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Wu K.,University of Science and Technology Beijing | Liu G.Q.,University of Science and Technology Beijing | Hu B.F.,University of Science and Technology Beijing | Li F.,University of Cambridge | And 4 more authors.
Journal of Materials Science | Year: 2012

The formation mechanism and coarsening behavior of fan-type structures in a new Ni-Cr-Co-based powder metallurgy superalloy were investigated by means of field scanning electron microscope, transmission electron microscope, electron backscattered diffraction, and differential scanning calorimetry. The results show that he fan-type structures consist of finger-shaped λ dendrites and c matrix between them. They nucleate in the chemical segregation regions on grain boundaries and grow by diffusion. There are three types of solute atoms flow: (a) rapid diffusion along grain boundary; (b) the diffusion from supersaturated c matrix to fan-type c0 phases; and (c) shortdistance diffusion from the previous formed λ0 phases at high temperature to c0 phases formed at low temperature within the branches of fan-type structures. These λ dendrites are perpendicular to grain boundaries and grow asymmetrically, resulting in grain boundary serration. In addition, the fan-type structures coarsen within the c0 depletion zone after the standard aging treatment. © 2012 Springer Science+Business Media, LLC. Source


Wu K.,University of Science and Technology Beijing | Liu G.,University of Science and Technology Beijing | Hu B.,University of Science and Technology Beijing | Li F.,University of Cambridge | And 4 more authors.
Materials Characterization | Year: 2012

The solidification characterization of a new rapidly solidified Ni-Cr-Co based superalloy prepared by plasma rotating electrode process was investigated by means of optical microscope, scanning electron microscope, and transmission electron microscope. The results show that the solidification microstructure changes from dendrites to cellular and microcrystal structures with decreasing powder size. The elements of Co, Cr, W and Ni are enriched in the dendrites, while Mo, Nb and Ti are higher in the interdendritic regions. The relationships between powder size with the average solid-liquid interface moving rate, the average interface temperature gradient and the average cooling rate are established. Microsegregation is increased with larger powder size. The geometric integrity of MC′ type carbides in the powders changes from regular to diverse with decreasing powder size. The morphology and quantity of carbides depend on the thermal parameters and non-equilibrium solute partition coefficients during rapid solidification. © 2012 Elsevier Inc. Source


Wu K.,University of Science and Technology Beijing | Liu G.,University of Science and Technology Beijing | Hu B.,University of Science and Technology Beijing | Li F.,University of Cambridge | And 4 more authors.
Materials and Design | Year: 2011

The hot compressive deformation behavior of a new hot isostatically pressed Ni-Cr-Co based powder metallurgy (P/M) superalloy was studied in the temperature range of 950-1150°C and strain rate range of 0.0003-1s-1 using Gleeble-1500 thermal simulator. The dynamic recrystallization-time-temperature (RTT) curve was developed and the constitutive equation of flow stress during hot deformation was established. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate. The flow stress represents as the characteristic of dynamic crystallization with the increasing of strain at the deformation temperatures lower than 1100°C and strain rates higher than 0.0003s-1. The beginning time of dynamic crystallization has no linear relationship with deformation temperature in the condition of strain rate lower than 0.01s-1. Besides, the experiments verify that the hyperbolic sine model including the variable of strain reflects the changing law of flow stress during the hot deformation process. © 2010. Source


Wu K.,University of Science and Technology Beijing | Liu G.,University of Science and Technology Beijing | Hu B.,University of Science and Technology Beijing | Zhang Y.,University of Science and Technology Beijing | And 3 more authors.
Xiyou Jinshu Cailiao Yu Gongcheng/Rare Metal Materials and Engineering | Year: 2012

The effect of solution cooling rate on the microstructure and microhardness of a novel type nickel-based P/M superalloy FGH98Iin subslovus plus superslovus and superslovus heat treatment condition was studied by means of optical microscope (OM), field emission scanning electron microscope (FESEM) and microhardness tester. The results show that subslovus pre-heat treatment makes the large grain boundary γ′ phase of as-forged alloy partial dissolution, and the grains grow a little, while the influence on the cooling γ′ precipitates of superslovus heat treatment is not obvious. The average sizes of secondary and tertiary γ′ precipitates decrease with the cooling rate increasing. Meanwhile, the shape of secondary γ′ precipitates changes from the butterfly-like to the spherical, the γ′ phase particle density increases and the area fraction decreases. It is also found that γ′ phases nucleate and precipitate in two stages if the cooling rate is not faster than 1.4°C/s. The higher the cooling rate, the higher the microhardness, and the microhardness increases more after aging. In addition, the average sizes of γ′ precipitates and the microhardness as a function of cooling rate are established. The above results can be used as a theoretical reference for the selection of actual superslovus processing of FGH98Idual-property disk. © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved. Source


Wu K.,University of Science and Technology Beijing | Liu G.,University of Science and Technology Beijing | Hu B.,University of Science and Technology Beijing | Li F.,University of Science and Technology Beijing | And 4 more authors.
Materials Characterization | Year: 2010

The hot deformation behavior of a new Ni-Cr-Co based P/M superalloy was studied in the temperature range of 950-1150 °C and strain rate range of 0.0003-1 s- 1 using hot compression tests. It was characterized by true stress-true strain curves, constitutive equation, strain rate sensitivity m contour maps, power dissipation η maps and hot processing maps. The microstructural validation of processing maps was also done. The results show that the flow stress decreases with increasing temperature and decreasing strain rate. The hot deformation apparent activation energy of the Ni-Cr-Co based P/M superalloy at peak stress is 805 kJ/mol. The m and η contour maps are similar, and the values of m and η in the peak zones increase with increasing strain. When the strain is 0.5, a domain with its peak η of 40% and peak m of 25% occurs at 1050 °C and 0.0003 s- 1, which corresponds to dynamic recrystallization and can be as an optimum condition for good workability. Crown Copyright © 2009. Source

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