Influence of coherency strain and applied stress upon diffusional ferrite nucleation in austenite: Micromechanics approach
Influence of coherency strain and applied stress upon diffusional ferrite nucleation in austenite: Micromechanics approach
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DOI：
10.1080/14786430902751569
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发表时间：
2010-01
影响因子：
1.6
通讯作者：
K. Lee;H. Lee;J. Lee
中科院分区：
文献类型：
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作者：
K. Lee;H. Lee;J. Lee
在开发超细晶铁素体钢的过程中，经常施加外部应力来控制奥氏体到铁素体的转变动力学。为了了解外加应力在扩散铁氧体形核中的作用，进行了微观力学分析。众所周知，在绝对零度下，奥氏体到铁素体的转变伴随着高达9%的体积增加。仅考虑体积变化的计算表明，相干铁氧体颗粒在Nishiyama-Wasserman （NW）取向下的应变能小于Kurdjumov-Sachs （KS）取向下的应变能，NW取向颗粒的优先形状依次为盘状、针状和球形。当引入外加弹性应力时，会产生两个相互作用项。第一个是由于fcc和bcc Fe之间弹性常数的差异而引起的非均匀性项，另一个是体积变化与外加应力之间的相互作用项。奥氏体弹性常数高体积模量软剪切模量的有趣特征与铁素体强弹性各向异性相结合，揭示了外加应力对铁素体形成能量学的不同影响。在一定的外加应力模式下，发现非均匀性和相互作用能项都降低了与铁素体粒子相关的自由能，促进了铁素体成核的增强。例如，与无应变情况下相比，相干应变单独降低了10−21个数量级的成核速率，但它与适当的外加应力的相互作用可以使成核速率提高约30倍。
In the quest to develop ultrafine-grained ferrite steels, an external stress is often applied to control the austenite-to-ferrite transformation kinetics. To understand the role of an applied stress in diffusional ferrite nucleation, a micromechanics analysis was performed. It is well known that the austenite-to-ferrite transition is accompanied by a volume increase of up to 9% at absolute zero. The calculation due to the volume change alone shows that a coherent ferrite particle has less strain energy in the Nishiyama–Wasserman (NW) orientation than in the Kurdjumov–Sachs (KS) orientation, and preferred shapes of NW-oriented particles are disk-like, acicular, and spherical in the order. When an applied elastic stress is introduced, two interaction terms arise. The first one is an inhomogeneity term due to the difference in elastic constants between fcc and bcc Fe, and the other is an interaction term between the volume change and the applied stress. The interesting feature of the austenite elastic constants–high bulk modulus but soft shear modulus–combined with the strong elastic anisotropy of ferrite, reveals the diverse influence of applied stress upon the energetics of ferrite formation. In certain applied stress modes, both inhomogeneity and interaction energy terms are found to lower the free energy associated with the ferrite particle, promoting enhanced ferrite nucleation. As an example, coherency strain alone decreases, when compared to a strain-free case, the nucleation rate by an order of 10−21, but its interaction with an appropriate applied stress can increase the rate by a factor of about 30.