CREEP OF BETA-COBALT AT LOW STRESSES

The creep of β-Co in vacuum has been studied for the ranges of stress up to 12 MN m-2, temperatures from 773 to 1113 K (0.44-0.63 TM), intercept grain size from 21 to 125 μm and test durations from 5 to 210 h. The measurements were made in situ on self-loaded helical specimens self-heated by electric current as reported previously for α-Fe. Creep rates at lower stresses were found to be proportional to stress in excess of a threshold stress which could be as large as 2.4 MN m-2 and grain size and temperature dependence of creep rate per unit stress indicate Coble creep as the dominant mechanism in this regime. Observed creep rates exceeded predicted values by factors less than ten, if the recent determination of the grain boundary diffusion coefficient in β-Co is used. Transition to creep with a stress exponent as high as six was observed especially at higher temperatures and larger grain sizes, the transition stresses ~5-9 MN m-2 then being factors of 1.3-3.5 below values derived by extrapolating reported high stress creep data fitted to the Dorn equation. Grain growth during creep occurred with an activation energy close to that for grain boundary diffusion. © 1979.