Abstract
The crystal structure of a martensite with a 4H-like structure, which was reported so far to exist not only in binary Ti–Ni but also in ternary Ti–Ni–Cu alloys in addition to the major martensite with a B19′ structure, has been examined by electron diffraction. As a result, the “4H” martensite was found to possess a monoclinic structure with a=0.491, b=0.720, c=0.931 nm and γ=93.3°, in which complicated atomic shuffles were expected to occur three-dimensionally. The crystal structure is thus not a simple 4H one as the so-called long period stacking order structure: The reciprocal lattice in the a*-b* plane was three times as dense as the corresponding one of, e.g. the γ2′ martensite in Au–Cd or Ag–Cd alloys. Lattice correspondence between the “4H” martensite and the B2 matrix was expected to be as follows: [100]M→⟨\bar111⟩B2, [010]M→⟨1\bar12⟩B2 and [001]M→⟨220⟩B2. The volume change associated with the B2→“4H” martensitic transformation, which was estimated from this electron diffraction study, was larger than that of the B2→B19′ transformation in absolute value. This may suggest that the thermal hysteresis of the former transformation is larger than that of the latter.
