Creep deformation of ultrafine grained Ni 75B 17Si 8

Abstract

Ultrafine grained, polycrystalline materials can be prepared by annealing of melt spun amorphous ribbons under carefully controlled conditions. A grain size of 6.5–12.10 −5 mm was attained in ribbons of Ni 75B 17Si 8, annealed at 600°C for 15 min, with a heating rate of 0.01–1 K/s. The samples were then creep tested at low stresses (< 400 kg/cm 2) and temperatures between 500 and 615°C. Coble creep, with an activation energy of 4 eV, was the dominant mechanism of deformation at low stress, and below 575°C. At higher stress the rate controlling mechanism became a dislocation climb mechanism. The transition was temperature and grain size dependent. Above 575°C, the dominant mechanism was found to be a nondiffusion controlled process, with an activation energy of 0.6 eV, close to dislocation glide activation energy in Ni.