A thick (5 mm) nanostructured Fe-40Al deposit was successfully elaborated by high velocity oxy-fuel (HVOF) spray forming of milled powder. Its microstructure was investigated by scanning electron microscopy (SEM), quantitative metallography, transmission electron microscopy (TEM), as well as micro-hardness measurements. The obtained results were compared to those of a thin (300 $\mu$m) nanostructured coating produced by the same HVOF spray parameters and feedstock powder in order to study the effect of the processing conditions on the microstructure formation and associated hardness. The thermal stability of the deposit was asserted by heat treatments and hardness measurements. Its high hardness, that is due to the retained nanostructured unmelted powder particles, is retained at temperature as high as 600 ℃.

Molecular dynamics simulations of the bulk and surface melting processes were performed for metal Cu. The variations of the structure and energy in the system during bulk melting process were analyzed. The movement of the solid/liquid interface position during surface melting process was observed. The interaction between atoms in the system adopts the embedded atom potential proposed by Mishin. The simulation results show that the structure and energy in the system vary discontinuously at 1585 K in bulk melting process and the solid/liquid interface remains unchanged at 1380 K in the surface melting process. The different mechanisms of the two melting processes induce lower thermodynamic melting point (1380 K) than the bulk melting point (1585 K). Surface melting is significant in real melting process, so the experimental datum measured is the thermodynamic melting point. The simulated melting point coincides well with the experimental one, thus it can be concluded that the present melting point simulation method is correct and effective, and the Mishin's embedded atom potential is suitable for dealing with complicated and disordered systems.

The critical driving force of martensitic pretransition has been calculated based on the Hamiltonian containing electron-phonon interaction. The calculated results shows that the values of the driving force is of the same level as that of the condensed phonon. Considered the electron-phonon interaction and the condensation of TA or LA phonon, the double sine-Gordon equation of atomic phase angle has been proposed to study the nonlinear characteristics of the pretransition and the electron-phonon coupling mechanism may be taken as its main mechanism.

After surface mechanical attrition treatment (SMAT) for 316L stainless steel, a gradient structure in its surface layer with grain size from nano-scale to micro-scale was obtained. The samples before and after the SMAT were annealed in vacuum at different temperatures for different durations, and the structural evolution as well as the hardness and the residual stress variations along the depth were analyzed. Experimental results show that when the annealing temperature is lower than 0.5Tm(Tm is the melt point), no obvious change can be found for the grain size in the gradient structure, except the martensite transformation in the affected layer due to the release of residual stress, and the hardness distribution along the depth remains unchanged. When the annealing temperature is higher than 0.5Tm, recovery, recrystallization and sharp drop of the residual stress occur in the gradient structure, which induce significant reduction of the hardness along the depth. The effect of annealing duration on the structure and property of the SMAT sample is less important comparing with the annealing temperature.

Based on the previous work reported by Zhu and Hong, a micro-scale cellular automaton (CA) model for modeling dendritic growth was further improved. In the present model, the solid/liquid interface equilibrium was determined using the Gibbs-Thomson equation for a simple binary alloy. It accounted for the effect of anisotropy in both interfacial kinetics and surface energy on the preferred growth orientation of a dendrite. The improved model was applied to simulate the dendritic features, in the cases of the free dendritic growth from an undercooled melt, competitive columnar growth in the directional solidification and equiaxed dendritic evolution. The simulation results show that the model can successfully predict the morphologies of both single and multi-dendrites with various preferred growth orientations.

Effect of high vertical magnetic field on the directional solidification structure of eutectic MnBi/Bi has been investigated. It has been found that high magnetic field has enhanced the formation of MnBi fiber and make the directional solidification structure of eutectic MnBi/Bi more regular; with the increase of the intensity of the magnetic field, the average spacing and diameters of MnBi fibers increase at same growth rate; magnetic field makes the morphology of MnBi phase change and enhances the faceted growth character of MnBi phase.

Solute compositional distributions at and ahead of the solid--liquid interfaces are analyzed during the plane--front growth for both the primary and peritectic phases as single phase and nucleation phase for peritectic alloys directionally solidified at low growth rates from initial transient to steady state. Taking the nucleation and compositional undercooling criterion and assuming that the maximum interface growth temperature of phase growth is more stability during the directional solidification, the nucleation transitions during the transient period are investigated and compositional range of band structure formation is determined. Considering the history--dependant growths of a phase, the band structure can be differentiated as cycling band and single band structure, which can explain the experimental phenomena appropriately. The calculated results for Ti-Al alloy are identical with the theoretical analyses.

Taking the nucleation and compositional undercooling criterion and assuming that the maximum interface growth temperature of phase growth is more stability during the directional solidification, microstructure selection maps for peritectic alloys from initial transient to steady state are developed, which takes into account the conditions when stable composition gradient has not been reached at the plane front directionally solidified at low growth rates and the history-dependant growth of a phase. These maps can explain the experimental phenomena appropriately due to consider the transitions of phase and microstructure during the whole process of solidification.

Quantitative characterization of microstructural evolution during deformation enhanced transformation in a low carbon steel SS400 was investigated on a Gleeble 1500 machine. General conclusions of the austenite transformation kinetics were formulated. It was shown that the transformation process can be divided into three stages according to the characteristics of transformation kinetics: The kinetics equations of two early stages fitted well in with the J-M-A equation. The kinetics of the first stage obeys Cahn's site saturation mechanism, the kinetics parameter n=4 which indicates the ferrite nucleates at austenite grain boundaries and triple points. Kinetics of the second stage doesn't obey Cahn's theory, the kinetics parameter n=1.0-1.5, corresponding to ferrite nucleating repeatedly at the high stored energy areas in front of the ferrite/austenite interface. The kinetics doesn't obey the J-M-A equation anymore in the final stage, and only few nucleation sites left at this moment. Deformation promotes the growth of ferrite grains, the growing rate of ferrite in the early stage of DEFT was calculated.

During uniaxial hot compression for a medium carbon steel, the deformation induced ferrite (DIF) appeared when deformed temperatures lower than A d3 (786 ℃). The volume fraction of DIF increases with decreasing deformation temperature, especially when deformed temperatures lower than 750 ℃ the volume fraction of DIF increases significantly, which is far beyond the equilibrium ferrite fraction of 54%. With the increase of DIF, more and more carbon atoms were rejected into the boundaries of ferrite and interfaces of ferrite/untransformed austenite. During isothermal holding below A1 (719 ℃) temperature just after deformation, supercooled austenite would decompose to different structures in three different ways: when the deformation temperature is higher than Ad3, conventional ferrite-lamellar pearlite structure was obtained; when the deformation temperature is lower than Ad3 whereas higher than Ar3 (645 ℃), retained austenite transformed to ferrite-lamellar or degenerated pearlite-grain boundary cementite; when the deformation temperature decreased to just above Ad3, a microstructure different from lamellar pearlite was obtained, which consisted of fine cementite particle and ferrite.

Electron energy loss spectroscopy (EELS) was used to study grain boundaries in steels. The normalized 3d occupancies of states of irons both in bulk and at grain boundary were calculated according to the EELS data which are related to the change in the bonding of grain boundaries and the fracture mode of the steels. It is found that if the grain boundary (GB) has a higher occupancy of 3d states of iron than that of the bulk, the sample has a weak bonding of GB and tends to intergranular fracture. Otherwise if the GB has almost the same occupancy of 3d state of iron as the bulk, the sample has a strong bonding of GB and tends to transgranular fracture.

Characteristics of the secondary precipitates on grain surfaces connected with segregation of boron and carbon elements in a Ni-15Co-10Cr-5.5W-5Mo-4.2Al-1.5Ti alloy (GH4049) were studied by using extraction carbon replica method. When B content was below 0.008\%, or the solid-solutioned B/C atom ratio was decreased to about 0.365, the high temperature ductility and stress rupture life were reduced because large MC films, dendritic flake M6C and fine M23C6 films precipitated at grain boundaries. When B contents were 0.011% and 0.018\%, or the solid-solutioned B/C atom ratios were 0.667 and 0.941, the amounts of these carbides were decreased or their precipitates were refrained, but particles M3B2 precipitated, which improves the high temperature properties markedly. When B content was excessively high, M3B2 particles precipitated densely, resulting in the decrease of high temperature properties. If decreasing the cooling rate after solution, B and C atoms can diffuse at long distance, which promotes the precipitate of M3B2 particles at grain boundaries, improves the morphology of MC, and depresses the precipitations of M6C and M23C6.

The structure evolution and phase transformation behavior of Ni-Ti-Nb shape memory alloys with 4.5%Nb (atomic fraction) were studied systematically. It is found that the structure evolution as well as the phase transformation characteristics of the alloys is very sensitive to the ratio of the atomic fractions of Ni and Ti. After appropriate predeformation, a wide hysteresis was observed in such low Nb alloys. The difference of the mechanical behaviors of the alloys with austenite or martensite states was investigated at the same temperature, and explained by the kinetics of thermoelastic martensitic transformation.

The isothermal oxidation resistance of Ti44Ni47Nb9 shape memory alloy (SMA) in air over the temperature range of 450-850 ℃ was studied by using XRD, SEM and EDS techniques. Light oxidation was found on the alloy surface at 450 ℃. The alloy was attacked by oxidation and the oxidation kinetic curves were approximately parabola within the temperature range of 600—800 ℃. Experimental results indicate that a multi-layered scale was formed, consisting of an outer rutile layer, a Nb rich intermediate layer and a thin inner Ni3Ti layer. It was found that the Nb rich intermediate layer was compact, which obviously improved the oxidation resistance of the alloy due to its effective inhibition to diffusion of oxygen and titanium. Furthermore, the internal oxidation of nickel element was also inhibited because of the existence of Nb rich layer, which reduced oxygen concentration in the alloy.

Based on the measured data of mould temperatures during continuous casting of round billet, an inverse problem model was developed. Through identifying the local thermal resistance and its distribution between the mould and the strand, the mould heat flux and the shell thickness profiles were calculated, and the relationship between them was also analyzed. The calculated results could preciously and correctly reflect the characteristics of non-uniform heat transfer around the mould circumference, which provided an applicable method for applying numerical simulation technique to monitor the solidification process and the“visual mould”technology in continuous casting.

Single crystalline, arc-melted and SPS poly-crystalline NbSi2 samples were prepared for oxidation experiment at 1023 K, and the effects of cracks, pores and grain boundary on the oxidation behavior of NbSi2 were investigated. For arc-melted poly-crystalline samples, NbSi2 fully turned into powders after 3 h exposure at 1023 K, which is known as the “pesting” phenomenon. As a comparison, no pesting was found in the dense SPS poly-crystalline samples and single crystals after 89 h. The oxide formed on NbSi2 at 1023 K consists of Nb2O5 and minor SiO2. The Nb2O5 is lose and nonprotective, which spalls from the sample during oxidation. The oxidation kinetics of all the NbSi2 samples at 1023 K follows a linear law. The oxidation rate is intrinsically determined by the reaction rate between the matrix and the oxygen in air, which in turn is determined by the exposure area of the sample. The NbSi2 poly-crystalline sample shows a much larger weight change than that of the single crystalline sample, indicating grain boundaries and pores increase the effective area of oxidation reaction. A relevant model was proposed.

The critical Al content for the selective formation of alumina scales on Fe-Al alloys has been analyzed by investigating the oxidation behavior of binary Fe-10Al and ternary Fe-5Cr-10Al and Fe-10Cr-10Al alloys (atomic fraction, %). A possible interpretation of the effect of the third-element Cr in Fe-xCr-10Al on promoting Al2O3 scale formation not involving a transition from the internal to external oxidation of Al is briefly presented.

The rate constant of oxygen transfer reaction between the molten iron oxide and CO2 gas on the surface of liquid slag was measured by the isotope exchange method. Also, according to the electrochemical mechanism in the reaction process, the effect of P2$O5 additive was discussed and a new model was established. From this model, due to participating the electron competition while volatilizing from slag, P2$O5 decreases the oxygen transfer rate. The results of analyzing and fitting to the experimental data have indicated that this new model is available to explain the effect of P2O5, and supported the view which there exits an electrochemical mechanism in this process.

FePt/Cu multilayers and FePt thin films were prepared by DC magnetron sputtering. The as-prepared samples were annealed in vacuum at a temperature range of 300-550 ℃ for 1 h. It is found that the addition of Cu could reduce the ordering temperature of FePt. The ordering parameter $S$ was evaluated to be 0.6, and the coercivity reached 421 kA/m in [FePt(4 nm)/Cu(0.2 nm)]10 multilayers annealed at 350 ℃. The Cu layer induced reduction of ordering temperature is ascribed to both kinetic and thermodynamic factors.

In order to develop a nondestructive technique observing domain switchings in microscopic scale, an in situ method was proposed based upon the principal of Raman spectroscopy. Two types of experiments were carried out to verify the proposed method. One is to measure the Raman spectra for a fixed grain in the PLZT ferroelectric sample before and after 90 rotation. The other is to observe variations of Raman spectra for a fixed grain under a DC field. The results revealed that by a 90 rotation the Raman spectra for grains in an in-plane polarized sample vary and the Raman spectra for grains in both un-poled and out-plane polarized samples remain unchanged. The results also indicated that the in situ Raman spectrum varies as the magnitude of the applied DC field increases. These results showed that the domain switching will leads to variations of Raman spectrum.

Negative thermal expansion compound ZrW2O8 was successfully synthesized by hydrothermal method with low temperature heat treatment at 500 ℃. The XRD result showed that the crystalline precursor ZrW2O7 (OH) 2 (H2O) 2 was formed when the concentration of HCl was equal to or greater than 6 mol/L. The thermal stability of the synthesized ZrW2O8 and its precursor ZrW2O7 (OH) 2 (H2O) 2 were studied by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA), which confirmed ZrW2O8 could be synthesized through sintering the precursor at low temperature of 500 ℃. Powder X-ray diffraction and FT-IR spectroscopy investigations confirmed that the synthesized product is single cubic ZrW2O8 phase.