It has been reported preiviously that an ordering reaction, the β→β₁ transformation, occurs first, followed by a martensitic transformation, β₁→β′, on rapid cooling from the β region in Cu-Al binary alloys. The possibility of this transformation process has also been observed in the case of isothermal transformation by means of dilatometric, electric resistance, and specific heat measurements. In order to determine the crystal structure of the β′ martensite with ordered configuration, a powdered specimen containing 24.06 at% Al close to an eutectoid composition was water-quenched from 950°C, and then analyzed by using the diffraction pattern obtained by use of an X-ray diffractometer.
The experiments showed the following results, (1) The superlattice lines observed in the diffraction pattern of β′ martensite has proved that the β₁→β′ transformation retains the Cu₃Al type ordering configuration, Cu₃Al type, when rapidly cooled. (2) The crystal structure of the β′ martensite is a distorted hexagonal closepacked structure, in which the (0001) direction makes an angle of about 3^° to the normal (0001) plane. The results obtained have been discussed with the martensitic transformations of Cu-Al, Cu-Zn, and Cu-Sn binary alloys and the similarity of the crystal structure and habit plane.