Mechanical properties of ru-ni-ai alloys
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Mechanical Properties of Ru-Ni-AI Alloys
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I.M. WOLFF and G. SAUTHOFF The Ru-Ni-AI system is significant for a number of reasons, not least of which is the interest in the ruthenium and nickel aluminides for high-temperature structural applications. The widely disparate properties of the B2 structured compounds NiA1 and RuA1, in particular, have enjoyed extensive study. Whereas various factors, including a limited number of slip systems, conspire to render NiA1 brittle at room temperature, Fleischer et al.V 5~ have drawn attention to the unusual room-temperature ductility and toughness that RuA1 exhibits in combination with high-temperature strength, specific stiffness, and oxidation resistance. Since alloys based on platinum group metals (PGMs) have apparent cost implications, some effort has been expended in seeking ways to replace some of the Ru without adversely affecting the intrinsic ductility. One approach is to seek some convergence in the properties of RuAI and its less ductile B2 counterparts by isostructural substitution for the Ru. t61 In this regard, the existence of a B2 NiA1 phase makes Ni an obvious candidate. Equally interesting is the possibility of exploiting the inherent ductility of RuA1 in composite microstructures. The incorporation of ductile phases into fine composite microstructures on the micrometer scale invokes a munber of synergies of interest to the exploitation of intermetallics as structural materials. Not only can such phases promote room-temperature toughness, they are also highly effective in developing strengths far in excess of that expected from simple rules of mixture. While the interfaces provided by these phases act as barriers to dislocation glide in the normal way,tvl other strengthening mechanisms can be inferred. These include a modified mechanical response, due to the constraint exerted on the phase by surrounding phases, I8,9~ and a dimensional effect (interphase spacing), below a critical value of which the interfaces contribute more to strengthening than do work-hardening effects within the limited glide dimensions of the phase.tto,~u Ductile phase strengthening finds particular tenability in eutectic structures, which can be engineered to exhibit very fine microstructures and have inherent phase compatibility. This is the subject of a parallel study,t121 in which select intermetallic composites based on Ru and Ir were found to exhibit greatly enhanced strengths, in some cases, in combination with good ductility, based on the controlled distribution of one or more ductile constituents. Although Ni is a logical substitute for Ru, Chakravorty and West tj31 have shown that continuous solid solution across the RuA1-NiA1 pseudobinary range is in fact inter-
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 ffir Eisenforschung GmbH, D-40074 Diisseldorf, Germany. Manuscript submitted June 30, 1995. METALLURGICAL AND MATERI
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