Analysis of thin-plate martensite microstructure in steel focusing on incompatibility and its visualization

The thin-plate martensite microstructure in an Fe-29.50Ni-0.34C (wt%) alloy was analyzed to understand the factors that affect the variant pairing tendency. We focused on the rigid body rotation Q _{l / k} at the variant/variant junction planes derived by the rank-1 connection, because the factor Q _{l / k} corresponds to the incompatibility in the junction plane. Theoretical analysis showed that three types of variant pairs (type I: V1/V17, type II: V1/V6, and type III: V1/V16) had a small degree of incompatibility ( θ). These pairs accounted for 70% of the total microstructure. The formation frequency followed the order type II > type I > type III in both thin-plate and lenticular martensite microstructures, applying a correction that makes the factors that reduce the correlation between the observed two-dimensional and actual three-dimensional formation frequencies of the variant pairs irrelevant. For the type III variant pairs, the crystal orientation did not strictly follow the orientation relationship that the invariant plane condition specified. We succeeded in mapping this deviation using data obtained by electron backscatter diffraction (EBSD). Both the deviation angle and axis were well explained by Q _{l / k}. The deviation angle was significant in the vicinity of the junction plane and decreased with distance from the junction plane. Although this could only be visualized for the type III pairs, its detection should be possible in other shape memory alloys through the use of a higher angular resolution for the EBSD measurements. Information regarding the spatial distribution of this incompatibility helps us understand the formation process of the martensite microstructure.