Formation of nanoscale layered structures and subsequent transformations during mechanical alloying of Ni60Nb40 powder mixture in a low energy ball mill

Abstract

Elemental Ni-Nb powder mixture containing 40 atom % Nb were mechanically alloyed in a low energy ball mill, allowing slower processing and easier progressive observation at intermediate milling times. The evolution of morphology and structure of the powders were investigated as a function of milling time by transmission and scanning electron microscopy and X-ray diffractometry. The results revealed that an ultrafine Ni/Nb layered structure with a typical thickness of ~30 nm, containing nanoscale size grains with a typical size of ~15 nm, develops in powder particles during mechanical alloying. This microstructure provides numerous high speed diffusion paths such as sub-grain boundaries and dislocation networks, allowing a high diffusion rate at low temperature and therefore permits different solid-state reactions to take place kinetically. Under mechanical alloying conditions used here continued milling led to a fully amorphous structure. Because of non-uniform plastic deformation the kinetic requirements for the amorphization reaction at the edges of particles is satisfied prior to the centres, resulting in an inhomogeneous progress of amorphization reaction from the edges towards the centres of particles.