Compressive creep behavior of spray-formed gamma titanium aluminide
- PDF / 898,153 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 340 Views
I.
INTRODUCTION
IN recent years, gamma titanium aluminides (TiAl), both single phase (g) and two phase (a2 1 g), have attracted considerable interest as a result of their attractive combination of properties, which include low density, good elevated-temperature strength, and excellent high-temperature creep properties.[1,2] Notably, two-phase (a2 1 g) gamma titanium aluminides with the composition Ti-(47 to 49)Al (atomic percentage is used herein unless noted otherwise) have a particularly high potential of widespread application in elevated temperature structures as a result of their ductility and fracture toughness characteristics.[3] Two-phase gamma titanium aluminides typically exhibit two types of microstructures: fully lamellar (a2 1 g) and duplex (primary g 1 lamellar). Fully lamellar (FL) microstructures generally exhibit higher fracture toughness and creep resistance relative to those of duplex microstructures. However, while the ductility of duplex microstructures may be improved to be as high as 4 pct through tertiary alloy additions, FL microstructures generally exhibit poor ductility and low room-temperature (RT) strength.[2,3,4] Accordingly, it is of interest to understand how to modify the microstructure of g-TiAl with the ultimate objective of raising the strength and the elongation levels of FL microstructures without sacrificing fracture toughness and creep resistance. For FL g-TiAl, there are two major microstructural variables that may influence the mechanical properties: (1) grain size and (2) a2/g interlamellar spacing. Inspection of available literature reveals that a good combination of mechanical properties in g-TiAl may be achieved through decreasing the grain size and the interlamellar spacing in a FL microstructure. For example, it has been reported that RT strength and ductility of a FL microstructure increase with decreasing grain size,[2,3] and fracture toughness inB. LI, Graduate Student, J. WOLFENSTINE, Assistant Professor, J.C. EARTHMAN, Associate Professor, and E.J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Manuscript submitted March 4, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
creases with decreasing interlamellar spacing.[5] The effect of grain size on fracture toughness was found to be complex.[5] In general, fracture toughness increases with grain size, but may become independent of grain size when the crack-tip plastic zone is embedded within an individual colony.[5] For creep resistance of FL g-TiAl, it was reported that creep rate at elevated temperatures, where dislocation climb is the dominant mechanism, is independent of grain size.[6] Instead, interlamellar spacing appears to be the controlling factor.[6] Unfortunately, the grain sizes of FL microstructures that are obtainable through conventional casting and heat treatment methods are generally coarse (≥500 mm), in contrast with the very fine grain size of duplex microstructures (10 to 40 mm).[2,
Data Loading...
