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

Abstract

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.