Tensile ductility of extrinsically toughened intermetallics
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INTRODUCTION
] N T E R M E T A L L I C - b a s e d alloys are attractive as future high-temperature structural materials because of their strength and creep resistance at elevated temperatures. I~ The tensile ductility and fracture toughness of these alloys are, however, generally low at ambient temperature. As a result, considerable efforts have been undertaken to develop means, which may involve new processing techniques, alloy additions, or both, for imparting tensile ductility and fracture toughness in brittle intermetallics. Techniques utilized to induce fracture toughness vary with individual intermetallic alloys; they include macroand microalloying t21 and compositing with a ductile phase, t31 Toughening mechanisms in intermetallic alloys can be considered either as intrinsic or extrinsic. 141 The former refers to processes, e.g., macro or microalloying, that improve the inherent fracture toughness of the material, while the latter refers to processes, e.g., crack bridging and deflection or microcracking, that improve the apparent fracture toughness by lowering the local driving force for fracture. Although both originate from the microstructure, delineation of intrinsic- and extrinsic-toughening mechanisms is desirable because of their different relationships and implications for other mechanical properties such as tensile ductility. Among others, extrinsic-toughening mechanisms include ductile-phase bridging 13-51 and shear ligament toughening. 161 The former is obtained by introducing a ductile phase in the intermetallic microstructure, while the latter, which is found in lamellar TiAl-alloys, is induced as the result of redundant deformation and fracture associated with ligaments that are formed between noncoplanar microcracks and the main crack. I~'7'~1 One of the characteristics of these two toughening mechanisms is that they lead to a resistance-curve fracture behavior. In both cases, the resistance-curve behavior is the consequence of the presence of intact ligaments in the crack wake that restrain crack surface opening due to the external load, thereby reducing the near-tip stress-intensity factor and increasing the apparent fracture toughness. K.S. CHAN, Staff Scientist, is with Southwesl Research Institute, San Antonio, TX 78228-0510. Manuscript received June 21, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
A class of intermetallic alloys that exhibits resistancecurve toughness is two-phase TiA1 alloys based on the lamellar a , + 3' microstucture. In these alloys, the K level at which a mode I plane-strain crack under tension starts to propagate is about 16 to 23 MPak/-m at 25 ~ 19-t-'j This critical K value, which corresponds to the onset of stable crack growth under monotonic loading and small-scale yielding, t~31 is designated as Ktc and often referred to as the initiation toughness, tj4'~Sj After initiation of crack growth, the stress-intensity level required to cause further crack growth usually increases with crack extension in the lamellar TiA1 alloys, leading to a fracture behavi
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