  The onset of plasticity in quenched martensitic microstructures is
characterized by a low initial yield stress followed by an extremely strong
initial hardening response, and then a sudden hardening saturation. Literature
attributes this behavior to residual stresses and dislocations inherited from
the martensitic transformation, or to microstructural heterogeneities causing
strength differences among the grains. Here, we argue that
orientation-dependent yielding of lath martensite due to inter-lath sliding,
which induces a substructure boundary sliding mechanism, may also contribute
significantly to the observed behavior. To demonstrate this, we systematically
study its quantitative contribution to the elasto-plastic transition behavior
in a numerical microstructural model. In the simulations, we employ an
effective laminate model for the martensite packets which takes into account
the yielding anisotropy due to sliding along the packet's habit plane
orientation. To account for the effect of carbon content, martensitic
microstructures with different levels of lath strength are considered. It is
shown that the martensite packets with a favorable habit plane orientation
start to yield earlier compared to those with an unfavorable orientation, which
initially remain elastic. As a consequence, the macro-scale response of the
microstructures exhibits a low yield stress, followed by a significant degree
of initial hardening which continues until the saturation stress level is
approached. The apparent work hardening rate depends on the contrast between
the in-habit plane and out-of-habit plane yield strength used in the model.
