针对熔化焊过程建立了宏微观耦合模型,模拟了熔池内不同区域凝固过程中随机取向枝晶的竞争生长过程.通过宏观三维有限元模型计算熔池中瞬态的传热传质过程,利用双线性插值算法将凝固参数传递给微观组织模型.采用元胞自动机法模拟随机取向的枝晶在熔池凝固条件下的竞争生长过程.模拟结果表明,所建立的微观模型能够精确模拟任意生长取向的枝晶.凝固条件中最大温度梯度方向对枝晶竞争过程有明显选择作用,生长方向与最大温度梯度方向相同或接近的枝晶在竞争中具有更大优势.焊缝中的晶粒组织由枝晶簇发展形成,晶粒组织的形貌演变取决于相邻枝晶簇之间的竞争过程,具有择优取向的枝晶簇会逐渐排挤非择优取向的枝晶簇并最终将其阻挡在凝固组织内部,宏观晶粒的取向与其内部枝晶簇的生长方向并不一定相同.熔池中心线附近区域在焊接过程中具有更小的温度梯度、更大的凝固速率以及更大的局部冷却速率,凝固后可以获得更加细小的焊缝枝晶组织.枝晶间距的模拟结果与相应凝固条件下的试验数据符合较好.

A macro-micro coupled model is developed to simulate the competitive dendrite growths in different areas of the welding pool in the solidification process. The transient solidification conditions in welding pool are obtained by the three-dimensional (3D) macro-scale FEM model. The thermal conditions used in the micro-scale cellular automata model is obtained from the macro-scale FEM model by using the interpolation algorithm. The simulation results indicate that the micro-scale cellular automata model developed in this paper can simulate the morphologies of dendrites with various growth directions accurately. The solidification conditions in welding pool have obvious effects on the competitive dendrite growth. The dendrites with their preferential orientations parallel to the direction of the highest temperature gradient are more competitive. The morphology of grain structure is determined by the competition among different dendritic arrays. The dendritic arrays with more fav

凝固过程中横向限制挡板的存在对晶体微观结构的演化存在重要的影响,不同性质的横向挡板将产生不同的限制效应,对最终凝固微观组织形成起着决定性作用.本文利用非等温相场模型,定性地模拟了纯金属Ni凝固过程中横向限制的存在对其枝晶微观形貌演化的影响,研究了不同尺寸及性质的横向挡板对枝晶微观结构形成的影响,讨论了横向限制对不同初始枝晶间距枝晶形貌发展的作用.计算结果表明,横向限制挡板的存在将直接影响凝固过程中微观组织的形貌演化过程并最终改变微观结构.随着横向挡板间距的减小,微观组织变化更加明显;挡板初始温度越低,枝晶形貌改变越明显;初始枝晶间距越大,形貌变化越明显;不同挡板高度对微观结构具有基本相同的影响.

@@@@Lateral constrain in the presence of melting has a significant effect on microstructure evolution of crystal growth, and this effect is related to the size and property of lateral constrain, thus determining microstructure formation during solidification. In the paper, mi-crostructure evolution in the presence of lateral constrain during the solidification of pure Ni metal is simulated using a non-isothermal phase-field model. Effects of size and properties of lateral constrain are simulated and studied, also microstructures formed at different initial dendritic arm distances are discussed. Results indicate that the presence of lateral constrain has a direct influence on pattern evolution which determines the microstructure formation. Microstructure changes significantly with lateral constrain distance turning small, initial constrain temperature low becoming low and initial dendrite arm distance growing. Different heights of lateral constrains have almost the same effects on micro

通过在自由能泛函中引入各向异性参数得到了一个基于高斯内核的改进晶体相场模型，并采用该模型研究了体心立方结构(BCC)枝晶生长的原子堆垛过程。结果表明，在BCC由正十二面体平衡形貌演化为枝晶组织过程中，形核位置经历了由面心({110}面)到尖端(?100?取向)的转移，进而发生界面失稳形成枝晶组织；枝晶生长过程中，新的固相原子首先在枝晶尖端附近形核，并快速向尖端及根部生长，枝晶尖端被新原子完全包覆后将再次诱发液相原子附着形核及生长；随初始液相密度的增加，固-液界面移动速率增加，速率系数的各向异性也增强。

On the basis of the Gaussian kernel phase field crystal model (PFC), we propose a modified PFC model. The atom-attaching process of three-dimensional body-center-cubic (BCC) dendritic growth is examined by using the modified PFC model. Our simulations indicate that in the process of the morphology evolution from regular dodecahedron to dendrite shape, the nucleation position of new layer is transferred from the center of {110} planes into the region of{110}plane near the?100?tips, and then the BCC dendritic morphology is obtained. In the process of dendritic growth, first, new solid atom absorption takes place near dendrite tips, then liquid atoms start to grow up on the existing solid phase rapidly. After the dendrite tips are completely occupied by new atoms, new nuclei begin to form again. Increasing the initial atom density n will increase the velocity coe?cient C and the anisotropy of C.

采用类金属透明模型合金丁二腈-1.8 wt%丙酮(SCN-1.8 wt%Ace)合金,研究了平行于生长界面前沿的液相对流对定向凝固胞/枝晶生长行为及胞/枝晶间距的影响.对于胞晶生长,在液相对流作用下,其尖端将会出现分岔,使得胞晶间距减小,并且液相对流流速越大,胞晶尖端分岔越明显,胞晶组织越细小,胞晶间距越小.至于枝晶生长,其生长行为与胞晶不同.当抽拉速度较小时,液相对流作用下枝晶两侧三次臂的生长速度将会超过枝晶尖端生长速度,形成新的枝晶列,使得枝晶一次间距减小,并且液相对流流速越大枝晶一次间距越小;当抽拉速度较大时,液相对流作用下迎流侧二次臂生长发达,且会抑制上游枝晶生长,使得枝晶一次间距增大,并且液相对流越强枝晶一次间距越大.

The cellular and dendritic formations are two kinds of typical morphology in the solidification, and there are many theoreti-cal models and experimental researches on them. Most models and researches are based on purely diffusive transport mechanism. However, convection effects are of importance in the evolution of cellular and dendritic growth. Since the metal materials are not transparency and the researches on microstructure only after quenching, it is difficult to observe the dynamic microstructure evolution in real time. In this paper, the effect of liquid flow on the cellular and dendritic growth was investigated by the in-situ observation of SCN-1.8 wt%Ace transparent alloy during the directional solidification under the liquid flow. The cellular tip splitting is found in the presence of liquid flow and the cellular microstructure is smaller after the cellular tip splitting. The cellular spacing decreases as the flow rate becomes larger, but the spacing will become steady ultima

本文采用差示扫描量热仪及透射电子显微镜研究了热处理工艺对NCu30-4-2-1合金组织及性能的影响。结果表明，固溶温度为850℃时，合金组织变化不明显；随着固溶温度的提高，枝晶间共晶组织逐渐溶解，当固溶温度为950℃时，枝晶组织基本消失，合金中β强化相基本溶入固溶体基体中，形成单相过饱和固溶体组织；继续提高固溶温度至1050℃时，晶粒异常长大。增加时效温度和时间，合金硬度值先增大，达到峰值后趋于平稳。合金经时效处理600℃×8 h，主要析出相为细小弥散分布的β′-Ni3 Si相，并与基体保持良好共格关系，合金强度硬度最大，断后伸长率较高。

Effect of heat treatment on microstructure and properties of NCu30-4-2-1 alloy was investigated by means of differential scanning calorimetry ( DSC) and transmission electron microscope ( TEM) , respectively.The results show that the microstructure has insignificantly change when solution temperature is 850 ℃ compared with the original cast microstructure.Inter-reticular dendritic distribution α+βeutectic dissolves gradually as the solution temperature increased.It shows that single phase supersaturated solid solution structure generates at 950 ℃along with the decomposition of dendrite structure and βstrengthening phase.When the solution treated temperature is raised to 1050 ℃, the grains begin grow abnormally.With increase of treated temperature and aging time, the hardness raises to peak value which finally decreases stably.The major precipitate is β′-Ni3 Si phase which disperses uniformly in the matrix when aged at 600 ℃for 8 h.Theβ′-Ni3 Si phase remains coheren