Orientation dependence of fracture behavior of Ti 3Al single crystals with D0 19 structure

Abstract

γ-based titanium aluminides exhibit remarkable mechanical properties at high temperatures and are deployed in jet engines. The microstructure of these alloys, consisting of alternating plates of the γ (TiAl) and α2 (Ti3Al) phases, exhibits highly anisotropic mechanical behavior. However, deep understanding of the associated micromechanics is lacking. The effect of differential strength of the slip/twin systems, structural length scales and temperature in determining the homogeneity of deformation at the intercolony and intracolony level has not been well understood. Hence, we establish structure–property relationships, under uniaxial compression and uniaxial tension, by crystal plasticity finite element techniques. The deviation from Hall–Petch behavior and the temperature-dependent yield stress anomaly are rationalized and included in the model. Our results reveal a complex interplay between lamellar orientation, structural length scale, temperature, and stress state. Key findings include: (1) Basal slip in the α2 phase can be activated by differential strengthening of slip systems at lamellar interfaces. Basal slip increases the propensity for crack nucleation due to coarse slip bands. (2) There is preferential activation of twinning in the γ phase under tension. These twins can improve ductility by increasing strain hardening. However, they can also promote crack nucleation. (3) Smaller lamellar widths at higher temperatures can improve strain homogeneity between the γ and α2 phases. (4) Yield stress anisotropy exhibited a non-monotonic variation with lamellar size and temperature. (5) Increasing the strength of slip/twin systems through solid solution or precipitation strengthening can improve intracolony deformation homogeneity, reducing crack nucleation. We suggest strategies for alloy and microstructure design to improve damage tolerance in these alloys.