第二相粒子含量对基体晶粒长大影响的计算机仿真研究

利用作者自行开发和改良的ＭｏｎｔｅＣａｒｌｏ计算机仿真算法,对含不同数量第二相粒子的虚拟材料基体中三维晶粒长大过程的影响进行了仿真和定量分析。结果表明,随粒子体积分数增另,晶粒长大指数下降,停滞状态的晶粒尺寸分布宽度减小,通过计算机仿真获得了极限晶粒尺寸和粒子参量间的定量关系,并由几何模型给出了证明,与文献中已有理论模型,计算机仿真及实验结果进行的比较分析表明,本文所得结果与实测结果符合得更好。     

含第二相颗粒的晶粒长大过程相场法

第二相颗粒对晶界有很强的钉扎作用,大量第二相颗粒的存在将影响系统正常的晶粒长大过程.本文采用相场法研究了第二相颗粒尺寸大小和体积分数对晶粒长大过程的影响.模拟结果表明第二相颗粒体积分数越大,对晶界的钉扎作用越强,稳态时晶粒尺寸越小.第二相颗粒体积分数达到一定范围时可能发生个别晶粒的异常长大.单个第二相颗粒尺寸越大,其对晶界钉扎作用越强,但当体积分数一定时第二相颗粒尺寸越小,总的钉扎效果越强,最终系统的晶粒尺寸越小.通过相场模拟获得了极限晶粒尺寸与第二相颗粒参数的定量关系,相场模拟结果与实验结果和Monte Carlo 模拟结果相符合.     

相场法模拟多个空间取向的棒状第二相粒子对晶粒长大的影响

采用相场法研究多个空间取向的棒状第二相粒子以及圆形第二相粒子对基体晶粒长大的影响.结果表明:在晶粒长大过程中,绝大部分棒状第二相粒子位于晶界处并与晶界方向一致,圆形第二相粒子大多位于三晶交点处;第二相粒子表现出强烈的钉扎晶界的作用,极限晶粒半径可以用Zener关系表示;在第二相粒子面积分数和粒子尺寸相同的情况下,当第二相粒子面积分数较小(＜5%)时,棒状与圆形第二相粒子对晶粒长大的钉扎作用没有明显差别;当粒子面积分数较大(＞5%)时,棒状第二相的钉扎效果好于圆形第二相的钉扎效果.     

正常晶粒长大的计算机模拟(Ⅱ)--第二相粒子形状及取向的影响

采用改进的跃迁概率和MonteCarlo加速算法,模拟了第二相粒子不同形状及其取向对基体晶粒长大的影响.模拟结果表明:针状粒子钉轧晶界的效果好于球状粒子,但随着第二相面积分数fA的增加,晶粒平均尺寸受第二相形状的影响逐渐减小,而晶粒形态受到的影响逐渐增强;取向对基体晶粒长大没有明显影响.     

不同尺寸二相粒子材料晶粒长大的元胞自动机仿真

在现有晶粒长大元胞自动机器（CA）模型的基础上制定新的转变规则，使之能够模拟第二相粒子对晶粒长大过程的影响，且对含有不同尺寸第二相粒子材料的晶粒长大过程进行仿真模拟和定量分析。模拟结果表明：第二相粒子对基体组织具有钉扎作用，当粒子体积分数一定时，总体钉扎作用随着粒子尺寸的减小而增强；对于单一粒子而言，大尺寸粒子比小尺寸粒子具有更强的钉扎效果，该结果能够准确地反映第二相粒子对晶粒长大过程的影响规律，与现有的相关理论相符合。     

第二相粒子形状对晶粒长大的元胞自动机仿真的影响

在正常晶粒长大元胞自动机模型的基础上添加圆形、正方形以及针形第二相粒子,并且对含有不同形状第二相粒子的晶粒长大过程进行仿真模拟和定量分析.模拟结果表明:第二相粒子对基体组织有钉扎作用,粒子面积分数越大,粒子总体钉扎作用越强;当第二相粒子面积分数较小时,针形第二相粒子的钉扎作用强于圆形和正方形第二相粒子;随着第二相粒子面积分数的增加,晶粒长大受第二相形状的影响逐渐减小;第二相粒子面积分数足够大时,不同形状的第二相粒子对晶粒长大的钉扎作用没有明显差别.     

相场方法研究硬质颗粒钉扎的两相晶粒长大过程

采用相场方法模拟第三相颗粒钉扎的两相耦合的晶粒长大过程,系统地研究了第三相颗粒体积分数和尺寸大小对两相晶粒长大过程的影响.模拟结果表明,第三相颗粒体积分数越大,对晶界的钉扎作用越强,且极限晶粒尺寸越小.单个第三相颗粒尺寸越大,对晶界钉扎作用越强.但当体积分数一定时第三相颗粒尺寸越小时,颗粒数目会越多,此时总的钉扎效果会越好,晶粒极限尺寸也越小.若晶粒长大系统同时引入两种不同大小的第三相钉扎颗粒,且两种颗粒所占比例相同时,钉扎效果最好.相场方法模拟所得到的二相多晶材料晶粒组织演化规律和晶粒生长指数、晶粒形态、生长动力学和拓扑结构特征与已有实验和理论结果相符合.     

相场法模拟不同形状的硬质颗粒对两相晶粒长大的影响

采用相场方法研究了不同颗粒体积分数及尺寸条件下不同形状的硬质颗粒对两相系统晶粒长大的影响,结果表明:球形颗粒大多处于三角晶界处,片状颗粒处于晶界处且沿晶界分布.不同形状的硬质颗粒对体积占优的α相晶粒长大无明显影响,对体积分数较小的β相晶粒长大的影响主要取决于颗粒数目.颗粒数目较少时,不同形状的硬质颗粒对β相晶粒长大无明显影响;颗粒数目较多时,片状颗粒比球形颗粒对β相晶粒长大的阻碍作用强烈.颗粒体积分数越大,颗粒对晶界的钉扎作用越强,稳态时晶粒的半径越小;颗粒尺寸越大,单个颗粒对晶界的钉扎作用越强,但总的钉扎作用越弱,稳态时晶粒的半径越大.     

第二相颗粒对晶粒长大影响的三维相场模拟

为了理解颗粒钉扎作用,应用多序参量晶粒长大相场模型,采用周期性边界条件,研究了三维条件下第二相颗粒的尺寸分布、体积分数、空间分布对基体晶粒长大的影响。结果表明,由于钉扎作用,含第二相颗粒的基体晶粒尺寸更加均匀,呈正态分布;与颗粒尺寸正态分布的基体相比,颗粒尺寸均匀分布的基体晶粒尺寸分布更宽;基体晶粒尺寸随第二相体积分数的增大而减小;晶粒平均半径随着颗粒分布不均匀程度的增加而降低,但当第二相颗粒均匀分布在基体中时,晶粒平均半径最小。     

相场法模拟球形和盘形第二相粒子对晶粒长大的影响

利用相场法模型,模拟研究含不同尺寸和面积分数的球形和盘形粒子的二维系统中晶粒的长大特征,揭示第二相粒子对晶粒长大的影响规律.结果表明:初始阶段晶粒长大符合长大指数n 为0.3～0.4的指数长大规律,其n与系统单位面积所含的粒子数量密切相关;晶粒长大过程中绝大多数粒子位于晶界处,其最终的平均晶粒半径可以用Zener关系表示;当粒子尺寸和面积分数一定时,粒子的形状对晶粒的长大过程没有明显影响.     

Effect of second-phase particle morphology on grain growth kinetics

Second-phase particles are often employed to inhibit grain growth in polycrystalline metals and ceramics. In this work, we studied the effect of second-phase particle morphology on the effectiveness of inhibiting grain boundary migration using the phase-field method. We employed a multi-order parameter phase-field model in combination with an efficient memory allocation strategy which allows large-scale and coalescence-free grain growth simulations. We analyzed the dependence of pinning forces on the particle size and shape, and performed computer simulations of grain growth in the presence of second-phase particles with different sizes and varying aspect ratios. We also discuss the relationship between the pinned grain size and size distributions.     

Phase-field theory of grain growth in the presence of mobile second-phase particles

We have developed a phase-field model for grain growth in the presence of mobile second-phase particles. In this model, each grain and particle is represented by a unique order parameter. The grain boundaries sweep the mobile particles during grain growth. The particle velocity is taken to be proportional to the driving force arising from the curvature of the phase boundary in the neighborhood of the particle. The proportionality factor is the constitutive parameter representing the mobility of the particle. We first study the model in a one-dimensional axisymmetric setting and compare the results with theoretical calculations. We then study the interaction of a bicrystal grain boundary with a dilute distribution of particles. Finally we show the effect of particles on polycrystalline grain growth.     

Ti(C,N)对Ni基高温合金组织结构、力学性能和抗氧化性的影响

基于相场法,以第二相颗粒平衡体积分数不同的合金为对象,研究第二相颗粒的沿晶析出、长大及其对基体相晶界的钉扎作用.结果表明,第二相颗粒的平衡体积分数越大,其形核孕育期越短,形核与粗化的速度也越快;然而在第二相颗粒的长大阶段,平衡体积分数较大(15％～25％)时第二相颗粒长大速度较慢,而平衡体积分数较小(10％)时则长大速度较快.随着第二相颗粒平衡体积分数的增大,其对晶界的钉扎作用也逐渐增强,但平衡体积分数增大到一定程度,钉扎作用将趋于稳定.第二相颗粒对晶粒极限尺寸的影响遵循Zener关系.     

Phase transformations in Al–Cu thin films: precipitation and copper redistribution

The (micro-)structural changes occurring in thin Al–Cu films of about 500 nm thickness and containing up to 1 at% Cu have been investigated. In particular, the copper distribution and the precipitation behaviour have been studied in situ during thermal cycling between 323 and 773 K. After slow cooling, large second-phase particles are observed which are mostly located at grain boundary triple points; the shape and size depend on the copper concentration. The average distance between these particles is about 16 μm, which is larger than the average grain size of approximately 1 μm. No second-phase particles have been observed within the aluminium grains. After cooling, a relatively large amount of copper (about 0.2 at%) as compared with bulk Al–Cu (0.001 at%) is not contained in second-phase particles. Most likely, this copper has segregated at grain boundaries, interfaces and dislocations. The temperature at which Al₂Cu starts to form in the thin films is well below the copper solvus for bulk material.     

A unified theory of recovery, recrystallization and grain growth, based on the stability and growth of cellular microstructures-II. The effect of second-phase particles

A simple theory which analyses discontinuous and continuous recovery, recrystallization and grain growth, and which enables the relationships between the various annealing phenomena to be explored, has been extended to include the effects of a dispersion of second-phase particles. Application to grain growth results in predictions which are consistent with those of more complex theories, and the model is also used to analyse the effect of a dispersion of second-phase particles on recrystallization occurring by either strain induced boundary migration or by particle-stimulated nucleation. An analysis of subgrain growth shows that this may occur discontinuously under conditions in which recrystallization is not possible, thereby suggesting the possibility of a thermomechanical processing route in which complete softening may occur without recrystallization.     

The Effect of Submicron Second-Phase Particles on the Rate of Grain Refinement in a Copper-Oxygen Alloy During Cold Spray

The effect of non-deformable submicron second-phase particles (d = 200-500 nm) on microstructural refinement during cold spray was examined. Using single particle impact testing, two types of splats were fabricated using two different feedstocks: a Cu-0.21wt.%O powder containing Cu₂O second-phase particles and a single-phase Cu. Microstructural evolution analysis using high-resolution electron backscatter diffraction shows grain refinement occurred at a higher rate in the Cu-0.21wt.%O powder. That was due to dynamic recrystallization initiated by particle-stimulated nucleation (PSN). High-strain-rate deformation of cold spray was found to be the key to activate PSN. The present study suggests cold spray is a possible technique to fabricate ultrafine-grained materials by using feedstock containing second-phase particles.     

Zr对高强度钢热影响区第二相粒子及韧性影响

研究了0.012 4%锆对低合金高强度钢焊接热影响区粗晶区第二相粒子和冲击韧性的影响.结果表明,模拟20 kJ/cm焊接线能量下无锆钢焊接热影响区粗晶区中第二相粒子为Al-Ti复合氧化物和（Ti,Nb） N析出物.而含锆钢则是Zr-Al-Ti复合氧化物及（Al,Ti,Nb） N和（Ti,Nb） N析出物.同时,定量数据分析表明含锆钢中氧化物和氮化物粒子密度更高且尺寸更加细小.这些高密度的细小的第二相粒子在焊接过程中能有效钉扎晶界移动,抑制奥氏体晶粒粗化,在焊接热影响区粗晶区中得到尺寸相对细小均匀的原奥氏体晶粒,使得含锆钢焊接热影响区粗晶区呈现韧性断裂和极好的低温冲击韧性.