Fatigue and Fracture of Nb 3 Al/Nb In Situ Composites Based on an Nb 18 at.% Al Alloy
- PDF / 3,875,399 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 19 Downloads / 209 Views
Fatigue and Fracture of Nb3 AI/Nb In Situ Composites Based on an Nb 18 at.% Al Alloy
D.L. Davidson, Southwest Research Institute, San Antonio, TX 78228 D.L. Anton, United Technologies Research Center, East Hartford, CT 06108
ABSTRACT Near Nb3 AI alloys, having Nb3 AI with the ordered A15 crystalographic structure within regions of Nb solid solution, can be considered as in-situ reinforced composites. Nb-18 at.%AI ingots were arc melted and heat treated to provide several microstructures. Small compact tension specimens were cracked in compression- compression cyclic loading and subsequently grown in tension-tension cyclic loading. Fatigue crack growth rates were found to be similar to crack growth through a TiAI-based alloy with a lamellar microstructure. Threshold stress intensity factor was estimated as 5.3 MPa4m, and fracture toughness as about 10 MPa'/m. INTRODUCTION The creep resistance of Nb3 AI (an A15 compound) is comparable to some nickel based superalloys at temperatures up to 11000C,1 making this material a candidate for gas turbine engines that would operate at temperatures well above the current maximum temperature of 1100 0 C. The physical metallurgy and slip characteristics of A15 compounds (Nb3 AI, V3 Si, Cr 3 Si) and near-Nb 3 AI compositions have been determined. 2 Elastic modulus, bend and yield strengthes, and elongation to fracture have recently been determined 3 and found to be within the range useful in turbine applications. Furthermore, Nb3 AI has been found to be more creep resistant than other intermetallic compounds melting above 14000C. 4 These results appear sufficiently promising that studies of possible alloying additions have been started 3 and a study of the phase stability of Nb-AI-Ti alloys is in progress. 5 A large government sponsored initiative is currently underway in Japan to refine processing and study the properties of Nb3 AI alloys. 6 The intermetallic Nb3 AI is very brittle at ambient temperature. The present study was undertaken to evaluate the fatigue and fracture toughness characteristics of a ductile phase toughened Nb3 AI alloy at ambient temperature. Through the introduction of Nb in solid solution, which is thermodynamically stable in this case, it is anticipated that both fatigue and fracture resistance can be significantly enhanced. DESCRIPTION OF MATERIALS AND EXPERIMENTAL PROCEDURES Several ingots of Nb-Al alloys were arc melted and cast in argon. Nominal composition of these alloys was Nb-18 at.%AI. The ingots, approximately 110 mm square by 8 mm thick, were then aged, cut into specimen blanks and examined for cracks and compositional uniformity. Small CT specimens (20 mm square, by - 5 mm thick) were removed from the remaining ingots by electric discharge machining. After polishing by typical metallographic preparation techniques, specimens were notched with a diamond saw to a depth (measured from the center line of the pin holes) of about 3 mm. Compression-compression (C-C) cyclic loading was used to initiate and grow a crack from the notch. The specimen w
Data Loading...
