The effect of Cu on microstructure transformation and microstructure refining of the weld metal of 690 MPa grade HSLA steel was investigated by OM, TEM, EBSD and thermal expansion instrument, and the mechanism of microstructure refining was discussed. Experimental results indicate that microstructure of weld metal is composed of granular bainite, lath bainite and residual austenite. The addition of Cu content from 0.24% to 0.53% in weld metal can decrease phase transition temperature, which induces the reduction of martensite-austenite (M/A) amount from 0.62% to 0.31%, the variation of M/A shape from small bulk and bar to granular shape, the increase of residual austenite amount, and the remarkable refining of microstructure. The increase of Cu content from 0.24% to 0.53% results in the decrease of the mean size of lath block from 2.18 to 1.99 μm, the decrease of the width of lath from 0.39 to 0.36 μm, and the increase of the amount of large angle boundary, which can inhibit crack propagation, from 68.5% to 71.0%. Analysis indicates that Cu can decrease phase transition temperature, increase the stability of austenite, raise the pontential difference between ferrite free energy (G_α) and austenite free energy (G_γ), reduce the critical size of crystal nucleation. Meanwhile, low phase transition temperature can retard the diffusion rate of carbon atom and slower the growth rate of crystal grains. These factors result in the refining of substructure.

The effect of austenitizing temperature on the microstructures and -20 ℃ impact toughness of HSLA100 steel was investigated by Gleeble-3500 thermal simulator. Its microstructures were observed by SEM and EBSD, and the relevant transformation kinetics was also analyzed by means of dilatometer. The results show that the microstructure of HSLA100 steel changes gradually from granular to lath bainite with increasing austenitizing temperature. The highest impact toughness of samples was achieved at austenitizing temperature of 1000 ℃, in which martensite-austenite (M/A) islands are finer and dispersed and the density of high angle boundaries is maximum. M/A islands, however, become coarser and this density lowers below 1000 ℃, beyond 1000 ℃, these islands are refined, being accompanied by a dramatic decrease of this density of high angle boundaries. Kinetics analysis indicates that with increasing austenitizing temperature, the transformation start temperature decreases but the transformation rate increases. Both lower start temperature and faster rate would facilitate M/A islands refining. All the transformation occurring in samples might be divided into two stages: bainite and martensite stages. The highest transformed fraction of bainite is achieved in the bainite stage at about 1000 ℃, resulting in the best impact toughness of HSLA100 steel. The crystallographic analysis of the well refined M/A islands at 1000 ℃ and 1300 ℃ shows that major high angle boundaries occur prior at the boundaries between different Bain groups belong to the same crystallographic group set to at austenite boundaries when covariance transformation occurring. When over-increasing austenitizing temperature, the covariance transformation products in coarser austenite grains are dominated by only one Bain group belong to the crystallographic group set, leading to the density of high angle boundaries and thus the impact toughness of HSLA100 steel decreasing.

Cable-type welding wire (CTWW) CO₂ gas shielded arc welding, which uses CTWW as consumable electrode, is an innovative arc welding process with high quality, high efficiency and low consumption, thus having significant potential of wide application in industrial manufacturing. So far, however, there is lack of deep study on this new welding technology, hindering its promotion. In this paper, the process mechanism of CTWW CO₂ gas shielded arc welding is studied through combining the experimental detection and numerical simulation. By using arc multi-information collection system, the characteristics of arc shape and behavior in CTWW CO₂ gas shielded arc welding process is acquired. The photographs of weld without defect are obtained. Based on the test results, the mechanism of CTWW CO₂  gas shielded arc welding is explained. A finite element analysis model suitable to CTWW CO₂ gas shielded arc welding is developed to simulate the temperature and stress field distribution. The results show that, there exist a unique bunchy electric arc in CTWW CO₂ gas shielded arc welding, which is formed through multi--arc rotating and coupling, leading to arc heat concentration; the calculated weld cross section agrees well with the experimental data, validating the accuracy of established heat source model. When the heat input for per unit length in CTWW CO₂ gas shielded arc welding is 2.9 times more than that in single welding wire (SWW) CO₂ gas shielded arc welding, the weld penetration and width are 4 times and 1.7 times of those in SWW CO₂ gas shielded welding, respectively. Under the same welding condition, the new welding process has a similar heat efficiency to submerged arc welding (SAW), but its weld penetration is greater, and its weld width, heat-affected zone width and peak temperature of thermal cycle are smaller. Besides, the residual stress field in CTWW CO₂ gas shielded arc welding is close to that in SAW.

SiC_f/TC17 composites were fabricated by a method of precursor wire with magnetron sputtering using a vacuum hot pressing (VHP) process and then exposed in vacuum at 973, 1023, 1073 and 1123 K for different times, respectively. The results show that element diffusions include interdiffusion caused by interfacial reaction and concentration gradient, and phase transformation diffusion in matrix. C and Ti mainly carry on reaction diffusion which is the reason of formation and growing up of reaction layer. Si, Al, Mo, Cr, Zr and Sn carry on downhill diffusion at interface of C-coating layer and reaction layer, but this type of diffusion is not obvious. Phase transformation diffusion in matrix lead to that Al diffuse to α phase, Mo and Cr diffuse to β phase, and Sn diffuse to Ti₃AlC, so the interfacial interdiffusions of these elements is suppressed. The results of the interfacial thermal stability show that the activation energy of reaction lay growing up is 138 kJ/mol, and the interface of SiC_f/TC17 composite is stable for long time while it is used not above 973 K.

Using the as-received P92 steel pipe steel samples with/without δ-ferrite as objects and EPMA-EDS+MPST (multiphase separation technique), this work investigated the effects of δ-ferrite on the volume fraction and< composition of M₂₃C₆ and Laves phases precipitated in the ageing treatment at 700 and 750℃ for 1000, 3000 and 5000 h, respectively. The results are as follows. The existence of δ-ferrite promoted aggregation and coarsening of M₂₃C₆ and Laves particles precipitated along boundaries with its neighboring lath martensite at the expense of consuming Cr, Mo and W in δ-ferrite during the ageing treatments. Varying rate of the volume fraction of M₂₃C₆ with the ageing time for the samples without δ-ferrite would chang at different ageing temperatures, and also the volume fraction of Laves phase would increase with decreasing ageing temperature for the samples with δ-ferrite. Owing to a few amount of Laves formed at higher temperature, the volume fraction of M₂₃C₆ increased in the samples with δ-ferrite. And increasing ageing temperature and time stimulated the coarsening of particles precipitated in the pipe samples investigated. It is expected that the related information from this study on the effects of existence of δ-ferrite on the microstructural stability and high temperature properties of P92 can be used as a reference when needed.

Formation of a special kind of complex ductile inclusions (CDI), which were character as oxides covered by sulphides, was investigated in a medium--high carbon steel. It was believed that fatigue and toughness properties of carbon steels were benefited from CDI formation, for oxides were separated from steel matrix. By means of thermodynamic calculation and inclusion microscopy in medium-high carbon steels with different S content and with Ca-treated, CDI in 0.55C steels has been researched. And thermal-stability of the CDI was researched by high temperature quenching. The results show that the frequency of CDI formation increase with S content changes from 0.001% to 0.01%. However, the fraction of covered oxides is no more than 90% without Ca-treated and can be promoted to about 100% in 0.006% S steel after Ca-treated. The reason of improved CDI formation by increasing S content and Ca-treated is that sulphides precipitation temperature could be promoted to higher than solid line of matrix, which is clearly showed by thermodynamic calculation. Furthermore, thermo-stability of CDI is affected heavily by heating history of matrix steel. Sulphide will dissolute from oxide when heat steel casting to an improper high temperature. Fortunately, the dissolution process could be stunted with redundant S content. It shows that thermo-stability of CDI could be received in Ca-treated steels with more than 0.006%S.

Many researches have demonstrated that the free volume have a great effect on the properties of bulk metallic glasses (BMGs). For different BMGs, however, quantitative measurement of free volumes and analysis of properties of BMGs using the measurement results are still difficult. In this work, the two types of characteristic free volumes, the free volume released in structural relaxation, ΔV_f-sr and the free volume generated in glass transition, ΔV_f-gt are given from the Δ(dV(T)/V₀) curve, where the Δ(dV(T)/V₀) is the thermal expansion difference between amorphous and crystalline samples measured by a cyclic thermal dilation test. In a series of Fe-(Er)-Cr-Mo-C-B BMGs, it is found that the BMG with the largest critical diameter (D_c) has also the largest ΔV_f-gt, and D_c increases sensitively with the decrease of ΔV_f-sr. More impressively, D_c² or D_c can be fitted with high regression coefficient of 0.998 by a negative exponential function of ΔV_f-sr. Hence, the characteristic free volume has a sensitive and close correlation with the glass forming ability of BMGs.

Fe-Ni base austenitic alloys have been widely used as structural materials in the hydrogen environment, which is strengthened by the precipitation of γ' [Ni₃(Al, Ti)]. η phase is present after solution treatment in some modified alloys, which is detrimental to hydrogen resistance performance of alloys. In order to eliminate the $\eta$ phase and optimize the alloy composition, the precipitation behavior of η phase in Fe-Ni base alloys with different Ti and Al contents was investigated in the range of 900 ℃ to 1030 ℃ using OM, SEM, TEM and EPMA. The results show that the formation ability of η phase is related to the content of Ti and Al. Increasing the (Ti+Al) content or the Ti/Al ratio in the alloys promotes the precipitation of η phase. Moreover, the η phase does not exist in alloys with the lower (Ti +Al) content or Ti/Al ratio, but still presents in the alloys with higher (Ti+Al) content and Ti/Al ratio after the standard solution treatment at 980 ℃, which is due to that the η phase precipitates at the expense of the γ' phase existing at higher temperature. In situ microstructure observations indicate that η phase and elemental segregation can be eliminated by the optimization of solution treatment condition.

The nanocrystalline nickel thin films with high density nano-scale growth twins were synthesized by means of pulse electrodeposition technology. Three samples were deposited at different bath temperatures of 30, 50 and 80 ℃, by keeping all the other parameters as constant, such as electrolyte, pH value, current density and on-time and off-time period. The effect of temperature on deposition rate, sample texture, grain size, and twin boundary length and twin lamella thickness were systematically analyzed by SEM, XRD and TEM techniques. The microscopical feature of twin boundary were investigated by HRTEM. The results show that the nano-twinned nickel films deposited at the rates ranging from 20 nm/s to 30 nm/s have a preferred growth plane of (220) when deposited at 30 and 50 ℃, but changes to (200) when the temperature increases to 80 ℃.  With increasing temperature, the grain size decreases from 900 to 300 nm, and the twin lamella thickness decreases from 60 to 28 nm. The relationship between deposition temperature and nanoindentation hardness for these films, moreover was determined. The nanoindentation hardness measurement indicates that the average indentation hardness of these films reaches a maximum value of 3.75 GPa at 50 ℃.

Nanocomposite films deposited by physical vapor deposition (PVD) methods have been attracting much attention worldwide in the last decade. Among these, Ti-Al-N is one of the most thoroughly studied hard film materials. Compared with such films as Ti-N, Ti-C-N and Ti-Zr-N, Ti-Al-N films are commercially available for various machining applications, due to their high hardness, relatively low friction coefficient, good oxidation and corrosion resistance. However, under certain conditions the loose of hardness and oxidation, the high internal stress are still serious drawbacks and restrict their industrial applications. Therefore, seeking for a new kind of film on basis of Ti-Al-N films becomes necessary, it is significant to study the< microstructure and properties of the films aim at specific application. In the present work, Ti-Al-Si-N films with various silicon contents were deposited on high speed steel substrates with the assistance of cathodic vacuum arc ion plating (AIP). The film structure, chemical and phase composition, mechanical and tribological properties are characterized by XPS, XRD, SEM, TEM, nano-indentation and Rockwell indenter. Combined with XRD and XPS analysis, the results indicated that the films were composed of crystalline TiAlN and amorphous Si₃N₄. With increasing silicon content in the film, a deterioration of the preferred orientation and a reduction of the grain size were detected. SEM observation of the film cross-sections showed that the microstructure changed from obvious columnar to dense nano-structure. Furthermore, with increasing silicon content, both the hardness and elastic modulus firstly increased a lot, within a certain ranges of silicon content changed to be steady, and then sharply decreased with more silicon addition in the film. The H³/E² ratio can be obtained on basis of measurements of hardness $H$ and Young's modulus E, it is proportional to the film resistance to plastic deformation. The results showed that the harder the film is, the higher the resistance to plastic deformation is in films. The adhesive strength between substrate and films was also studied by scratch tests, the values decreased firstly, and then increased again with increasing silicon content. The essence of above phenomena is attributed to the variations of microstructure and morphologies in the films induced by increasing silicon content. This whole study implies that these Si-doped Ti-Al-N films deserve some cautiousness before its application for\linebreak wear resistance.

An antiferromagnetic Fe24Mn4Al5Cr covar alloy has been impulse-passivated by an alternating current (AC) voltage overlapping a direct current (DC) voltage, in order to improve its corrosion resistance due to the poor corrosion property of austenite matrix with a high Mn content, a low Cr and/or Al content. The impulse-passivated films were obtained in 1.0 mol/L Na₂SO₄ solution at an AC voltage with modulation amplitude of 180-480 mV during alternating period of 200-400 ms for modification time of 5-15 min, and simultaneously at a DC voltage of 620 mV. The impulse-passivation parameters in 1.0 mol/L Na₂SO₄ + 0.5 mol/L Ha₂SO₄ solution were optimized as amplitude of 380 mV, period of 300 ms, and time of 10 min, by using the potential decline curves. The impulse-passivated films on the covar alloy under the optimum parameters were characterized by Auger electron spectroscopy and X-ray photoelectron spectroscopy (AES/XPS), and evaluated by anodic polarization and electrochemical impedance spectroscopy (EIS), respectively, and compared with those of the constant-passivated films at DC voltage of 620 mV for modification time of 15 min. In the impulse-passivated films on the covar alloy, an enrichment of elements Al and Cr, a lack of Mn in a thick barrier film composed of oxides Al₂O₃ and Cr₂O₃. The anodic polarization curves of the impulse-passivated films have a self-passivation with the higher corrosion potential of -100 mV and lower passive current density of 0.7 μA/cm², than those of -360 mV and 2.6 μA/cm² for the constant-passivated films. These electrochemical polarization behaviors were similar to those of the AISI 304 austenitic stainless steel. The EIS of the impulse-passivated films has larger diameter of capacitive arc, higher impedance modulus $|Z|$, and wider phase degree range,  relative to the constant-passivated films. Correspondently, the impulse-passivated film resistant R_p increased to 54.0 kΩ·cm² from 14.8 kΩ·cm² and effective capacitance decreased to 10.2 μF/cm² from 14.0 μF/cm², using an equivalent electric circuit of< R_s-(R_p//CPE). The high insulation of the impulse-passivated films on the covar alloy led to an improved corrosion resistance of the cover alloy. The impulse-passivated antiferromagnetic Fe24Mn4Al5Cr covar alloy exhibits an application potential in wide industrial field.

The strategy for disposal of high-level radioactive waste in china is to enclose the spent nuclear fuel in sealed metal canisters which are embedded in bentonite clay hundreds meters down in the be-rock. The choice of container material depends largely on the redox conditions and the aqueous environment of the repository. One of the choices for the fabrication of waste canisters is copper, because it is thermodynamically stable under the saline, anoxic conditions over the large majority of the container lifetime. However, in the early aerobic phase of the geological disposal the corrosion of copper could take place, and the corrosion behavior of copper would be influenced by the complex chemical conditions of groundwater markedly. Pitting corrosion of copper often takes place in power plants or air-conditioning condensate water. The corrosion environment usually contains HCO₃^-, SO₄^2- and Cl^-. In the early stage of geological disposal, if the aerobic water with HCO₃^- , SO₄^2- and Cl^- immersion repository, the pitting corrosion of copper may occur. Some researchers believed that SO₄^2- and Cl^- would promote the occurrence of pitting corrosion of copper, and HCO₃^- will lead to surface passivation and inhibit pitting. It is considered that in the solution with HCO₃^- and SO₄^2-, HCO₃^- could firstly promote and then inhibit pitting. However, there is no systematic work about pitting in the solution with HCO₃^- and Cl^-. In this work, the cycle polarization behavior and surface morphology of pitting on copper has been investigated in HCO₃^- and Cl^- mixed solution, respectively by electrochemical cyclic polarization test and SEM. The results showed that the circular polarization curves of copper could be divided into four types. The pitting on the surface of copper occurs only in the environment with both Cl^- and HCO₃^-. In the area of active dissolve pitting, the pitting susceptibility increased with the increase of concentration of Cl^-, while it increased then decreased with the increase of the concentration of HCO₃^-. In the area of passive film rupture pitting area, pitting susceptibility increased with the increase of concentration of Cl^- and with the decrease of the concentration of HCO₃^-.

Porous copper with long cylindrical pores fabricated by unidirectional solidification of metal-gas eutectic system can be used to manufacture a special kind of micro-channel heat sink. In order to simplify the heat transfer analysis, a fin model was introduced into the theoretical study on heat transfer performance of directionally solidified porous copper heat sink. The heat transfer performance of porous copper heat sink was also tested by experiments, and it was found that experimental values are far less than theoretical predicted ones. That is because the structure of porous copper might deviate from its ideal structure, such as, some pores are not penetrated, and the distribution of pore size and pore location is not uniform. After the model was modified by introducing area ratio of penetrating pores and mean diameter of penetrating pores, the theoretical results were consistent with the experimental results. Thus the analytical method based on the fin model in this paper can be used to predict the heat transfer performance of directionally solidified porous copper heat sink. According to the theoretical analysis, porous copper used for heat sink with excellent heat transfer performance should have the following porous structure: the pore diameter is 0.1-0.6 mm, the porosity is 30%-70%, the height of porous copper is more than 4 mm when its length along the direction of pore axis is 20 mm.

The Ni-Ag alloy has good mechanical properties, high corrosion resistance and electrical conductivity. It is an excellent candidate to be used in many high-tech fields of aerospace, energy resource and chemical engineering etc. This alloy, however, is a typical monotectic system. Generally, the liquid--liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation. The manufacturing of this alloy is thus extremely difficult. Injection casting has already been carried out with the Ni-Ag monotectic alloy. The sample with composite microstructure, in which Ag-rich particles dispersed homogeneously in Ni matrix has been obtained. A model describing the microstructure evolution during injection casting of the Ni-Ag monotectic alloy has been proposed. The process of microstructure formation has been simulated and discussed in details. The results indicate that the Ostwald coarsening of Ag-rich droplets is very weak during cooling in miscibility gap under injection casting cooling conditions. The dispersivity of the primary Ag--rich phase is controlled by the nucleation of Ag-rich droplets during the liquid-liquid transformation. The number density ($N$) and average radius (<R>) of primary Ag--rich particles depend exponentially on the cooling rate of the alloy during the nucleation of Ag-rich droplets ($\dot{T}_{\rm Nuc}$) according to $N\propto\dot{T}_{\rm Nuc}^{1.8}$ and $\langle R\rangle\propto\dot{T}_{\rm Nuc}^{-0.6}$.

The effects of melt mixing with high-temperature and low-temperature melts on Au-20Sn (mass fraction, %) eutectic alloy solidification microstructures have been studied. The solidification microstructure evolutions of Au-20Sn eutectic alloy by the temperatures of high-temperature melt were investigated. Melt mixing with high-temperature and low-temperature melts can effectively improve the solidification microstructure of Au-20Sn alloy. Adopting an appropriate melt mixing condition, which is high-temperature melt with 350 ℃ and low-temperature melt with 283 ℃, the precipitation of primary phase ζ'-Au₅Sn will be inhibited during solidification process, and the full lamellar eutectic microstructure was obtained. When the temperature of high-temperature melt is high (360 ℃) or low (340 ℃), the primary phase ζ'-Au$_{5}$Sn will also exist in the solidification microstructure. Melt mixing with high-temperature and low-temperature melts can effectively decrease the Au atom segregation and modify the precipitation behavior of primary phase ζ'-Au₅Sn. The compressive behavior at 220 ℃ exhibits a low yielding stress and a low stress platform for the alloy with full lamellar eutectic microstructure prepared by melt mixing, which indicates that the hot--workability of Au-20Sn alloy can be improved by melt mixing.

Si-Zr-Y co-deposition coatings on an Nb-Ti-Si-Cr base ultrahigh temperature alloy were prepared by pack cementation processes at 1250 ℃ for 8 h with different halide activators in the packs. The structure, constituent phases and formation process of the coatings were investigated. The results show that all the coatings are mainly composed of an (Nb, X)Si₂ (X represents Ti, Cr and Hf elements) outer layer, a (Ti, Nb)₅Si₄ transitional layer, and an Al, Cr and Y-rich (Nb, X)₅Si₃ inner layer located between the coating and the substrate. In the five kinds of (NaF, NH₄F, NH₄Cl, NaBr and NaCl) activators investigated, the coatings prepared respectively with NaF and NH₄F activators are thicker and denser, and furthermore, the contents of both Zr and Y elements are larger in the coating prepared with NaF as the activator. However, the coatings prepared respectively with NH₄Cl, NaBr and NaCl as the activators are thiner, and lots of holes and ZrO₂ and HfO₂ particles are observed in the surface layer of the coatings. The higher equilibrium partial pressures of gaseous halide silicides in the packs resulted in thicker and denser co-deposition coatings, while the equilibrium partial pressures of  gaseous halide Zr and Y in the packs have less effect on their contents in the coatings.

As the increases of alloying elements, the highly alloyed Ni-based superalloys are very difficult to deform due to the low deformation plasticity, high flow stress and narrow deformation temperature interval. The hot deformation behavior of highly alloyed as-cast and homogenization treated GH742 alloys was investigated by isothermal compression conducted on a Gleeble-3500 thermo-simulation machine. The ingots were homogenization treated at 1140 ℃ for 6 h or at 1100 ℃ for 30 h and at 1160 ℃ for 40 h, followed by furnace cooling to 800 ℃ and then air cooling to room temperarue. The GH742 alloys possess lower flow stress, higher plasticity and larger recrystallization degree as the homogenization degree increases during the deformation process. High dislocation density and deformation bands are formed due to the elemental segregation and the precipitates of γ' and δ phase in the interdendritic regions, which enhance the flow stress. Mircocracks are initiated when the brittle precipitates such as Laves phase and Ni₅Ce phase are crushed by compression. Stress concentration area as well as the banded structure of carbides is formed when the primary MC type carbides are deformed. The two-step homogenization treatment via low temperature pretreatment followed by high temperature diffusion presented by this paper can not only eliminate the elemental segregation and the detrimental precipitates, but also improve the dimension and distribution of MC type carbides, which decreases the flow stress, increases the hot deformation plasticity remarkably, and obtains homogeneously recrystallized microstructure.