Strength and Toughness of Composite Materials Based on Nickel Aluminide Matrices
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STRENGTH AND TOUGHNESS OF COMPOSITE MATERIALS BASED ON NICKEL ALUMINIDE MATRICES J.D. RIGNEY, P.S. KHADKIKAR, J.J. LEWANDOWSKI and K. VEDULA Department of Materials Science and Engineering Case Western Reserve University Cleveland, OH 44106 ABSTRACT Several nickel aluminide matrix composites were prepared using vacuum hot pressing techniques. The matrix compositions, based on N3A1, Ni3AI+B and NiAl, were reinforced with 10 volume % TiB 2 particles. Both smooth and notched bend tests were conducted at room temperature on the monolithic as well as the reinforced materials in order to determine the effects of TiB 2 reinforcement on both the smooth bend and notched bend properties. TiB 2 additions were shown to improve the smooth bend strengths regardless of the matrix composition while notched bend tests, conducted to provide estimates of fracture toughness, revealed somewhat lower values for the composites in comparison to the monolithic materials. Fractographic analyses and in-situ fracture observations of the composites revealed that preferential fracture in regions of clustered TiB 2 particles may significantly affect the measured toughnesses. 1 INTRODUCTION Nickel aluminide intermetallics, Ni3A1 and NiAl, possess combinations of properties which are potentially useful for high temperature service. Although these materials have low densities and excellent oxidation resistance, they typically exhibit inadequate strength at high temperatures and brittleness at low temperatures. Doping with boron has been effectively used to overcome the low temperature brittleness of Ni3AI [1], whereas the brittleness of NiA1 is still largly unresolved except for recent studies of stoichiometric cast and extruded NiAl [2]. Of particular significance is that little is known about the fracture behavior of these materials under more severe stress states, although recent work [3,4] has begun to address these issues. The introduction of a reinforcement into an intermetallic matrix may offer improved high temperature strengths and low temperature toughnesses when compared to the monolithic materials. However, little is known regarding the behavior of an intermetallic matrix reinforced with hard, non-deforming constituents. The present work investigated the effects of reinforcement on low temperature deformation and toughness of a "brittle" matrix (e.g.NiAl, Ni 3A1) in comparison to a "ductile" matrix (e.g.Ni 3Al+B). 2 EXPERIMENTAL PROCEDURES The monolithic and composite materials were prepared using vacuum hot pressing techniques. The matrix materials were fabricated using prealloyed Ni3AI and NiAl powder (-80 +325 mesh) obtained from Alloy Metals, Inc., Troy, MI. Elemental boron powder was blended Mat. Res. Soc. Symp. Proc. Vol. 133.
1989 Materials Research Society
604
with Ni3 A1 to form the powder for the Ni3AI+B matrix using a twin shell blender. The powders used to manufacture the composites were prepared via blending of the matrix powders with 10 volume % TiB 2 powder (average size 5 gim) obtained from Union Carbide Corporation,
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