Abstract
The allotropic, martensitic phase transformation (hcp → fcc) in cobalt was investigated by differential scanning calorimetry (DSC) upon isochronal annealing at heating rates in the range from 10 K min−1 to 40 K min−1. The microstructural evolution was traced by optical microscopy and X-ray diffractometry. The kinetics of the phase transformation from hcp to fcc Co upon isochronal annealing was described on the basis of a modular phase transformation model. Appropriate model descriptions for athermal nucleation and thermally activated, anisotropic interface controlled growth tailored to the martensitic phase transformation of Co were implemented into the modular model. Fitting of this model of phase transformation kinetics to simultaneously all isochronal DSC runs yielded values for the energy of the interface separating the hcp and fcc Co phase and the activation energy for growth.
Highlights
Pure cobalt exhibits an allotropic phase transformation at the equilibrium temperature T0 with the hcp modification as low temperature phase and the fcc modification as high temperature phase
The present paper focuses on the hcp→fcc phase transformation using “stabilized” specimens rR
In order to establish a microstructural reference state, each specimen used for the kinetic analysis, was initially exposed to isochronal transformation cycles with a cooling/heating rate of ±50 K min-1 in a temperature range from 523 K to 893 K
Summary
Pure cobalt exhibits an allotropic phase transformation at the equilibrium temperature T0 (at constant pressure) with the hcp modification as low temperature phase and the fcc modification as high temperature phase. On partial (SP) dislocations, with Burgers vectors, of type 1 6 11 2 , on every second closest packed ly plane [6, 19] This process can be called “ordered glide” as an ordered array of Shockley partials is required for the phase transformation. A critical size (a critical value of r) does not occur (cf Eq (5)): The transformation can take place “spontaneously”, i.e. without overcoming an energy barrier by ev thermal activation, provided that the energy difference between the product phase (lying in-between the dissociated dislocation arrays) and parent phase per unit area, ∆GA, becomes negative, e.g. by a iew change of temperature In order to establish a microstructural reference state (see section 4), each specimen used for the kinetic analysis, was initially exposed to isochronal transformation cycles with a cooling/heating rate of ±50 K min-1 in a temperature range from 523 K to 893 K.
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