FORMATION OF AMORPHOUS ELECTRODEPOSITED NI-W ALLOYS AND THEIR NANOCRYSTALLIZATION

Ni-Walloys containing about 5 to 30 at. % W were prepared by electrode position. The plating bath for the electrodeposition contained nickel sulfate, citric acid, sodium tungstate and ammonium chloride, and was operated at various bath concentrations and conditions of electrolysis . X-ray diffraction peaks of the deposited alloys broadened with incre asing tungsten content and an amorphous pattern appeared at a tungsten content of about 20 at. %. Nanocrystallization behavior of the amorph ous Ni-25.0 at. % W alloy has been studied by X-ray diffraction, high resolution transmission electron microscopy and small angle X-ray scat tering. The Hall-Fetch strengthening mechanism was observed for the ha rdness extending to a finest grain size of about 10 nm. When the grain size was less than about 10 nm, decrease of the hardness was observed . This decrease may be due to the significant increase of the intercry stalline volume fraction, especially the fraction associated with the triple junction. (C) 1998 Acta Metallurgica Inc.