Ductile-phase toughening in V-V 3 Si in situ composites

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TRODUCTION

STRUCTURAL applications of many monolithic intermetallic compounds are limited by their low intrinsic fracture toughness. Therefore, ductile-phase-toughened intermetallic composites have been examined widely in recent years as an approach to produce intermetallic-based composites with substantially improved fracture toughness. Such composites have been generated using both extrinsic[1– 4] and in situ[5,6,7] methods. The in situ approach, in which the ductile phase dispersion is generated on solidification, is the focus of the present study because of its potential for relatively low cost production of components. Further, in situ composites are intrinsically thermodynamically stable.[7] However, the extent to which the microstructures can be tailored is more limited for in situ composites than for extrinsic ones. Solidification conditions and alloy composition are two primary variables through which in situ composite microstructures can be optimized. The V-Si system was chosen as a model for studying ductile-phase-toughened in situ composites.[8,9] This system was chosen rather than the more technologically important and well-studied Nb-Si system, because the Nb-Si phase diagram is relatively complex, resulting in complex microstructures.[7,10] In contrast, the V-Si phase diagram[11] contains a simple eutectic between the V3Si intermetallic and the terminal metallic solid solution, and this allows the generation of simple composite microstructures. Specifically, the eutectic reaction occurs at approximately 1870 7C and 7.6 wt pct Si (all compositions given in this article are in weight percent). At the eutectic temperature, the bcc V-Si solid solution, denoted (V), contains 4 pct Si, and the V3Si G.A. HENSHALL and M.J. STRUM, formerly with the Lawrence Livermore National Laboratory, Livermore, CA 94551, are with the Hewlett-Packard Company, Palo Alto, CA 94304. B.P. BEWLAY and J.A. SUTLIFF are with the General Electric Company, Schenectady, NY 12301. Manuscript submitted September 30, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

(A15 structure), which also exists as a solid solution and is hereafter denoted (V3Si), has a composition of 11 pct Si. The phase concentrations change as the temperature decreases from the eutectic, but below about 1400 7C, the compositions of the two solid solutions remain nearly constant at 2.7 pct Si for the (V) and 13.1 pct Si for (V3Si). V-V3Si composites were synthesized by (1) conventional arc melting (AM), (2) cold-crucible induction melting (IM), and (3) cold-crucible directional solidification (DS). Through the use of these different casting methods, and by varying the alloy Si concentrations, composites were produced with a wide range of microstructures, interstitial impurity concentrations, and volume fractions of the ductile (V) phase. The fracture toughness of the V-V3Si composites is discussed in relation to current theories of ductile-phase toughening, for which crack bridging is the most prominent assumption. The toughness enhancement provided by ductile-pha

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