ELECTRON-MICROSCOPY STUDY OF TYPE-II TWINS IN GAMMA-1'-CU-AL-NI MARTENSITE

The lattice invariant shear in the beta1-->gamma1' martensitic transformation in a Cu-Al-Ni alloy is confirmed to be [111]gamma1' Type II twinning by X-ray diffraction and two-surface analysis using single crystals. However, Type II twins have not been observed by electron microscopy in the past. To resolve this conflict, the twins in the martensite were reinvestigated by electron microscopy. Since unambiguous twin diffraction patterns for Type II twins are available only for specific orientations, well-prepared single crystals with the {101}beta1 surface were used. As a result, the presence of Type II twins were unambiguously proved by the twinned diffraction patterns from the unique eta1 direction and by those, in which eta1 is normal to the electron beam. The reason for non-observation of Type II twins in the past were also discussed. The widths of Type II twins were found to be one order of magnitude larger than those of Type I twins. Thus, the boundary energy of Type II twin is suggested to be larger than that of Type I twin in the present martensite.