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

Abstract

Ambient-temperature fracture toughness and fatigue crack propagation behavior are investigated in a wide range of (γ+α 2) TiAl microstructures, including single-phase γ, duplex, coarse lamellar (1 to 2 mm colony size (D) and 2.0 µm lamellar spacing (λ)), fine lamellar (D ∼ 150 µm, λ=1.3 to 2.0 µm), and a powder metallurgy (P/M) lamellar microstructure (D=65 µm, λ=0.2 µm). The influences of colony size, lamellar spacing, and volume fraction of equiaxed γ grains are analyzed in terms of their effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth properties, while duplex and single-phase γ microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth. These mechanisms primarily involve cracktip shielding processes and include crack closure and uncracked ligament bridging. However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<300 µm) cracks, the distinction in the fatigue crack growth resistance of the lamellar and duplex microstructures becomes far less significant.