The nucleation of intragranular acicular ferrite (IAF) on complex titanium–containing inclusions for titanium deoxidized steel was observed dynamically by confocal scanning laser microscope (CSLM), and the effects of average austenite grain size and cooling rate on the nucleation of IAF was systematically investigated by using OM, FE–SEM and EDS. Furthermore, the mechanism of IAF nucleation on the complex titanium–containing inclusions was studied by using EPMA in detail. The results show that the starting formation temperature of IAF firstly increases with increasing austenite grain size, then decreases. With the increase of cooling rate, the volume fraction of IAF increases obviously due to the increase of the starting formation temperature. Mn–deplete zone (MDZ) is considered to be a dominant driving force for the nucleation of IAF. In addition, it is found that the range of starting formation temperature of IAF is 550—633 ℃ with austenite grain size in the range of 132—255 μm. Volume fraction of IAF obtained at room temperature is up to 90% with austenite grain size of 176 μm and a cooling rate of 5 ℃/s in this experiment condition.

The effect of sulfur contents on fracture toughness of railway wheel steels for high speed train was investigated. The results show that, with the same production process, sulfur content increase to some extent in wheel steels resulted in a great improvement in fracture toughness. The analysis of non–metallic inclusions revealed that, in the steel with sulfur content of 0.001%, a large amount of Al₂O₃ and complex Al₂O₃+(Ca,Mg)O inclusions were found, while MnS inclusions were rarely observed. When S content was increased from 0.001% to 0.006%, the type and morphology of inclusions were changed, MnS inclusions with core of Al₂O₃ were often observed and naked oxide inclusions were seldom found. This investigation shows that the fracture toughness improvement were closely associated with the type and morphology of inclusions due to sulfur content variation. The calculation showed that, with a cover of ductile MnS, stress concentration around complex inclusions decreased, which contribute to toughness improvement.

The most common texture formed in rolled bcc metals is 45^? rotated cubic {001}<110>, the detailed deformation structure and microhardness of these grains in cold-rolled Ta–7.5%W alloy foils were carefully investigated. It was found that when cold reduction was 10%, the typical cold deformation microstructures were found in the grains, including dislocation dipoles, a"scissors" type dislocation reaction and dislocation debris. As the reduction increased, high dense dislocation walls were formed firstly and microbands formed subsequently in one kind of grains while typical dislocation cells were formed in another kind of grains. Based on energy analysis, dislocation mesh structures would relax into dislocation cells because the elastic energy of the dislocation cells is always lower than that of the dislocation mesh. It is probable that the microbands are derived from the high dense dislocation walls rather than dislocation cells while the dislocation density in the grains must reach a critical value which is about 1014/m2 in Ta–7.5%W alloy. The microhardness test showed that the work hardening rates were different in these grains. Compared with grains with cell structures, the work hardening rate was higher in grains with microbands.

In this work, microstructure and texture evolution in the sheets of Mg alloy AZ31 was studied by means of bidirectional cyclic in–plane bending at 423 K followed by annealing at 523 K. The deformed and subsequent annealed microstructures were investigated by OM and SEM/EBSD examinations. The results showed that twinning is the dominant deformation mechanism. With the increase of deformation pass, more and more twins were produced in the grains near the surfaces of the sheets and finally these grains were serially divided up by twinning intersections. But only a few twins were formed in the grains of the middle of the sheets due to relatively lower strain. Finally, a gradient structure with high–density twins in the regions near the surfaces and, in contrast, lower density ones in the center of the sheets were induced. After 6 pass bending at 423 K followed by a subsequent annealing at 523 K, the average grain size near the surfaces was reduced to about 10 μm from the original size of 46 μm due to operation of static recrystallization at twin intersections and compression twins. Particularly, the relative intensity of the strong texture developed in the sheets was severely weakened by repeated bending, and this led to an increase of fracture elongation from 19.01% to about 24% with little change of tensile strength.

With the rapid development of electricity, petroleum and transport industry, more and more pipelines are buried in parallel with high–voltage transmission lines or electrified railways, and the resulting AC corrosion phenomenon has attracted more attentions. In this work, the effects of AC current density, electrolyte composition and AC current frequency on corrosion potential of Q235 steel were studied through simulation experiments, and the results are as follows: AC current makes corrosion potential of Q235 steel deviate positively or negatively depending on polarization rate of anode/cathode within different electrolytes, and the offset of corrosion potential becomes more significant as the AC current density increases or the AC current frequency decreases. Besides, the inherent mechanism of such offset induced by AC current was also studied through double layer model of interface and corrosion products analyses.

A cluster–plus–glue atom model was employed to design Ni–Nb based ternary bulk metallic glasses. The binary eutectic point Ni_59.5Nb_40.5 was first interpreted by the model in form of a cluster formula [(Ni_0.5Nb_0.5)–Ni₆Nb₆]Ni₃, where the cluster is (Ni_0.5Nb_0.5)–centered icosahedron derived from a eutectic phase Ni₆Nb₇ (Fe₇W₆ type). It was then pointed out that the best binary glass former Ni₆₂Nb₃₈ could be interpreted based on the eutectic cluster formula by replacing the cluster center Nb_0.5 with Ni_0.5, namely [Ni–Ni₆Nb₆]Ni₃=Ni_62.5Nb_37.5. To further improve the glass–forming ability, Zr, Ta and Ag are selected as alloying additions to partially replace Nb in the [Ni–Ni6Nb6]Ni3 cluster formula, and glassy rods with a critical size of 3 mm are achieved at appropriate ternary compositions by copper–mould suction–casting. DTA measurements indicate these bulk metallic glasses exhibit high thermal stabilities, among which the [Ni–Ni₆Nb₅Ta]Ni₃ alloy has the highest T_g (glass transition temperature)of 935 K and T_x (crystallization temperature) of 952 K. Room–temperature compressive curves of [Ni–Ni₆Nb₅Zr]Ni₃ and [Ni–Ni₆Nb₅Ta]Ni₃ alloys show they have limited plasticity with a elongation of about 0.3%, fracture strength of the [Ni–Ni₆Nb₅Zr]Ni₃ and [Ni–Ni₆Nb₅Ta]Ni₃ BMGs are about 3.2 GPa and 3.4 GPa, respectively.

Pitting mechanism and behaviour of X70 pipeline steel in humid storage environments were investigated using electrochemical polarization curves, electrochemical impedance spectrums (EIS), immersing corrosion tests and corrosion morphology observation through SEM. It was demonstrated that pitting of X70 pipeline steel occurred in simulated moist storage environments, for which the corrosive substances came from the residual species in laminar cooling water introduced during steel manufacture processes. HCO₃ ^- and NO₃⁻ are passivating agents, Cl⁻ and SO₄²⁻  would destroyed the passivation layer, which could lead to pitting. In solution with 0.5 mol/L NaHCO₃, 0.02 mol/L Cl⁻ was enough to break the passivation layer. Cl⁻ concentration is a key factor for pitting initiation and propagation. When the Cl⁻ concentration was relatively low, pitting could initiate but was hard to grow up. When the Cl⁻ concentration was moderate (about 0.149 mol/L), pitting sensitivity was the highest because pitting was easy to grow up. However, if the concentration of Cl− was too high, uniform corrosion occurred.

Tensile tests on a FeNi–base austenitic alloy, with different amount of twin boundaries, were conducted at different temperatures and three strain rates, respectively. The results show that serrated flow occurs at temperatures from 300 to 700 ℃. This serrated flow exhibits bulge–like serrations at temperatures from 300 to 600 ℃ and stress–loss serrations at 700 ℃, which manifests the nature of thermal activation, i.e. higher temperatures boost serrations and higher strain rates depress them. Investigations on samples deformed at room temperature (no serrated flow) and 400 ℃ (prominent serrated flow) indicate that twin boundaries are strong enough to block slip deformation at 400 ℃. As a result, stress accumulates on twin boundaries and bulge–like serrations appear on the tensile curves. Effect of twin boundary amount on the morphologies of serrations testified this mechanism.

Since its advent, TRIP steel has been considered as the best automotive steel to improve the automotive safety dramatically because of its excellent mechanical properties. However, TRIP steel was not widely used and the main reason is that the large amount of Si in the steel led to surface quality problems of the plate. In this work a kind of C–Mn–Al–P TRIP steel with Si being replaced by Al and P was designed and subjected to continuous annealing. The macro and micro mechanical performance was analyzed. The results showed that the mechanical properties were excellent with tensile strength between 740 and 790 MPa and elongation above 25%, indicating that the production of TRIP steel without Si addition was feasible. Compared with traditional TRIP steel with Si, the designed steel demonstrated higher yield strength of 550—600 MPa, which was a little higher than that of high Si steel and much higher than that of medium or low Si steel. The micro hardness of ferrite was measured by nano mechanics probe and it was 0.6 GPa higher than that of medium Si TRIP steel. The IF steel whose strength was already known was used as a calibration and the conclusion was that the 0.6 GPa difference in nano hardness could result in 75 MPa difference in yield strength and 80 MPa division in tensile strength. Through further analysis, it was believed that the higher strength of ferrite was mainly caucsed by C, not P.

The corrosion behavior of Ni–10Al and Ni–xCr–10Al (x=3, 5, 10) alloys at 700—800 ℃ in H₂–H₂S–CO₂ mixtures was studied. The addition of Cr resulted in a ignificant reduction of the corrosion rate of the Ni–10Al alloy: at 700℃ the corrosion of Ni–10Al alloy showed a linear kinetic law, but Ni–xCr–10Al followed the multi–stage parabolic rate law; at 800 ℃ the corrosion of all the alloys followed a quasi–parabolic rate law. Ni–10Al alloy formed a complex scale of Ni₃S₂ and Al₂O₃, whereas Ni–3Cr–10Al formed an irregular scale of Al₂O₃ with a CrS inner corrosion region, and a thinlayer of Al₂O₃ formed on Ni–5Cr–10Al and Ni–10Cr–10Al alloys. The equilibrium partial pressures of oxygen and sulfur in the mixed gases were calculated to estimate the possible reactions between the alloy components and the gaseous atmosphere, and the corrosion mechanisms were also discussed.

The microstructures and compressive properties of arc–melted Fe_100−xNb_x(x=6, 9,12, 15, 17) alloys, which are relevant to eutectic reaction (L → α–Fe+Fe₂Nb), were investigated by SEM, XRD and test machine system 810 (TMS 810). The results showed that the microstructure of the ingot sample consisted of primary phase (α–Fe or Fe₂Nb) and sub–micro–sized eutectics. Fe₉₁Nb₉ alloy had the best comprehensive properties with ultimate strength about 1.63 GPa, yield strength about 1.04 GPa and compressive strain ductility about 23%. For alloy Fe₉₁Nb₉, the substitution of Hf element for Nb element produced no observable effects on both the microstructure and the compressive mechanical properties; while the substitution of Y element for Fe element resulted in obvious change in the microstructure and dramatic deterioration in the mechanical properties.

A new technology was developed based on carburization and succedent low–temperature austempering to produce hard bainitic microstructure in the surface layer of low–carbon steel 20CrMnMoAl. The microstructure and property of the hard bainite were investigated. The samples were carburized in a gas carburizing furnace at 930 ℃ for 8 h. The carbon content in the surface layer increased to 0.81%(mass fraction) after carburization. The carbonized samples were heated at 930℃ for austenitization and then isothermally quenched at different temperatures in salt bath. The microstructures were investigated by OM, TEM and XRD, and the hardness was tested by HV–sclerometer. The results show that for the samples which were isothermally quenched at 220 and 250℃, ultrafine hard bainite was obtained in the surface layer with a hardness of 630 HV. The plate of ultrafine bainitic ferrite was as thin as 70 nm and film of retained austenite had a thickness of several nanometers. Low carbon lath martensite was obtained in the center and mixstructures of the ultrafine bainite and low carbon martensite in the transition layer.

It is generally recognized that welding heat affected zone (WHAZ) is the poorest toughness region in the welded joint of low carbon bainitic steels. The thermomechanical simulator was employed to simulate the welding thermal cycle processes of different sub–regions in WHAZ of low carbon bainitic steel in this work. The toughness of simulated specimens were tested on the instrumented drop weight impact tester with oscilloscope, and miscrostructure features were observed by means of OM, SEM, TEM and EBSD. The results showed that when cooling time (t_8/5) was 30 s, the crack initiation energy of various sub–regions was similar, and the range of their values was between 40 and 70 J. However, fine grained heat affected zone (FGHAZ) exhibited excellent crack arrest properties because the impact load–time curve included wide crack ductile propagation and crack brittle propagation stages. By contrast, the crack propagation energy of intercritical heat affected zone (ICHAZ) and coarse grained heat affected zone (CGHAZ) obviously deteriorated. With the increase in cooling time, both crack initiation energy and crack propagation energy of various sub–regions decreased, in which the crack initiation energy of CGHAZ and the crack propagation energy of FGHAZ decreased notably. Under different cooling rates, the variation of morphology and size of M–A constituents was mainly responsible for the deterioration of crack initiation energy. As for crack propagation energy, the FGHAZ had a good resistance to crack propagation due to high density of high angle grain boundary. Therefore, its crack propagation energy was far superior to other sub–regions. There was uneven effective grain size in the ICHAZ and ferrite grain grew with the decease in cooling rate, which decreased the crack propagation energy. In the CGHAZ, prior austenite grains coarsened and the density of high angle grain boundaries decreased greatly, which resulted in the decrease in crack propagation energy.

Zinc–indium alloy electrodes based on nickel substrate were prepared by a simple electroplating technique. The content of indium element in zinc–indium alloy became higher with increase in electroplating time. The effects of indium in zinc–indium alloy coating with different electroplating times on the electrochemical behaviors of zinc in alkaline solutions were investigated by inductively coupled ICP, SEM, EDS, LPS, CV and EIS. The results showed that ten minutes was the best electroplating time among the investigated electroplating times. At the plating time of ten minutes, uniform and smooth Zn–In alloy coating could be formed on the surface of nickel substrate. However, as electroplating time went on the uniformity and homogeneity of Zn–In alloy coating became much poorer because of the formation of the local large indium particles resulting from the priority growth. The electrochemical measurements showed that indium in zinc–indium alloy coating could enhance the overpotential of hydrogen evolution and the anodic dissolution resistance of zinc. This means that indium could inhibit effectively the conjugated reaction of zinc corrosion in alkaline electrolyte. Besides, indium could provide the skeleton support when zinc–indium alloy dissolved in alkaline solution. The skeleton support offered the path for OH− species passing through the surface layer of zinc–indium alloy to react with zinc in alloy inner. These effects made the passivation potential of zinc in alkaline solution shifted in positive direction. Indium could also broaden the potential region for the active dissolution of zinc and postpone the passivation of zinc. The depth of discharge of zinc in alkaline electrolyte could be improved in the presence of indium in zinc–indium alloy coating with appropriate content. Accordingly, the discharge capacity of zinc in alkaline electrolyte could be raised to a certain degree. The indium with appropriate contents in Zn–In alloy coating could favor the reduction of zinc oxide products. The charge–discharge performance of zinc could also be improved to a great extent.

During the controlled pulse key–holing plasma arc welding (PAW), keyhole shape and size change with the welding current dynamically, and undergo variation of the "establishing–expanding–contracting–closing"process. Numerical analysis on such dynamic variation process of keyhole shape and size can provide insight into the process mechanism and basic data for optimizing the process parameters. In this study, a three–dimensional transient model is developed to conduct numerical simulation of welding temperature field, weld pool geometry, and keyhole shape and size in controlled pulse PAW. The keyhole shape and size are computed by analyzing the force–action on the weld pool surface, and two situations are considered to deal with partial keyhole and open keyhole. The dynamic variation features of three–dimensional keyhole shapes in weld pool in a pulse cycle are numerically calculated. Experiments are conducted to validate the numerical simulation result of key–holing duration.

Linear polarisation resistance (LPR) was used to trace the corrosion rate of 3Cr pipe line steel during corrosion process. EIS and SEM were used to detect the structure of protective CO₂ corrosion product films. The appearance and evolvement of the loose internal–layer with lower Cr/Fe ration which was observed by SEM, was analyzed by EIS. Amorphous corrosion product film grew from outside to inside with time. The increase of the Cr/Fe ratio and the thickness of the film cause the decrease of momentary corrosion rate. The chromic compound produced by a certainty Cr³⁺ concentration. When the pitting corrosion suppressed, high corrosion rate in the pit resulted in high Cr³⁺ concentration which would produce much chromic compound, and in this way, the development of the pit would be prevented.

The influences of laser rapid solidification and annealing treatment at 600—1000 ℃ on the microstructure and properties of laser clad FeCoNiCrAl₂Si high–entropy alloy coating were investigated. The experimental results indicate that the precipitation of intermetallic compounds in the coating could be effectively inhibited by laser cladding with rapid solidification. The coating had simple ordered bcc solid solution phases with high microhardness (900 HV_0.5), good high temperature phase stability and softening resistance. The coating was mainly composed of dendrites, Fe, Cr and Si are enriched in interdendritic region, Ni, Co and Al are enriched in dendritic region. As the annealing temperature increase, the segregations of Al and Si increase, but segregations of Fe, Cr, Ni and Co changed little. Massive low angle grain boundaries were distributed at the interface between dendritic and interdendritic microstructures. After annealing at 600 ℃ for 5 hours, the microstructure was greatly refined, and the grain boundary misorientation converted from low to high angles.

Structural changes in Ti films covering over Ti or Al atomic layer on the TiAl substrate during heating were investigated by molecular dynamics simulations within the framework of embedded atom method (EAM). On the basis of analyses of bond–pair types, mean square displacements, atomic density functions, and atom packing in different shells, it is shown that the structural changes in the Ti film could involve several stages owing to atomic position interchanging. In addition, the Ti or Al layers play an important role in the structural changes in Ti films.

Utilizing the high–temperature chemical reaction between Ti and AlN, TiN reinforced Ti₃Al composite coating was in situ synthesized on TC4 alloy substrate by laser cladding technique. The phase constitution and microstructure of treated samples were examined by XRD and SEM. The results reveal that the coating is mainly composed of TiN and Ti3Al phase. When the molar ratio between Ti and AlN is 4∶2, the content of TiN reduces as the laser power density increases; whereas when the molar ratio between Ti and AlN is 4∶1, the content of TiN increases with increasing the laser power density. SEM observation shows that the morphology of TiN changes gradually from bar–shaped to granular with the increase of laser power density. When the molar ratio between Ti and AlN is 4∶1 and the laser power density is 15.28 kW·s·cm−2, the macro-morphology of surface coating is better, no pores and cracks appear. The average microhardness of laser cladding coating is 3.4 times larger than that of TC4 alloy substrate.

Precipitation strengthening can occur by means of two mechanisms, depending on the size. First, the shearing mechanism is active for small precipitates and the strengthening increases with the increase in the precipitate size. Second, the Orowan bypass process, where dislocation loops around precipitates is active for larger precipitates and the strengthening decreases with increase in the precipitate size. Therefore, there must exists a critical transition size of precipitates, in which the strengthening reaches the maximum. V–Nb–Mo micro–alloyed steels were isochronally tempered for 4 h at different temperatures and isothermally tempered for different times at 650 ℃ after solution treatment at 1200 ℃ for 0.5 h. The tempering conditions where the transition of precipitate strengthening mechanism occurred could be ascertained by the isochronal and isothermal hardness curves. The critical transition size of precipitates was determined to be about 1.0 nm by three dimensional atom probe (3DAP). Based on the precipitation strengthening theory and shearing mechanism dominated by coherency strengthening, the critical transition size of precipitates was calculated to be about 1.0 nm, which was in good agreement with the value measured by 3DAP. It indicates that the size of second phase can be well predicted by the precipitation strengthening theory when the micro–alloy steel reaches the maximum precipitation strengthening effect.