Single Crystal Processing of Intermetallics for Structural Applications
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SINGLE CRYSTAL PROCESSING OF INTERMETALLICS FOR STRUCTURAL APPLICATIONS
EDWARD H. GOLDMAN GE Aircraft Engines, Cincinnati, OH 45215 ABSTRACT
A number of techniques are available for making metals, non-metals, and intermetallic materials into high-purity single crystals. The most common of these for producing large crystals involve solidification from the melt. The high melting temperatures of most intermetallics of interest for structural applications result in the expected problems of achieving the required high temperatures and temperature gradients while containing the molten material in a chemically, thermally and mechanically stable environment. Processes which have produced intermetallic single crystals, and the materials which have been crystallized, are reviewed. The largest known single crystals of a high temperature intermetallic have been produced in alloys based on NiAl using a modified Bridgman-type directional solidification process, an evolution of the process commonly used to create large jet engine turbine airfoils in Ni-base superalloys. Issues related to processing are described, and the resultant solidification structures are compared with those typical of superalloys. Finally, the prospects for the various processes, and the advances required to push them toward more practical applications, are addressed. INTRODUCTION
The high melting temperature, low density, and good environmental resistance of certain intermetallic compounds make them attractive materials for high temperature (>2000F [11 00C]) applications, such as those in the hot section ofjet engines. The traditional barriers for use of intermetallics as structural materials have been low room temperature plasticity and low high temperature strength. Directional solidification is a tool that can help to overcome these barriers. In particular, high temperature mechanical properties such as creep resistance are generally grain boundary-limited; the absence of boundaries normal to the induced stress can significantly improve performance at temperatures approaching the melting point. This is the case for Ni-base superalloy turbine blades in jet engines, where equiaxed castings are replaced by the now-common columnar-grain (CG) and single crystal (SX) castings (Figure 1).
Figure 1. Turbine blades with equiaxed (left), columnar grain (middle), andsingle crystal (right) structures.Blades infront are hollow, with thin walls and serpentinecooling passages. (Photo courtesy of Howmet Corp.)
In addition, the absence of grain boundaries opens the door to alloying additions which may otherwise be impossible due to segregation at grain boundaries, or, as in the case of superalloys, may allow the removal of alloying elements which are no longer required for grain boundary enhancement and which reduce the melting point or otherwise compromise properties or processibility. In some intermetallic systems, the absence of grain boundaries may improve low temperature ductility and/or toughness. Mat. Res. Soc. Symp. Proc. Vol. 288. ©1993 Materials Research
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