Effect of Hydrogen on the Substructure of Lenticular Martensite in Fe-31Ni Alloy

This study investigated the effect of hydrogen on the substructure of martensite in Fe-31Ni alloy. In both the hydrogen-charged and uncharged specimens, typical lenticular martensite plates formed after subzero cooling. However, we found that the fraction of twinned region (including the area of midrib) in lenticular martensite plate increased with increasing hydrogen content. In addition, the width of individual twins in the hydrogen-charged specimen was slightly smaller than that in the uncharged specimen. These results indicated that the existence of hydrogen facilitated twinning deformation as a lattice invariant deformation. We presented a comprehensive discussion about the reason why hydrogen enhanced twinning deformation. Even though tetragonality of martensite in the hydrogen-charged specimen could not be confirmed by X-ray diffraction, the transmission electron microscopy observations and the first-principles calculations suggested that hydrogen might increase the tetragonality of martensite. We proposed that solid solution hardening and an increase in the tetragonality of martensite by the existence of hydrogen were the possible reasons for facilitating twinning deformation as a lattice invariant deformation in martensitic transformation.