Application of a modified jogged-screw model for creep of TiAl and α-Ti alloys

Abstract

Stress exponents for creep, in the range of 5, are typically associated with dislocation creep processes, normally associated with a strong tendency for subgrain formation. In this article, we will demonstrate that there are several important alloy systems that have similar stress dependence and, yet, lack this tendency for subgrain formation. Specifically, dislocations in the intermetallic compound γ-TiAl and the hexagonal close-packed (hcp) α phase of the commercial Ti alloy, Ti-6242, tend to be homogeneously distributed with a tendency for alignment along screw orientation. In both alloy systems, the screw dislocations exhibit a large density of pinning points, which detailed transmission electron microscopy (TEM) investigation indicate are locations of tall jogs. These observations suggest that the jogged-screw model for creep should be appropriate after suitable modification for the presence of these tall jogs. This modified jogged-screw (MJS) model is presented, together with a discussion of the assumptions made, and the results of this model are shown to compare favorably with experiment for both alloy systems. The possible criteria for the formation of tall jogs are also described, and the potential application of this modified model to other alloy systems is discussed.