Mechanical evaluation of FP alumina reinforced NiAl Composites
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MECHANICAL EVALUATION OF FP ALUMINA REINFORCED NiA! COMPOSITES D.L. Anton* and D.M. Shah** * United Technologies Research Center, E. Hartford, CT 06084 ** Pratt & Whitney, E. Hartford, CT 06084 ABSTRACT NiAl is well known for its low density, superb oxidation resistance and low ductile to brittle transition temperature. It is equally renowned for its low room temperature fracture strength and poor high temperature creep strength. A compositing approach has been used to introduce chopped and aligned FP AI 2O3 into a matrix of fine grain NiAl via a powder metallurgical approach. This resulted in composites with approximately 40 vol. per cent undamaged alumina reinforcement. Fiber orientation effects on strength have also been characterized. Room temperature tests resulted in yield strength increases of 425% for chopped FP reinforcements and 800% for aligned composites. Elevated temperature tests conducted at 1200*C were even more dramatic in their strength increment with 200% and 3600% increases respectively. Fractographic results show matrix ductility, fiber-matrix decohesion and minimal fiber pull-out. INTRODUCTION For high melting point intermetallics to become commercially viable as structural materials, their well known lack of low temperature ductility and fracture toughness will need to be addressed. One method of imparting damage tolerance into brittle ceramics is by incorporating strong fibers into them. These reinforcements, typically SiC, Si3N4 or A120,, impart damage tolerance through a fiber pull-out mechanism [1,2]. In addition to damage tolerance, NiA1 which has excellent oxidation resistance and a leading candidate for gas turbine application, also will need to be strengthened at high temperatures where its creep resistance has been shown to be very poor [3]. Thus the introduction of strong fibers into an NiAl matrix can lead to both enhanced damage tolerance as well as increased high temperature strength. NiAl was also chosen as an ideal matrix candidate because a great deal of information is available in the literature on its ambient and high temperature properties. A recent study [4] of W(218) and Saphicon® fiber reinforced NiAl illustrated the effects of composite ply geometry on the strength of large fiber diameter, 250Mm, composites. Much of this effect stemmed from inter-fiber defects in the matrix. Alumina fibers were chose as the preferred reinforcing phase because of their chemical stability [5] and commercial availability. Specifically, DuPont's FP1 alumina fibers were used because of their fine diameter, approximately 12pm, which would allow them to be incorporated easily into thin wall structures. As such, two reinforcement geometries were studied in this system. Specifically, aligned FP fibers and chopped and randomly aligned FP fibers. In this study we will conform to the following convention for referring to the composites: Matrix/Reinforcement, where the reinforcement will be designated AFP for Aligned ER and CFP for Chopped aP. EXPERIMENTAL PROCEDURES The NiAl powder was received fro
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