Delay Time in the Alpha to Gamma Transformation of Iron

Author: Boedtker, Olaf Alexander

Year: 1961

Degree: Dissertation (Ph.D.)

Advisor: Duwez, Pol E.

Committee Member: Unknown, Unknown

Option: Mechanical Engineering; Physics

DOI: 10.7907/Z8WG-R830

Abstract

Iron, containing a certain amount of carbon and nitrogen, was pulse-heated from an initial temperature in the alpha phase into a final temperature within the gamma phase. The bulk of the allotropic transformation took place a certain time after the final temperature had been reached. The relationship between this delay time and the final temperature follows an Arrhenius type rate equation with a heat of activation of 20,500 ± 1400 cal per mole. It was also established that under the present experimental conditions alpha iron can be heated directly into the delta phase without going through the gamma phase. The effect of the rate of heating on the alpha to gamma transformation temperature can be deduced from the measurements of the delay time.

An interpretation of a possible transformation mechanism is given. It is assumed that the transformation takes place in two steps: (1) A relatively slow diffusion-like growth of nuclei of the new phase in the old matrix to their critical size, followed by (2) a very, fast shear transformation.

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