探究二元共晶的生长过程：实时原位观察、数值模拟与解析解研究

Exploring the Growth of Binary Eutectic: Real Time In-situ Observation,Numerical Simulation and Analytic Solutions Calculation

由定向凝固法在一定凝固速度范围内制备得到的共晶通常为层状或棒状结构共晶。层状和棒状二元共晶合金具有优良的物理、化学和力学性能,被广泛应用于航空航天和汽车等领域,应用前景广阔。值得指出的是共晶合金已被应用于飞机发动机的涡轮叶片。近几十年来,国内外研究人员已经从共晶生长的实时原位观察实验、数值模拟和解析解计算三个方面展开了大量研究。共晶间距对共晶合金的性能具有显著影响,进而影响共晶合金在实际中的应用。在共晶生长过程中,工艺参数如凝固速度、共晶成分、固相体积分数、温度梯度、合金的初始浓度等是共晶间距的重要影响因素。此外,共晶形貌受共晶生长的稳定性和相界各向异性的影响。因此,研究人员在进行共晶生长的实时原位观察实验、数值模拟和解析解计算时主要关注工艺参数对共晶间距和共晶形貌的影响规律。大量的实时原位观察实验和数值模拟的研究结果表明,当凝固速度和合金的初始浓度增大时,层状共晶或棒状共晶间距减小。共晶成分发生变化时,共晶结构在层状和棒状之间发生转变。此外,共晶结构的转变条件依赖于α和β固相体积分数,当α和β两固相体积分数相差较大时,共晶倾向于形成棒状结构。当α和β两固相体积分数相接近时,共晶倾向于形成层状结构。同时,研究人员已经建立了共晶生长的数学模型并使用数学方法计算获得了共晶生长的解析解,进而研究了工艺参数对界面形貌和临界共晶间距的影响规律。本文从定向凝固条件下层状共晶和棒状共晶生长的实时原位观察的实验研究、数值模拟和解析解计算三方面分别阐述了工艺参数对共晶间距和共晶形貌的影响规律;重点阐述了在层状共晶和棒状共晶生长过程中,工艺参数如凝固速度、共晶成分和固相体积分数对共晶间距的影响规律;概述了相界的各向异性及共晶生长的稳定性对共晶形貌的影响规律。此外,对一种特殊的二元棒状共晶生长的实时原位观察和解析解计算进行了介绍,指出了目前该研究领域存在的一些问题,并对未来的研究工作进行了展望。

It is generally known that lamellar or rod-shaped eutectic alloys can be obtained by directional solidification in a certain range of solidification rate.The lamellar and rod-shaped binary eutectic alloys exhibit plenty superior physical,chemical and mechanical properties,and have been widely used in aerospace and automobile fields,showing a broad prospect of application.It is worth mentioning that the eutectic alloys have been already utilized as the turbine blades.In recent years,huge efforts have been put into the research of eutectic growth through real time in situ observation,numerical simulation and analytic solutions calculation.The eutectic spacing will significantly affect the properties of the eutectic alloys and further affect their practical application.The process parameters including pulling velocity,thermal gradient,volume fraction of solid phase,eutectic composition and initial concentration of alloy are the key influence factors of the eutectic spacing.Moreover,the eutectic morphology also can be affected by the stability of eutectic growth and the inter-phase boundary anisotropy.Therefore,researchers mostly focus on the influence of the process parameters on the eutectic morphology when they study the eutectic alloys through real time in situ observation,numerical simulation and analytic solutions calculation.Numerous research results show that the spacing of lamellar and rod-shaped eutectic decrease with the increase of pulling velocity and the alloy initial concentration.The structure transformation between rod-shaped and lamellar of eutectic can be realized by the composition variation of eutectic.Moreover,the eutectic structure transition condition depends on the solid phase volume fraction ofαandβphases.When the solid volume fraction ofαandβphases is in great difference,the eutectic tends to form the rod-shaped structure.When the solid phase volume fraction ofαandβphases is close,the eutectic alloy tends to form the lamellar structure.Meanwhile,the mathematic models of the eutectic growth have been established and the analy-tic solutions have been calculated through mathematic method.Furthermore,the effect of the process parameters on the interface morphology and the critical eutectic spacing has been investigated.In order to study the influence of the process parameters on the eutectic morphology,a state-of-the-art review on in situ experimentation studies with numerical simulation and analytic solutions calculation of the binary eutectic is presented.This review mainly focus on the effect of solidification velocity,eutectic composition and the volume fraction of solid phase on the eutectic spacing.The effect of the interphase boundary anisotropy and stability of the eutectic growth on the eutectic morphology are also analyzed.In particular,an in situ experimentation studies and analytic solutions calculation of the growth of a special rod-shaped binary eutectic is introduced.Problems in current researches and new perspectives for future research are proposed as well.