Experimental Evidence of the Effect of a High Magnetic Field on the Stray Grains Formation in Cross-Section Change Regio
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oys have been widely used as turbine blade materials for advanced aero-engines and industrial gas turbines due to their excellent high temperature strength and creep strength. However, one of the major problems encountered during directional solidification is the formation of stray grains in the platform ends or shroud regions of turbine blade.[1] Since the random orientations of stray grains which may form high-angle boundaries, the mechanical properties would be decreased. The formation of stray grains in the platform region of turbine blade has been investigated by many scientific workers.[1–10] It is found that local sudden changes in the solidification conditions of cross-section change regions may lead to the occurrence of heterogeneous nucleation which forms stray grains. Therefore, many efforts have been made to eliminate the formation of stray grains in the platform regions of turbine blade, such as optimization of withdrawal velocity or thermal gradient.[1–7] However, this key problem still cannot be effectively solved. In recent years, high magnetic field has been widely used in solidification processing. It is reported that in the WEIDONG XUAN, Engineer, ZHONGMING REN, Professor, and CHUANJUN LI, Lecturer, are with the Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 21, 2014. Article published online February 10, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
case of paramagnetic pure aluminum and Al-Cu alloys, the nucleation temperature of primary phase decreased significantly with a high magnetic field up to 10 T.[11,12] For diamagnetic Bi, its nucleation temperature increased.[13,14] The results suggest that high magnetic field may suppress nucleation of grain during solidification of paramagnetic alloy. Therefore, it is much possible to utilize a high magnetic field to suppress the formation of stray grains in the platform regions during directional solidification of superalloy turbine blade which is paramagnetic. Unfortunately, up to date, little work has been done about the effect of a high magnetic field on stray grains formation in cross-section change region. Hence, the aim of present work is to investigate the effect of a high magnetic field on stray grains of superalloy DZ417G in the cross-section change region during directionally solidification. The chemical compositions of alloy DZ417G used in present work are Cr 8.96, Co 9.72, Mo 3.08, Al 5.41, Ti 4.50, B 0.015, V 0.86, Fe 0.23, C 0.18 (wt pct), and Ni as balance. The schematic illustration of the sample for directional solidification is shown in Figure 1(a). The single crystal seeds from superalloy PWA1483 were used to control the grain numbers and orientations of crystals. The chemical compositions of PWA1483 are Cr 12.2, Co 9.0, Mo 1.9, W 3.8, Al 3.6, Ti 4.2, Ta 5.0, C 0.07 (wt pct), and Ni as balance. The seeds did not affect the chemical composition of DZ417G after casting since their similar compositions. Similar technique
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