  A novel complex-phase steel alloy is conceived with a deliberately unstable
austenite, $\gamma$, phase that enables the deformation-induced martensitic
transformations (DIMT) to be explored at low levels of plastic strain. The DIMT
was thus explored, in-situ and non-destructively, using both far-field
Three-Dimensional X-Ray Diffraction (3DXRD) and Electron Back-Scatter
Diffraction (EBSD). Substantial $\alpha'$ martensite formation was observed
under 10% applied strain with EBSD, and many $\varepsilon$ grain formation
events were captured with 3DXRD, indicative of the indirect transformation of
martensite via the reaction $\gamma \rightarrow \varepsilon \rightarrow
\alpha'$. Using $\varepsilon$ grain formation as a direct measurement of
$\gamma$ grain stability, the influence of several microstructural properties,
such as grain size, orientation and neighbourhood configuration, on $\gamma$
stability have been identified. Larger $\gamma$ grains were found to be less
stable than smaller grains. Any $\gamma$ grains oriented with {100} parallel to
the loading direction preferentially transformed with lower stresses. Parent
$\varepsilon$-forming $\gamma$ grains possessed a neighbourhood with increased
ferritic/martensitic volume fraction. This finding shows, unambiguously, that
$\alpha$/$\alpha'$ promotes $\varepsilon$ formation in neighbouring grains. The
minimum strain work criterion model for $\varepsilon$ variant prediction was
also evaluated, which worked well for most grains. However,
$\varepsilon$-forming grains with a lower stress were less well predicted by
the model, indicating crystal-level behaviour must be considered for accurate
$\varepsilon$ formation. The findings from this work are considered key for the
future design of alloys where the deformation response can be controlled by
tailoring microstructure and local or macroscopic crystal orientations.
