连续升温过程中γ-Fe→δ-Fe→液态Fe相变的分子动力学模拟

MOLECULAR DYNAMICS SIMULATION OF PHASE TRANSFORMATION ofγ-Fe→δ-Fe→LIQUID-Fe IN CONTINUOUS TEMPERATURE-RISE PROCESS

采用所构建的长程F-S势函数,对连续升温过程中γ-Fe→δ-Fe→液态Fe的相变过程进行了分子动力学(MD)模拟。结果表明,计算得到的γ-Fe,δ-Fe以及液态Fe的微观结构(径向分布函数、配位数)和宏观物性(密度)都能与实验结果吻合很好,但相变温度点与实验值的偏差较大,推测是由过快升温速度造成的过热度过高所致,从微观结构、瞬态能量及密度的分析出发,讨论了固态相变(γ-Fe→δ-Fe)和固-液相变(δ-Fe→液态Fe)的具体过程,其中,固态相变主要形成于晶格扭曲和滑移,而固-液相变即熔化过程起始于固体岛颗粒的边缘,并逐渐向其中心扩散,在相变演化过程中,通过瞬态能量和密度的起伏观察到明显的孕育过程。

Understanding high-temperature phase transformations of pure Fe is fundamental for quality control and product design of steels.Various theoretical methods have been used to determine dynamically the mechanism of phase transformations in pure Fe includingγ-Fe toδ-Fe andδ-Fe to liquid-Fe.Among these methods,molecular dynamics（MD） simulation has become a prospective method,in which atomic interactions play a key role in phase transformations.However,most attention was focused on the MD simulation of temperature-drop phase transformations rather than temperature-rise phase transformations before.In the present study the isothermal-isobaric MD simulation at a wide temperature range ofγ-Fe→δ-Fe→liquid-Fe transformations in pure Fe was carried out by giving a set of long-range Finnis-Sinclair potential parameters.The results show that a better agreement between simulation and experimental results for the microstructures（including radial distribution functions and coordination numbers） and densities of transformed phases validate that the set of potential parameters for the MD simulation are reasonable.The larger difference between the calculated and experimental transformation temperatures is attributed to the effect of superheat degree induced by ultrafast heating speed in the MD simulation.Evolvement of microstructures exhibits lattice-distorting and sliding induced byγ-Fe toδ-Fe phase transformation and melting ofδ-Fe islands fromδ-Fe to liquid Fe.Finally,in the MD simulation stronger and stronger fluctuations of instantaneous energy and density just before transformations,especially melting,show an apparent pregnant process in phase transformations.