Materials Science Forum Vols. 558-559

Abstract: The novel coupling recrystallization model is proposed in this study. First, the deformation microstructure was simulated by the finite element method based on the strain gradient crystal plasticity theory. The calculated dislocation density and crystal orientation were transferred to the recrystallization phase-field simulation. The initial subgrain structures used in phase-field simulation were determined by a relationship between dislocation density and subgrain size with the dislocation density distribution calculated by crystal plasticity simulation. The so-called KWC phase-field model, which can introduce both subgrain rotation and grain boundary migration, was employed, and spontaneous nucleation and grain growth were simulated simultaneously.

Abstract: The present work concerns the simulation of metallurgical evolutions in 3D multi-pass forming processes. In this context, the analyzed problem is twofold. One point refers to the management of the microstructure evolution during each pass or each inter-pass period and the other point concerns the management of the multi-pass aspects (different grain categories, data structure). In this framework, a model is developed and deals with both aspects. The model considers the microstructure as a composite made of a given (discretized) number of phases which have their own specific properties. The grain size distribution and the recrystallized volume fraction distribution of the different phases evolve continuously during a pass or inter-pass period. With this approach it is possible to deal with the heterogeneity of the microstructure and its evolution in multi-pass conditions. Both dynamic and static recrystallization phenomena are taken into account, with typical Avrami-type equations. The present model is implemented in the Finite Element code FORGE2005®. 3D numerical simulation results for a multi-pass process are presented.

Abstract: Plastic deformation induces the dislocation and residual stress fields, which rest in a material after releasing of applied external forces. One can distinguish the stored energy connected with dislocation density and that with residual stresses. The stored energy distributions can be determined experimentally by diffraction experiments and also can be predicted by deformation models. The so obtained distributions of the stored energy versus crystal orientation were correlated with deformation and recrystallization textures of low carbon steel.

Abstract: The kinetics and microstructure evolution during static recrystallization (SRX) of hot-deformed austenite in a low carbon steel are simulated by coupling a cellular automaton (CA) model with a crystal plasticity finite element model (CPFEM). The initial deformed characteristics, which include the stored energy of deformation and the crystallographic orientation induced by a plane strain hot compression are simulated using a crystal plasticity finite element model. These data are mapped onto the CA regular lattices as the initial parameters for SRX simulation. The coupled simulation results reveal that the heterogeneous distribution of the stored energy of deformation results in non-uniform nucleation and a slower kinetics. The influence of non-uniform distribution in stored energy on the SRX kinetics and microstructure evolution is discussed based on a microstructural path (MP) analysis.

Abstract: An improved Monte Carlo (MC) Potts model algorithm has been implemented allowing an extensive simulation of three-dimensional (3D) normal grain growth. It is shown that the simulated microstructure reaches a quasi-stationary state, where the growth of grains can be described by an average self-similar volumetric rate of change, which depends only on the relative grain size. Based on a quadratic approximation of the volumetric rate of change a generalized analytic mean-field theory yields a scaled grain size distribution function that is in excellent agreement with the simulation results.

Abstract: The grain growth kinetics of silica and calcia doped alumina at 1400oC and their grain boundary complexion is characterized. These data are compared to predictions of both diffusion controlled and nucleation limited interface controlled grain growth theory. It is deduced from the indicators that the mechanism for normal and abnormal grain growth in these aluminas is diffusion controlled.

Abstract: The effect of carbon addition on the grain growth and ordering kinetics of FePt film has been experimentally studied by sputter-depositing a monolithic FePt-20at.%C film of 24 nm. Carbon addition of 20at.% to FePt thin film in a form of FePt (20 nm)/Cn (4 nm) (n = 1, 4) significantly reduced both the grain growth and ordering kinetics. Reducing the thickness of carbon layer, i.e. from n = 1 to n = 4, led to a much finer grain size distribution as well as to a finer grain size. The Monte Carlo simulation study indicated that the decrease of grain growth and ordering kinetics is primarily due to a continuous decrease of the mobility of order – disorder inter-phase with the progress of ordering reaction. This can eventually lead to a stable 2-phase grain structure inter-locked by low mobility inter-phases and is responsible for the formation of a fine grain size distribution in the FePt/Cn film with n = 4.

Abstract: Use of silver (Ag) nanoparticle suspension for various applications such as ink-jet printing of electronic circuits has been of prime interest. We observed the microstructure evolution of the inkjet-printed Ag thin films on Si substrates under various annealing conditions using the field-emission scanning electron microscopy (FE-SEM). Abnormal grain growth characteristics were identified when annealed at about 240 oC under ambient air. Growth characteristics of pores were found to be in accordance with that of grains. Competition between grain and pore growth is attributed to small grain sizes, low packing density and high porosity, which are characteristic of inkjet-printed Ag films as dried.

Abstract: Multi-component ceramic composites consisting of two, three and four phases, based on duplex microstructures of zirconia and alumina, were fabricated by a polymer complexation route employing polyethylene glycol (PEG) as a polymeric carrier. The polymer complexation route provided porous and soft powders and they were sintered after a simple ball milling process. In this study, the microstructures and flexural strengths of the multi-component (Al2O3-ZrO2-Y2O3-SrO) ceramic composites were examined on the processing variations of mole ratio and sintering temperature. The composites showed various grain morphologies according to the sintering temperature, and flexural strength of 410 MPa was obtained in the Al2O3·ZrO2·0.5Y2O3·0.4SrO composite sintered at 1600 °C for 1 h. In particular, needle-shape grains were observed in the four-component composites sintered at 1500 °C.

Abstract: Mineral wool products can be used for thermal and acoustic insulation as well as for fire protection. The high temperature properties and the crystallization behaviour (devitrification) of the amorphous fibres during heating have been examined. Commercial stone wool and commercial hybrid wool (stone wool produced by a glass wool process) have been compared, as well as specially produced stone wool fibres. The fibres differed in chemical compositions and degree of oxidation given by Fe3+/Fetotal ratios. The materials were studied by thermal stability tests, X-ray diffraction, Mössbauer spectroscopy, secondary neutral mass spectroscopy, differential scanning calorimetry and thermal gravimetric analysis. When stone wool fibres were heated at 800 °C in air, oxidation of Fe2+ to Fe3+ occurred simultaneously with migration of divalent cations (especially Mg2+) to the surface. Decreasing Fe3+/Fetotal ratios resulted in increasing migration and improved thermal stability. The cations formed a surface layer mainly consisting of MgO. When heated to above 800 °C, bulk crystallization of the fibres took place with diopside and nepheline as the main crystalline phases. Commercial stone wool and the specially made fibres were considerably more temperature stable than the commercial hybrid wool. Commercial hybrid wool has a high Fe3+/Fetotal ratio of 65% resulting in less migration of cations during heat treatment.