| The addition of slow diffusing alloying elements (X) to the Fe-C system can have a marked effect on the formation of pearlite. An understanding of this effect usually begins with an analysis of how the alloying element redistributes, if at all, between the austenite (γ) and the growing ferrite (α) and cementite (M3C; M=Fe,X). The tendency for a substitutional alloying element to partition during the γ → α + M3 C transformation is implicitly captured by the form of the ternary Fe-C-X phase diagram. In the case of Mn additions, a (γ+α+M3C) three phase field is opened and the tendency for Mn to partition from a to M3C is highlighted.; The formation of pearlite within the (γ+α+M3C) three phase field (which is necessarily less than 100% pearlite at equilibrium) exhibits some very interesting features, namely a growth rate which decreases continually in time and an interlamellar spacing which increases in time. The term ‘divergent pearlite’ has been coined to describe the transformation product. Hillert has offered a qualitative model to describe the formation of divergent pearlite assuming the transformation is governed by the partitioning of Mn between the α and M3C and that local equilibrium conditions prevail at the moving transformation interface. The model suggests that pearlite growth within the (α+M3C) two phase field should grow with steady state conditions and a constant interlamellar spacing.; This dissertation describes work carried out to critically test the local equilibrium model for pearlite growth in a quantitative manner through comparison of pearlite formation within the (α+M3C) two phase and (γ+α+M3C) three phase fields. Pearlite growth within the (α+M3C) two phase field does occur under steady state conditions for much of the transformation, but analytical transmission electron microscopy (ATEM) measurements of the Mn contents inherited by the a and M3C at the growth front indicate that local equilibrium conditions do not prevail at the interface. Growth within the (γ+α+M 3C) three phase field does occur with a growth rate which continually decreases in time and an interlamellar spacing that increases in time but again, ATEM measurements of the Mn contents inherited by the growing a and M3C indicate that local equilibrium conditions do not prevail at the interface. In each case, the a is too rich in Mn and it is likely that this is a consequence of the formation of non-equilibrium volume fractions of a rather than incomplete partitioning of Mn at the transformation front. A qualitative explanation of why such a situation may arise is proposed, based on the kinetic advantage of reducing the diffusion distance of Mn from the a to the M3C. |
