Abstract
In order to establish the crystal structure of Cu30Au25Zn45 martensite, a crystallographic study has been carried out by means of optical and electron microscopy, selected micro-area electron diffraction and X-ray diffraction. From the electron diffraction study, the crystal structure is determined to be of a modified 18R type of long period stacking order with monoclinic unit cell, and from the X-ray diffraction study the lattice parameters are determined to be a=4.559, b=5.631, c=39.91 Å and β=87.5°. Two kinds of planar defects have been found in the martensites by electron microscopy. One is stacking faults parallel to the basal plane with a low density, and the other is antiphase domain boundaries inherited from the matrix phase with a high density.
Based upon the rigid sphere model of atoms, the basal plane of the martensite is inferred to possess atomic sites at (000) for Au, (0, 1/2, 0) for Zn and (1/2, 1/4, 0) and (1/2, 3/4, 0) for Cu, which are different from those reported so far. According to these atomic sites in the martensite, an atomic configuration in the matrix phase is proposed, which is almost identical to that of the original Heusler alloy, Cu2MnAl.
