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.
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机译：向Fe-C系统中添加慢扩散合金元素（X）可以对珠光体的形成产生显着影响。通常从分析合金元素如何在奥氏体（γ）与生长的铁素体（α）和渗碳体（M 3 C; M = Fe，X）。通过三元Fe-C-X相图的形式隐式地捕获了替代合金元素在γ→α+ M 3 C转变过程中分配的趋势。在添加锰的情况下，一个（γ+α+ M 3 C）三相电场打开，Mn从a分配到M 3 C的趋势为突出显示。在（γ+α+ M 3 C）三相场中形成珠光体的过程（平衡时必须小于100％珠光体）表现出一些非常有趣的特征，即生长速率不断降低时间间隔和层间间距会随着时间增加。术语“发散的珠光体”是为了描述转化产品而创造的。希勒特提供了定性模型来描述发散性珠光体的形成，假设该转变受Mn在α和M 3 C之间的分配支配，并且在移动的转变界面处存在局部平衡条件。该模型表明，在（α+ M 3 C）两相场中的珠光体生长应在稳态条件和恒定的层间间距下生长。本文通过比较（α+ M 3 C）两相和（γ+α+ M）内珠光体的形成，定量地严格检验珠光体生长的局部平衡模型。 3 C）三相场。 （α+ M 3 C）两相场中的珠光体生长确实在稳态条件下发生了大部分转变，但是通过分析透射电子显微镜（ATEM）测量了α所继承的Mn含量。在生长前沿的M 3 C表示界面处不存在局部平衡条件。确实发生了在（γ+α+ M 3 C）三相场中的生长，其生长速率随时间不断减小，层间间距随时间增大，但是再次通过TEM测得Mn含量a和M 3 C的生长所继承的结果表明，界面处不存在局部平衡条件。在每种情况下，a中的Mn含量都太高，这很可能是由于在转化前沿形成了锰的非平衡体积分数而不是不完全分配的结果。基于减少Mn从a到M 3 的扩散距离的动力学优势，提出了为什么会出现这种情况的定性解释。
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