Fe合金FCC-BCC原子尺度台阶型马氏体相界面迁移行为的分子动力学模拟研究

Molecular dynamics simulation of migration behavior of FCCBCC atomic terrace-step phase boundary in iron-based alloy

两相界面的原子尺度结构对相界面迁移行为具有重要影响.高分辨透射电子显微分析表明钢中马氏体相界面具有高度为若干原子层间距的台阶结构,然而目前Fe合金马氏体相变的模拟研究工作中绝大多数使用非台阶型相界面结构作为模拟初始模型.本文基于拓扑模型和相变位错理论构建了Fe合金FCC/BCC台阶型相界面初始模型,采用分子动力学模拟方法研究了Fe合金马氏体相界面的迁移行为.研究结果表明,当两相界面具有约束共格匹配关系及台阶结构时,体系发生FCC→BCC马氏体相变并呈现典型的非扩散切变特征;相变过程中FCC/BCC宏观尺度相界面沿其法线方向以(4.4±0.3)×10^2 m/s的速度迁移,且相界面在迁移过程中始终保持稳定的台阶结构和相对平直的宏观界面形貌特征;相变位错的滑移速度高达(2.8±0.2)×10^3 m/s,相变位错阵列沿台阶面的协同侧向滑移不仅是马氏体台阶结构宏观相界面迁移的微观机制,也是马氏体相变宏观形状应变的主要来源;采用分子动力学模拟方法获得的Fe合金马氏体相变晶体学特征参量与拓扑模型的解析解数值非常接近,相变产生的整体宏观形状应变由平行于相界面的剪切应变和垂直于相界面的法向应变两部分组成.

The martensitic transformation between the high-temperature face-centered cubic(FCC)phase and the low-temperature body-centered cubic(BCC)phase in iron-based alloys has been studied for years,which plays a critical role in controlling microstructures and hence properties of the alloys.Generally,the BCC structure martensitic phase forms from the FCC parent phase,involving a collective motions of atoms over a distance less than the interatomic distance in the vicinity of the interphase boundary.Thus the structure of interphase boundary separating the FCC and BCC phases is the key characteristics to quantitatively understanding the mechanism and kinetics of martensitic transformation.Due to the difficulty in observing the atomic motions taking place at a velocity as high as the speed of sound,the experimental investigation on the migration of FCC/BCC interphase boundary during the transformation is as yet limited.Noteworthily,molecular dynamics(MD)simulation has been applied to studying the martensitic transformation,in particular for investigating the mobility of the FCC/BCC interphase boundary in iron.However,in most of the MD studies the atomistically planar interfaces of{111}FCC//{110}BCC are considered as the initial configuration of the interphase boundary between FCC and BCC phases,which is in contradiction to the high-resolution TEM observations.In fact,the FCC/BCC interphase boundary,which is known as the macroscopic habit plane,is a semi-coherent interface consisting of several steps and terrace planes on an atomic scale.In the present work,the atomic configuration of a terrace-step FCC/BCC interphase boundary of iron is built in terms of the topological model.The MD simulation is conducted to clarify the mechanism of interphase boundary migration in the FCC-to-BCC transformation.The results show that the FCC/BCC boundary migrates along its normal at the expense of FCC phase as a result of the lateral motions of the transformation dislocations.Meanwhile,the interphase boundary maintains the stable terrace-step structure during the transformation.Further examinations reveal that the transformation dislocations move steadily at a velocity as high as(2.8±0.2)×10^3 m/s,affecting the migration of the interphase boundary with a constant velocity of about(4.4±0.3)×10^2 m/s.The effective migration velocity of FCC/BCC interface exhibits dynamic properties consistent with the characteristic features commonly observed in a displacive martensitic transformation.Additionally,the motion of transformation dislocations gives rise to the macroscopic shape strain composed of a shear componentГyz=0.349parallel to the boundary and a dilatationГzz=0.053 normal to the boundary in the MD simulation,which is close to the crystallographic calculations by the topological model.