Pressure Casting of Ni 3 Al/Al 2 O 3 Composites
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PRESSURE CASTING OF Ni3AVAi20
3
COMPOSITES
Said Nourbakhsh, Fei-Lin Liang and Harold Margolin Department of Metallurgy and Materials Science, Polytechnic University, 333 Jay Street, Brooklyn, NY 11201 ABSTRACT A series of Du Pont PRD-166 and FP fiber reinforced Ni and nickel aluminide matrix composites, containing Ti or Y as a wetting agent, were produced by pressure casting. Modulus values determined under compression varied from 273 to 287 GPa in a direction parallel to the fibers and ranged from 148 to 159 in a direction normal to the fibers. Modulus values parallel to the fiber direction were approximately 5% lower than that predicted by the rule-of-mixtures. The much lower values of modulus normal to the fibers has been attributed to a number of sources which include fiber/matrix and fiber/fiber debonding and stress concentration resulting from voids existing at fiber/fiber bonds. It was observed, qualitatively, that good bonding between matrix and fiber occurred when Ti diffused from the matrix into fibers. The presence of Al in the matrix tended to inhibit Ti diffusion, increasingly as Al content increased. Thus fiber/matrix bond strength was poorer in the aluminides than in an alloy containing no Al. Y was not found to improve bonding, and an interface reaction zone was obserced in some areas where liquid metal initially entered the fiber preform. INTRODUCTION There is considerable interest in developing high temperature metal-matrix composites in order to extend the operating temperature beyond current limitations of Ni based superalloys. Powder metallurgy approaches to the development of Ni 3AI matrix composites have recently been described [1,21. These efforts have been confined to the use of particulate rather than continuous fibers. Pressure casting offers an opportunity to fabricate continuous fiber reinforced metal-matrix composites. Extensive experiments have been carried out with Al and Mg alloys reinforced by A120 3 fibers [3-51. Recently, an apparatus has been constructed which permits high temperature Ni based metal-matrix composites to be cast under pressure [6,7]. Briefly, the process is as follows. Ni or Ni 3AI is melted, under vacuum, in a crucible which is situated above the fibers which are heated simultaneously but separately from the metal. When the molten metal and fibers reach the desired temperatures, 100 'C above the melting temperature of the matrix, the liquid metal is poured onto the fibers. At this point argon gas is admitted until a pressure of 3 MPa is reached. The gas pressure forces the melt, alloyed with Ti or Y to facilitate wetting, to infiltrate the fibers.
EXPERIMENTAL PROCEDURE Materials- Four separate matrix compositions and two different fiber types were used in these studies. These compositions (wt%) and fibers are given in Table I. The matrices of Composites A(B) and C in the absence of wetting additions, Ti or Y, are commercial materials known as IC50 and IC218, respectively. They are both single phase nickel aluminides having an L1 2 crystal structure. Table I
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