Masters Theses

Date of Award

12-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Ben F. Oliver

Committee Members

Charlie R. Brooks

Abstract

With the upper temperature limit of the Ni-based superalloys attained, a new class of materials is required. Intermetallics appear as likely candidates because of their attractive physical properties. With a relatively low density, high thermal conductivity, excellent oxidation resistance, high melting point, and simple crystal structure, nickel aluminide (NiAl) appears to be a potential candidate. However, NiAl is limited in structural applications due to its low room temperature fracture toughness and poor elevated temperature strength. One approach to improving these properties has been through the application of eutectic composites. Researchers have shown that containerless directional solidification of NiAl-based eutectic alloys can provide improvements in both the creep strength and fracture toughness.

Although these systems have shown improvements in the mechanical properties, the presence of refractory metals increases the density significantly in some alloys. Lower density systems, such as the carbides, nitrides, and borides, may provide NiAl- based eutectic structures. With little or no information available on these systems, experimental investigation is required.

The objective of this research was to locate and develop NiAl-carbide eutectic alloys. Exploratory arc-melts were performed in NiAl-refractory metal-C systems. Refractory metal systems investigated included Co, Cr, Fe, Hf, Mo, Nb, Ta, Ti, W, and Zr. Systems containing carbides with excellent stability (i.e., HfC, NbC, TaС, TiC, and ZrC) produced large blocky, cubic carbides in an NiAl matrix. The carbides appeared to have formed in the liquid state and were randomly distributed throughout the polycrystalline NiAl. The Co, Cr, Fe, Mo, and W systems contained NiAl dendrites with a two-phase interdendritic microconstituent present. Of these systems, the NiAl-Mo-C system had the most promising microstructural for in-situ composites.

Three processing techniques were used to evaluate the NiAl-Mo-C system: arc- melting, slow cooling, and containerless directional solidification. Arc-melting provided a wide range of compositions in an economical and simple fashion. The slow cooled ingots provided larger ingots and slower cooling rates than arc-melting. Directional solidification was used to produce in-situ composites consisting of NiAl reinforced with molybdenum carbides.

Dramatic changes in microstructures were observed for small compositional variations (<0.1 at.%) in the arc-melted ingots. Regions containing specific primary phases or two-phase microconstituents were identified. A unique structure to this system was recognized: the "spline". Basically a large, broken lamellar sheet surrounded by a sheath of NiAl, the spline was observed in arc-melts, slow cooled ingots, or spilled liquid during directional solidification. In fact, these splines, still surrounded by NiAl, remained after high temperature heat treatments. Heat treatments also revealed Widmanstatten-type precipitation in the slow cooled NiAl-1Mo-1C ingots.

The slow cooled ingots contained large regions of aligned carbides and graphite in an NiAl matrix. The existence of a broken lamellar eutectic in this system is possible. Morphology, solidification behavior, and volume fraction all support this theory but uncertainty remains. The Widmanstatten precipitation may also be confusing the results. Although MoC has been identified by x-ray diffraction as the reinforcing phase, it is possible that more than one carbide is present.

In-situ reaction of graphite with the liquid zone during containerless directional solidification proved unsuccessful. A graphite coating, with high emissivity, coated the surface of the liquid thus limiting induction melting. Two ingots, NiAl-1Mo-0.61C and NiAl-1Mo-0.73C were successfully directionally solidified. Although consisting primarily of polycrystalline NiAl with a fine dispersion carbides, regions near the end of processing contained aligned carbides. Directional solidification of NiAl-1Mo-1C ingots was impaired by the formation of a coating on the liquid zone. This coating is most likely graphite, but could be a carbide precipitating from the liquid.

Four-point flexure testing and creep compression testing were performed on the directionally solidified NiAl-Mo-C ingots. Fracture toughness values ranged from 10.4 to 13.5 MPa√m compared to 4 to 6 MPa√m for polycrystalline NiAl. Although much less than many of the other NiAl-based eutectic systems, the creep strength was also greater than that of single crystal NiAl. However, these mechanical properties do not represent the optimum morphology, volume fraction, or growth conditions for this system.

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