Observations of crack tip process zones in cubic titanium trialuminide intermetallics

Abstract

Despite the fact that the single-phase L1 2-ordered titanium trialuminides, derived from D0 22-ordered Al 3Ti by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn have a cubic lattice structure, their room temperature fracture toughness remains quite low (4–5 MPa m 1/2). In general, process zones developed at the crack tips determine the fracture toughness of a material. In this work the results of the crack tip fracture studies of cubic (L1 2) Al 3Ti alloys stabilized with Mn are presented. The process zones at the crack tip in nearly stoichiometric single-phase L1 2 9Mn–25Ti (at.%) titanium trialuminides were not observed in most of the specimens studied. Occasionally, two types of process zones were observed: either small, heavily localized process-plastic zones accompanied by a crack tip “collapse”, or “pseudo-bifurcated” ones, reminiscent of those in brittle ceramics. Observations of the crack tip process zones in multiphase, high Ti (up to ∼33 at.%), B-doped trialuminides, exhibiting increased fracture toughness (∼7 MPa m 1/2), show the presence of secondary microcracks in the zone ahead of the crack tip and adjacent to the propagating crack, and more plasticity at the crack tip.