High-temperature behavior of precious metal base composites
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I.
INTRODUCTION
A. Ductile-Phase Strengthening and Toughening STRATEGIES to ductilize or strengthen intermetallics usually involve alloying additions or an adjustment in composition to take advantage of secondary phases. This often leads to a trade-off in the inherent density, melting point, and high-temperature strength advantages of the base alloy. Eutectic systems therefore enjoy some potential as alloys for sustained high-temperature use in that they are thermodynamically stable, can be engineered to exhibit a high interface density, have inherent interphase compatibility (as opposed to metal matrix composites), and are often modifiable by alloying (for example, small additions of Mo, W, or V to NiA1-Cr change the eutectic morphology from round rods to faceted rods and platest~) or solidification processing to give directional structures.t2-6] Furthermore, eutectic structures can take advantage of strengthening or ductilizing phases intrinsic to the system and are particularly suitable vehicles for ductile phase toughening concerns. Ductile secondary phases have been shown to be effective in enhancing toughness in a number of intermetallic systems by acting as obstacles to growing cracks tT-~~ or acting as compliant interface layers,t',~21 Interestingly enough, there is also evidence to support the idea that such a ductile phase can contribute to strengthening at the phase boundaries. This extends from the observation that many two-phase systems exhibit strengths in excess of that predicted by a simple rule of mixtures. In coarse, two-phase systems, this has come to be associated in the first place with the constraint experienced by coexisting phases. In essence, such phases exhibit greater strengths and a modified deformation response (relative to when they exist in isolation) owing to the constraint imposed on deformation by the surrounding hard phase, effectively increasing their
I.M. WOLFF, Assistant Director, on leave from the Physical Metallurgy Division, Mintek, Randburg 2125, South Africa, and G. SAUTHOFF, Group Leader, are with the Max-Planck-Institut fiir Eisenforschung GmbH, D-40074 Dfisseldorf, Germany. Manuscript submitted June 29, 1995. 2642--VOLUME 27A, SEPTEMBER 1996
yield strength, t131Furthermore, soft particles embedded in a brittle intermetallic matrix can clearly strengthen the alloy in the same way as hard particles in a conventional disordered alloy, i.e., by acting as discontinuities in the path of dislocations (for example, Jung and Sauthofltt141). However, anomalous strengthening effects have also been associated with a critical interphase dimension. Kl6wer and Sauthof~3,4] have found that under conditions of creep, there exists an interlamellar spacing in (y + /3) Ni-Fe-A1 alloys below which the interfaces contribute more to the composite strength than do the constituent phases. This can be understood in terms of threshold stresses associated with strain incompatibilities at the interfaces. Eutectics have the advantage of allowing the attainment of such structures on a microstruct
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