Orthorhombic martensite ($\alpha^{\prime\prime}$, space group Cmcm) in $\beta$-stabilized Ti alloys gives rise to shape memory effect, superelasticity and tailorable thermal expansion. However, a unified understanding of the crystal structure of $\alpha^{\prime\prime}$ across different Ti alloys has not been reached until recently. Accordingly, not only has it so far been very difficult to assess the stability of $\alpha^{\prime\prime}$ martensite relative to austenite ($\beta$, bcc), but also unfeasible to reliably compare $\alpha^{\prime\prime}$ martensites across different alloy systems, inhibiting effective alloy design.

Here, we present a novel metric ($\delta$) that eliminates the structural ambiguity of $\alpha^{\prime\prime}$. By employing a straightforward yet highly instructive solid sphere model for the basic tetrahedral structural unit the crystal structures involved in the bcc - Cmcm - hcp martensitic transformation are categorized into several intermediate configurations. Beneficially, the new metric uniquely describes the current configuration of $\alpha^{\prime\prime}$ along the bcc - Cmcm - hcp transformation path independent of chemical composition. Further, the exclusive use of relative quantities to describe unit cell geometry and atom positions renders the here developed approach independent of alloy content.

Via the novel methodology $\delta$-contour plots are created to analyze and to compare the crystal structure of $\alpha^{\prime\prime}$ across a broad range of Ti alloys from experimentally measured lattice parameters. Systematic analysis of more than 100 alloy formulations from several alloying systems reveals that a large fraction of structural configurations along the bcc - Cmcm - hcp transformation path is not observed in quenched alloys. The threshold between missing and observed configurations is identified with an ideal Cmcm crystal structure, relative to which the experimentally found $\alpha^{\prime\prime}$ is compressed along its c-axis.