Elastic accommodation and anomalous tetragonality of thin-plate martensite in an Fe-31Ni-10Co-3Ti alloy revealed by in situ neutron diffraction at cryogenic temperatures

In this study, in situ neutron diffraction measurements during cryogenic cooling were performed on Fe-31Ni-10Co-3Ti alloy to evaluate changes in the austenite lattice constant and the tetragonality of martensite during the formation of thin-plate martensite. The average lattice constant of austenite changed only slightly during cooling owing to the Invar effect; however, it decreased below the Ms temperature (101 K<Ms<110 K), slightly deviating from the value expected from thermal contraction. An increase in full width at half maximum (FWHM) of austenite peaks was observed during the martensitic transformation. Additionally, EBSD mapping revealed localized plastic accommodation in austenite near intersecting martensite regions. This indicates that the deviation of the austenite lattice constant is mainly associated with elastic accommodation of transformation strain, with a minor contribution from the introduction of dislocations. For martensite, the a -axis lattice constant decreased with decreasing temperature, whereas the c -axis increased anomalously, resulting in an increased tetragonality below 35 K. During subsequent heating, the c -axis decreased, resulting in a reduction of the axial ratio. This anomalous c -axis behavior and the observed tetragonality are inferred to be related to Bain strain introduced during twin formation in martensite, and may also be influenced by interfacial stress with surrounding austenite. These findings provide additional insight into how elastic accommodation for transformation misfit strains govern lattice strain behavior during thin-plate martensitic transformation in the Fe-Ni-Co-Ti alloy. • In situ neutron diffraction revealed lattice strain evolution during transformation. • Austenite lattice deviation below Ms is dominated by elastic strain accommodation. • FWHM evolution suggests predominantly elastic strain accommodation in austenite. • Martensite c -axis shows anomalous inverse temperature dependence at cryogenic range. • Tetragonality evolution is linked to Bain strain and transformation constraints.