A STUDY ON THE DIFFUSION OF CARBON IN γ-IRON BY METHOD OF INTERNAL FRICTION

Abstract

In γ-iron (containing 1.7% of Mn) an internal friction peak was observed around 240℃with a frequency of vibration of about 2 cycles per second. The height of the peak was found to increase with an increase of the carbon content and was found to decrease when the specimen was annealed at an elevated temperature. The activation energy associated with this peak was found to be 34,000±2,000 calories permole which is close to the activation energy of diffusion of carbon in y-iron determined in macro-diffusion experiments. It is concluded, thus, the observed internal frictionpeak is associated with the stress-induced diffusion of carbon in y-iron.The coefficient of diffusion D of carbon in y-iron in the temperature range of 200-260℃ was calculated according to data given by internal friction measurements. When plotted with logD against 1/T, these values of D were found to lie on the same straight line with those values of D in the temperature range of 1000-1200℃ determined in macro-diffusion experiments.The height of internal friction peak appeared to vary with the square of the carbon content in the specimen. This seems to indicate that two carbon atoms are participating in the elementary act associated with the stress-induced diffusion. A preliminary model for the observed internal friction peak was suggested thereupon: Owing to the presence of holes (vacancies) in the crystal lattice, two carbon atoms are apt to form an atomic-pair and this pair produces lattice distortions which are different along three crystallographic directions. Rotation of such an atomic-pair under the action of stress gives rise to internal friction.