Influence of solidification rate on precipitation and microstructure of directional solidification IN792 + Hf superalloy
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Influence of solidification rate on precipitation and microstructure of directional solidification IN792 + Hf superalloy W.R. Sun Department of Superalloys and Special Castings, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110015, People’s Republic of China
Z.Q. Hu State Key Laboratory of Rapidly Solidified Nonequilibrium Alloy, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110015, People’s Republic of China
J.H. Lee Department of Metallurgy and Materials Science, Changwon National University, 9 Sarim-Dong, Changwon, Kyungnam, 641-773, Korea
S.M. Ceo and S.J. Choe High Temperature Materials Laboratory, Korea Institute of Machinery and Materials, 66 Sangnam Dong, Changwon, Kyungnam, 641-010, Korea (Received 9 November 1998; accepted 19 July 1999)
The effect of solidification rate on the precipitation and microstructure of directional solidification IN792 + Hf alloy was studied. The solidification sequence and the initial precipitation temperature of different phases were determined by the observation of the quenched microstructure combined with the differential thermal analysis measurement. The script carbide was turned into faceted carbide with the drop of solidification rate. It was concluded by microstructure analysis that the faceted carbide was pushed by the ␥ solid front before it was captured. The incorporation of ␥ phase into the faceted carbide was due to the dendrite growth of the carbide toward one point and the mergence of the dendrites. Some long carbide bars were formed along the grain boundaries by continual reaction of eutectic (␥ + MC carbide) at a solidification rate of 0.5 m/s. Two zones, the ␥⬘ forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic ␥/␥⬘ nucleated in the ␥⬘ forming elements enriched zone. The -phase precipitation was controlled by the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The growth of eutectic ␥/␥⬘ increased the ratio and induced the -phase precipitation. A lower solidification rate decreased the ratio by sufficient diffusion and hence efficiently suppressed the -phase precipitation.
I. INTRODUCTION
With the application of the directional solidification (DS) technology, the mechanical properties of superalloys have been significantly improved because of elimination of the transverse grain boundaries normally perpendicular to the applied stress.1 In addition, the DS plus quenching supplies an elegant method for studying the solidification sequence of the alloys. The microstructure of Ni-base superalloy is generally made up of the ␥ matrix, ␥⬘ phase precipitated coherently in the ␥ matrix, carbides, and eutectic ␥/␥⬘. Some harmful phases such as the phase and the phase tend to precipitate in the alloys when the composition or the solidification conditions are not controlled properly.2 The microstructure of the DS superalloys generally depend on the composiJ. Mater. Res., Vol. 14, No. 10, Oc
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