Papers by Author: Lin Liu

Abstract: The stress rupture life of DZ125 nickel-based superalloy that was prepared by directional solidification process under the temperature gradient of 500 K/cm has been studied at 900°C and 235MPa. The results showed that with the increase of directional solidification rate from 50 μm/s to 800 μm/s, the primary dendrite arm spacing reduced from 94 μm to 35.8 μm and γ' precipitates reduced and more uniformed in size. The high temperature stress rupture life of as-cast sample increased firstly and then decreased and reached its maximum at the solidification rate of 500 μm/s. The dislocation configuration of sample with refine dendritic structure after stress rupture was investigated and discovered that the dislocations in different parts of sample had different morphology and density, which indicated that the deformation of as-cast samples were uneven during high temperature stress rupture. A lot of dislocations intertwined around carbides and at the interface of γ/γ', and the dislocation networks were destroyed and the dislocations entered γ' precipitate by the way of cutting.

Abstract: A Re-containing single-crystal superalloy was processed with conventional radiation high rate solidification (HRS) and liquid-metal cooling (LMC) techniques, respectively. Compared with the HRS castings, LMC castings exhibited the greatly refined dendrite structures and lower segregation. At the withdrawal rate of 50 μm/s, the primary and secondary dendrite arm spacing of LMC casting was 103 and 19 μm, which were reduced by 60 % and 53 % compared to the counterpart of HRS castings. Moreover, higher withdrawal rates significantly shortened the characteristic diffusion distance and reduced the residual segregation of refractory elements after homogenization-solution treatments, which is beneficial for improving the local phase stability of Re-containing superalloys.

Abstract: The well-aligned growth structures which derive from directional solidification of ceramic eutectics are of great interests due to their potential use in electronic devices and as structural materials at high temperatures. Because of the complexity of the component system and very high melting points, the solidification behavior on the oxide ceramic eutectic is still unclear up to date. In the presented paper, the Al₂O₃-Y₂O₃-ZrO₂ ternary eutectic ceramic is remelted by a DTA apparatus. The maximal heating temperature is 1950 °C. The melting and solidification behavior are investigated by the DTA analysis. The solidification microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The results show that solidus temperature and the liquidus temperature are 1738.4 °C and 1750.1 °C, respectively. The formation path of eutectic phase is discussed. The microstructure of as-solidified eutectic ceramic shows a divorced ternary eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. Furthermore, the microstructural comparison with directionally solidified ternary eutectic ceramic is presented and discussed.

Abstract: A series of Ni-base single crystal superalloys with increasing contents of Re (3-6 wt. %) were directionally solidified under a high thermal gradient of approximately 250 K/cm. It shows that Re additions slightly increases the liquidus temperature, but decreases the eutectic reaction and γ′-solvus temperature of the experimental alloys. EPMA analysis indicates that Re addition strongly influences the segregation ratios of the constituent elements, particularly for Cr, Mo and W, and thus results in more severe dendrite segregation and large amount of eutectic formation in the as-cast microstructures.

Abstract: The relativity between the alloy melt state and solidification microstructure has been received great interesting recently. Some previous studies proved that control the state of alloy melt can improve the microstructure and properties. In this paper, the character of melt of A357 alloy between 700°C-1150°C was studied by using DSC. The result indicates that there are exothermic reactions over liquidus which corresponds to dissolution of crystalline particle or cluster in the melt. In order to investigate the effects of character of melt on the microstructure, the alloys were first superheated at 720°C，820°C and 925°C respectively with different character of melt, held for 20min, then the melts cooled to 720°C before quenching in Ga-In-Sn melt. The microstructure of samples was analyzed. The results show that with the increase of superheating temperature, the secondary dendrite arm spacing (SDAS) of -Al trends to decrease, and the eutectic silicon becomes fine and dispersion and its morphology becomes spherity instead of coarse platelike. These results can be explained by improving of uniformity of the melt and the changing of the solidification parameters.

Abstract: Inconel 718C is a nickel-base alloy that is difficult to machine. This paper presents a study of the influence of grain size in as-cast workpieces on cutting forces of Inconel 718C. The end milling tests were performed to understand the effect of various grain structures on machinability of nickel-base superalloy under wet condition using carbide insert. The collected data of cutting forces were analyzed using polynomial regression methods. The results show that grain refining of Inconel 718C can effectively decrease cutting force and improve the machinability of nickel-base superalloy.

Abstract: Directional solidification experiments on Pb-Bi peritectic alloys have been conducted at very low velocity (V=0.5 μm/s) and high thermal gradient (G=25 K/mm). Incomplete banded and oscillatory structures have been observed in both of hypoperitectic and hyperperitectic compositions over several millimeters of growth. These structures resulted from the repeated nucleation and competition between properitectic α- and peritectic β-phases. The banded or oscillatory structures are found to be transient and the final steady-state phase was only the peritectic β-phases. With an increase in composition, β phase formed and α phase disappeared at a lower solidified distance. Composition variations in the banded structure are measured to determine the solute distribution along the growth direction.