Advancement of the Directional Solidification process of a NiAl-Alloy
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Advancement of the Directional Solidification process of a NiAl-Alloy
F. Scheppe, I. Wagner and P.R. Sahm RWTH Aachen, Giesserei-Institut, Intzestr. 5, D-52056 Aachen Germany ABSTRACT The directional solidification process of high temperature intermetallic alloys was investigated and discussed for a hypo eutectic NiAl-Cr-alloy. An unexpected solidification behavior was found which may be peculiar to the selected alloy group. The attempts show that due to the required high furnace temperatures only a narrow range of tolerance exists, in which the variation of the process parameters led to directional solidification. Aligned primary NiAl dendrites were then observed, embedded in randomly oriented interdendritic lamellas as known from conventional equiaxed castings. The directional solidification process was basically evaluated for high temperature intermetallics.
INTRODUCTION In order to improve the efficiency of modern gas turbines for energy transformation with simultaneously decreasing ecological damage, higher material demands are inevitable [1]. Intermetallic compounds such as NiAl offer new opportunities for developing low density, highstrength structural alloys with higher temperature capability when compared to conventional Tiand Ni-base alloys. The advantages of NiAl-base intermetallics are the high melting points of up to 1650 °C which are about 100 K to 250 K higher than these of the conventional Ni-base superalloys, a thermal conductivity of about four times than that of Ni-base alloys as well as an excellent oxidation and hot corrosion resistance. Furthermore, these materials exhibit a density of 6.20 g g/cm3 to 6.35 g/cm3 which is approximately 75% the density of state of the art superalloys and high temperature strength is provided above the ductile-to-brittle transition temperature of 900°C to 1000°C. In contrast to Ni-base alloys NiAl-base intermetallics exhibit an excellent microstructural stability without coarsening or dissolution of second phases like Ni3Al at temperatures up to 1350°C. The high strength, however, is usually associated with low ductility at room temperature [3]. This requires a special adjustment of the mechanical machining due to the high hardness. Intermetallic NiAl-base compounds like NiAl-Ta-Cr [5] are subject of an ongoing development for low density, high strength structural alloys with additional second phase strengthening for applications in gas turbines. Strong bonding between aluminum and nickel, which persists at elevated temperatures yields excellent high temperature properties with specific strength competitive to superalloys and ceramics. Thus, these alloys offer new opportunities for the application in gas turbines at temperatures higher than currently possible with conventional Nibase superalloys [2]. At the foundry institute of RWTH Aachen the casting technology of NiAl-base intermetallics has been developed and optimized in the last years for different applications. High quality N5.7.1
components for land based gas turbines can be produced reliably by an adap
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