Infiltration and directional solidification of CMSX-4 through a particulate ceramic preform
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. INTRODUCTION
IN turbine engines, efficiency is often reduced due to gas leakage between the turbine blade tips and the engine shroud. One practice to reduce this leakage is the use of an abradable system, in which abrasive tips are attached to the turbine blades and the shroud is covered with a relatively soft ceramic. Early in the life of the engine, the blades cut into the shroud so that the gas flow past the blade tips can be reduced. In common practice, the abrasive tips are sintered cermet, in which particles of a ceramic (such as c-BN and alumina) are in a matrix of a superalloy similar to the blade alloy. The cermet typically is brazed onto the end of the blade,[1] although there are other methods (e.g., electroplating and plasma spray[2–5]) that are not yet commonly used in industry. The drawback of brazing is the weakness of the cermet/blade interface. In this research, a potential method for producing abrasive tips in-situ in the blade casting process was investigated. Particulate ceramic preforms were infiltrated by liquid superalloy under a gas pressure of about 1 atm. The alloy in the preform was directionally solidified, and microstructure development in the presence of the ceramic preform was studied. There have been a few studies on solidification of alloys in metal-matrix composites. Trivedi and Sekhar[6,7] investigated dendritic solidification in narrow channels between solid fibers or particles using the transparent-model organic system of succinonitrile-acetone. When the dendrite-tip radius (R) was much smaller than the channel-wall spacing (), the channel did not affect the growth of the dendrite. When R was similar to , the perturbation in the solute field due to the presence of fibers or particles changed the dendrite-tip radius, which can give rise to a dendrite-to-cell transition. Mortensen and Flemings[8,9]
RUI SHAO, Graduate Research Assistant, MATTHEW J.M. KRANE, Associate Professor, and KEVIN P. TRUMBLE, Professor, are with the School of Materials Engineering, Purdue University, West Lafayette, IN, 47907-2044. Contact e-mail: [email protected] Manuscript submitted June 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
studied the solidification of a hypoeutectic Al-4.5 pct Cu alloy within an Al2O3 fiber preform and observed no nucleation on the preform and reduced microsegregation in the channels, which were of a size similar to or smaller than the primary dendrite-arm spacing. Three mechanisms for the reduction of microsegregation were proposed and explained[9] with analytical models based on the Scheil equation. When R V , microsegregation can be reduced due to dendrite coarsening. When R is similar to or the primary dendrite-arm spacing is larger than , microsegregation can be reduced due to the dendrite-to-cell transition. At sufficiently low cooling rates, microsegregation can be reduced or eliminated by backdiffusion in the primary solid phase. In this research, the wetting, infiltration, and solidification of the single-crystal superalloy CMSX-4 in two different p
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