China low activation martensitic (CLAM) steel has been considered as the primary candidate structural material for application in fusion systems because of its good thermal conductivity and low thermal expansion ratio. In this work, the tensile behavior of the CLAM steel in liquid lead-bismuth eutectic was investigated to assess the compatibility of CLAM steel with liquid metal. The CLAM steel was tempered before test. The tensile tests were performed in liquid lead-bismuth eutectic and argon gas respectively at different temperatures ranging from 200 ℃ to 500 ℃ under different strain rates. All the specimens ruptured in ductile manner in argon gas environment, exhibiting obvious necking and dimples on the fracture surface. For those tested in liquid lead-bismuth eutectic, the specimens behaved ductile fracture when the test temperature was below 250 ℃, but fractured in brittle cleavage manner in the temperature range of 300~450 ℃. The embrittlement mainly occurred after necking, showing typical river pattern on the fracture surface with slight necking trace, and obvious cracking points were observed to initiate at the fracture edge and propagated towards the center of the specimen, namely, the appearance of the ductility trough that shows significant degradation in total elongation while no noticeable differences in strength compared with the tested specimens in argon gas environment. Furthermore, the brittle fracture disappeared and total elongation recovered when the tensile tests were performed out of the embrittlement temperature range. In slower strain rate tensile (SSRT) tests, the temperature range of the ductility trough greatly expanded and brittle fracture occurred at temperatures below 250 ℃. The results indicate that CLAM steel is susceptible to embrittlement in liquid lead-bismuth eutectic. This is because the contact of the liquid metal with the cracking tip leads to a decrease of the interfacial energy, which further reduces the critical cleavage stress and facilitates the brittle fracture. Both temperature and strain rate are evidenced in this work to have an effect on the embrittlement of CLAM steel.

More and more attentions have been paid to porous stainless steel, as the excellent performance in the physical, chemical and mechanical properties, in the field of solid oxide fuel cell, medical drug for implantable devices and so on. In this work, a new method called physical vacuum dealloying has been applied to produce porous stainless steel. Firstly, 316L-50Mn initial alloy was successfully melted by vacuum induction furnace, then the porous stainless steel was developed by 316L-50Mn after heat treatment in vacuum environment in this experiment. SEM, EDS and XRD were used to analyze the porous stainless steel made by physical vacuum dealloying method. Meanwhile, the effects of temperature and time on the formation, development and morphology of pores during the dealloying process were also studied. The results show that it was effective to produce porous stainless steel by physical vacuum dealloying method. The porosity of micro pores in porous stainless steel is 30%~60%, with 0.5~3 μm pore size, and the 15~60 μm thickness. The temperature mainly affects the hole formation and development by influencing the evaporation and bulk diffusion rate of Mn element, and time plays a major role in the thickness of the porous layer in the process of preparing porous stainless steel.

Duplex stainless steels are widely used in nuclear industry for their excellent mechanical behavior, good weldability and superior ressistance to corrosion, the fracture toughness of which will be deteriorated with ageing time, as they are exposed to a certain temperature (204~538 ℃). In the present work, hot forging will be employed to induce the change of ferrite grain orientation and refinement of austenite grains; it is expected to improve the impact toughness after long-term thermal ageing. The microstructure and impact surface morphology of Z3CN20-09M duplex stainless steel were investigated by using SEM and EBSD. The micro-mechanical properties and impact properties of Z3CN20-09M duplex stainless steel at different thermal ageing time were tested by a nano-indenter and an instrumented impact tester. The results show that the crystal orientation of ferrite changes obviously and the austenite is changed from the original coarse columnar grains to the fine equiaxed grains after hot working. The im pact toughness of cast materials and forged materials decreases greatly with ageing time. The charpy impact energy of both aged and unaged forged-materials is higher than that of cast material. Cast material and forged material exhibit microvoid coalescence fracture in the early of thermal ageing; after 3000 h thermal ageing, the impact fracture features changes from ductile dimples to brittle cleavages in ferrites and tearings or dimples in austenites. However, cleavage features in forged material are significantly less than those in cast material due to the difference in ferrite crystal orientation.

Microstructure and texture evolution of Hi-B steel have been extensively studied in the past decades, and the microstructures are ordinarily characterized only using a single two-dimensional plane of polished or thin foil specimen. Much information on the morphologies is lost owing to the fact that a large part of microstructure is embedded beneath the polished surface, or removed during specimen preparation. Recently, computer-aided three-dimensional morphologies have been developed which can visualize microstructure in metals. The three-dimensional visualization promotes a better understanding of the actual information of polycrystalline materials, especially when the grain morphologies and size were required in three dimensions. In this work, three-dimensional morphologies of different oriented grains which include Goss, brass, {411}<148> and {111}<112> oriented grains in Hi-B steel formed during early stage of secondary recrystallization annealing were investigated by a combination of serial sectioning, computer-aided reconstruction and visualization, and electron back-scattered diffraction technique, and then the growth behavior of Goss oriented grains before abnormal growth was discussed. The results show that Goss oriented grains mainly exhibit pagoda shape, brass oriented grains are similar to inverted taper shape, which the grain sizes reduce gradually from the surface of the sample to the internal along normal direction, and {411}<148> oriented grains also exhibit pagoda shape and inverted taper shape. However, the morphologies of {111}<112> oriented grains show irregular shape. Compared with other oriented grains, Goss oriented grains have no size advantages on three-dimensional scale, and the growth of Goss oriented grains is mainly controlled by curvature before they grow up abnormally.

Temperature reverting rolling process (TRRP) is a newly developed technology for producing heavy steel plate with ultrafine grained surface layer. With hybrid structures along thickness direction, TRRP steel plate has excellent fracture toughness with crack arrestability which arouses interest recently. However, the crack arrest mechanism of the surface layer is still unclear to date. In this work, two types of steel plate produced by TRRP and traditional thermo mechanical control process (TMCP) were studied in order to get a comprehensive understanding of the crack arrest mechanism. The mechanical property tests demonstrate that the toughness of surface layer of TRRP steel is significantly higher than that of TMCP steel, while the mechanical properties at 1/4 thickness position of the two types are quite close. It's worth noting that ductile-brittle transition temperature of the TRRP steel surface layer is as low as -100 ℃. Microstructure analysis of the TRRP surface layer shows a coexistence of equiaxed ferrite grains with grain sizes of about 2 μm and dispersed M-A constituent. Numerical simulation of the temperature field of TRRP intermediate slab reveals the microstructure forming process. First, the surface layer is cooled lower than phase transformation temperature, which results in the generation of bainite ferrite. Subsequently, dynamic recrystallization of ferrite takes place in rolling process and leads to the formation of ultrafine grains. Instrumental impact test at -60 ℃ shows that the crack propagation of TRRP steel is effectively inhibited after a steady developing stage. The morphological analysis of the cross section of fracture shows significant plastic deformation in the surface layer, which means crack propagation energy is absorbed. As a result, the crack propagation is efficiently arrested. The statistical study of the grain orientations in the surface layer of TRRP steel indicates a randomly distribution of the ultrafine grains, which can hinder the crack propagation effectively. The nano indentation test shows that the hardness distributions of TRRP steel are mainly below 2.0 GPa. This means the microstructure is characterized by a small amount of hard phase dispersing in soft matrix, thus the crack initiated at the interface of phases can hardly propagate.

The minor element in alloy greatly aggravate the segregation of main elements and formation of harmful phase, resulting the deterioration of mechanical properties. Low segregation technology of cast superalloy was pioneered by Prof. Shi Changxu and co-workers in the early eighties. The technology is to control the content of minor element, such as P, Si, B and Zr, to lower the solidification segregation in the super-alloy. The working temperature and mechanical properties of superalloy can be increased greatly by using the low segregation technology. A series of alloys, such as M17 and GH738 with low segregation and excellent properties, had been developed. This study extends low segregation technology to 30Cr2Ni4MoV steel of large shaft for thermal power equipment, 690 alloy for steam generator tube in nuclear power plant, and uranium alloy for nuclear fuel. The solidification and segregation behaviour in the 30Cr2Ni4MoV steel was investigated, it is found that the minor elements of O and Al are essential for the formation of serious solidification segregation in the steel. Moreover, the solidification behavior of 690 alloy has been studied. S and N increases solidification interval, and the effect of S is greater than that of N. The solidification segregation of 690 alloy can be alleviated by controlling the contents of the S and N. Finally, the solidification temperature interval of high carbon uranium is calculated. With the car bon content increasing from 0.01% to 0.03%, the solidification interval is from 40 ℃ to 75 ℃. Thus, for the radioactive uranium alloys, minor elements show segregation to some extent in the residual liquids of final solidification zone. The minor elements in U-6Nb alloy are C, N and O. For uranium with high carbon content, the minor elements are C and O.

The Q235A mild steel and AISI304 austenite stainless steel were subjected to solid diffusion welding by vacuum diffusion bonding approach to investigate the influence of welding temperature on the interfacial morphology, microstructural constituents and mechanical properties. The results show that the single ferrite layer (zone II) and carbon-enriched layer (zone III) were formed nearby the bonding interface of Q235A mild steel and AISI304 austenite stainless steel, and heterogeneous microstructure on both sides of interface formed a common grain boundary by diffusion. The strength and toughness of the bonded joint reached the highest values, for welding temperature of approximately 850 ℃, welding pressure of beyond 10 MPa, and welding time of approximately 60 min, which was larger than those of the Q235A mild steel layer. Otherwise, the Cr₂₃C₆ carbide easily formed at a relatively lower temperature (≤800 ℃), whereas the secondary carbides and intermetallic compounds formed at a relatively higher temperature (≥900 ℃). Both cases would dramatically deteriorate the strength-toughness of the bonded joint. Therefore, it was proposed that the brittle precipitate phases can be effectively avoided by controlling the welding temperature to approximately 850 ℃, thus ensuring the resulting performance of the bonded joint.

The rapid development of energy, electricity, and transportation industries has created a market for steel pipes; however, buried steel pipelines near high-voltage transmission lines and electrified railways often experience alternating current (AC) corrosion at the damaged coating of pipelines; such phenomenon is mostly due to the resistance between the capacitance and inductance coupling, especially for long-distance pipelines in parallel operation. AC corrosion can cause pipeline corrosion perforation and stress corrosion cracking (SCC) in some cases, which has been a vital threat to the pipeline safety. In this work, the influence of AC on corrosion behavior of X80 pipeline steel was investigated in NS4 near-neutral solution by data acquisition technique, electrochemical test, immersion tests and surface analysis techniques. Results show that with the increasing of AC density, corrosion morphology changed from uniform corrosion to localized corrosion with many pits. Under the full AC interference, X80 steel occurred cathodic and anodic polarization which resulted in iron dissolution and hydrogen precipitation. The negative half wave AC would lead to hydrogen evolution and hydrogen induced anodic dissolution, the pits in X80 steel surface present sharp. However, under disturbance of positive half-wave AC, only anodic dissolution occurred and the pitting appeared spill shape and smoothly. Under various AC waveform interference, the corrosion products of X80 steel surface were different. Under full AC wave and positive half-wave interference, the corrosion products were loose, had have no α-FeOOH and occurred cracks; however, under negative half-wave AC interference, the corrosion products were denser and contained α-FeOOH which has protective effect on substrates.

TiAl based alloys have been widely used as promising aerospace structural materials, which benefit from their unique combination of mechanical properties. However, they yield poor plasticity and low process ability, thus restricting the wide application. In this work, an efficient way was proposed by which direct current (DC) was imposed on the solidification process of TiAl-based alloy. Influences of DC on the microstructure and properties of directionally solidified Ti-48Al-2Cr-2Nb alloy using water cold crucible directional solidification equipment has been investigated. The changes of solidification microstructure, phase structure and composition of the alloy and γ/α₂ interlamellar structures were characterized by OM, XRD, SEM and TEM. The effect of DC on the size of eutectoid colony, interlamellar spacing and relative content of α₂ phase had been studied by Image Pro Plus. Furthermore, the mechanical properties of the directionally solidified Ti-48Al-2Cr-2Nb alloy at 800 ℃ were performed. The results revealed that the DC can evidently promote the homogeneity of the solidification component and refiner the structure, and the segregation in lamellar colonies can be efficiently reduced or eliminated to a certain extent. With the increasing of the current density, the grain size and lamellar spacing decreased first and then increased, however, the α₂ phase content showed a totally different trend. Moreover, the microhardness, compression yield strength and the fracture strength of the alloy also revealed a trend of decrease after the first increase too. With the current density increasing, the average grain size and interlamellar spacing declined to the lowest of 0.46 mm and 0.19 μm, respectively, and the content of α₂ phase increased from 18.5% to 39.4%. The microhardness of sample reached 542 HV, the compression yield strength and the fracture strength were remarkably improved, and the maximum values reached 1200 and 1365 MPa, respectively. DC can cause a reduction of the supercooling in front of the liquid phase during the solidification process. The results can be seen as the peritectic reaction L→β+L→α+β moving a tiny drift to the direction of the Al-rich side in TiAl binary phase diagram, consequently, the primary β-phase increased, and the content of α₂ phase, microstructure under room temperature, increased evidently.

Electromigration (EM), which describes the mass transport due to the momentum exchange between conducting electrons and diffusing metal atoms under an applied electric field, has become a serious reliability issue in high-density packaging. With the increasing demands for miniaturization, liquid-solid (L-S) EM will pose a critical challenge to the reliability of solder interconnects. In this work, The interfacial reactions and diffusion behaviors of In, Sn and Cu atoms in Cu/Sn-52In/Cu interconnects during L-S EM under a current density of 2.0×10⁴ A/cm² at 120 and 180 ℃ have been in situ studied by using synchrotron radiation real-time imaging technology. During L-S EM, since there was no back-stress, the In atoms directionally migrated toward the anode due to the negative effective charge number (Z^*) of In, which is different from the In atoms directionally migrated toward the cathode due to the back-stress induced by the preferential migration of the Sn atoms over the In atoms toward the anode during the solid-solid (S-S) EM. Furthermore, a modified expression for calculating the effective charge number Z^* of liquid metals was proposed based on the enthalpy changes of melting process. The Z^* of In atoms was calculated to be -2.30 and -1.14 at 120 and 180 ℃, respectively, which was consistent with the migration behavior of In atoms. The model provides a theoretical basis for determining the direction of the EM. The polarity effect, evidenced by the IMC layer at the anode growing continuously while that at the cathode was restrained, was resulted from the directional migration of In and Cu atoms toward the anode during L-S EM, which was more significant at high temperature. The consumption of cathode Cu during L-S EM followed a parabolic relationship with the EM time, and the consumption rate was magnitude higher at high temperature. The migrations of In atoms was discussed in terms of diffusion flux.

Fe-Cr based damping alloys have high mechanical properties and good corrosion resistance, which have been applied to reduce vibration and noise. Their high damping behavior is primarily attributed to the stress-induced irreversible movement of 90° magnetic domain walls. Most researches mainly focused on the damping behavior of these alloys. However, little attention has been paid to the mechanical properties, which are the important consideration for engineering applications. Recently, a FeCrMo damping alloy with Cu addition was found to possess higher damping capacity and higher mechanical properties. In this work, scanning transmission electron microscopy (STEM) and dynamic mechanical analyzer (DMA) were used to investigate the Cu precipitation and its influences on damping capacity and mechanical properties of FeCrMoCu alloy (1.0% and 2.0% Cu addition, mass fraction) with dif ferent cooling rates. The results show that the Cu element in 1.0Cu alloy is fully dissolved in the matrix. When the cooling rate is slow (furnace cooling), there will precipitate a small amount of second phases, which are small in size (<5 nm) and contain relatively few Cu atoms (3.7%). As for 2.0Cu alloy, with decreasing cooling rate (from water cooling to air cooling, and to furnace cooling) there will firstly precipitate a small amount of second phase with small size (<5 nm); subsequently, the particles grow into a spherical shape (10~15 nm) and their number increases; at last, the particles transform into round bar with coarse size of 100~400 nm and the precipitate number decreases obviously. The Cu content of the latter two precipitates increased obviously (about 30%~40%). These precipitates will significantly increase the average internal stress of the experimental FeCrMoCu alloy, which will obviously decrease the damping capacity. Therefore, the damping capacity of 2.0Cu alloy is much lower than that of 1.0Cu alloy. Meanwhile, the precipitate will obviously improve the strength. Compared with coarsen Cu-riched phase, the finer second phase has better hardening effect and its influence on ductility and toughness is relatively small. The FeCrMoCu alloy with addition of 1.0% Cu can obtain better damping capacity and mechanical properties at the same time.

Fe-Pt-B nanocomposite magnets have attracted much attention because of their excellent hard magnetic properties, in which the face-centered-tetragonal FePt (L1₀) phase ensures high coercivity (_iH_c) and the Fe₂B phase provides high magnetic saturation. A high _iH_c, however, is hard to reach at low Pt concentrations in these nanocomposite magnets. It is known that a high concentration of B favors the formation of L1₀ phase in Fe-Pt-B alloys with low Pt concentration, but the annealed microstructure is usually coarse-grained due to their low amorphous-forming abilities, and the magnetic properties get deteriorated. Replacement of Fe with Co is expected to enhance the amorphous-forming ability of Fe-Pt-B alloys with low Pt and high B concentrations, and to improve their magnetic properties. In this work, the structure and magnetic properties of as-quenched and annealed Fe_55-xCo_xPt₁₅B₃₀ (x=0~45, atomic fraction, %) alloys have been investigated. Melt-spun ribbons were prepared by melt spinning, followed by vacuum annealing at different temperatures. The structure and magnetic properties of the samples were examined by XRD, TEM and a vibrating sample magnetometer (VSM). The results indicate that single amorphous phase is formed in the alloys at x=15~45. After appropriate annealing, a nanocomposite structure consisting of L1₀ and (Fe, Co)₂B phases is obtained at x=0 and 15, and an additional (Fe, Co)B phase gets formed at x=30 and 45. A fine microstructure with mean grain size of ~18 nm has been obtained in the annealed alloys with x=15~45. In these nanocomposite alloys, the best hard magnetic property with an energy product of 94.4 kJ/m³ is reached at x=15. With increasing Co content, the _iH_c gradually increases to a maximum value of 413.7 kA/m at x=30, and then decreases at higher Co contents, which are attributed to the change of the magnetocrystalline anisotropy in L1₀ phases with different c/a ratios.

SnAgCu solder alloys, such as Sn3.0Ag0.5Cu, Sn3.8Ag0.7Cu and Sn3.9Ag0.6Cu, are widely used for consumer electronics due to their good wettability, high mechanical properties and excellent thermal fatigue reliability. However, the high Ag content in SnAgCu solder can bring about a relatively high cost and poor drop impact reliability because of the formations of thicker brittle Ag₃Sn compound during soldering. Therefore, the development of low Ag content SnAgCu solders to satisfy the requirements of electronic production has become a hot topic in this field. In this work, the effects of Sn0.3Ag0.7Cu, Sn1.0Ag0.5Cu and Sn3.0Ag0.5Cu solder on the melting character, wettability, mechanical properties and microstructures, phase composition were investigated by DSC, micro-joint strength tester, SEM, EDS and XRD. Under -55~125 ℃ cyclic conditions, the interfacial layer change of Sn-Ag-Cu solder joints was measured by TL-1000 high and low temperature test chamber. The results showed that, with the Ag content increased, the melting point was not changed, the wetting angle significantly decreased. And the wettability of three solders was improved under N₂ atmosphere. Moreover, the mechanical properties of three solder joints were enhanced with the increase of Ag content. The matrix structure of Sn0.3Ag0.7Cu and Sn1.0Ag0.5Cu solder joint have a small amount of Ag₃Sn and large Cu₆Sn₅ particles, and the distribution of particles were disordered. However, the matrix structure of Sn3.0Ag0.5Cu solder joint was obviously uniform. This is the reason that the mechanical properties of Sn0.3Ag0.7Cu and Sn1.0Ag0.5Cu solder joints were lower than that of Sn3.0Ag0.5Cu. In addition, the solder joints were subjected to a thermal cycling reliability test, it was found that the thickness of intermetallic compounds (IMCs) increased, and the morphology of IMCs was gradually changed from scallop-like to planar-like.

Based on the empirical electron theory (EET) of solids and molecules, the valence electron structure caculation results of Fe-Cr alloy containing (0~30%)Cr were analyzed semi-quantitatively. The electron density differences of interface (Δρ) between Fe-Cr alloy and Cr₂O₃, Fe₂O₃ passivation films were calculated. According to the results, adding Cr to α-Fe matrix can strengthen the matrix by improving the number of hybid atomic orbitals σ_n, the number of the strongest bond covalent electron pairs n_A and the strongest covalent bond energy E_A of Fe-Cr alloy. Once the content of Cr rises up to 12.52% and 24.3%, the corrosion resistance of Fe-Cr alloy is improved because of Cr being changed to a higher hybrid level, where Cr becomes more unstable and easily reacts with environment to form a complete passivation layer of Cr₂O₃. Moreover, among the electronic density differences of 24 low-index faces between Fe-Cr and Cr₂O₃, Fe₂O₃, only the Δρ of Fe-Cr(112)/ Cr₂O₃(0001), Fe-Cr(112)/Cr₂O₃ (101?0)_Cr,Fe-Cr(112)/Fe₂O₃(112?0) are lower than 10%. For the matrix with same content of Cr, the Δρ between Fe-Cr(112) and Cr₂O₃(101?0)_Cr is the lowest, but the number of hybid atomic orbitals σ satisfied Δρ<10% is the largest. Δρ (σ) of Fe-Cr(112)/Cr₂O₃(0001) and Fe-Cr(112)/Fe₂O₃(112?0) is decreased (increased) with the increase of Cr, therefore the interface bonding strength between Cr₂O₃, Fe₂O₃ and matrix will be enhanced, it has been found that the corrosion resistance of Fe-24.3%Cr is better. The calculation results of variation of Fe-Cr corrosion resistance with Cr content are in better agreement with Tammann's law.

Variable polarity plasma arc (VPPA) is a kind of source to provide heat and force at welding process. It can remove the oxide layer with high melting point on the surface of base metal using the cleaning action of cathode spots (the special property of VPPA). So variable polarity plasma arc welding (VPPAW) is a very suitable method to join aluminum alloys which always have extremely tenacious surface oxides. It is great significant to understand clearly the physical characteristics of VPPA for predicting welding defects and making the welding process stable. Therefore, modeling and simulating VPPA are necessary and helpful to understand welding process theory and promote its application further. In this work, a three dimensional transient calculated model of VPPA was established. To describe the electrical characteristics of VPPA at different polarities, a sequential electric conducting model was proposed. With finite difference method, the temperature field, fluid flow and current density of VPPA were solved out. And the distribution of plasma arc pressure on the anode surface, as well as its evolution process as the time going on were analyzed. Arc pressure was measured experimentally to verify the calculated model. The results show that the arc temperature field of electrode negative (EN) is more compressed than that of electrode positive (EP). The range of high temperature at EN is a little larger. Arc pressure and current density of EN at central area are both higher than EP. Nonetheless, the magnitude of these values begins to reverse at a certain distance to center in radial direction. Moreover, the arc pressure rapidly responses to welding current. Pressure at EP is about 20% lower than that of EN. The pressure reduces to the lowest value when the current pass through 0. After that, while the current reaches to normal value, the pressure will immediately impact to a larger value, then quickly recover to an average value. Otherwise, to compare the experimental results with calculated results of arc images and arc pressure, they are in good agreement with each other.