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INTERMETALLIC/METALLIC POLYPHASE IN-SITU COMPOSITES D.R. JOHNSON, S.M. JOSLIN, B.F. OLIVER The University of Tennessee, Knoxville, TN 37996-2200 Materials Science and Engineering Dept. R.D. NOEBE AND J.D. WHITTENBERGER NASA-Lewis Research Center, Cleveland, OH 44135

ABSTRACT To evaluate various in-situ reinforcement schemes, a computer controlled containerless directional solidification system has been used to produce NiAl-based polyphase composites containing up to two intermetallic phases and at least one ductile phase. Systems evaluated include Ni-Al-Cr, Ni-Al-Mo, Ni-Al-V ternary systems that form NiAl/a-refractory metal eutectics and a three phase eutectic in the Ni-Al-Cr-Nb system. Initial screening of these in-situ composites has included morphological characterization, four point bend testing, temperature dependent yield strength evaluation and compressive creep testing. Occasional growth defects termed "banding" currently interrupt the continuity of these composite structures and limit the attainment of optimum properties. However, both the creep strength and toughness of NiAl were improved by in-situ reinforcement. INTRODUCTION While in-situ composites have been studied for some time (1,2], interest in them has been expressed for possible high temperature applications. The same is true of the B2 compound NiAl. However, limited low temperature toughness and poor elevated temperature strength render NiAl-based alloys inadequate as structural materials. These deficient properties may be simultaneously improved by reinforcing NiAl with "appropriate" second phases to form a composite material. This paper is part of an ongoing program to evaluate NiAlbased eutectics. These in-situ composites have been generated in a computer-automated, containerless levitation zone processor utilizing ultra-pure atmospheres. Thus, the possibility of contamination of the ingot from crucible materials is eliminated. This preliminary report includes data for NiAl based eutectics whose compositions are given in Table 1. Hypereutectic alloys NiAl-12Mo and NiAl-15Mo were also investigated with the intent to increase the volume fraction of metallic reinforcement. A number of these alloys were deliberately doped with Zr to improve the high temperature strength of the NiAl matrix. Ternary additions of Zr to binary NiAl have been shown to greatly improve the strength of the resulting alloy [3] as well as to getter impurities from the alloy. The existence and composition of a three phase eutectic was determined from a series of arc-melted buttons. The matrix phase of this eutectic, (Cr,Al)NbNi, was found to have a Laves or C14 crystal structure by X-ray analysis. Mechanical Mat. Res. Soc. Symp. Proc. Vol. 273. @1992 Materials Research Society

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properties are reported for all of these systems and compared to more conventionally processed NiAl alloys. Table 1:

NiAl-Based Eutectics Investigated

Alloy Composition (atomic %)

Morphology

NiAl-9Mo-O.lZr NiAl-12Mo-O.lZr NiAl-15Mo-O.lZr NiAl-33Cr-O.lZr NiAi-40V

Mo rods Mo rods Mo rods Cr