The structural ceramics can be classified as oxide ceramics, nitride ceramics and carbide ceramics, which all possess low density, high anti-wearability and strong ability of anti-high temperature erosion, so they are a kind of materials that can be used in severe environments. Their compositions, crystal structures and properties were simply introduced, and the present research progress as well as application was also reviewed.
Other character of the structural ceramics is very low plasticity, resulting in especially hard to form complicate and large--scale parts. The research of the superplasticity of structural ceramics is one of the hotspots in the world. As examples, Y-TZP, Al2O3, Si3N4 and SiC were analyzed and the content emphasizes on tensile (compress) mechanical tests, microscopic mechanism and effects of crystal structure and additions on structural ceramics' superplastic deforming. In addition, its mechanical measurements and advanced research were also overviewed.
Two-phase NiTi base alloys (NiTi)50-0.5xNbx (x=5, 10, 15, 20) with both high damping capacity and high yield strength have been prepared by addition of Nb. With increasing Nb content, the fraction of eutectic (NiTi+β--Nb) increases, and yield strength in martensite state rises. When x=15, the yield strength reaches the highest value of 289 MPa. And the alloys also exhibit high damping capacity, tanδ>0.01. According to the damping mechanism for the shape memory alloys, the relationship between the damping properties and temperatures of NiTiNb alloys was explained, and phase boundary damping from the β-Nb and NiTi phases was discussed.
Upper bainite, lower bainite and granular structure in isothermally treated steels have all themselves C curves. A single microstructure is always produced in the primary stage, whereas the nearby two-microstructure complex is usually produced in the medium or final stage of the transformation. Only a single microstructure and Arrhenius equation must be used to deduce the overall activation energy for the transformation product. A combination of overall activation energy, morphology and free energy curve can explain the bainitic transformation mechanism and granular structure formation mechanism. The former follows military atom diffusionless martensite-like shear, which occurred in carbon-depleted region controlled by carbon atom diffusion in austenite, and the latter results from civilian atom diffusionless interface control transformation, which occurred in the most carbon--depleted region controlled by carbon atom diffusion in austenite.
Similarly to steels, in the isothermally treated $\beta$ brasses, the plate-like α1 (bainite) and rod-like α have all themselves C curves. A single phase is always produced in the primary stage, whereas the nearby two-phase complex is usually produced in the medium or final stage of the transformation. Only a single phase and Arrhenius equation must be used to deduce the overall activation energy for the transformation product. By a combination of overall activation energy, morphology and change of free energy, it can be explained that the plate-like α1 (bainite) transformation mechanism follows military atom diffusionless martensite-like shear, occurring in solute-depleted region controlled by solute atom diffusion in parent phase β', and the rod-like α phase results from civilian atom diffusionless interphase control transformation, occurring in the most solute-depleted region controlled by solute atom diffusion in parent phase β'.
The martensitic transformation behavior under coupling temperature and high magnetic field in TiNi paramagnetic shape memory alloy was studied by using the modified Landau model. In order to introduce the effect of high magnetic field, the Fermi surface total density of state (DOS) of TiNi alloy under different phase--transformed shear strain (order parameter) was calculated by using the first--principles calculations, then the relationship between magnetic susceptibility and shear strain was carried out. The calculation results from the modified Landau model involving the effect of magnetic free energy indicated that the martensitic transformation temperatures (Ms and T0) suddenly increase. It can be mainly contributed to the parabola increasing of martensitic transformation driving force under coupling temperature and high magnetic field. Also, it is found that the oriented growth of martensitic variants occurs under coupling temperature and high magnetic field due to the increased difference of free energy between variants, which is consistent with the TEM observation that some vertical two--variants are present in TiNi alloy under 5 T magnetic field.
A two-dimensional (2-D) model coupling cellular automaton-lattice Boltzmann method (CA-LBM) was developed for the simulation of dendritic growth in the presence of natural convection. The present model adopts the CA approach for the simulation of dendritic growth and the LBM for the numerical solution of flow dynamics as well as the species and heat transports controlled by both diffusion and convection. The validation of the LBM was performed by testing the calculated natural convection in a square cavity. The CA-LBM model was applied to simulate single and multi-dendritic growth in alloys under natural convection. The simulated single dendritic steady-state growth data of the upstream tip can be compared well with the analytical predictions. It is found that the dendritic growth is obviously influenced by natural convection.
The evolution of the second phase in 2.25Cr–1Mo–0.25V steel weld metal during holding at 700 ℃ for a series of durations was studied by TEM and EDX. The precipitates in the as– welded specimen are M3C carbides, and during tempering M7C3 and M23C6 carbides appeared. After long term tempering M3C carbides spheroidized and disappeared. In the early tempering stage M7C3 carbide with low wCr/wFe is metastable phase. M23C6 carbide can exist only under low tempering parameter, and M7C3 carbide with high wCr/wFe formed in the late stage is the stable phase. M3C carbides are often sphere–liked, M7C3 and M23C6 carbides are rod–shaped and lump–shaped.
The modifications of basal texture in AZ31 magnesium alloys processed by asymmetrical rolling and equal channel angular rolling (ECAR) are investigated. Deformation mechanisms are determined by microstructure observation and macro- and micro-texture measurements. Results show that during asymmetrical rolling the basal texture is gradually rotated around TD (transverse direction) by basal slip activated by shear strain on the rolling plane, whereas the prism texture is induced by tension twinning activated by shearing parallel to the plane separating the two channels during ECAR. So both the special processing techniques may improve the plasticity of magnesium alloy. The effects of rolling reduction and annealing on textures are also studied and discussed.
The particle swarm optimization (PSO) was successfully applied for the modified maximum entropy method (MMEM) of the quantitative texture analysis. Using two incomplete pole figures or only one incomplete pole figure, the complete orientation distributions of pure copper samples can been obtained. Such an improvement has been verified to keep the merits of primary MMEM, i.e., the complete orientation distribution function (ODF) of samples can be calculated using lesser experimental data, furthermore, the difficulty to choose a set of purposely initial solutions has also been overcome when solving the nonlinear equation, which makes the MMEM more versatile and practicable.
A new mathematical model constituting temperature schedule was established in order to realize flexible rolling of heavy and medium plate, in which the architecture of artificial neuron network (ANN) with optimal ability to predict mechanical properties was first constituted, and then, temperature schedule was constituted by genetic algorithm (GA). It is shown that mechanical properties can be well-predicted by recursive functions and ANN, and the precision predicted by ANN is higher than that by recursive functions. Finishing temperature and final cooling temperature among schedule ones are most important factors for mechanical properties. Mechanical properties calculated by temperature schedule and recursive mechanical-property functions agreed well with desired results. The temperature schedule constituted can be used to produce some steel plates with the same compositions but different strengths.
Gigacycle fatigue properties of bearing steel GCr15, were investigated under cantilever-type rotary bending fatigue tests in an open environment at room temperature. S-N data obtained from fatigue test showed continuous gradual decline and large dispersion. These data could not be described by a step wise S-N curve. From the results of fractography, fatigue fracture mainly occurred at surface flaws or an inclusion in the region of high stress amplitude level, whereas it mainly occurred at a subsurface inclusion in the region of low stress amplitude level. Fish-eye marks were always observed around the subsurface inclusions on the fracture surface, and for most subsurface inclusions, a granular bright facet (GBF) area was observed in the vicinity around them. The experimental results were analyzed and showed that a larger scatter of inclusion size and clusters of smaller inclusion particles are key factors influencing the scatter of fatigue life. Influence of inclusion size on the dispersion of fatigue life can be quantitatively analyzed by using the estimated GBF growth rate from the S-N data.
The flow stress-strain behavior of GH761 alloy was investigated via hot compression testing. The peak stress σp, starting steady--state stress σs, and corresponding strain εp, εs decrease with decreasing strain rate ε at constant temperature. At constant strain rate, σp, σs and εs drop with rising temperature, but εp does not change obviously. On the basis of reducing original grain size, lowering deformed temperature and enhancing strain rate can well refine structure. The microstructure will be most homogenous and finest when the strain reaches the level that DRX is finished exactly. Further increasing the strain will promote the grain growth. The hot deformation constitutive equation obtained is as follows: ε=6.5×106σp4.86exp(-461×103/RT).
The corrosion rates of X65 steels with different Cr contents were measured in CO2 environment under high temperature and high pressure condition. ESEM, EDS, XPS and SEM were employed to analyze the morphologies and characteristics of corrosion scales on the steels. The results show that the corrosion rate significantly decreased with increasing Cr content in the steels. An increase in Cr content< leads to a lower susceptibility to local corrosion. When the Cr content reaches up to 3%, the local corrosion can be eliminated. The competitive deposition of FeCO3 and Cr(OH)3 on low-chromium steels results in a multilayer structure of the scales. The scales on 1Cr-X65 and 3Cr-X65 steels have three-layer structure and scale on 5Cr-X65 steel has two-layer structure. Cr exists mainly as amorphous compound Cr(OH)3 in specific layer of the corrosion scales on low-chromium X65 steels. The Cr(OH)3 content of the scale increases remarkably with increasing Cr content in the steels. The high Cr(OH)3 content improves the protection performance of the scales and the corrosion resistance of low-chromium X65 steels. The rate of general corrosion and susceptibility to local corrosion are mainly dependent on the formation of codeposition layer of FeCO3 with Cr(OH)3 on low-chromium steels.
The addition of 4% niobium in Fe-Co-Nb--B amorphous alloy may retard the crystallization process, raise the crystallized temperature and enhance the thermal stability. The nucleation of Fe3B crystallized phase is checked while the nucleation and growth of Fe23B6 phase is promoted. The average grain sizes can be reduced from 30-60 nm to about 14-20 nm. The crystallization activation energy calculated by the onset crystallization temperatures decreases obviously. The nucleation process of α-Fe(Co), Fe3B and Nd2(Fe, Co)14B phases is more difficult than the growth process, while the growth process of α-Fe(Co), Fe23B6 and Nd2(Fe, Co)14B phases is more difficult than the nucleation process caused by the addition of niobium. However, the mechanism of the nucleation and growth of the crystallized phases is almost unchanged. The crystallization process is mainly dominated by one-dimensional nucleation and three--dimensional growth with decreasing nucleation rate.
The metallic nitrides Mn3(Cu1-xGex)N were prepared by solid-state sintering in the pure nitrogen atmosphere at 1073 K. The X-ray diffraction analysis shows that all of the sintered polycrystalline samples present an antiperovskite structure of Mn3CuN phase. The linear thermal expansions of compounds Mn3(Cu1-xGex)N (0.40≦x≦0.60) measured by using Michelson interferometer, exhibited negative thermal expansion near Neel temperature. With increase in the content of Ge, the temperature at which the negative thermal expansion occurred increases while the temperature range widens, but the thermal expansion coefficient decreases. The sample with x=0.60 presents negative thermal expansion in the region of 250-290 K (around 273 K), the linear expansion coefficient can reach to -65×10-6 K-1, shows a potential for practical application. The temperature dependence of the magnetization indicates that the negative thermal expansion behavior of Mn3(Cu1-xGex)N compounds occurs in the gradual transition from antiferromagnetic to paramagnetic state. The negative thermal expansion is caused by magnetovolume effect, originating from magnetic spin order disappearance and magnetic moment decrease near the Neel temperature.
Based on die casting experiments, metal--die interfacial heat transfer coefficient (IHTC or h) was determined, and it is found that the IHTC changes linearly with the solidification rate v, i.e., h=kh-vv+ω, where kh-v is a function of the initial die surface temperature and the casting thickness while ω is a constant. Both kh-v and ω can be calculated by applying the regression method. Such relationship between h and v is suitable for both AM50 magnesium alloy and ADC12 aluminum alloy.
Considering the thermal action characteristics of pulsed arc in laser+pulsed GMAW (gas metal arc welding) hybrid welding, the arc heat flux is divided into two double-elliptic distribution modes with different parameters corresponding to pulse current duration and base current duration. Meanwhile, the thermal conductivity at the weldment surface is appropriately lowered to take account of intermittent action of pulse and base current indirectly. Based on the level of averaged welding current, the distributed region for double-ellipsoid of droplets heat content is determined, and the action location of laser heat input is taken into consideration. The previous heat source modes have been improved through dealing with the abovementioned aspects, and two new kinds of adaptive combined volumetric heat source modes are developed. The weld geometry and dimensions are numerically simulated under different conditions in hybrid welding process by using the improved heat source modes. The predicted weld penetration depth, width and fusion line locus all agree well with the experimental results. Thus, numerical simulation accuracy for hybrid welding has been greatly improved.
Effects of processing conditions on densification behavior and microstructural features of laser sintered micron/nano-sized C-W powder mixture were investigated. Reasonable increase in the laser power or decrease in the scan speed leads to a higher sintered density and a more homogeneous microstructure. Decreasing the scan line spacing to 0.15 mm improves the surface finish of the sintered component. Lowering the powder layer thickness to 0.15 mm yields a more coherent inter-layer bonding. Under the suitable processing conditions determined, the highest densification level reaches 95.2%. A series of regularly shaped W-ring/Cu-core structures are also formed in laser sintered component, and the forming mechanism was discussed.
Thermal expansion behaviors of carbon fabric/Mg-2.0Re-0.2Zr (2D) and carbon fabric/Mg-2.0Re-0.2Zr (1D) composites fabricated by squeeze casting technology were measured, and the results show that the anisotropy is obviously improved by using carbon fabric. Coefficient of thermal expansion (CTEs) of 0°/90° of 2D composite between 50 and 350 ℃ changes from 4.03×10-6 ℃-1 to 1.83×10-6 ℃-1, and the 45° CTEs decrease from 4.53×10-6 ℃-1 to 2.31×10-6 ℃-1. CTEs of 0°/90°of 2D composite between 20 and 150 ℃ are in a good agreement with the model derived by the authors. Dimensional stabilities of composites were evaluated by thermal cycling method between 20 to 150 ℃. Strain hysteresis of 2D composite is observed during thermal cycling, and residual stain is mainly matrix plastic deformation generated by thermal stress. The net strain shows little change with increasing cycling number, which demonstrated 2D composite has good dimensional stability.
The bulk metal forming processes are calculated and analyzed by using a multi-step finite element method (FEM) based on deformation theory of plasticity. In this method, FEM solution is implemented to minimize approximated plastic potential in static equilibrium by constraint variation principle, for incompressible rigid-plastic materials. The multi-step simulation deals with the fictitious sliding constraints for intermediate configurations and iterations step by step along the deformation path, considering the contact and deformation history, which could provide rapid analysis for more complicated bulk forming problems. The one-step and multi-step forward simulations of several typical bulk metal forming problems are performed by this method, the calculated results of which are compared with those obtained by incremental FEM. The results indicate: multi-step FEM simulation of the bulk metal forming processes could give the reasonable answers with a small amount of computing time, the errors of which are less 10% compared with those of incremental FEM.
