Microstructure of a rapidly-solidified Ni-base eutectic superalloy
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I.
INTRODUCTION
RAPIDsolidification of alloys frequently leads to the development of unique microstructures unobtainable by conventional casting. Some possible characteristics of these microstructures are submicron grains, ultrafine precipitates, and suppressed macroscopic solute segregation, l Recently, there has been much interest in using rapid solidification to produce novel structures in nickel-base superalloys. 2-6 Evidence exists that rapid solidification can enhance the mechanical properties of these alloys, improving properties such as hot forgeability, fatigue life, strength, and ductility. 7,s,9 The technique of melt spinning has been shown to be a particularly convenient method to investigate the rapid solidification behavior of Ni-base superalloys. 2~ This process produces the alloy in the form of a thin ribbon with constant thickness. Therefore, the microstructure is uniform along the ribbon length, in contrast with the situation for rapidlysolidified powders, where a range of powder sizes and hence microstructures are produced. In addition, the narrow thickness of the ribbon may make it possible to achieve a uniform microstructure across the ribbon thickness. In order to achieve maximum strength at the high temperatures where Ni-base superalloys are usually employed, it is desirable to have a fine dispersion of stable precipitates. Previous investigations have shown that fine carbide particles can be produced in Ni-base superalloys by rapid solidification. 2~ In an attempt to optimize the carbide particle distribution, a y-3,'-MC eutectic alloy ~~ which was originally developed for directional solidification was chosen for study in the present investigation. After directional solidification, this alloy consists of aligned Ta-rich MC carbide ERNEST L. HALL and SHYH-CHIN HUANG are Members of the Technical Staff at the General Electric Corporate Research and Devel9opment Center, P.O. Box 8, Schenectady, NY 12301. This paper is b a ~ d on a presentation made at the symposium "Physical Metallurgy of High Temperature Alloys" held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 3 and 4, 1983 under the TMS-AIME High Temperature Alloys Committee. Manuscript submitted July 5, 1984.
METALLURGICAL TRANSACTIONS A
rods in a y - y ' matrix. The MC phase is stable up to the melting point of the alloy and hence increases the temperature capability of this alloy. During conventional casting of this alloy, carbon would segregate to interfaces, causing blocky precipitation at grain boundaries and leading to suboptimal properties. Rapid solidification, on the other hand, has the potential of producing the desired distribution of fine carbides. This paper describes the microstructures produced by melt-spinning in a y - y ' - M C eutectic alloy. Analytical electron microscopy is used to determine the distribution, chemistry, and crystallography of the phases in the ribbon. The as-spun microstructure is studied in detail, and the effect of heat treatment on that microstructure is shown.
II.
EXPERIME
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