Microstructure and order in NiPt3

The L1-type ordering of a Ni-75.2 at.% Pt single crystal quenched from 1123 K was followed using X-ray diffraction and transmission electron microscopy (TEM). In spite of sample stoichiometry it turned out impossible to achieve a large degree of long-range order, a Bragg–Williams order parameter of only 0.56 was obtained by X-ray scattering following a step-wise ageing at 613 and 573 K for a total of 2150 h. In a subsequent isochronal heating run, the order–disorder transition temperature was determined to be about 690 K. TEM studies were performed for specimens aged at 613 K for time periods up to 250 h. They revealed material heterogeneity without any variation in composition. The L1-ordered regions homogeneously distributed in the matrix and decorating dislocations reached sizes of 5–10 nm at a volume fraction below 50%, which is consistent with the value of homogeneous long-range order.