Processing and mechanical properties of AI 2 O 3 fiber-reinforced NiAl composites
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I.
INTRODUCTION
IN recent years, vast resources have been directed toward the development of structural materials with use temperatures significantly above those of current generation superalloys. Gradual evolution of proven materials is a low-risk approach for achieving increased material performance but offers only limited improvements in temperature capabilities. Conversely, while the low density and high melting temperatures of intermetallic compounds offer substantially higher use temperatures, development of these systems presents a proportionally greater challenge. Of the numerous intermetallics under consideration, NiAl is especially attractive because of its combination of low density (5.9 g/cm3), high melting temperature (1910 K), high thermal conductivity, and excellent oxidation resistance, m Although the potential of NiA1 is intriguing, its viability is in doubt because of its low elevated-temperature strength and poor ambient-temperature toughness. While microalloying has proven successful in improving hightemperature properties, these improvements have been possible only at the expense of toughness, f21 Similarly, increased low-temperature toughness has been achieved only with a concomitant decrease in flow strength. An alternative approach for increasing both strength and damage tolerance is through the use of composites. Most NiA1 composites investigated thus far can be categorized as follows: (1) in situ composites containing a continuous, ductile, second phase, such a s Ni3A1,131 Mo, 141 or Cr; tSj (2) discontinuously reinforced composites, where the NiA1 matrix is strengthened by a fine dispersion of either particulates (such as TiB2, t61 A1N, tTI or ZrO:l% or A1203 whiskers; 191 and (3) continuous-fiber-reinforced R.R. B O W M A N and S.M. ARNOLD, Research Engineers, are with the NASA Lewis Research Center, Cleveland, OH 44135. A.K. MISRA, Research Engineer, is with NYMA Inc., LeRC Group, Brookpart, OH 44142. Manuscript submitted March 17, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
NiAl.im ~3] One impetus for the current study has been the interest in continuous-fiber reinforced NiA1 as a potential material for the High-Speed Civil Transport (HSCT) program, ll4] Any potential reinforcing fiber must possess hightemperature strength and be compatible with the matrix. The compatibility requirement refers to both mechanical as well as chemical compatibility. Mechanical compatibility is achieved by having similar coefficients of thermal expansion (CTE), thereby minimizing thermally generated stresses in both the matrix and fiber. Elimination of these stresses is of paramount importance in NiAl-base systems (CTEj200 K = 15 x 10-/K[15~__be cause of NiAl's low fracture toughness ( - 5 MPaX/m t~6]) and limited failure strain. Chemical compatibility is required for long-term stability of the composite system under a variety of thermal histories and environments. Additionally, the fiber must be readily available in quantities sufficient for developmental studies. At present, single crystal A1203
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