【作者】 余勇；

【导师】 戎咏华； 陈裕泽；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2008， 博士

【摘要】 本文开展了氦和位错对HR-2合金力学性能影响的多尺度模拟研究。在微观尺度上,利用修正的嵌入原子势(MEAM)与分子静力学(MS)计算了氦团簇的形成能与原子组态,利用动力学蒙特卡罗(KMC)模拟了氦团簇的演化过程。在细观尺度上,建立了三维离散位错动力学(3DDD)模型,并模拟了位错克服氦团簇的热激活滑移过程。在宏观尺度上,建立了基于位错密度演化的HR-2合金物理本构模型,并将氦团簇的热激活应力引入到本构方程以描述氦对HR-2合金力学性能的影响。在数值模拟中,对相关的模型及计算方法进行了研究。并开展了HR-2合金力学性能试验研究与位错组态分析,探讨了HR-2合金力学行为的微细观机制。主要有如下一些结论:a) MS计算结果表明间隙He原子(HeI)在α-Fe和γ-Fe中均以四面体间隙位最稳定。氦-空位(He-V)团簇的形成能随He/V值增大而增大,空位对He-V团簇有稳定作用,而HeI的加入使He-V团簇能量上升而引发自捕陷反应。b) KMC模拟表明热空位与热激活能对He的演化起着不同的作用。高温时,大量的热空位捕获了大多数的HeI,使之不能充分聚集而实现自捕陷;而温度较低时,热激活能随温度升高而增大,开动了不同类型的自捕陷反应,而使团簇随之变大。c)建立的3DDD模型能较准确地模拟Frank-Read位错源的临界剪切应力、临界位错组态,且与试验值接近。模拟了不同应变率加载下FCC晶体的位错演化与应力-应变曲线,所建的数值计算方法能有效地模拟位错偶以及低应变率加载下的力学行为。d)位错演化微分方程与力学状态微分方程所组成的方程组具有严重的刚性性质,即外应力的变化远高于位错密度的变化,这与真实材料的性质相一致。e)用Langevin力来描述温度对位错的热效应,模拟了位错克服Peierls应力的热激活滑移过程,并拟合了Peierls阻碍势垒,所得结果与试验值相当。f) HR-2合金具有明显的应变强化、应变率强化和温度软化效应,这是由于位错滑移的短程阻碍势垒小引起的。温度和应变率对位错组态演化有不同的作用,即温度升高位错组态逐渐从网格状位错墙向位错胞方向发展,而应变率升高则使位错运动方式以平面滑移为主,当应变率足够高时则出现孪晶,以满足变形速率要求。g)根据FCC晶体塑性变形的林位错切割机制,建立了基于位错密度演化的HR-2合金物理本构模型。拟合的模型参数与材料微结构特性相一致,并且模型能在较宽温度与应变率范围内描述该合金的力学行为。h)在KMC与位错动力学模拟基础上,拟合了氦团簇的热激活应力本构方程,并引入到HR-2合金的本构模型中以描述了氦对HR-2合金的强化效应。结果表明,利用热激活本构方程评估的强化效应比临界分切应力(CRSS)评估的更有效。本文将串行的多尺度模拟方法应用于模拟氦对HR-2合金力学性能的影响。通过微观模拟得到氦团簇的分布,然后利用位错动力学模拟位错与氦团簇间的相互作用得到氦团簇引起的热激活应力,最后将其引入到HR-2合金的物理本构模型中以模拟氦对宏观力学性能的影响。该方法一方面兼顾了物理本构模型对宏观力学行为描述的准确性,另一方面兼顾了微细观模拟(如位错动力学)对材料微结构特性的描述。更多还原

【Abstract】 The multiscale simulations of the effect of helium and dislocation on the mechanical properties of HR-2 alloy are carried out in this paper. At the microscale, the modified embedded atom method (MEAM) and molecular statics (MS) are used to calculate the formation energies and the atom configurations of the helium clusters. And, the kinetic Monte Carlo (KMC) is used to simulate the evolution of helium clusters. At the mesoscale, the three-dimensional discrete dislocation dynamics (3DDD) is constructed and used to model the dislocation sweep the helium clusters. The constitutive equation for describing the thermally-activated stress of helium cluster is fitted. At the macroscale, based on the evolution of dislocation density, a physically based constitutive model of the HR-2 alloy is constructed. The thermally-activated stress of helium clusters is added into the constitutive model for describing the effect of helium on the mechanical property of HR-2 alloy. In the above simulation, the models and the numerical methods are investigated. In addition, the mechanics and the dislocation configuration of HR-2 alloy are investigated for discussing the micro-mechanism of the mechanical behavior. Main conclusions are described as following:a) The MS calculation results indicate that the tetrahedral interstitial position inγ-Fe orα-Fe is the most stable site for interstitial helium atom (HeI). The formation energies of the He-V clusters are increase with increasing the values of the He/V. The vacancy can stabilize the He-V cluster, but the HeI added into the cluster will increase the cluster energy and bring about the self-trap action.b) The KMC simulation demonstrates that thermal vacancies and thermal activation energy have different effects on the helium behavior. At the high temperature, plenty of thermal vacancies will trap the most of HeI, which make the HeI can not congregate and can carry out self-trap. At the low temperature, as the thermally-activated energy increases with elevating the temperature, the bigger He-V clusters are formed because the different self-trap actions are carried out.c) The 3DDD model can accurately simulate the critical shear stress and dislocation configuration of the Frank-Read dislocation source, which are consistent with the experiment. The mechanical behavior and the dislocation evolution in FCC crystal are simulated. The simulation results indicate that our numerical algorithm can efficiently simulate the dislocation dipole and mechanical behaviour under the low strain rate loading.d) The differential equation group consisted of dislocation evolution and mechanical state has serious stiff property, namely the external stress changes more quickly than dislocation evolution, which is consistent with the property of the real material.e) The Langevin force is used to describe the thermally-activated effect of temperature on the dislocation. The thermally slip of dislocation overcoming the Peierls stress is simulated, and the activation energy of Peierls stress is fitted, the result accords with experiment data.f) Due to the barrier energy of the dislocation slipping is relative low, HR-2 alloy has the evident strain strengthen, strain rate strengthen and temperature soften. The temperature and the strain rate have different effects on the evolution of dislocation. With the temperature elevating, the configuration of dislocation changes from the wall to the cell. While with the strain rate increasing, the dislocation mostly plane slips. If the strain rate is enough high, the deformed twins are appeared in order to satisfy the deforming rate.g) According to the forest dislocation cutting mechanism of FCC metal, a physically based constitutive model of HR-2 alloy is developed, based on the dislocation density evolution. The model parameters are consistent with the microstructural properties of the HR-2 alloy, and this model can accurately predict the complicated mechanical behavior of the alloy in a rather wide range of temperatures and strain rates.h) Based on the simulations of KMC and 3DDD,, the thermally-activated constitutive equation of the dislocation overcoming the helium clusters is fitted. Induced the thermally-activated stress caused by the helium clusters into the HR-2 constitutive equation, the hardening effect of helium on the HR-2 alloy is described. The result indicates that using the thermally-activated constitutive equation to evaluate the hardening effect is better than using the critical resolved shear stress (CRSS).In this paper, a serial multiscale model is used to simulate the effect of helium on the mechanical properties of HR-2 alloy. That is using the mircroscale simulation to obtain the distribution of helium clusters, and using the dislocation bynamics to model the interaction between the dislocation and helium cluster for obtaining the thermally-activated stress caused by the helum cluster, lastly added this stress into the physically based constitutive model for describing the effect of helium on the mechanical property. This method gives attention not only to the describing ability of the physically based constitutive equation for the mechanical property, but also to the properties of the microstructures simulated by the micro- or meso-scale models, such as dislocation dynamics.更多还原