Different microstructures of mobile twin boundaries in 10 M modulated Ni–Mn–Ga martensite

The morphology and microstructure of a single, macroscopically straight twin interface with a twinning stress of about 1 MPa was analysed in detail by differential interference contrast optical microscopy and X-ray diffraction. The interface was identified as a Type I macrotwin boundary between two variants with a/b-laminates and constant modulation direction, in contrast with a highly mobile twinned interface consisting of Type II macrotwin boundary segments with changing modulation direction and a/b-laminate reported earlier. Theoretical analysis using elastic continuum theory shows that only pure Type I or Type II boundaries are fully compatible with a/b-laminate. Other hypothetical twin microstructures combining these two mobile interfaces are shown to be incompatible to various degrees. A weakly incompatible combination of Type I and II boundaries was experimentally observed. The large difference in mobility between Type I and Type II macrotwin boundaries created at the same location of the same sample indicates that the mobility depends on the internal structure of these boundaries. A possible origin of this different mobility is discussed.