Papers by Keyword: Coarsening

Abstract: Microstructure evolution of high energy milled Al-50wt%Si alloy during heat treatment at different temperature was studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) results show that the size of the alloy powders decreased with increasing milling time. The observable coarsening of Si particles was not seen below 730°C in the high energy milled alloy, whereas, for the alloy prepared by mixed Al and Si powders, the grain growth occurred at 660°C. The activation energy for the grain growth of Si particles in the high energy milled alloy was determined as about 244 kJ/mol by the differential scanning calorimetry (DSC) data analysis. The size of Si particles in the hot pressed Al-50wt%Si alloy prepared by high energy milled powders was 5-30 m at 700°C, which was significantly reduced compared to that of the original Si powders. Thermal diffusivity of the hot pressed Al-50wt%Si alloy was 55 mm2/s at room temperature which was obtained by laser method.

Abstract: Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for second stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position of the blade and mechanical tests of high temperature tensile test. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP phase formation occurred and progressed with increasing service time, and especially the leading and trailing edge of top layer should be a check points for used blade. High temperature tensile results of 25,000 and 52,000 hrs used blades were also compared with serviced time and position in each blade.

Abstract: A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.

Abstract: Gradient annealing experiments of a near-eutectic AlCu30 alloy with artificial stirring induced by a rotating magnetic field (RMF) of 6 mT were performed. The specific surface area of the primary phase was measured on metallographic sections perpendicular to the sample axis with a fixed amount of fraction solid. The variation of the specific surface area with fluid flow is compared to flow free experiments: the specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and discussed with models of convective coarsening of dendrites.

Abstract: Microsegregation induced inhomogeneity of the coarsening of cuboidal γ’(Ni3(Al,Ti)) precipitates was studied in a single crystal nickel-based superalloy CMSX-4 at temperatures ranging from 850 to 1000 °C and for an ageing time up to 5000 h. Experimental results showed a significant statistical difference in the size of the γ’ precipitates between the dendrites and interdendritic region. The achieved results are analyzed and discussed from the point of chemical heterogeneity, activation energy for coarsening, time exponent, effective diffusion coefficient and γ/γ’ interfacial energy which control coarsening kinetics of the cuboidal γ’ precipitates within the dendrites and interdendritic region.

Abstract: A phase-field simulation is performed to examine the effect of elastic inhomogeneity between the  and ’ phases on coarsening of the ’ phase in Ni-based superalloys. In the calculation of elastic strain energy, the mechanical equilibrium equation in elastically inhomogeneous system is solved by an iterative-perturbation scheme. On the basis of the elastic constants of a practical Ni-based superalloy, a series of simulations is performed in which both elastic anisotropy and shear modulus are varied independently. The variation of elastic anisotropy gives significant effect on both morphology and size distribution function of the ’ particles, whereas the variation of shear modulus gives little effect on them. Furthermore, it is found that the coarsening rate constant of the cubic growth raw changes and increases with increasing the standard deviation of the ’ size distribution.

Abstract: A new process for fabrication of semisolid billets is introduced, which involves equal channel angular pressing and isothermal heating in the semisolid state. The process leads to a relatively fine globular microstructure. The microstructure evolution during isothermal treatment is studied and it is shown that dendrites breaking up has happened during equal channel angular pressing in semisolid state. The microstructural evolution during isothermal heating and the mechanism for the formation of the globular structure is tried to be understood and also modeled.

Abstract: Approaches used for simulation of precipitates evolution in multicomponent systems are analyzed in brief. Simulation of precipitates growth and dissolution in such systems, simulation of a polydisperse particle ensemble evolution at different stages and possibilities of taking into account nucleation during precipitates growth from a supersaturated solid solution are considered by an example of iron-carbon alloys. The problems to be solved are formulated.

Abstract: Coarsening of particles during liquid phase sintering is known to be an example of Ostwald ripening. This coarsening process, in a fully wetting system, is simulated in three dimensions with a kinetic Monte Carlo model. The results from the simulation for microstructures, kinetics and particle size distributions are compared to available experimental findings. It is found that the average particle volume increases linearly with time and that the particle size distributions are consistent with those obtained experimentally, as in the W-Ni-Fe and Sn-Pb systems.