The effect of constraint-induced normal stress on the failure of notched TiAl components
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Introduction alloys are under consideration as lighter-weight replacements for nickel superalloys in some aircraft engine components. Their specific stiffness, E /, is approximately 50 pct higher than the materials they are intended to replace. Initial efforts directed toward transitioning titanium aluminides into production engines have focused on the low pressure region of commercial aircraft engines, where the temperature capabilities of existing TiAl alloys are not exceeded.[1] Limited tensile ductility and its role at stress concentrations is a significant concern for titanium aluminides. In unnotched tensile tests, plastic strains at failure are typically in the range of 0.3 to 1.8 pct. This is significantly lower than failure strains of 15 to 20 pct seen in nickel superalloys. In addition to low average failure strains, variability in tensile ductility is a concern. Occasionally, specimens are seen that experience macroscopic plastic strains at failure much less than 0.3 pct. Separating contributions to variability from extrinsic (e.g. handling and surface preparation) and intrinsic (e.g. microstructure and grain size) effects and using this knowledge to decrease the overall variability in ductility seen in specimens and components is a primary goal in developing TiAl alloys. The issues of intrinsic effects and the choice of casting parameters to control tensile ductility have been considered by Rishel et al.[2] and Raban et al.[3]
TiAl
JORGE G. MILKE, formerly Graduate Student, Department of Mechanical Engineering, Carnegie Mellon University, is Design Engineer, GE Medical Systems, Waukeska, WI 53188. JACK L. BEUTH, Professor, Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. NICHOLAS E. BIERY, formerly Graduate Student, Department of Mechanical Engineering, Carnegie Mellon University, is Research Engineer, ExxonMobile Upstream Research Company, Houston, TX 77252. HUANG TANG, Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted May 1, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
The role of limited ductility for TiAl components containing stress concentrators has been considered in previous work by the authors and coworkers.[4,5] This previous work addresses two issues for components subjected to monotonic loading: the amount of ductility needed in notched component design and notch strengthening. In Reference 4, it is demonstrated that plastic strains of 0.8 pct are large enough to effectively blunt elastic stress concentrations as large as 2.4 in notched TiAl components. For notched components with elastic stress concentrations of 2.4 or less, alloy development to increase plastic straining capability beyond this limit will not result in substantial increases in load-carrying capability. Room temperature tensile tests carried out by Knaul et al.[4] on a Ti-47.9Al-2.0Cr-2.0Nb alloy with a predominantly equiaxed gamma grain microstructure also suggest that a notch strengthening e
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