The tensile deformation behavior and the axial lattice strain response of 1250 MPa ultra-high strength steels with and without hydrogen charging were comparably investigated using time-of-flight neutron diffraction together with the fracture morphology and microstructure observation. Before tensile loading, the axial (110) lattice plane spacing of hydrogen charged specimen was found larger than that of non-charged specimen while the axial (200) lattice plane spacing of the former was smaller than that of the latter, suggesting that the hydrogen atoms occupied the tetrahedral site promoted the increment of axial (110) lattice plane spacing while the balanced internal stress resulted in the proper decrement of axial (200) lattice plane spacing. The necking and ductile fracture after approaching the 1250 MPa tensile strength occurred in the non-charged specimen, while the brittle fracture occurred in the 8.0×10^-6 hydrogen charged specimen at 500 MPa holding during step-by-step loading. The neutron diffraction analysis showed that in the non-charged specimen, the linear elastic deformation was kept up to 500 MPa loading, the nonlinear elastic deformation was observed preferably on the axial (200) reflection at 700 MPa, and then on the axial (110) reflection at 800 MPa; the axial {200} (i.e. <200>//TD, TD—tensile direction) grain orientation-dependent microyielding was observed preferably at 800 MPa while the (211) reflection was still under linear elastic deformation. Comparably, in the hydrogen charged specimen, the nonlinear elastic deformation was observed preferably on the axial (110) reflection at 300 MPa, and then on the axial (200) reflection at 400 MPa; the axial {110} grain orientation-dependent microyielding was observed preferably at 400 MPa while the axial (211) reflection was still under linear elastic deformation. The longitudinally sectioned microstructure observation under fracture surface confirmed the typical <110>-oriented tensile fiber texture in the non-charged specimen while the intergranular cracks along grain boundaries, quasi-cleavage/cleavage cracks and local crystal rotation in various grains of the hydrogen charged specimen. A concept about crystallographic orientation dependent microyielding was employed here to explain the above results, i.e. the hydrogen charging promoted the axial {110} grain orientation-dependent microyielding rather than axial {200} grain orientation-dependent microyielding, and the diffusible hydrogen embrittled the matrix microstructure, accompanying with local plastic deformation.

After quenching and proper intercritical tempering, ZG06Cr13Ni4Mo martensitic stainless steel is composed of tempered martensite matrix and reversed austenite. The deformation induced martensitic transformation of reversed austenite occurring during the deformation results in the transformation induced plasticity (TRIP) effect, which is beneficial to the mechanical properties of this steel. However, studies on the TRIP effect of reversed austenite are limited to description of phenomenon and mechanism behind is not clear. In order to reveal the mechanical stability and transformation induced plasticity of the reversed austenite during tension test in tempered ZG06Cr13Ni4Mo steel, a custom-built mini tensile instrument has been designed and installed on Shanghai Synchrotron Radiation Facility to conduct the in situ synchrotron high energy X-ray diffraction (SHXRD) experiment during the uniaxial tension. Three samples, which were tempered at 620 ℃ with different holding times and cooling rates in order to obtain different volume fraction of reversed austenite, were used to investigate the relationship between the deformation induced martensitic transformation and work hardening behavior. The integral intensity and the full width at half maximum of diffraction peaks of the reversed austenite and tempered martensitic matrix under different engineering stress were recorded. The gradual decrease in the integral diffraction intensity of reversed austenite with increase in tensile stress indicates that the reversed austenite has been induced to transform into martensite during the tension deformation. Furthermore, the volume fraction of reversed austenite during tension was quantitatively calculated by fitting the whole diffraction spectra of reversed austenite and tempered martensitic matrix with the Rietveld refinement method. The evolution of the reversed austenite fraction indicates that the deformation induced martensitic transformation initiates at the macro-elastic stage and through the whole deformation, which is different to the retained austenite in TRIP steel. Meanwhile, the work hardening exponents of three samples with different volume fraction of reversed austenite have been compared. It is found that the deformation induced martensitic transformation of reversed austenite increases the dislocation density of martensitic matrix and results in the increase in the work-hardening exponent during the plastic deformation, which enhances the ductility of ZG06Cr13Ni4Mo martensitic stainless steel.

0Cr16Ni5Mo steel is the most popular material used for fasteners and bolts in the marine engineering equipment. With the light weight trend of equipment, the strength grades of the steel become higher. 0Cr16Ni5Mo steel combines high strength, high hardness and high fracture toughness with good ductility. However, high strength steel is prone to degradation by hydrogen, resulting in the loss of its excellent mechanical properties. And the presence of diffusible hydrogen near a notch tip is easily to cause crack propagation. The susceptibility to hydrogen embrittlement of steel is largely determined by the hydrogen diffusivity and the behaviors of hydrogen trapping in the steel. Therefore, the hydrogen trapping behaviors of 1000 MPa grade 0Cr16Ni5Mo steel have been investigated by means of thermal desorption spectroscopy (TDS). Meanwhile, the hydrogen embrittlement susceptibility of the notch and smooth specimens was evaluated by slow strain rate tests (SSRT), and the fracture morphology was also observed. The results showed that the main hydrogen traps of experimental steel was contained dislocations and grain boundaries. The elongation of hydrogen charged specimens was decreased obviously rather than tensile strength. With the increase in hydrogen concentration, the fracture surfaces of hydrogen charged specimens was displayed a transition from ductile microvoid coalescence to a mixed morphology of dimples, quasi-cleavage and intergranular features. The steel had little irreversible hydrogen due to less C content, and had much susceptibility with reversible hydrogen contained. The model of hydrogen induced stress was calculated on basis of Eshelby equivalent inclusion, validating the relationship between stress concentration and hydrogen concentration.

The relationship of the evolution of the phase parameters (area fraction ? M 23 C 6 and equivalent width W ) of grain boundary M₂₃C₆ plates with the embrittlement of HR3C super-heater tube samples in service was studied. Based on the ASTM E112 standard charts, the total length of two dimensional austenite grain boundaries (L_gb) corresponding to each grain size number (G_L) was determined in the observed area of the metallographic images and expressed as L_gb (G_L). Making use of the SEM-SE images of the samples, the ? M 23 C 6 and W were determined. The relationships of W with G_L and ? M 23 C 6 were established as W(G_L, ? M 23 C 6 ). Combined with the result from a Charpy impact test, the function of the impact value (a_KV) as the W was obtained. In addition, the grain boundary elastic modulus (E_r) was measured by a nano-hardness test. The result shows that intergranular fracture occurred on all the room temperature impact test specimens taken from the super-heater tubes exposed under the operating conditions. The W was increased with the decrease of G_L and the increase of E_r at a constant ? M 23 C 6 , causing a corresponding decrease of a_KV, and hence promoting the embrittlement of the HR3C super-heater tubes. The related mechanism for the intergranular fracture caused by the increase of the equivalent width W of grain boundary carbides (carbide coarsening) can be explained through the application of the proposed method.

Due to its excellent ductility and moderate strength, QT400-18L ferrite ductile iron has been widely used in producing core components of wind power equipment such as the hub of a wind turbine. Most of the researches have focused on the exploration of mechanical properties at low temperature, but none of them give the explanation on microcosmic mechanism of ductile iron during low temperature impact and the mechanism of crack nucleation and propagation of ferrite ductile iron during impact fracture has not been analyzed. In this work, the impact toughness of QT400-18L ferrite ductile iron was measured by V-notch Charpy impact test at different temperatures, the influence of low temperature impact toughness and the fracture behavior of ferrite ductile iron were discussed. The results show that the cleavage fracture resistance of QT400-18L ferrite ductile iron is reduced with the decrease of impact temperatures. Above ductile-brittle transition temperature (DBTT), most of the total fracture energies are expended during the crack propagation process. Below DBTT, both crack initiation energy and crack propagation energy decrease obviously. By using in situ fracture metallographic observation method, crack initiation and propagation of QT400-18L ferrite ductile iron under different temperatures were analyzed. Above DBTT, graphite nodules play the role of crack blunting and reducing crack propagation rate; in DBTT range, the fracture morphology shows mixed fracture with cleavage and dimples, which are related to graphite nodules; below DBTT, deformation twins lead to the nucleation of microcrack and result in cleavage fracture, the deformation twinning could possibly play a significant role in the ductile to brittle transition of QT400-18L ferrite ductile iron.

Austenite-ferrite transformation in low carbon steels has a fundamental role in phase transformation and is industrial importance. The kinetics of austenite transformation can be described by the kinetics of austenite-ferrite interface migration. Two classical models, the diffusion-controlled growth model and the interface-controlled model, can be used to describe the growth of proeutectoid ferrite during g→a isothermal transformation. The austenite transformation in continuous cooling process is more common in production. In continuous cooling process, the equilibrium carbon concentrations in austenite and ferrite change with temperature and the kinetics of austenite transformation is different from that in isothermal process. Based on the models for g→a isothermal transformation, a diffusion control model is established for the growth of proeutectoid ferrite during the decomposition of supersaturated austenite in continuous cooling process. The interface position of proeutectoid ferrite varying with temperature is described with the model. The soft impingement effect at the later stage of transformation is considered. The carbon concentration at the austenite side of interface is difficult to reach the equilibrium carbon concentration when the cooling rate is high. A parameter as the function of cooling rate is proposed to modify the carbon concentration at the austenite side of interface. The polynomial diffusion field approximation is assumed in front of the interface. Simulation is done by utilizing the model to analyze the growth of proeutectoid ferrite in continuous cooling process with different bulk concentrations, austenite grain sizes and cooling rates. The interface position of proeutectoid ferrite as a function of temperature or time is obtained under different cooling conditions. Also, carbon diffusion length at the austenite side of interface as a function of time and carbon profile as a function of interface position are obtained under different cooling conditions. Furthermore, the proeutectoid ferrite fraction as a function of temperature can be acquired. The change law of carbon diffusion length with interface position and the change law of interface position with square root of time are discussed. The simulation results of diffusion control for austenite transforming in Fe-0.17C (mass fraction, %) alloy with grain size of 17 mm and different cooling rates show a good agreement with the literature results previously reported.

Ferritic stainless steel (FSS) containg (11%~13%)Cr with low C and N has excellent comprehensive performances and thus can be widely applied in extensive fields such as automobile exhaust systems, containers, buses and so on. Among them, 409L steel containing 11%Cr has been increasingly used in applications for tail pipes in the cold end parts of automobile exhaust systems because of its good corrosion resistance and moderate price. During the manufacture process for these tail pipes, improper heat treatments and welding operations cause the precipitation of some detrimental phases such as carbides, nitrides, which leads to a reduction on the resistance to intergranular corrosion (IGC) due to the presence of Cr-depleted zone in the grain boundaries. In this work, the precipitates in grain boundaries of 409L steel aged at 600 ℃ were investigated using TEM, EDS and SAED. The double loop-electrochemical potentiokinetic reactivation (DL-EPR) method was extended for evaluating the IGC susceptibility of 409L steel. The operating conditions of the DL-EPR test for 409L steel were optimized by investigating the influences of the main test parameters, such as scanning rate, solution composition, solution temperature. The experimental results showed that the IGC occurred in aged 409L steel due to the precipitation of M₂₃C₆ along grain boundaries. The optimized DL-EPR test could evaluate the IGC susceptibility of 409L steel quantitatively with high reproducibility. With the increase of aging time, much more M₂₃C₆ precipitated along grain boundaries, which resulted in 409L steel more susceptible to IGC.

DP600 steel sheet with high strength has drawn much attention in the automotive industry, but the shape change following forming and unloading has not been known widely. The time-dependent springback of DP600 steel sheet was investigated under different pre-strains by uniaxial tension. According to the viscous behaviors under the elastic and plastic loading tests, the lower limit of integration in the constitutive equation of linear viscoelasticity was modified and the creep compliance was gained from the creep curve at a constant stress level of 309 MPa at room temperature. The predicted curve was acquired by using the superposition of the unloading impulse and the historical loading curve. The results reveal that the strain rates with high initial values gradually decreased following unloading at room temperature. As the pre-strain went up, both the absolute anelastic strain of time-dependent springback and the anelastic proportion of the total springback increased. Meanwhile, the direction of the time-dependent springback was same as that of the initial springback and opposite to the loading direction. In the same springback period, the ratio of the unloading stress to the creep springback strain tended to vary more linearly with the pre-strain than those obtained from immediate unloading. The simulated results using the revised model are in good agreement with the experimental data.

A 2D axial symmetrical mathematical model was developed for stationary keyholing plasma arc welding (PAW), to describe the transport process in coupled high-temperature flow arc and molten pool in the workpiece. The evolutions of electric, magnetic, velocity and temperature fields were simulated. The simulated fusion line of the weld bead is in quite good agreement with the experimental results, validating the mathematical model. It turns out that, both the current density and the temperature reach the maximum values near the tip of the tungsten cathode. The arc displays a typical bell-shape above the workpiece, but becomes slim cone-shape near the central axis as the arc enters the keyhole. The argon plasma slows down sharply when it strikes the inner wall of the keyhole, so high pressure appears in the keyhole and some argon plasma flows back. The combination of fluid flow and heat transfer contributes to the reversed bugle shaped fusion line. The simulation of orthogonal test was further conducted to study the effects of operational and structural parameters of the weld torch. The range analysis shows that the structural parameters of weld torch are more influential than the operational parameters. That is, more attention should be paid to control the gap between two electrodes, the electrode shrinkage and the nozzle diameter to guarantee the welding quality.

Mo₅SiB₂ (T2) can be used as a promising elevated-temperature structural material because of its high melting temperature (about 2200 ℃), and excellent resistance to oxidation and creep. The Mo₅SiB₂ (T2) alloy was prepared by both spark plasma sintering (SPS) and tube furnace sintering (TFS), and then the microstructures were characterized by XRD, SEM-EDS and TEM. The results show that the rapid heating rate is one of important dynamic conditions responsible for the synthesis of T2. Compared with traditional methods, SPS can provide the fast synthesis in a particular way of labilized plasma sintering so that the sample can be heated to the expected temperature of 1500 ℃ with a short period. The melted Si can rapidly react with Mo and B to synthesize T2 in the solid-liquid state prior to the formation of binary phases (Mo₃Si, Mo₅Si₃, MoB, etc.) in the solid state in the range of 600~1200 ℃. The average size of grains is equal to 1.44 μm. The boundaries are clear and have the shape of a straight line without transition zones. Moreover, no defects such as dislocations were found in the T2 alloys prepared by SPS.

The properties of the glasses and crystals of Ge_xSe_90-xSb₁₀ (x=20, 23, 25, 30) prepared with melt quenching method have been analyzed by DSC. The glass transition temperature (T_g) of Ge_xSe_90-xSb₁₀ glasses at different heating rates, the specific heat capacity and the entropy of the glasses and the crystals have been obtained. And on this basis, the kinetics ideal glass transition temperature (T₀), the thermodynamics ideal glass transition temperature (T_K) (Kauzmann temperature) and the kinetic fragility index (m) of Ge_xSe_90-xSb₁₀ glasses have been determined. It is found that T_g, the specific heat capacity and the entropy increase with increasing the heating rate. T_g and T₀ first increase and then decrease with increasing the Ge content, while T_K increases linearly with the increase of Ge content. The m, T_g/T_K, T_g/T_m and T_g/T₀ are estimated to be 20.7~23.2, 1.183~1.352, 0.678~0.742 and 1.006~1.019, respectively. For each Ge_xSe_90-xSb₁₀ glass in this work, its m is smaller than 30 and T_g/T_K is larger than 1.1, which means that the Ge_xSe_90-xSb₁₀ glass should be considered as a strong melt. The value of T_g/T_m is larger than 2/3, which indicates that amorphous Ge_xSe_90-xSb₁₀ can be formed easily.

As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of microstructure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed b phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of distance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than b phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and residual b phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b phases with smaller size are attained in the weld bottom owing to the dynamic recrystallization, and the distribution of b phase on primary a matrix is more homogeneous. When the rotational speed reaches 350 r/min, the area where the peak temperature is higher than b phase transus temperature becomes wider along the thickness direction, which makes the size and quantity of lath-shape a phase increase and then the lath-shape a clump appears. Lath-shape a phase with different orientations hinder the propagation of crack and be beneficial for the tensile strength of FSW joint.

The 6××× series aluminum alloys Al-Mg-Si-Cu are widely used in the transportation and building industries due to their comprehensive mechanical properties, adequate formability, high corrosion resistance and good weldability. For decades, ultrafine grain structure (UFG) produced by severe plastic deformation (SPD) has been proved to be a promising way in strengthening Al alloy materials. Although this method can guarantee a great improvement in strength, the obtained ductility is always disappointing. Besides, this method has a limitation to fabricate products suitable for practical use. Recently, combining deformation and aging has been proposed to produce high-strength Al alloys. This strategy is very effective in achieving Al alloys with strength-ductility synergy even through conventional producing process, for example, rolling and aging. The strain ratio of deformation is critical in tuning the mechanical properties which could be acquired by the above method. The effect of deformation strain ratio on the age-hardening behaviors and microstructure in Al-Mg-Si-Cu alloy produced by combining cold-rolling and aging are investigated using hardness test, tensile test, EBSD and TEM in this work. The results show that the as-rolled hardness increases gradually with deformation strain ratio. The age-hardening potential declines with the increase of strain ratio, though post-aging could further strengthen the as-rolled alloys. The grains elongate along the rolling direction during deformation and finally have a lamellar structure. Fragmentation and extensive defects like sub-grain boundaries occurs inside the grains. The dislocations become denser inside the alloy with the increase of the deformation ratio. When the deformation ratio is large (above 60%), formation of dislocation tangling and sub-grains are observed. Deformation-induced change of the dislocation configuration affects the precipitation significantly. Due to the interaction between solutes precipitation and defects annihilation, the distribution of precipitates undergoes a change from being isolated to a continuous manner.

Bulk metallic glass (BMG) composites containing B2-CuZr phase are of interest due to they behave large plastic strain and apparent work hardening in tension. Nevertheless till now most BMG composites containing B2-CuZr phase are based on Cu_47.5Zr_47.5Al₅ or Zr₄₈Cu_47.5Al₄Co_0.5 BMG, which has limited glass forming ability (GFA). The prepared sample size is small, which restricts their potential engineering structural applications. In this work, Zr-Cu-Al-Y quaternary system is selected due to its high GFA. By tuning composition close to CuZr alloy in Zr-Cu-Al-Y quaternary system, Zr_46.9Cu_45.5Al_5.6Y_2.0 BMG is selected because it has proper GFA (critical diameter D_c=5 mm) and relatively large fracture toughness (K_Q=(49±3) MPam^1/2). By decreasing the cooling rates of the melt via increasing diameter of casting rods, large-sized in situ Zr_46.9Cu_45.5Al_5.6Y_2.0 BMG composites containing 13% and 25% volume fractions spherical B2-CuZr phase were prepared in the casting rods with 6 and 7 mm in diameters, respectively. In compression testing, the in situ BMG composites containing 25%B2-CuZr phase promote multiple shear bands within glass matrix and remarkable global plastic deformation, accompanied by a large compressive plastic strain as 6.5%. Nevertheless in tension testing no obvious global ductility was achieved, which attributes to the low mode I fracture toughness and small plastic zone size (R_P=88 mm, R_P=(1/3π)(K_Q/s_y)² ) of glass matrix. Three point bending test results show that Y has an adverse effect on the fracture toughness and plastic zone size of Zr-Cu-Al BMGs. In contrast to Zr_46.9Cu_45.5Al_5.6Y_2.0 BMG, fatigue pre-cracked Zr₄₈Cu₄₅Al₇ BMG plate samples can be prepared and exhibit a high fracture toughness (K_Q=(62±3) MPam^1/2) and a large plastic zone size (R_P=150 mm) in plane strain state. Our results show that GFA and fracture toughness of glass matrix should be balanced when designing new BMG composites containing B2-CuZr phase.

Micro-computed tomography is a new three-dimension high-resolution imaging device, which due to its X-ray brightness generated by a compact electron impact X-ray source. To achieve higher X-ray brightness, the size of the X-ray source should be as small as possible. However, the X-ray brightness is fundamentally limited by the maximum possible heat dissipation of the X-ray target. As an electron beam strikes a metallic target, the power density of target is increased with the decreasing of the spot size of electron beam, which results in the decrease of the X-ray brightness by significant temperature elevation of the target surface. A practical solution for these requirements is the use of a multi-film target consisting of a thin-film target on a thicker substrate film. The substrate should be composed of a light material with high thermal conductivity to prevent absorption of the signal X-rays and to elevate the target temperature. In such a multi-film target, several factors as following must be considered to choose the materials and thicknesses of the multiple films: the highest power density of the target can sustain without performance degradations or damage, and the efficiency of the X-ray generation in the target material including any self absorption effects. The present work designed a basic structure of tungsten-aluminum transmission target, according to the theoretical model of the end-window transmission target for Micro-CT. The thicknesses of tungsten target surface and aluminum substrate are determined by the Geant4 simulation results and the Müller calculation model of temperature rise, respectively. According to the structure parameter of tungsten-aluminum transmission target of YXLON, the tungsten film with the thickness of 2, 5 and 8 μm are prepared on the aluminum substrate by the magnetron sputtering method. The density and evenness of tungsten film both are well by the SEM analysis. The performance of three kinds of target with different thicknesses is carried out on the X-ray tube of YXLON. The results show that the optimal thickness of tungsten film is 5 μm, and the X-ray emitting efficiency of tungsten-aluminum transmission target is the biggest, which the corresponding production power of X-ray is the lowest. On this basis, the contrast experiments of X-ray emitting efficiency and X-ray imaging effect are carried out between the tungsten-aluminum transmission target of homemade and that of YXLON. The experimental results indicate that the X-ray emitting efficiency, the corresponding X-ray production power and the X-ray imaging effect of homemade target all are superior to that of YXLON, which could be satisfied the application requirements of high quality target for Micro-CT.