Doctoral Dissertations

Date of Award

5-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Ben F. Oliver

Committee Members

W. T. Becker, C. R. Brooks. A. J. Pedraza, C. T. Liu

Abstract

Many ordered intermetallic alloys possess an unique combination of properties malring them extremely attractive for high temperature applications. These advantageous properties include: high melting point, high thermal conductivity, low density, excellent oxidation resistance, and low material costs. However, the use of ordered intermetallic alloys for structural applications have been limited due to their brittleness at room temperature. Also, the elevated temperature strength of many intermetallic alloys such as NLAl is poor. The objective of this research was to improve both the room temperature fracture toughness and elevated strength of NiAl-based materials by producing in-situ composites from directional solidification of eutectic alloys. Directional solidification was performed by containerless processing techniques in a levitation zone refiner to minimize alloy contamination. Three classes of NiAl-based in-situ composites were examined. These were the NlAl-Cr eutectic system, the NiAl plus Laves phase eutectics, and ternary eutectic systems containing NiAl, a refractory metal phase, and a Laves phase. Room temperature fracture toughness of these materials was determined by four-point bend test. Preliminary creep behavior was determined by compression tests at elevated temperatures, 1200-1400 K. In-situ composites based on the NiAl-Cr eutectic system were successfully produced by containerless processing and evaluated. Molybdenum additions of 0.6 to 6 at. % were used to change the eutectic microstructure. The NiAl-Cr alloys had a fibrous microstructure while the NiAl-(Cr,Mo) alloys containing 1 at. % or more molybdenum exhibited a lamellar structure. The effect of eutectic morphology on both the room temperature fracture toughness and the 1300 K creep strength was investigated. The NiAl-28Cr-6Mo alloys exhibited the best creep resistance and additional creep testing was performed at 1200 and 1400 K. This NiAl-(Cr,Mo) eutectic displays a promising high temperature strength while still maintaining a reasonable room temperature fracture toughness when compared to other NiAl-based materials. The Laves phase NiAlTa was used to strengthen NiAl and very promising creep strengths were found for the directionally solidified NiAl-NiAlTa eutectic. The eutectic composition was found to be near NiAl-15.5Ta (at.%) and well aligned microstructures were produced at this composition. An off-eutectic composition of NiAl-14.5Ta was also processed. The off-eutectic composition resulted in microstructures consisting of NiAl dendrites surrounded by aligned eutectic regions. The room temperature toughness of these two phase alloys was similar to that of polycrystalline NiAl even with the presence of the brittle Laves phase NiAlTa. Evidence of a ternary peritectic reaction: NiAl+NiAlTa+liquid=Ni2AlTa was also found from cast microstructures of Ni-Al-Ta alloys. Polyphase in-situ composites were generated by directional solidification of ternary eutectics. This work was performed to discover if a balance of properties could be produced by combining the NiAl-Laves phase and the NiAl-refractory metal phase eutectics. The systems investigated were the Ni-Al-Ta-X (X=Cr, Mo, or V) alloys. Ternary eutectics were found in each of these systems and both the eutectic composition and temperature were determined. Of these ternary eutectics, the one in the NiAl-Ta-Cr system was found to be the most promising. The fracture toughness of the NiAl-(Cr,Al)NiTa-Cr eutectic was intermediate between those of the NiAl-NiAlTa eutectic and the NiAl-Cr eutectic. The creep strength of this ternary eutectic was similar to or greater than that of the NiAl-Cr eutectic.

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