An off–normal fiber–like texture, also regarded as the fourth texture, that is named axiotaxy. Axiotaxies concern the preferred grain orientations along the axes in crystallographic coordinate system, rather than the axes in sample coordinate system. They reflect the extent of phase boundary matching during, e.g., martensitic transformation or thin film deposited on single crystal substrates as well as the selection of variants. The excellent properties of high manganese TRIP steels with a good combination of strength and elongation attract great interest of automobile industry and thus the research on this type of steels is increasing. The reason responsible for the super properties of the steels is mainly due to the presence of two types martensites ( α'– and ε–martensite) which can form either thermally during quenching or strain–induced. Therefore, the Axiotaxies related with these two types of martensites are more complicated and may show more information. This paper investigated the axiotaxies revealed by EBSD orientation mapping during the study of orientation relationship and variant selection in martensitic transformation of two high manganese TRIP steels with 18%Mn and 22%Mn together with 3%Si and 2%Al. It is shown that the quasi–fiber textures in these steels can be caused in 4 ways. First, they were caused by the α–martensite variants formed in an ε-martensite plate which were generated from an austenitic {111} plane by K–S relationship. The center of axiotaxy corresponds to the normal of habit plane {111} and this type of axiotaxy was observed only in the pole figures of α'–martensite. Secondly, they were due to the ε–martensite transformed from austenitic deformation twins, this indicates an important role of twinning in martensitic transformation. Thirdly, they were caused by reularly mis–indexing of phases due to the strain aroused by martensitic trans- formation, namely, the real axiotaxy existed in α'–martensite was transferred erroneously into the pole figures of α'–austenite or ε–martensite. Fourthly, they may also be due to the accommodation of lattice planes for low misfits by means of external deformation. All of axiotaxies detected were related with martensitic transformation and the significance of analyzing axiotaxy lies in that it reveals much information about phase boundary match, vrint selection or the effects of deformation during quenching or during deformation.
The texture of Mg alloy dendrites is quite different from that of fcc or bcc metals because of the influence of hcp crystal lattice on the dendrite morphology evolutions during solidification. Although the simulations of dendrite morphologies for fcc or bcc metals by cellular automaton (CA) methods have been widely reported, CA simulations of magnesium alloys with hcp crystal lattice have just appeared in recent years. When performing the simulation of Mg alloy dendrites with a CA method on a square mesh, the artificial anisotropy of growth kinetics introduced by the square mesh makes it hard to reflect the texture of Mg alloy dendrites, which shows the six–fold symmetry instead of four–fold symmetry of bcc or fcc metal dendrites.
In the present paper, a two dimensional CA model has been developed for simulating the dendrite morphology evolution of castMg alloys. The model employs two sets of meshes to perform the numerical simulation, where a hexagonal mesh is used to perform CA calculation to reflect the texture of Mg alloy dendrites, and an orthogonal mesh is used to solve the mass transportation equation. The two sets of meshes are coupled by an interpolation method. By employing the two–set mesh method, the texture of Mg alloy dendrites is well reflected and the undesired artificial growth kinetics introduced by square mesh is avoided. In the model, the growth kinetics of dendrite tips was determined by the difference between local equilibrium and local actual compositions obtained by solving the solute transport equation. With this calculation method for growth kinetics, the solid fraction of interface CA cell can be obtained directly from the solute field, which decreases the computational cost greatly. The model was applied to simulate the single dendrite evolution and columnar dendrite growth of AZ91D Mg alloy, as well as multi–dendrite growth and grain size of Mg–10Gd–2Y–0.5Zr (mass fraction, %) alloy step–shaped castings. To validate the current model, permanent mold sample castings of AZ91D Mg alloy and step–shaped castings of Mg–10Gd–2Y–0.5Zr alloy were produced. Optical metallographic examinations were performed on specimens of these two Mg alloys, and grain sizes were measured on solution treated specimens of Mg–10Gd–2Y–0.5Zr alloy. The simulated and experimental results were compared.
The law of grain growth is one of the classic problems in materials science. In 1952, an exact formula named von Neumann relation was derived for grain growth in two–dimensional space that the growth rate of a grain depends only on its number of sides n. In three dimensions, topology– dependent rate equations of grain growth are usually proposed to describe the individual grain growth. Such equations can describe the mean growth rate of grains within the same topological class and can be used to derive general properties of polycrystals. In this paper, based on average N–hedra (ANHs) model proposed by Glicksman recently, the law of the change of grain surface area was studied and a topology–dependent rate equation of grain surface area change was derived. Both the contributions of the grain boundary motion and grain edges motion to the grin groth were considered in the derivation. This topology–dependent rate equation can be expressed through a simple relation that the change rate of grain surface area is proportional to the square root of the number of grain faces. This result can assist in a better understanding of the process of grain growth rom a statistical point of view and is similar to the reported topology-dependent grain growth equations.
Lath martensite with a dislocation substructure is one of the most common forms of martensite in structural steels. Surface relief has been regarded as an important characteristic in the martensitic transformation. Crystallographic features on surface relief are essential to get into deep insight of the long range strain field in the transformation, and explore the mechanism of the phase transformation. However, very limited experimental data on the shape strain associated with the formation of surface relief caused by the lath martensite have been reported so far, especially for the quantitative study of the displacement vector. The present investigation was carried out to study the shape deformation in the formation of the lath martensite on the austenite matrix in an Fe–20.2Ni–5.5Mn (mass fraction, %) alloy. The shape strain accompanying surface relief, such as the magnitude and direction of the displacement vector, has been concerned in a quantitative way. The morphology of the relief was studied by the optical microscope (OM) and the atomic force microcope (AFM). The orientations of the matrix grain and the lath were measured by the electron backscattered diffraction (EBSD), respectively, which was used to determine the orientation of the habit plane, and the orientation relationship (OR) between the lath martensite and its neighboring matrix. Combing the data from EBSD and AFM, it is concluded that the relief is produced by a single bcc crystal, which exhibits a tent-shaped relief. Based on an electron backscattered diffraction analysis, the ustenite/martensite orientation relationship is found to be in the closer vicinity of K–S orientation relationship, which is consistent with that in bulk materials obtained by transmissin electron microscope (TEM), and the habit plane is determined to be near (111)f . The largest shear angle for the relief is calculated to be 27.49°, and the directions of comined displacement vector are scattered around [121]f . However, the bserved maximum surface tilt angle is 22.41°, which is smaller than the calculated value. Considerinthe habit plane is not perpendicular to the pre–polishing surface, the measured smaller value f tilt angles is reasonable.
Many alloys show a phase diagram characterized by the appearance of a miscibility gap in the liquid state. Some of them have great potentials to be used in industry. But when a homogeneous, single–phase liquid is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally the liquid–liquid decomposition causes the formation of the microstructure with serios segregation. The microstructure evolution during the liquid—liquid phase transformation of monotectic alloys is a result of the common actions of manfactors. It is very complex, especially the convection of the matrix and the motion of the minority phase droplets migt mix thigs p and make it extremely difficult to ivestigate te solidification process of monotectic alloys. In this paper, the effects of convection and minority phase droplet motion on the solidification of monotectic alloys were introducedthe current situation of researces in this field was overviewed.
Solid compounds added into liquid steel can be utilized as substrates for primary ferrite phase or primary austenite phase nucleation during solidification. The effect of solid compounds promoting heterogeneous nucleation can be interpreted as an electrostatic effect between substrates and nucleated phases, with heterogeneous nucleation being considered as caused by the free energy change due to the redistribution of free electrons at the interface of substrate and nucleated phase. In order to evaluate the electrostatic effect, Yu’s empirical electron theory was introduced. With concepts of lattice electron and atomic state hybridization brought forward by Yu, the bond length difference method was applied to calculate valence electron structures of substrates and nucleated phases. The electrostatic effect was quantified as a electron transfer rate at the interface of substrate and nucleated phase. Parameter Δρ was proposed to represent the electron transfer rate. In this study,23 compounds commonly found in liuid steel were selected as the substrates, along with δ–Fe and γ–Fas the nucleatd phases. The valence electron structures of sbstrates and nucleatephases were calculated on the basis of crystal structure data obtained by experimnts. Parameter Δρ between each substrate and δ–Fe/ γ–Fe was calculated from the valence eectron structures. The results show ha, as the parameter Δρ increases, the work of heterogeneous nucleation derived from experimental data decreases; the larger Δρ is, the more effective the substrate is for promoting nucleatio.
The kinetic law of austenite grain growth in the X12CrMoWVNbN10–1–1 ferrite heat–resistant steel, which has been used as the high and medium pressure rotor of ultra–supercritical generating units, has been studied by quantitatively measurement of the austenite grain size after austenitized from 1010 ℃ to 1200 ℃ with holding time from 5 to 1200 min. The results show that the grain grows in a normal grain growth (NGG) mode when the austenitizing temperature is lower than 1050 ℃, and the homogeneous small grains can be obtained even the holding time reaches 1200 min. When the austenitizing emperature lies between 1050 ℃and 1120 ℃with different holding tme, the bnormal grain growth (AGG) can be bserved. At even higher temperature than 1150 ℃, the austenite grains grow rapidly with a NGG mode. The austenitizintemperature and holdng time are thus determined or different austeize grain states, and the parameters in te NGG knetic equation are fit.
Superalloy Inconel 718 is an important material used for aero–engine high temperature turbine disks. The grain refining of Inconel 718 becomes critical because of the improvement in the quality and reliability of aero–engine. Inconel 718 turbine disks are manufactured by multi–stage hot deformation processes, in which the recrystallized grain grows up in next passes. Therefore, it is difficult to obtain a uniform and refined microstructure by recrystallization refining. The δ phase in Inconel 718 can control grain size through the strong pinning effect. Thus, the Delta process (DP) has been applied for the forging of Inconel 718. In this paper, for the DP of Inconel 718, the evolution of δ phase during isothermal compression deformation at temperature of 950℃ and strain rate of 0.005 s−1, was studied by using optical microscope (OM), scanning electron microscope (SEM) and quantitative X–ray diffraction (XRD) technique. The results show that spherical or rod–shaped δ phase particles in the interior of grains precipitated in the aging treatment disappear during the heating and holding time before deformation, and thcontent of δ phase decreases from 8.14% to 7.05%. Dissolution of δ phase occurs during the deformation, and the content of phase decreases from 7.05% to 5.14%. The spheroidization of plate–like or needl–like δ phase takes place due to the effect of deformation and dissolution breakages, and the plate–like or needle-like δ phase transferrs to sphrical or rod–shaped δ phase. In the centre with the largest strain, the plate–like or needle–like δ phase disappears and spherical or rod–shaped δ hase appears in the interior of grains and grain boundais.
Super duplex stainless steels have been extensively used in many applications owing to their excellent mechanical properties and corrosion resistance. But subjected to “aging” treatments at temperatures of 600 to 1000℃, duplex stainless steels will precipitate a certain amount of intermetallic phases such as σ phase, χ phase and chromium nitride, etc., which decrease mechanical properties and corrosion resistance of these steels. In this paper, the effects of aging treatments on the type, size and quantity of precipitation particles of a cast super duplex stainless steel after solution treatment were studied by means of OM, SEM, XRD and TEM. The results indicate that the soloution annealing cast super duplex stainless steel is aged in temperatures ranging from 650 to 950℃, no phase transformation occurs in the original austenite but the intermetallic phases are precipitated in ferrite and at ferrite/austenite interfaces. The precipitated intermetallic phases are mainly χ phases during aging at 650℃. They are σ and χ phases at aging temperature of 750℃. When temperature increased to 850℃, the intermetallic phase is only σ phase. At temperature of 950℃, there is a little σ phase precipitated at ferrite/austenite interfaces. The formation of intermetallic phases is analyzed by thermodynamics, which shows that metastable χ phase can invert to σ phase at aging temperatures from 650 to 750℃. When temperature ranging from 850 to 950℃, σ phase can be directly precipitated.
The fatigue damage under the stress amplitude below fatigue limit is evaluated based on the modified Miner’s rule usually. The medium carbon railway axle steel shows obvious cyclic softening or hardening behavior during fatigue process. The result of rotary bending fatigue test on medium railway axle steel shows that the stress–life data above the fatigue limit tally with three parameter mode. Therefore the fatigue damage of this steel under variable amplitude stress can’t be evaluated based on the stress linear cumulate damage rule. According to the rotary bending fatigue data obtained from the rotary bending fatigue test under constant and high–low two levels periodic variable amplitude stress, a method oevaluating variable rotary bending fatigue damage of mterial whose icline part of the S–N curve under constant amplitude stress loading is curve, was presented. Considering the cyclic hardening or cyclic softening durig the ftigue process, the stress–life data were transformed to the inear plastic strain–life data according to the cyclic constitutive retionship of this material. Afterward the fatigue damage under varible amplitude stress was evaluated by plastic strain obeying the strain linear cumulate damage rule.
NaF was used as the accelerant to accelerate the conversion coating formation on 6063 Al alloy in Ce(NO3)3 and KMnO4 solution. Orthogonal experiments were conducted to find out the optimal process for prepare Ce–Mn conversion coating on Al alloy surface. Coating thickness and anti–corrosion time were taken as the indexes of performance assessment. Two better solution components and coating formation times at room temperature and pH=2.0 were selected to be 10 g/L Ce(NO3)3+2 g/L KMnO4+0.06 g/L NaF, 12 min and 7 g/L Ce(NO3)3+1 g/L KMnO4+0.06 g/L NaF, 9 min. The anti–corrosion ability of coating was evaluated by dropping test, polarization curve and electrochemical impedance spectroscopy. The increase of ΔE (the different between pinhole corrosion and corrosion potentials) and the decrease of corrosion demonstrate that the anti–corrosion ability of 6063 Al alloy with Ce–Mn conversion coating is greatly enhanced since the cathodic current (ic) and anodic corrosion current (ia) decrease. Ce–Mn conversion coating serves as an effective barrier to prevent corrosion attack. Generally, lower C (Capacitance) points out relatively higher degree of surface homogeneity which yields an almost closed capacitive arc. The addition of NaF make C become less, conversion coating resistance (Rc) and charge transfer resistance (Rct) become higher. A thicker and denser coating was formed on the surace of l alloy, which presents a barrier to O2 or CO2 or Cl− permeation, bring better protection to Al 6063 alloy. The surface hardness was determined by micro–hardness test, the micro–morphology, and compositions of coatings were analysed by SEM and EDS. With NaF added, the surface hardness becomes stronger. Formation time was also an important factor to prepare a high–quality coating, corrosion resistance of Ce–Mn conversion coating was more effective when formation time is 9 min than when it is 15 min. The results of orthogonal experiments show that the optimal coating processing is 7 g/L Ce(NO3)3+ 1 g/L KMnO4+0.06 g/L NaF, 9 min. The additioof NF can accelerate the coatig formation, increase the Ce and Mn content in coating and thus improve the coatig anti–corrosion performance. It is found that the surface icro–hardness increases from HV72 of pure Al surface to HV532 of Al alloy surface with Ce–Mn conversion coating.
Rare–earth–iron alloy TbDyFe is an advanced magnetostrictive material to date because of its giant magnetostriction, high energy density, and rapid response at room temperature and low magnetic field. Due to the high sound velocity and eddy current losses of TbDyFe alloy under high frequency, its applications are limited. The bonded giant magnetostrictive materials are expected to exhibit high resistivity to reduce the eddy current loss. In the present study, the bonded giant magnetostrictive materials were prepared by mixing TbDyFe alloy particles with epoxy resin. The electrical resistivity, impedance and eddy current losses of the bonded materials have been primarily analyzed. The optimized magnetostriction is observed to be 723.0×10−6 at magnetic field of 400 kA/m in the bonded material witparticle mass fraction of 90% and particle size of 74—150 μm. TbDyFe particle size and mass fraction show a significant influence on the magnetostriction of the bonded mateials. Compared to the advanced oriented TbDyFe allo, the electrical resistivity is 5 orders of magnitude greater, and the sound velocity is 1/3 lower under the applied magnetic field of 32.7 mT, ad the eddy current loss factor is reduced by 90% at 2×105 Hz, and by nearly 50% at 1×17 Hz.
Directional solidification experiments were conducted for Ti–(44%—54%)Al (atomic fraction) alloys in a wide range of ratios between temperature gradients and growth rates. Interfacial morphology evolution, microstructure formation and the final lamellar orientatios were investigated undedifferent solidification conditions. Nucleation and compositional undercooling citerion were used to clculate the phase selection map for Ti–Al alloys which gives phases and corresponding microstructures at different initial compositions and solidification parameters. The map is in good agreement with the experimental results and gives important criteria for determing phase compositions and solidification parameters in lamellar orientation control of Ti–Al alloys.
Functional gradient materials have been applied in many engineering fields, such as hot forging moulds. The CO2 surfacing welding technology is an important technique to fabricate functional gradient materials. The CO2 surfacing welding experiment on Q235 low carbon steel plate has been carried out with additional pulsed AC longitudinal magnetic field (electromagnetic stir, EMS). Through the analyss of microstructure, hardness, wear resistance and thermal mechanical properties of surface clad layers, the effects and functions of EMS have been studied at CO2 welded joints. The rslts show that EMS can improve the interficial mcrostructure of surface clad layers, refine their grains, increase the hardness and wear resistance of surface layers, and enhance ther therml echanical properties. This surfacing welding can be applied to fabricate new high–qualty double–etal hot forging moulds and heat stretch extension mouds.
