Steels are the most widely used metallic materials in various applications, including construction, automotive, and machinery, due to their superior strength, workability, and cost-effectiveness. The austenite phase formed at high temperatures in common steels transforms into the high-strength martensite phase when rapidly cooled. The internal stresses generated during the martensitic transformation are believed to influence the mechanical properties and have been discussed for decades; however, they remain unclear. This study aims to reveal the relationships among lattice parameters, internal stresses, and crystal defects during martensitic transformation and subsequent tempering through in-situ neutron diffraction. The experiments were conducted using the engineering materials diffractometer “TAKUMI” equipped with a thermomechanical processing simulator at J-PARC (Fig. 1a, b).
 Fig. 1c illustrates the temperature history during the partial martensitic transformation (Step 1) and the subsequent cyclic heating-cooling processes (Steps 2‐4). During the partial martensitic transformation, the austenite exhibits compressive lattice strain (Fig. 1d), whereas the martensite displays tensile lattice strain (Fig. 1e). These changes in the lattice parameters were clarified to be caused by the formation of vacancies in the austenite phase and dislocations in martensite, respectively. After the removal of those crystal defects through subsequent tempering, martensite was found to exhibit compressive stress, that balances the tensile stress of the austenite, revealing their true nature.
 This research enhances the fundamental understanding of internal stress associated with phase transformations in steels and will contribute to the microstructural control of high-strength steels.

This work was supported by the Elements Strategy Initiative for Structural Materials through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (project number JPMXP0112101000).