The influences of Ru and Cr as well as their interaction on the elemental partitioning ratio and microstructural evolution have been investigated in six Ni–based single crystal experimental superalloys with various levels of Ru (0—5.1%) and Cr (0—5.7%) additions (mass fraction). The results indicate that γ′ precipitates are nearly spherical in the dendrite core of the base alloy (Ru and Cr–free), which has a low Re partitioning ratio and near zero lattice misfit, after aging treatment at 1100 ℃ for 8 h. The lattice misfit and Re partitioning ratio increase slightly and the γ′ precipitates change to be more cuboidal with the addition of 5.1%Ru in both Cr–free and Cr–containing alloys. Meanwhile, the Re partitioning ratio increases significantly with increasing the Cr content in both Ru–free and Ru–containing alloys, which in turn results in more negative lattice misfit and more cuboidal γ′ precipitates. After long–term thermal exposure at 1100 ℃, the nearly spherical γ′ precipitates with near zero lattice misfit in the alloy have no change in morphology, and are coarsened after a longer exposure time, while the alloy with intermediate γ′ precipitates and low lattice misfit is coarsened more severely. However, a nearly–rafted structure tend to form in the alloy with nearly cuboidal′ precipitates and intermediate misfit after heat treatment for 800 h. The time to form the rafted structure is significantly reduced in the alloys containing both Ru and Cr with high Re partitioning ratio and high lattice misfit as well as cuboidal or rectangular γ′ precipitates. The alloy containing high Ru and intermediate Cr exhibits a rafted trend after heat treatment for 200 h while the rectangular γ′ precipitates are rafted after heat treatment for only 50 h in the alloy containing high levels of Ru and Cr additions with the highest lattice misfit.

DZ417G is a directionally solidified (DS) superalloy developed for low–pressure blade applications in gas turbine engines. The crystallization microstructures of DZ417G samples caused by sand–blasting and machining were investigated. The tensile properties of the alloy after crystallization were tested at room temperature and 900 ℃and its stress–rupture performance was examined under conditions of 980 ℃/216 MPa and 760 ℃/725 MPa. The results show that after solution treatment equiaxed recrystallization grains form on the surface of specimens machined from directionally solidified alloy bars. After aging treatment, cellular recrystallization takes place on the surface of specimens pretreated by sand blasting. Both the yield strength and tensile strength at room temperature decrease after recrystallization, while those at 900 ℃ are slightly affected by recrystallization. The recrystallization depth increases after stress–rupture tests, which may be attributed to migration of recrystallization boundaries driven by high temperature and stress. For samples without recrystallization microstructure, the fracture mode is transgranular, which is controlled by the propagation rate of the cracks initiated both on surface and at inner microstructure discontinuities. While for samples with recrystallization microstructure, the cracks prefer to be initiated on transverse recrystallization grain boundaries and propagate along the recrystallization boundaries into the matrix, which may accelerate the propagation rate. TRF (transverse recrystallization area fraction) is a factor to evaluate the effect of recrystallization on the stress–ruptured performance. The stress–ruptured performance is decreased with the increase of TRF between 0 and 0.5. For samples with a TRF of 0.5, second cellular recrystallization forms in the first equiaxed recrystallization grain. The cracks initated at interfaces of
cellular microstructure have a high density, which impair the stress–ruptured performance of DZ417G alloy.

Near–isothermal forging experiments of the TG6 titanium alloy have been conducted at the deformation temperatures ranging from 850 to 1075 ℃with a constant strain rate and deformation degree. The primary α phase content in the alloy after solution, deformation and heat treatment, and the thickness of lamellar α phase in the alloy after heat treatment were measured by OM and image analyzer. The results show that compared to solution state, both forging and forging + heat treatment cause the decrease or increase of primary α phase content when forging temperatures are below or over 1000 ℃. The thickness of the lamellar α phase increases with the increase of forging temperature, which is caused by the decrease of nucleation density of secondary α phase. The β phase grains are so big when the forging temperature is 1075 ℃, while their microstructures are non–uniform. The room temperature and 600℃ tensile properties, impact property and fracture toughness of TG6 alloy have also been measured. It is found that the tensile strength of the TG6 alloy is not very sensitive to forging temperature. The plasticity decreases but the fracture toughness increases with the increase of forging temperature. There is no obvious change in the impact toughness when forging in the α and β double phase field, however it decreases when forging temperature is near the β transformation temperature. The change in mechanical property was explained by SEM, TEM and OM experiments.

The presence and distribution of Ni₄Ti₃ particles in NiTi alloys have a significant influence on martensitic phase transformation path by favoring the formation of R–phase rather than B19’ phase since the latter produces larger lattice deformation. To deeply understand the above, some experimental studies have been done by using differential scanning calorimetry (DSC) and in situ transmission electron microscopy (TEM). Meanwhile, some preliminary simulations have also been performed focusing on the morphology evolution of single and multiple Ni₄Ti₃ variants in single NiTi alloy system as well as considering the effect of external loads on selective precipitate growth. Whereas, in engineering application, the NiTi alloys often undergo the external load which may affect the growth kinetics of Ni₄Ti₃ precipitates. Therefore, it is necessary to investigate the effect of applied load on growth kinetics of Ni₄Ti₃ precipitates. In this paper, the phase field method has been extended to study the microstructure evolution and growth kinetics of Ni₄Ti₃ precipitates in NiTi alloys during zero–stress and stress–assisted aging. The simulation results show that during stress–free aging, four groups of the variants precipitate along the corresponding (111)_B2 habit plane; when the NiTi matrix is under <111>_B2 comprssive stress–assisted aging, there is only one group of the variants with the normal lines parallel to <111>_B2 to be precipitated. Although the uniaxial compressive stress apparently promotes the nucleation and slightly accelerates the growth of Ni₄Ti₃ variants in each group, the trends of aging time dependences of the area fraction, variant length, variant width and length–width ratio seem unchanged. The larger stresses can cause length and width of the variant slightly larger, but the area fraction of the Ni₄Ti₃ particles increases with increasing stress level. The simulation results are in good coincidence with the experimental results available.

In metallography the shape of high carbon martensite is commonly considered to be ens–like in three dimensions, which is of needle or bamboo leaf on the OM micrographs of section lanes. However, a round martensite on OM micrographs have never been seen so far. This question as not been answered over the years. It has not been theoretically explanined why is martensite ath–like for low carbon steels and lens–like for high carbon steels on OM micrographs. For answering hese questions, the martensitic morphology of a steel with carbon content 1.37% was observed in the ame view by OM using a method of multiple sectioning of one sample. The length, width of the aimed artensite and sectioned thickness of samples were measured before observing. It is found that the artensite is a flat ellipsoid rather than lens–like as described in traditional textbooks. The ratio of a/b is about 3 and a/c is abot 20 for the martensite ellisoid. The thermal dynamic analyss ndicates hat the nucleation energy for a martenste s closely related to its shape. Ithe austenite with the ame grain size, the nucleation energy (ΔG^∗) of martensite will be lower with ellipsoid shape than with lens shape. The drvng force for martensitic transformation will reduce with the decrease of carbon ontent, resulting n the formation of lath–like martensite in steels with low carbon content.

TiC dispersion–strengthened 304 stainless steel was fabricated using the technique of in situ reaction during melting of the steel. Microstructure observation reveals that the distribution of the added TiC particles with the size of 3—10 μm is basically uniform in the matrix grains, but slight aggregation of particles is observed in a few areas in the microstructure. The addition of TiC particles into 304 stainless steel results in the increase of ultimate strength, but the decrease of ductility. When the ingot of the TiC dispersion strengthened 304 stainless steel prepared is remelted by the technique of electroslag remelting, TiC particles in the steel are significantly refined and the distribution of them becomes more homogeneous, therefore the tensile properties of the steel are further improvd in comparison with that before electroslag remelting. Some tiny TiC particles with nano–scale were observed in the microstructure of the steel after electroslag remelting. Introduction of TiC particles to 304 stainless steel also causes a notable increase in creep resistance at thtemperature of 650 ℃ and applied stress of 150 MPa and the further improvement on creep properties of the steel is obtined after electroslag remelting.

Duplex stainless steel (DSS) has been used widely due to its very excellent combination of mechanical and corrosion resistance properties. However, the use of DSS is limited by their susceptibility to the formation of dangerous intermetallic phases, such as χ phase and phase, resulting in the detrimental effects on mechanical properties and corrosion resistance. With the application of the super duplex stainless steel (SDSS) with a pitting resistance equivalent number more than 40. The harmful effects of intermetallic phases easily precipitated in DSS on mechanical properties and corrosion resistance are a research focus. This paper, based on prior study, investigates the effects of intermetallic phases formed during aging on the mechanical properties and corrosion resistance of cast 2507 super duplestainless steel by means of optical microscope (OM), scanning electron microscope (SEM), tensile test and electrochemical test. The results indicate that for the solution annealing cast 2507 super duplex stainless steel aged in the temperature range 650—950 ℃, small and depressive distribution χ phase precipitatd from ferrite can increase its tensile strength, but σ phase causes the opposing effect on its plasticity owing to precipitated along grain boundaries. Both of them can rapidly deteriorate its plasticity, and cause brittle failure. The effect of σ phase on corrosion resistance of 2507 SDSS is weaker than phase. The more the σ phase is, the worse the corrosion resistant could be.

The electrochemical impedance spectroscopy (EIS) was used to analyze the corrosion evolution of pre– and post–transition of the annealed Zr–4 alloy in 0.01 mol/L LiOH aqueous solution at 360 ℃/18.6 MPa. The results show that oxidation weight gain of Zr–4 alloy increases slowly before corrosion transition. When Zr is oxidized into ZrO2, the stress appears in the oxide film due to volume expansion. With gradually thickening of the oxide film, accumulation of micro–defects will relax the stress in outer layer of oxide film. As a result, the uniform and compact oxide layer turns into double lyer with a porous outer lyer and a dense inner one. Meanwhile the impedance spectra evolve from single arc into doble capacitive arcs. When the oxide film forms macro–defects such as cracks, corrosion trasition tkes place, giving rise to an accelertion of oxidation weight gain and the rapid drop of impedace nd corrosion potential. Corrosion potential and electrochemical impedace spectroscopy are able to obtain in situ some informatioof the trasition point.

Potentiodynamic polarization technique and slow strain rate testing (SSRT) were employed to study the stress corrosion cracking (SCC) behavior of a welded X80 pipeline steel in Ku’erle soil simulated solution. Fracture surfaces were observed by SEM under different applied potential conditions. The results show that the polarization curves of the base metal and weld joint represent the typical characteristics of active dissolution. It is found that cracks are generally initiated at corrosion pits and inclusions under anode polarization and open circuit potential. The crack generation mechanism of X80 pipeline base steel and weld metal are attributed to the dissolution at anode. When the applied potential is −900 mV (vs SCE), the base metal exhibites lower SCC sensitivity due to cathodic protection while under the same condition welded joins higher SCC sensitivity. Both the base metal and weld joins exhibite higher SCC sensitivity under −1200 mV (vs SCE) polarization potential and their cracking generation mechanism is hydrogen induced cracking (HIC) due to the synergistic action of stress and hydrognCommonly the weld joint is more sensitive to SCC than the base etal under the same applid potential, and fractures are usually presented in heat affected zone (HAZ) and this is attributed to metallurgical phase transformation and residual stress enerated during welding process.

The effects of cooling rate and deformation on microstructures of an X80 pipeline steel were investigated by thermo–mechanically simulated tests heated up to the same temperature and cooled at different rates under 0 and 0.7 true strain deformation. The results reveal that the start temperature for bainite transformation will be increased by 30—80 ℃higher under 0.7 true strain than under no deformation. But the increase of cooling rate depresses the start temperature for bainite transformation and accelerates the progress of its transformation. It is found that the microstructure of the cooled samples without deformation is all composed of bainite with prior austenite grain boundaries (PAGB), but it is complicated for the cooled samples under 0.7 true strain. In the range of 1—40 ℃/s, with the increase of cooling rate, microstructures appeare successively such as polygonal ferrite, quasi–polygonal ferrite, massive ferrite, granular bainite, acicular ferrite (+ granular bainite) and athy bainite. Full martnsite steel can be acheved by quenching. Under current experiment conditions, acicular ferrite can be obtained n the steel at cooling rates of 10—20 ℃/s. In this cooing ate range, the fraction of high angle grain boundaries (HAGBs) inceases and the grain size decreases with the increase of cooling rate.

The micro–fabricating process and electromagnetic properties of two kinds of Fe powders with different grain sizes and internal strains were studied. The results show that the homemade Fe powders with smaller grain size, larger internal strain and surface roughness than the carbonyl Fe powders, evolve into big and thin Fe flakes by micro–fabricating for 10 h with the help of the process control agent (PCA). When micro–fabrication for t=12 h, these Fe flakes will fracture and their average width will reduce due to strong internal strain. When t is further prolonged to 24 h, these Fe flakes become thinner, finally, those with larger aspect ratio and smaller width can be obtained, which show high permeability and low permittivity. The epoxy resin-based microwave absorbing materials containing such Fe flakes of 20% (volume fraction) exhibit a reflection loss less than −10 dB in the 9.4—17.9 GHz frequency range with a minimal peak of −14.6 dB, whose average thickness is 1.5 mm.

The nanocrystalline SmCo₂ bulk alloy with an average grain size of about 22 nm was prepared by combining high–energy ball milling and spark plasma sintering (SPS), in preparation the amorphous alloy powder obtained by milling was densified and crystallized during the SPS process. The microstructure, crystal structure and phase structure of such SmCo₂ were determined. These results show that the nanocrystalline SmCo₂ is not able to exist stably as a single phase at room emperature and decoposes into SmCo₃ and Sm₉Co₄ phases, different from the conventional coarse–grained SmCo₂ phase, which exiss stably at room temperature. Phase stability and transformation behavior have been explained by he nanocale thermodynamic simulation. Comparing with the coarse– grained SmCo₂ with very weak magnetic properties, the nnocrystalline SmCo₂ exhibits remarkable permanent magnetism and good overall magnetc properties.

In metal matrix composites (MMCs), the reinforcement/matrix interface has a profound effect on its overall properties. The proper selection or design of the interfacial phase plays a vital role in optimizing the final properties of MMCs, in which high damping capacity is one of the most important properties of materials used in engineering structure. Although the damping mechanism of discontinuously reinforced aluminum composites has been extensively studied, only a few of researches have been reported on the effect of whisker coatings on the damping capacity of MMCs. In this paper, the pure aluminum matrix composites reinforced by alumina borate (Al₁₈B₄O₃₃) whisker with and without Bi₂O₃ coatings were separately fabricated by squeeze and the volume fraction of Al₁₈B₄O₃₃ whisker in these composites is about 20%. The mass ratios between Al₁₈B₄O₃₃ whisker and Bi₂O₃ were set as 6∶1, 10∶1 and 20∶1, respectively. The effects of coating contents and strain amplitudes on the damping properties of the coated composites at various temperatures and frequencies were examined. Two damping peaks (P1 and P2) in the coated composites at around 80 and 285 ℃ were observed. The results of damping characterization indicate that the damping capacity of the coated composites strongly depends on the content of coatings and strain amplitudes. Moreover, P2 increases more rapidly with increasing the coating content and strain amplitude. P1 is related to dislocation motion and interfacial slip between the whisker and Bi phase. The damping mechanism of P2 changes with the increase of stain amplitude.

The behaviors of keyhole formation determine to a great degree the deep penetration welding process and the weld depth–width ratio in welding of medium and large thickness plates. Obtaining a deep insight into the dynamic keyholing process in the weld pool in plasma arc welding is of great significance for widening the process parameter–widow and improving the process robustness as well as weld quality stability. Based on the Level–Set theory, a numerical model is developed to describe the keyhole behaviors in the weld pool in stationary plasma arc welding (PAW), and employed to track the evolution process of keyhole boundary. In this paper, the fluid flow fields in both the plasma and the keyhole regions are constructed according to the experimental and simulation data in the previous literatures. The combined volumetric heat source model is used to numerically analyze the transient temperature field and then to determine the weld pool geometry. The algorithm of Level–Set theory combined with the transient thermal conduction model is used to determine the evolution of both keyhole and weld pool geometry at different time steps. The dynamic information on the weld pool and keyhole geometry and sizes under a few process conditions is obtained by the numerical simulation of keyholing phenomena in welding of stainless steel plates. It is found that a complete keyhole is established at 2.7 and 2.5 s for the current levels of 170 and 180 A, respectively. During the stationary PAW process, the cross–section geometry of keyhole transforms from U–shape at initial stage to V– shape later and shows finally a hyperbola shape after a complete keyhole is formed. The numerical analysis of keyhole formation is verified by measuring the efflux plasma voltage signals at the moet of keyholing.

Under the action of controlled pulse current waveform, the welding current varies dynamically with the real–time key–holing situation, and there is a time lag between current signal and keyhole generation in plasma arc welding process, which is caused by the keyhole thermal lag effect. In this case, there are many pulse parameters like peak current level, duration and its dropping rate, which inevitably complicate the process. Determining the keyhole thermal lag time under various welding conditions is of great significance to ensure penetration depth and weld quality. In this paper, an experimental system for controlling pulse key–holing plasma arc welding has been developed, which measures both the welding current and efflux plasma voltage signals in real–time for characterizing the keyhole situation. The keyhole thermal lag time is defined as the difference between the instant at which the efflux plasma voltage is at 70% of its peak value and the intant at which the weldincurrent is at its peak level. The experiments are conducted to determine the valus of keyhole theral lag time by the in–rocess sampling during welding of constant–thickness and varied–thickness test plates of stainless steel. For constant–thickness ones, the average lag tme is 0.36 s, for varied–thickness ones, the welding current peak value is lowered with plate thicknss thinned gradually, but the keyhole thermal lag time is kept at an averaged value of 0.22 s with a lss deviation.

The effects of Ti–containing second–phase particles on the microstructure and properties of simulated welding heat affected zone (WHAZ) in a microalloy steel have been systematically investigated. The results show that the Ti deoxidized steel has a better low temperature impact toughness of simulated WHAZ than the Al deoxidized steel even in a longer phase change cooling time (t_8/5=120 s, correspond to a welding heat input of 100 kJ/cm). The role of Ti–containing second–phase particls on improving the proprties ofWHAZ is mainly manifested in the following two aspects. First, theris a large number of Nb–containing TiO_x–MnS particles with the size about several hundreds nanometers and Ti oxide containing NbC particles with the size less than 50 nm dispersed in the Ti deoxidized steel, these two kinds of small second–phase particles restrain the growth of austenite grain and refine the microstructure of WHAZ. Second, TiO_x–MnS composite particles with the size between 1 μm to 3 μm can induce the nucleation of intragranular acicular ferrite (IAF) which could divide the prior autenite grain and refine tmicrostructure effectiely. As a result of the above two effects, the low temperature toughness of HAZ in Ti deoidized steel is significantly improved compared with the Al deoxidized steel.

The computational fluid dynamics (CFD) method was used to research the velocity and concentration fields in the multiphase flow tubular stirred reactor. A mathematical model of liquid/solid two–phase flow was established and the simulated results obtained by usig this model are in better agreement with the experimental ones. The results show that the maximum velocity of solid–liquid phase flow is 1.58 m/s when the rotation speed is 150 r/min and there is no flow dead zone in the tubular stirred reactor because that turbulent flow is stregthened by the stirrig action. Radiamixture is better when the rotation speeds are 150 and 250 r/min than when the rotation speeds are 25 and 350 r/min, a proper rotation speed is need to obtain the prefered mixture. The critical impeller speeds are 35 and 38 r/min when the soiconcentration are 2% ad 75%, respectively

The spreading properties of unsteady state temperature wave, heating flux wave and heating shock phenomena during continuous hot slab rolling have been numerically simulated for three types of commonly used composite rolls by use of various coordinate systems and user–defined functions based on Fluent 6.3 software. The results indicate that the temperature wave can be divided into a high–frequency and low–frequency unsteady state temperature wave to describe the temperature distribution and spreading properties in rolls. It is found that the wave amplitude of the high–frequency temperature wave will quickly decrease with the increase of propagation depth, and its propagation depth of maximum impact is less than 7.5 mm. But, under the same rolling conditions, the amplitude attenuation of the low–frequency temperature wave is slower, and its propagation depth of impact is more than 20 mm. The heating shock decreases rapidly with the increase of distance from the surface of work roll. The heating shock depth is the deepest for the high speed steel composite roll, but the shallowest for the high nicked chromium cast iron composite roll under the same rolling conditions. The absorbed and resided heat by work–rolls during each slab hot rolling is down, but the dissipated heat by rolls is up with rolled slab number increasing.

In order to control stability of molten metal surface during electromagnetic continuous casting, the stabilities of melting surface under applications of an alternating magnetic field and a compound magnetic field were investigated experimentally. The melting surface behavior of the Sn–32%Pb–52%Bi alloy with low–melting point was measured by using a laser displacement sensor and visualized by using a high–speed video camera. The Fourier analysis method was introduced to reveal the fluctuation characteristics of free surface. It is experimentally found that when only alternating field is applied, the free surface keeps fluctuating. However, with increasing the alternating magnetic flux density, two statuses of the free surface, the stable camber deformation and the swinging phenomenon, can be observed orderly, and during swinging, the square–like or triangular prism–like deformations appear stochastically. The Fourier analysis indicates that the dominant fluctuation frequency at free surface center increases with increasing the alternating magnetic flux density. After simultaneously superposing a transverse static magnetic field within 0—1.44 T, the unstable swinging behavior of free surface can be remarkably controlled. Increase of static magnetic flux density can make the swinging amplitude reduce. Also the static magnetic field can effectively damp the free surface fluctuation. The stretch phenomenon was observed due to application of high static magnetic field in compound field, also a series of regular surface fluctuations appeared on free surface. There is an appropriate range of the static magnetic flux density to obtain more stable free surface, lower fluctuation amplitude and dominant fluctuation frequency.