Comparison of grain size distributions obtained by XRD and TEM in milled FCC powders

Measurements of grain size in powders of the Cu-15at%Al, Cu-20at%Ni, Cu and Ni systems, milled for different times were conducted. X-ray diffraction (XRD) (Warren-Averbach method) and transmission electron microscopy were used for that purpose. From both techniques, distributions of grain diameter (length) were obtained, which permitted to compare both results. Such results were interpreted by considering previous studies of particle size evolution and microstructural evolution. A better comprehension of phenomena that occur in the mechanical alloying, as the grain refinement and the solid solution formation, is attained.

The grain size results obtained by the two techniques were coincident for the Cu-15at%Al system. This system did not present adherence of the powders to the milling media. In the case of the Cu-20at%Ni, Cu and Ni systems, that presented adherence, the results were coincident solely in the powder milled for 864 ks. This is explained due to the microstructural homogenization, reached until that time.

The grain size as a function of the milling time of the two classes of FCC systems, being referred to the adherence phenomenon, presented opposed trends. For example, for the Cu-at15at%Al system, the 70% of the population of grains had a grain size smaller than 7.5, 12 and 20 nm for the milling times of 180, 360 and 864 ks respectively. In the case of the Cu-20at%Ni, Cu and Ni systems, the 70% of the population of grains had a grain size smaller than 35, 22 and 16 nm for the milling times of 180, 360 and 864 ks respectively. These contrary trends reflect different mechanisms of grain size refinement.

A maximum value of grain size of 20 nm required to form the solid solution in the Cu-at15%Al and Cu-20%atNi systems was found.