The effect of microstructure on fracture toughness and fatigue crack growth behavior in γ-titanium aluminide based intermetallics

摘要

Ambient-temperature fracture toughnessand fatigue crack propagation behavior areinvestigated in a wide range of (γ+α2) TiAlmicrostructures, including single-phase γ, duplex,coarse lamellar (1 to 2 mm colony size (D) and 2.0μm lamellar spacing(λ)), fine lamellar (D-150μm,λ=1.3 to 2.0μm), and a powder metallurgy (P/M)lamellar microstructure(D=65μm, λ=0.2μm). Theinfluences of colony size, lamellar spacing, andvolume fraction of equiaxed γ grains are analyzedin terms of their effects on resistance to thegrowth of large (>5mm) cracks. Specifically,coarse lamellar microstructures are found toexhibit the best cyclic and monotonic crack-growthproperties, while duplex and single-phase γmicrostructures exhibit the worst, trends whichare rationalized in terms of the salientmicromechanisms affecting growth. These mechanismsprimarily involve crack-tip shielding processesand include crack closure and uncracked ligamentbridging. However, since the potency of thesemechanisms is severely restricted for cracks withlimited wake, in the presence of small 9<300μm)cracks, the distinction in the fatigue crackgrowth resistance of the lamellar and duplexmicrostructures becomes far less significant.