Effects of R-ratio on the fatigue crack growth of Nb-Si (ss) and Nb-10Si In Situ composites
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INTRODUCTION
Although superalloys are the current choice for hightemperature (1073 to 1273 K) structural applications in aircraft engine technology, the need for more efficient engine performance has necessitated a search for materials exhibiting lower densities, higher moduli, and higher temperature capabilities. Intermetallic alloys such as nickel aluminides and titanium aluminides have received significant attention for possible applications at temperatures below 1273 K;[1–5] however, for applications above 1273 K, attention has been given to other intermetallics such as refractory metal silicides.[6–16] One such intermetallic silicide that has received recent attention is Nb5Si3. In addition to its high melting temperature (Tmp 5 2753 K), Nb5Si3 also exhibits a lower density than nickel-base superalloys. Although Nb5Si3 is expected to exhibit high-temperature strength and creep resistance, the low fracture toughness (1 to 3 MPa=m),[10,11] ductility, and estimated fatigue threshold behavior (DKth , 3 MPa=m) at ambient temperatures would severely limit the usefulness of such a material. In order to improve upon the low-temperature fracture toughness of the Nb5Si3 intermetallic, the introduction of ductile reinforcements was proposed.[6,7,10,11] One method of introducing the ductile reinforcements was via in situ formation during alloy solidification. Since Nb5Si3 exhibits thermodynamic stability with Nb, as shown in the NbSi phase diagram[17] in Figure 1(a), the composites formed would consist of Nb3Si and a metallic Nb phase with Si in solid solution. Extrusion and heat treatments could then W.A ZINSSER, Jr., formerly Graduate Student, Department of Materials Science and Engineering, Case Western Reserve University, is with Cessna Aircraft Co., Wichita, KS 67226. J.J. LEWANDOWSKI, Professor, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. This article is based on a presentation made in the symposium ‘‘Fatigue and Creep of Composite Materials’’ presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
be performed to break up the composite microstructure and transform the Nb3Si to Nb5Si3, as described elsewhere.[18] While previous work on these composites was focused on the fracture toughness and R-curve behavior of the Nb10 at. pct Si composite,[15,19] as well as the toughness and cleavage fracture stress of the Nb-Si(ss) constituent, little information exists on their fatigue crack growth and threshold behavior. The purpose of this work was to characterize the fatigue crack growth behavior of a Nb-10 at. pct Si composite as well as a bulk alloy of Nb-Si(ss), which is the main toughening constituent present in the composite. Fatigue crack growth experiments were performed to determine the effects of DK and R ratio on the resultant fatigue curve, threshold, and fatigue fracture morphology. Qualitative fracture
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