A general model for the crystal structure of orthorhombic martensite in Ti alloys

The present work develops a novel unified approach to describe the crystal structure of orthorhombic martensite (α′′) in Ti alloys independent of chemical composition. By employing a straightforward yet highly instructive solid sphere model for the basic tetrahedral structural unit the crystal structures involved in the β ↔ α′′/α′ martensitic transformation are categorized into several intermediate configurations. Importantly, a new metric is introduced, δ, which unambiguously characterizes the atomic positions inside the orthorhombic unit cell depending on unit-cell geometry. Furthermore, the exclusive use of relative quantities to describe unit-cell geometry and atom positions renders the approach developed herein independent of alloy content. In this way, shortcomings of commonly suggested structural metrics for α′′ are eliminated. Subsequently, the novel methodology is applied to analyse and compare the crystal structure of α′′ across a broad range of Ti alloys based on experimentally measured unit-cell parameters. From this analysis it emerges that a large fraction of structural configurations along the b.c.c.–Cmcm–h.c.p. transformation path is not observed in quenched alloys. The threshold between the not-observed and the remaining well observed configurations is identified with an ideal Cmcm crystal structure, relative to which the experimentally found α′′ is compressed along its c axis.