Processing and Microstructure of Melt Spun Niai Alloys
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PROCESSING AND MICROSTRUCTURE OF MELT SPUN NIAI ALLOYS I.E. Locci5 +, R.D. Noebe +, J.A. Moser +, D.S. Lee + and M. Nathal+, *Case Western Reserve University, Cleveland, OH,44106 +NASA Lewis Research Center,Cleveland, OH., 44135. ABSTRACT The influence of various melt spinning parameters and the effect of consolidation on the microstructure of melt spun NiAl and NiAl + W alloys have been examined by optical and electron microscopy techniques. It was found that the addition of 0.5 at.% W to NiAl results in a fine dispersion of W particles after melt spinning which effectively controls grain growth during annealing treatments or consolidation at temperatures between 1523 and 1723 K. Increased wheel speeds are effective at reducing both ribbon thickness and grain size, such that proper choice of both composition and casting parameters can produce structures with grain sizes as small as 2 [t m. Finally, fabrication of continuous fiber reinforced composites which used pulverized ribbon as the matrix material was demonstrated. INTRODUCTION Because of their excellent oxidation resistance [1], relatively low density and high melting point, alloys based on the binary NiA1 intermetallic system are of considerable interest as alternative high temperature structural materials. Even with these potential benefits, however, NiA1 alloys are not presently used in any structural application because of inadequate ductility and toughness at ambient temperature. The inability to adequately process nickel-aluminides was originally a problem in their early development as well [2,3]. Today more sophisticated and non-traditional processing techniques are available and being employed in the fabrication of these materials. In tandem with new processing techniques, macroalloying to promote a change in slip systems [4,5], composite fiber reinforcement [6], the use of second phases to promote additional uniform slip [7], and grain refinement [8] are all being examined as possible methods for improving the low temperature ductility or toughness of NiA1. The primary focus in the first stage of this ongoing investigation is to determine the effect of rapid solidification processing (RSP) via a chill block melt spinning process on the microstructure and properties of NiA1 and a NiAl + W alloy. The advantages of a rapid solidification process in the production of an alloy are primarily increased homogeneity, decreased grain size and the ability to uniformly distribute fine precipitates throughout the material. In this study, fine W precipitates were used to pin grain boundaries, nearly maintaining the original fine grain size of the rapidly solidified material even after elevated temperature exposure. The potential benefit of a fine grain size on the low temperature properties of a brittle intermetallic was first proposed by Schulson [9], who suggested that a critical grain size should exit in these materials, below which the stress necessary to nucleate cracks is less than the stress needed to propagate them. Schulson, et al.,[8] did observe a crit
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