The role of parent austenite grain size on the variant selection and intervariant boundary network in a lath martensitic steel

This study investigated the influence of parent austenite grain refinement on the intervariant boundary network (population and connectivity) in a lath martensitic steel. Parent austenite grain refinement revealed a progressive reduction in the fraction of the 60 degrees misorientation boundaries in martensite, which was linked to a decrease in the 60 degrees/[110] intervariant boundary population. The phenomenological theory of martensite crystallography demonstrated that the variant selection mechanism altered from the 3-variant clustering (V1V3V5) in the coarse parent austenite towards the 4-variant clustering (V1V2V3V4) in the fine parent austenite grain, due to the change in the lattice parameter of the parent and daughter phase in which the martensite transformation occurs, as measured using in situ neutron diffraction. The change in the variant clustering arrangement with the parent austenite grain refinement led to a progressive promotion of 60 degrees/[111] and 10.5 degrees/[011] intervariant boundaries at the expense of 60 degrees/[110] martensite intervariant boundaries. Subsequently, the connectivity of low energy {110} tilt intervariant boundaries gradually increased through the refinement of parent austenite grain size, eventually reducing the high energy {110} twist boundary connectivity. This change improved the impact toughness of martensite produced from the fine-grained austenite as the weak connectivity of high energy boundaries delays the coalescence of voids, promoting ductile fracture.

Automated summary

This study investigated the influence of parent austenite grain refinement on the intervariant boundary network in a lath martensitic steel. It found that refining the parent austenite grain led to a decrease in the fraction of certain boundaries in the martensite and an increase in the connectivity of low energy boundaries, ultimately improving the impact toughness.