Kinetics of the allotropic hcp–fcc phase transformation in cobalt

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 light optical microscopy (LM) and X-ray diffractometry (XRD). 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.

Kinetics of the fcc → hcp Phase Transformation in Cu-Ge Solid Solutions Upon Isothermal Aging

“…[7,25] In the present work, a value of r for a-Cu(Ge) with 10.8 at. pct Ge was obtained by adopting the procedure given in Reference 30 that related the SFE to the solute concentration in a Cu(Ge) solid solution: r = 0.0058 ± 0.0052 mJ m À2 (taken as constant for the annealing temperature range in the present work).…”

“…It can be shown that a certain critical size (half of the separation distance of the SPs of a pair gliding in opposite directions) does not occur: the nucleation is athermal (i.e., ''spontaneous nucleation''). [7,20] The larger the height of the dislocation array, the smaller the undercooling required to ''activate'' the corresponding array of dislocations, implying that the minimum, critical height decreases with temperature. Hence, the nucleation during cooling can be conceived as ''site saturation'' at each temperature, where arrays of specific height start to operate.…”

“…The fcc fi hcp transformation is established by ''ordered glide'' of an array of Shockley partial dislocations (SPs), originating from the dissociation of a perfect dislocation, on every second closed-packed plane (where it is assured that microscopic shear cancels out over short distances [7,25] ). The region between the two separated arrays of SPs can be conceived as two product (hcp) phase particles (References 7 and 20 see Figure 2).…”

“…The volume and interfacial surface of the particle can be calculated from the dimensions of the particle as given in (b) adapted from Ref. [7], Taylor & Francis Ltd www.tandfonline.com.…”

“…[1] In the Cu(Ge) system, the hcp product phase is called f-phase. The precipitation of the f-phase from supersaturated Cu(Ge) solid solutions complies with the same crystallographic orientation relationship as holds for the fcc fi hcp phase transformations observed in Co, Ag-Al, etc., i.e.,ð11 1Þa==ð0001Þf; ½ 110 a==½11 20 f. [1,2,[4][5][6] In contrast with the allotropic fcc fi hcp phase transformation occurring in Co, [7] the transformation in the Cu-Ge system is associated with compositional changes [1,8] (cf. Figure 1) and therefore the transformation mechanism is an intermediate class of a diffusionless, displacive, martensitic fcc fi hcp phase transformation and a diffusional phase transformation: Ge must diffuse to the f-phase nucleation sites and/or the growing f-phase plates to establish the f-phase composition.…”