning electron microscope, inductively coupled plasma-optical emission spectrometer, and optical microscope. The mechanical properties and corrosion resistance were investigated by universal tensile machine, Vickers hardness tester, and electrochemical workstation. Results show that CoCrFeNi HEA prepared by MA-VHPS exhibits excellent ultimate tensile strength and elongation, compared with those prepared by the arc-melting method and the combined method of the electrochemical reduction (FFC) with VHPS. The hardness of CoCrFeNi HEA prepared by MA-VHPS is twice larger than that prepared by arc-melting. The CoCrFeNi HEA prepared by MA-VHPS displays corrosion resistance in 0.5 mol/L H₂SO₄, 1 mol/L KOH, and 3.5wt% NaCl solutions, comparable to that of the 304 stainless steel and CoCrFeNi HEAs prepared by FFC-VHPS or arc-melting.

Abstract:The microstructure, mechanical properties, and corrosion resistance of ultra-high strength cold-rolled thin strips of Al-7.88Zn-2.05Mg-1.70Cu-0.19Er alloy were investigated. Results reveal that the excellent comprehensive properties of the cold-rolled thin strips with 0.5 mm in thickness are achieved after solid solution treatment of 475 °C/1 h/water quenching and double aging treatment of 120 °C/6 h+150 °C/24 h. The hardness, ultimate tensile strength, yield strength, and elongation of the thin strips are 1859.1 MPa, 669.4 MPa, 624.1 MPa, and 11.2%, respectively. The mechanical properties of double-aged strip are comparable to those of alloy after peak-aging treatment. The electrical conductivity, exfoliation corrosion rating, and stress corrosion cracking sensitivity of the thin strip are 35.5%IACS, EA, and 4.07%, respectively. The fine spherical Al₃(Er, Zr) and η? phases are uniformly distributed in the Al matrix, and most precipitates are coherent with Al matrix lattice. The tiny discontinuous grain boundary precipitate η phase mainly causes the lower stress corrosion sensitivity of the studied alloy.

Abstract:The 6061 aluminum alloy and QP980 steel were lap-welded by friction stir welding (FSW) technique, and the effect of different probe lengths of 1.5 and 2.1 mm on the microstructures and properties of welded joints was investigated. Results show that the FSW lap-welded joints of 6061 aluminum alloy/QP980 steel consist of three layers: the upper layer is aluminum alloy, the middle layer is composed of Fe, Al, and intermetallic compounds, and the under layer is steel. When the probe length is 2.1 mm, the aluminum layer contains scattered steel fragments. Two kinds of intermetallic compounds can be detected: the dark gray layer close to Al is Fe₄Al₁₃ phase, and the one close to steel is Fe₂Al₅ phase. With extending the probe length, the fracture load of the joints is decreased from 4 kN to 3 kN. The joints welded by short probe fracture at the bonding interface, whereas those welded by long probe fracture at the mixture zone of Al and steel. The fracture position change is caused by the porosities and steel fragments. In addition, the steel fragments embedded in the Al matrix promote the stress concentration and crack initiation during the deformation process, therefore decreasing the mechanical properties of the joints.

Abstract:The optical microscope and X-ray diffractometer were used to study the microstructure evolution of Cu0.6Cr alloy prepared by low-temperature equal channel angular pressing (ECAP) with extended route. The scanning electron microscope and energy dispersive spectrometer were used to study the grain size, precipitate distribution, and fracture characteristics of Cu0.6Cr alloy after different aging heat treatments. The tensile strength, hardness, and conductivity of the alloy prepared by low-temperature ECAP and low-temperature ECAP+aging heat treatment were analyzed. Results show that the obviously refined and intersecting fibrous structure is formed in Cu0.6Cr alloy after low-temperature ECAP. The alloy maintains the preferential orientation of (111) crystal plane during deformation. In the aging heat treatment, the larger the alloy deformation, the larger the number and size of the precipitates, and the faster the precipitation rate of the secondary phase. After aging at 450 °C for 2 h, the tensile strength of the 5-pass alloy is 568.1 MPa, the Vickers hardness is 1624.8 MPa, and the conductivity is 82%IACS.

Abstract:The nozzle clogging in vacuum induced melting gas atomization (VIGA) process was investigated. To understand the influence of tip shape of the delivery-tube on nozzle clogging more accurately, the interface tracking method of computational simulation fluid volume was adopted to simulate the two-phase flow in the primary atomization region. Results show that the small platform at the end of the draft tube is the key factor leading to the nozzle clogging. Therefore, the expansion angle (30°, 35°, 40°, 45°) of the delivery-tube is improved to shorten the width of the small platform. Thus, the nozzle clogging is solved, the atomization continuity is realized, and the atomization efficiency is improved. Additionally, when the expansion angle is 40°~45°, the powder has better morphology with the particle size of 21~25 μm. The numerical simulation results under different modification aspects display similar trends to the experiment ones. This research is of guidance significance and reference value to understand the nozzle clogging process of VIGA process.

Abstract:The element diffusion process of the binary systems (Fe-Cu and Fe-Ni) in the vacuum brazing of high-nitrogen stainless steel with AgCuNi filler was investigated by Lammps software for molecular dynamics simulation analysis. Results show that the mutual diffusion phenomena of Fe-Cu and Fe-Ni binary systems are obvious, and the thickness of the diffusion layer is increased with increasing the diffusion time. In the Fe-Cu diffusion process, only the mutual atom diffusion occurs; whereas not only the mutual atom diffusion, but also the formation of mesophase occurs in the Fe-Ni diffusion process. In the Fe-Cu binary system, the mean square displacement (MSD) and diffusion coefficient of Fe atom are greater than those of Cu atom, so the diffusion ability of Fe atom is better than that of Cu atom. Similarly, in the Fe-Ni binary system, MSD and diffusion coefficient of Fe atom are greater than those of Ni atom, so the diffusion ability of Fe atom is also better than that of Ni atom. With increasing the diffusion temperature, MSD and diffusion coefficient of atoms are increased, and their diffusion ability is enhanced.

Abstract:The microstructure and mechanical properties of Al_0.1CoCrFeNi single crystal high entropy alloy (HEA) under axial tensile loading at room temperature (300 K) were investigated by molecular dynamics method. The tensile properties of the single crystal HEA were analyzed by changing the simulated strain rate and temperature. The microstructure and tensile properties of the single crystal HEA with small surface cracks were studied by simulating tensile experiments at room temperature. Results demonstrate that the tensile strength is increased with increasing the strain rate in a certain range; the Young's modulus and tensile strength are increased with decreasing the temperature at strain rate of 10¹⁰ s^-1. The single crystal HEA with small through cracks on surface presents a necking phenomenon after stretching for a period, and the stress concentration occurs at the crack tip along with the rapid development of a large number of slip dislocations, resulting in the rapid fracture.

Abstract:The molecular dynamics method was used to simulate the machining response of nanoscale γ-TiAl. The rule-generated rough workpiece surface was used to study its effect on atom removal mechanism. The cutting process was investigated by varying the texture density and tool radius. Results show that the surface morphology of the workpiece has a non-ignorable influence on the generation of subsurface defects and the atom removal: the rough surface in the shear mode affects the generation of laminar fault shear zone. The increase in texture density increases the number of subsurface defects, and the integrity of the machined surface is different corresponding to different cutting patterns. The relative tool sharpness has an effect on the cutting mechanism and texture effects.

Abstract:A new piercing method (two-high rotary piercing) for magnesium alloy seamless pipes was proposed, and the two-high piercing process was simulated by DEFORM-3D finite element analysis software. Results show that the simulated microstructures and the experiment ones all exhibit the similar evolution rules, and their average grain sizes are close to each other's, suggesting that the two-high piercing process is a feasible method to pierce AZ31 magnesium alloy seamless tubes and the DEFORM-3D software is a reliable tool to simulate the two-high piercing process.

Abstract:The constitutive model of TA32 titanium alloy was established, and different drawbeads were added on the die to eliminate the wrinkles. The Barlat 89 and Hill 48 yield criterions were used to compare the prediction accuracy of the finite element simulations. The saddle shape part of TA32 titanium alloy was hot-stamped, and the thickness distribution was measured and compared with the simulation results. Results show that the wrinkles can be effectively eliminated by adding the drawbeads along X-axis and Y-axis. The saddle shape part can be precisely formed without defects. The finite element model with Barlat 89 yield criterion has better prediction accuracy than that with Hill 48 yield criterion does, indicating that the finite element simulation has good theoretical prediction significance. The mechanical properties and microstructure of the hot-stamped part were investigated and it is found that they all meet the practical engineering requirements.

Abstract:TA19 titanium alloy was ignited by friction oxygen concentration method, and two ignition damage samples after critical ignition and sustained combustion under oxygen enriched conditions were obtained. The phase composition, microstructure morphology and its formation mechanism of TA19 titanium alloy combustion products were studied by scanning electron microscopy (SEM), energy spectrum analysis (EDS) and X-ray diffraction (XRD). Results show that: The critical ignition products of TA19 titanium alloy mainly include TiO, Ti3O, rutile type and anatase type TiO2, and the initial combustion temperature is about 500℃. While the sustained combustion products of TA19 titanium alloy mainly include TiO, rutile type TiO2. Combined with the tetragonal structure ZrO2 formed in the fusion zone, the continuous combustion temperature is judged to be above 1170℃.Four distinct zones form from the combustion surface to the alloy matrix, and they are in the sequence of combustion zone, fusion zone, heat-affected zone and transition zone. During critical ignition, Zr solid solution rich in O, α-Ti solid solution rich in O and β phase rich in Al/Sn/Mo are formed in the fusione zone; A large amount of α-Ti solid solution rich in O and a small amount of β phase rich in Al/Sn/Mo are formed in the heat-affected zone. During sustained combustion, Al solid solution rich in O, Zr solid solution rich in O, α-Ti solid solution rich in O and β phase rich in Al/Sn/Mo are formed in the fusion zone; A large amount of α-Ti solid solution rich in O and a small amount of β phase rich in O and Zr are formed in the heat affected zone, finally they form a dense layered structure that prevents the inward diffusion of O and outward diffusion of Ti. In the combustion process of TA19 titanium alloy, the element Zr first diffuses into the interface of the fusion zone, followed by Al. This is different from that Al first diffuses to the interface of the fusion zone during the combustion of TA15 titanium alloy. The reason may be that the Zr content in TA19 titanium alloy is twice that in TA15 titanium alloy, and the activity of Zr is higher. The main combustion mechanism of near α type titanium alloy is the formation of O- rich solid solution. From the perspective of microstructure, higher equiaxed phase content is conducive to the dissolution of oxygen; From the point of view of alloy composition, proper control of the content of Ti, Al and Zr is beneficial to obtain dense O-rich solid solution.

Abstract:High power pulsed magnetron sputtering technology can increase the ionization rate of the sputtering particles to enhance the mechanical properties of the film, but the loss of deposition rate has seriously affected the industrialization of this technology. The main purpose of this study is to achieve the functional goal of increasing the ionization rate of the sputtering particles and taking into account the high efficiency goal of the deposition rate. The gas discharge above the cathodic target surface is changed from glow to arc through the adjustment of the electric field parameters using a new type of dual-stage pulsed electric field. The high-density plasma generated by arc discharge enhances the impact kinetic energy of Ar+ and the accumulation of Joule heating on the target surface, and induces the target material to escape from target surface in a thermal emission mode with high ionization rate and high yield. The results showed that when the target current density of the Cu target and the Ti target increased, the volt-ampere characteristics between the cathodic target and the anodic vacuum chamber changed from a proportional to an inverse relationship, indicating that the gas discharge changed from glow to arc, and the escape-target mode of target materials could be transformed into a thermal emission mode. When the dual-stage pulsed electric field was used to induce arc discharge, the morphology of the Ti target surface changed from a polygonal pit structure with linear stripes to a large-area pit structure with linear stripes and ripple structure. It is indicated that the target material escaped from the target surface by sputtering and thermal emission, and the deposition rate of the Ti film had significantly increased from 6 nm/min to 26 nm/min.

Abstract:Selective Laser Melting (SLM) and Selective Electron Beam Melting (SEBM) were used to fabricate pure tungsten. The microstructure and mechanical properties of pure tungsten were compared under different process parameters. The crack initiation location and formation mechanism were analyzed by SEM and EBSD. The results show that the crack generation of pure tungsten printing process can be effectively reduced by adjusting the process parameters in SLM and SEBM process. The relative density and hardness of pure tungsten samples are proportional to the energy density input by SLM and SEBM. Compared with SLM, SEBMed pure tungsten has lower temperature gradient, less thermal stress accumulation and fewer cracks. There were many cracks in SLMed samples, and that mostly existed in the lap areas. Cracks in SLMed and SEBMed samples were distributed along grain boundaries, most of which were initiated at high-angle grain boundaries.

Abstract:In this work, Inconel 625 alloy is used as a reference object, based on the alloy design ideas of interface control, a model alloy with lower stacking fault energy is designed by Thermodynamic software. The influence of stacking fault energy on the evolution mechanism and grain refinement mechanism of nickel-based alloy with gradient structure was studied by surface mechanical grinding treatment (SMGT) technology. The results show that a 450 μm, 300 μm and 250 μm thick gradient structure layers were prepared in Inconel 625 alloy, M1 and M2 alloys by means of SMGT, respectively. Moreover, the evolution mechanism of the gradient structure of different alloys is also different. For the Inconel 625 alloy, with the gradual increase of strain, its microstructure refinement mechanism gradually changed from dislocation segmentation mechanism to shear band and/or twin segmentation mechanism. However, for the M alloy, the deformation twin splitting mechanism is dominant. Therefore, based on the design concept and the corresponding microstructure evolution mechanism, it can provide a reference for the composition design of low stacking fault energy nickel base wrought superalloy.

Abstract:The deformation mechanism will be more intricate by the size effect when the characteristic size of metal parts is reduced to a micro scale. In this paper, the aging Inconel 718 foil with thickness of 0.1mm was selected as the study subject and tested for its mechanical properties. Based on the data mechanical test, the anisotropy, elongation, yield strength and maximum tensile strength of aging Inconel 718 foils were explored under different tensile direction and uniform strain rate. And the predicted model of anisotropy and yield strength and the hardening model for considering the strain and strain rate are established. The research results show that the aging Inconel 718 foils have conspicuous anisotropy. The tensile direction of 45° is an extreme point for the anisotropy, elongation, yield strength and maximum tensile strength. The elongation and anisotropy firstly increase and then decrease with the increase of angle between tensile direction and rolling direction. But the change rule of yield strength and maximum tensile strength is opposite to elongation and anisotropy. In order to accurately predict the anisotropy and yield strength, two different sets of material parameters are needed due to the size effect. When the strain rate is higher than 0.1 s_^-1, the material yield strength shows obvious sensitivity for strain rate. In this moment, the above hardening model is not applicable.

Abstract:Based on Gleeble isothermal hot compression test, combined with OM, SEM and EBSD analysis methods, the flow characteristics and microstructure evolution of a new hot extruded third generation nickel-base powder superalloy FGH4113A (WZ-A3) during superplastic compression deformation was systematically studied. The superplastic deformation behavior and deformation mechanism at different temperatures of 1050 and 1100 ℃, different strain rates of 0.001 and 0.005 s_^-1, and different amount of deformation (strain) were presented. The results shown that FGH4113A (WZ-A3) alloy exhibited good superplasticity during hot compression deformation, and no cavities or cracks were observed. The grain size tends to grow at 1100 ℃/0.001 s_^-1 after large deformation (60%-80% deformation), while grain size has little change under other deformation conditions. In the process of superplastic compression deformation, the accumulated dislocation is annihilated mainly by dynamic recovery and dynamic recrystallization, and grain boundary slip is the main reason for large deformation without cracking. The results lay a good foundation for the development of superplastic isothermal forging process of FGH4113A (WZ-A3) alloy.

Abstract:Based on the Microscropic Phase-field method and the interatomic potential equation, we calculate the first near-neighbor interatomic potentials of the L10 structure in Co-Pt alloy, and the results show that the first near-neighbor interatomic potential of the L10 structure in the Co-Pt alloy increases with temperature, and increases with concentration. The first near-neighbor interatomic potential, which linearly changes with the temperature and concentration, is close to other literature result. Using the interatomic potential to simulate the Co-Pt alloy precipitation process and the final morphology, the simulation results can get L10 and L12 structure. According to the sequence parameters, we can get that the L10 structure precipitation mechanism characteristics are spinodal decomposition, and then coarsed. The final two-phase volume fraction is close and the alloy precipitation profile and experimental results are consistent. The calculation of the interatomic potential for L10 structure in Co-Pt alloy using Phase-field method can expand the application range of Phase-field method in alloy design with L10 structure.

Abstract:The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were adopted to investigate precipitates in 46Fe-33Ni-15Cr and 50Fe-27Ni-20Cr valve alloys. The results show that 46Fe-33Ni-15Cr alloy contains γ", σ and MC phase after solution and aging, however 50Fe-27Ni-20Cr alloy contains γ", σ, MC and Laves phase. The γ" phase has two patterns, spherical and cellular in 46Fe-33Ni-15Cr alloy. But only one pattern of γ" phase spherical exists in 50Fe-27Ni-20Cr alloy, and its size is about 20nm~40nm,smaller than that in 46Fe-33Ni-15Cr alloy. The σ phase distributes in strip on the grain boundary in 46Fe-33Ni-15Cr alloy, but it precipitates in needle shape along the grain boundary in a certain orientation. Within 2000 hours aging under 700℃, the amount of σ phase in 46Fe-33Ni-15Cr increases more rapidly than that in 50Fe-27Ni-20Cr alloy with the increasing aging time.

Abstract:The excellent catalytic activity of Platinum (Pt) nanoparticles have been widely used in energy and energy storage. The investigation demonstrates that the catalytic capacity of Pt depends on the number and type of active sites on the surface, however, the regulation mechanism of its surface activity was not fully understood. In this paper, molecular dynamics method was used to study the microstructure and phase transformation of Pt nanoparticles based on LAMMPS software. The results show that the proportion of disordered atoms of Pt nanoparticles decreases with the increase of particle radius, and the melting temperature of Pt nanoparticles decreases with the decrease of particle radius. In addition, the particles can be further divided into two parts: the surface shell and the inner core. Like the bulk material, the coordination number of the inner core is also 12. The thickness of the shell was about 0.27 nm with the thickness close to 2 layers of atoms and the coordination number decreases with the increase of the core distance. This unique core-shell structure resulting that the potential energy surface atoms was approximately higher than the core. In this study, we found that the Pt shell atoms with particle radius of 3nm can melt at 1300K, whereas the inner atoms can not melt. Therefore, the structure characteristics of Pt catalyst can be regulated by the phase transformation law, which provides a theoretical basis for the regulation of surface activity.

Abstract:In order to investigate the effect of pulsed magnetic field on precipitation behavior of as-rolled TC4 titanium alloy, a pulsed magnetic field was applied during aging.The effects of electromagnetic energy on microstructure, strength and plasticity of TC4 during aging were studied by SEM,TEM and tensile testing machine.Observation and analysis were carried out at nucleation stage and growth stage respectively.The mechanism of electromagnetic energy in aging process was analyzed by using classical nucleation theory and the first law of diffusion. The results show that the precipitation rate of TC4 is significantly increased due to the addition of electromagnetic energy. The aging time is reduced on the premise of ensuring the mechanical properties. Compared with 4 h without pulse magnetic field, the plasticity of 2 h with pulse magnetic field is increased by 22.67%, and the tensile strength is approximately the same. The aging process under the coupling of pulse magnetic field and temperature field can lead TC4 to over aging stage (2h). In the aspect of tensile fracture, the fracture morphology of fracture is dimple fracture before applying pulse magnetic field 2 hours ago, but when the time increases to 2h, the fracture surface gradually changes to the fracture morphology in quasi solution.The results reveal that the secondary phase precipitation and growth of TC4 can be promoted by pulsed magnetic field through electromagnetic energy, which is due to the synergistic effect of reducing critical nucleation work and promoting atomic diffusion. Overall, electromagnetic energy can promote the precipitation of secondary phase and reduce the aging time.

Abstract:Non-isothermal oxidation behavior of Ti-Al-Mo titanium alloy’s high temperature weld metal after molten pool solidified during cooling process at different temperatures in the range of 500~1300℃ was studied, and the oxidation mechanism in air and CO2 atmosphere was compared and analyzed. The surface oxidation degree of the sample, the morphology and phase of the oxide layer were studied by means of oxidation weight gain method, XRD, SEM and XPS. The results show that the oxidation degree of the sample in CO2 atmosphere is lower than that in air atmosphere, and the oxide particle size of the oxide layer in CO2 atmosphere is smaller and more compact. The main phase of oxide layer in both atmospheres is rutile TiO2. Besides, the mixed oxide of (TiO2+Al2O3) is observed in the surfacial layer, and the proportion of Al2O3 in CO2 atmosphere is higher. The oxide layer porosity in CO2 atmosphere is lower , which reduces the internal matrix diffusion of CO2. And the reaction thermodynamics inclination of the titanium alloy with CO2 is significantly lower than that with the air, so titanium alloy weld metal in the environment of CO2 atmosphere will have lower oxidation tendency. It’s proved that CO2 can be used as a protective gas in a certain temperature range during the cooling process of titanium alloy weld metal.

Abstract:The effect of heat treatment temperature and cooling rate on the basket-weave structure and tensile properties of TC21 titanium alloy is studied by triple heat treatment experiments. The results indicated that the microstructure of TC21 alloy presents a typical basket-weave structure, which is achieved by high temperature (990℃) solid solution in the β single-phase region, and then through high temperature(870~910℃)aging in the (α + β) phase region and low temperature (590℃) aging. With the increasing of the second heat treatment temperature, the content and length of the flake α phase decrease significantly, and the thickness of the flake α phase increases slightly, the strength of the alloy increases, the ductility decreases. After being treated with different cooling rates, the microstructure of TC21 titanium alloy consists of α phase, β phase and α′martensite in both water cooling and air cooling samples, but only α phase and β phase are identified in furnace cooled sample, Compared with the tensile properties of the three samples, the water cooling and air cooling samples showed better strength and poorer ductility, furnace cooling samples show good ductility and poor strength. The better triple heat treatment process for TC21 alloy is 990℃/1 h AC+870℃/1 h AC+590℃/4 h AC.

Abstract:Abstract:SiCf/Ti-6Al-4V composites prepared by laser cladding have short reaction time and diffusion time. The interfacial properties and micromechanical properties of SiCf/Ti-6Al-4V composites under different vacuum thermal exposure conditions were studied. By means of scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM) and nanoindentation instrument, the thickness of the reaction layer, chemical composition at the interface and micromechanical properties at different temperatures and times after vacuum thermal exposure were characterized. It is found that the interfacial reaction between reinforcer, C coating and matrix occurs in the process of laser cladding laminated composites, and the main interfacial reaction product is TiC. Short time vacuum thermal exposure increases the thickness of the interfacial reaction layer and decreases the thickness of the C coating. With the extension of thermal exposure time, the growth of the reaction layer conforms to a parabolic law. The kinetic parameters of the reaction are frequency factor k0 =6.005×10-3 m.s-1/2, and activation energy Q=143.13 kJ.mol-1. The hardness and elastic modulus of fiber, C coating, substrate and interfacial reaction layer do not change obviously after 900℃ short-time vacuum thermal exposure, indicating that the material has good short-time thermal stability at 900℃ and below.

Abstract:This paper studies and analyzes the cracks in the process of cross-rolling piercing production of domestic 825 alloy pipes. Through the use of metallographic observation and finite element analysis methods, the causes of the cracks are discussed, and it is found that the local temperature rise and increased shear stress is the main cause of cracking. In addition, from the perspective of three process parameters such as the initial diameter of the tube, the initial temperature, and the lubrication conditions of the plug, the regularity of the cracking phenomenon of the tube is studied, and the most important factors affecting the local temperature rise of the tube It is the plug lubrication condition, and the local temperature rise phenomenon can be significantly reduced by reducing the friction coefficient of the plug. However, the influence of process parameters on the maximum shear stress is not obvious. Therefore, it is believed that the temperature rise caused by the cross-rolling piercing process is the most critical influencing factor for the quality control of the tube.

Abstract:A high Al+Ti containing Ni-based wrought superalloy ingot was prepared by a vacuum induction furnace. Then the ingot was remelted into three with different Zr contents in the same furnace. After as-cast microstructures observation, these ingots were homogenized. The effect of Zr on microstructures evolution during the homogenization process was emphatically studied. The results show that the Zr addition has no obvious influence on the initial grain size of the ingots, but significantly promotes the non-equilibrium solidification eutectic (γ + γ′) and Zr rich phase precipitation, especially at the grain boundary. Besides, Zr significantly promotes the formation of micropores in the interdendritic region. After homogenization, the non-equilibrium phases have been completely dissolved, the dendrite segregation has been obviously reduced and the original grains have grown obviously. Interestingly, the higher the Zr content is, the more obvious the grain growth is. Besides, the size and area fraction of micropores have increased markedly compared with those in the as-cast microstructure, and the micropores formation in the as-homogenized microstructure also increases significantly with the increase of Zr content. Zr increases the micropores formation in the as-cast alloy is mainly related to the enrichment of Zr in residual liquids which retarding the alloy solidification. The principal reason why Zr promotes the grain growth and micropores formation during homogenization is that the more eutectic (γ + γ′) and Zr-rich phase formation aggravates the interdiffusion between solute elements.

Abstract:The Scanning Electron Microscope (SEM) with energy dispersive spectrometer (EDS) and Electron Back-Scatter Diffractometer (EBSD), Optical Microscope (OM) and the thermal simulation testing machine were used to study the effect of the powder size distribution on the hot deformation behavior and the organizational differences of a novel nickel-based powder metallurgy superalloy (WZ-A3). The results show that: compared with powder preparation of HIP-01, the dendrite structure of coarse powder is obvious and the composition segregation is serious. The residual primary γ′ of the HIP-02 made by the coarse samples is more than the HIP-01. At the low temperature (1050℃, 1080℃) and high strain rates (1/s, 0.1/s), the peak stress of HIP-01 is higher than that of HIP-02. After hot compression, the crack situation at the edge of HIP-01 is more serious than that of HIP-02. Most of the original HIP-01 microstructure are retained, and the crack is generated at the Prior Particle Boundary (PPB), while some recrystallized structure appears at the edge of HIP-02. At 1080℃-0.001/s, the peak stress of HIP-02 sample is about 30MPa lower than that of HIP-01. The recrystallization phenomenon of HIP-02 during hot compression is obvious. The recrystallized grains of the HIP-02 are uniform. The HIP-01 has necklace crystal structure, and the recrystallization is not sufficient. The coarse γ" can promote the recrystallization of HIP-02 sample at 1050℃ and 1080℃. At high temperature (1150℃) and low strain rates (0.001/s, 0.01/s), the γ" dissolve into γ matrix, and the thermal deformation behavior of the two samples is similar. No cracks occur in the samples, and the microstructure is fully recrystallized. The abnormal grown grains were found in 1150℃-0.001/s at the both two samples.

Abstract:Effect of solution and aging heat treatment on microstructures and stress rupture properties of a fourth generation Ni-based single crystal superalloy with content of 4.0% (in weight percent) Re and Ru has been investigated.The results show that the as-cast alloy exists significant composition segregation.Due to less Al and Ti elements, interdendritic γ/γ’ eutectic content is low.During the solution heat treatment,the eutectic is almost dissolution after 1310℃.However,the differences between dendrite core and interdendritic is completely eliminated until 1340℃ and the segregation of other elements except Re is significantly improved.The advantage of the fourth generation Ni-based single crystal is to improve the mechanical propertie at high temperature,especially stress rupture properties.After solution and aging heat treatment,the stress rupture lives of DD476 is about twice that of the second generation single crystal superalloy at 950℃/300MPa and is more than 10 times comparaed with the second generation single crystal superalloy MC2 at 1150℃/100MPa.There is no TCP(topologically close-packed) precipitation during the whole heat treatment because of the addition of Ru element in DD476.

Abstract:U-5.7Nb-xTi (wt. %) alloys with different Ti contents were fabricated by arc-melting. With the increasing of Ti contents, γ0 phase was first to appear in the α″ phase matrix, followed by its propagation. After completely transforming to γ0 phase at U-5.7Nb-1.2Ti, additional Ti alloying led to the formation of the equilibrium γ12 phase. After the Ti atom was equivalently converted to the equilibrium niobium concentration Nbeq, the boundary of each phase composition was very consistent with the U-Nb alloy. Meanwhile, the hardness of U-5.7Nb-xTi alloys were similar with those of U-Nb alloys with same total atomic contents. By analyzing the mechanism of transformation of the U-alloy and the theory of solid solution strengthening of the metal materials, the phase structure and mechanical behavior were considered to be closely related to the yield strength of the parent phase and twinning energies, respectively. Due to their similar physical parameters including atomic radii and bulk modulus, the solution strengthening effects of the parent phase and variation of the twinning energies led by Ti or Nb alloying were similar, which were responsible for the similarities of the phase structure and the mechanical properties in U-Nb-Ti and U-Nb alloys. According to the same physical mechanism, U-Nb-X ternary alloys (or multi-element alloys) can be more accurately predicted the phase structure and the mechanical properties by analyzing physical parameters such as the atomic radius and bulk modulus of alloying element X. These results and analyses provided theoretical basis and guidance for further strengthening design of U-5.7Nb alloy.