Materials Science Forum Vols. 561-565

Abstract: In this paper, 3-dimension finite element model was applied to analyze the commercial pure Ti billet subjected to four-pass equal channel angular extrusion process at 400°C with Bc route. The effective strain distribution and the damage within up to four-pass were described with friction conditions in 3D. The propensity for cracks was predicted, which did correlate approximately to the experiment.

Abstract: L10 phase FePt alloy is regarded as one of the most promising materials for ultra high density magnetic recording media. However, structural point defects, which would reduce the media's signal to noise ratio, are inevitable in non-stoichiometric L10 FePt alloy. Hence, possible types of point defect (vacancy and anti-site defect) in non-stoichiometric ordered FePt alloy were fully studied using density functional theory. Investigation over vacancy shows the formation energy of Fe and Pt vacancy is respectively 2.58eV and 3.20eV. Geometry relaxation implies Fe vacancy has a stronger deformation force upon the original lattice. Meanwhile, anti-site defect study shows that the formation energy of Fe anti-site (Fe occupation in Pt sublattice) and Pt anti-site (Pt occupation in Fe sublattice) is respectively 1.05eV and 0.66eV. Therefore, for Fe-rich and Pt-rich alloy, the preferred structural point defects are both anti-site substitution rather than vacancy due to the much lower formation energy.

Abstract: The goal of this work is to describe the cosmic micromechanics connection with irreversible deformation processes in spatially extended polycrystalline systems, where the nature of the crystalline structure of the universe in a relativistic framework at Max Plank scale and Edwin Hubble scale play and important role. In this physical construction by applying the theoretical model of Muñoz-Andrade the activation energy for irreversible deformation processes in spatially extended polycrystalline systems is obtained. Consequently, the main results of this work are analyzed in the context of the unified interpretation of Hubble flow, plastic flow and super plastic flow.

Abstract: The mechanical, thermal and electronic properties of the nanoscale materials are studied using an ab initio molecular dynamics (TBMD) method and statistical moment method (SMM). We investigate the mechanical properties of nanoscale materials like carbon nanotubes (CNT), graphens and nanowires in comparison with those of corresponding bulk materials. The electronic density of states and electronic transports of the nanoscale materials, with and without the atomistic defects are also discussed. We will show that the thermodynamic and strength properties of the nanoscale materials are quite different from those of the corresponding bulk materials.

Abstract: Phase Field Method is combined with the Cluster Variation Method within the square approximation, and the multiscale ordering behavior from atomistic to microstructural evolution process of ordered domains in the two dimensional square lattice is investigated. The transition temperature is determined at 1:1 stoichiometric composition and it is confirmed that the transition is of the second order. The growth process of the ordered domains is visualized and it is revealed that the sharp decrease of the free energy takes place during the process.

Abstract: In this paper, the thermodynamic and kinetic requirements of heterogeneous and homogeneous nucleation of metallic melting were suggested. Based on the kinetic requirements of nucleation, the mathematic model of wetting angle of heterogeneous nucleation was developed, Based on the wetting angel model, it was predicted that the maximum undercooling of homogeneous nucleation for melts is two thirds of melting temperature. With the wetting angel model, the wetting angles of different catalysts in liquid iron were calculated, and calculation results are in agreement with that of other researchers.

Abstract: The bending tool design of a cold roll-formed blank has been discussed in this paper. The cold roll-formed blanks give a up and down bending line instead of a conventional straight line, The problems of wrinkling and edge irregularity occured frequently in such bending operation. A die design and blank pre-cutting methods are proposed in this paper to eliminate the edge irregularity. The radii of die and punch, punch profile, the gap of tools are considered as the design factors. The CAE method is adopted to obtain the measurements of the design of experiments (DOE). A fourth-order Bezier curve is proposed to desing the pre-cutting curve systemtically and automaticaly. The Golden section method is adopted to improve the pre-cutting curve design. The edge iregularity of a cold roll-formed balnk can be minimised by the proposed method. The wrinkling defects can be decreased but are not able to fuly elimiated.

Abstract: The conventional method of bending the large crankthrow was investigated by computer simulation combined with manufacturing trial, and the typical forging defects, such as constricted waist, folded cracks and horn mouth on forged blank were analyzed. On the basis of these results, a novel forging shape of preformed blank was proposed using anti-transformation method by computer simulation. The FEM simulated results show that all the above defects can be avoided by carrying out the novel method, furthermore, the maximum resistance of the novel bending deformation was reduced to 72% of conventional process, and the weight of forging blank can be decreased by 15%. Finally, the optimum forging shape was applied to the actual process, the simulation results were confirmed by manufacturing trial, and qualified forged piece was gained. The FEM model established can be used for further optimization of other types of crankthrow.

Abstract: A physical model for austenite recrystallization of steel concerning TMCP is developed. Dislocation density plays a key role as recrystallization driving force. The dislocation density change is a result of competition between dislocation generation and dynamic recovery. Recrystallization is described as a nucleation-growth process. An abnormal subgrain growth mechanism is introduced for nucleation. A few subgrains fulfilling abnormal growth conditions will stand out and become nuclei of recrystallization. The recrystallized grain grows to the deformed materials driven by the stored energy. Oswald ripening occurs for grains surrounded by recrystallized grains. The models were verified by laboratory simulation results for selected austenite stainless steels. It showed good agreement between predicted and experimental results.

Abstract: Functionally gradient material (FGM) developed for heat-shielding structure is often used in the very high temperature environment. Therefore, the material property parameters are not only functions of spatial coordinates but also ones of temperature. The former leads to partial differential equations with variable coefficients, the latter to nonlinear governing equations. It is usually very difficult to obtain the analytical solution to such thermal conduction problems of FGMs. If the finite element method is adopted, it is very inconvenient because material parameter values must be imputed for each element. Hence, a semi-analytic numerical method, i.e., method of lines (MOLs) is introduced. The thermal conductivity functions do not need to be discretized and remain original form in ordinary differential equations. As a numerical example, the nonlinear steady temperature fields are computed for a rectangular non-homogeneous region with the first, the second and the third kinds of boundary conditions, where three kinds of functions, i.e. power, exponential and logarithmic ones are adopted for the thermal conductivity. Results display the important influence of non-linearity on temperature fields. Moreover, the results given here provide the better basis for thermal stress analysis of non-homogenous and non-linear materials.