Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy

TL;DR

This study investigates dislocation interactions at crack nucleation sites in a fatigued near-alpha titanium alloy, revealing mechanisms involving pile-ups, phase boundaries, and dislocation transmission that influence crack initiation.

Contribution

It provides new insights into the dislocation mechanisms and phase boundary interactions responsible for crack nucleation in near-alpha titanium alloys under fatigue.

Findings

Dislocation pile-ups occur at alpha/phase boundaries during fatigue.

Crack nucleation is linked to subsurface double-ended pile-up mechanisms.

Phase regions allow dislocation transmission, affecting crack initiation.

Abstract

Dislocation interactions at the crack nucleation site were investigated in near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic plastic strain in the alloy resulted in dislocation pile-ups in the primary alpha grains, nucleated at the boundaries between the primary alpha and the two-phase regions. These two phase regions provided a barrier to slip transfer between primary alpha grains. We suggest that crack nucleation occurred near the basal plane of primary alpha grains by the subsurface double-ended pile-up mechanism first conceived by Tanaka and Mura. Superjogs on the basal <a> dislocations were observed near the crack nucleation location. The two phase regions showed direct transmission of a3 dislocations between secondary alpha plates, transmitted through the beta ligaments as a[010], which then decompose into (a/2)<111> dislocation networks in the beta. The beta ligaments themselves do not appear to form an especially impenetrable barrier to slip, in agreement with the micropillar and crystal plasticity investigations of Zhang et al.