Recently, variant pairing in lenticular and thin-plate martensite was investigated in terms of the compatibility condition: QU_i – U_j = b ⊗ n (Q: rigid body rotation, U_i, U_j: deformation gradient matrices of two different variants, n: junction plane normal, b: (QU_i – U_j)n). The results showed that pairs with smaller rotation Q are formed more frequently. 

 It has been reported that the preferred lath variant pairing in the early stage of the transformation is different from that in the final stage. It is important for microstructural control to clarify the mechanism by which the preferred variant-pairs change as the transformation progresses. In order to tackle this problem, it is useful to first clarify the effect of the compatibility on variant pairing in lath martensite. In this study, we investigate the crystallographic characteristics and frequency of variant pairs in lath martensite microstructure of Fe-Ni-Cr-C alloys using electron backscattering analysis (EBSD), to clarify the effect of the compatibility on variant pairing in lath martensite.

 Deformation gradient for each variant was calculated based on the phenomenological theory of martensite crystallography (PTMC): U_i = RBL₁L₂ (R: rotation, B: lattice deformation, L₁, L₂: lattice invariant deformation). Following the model proposed by Kelly [3], L₁: (112) [1 -1 -1]α' and L₂: (110) [1 -1 -1]α' are assumed as the lattice invariant shear systems.

  For V1/V2, V6, V16 pairs, the theoretical junction plane normals derived by the compatibility condition were in good agreement with the experimentally determined junction planes. The V1/V2, V1/V4, V1/V6 pairs, for which θ is smaller, dominates 65% of the observed pairs. V1/V17 pair was, however, rarely observed despite its smaller θ. Geometrical aspects of variant pairing will be discussed in detail basing on the above results.