K. R. Kinsman

Thickening kinetics of proeutectoid ferrite plates in Fe-C alloys

Thermionic electron emission microscopy was used to measure directly the thickening kinetics of proeutectoid ferrite sideplates in Fe-C alloys. These kinetics were found to be exceedingly irregular. During the first few seconds of growth, the thickening rate is 5 xl0-5±1 cm/s; afterwards it usually diminishes to 1 - 30 × 10-6 cm/s. As predicted by a general theory of precipitate morphology, thickening was shown to occur only by the ledge mechanism, despite the relatively poor matching of the austenite and the ferrite lattices. Ledges were observed to lengthen at rates controlled by the diffusion of carbon in austenite. Tent-shaped and other more complex surface relief effects, rather than the invariant plane strain relief, were found to predominate. These features are shown to be the expected result of a diffusional transformation occurring by means of a ledge mechanism.

Influence of al, co, and si upon the kinetics of the proeutectoid ferrite reaction

The effects ofca. 3 at. pct of Al, Si, or Co upon the kinetics of grain boundary ferrite allotriomorph formation (and thus upon hardenability) relative to those in Fe-C alloys of comparable carbon content were evaluated. All three alloying elements displace the TTT curve for the initiation of transformation to shorter times at the higher reaction temperatures. Both aluminum and silicon increase the parabolic rate constant for allotriomorph thickening,α, relative to that in their counterpart Fe-C alloys; the influence of cobalt uponα, if any, is appreciably less. In the Fe-C-Al and Fe-C-Si alloys, thickening proceeds noticeably less rapidly than volume diffusion control (as assessed by Atkinson’s analysis of the growth of an oblate ellipsoid) allows. In the Fe-C-Co and Fe-C alloys, the average calculated and experimental α’s are in better agreement but, evidently as a result of the presence of dislocation facets at a broad face of allotriomorphs, some allotriomorphs actually thickened more rapidly than calculated. The substantial scatter inα in all alloys was also attributed to these facets. Indirect determinations indicated that all three elements increased the rate of nucleation of ferrite allotriomorphs.

On the growth kinetics of grain boundary ferrite allotriomorphs

Previous work has shown that the thickening kinetics of proeutectoid ferrite allotriomorphs in an Fe-0.11 pct C alloy are often more rapid than the kinetics calculated for volume diffusion-control from the Dube-Zener equation for the migration of a planar boundary of infinite extent, assuming the diffusivity of carbon in austenite,D, to be constant at that of the carbon content of the Ae3. Recalculating the thickening kinetics, using a numerical analysis of the infinite planar boundary problem previously developed by Atkinson in which the variation ofD with composition is taken fully into account, was found to increase this discrepancy. Measurements were then made of the lengthening as well as the thickening kinetics of grain boundary allotriomorphs in the same alloy. Application to these data of Atkinson’s numerical analysis of the growth kinetics of an oblate ellipsoid, in which the composition-dependence ofD is similarly considered, produced an acceptable accounting for nearly all of the data. It was concluded that the growth of ferrite allotriomorphs is primarily controlled by the volume diffusion of carbon in austenite; the presence of a small proportion of dislocation facets along one of the broad faces of the allotriomorphs, however, usually results in growth kinetics which are somewhat slower. An alternate treatment of the lengthening and thickening data upon the basis of the theory of interfacial diffusion-aided growth of allotriomorphs indicated that, in the temperature range investigated (735° to 810°C),the diffusivities of carbon along γ:γ and γ:α boundaries required for this mechanism to make a significant contribution to growth are too high to be physically plausible.