Materials Science Forum Vols. 475-479

Abstract: With the microscopic phase-field model, the early precipitation mechanisms of the ternary Ni75AlxV25-x alloys with middle Al composition were explored by computer simulation in this paper. Through the simulated atomic pictures and composition order parameters of precipitates, we can explain the complex precipitation mechanisms of θ (Ni3V) and γ′ (Ni3Al) ordered phases. Simulated results also show that the precipitation characteristic of γ′ phase transforms from non-classical nucleation and growth to congruent ordering + Spinodal decomposition gradually, otherwise, the precipitation characteristic of θ phase transforms from congruent ordering + Spinodal decomposition to non-classical nucleation and growth mechanism gradually.

Abstract: Near-edge structure of X-ray absorption spectrum (NEXAFS) of various Ti-oxides were investigated by combined with first principles orthogonalized linear combinations of atomic orbitals (OLCAO) method. From experimental and theoretical studies on the NEXAFS of the tetravalent and trivalent Ti-oxides, including rutile, anatase, brookite, columbite, and Ti2O3, it was found that the valence state of Ti can be identified by regarding the positions of the spectral onset and the shoulder in the main-peak of Ti-K NEXAFS.

Abstract: Employing the first principles discrete variational method(DVM), we investigated the electronic structure of LaNi5 hydrogen storage alloys containing various alloying elements, M=Mn,Fe,Co. The results showed that s electrons of H mainly interact with s electrons of hydride-non-forming element Ni, though hydride forming element La have stronger affinity to hydrogen atom. And alloying elements strengthened the bond between B-H, so decreased the capacity of doped-system.

Abstract: Theoretical investigation of the phase equilibira of three kinds of Fe-based alloys, Fe-Ni, Fe-Pd and Fe-Pt systems is attempted by combining FLAPW total energy calculations and Cluster Variation Method. It is revealed that the magnetism plays a crucial role in the phase stability and spin polarized calculation is indispensable. The experimental L10-disorder transition temperatures are reproduced with fairly high accuracy. Thermal vibration effects incorporated based on the Debye-Gruneisen model further improve the calculated transition temperatures. Furthermore, the influence of the various effective cluster interactions on phase stability is calculated systematically.

Abstract: The valence electron structure of GP Zone in Al-Zn alloy was calculated according to the Empirical Electron Theory(EET). The result of valence electron structure was further applied to analyze the interface energy between GP Zone and matrix. The reason that the GP Zones were formed even at the highest quenching speed and the hardness was stably ascended as soon as GP zones were formed and reached the maximum before the precipitation of metastable phase, was that the amoumt of the strongest covalent bond in the GP Zone was far more than that in α-Al cell and the Al atom in GP Zone was easily prone to forming covalent bond with Zn atom. It was an easy and effective method to apply the EET theory with the hardball model to calculation of interface energy.

Abstract: Dendritic is the most observed microstructure in metallic materials. Traditional Cellular Automaton model can only predict the grain structure and grain size. It is necessary for us to modify the original model to reflect the real shape of dendrite. In the paper, a mathematical model was established to describe the evolution of the dendritic shape, in which the influence of microsegregation and curvature on undercooling was taking into account. In addition, the growth model was proposed based on the minimum free energy principle. Modeling results indicated the proposed models can predict not only grain structure, but also dendritic morphology. Free growth of equiaxed grains and the competitive growth of columnar grains were simulated. The modeling results were also validated with experiments.

Abstract: A Cellular-Automaton Finite-Volume-Method (CAFVM) algorithm has been developed, coupling with macroscopic model for heat transfer calculation and microscopic models for nucleation and growth. The solution equations have been solved to determine the timedependent constitutional undercooling and interface retardation during solidification. The constitutional undercooling is then coupled into the CAFVM algorithm to investigate both the effects of thermal and constitutional undercooling on columnar growth and crystal selection in the columnar zone, and formation of equiaxed crystals in the bulk liquid. The model cannot only simulate microstructures of alloys but also investigates nucleation mechanisms and growth kinetics of alloys solidified with various solute concentrations and solidification morphologies.

Abstract: Thermodynamic and kinetic data are generally essential for quantitative modeling of materials processing, structure, and property. Thermo-Calc program interfaces, including TQ, TCAPI, and TC MATLAB Toolbox, provides a hierarchy of APIs for application programmers to access thermodynamic and kinetic data via the kernel of Thermo-Calc and DICTRA, the most widely used software and database system for multi-component phase equilibrium and phase transformation calculations. With these program interfaces, variation of thermodynamic and kinetic properties can be directly obtained in real time as the local temperature, pressure, or composition changes. The structure and usage of the Thermo-Calc program interfaces will be introduced in this article. Successful application examples will be illustrated.

Abstract: A Monte Carlo Potts model has been used to investigate cube-texture strengthening during grain growth in rolled high-purity Ni-tapes. The initial conditions for the simulations have been taken from electron back-scatter pattern (EBSP) orientation maps of already fully recrystallized samples. Experimentally, grain growth leads to an increase in the cube volume fraction to >95% , accompanied by an approximately ten-fold increase in the grain size. High cube volume fractions can be predicted under a number of conditions, though a small surface energy advantage of just 2% for cube-oriented grains is required to match the texture strengthening to the grain size change. An additional issue of interest is the influence on the grain growth of the large area-fraction of twin boundaries in the fully recrystallized condition. The presence of boundaries with low energy has a strong influence on the simulated microstructural evolution.

Abstract: A new approach to simulating the orientation distribution function (ODF) using one diffraction frame collected by X-ray two-dimensional detector is proposed. Based on the concept that the texture component presents a normal distribution form, the volume fraction and the orientation spread of texture component are obtained using curve fitting method from diffraction profile and, the pole figures as well as ODF can be calculated. Thus the texture of grain-oriented silicon steel is determined rapidly and conveniently. A comparison of the simulation ODF with that reconstructed from experimental pole figures by series expansion is presented and the results show that the new approach works correctly.