Measurements of experimentally grown 3-D xenon multiplets and comparative phase-field simulations

Experimentally observed growth morphologies (dendrites, doublons, and seaweed) of three-dimensional (3-D) xenon crystals during free growth into pure supercooled melt were investigated in this study. Measurements of the 3-D reconstruction of dendrites were compared to analytical predictions and scaling parameters were determined. For two different phase-field models morphology diagrams were derived and compared to analytical model calculations. By using special initial conditions it is possible to reproduce the growth morphologies found in these experiments.     

Growth morphologies of decagonal quasicrystal in highly undercooled Al_(72)Ni_(12)Co_(16) alloy melt

The electromagnetic melting and cyclic superheating method was applied to undercool the Al72Ni12Co16 alloy melt, and a maximum undercooling, 180K was obtained. Growth morphologies were characterized by using optical microscopy, scanning electron microscope(SEM) and transmission microscopy(TEM). The microstructural morphologies indicate that a continuous growth mode of D-phase along the periodic orientation of ten-fold axis is preferred at large undercoolings. According to the Toner's step growth mode of quasicrystal, the preferred continuous growth along the periodic orientation of ten-fold axis is caused by the loss of potential barrier for nucleating steps along this direction.     

相场模型参数对枝晶形貌的影响

定量研究了相场模型中过冷度、温度对相场的耦合系数、各向异性系数等参数对晶粒形貌的影响，讨论了不同枝晶形貌和海藻状形貌的产生和转化，研究了生长速度与晶粒形貌及相场参数的联系，分析了枝晶生长尖端的分叉及边枝的产生原理。     

Orientation selection in solidification patterning

Crystal growth has been recognized as a paradigm for non-equilibrium pattern formation for decades. Scientific interest in this field has focused on the growth rates and curvature of branches in snowflake-like structures patterned after a solid’s crystallographic orientations. In reality, there exists a much richer variety of crystal patterns in nature. Investigations of dendritically solidifying alloys reveals structures that continuously change orientation between different growth directions, some of which are not along preferred crystallographic directions. The selection mechanism of such patterns is poorly understood. In this paper we demonstrate computationally and experimentally that a material’s surface tension anisotropy can compete with anisotropies present in processing conditions during solidification to produce a continuous transition from dendritic to seaweed and fractal-like structures. The phase space of such morphologies is characterized and the selection principles of the various morphologies explored are explained. These results have direct relevance to microstructure formation in commercial lightweight metal castings.     

Revealing the Diversity of Dendritic Morphology Evolution During Solidification of Magnesium Alloys using Synchrotron X-ray Imaging: A Review

In this paper, the diversity of complicated dendrite microstructure and its evolution behavior during solidification in different magnesium alloys under various processing conditions were illustrated using synchrotron X-ray imaging technique. A variety of dendritic morphologies and branching structures were revealed, i.e., sixfold plate-like symmetric structure in Mg-Al-based structure, 12-branch structure in Mg-Zn-based alloys and 18-branch structure in Mg-Sn- and Mg-Ca-based alloys as well as seaweed like hyper-branched structure in Mg-38wt%Zn alloy. In addition, a dendrite morphology and orientation transition with increasing addition of Zn content were also observed in Mg-Zn alloy, with dendrite growth pattern transform from anisotropy (low Zn addition) with sixfold symmetric snow-flake structure to relative isotropy (intermediate Zn addition) where seaweed morphology presented and then back to anisotropy (high Zn addition) when only 12 branches with preferred < $11\overline{2}1$ > orientations were observed. The phase-field model representing the typical dendritic morphologies and branching structures under various conditions was also depicted and discussed. Further, the two-dimensional (2D) real-time dendrite growth dynamics in different Mg-based alloys captured using synchrotron X-ray radiography for unveiling the originate of the α-Mg dendrite was reviewed. Following this, the four-dimensional (3D + time) synchrotron X-ray tomographic in situ observation of dendritic morphology evolution indicating the formation mechanism of the diverse dendritic morphology during Mg-Sn- and Mg-Zn-based alloys was also summarized. Finally, the future study on exploring the complicated dendritic morphologies and their origination during solidification of Mg-based alloys is prospected.     

Co-Sn单相合金深过冷凝固时Co3Sn2 相生长形貌的演变机制

开展了(Co₆₀Sn₄₀)_100-xNb_x (x=0,0.4,0.6,0.8,at%)单相合金的深过冷凝固实验,研究了Co₃Sn₂相生长形貌的演变机制。结果表明,在小过冷度下,Co₃Sn₂相在x=0,0.4以海藻状的模式进行生长,随着添加的Nb含量增加至0.6at%,其生长形貌转变为树枝晶,并在x=0.8进一步转变为分形海藻晶,这主要是由于界面能各向异性和动力学各向异性的变化。随着过冷度的增加,(Co₆₀Sn₄₀)_99.4Nb_0.6合金中Co₃Sn₂相生长形貌在过冷度大于28 K时从树枝晶转变为分形海藻,当过冷度高于143 K时转变为密集海藻。少量的Nb添加在小过冷度和中间过冷度时能提高Co₃Sn₂相的生长速度,但是在大过冷度下会显著降低生长速度。Co₃Sn₂相生长速度随过冷度变化规律的转变对应其生长形貌从分形海藻向密集海藻的转变。     

Influence of ordering kinetics on dendritic growth morphology

The effect of interfacial ordering kinetics on growth morphology is examined for undercooling solidification of intermetallics. The focus is on NiAl, which has been found to solidify into completely irregular seaweed-like morphologies at growth velocities of ～2 m s^?1. The transition from regular dendritic to seaweed growth in this material is simulated using a hybrid deterministic–probabilistic model of microstructure evolution, taking account of the interfacial kinetic effects. Modelling shows that seaweed formation in intermetallic forming systems can be attributed in part to transient entrapment of the chemical and orientational disorder at the solid/liquid interface. Both types of disorder trapping give rise to strongly preferential depression of the interfacial temperature, while orientational disorder trapping also reduces the definitiveness of crystal orientation at the interface. These effects counterbalance the role of crystallographic anisotropy in promoting growth into preferred directions, and hence lead to relatively easy breakdown of regular dendritic patterns in highly ordered materials.     

Effects of rate on crack growth in a rubber-modified epoxy

The rate-dependent fracture behavior of a 10-phr rubber-modified epoxy was investigated using double-cantilever-beam tests at various crosshead speeds. Dramatic rate effects were observed in the R-curve behavior and in the relationship between the applied energy-release rate and the crack velocity. Furthermore, a transition between fracture with toughening mechanisms operating (kinetic crack growth) and brittle behavior (dynamic crack growth) was observed. This transition depended on the crack velocity and applied energy-release rate. Such behavior is expected to depend on how the intrinsic toughness and/or the extrinsic toughening mechanisms are influenced by strain rate. It was shown that the size of the process zone was only weakly dependent on the crack velocity until the onset of dynamic fracture. Furthermore, the extent of void growth was virtually independent of the crack velocity in the kinetic regime. These results appear to rule out the notion that crack-tip shielding is significantly affected by rate effects in this rubber-modified epoxy. Rather, the rate effects may arise from a rate-dependent intrinsic toughness. It was observed that the intrinsic toughness decreased significantly with increasing crack velocity. The crack instability was shown to be associated with an abrupt cessation of the development of the process zone, with both cavitation and void growth being totally suppressed.     

突扩管条件下材料的冲刷腐蚀机理研究Ⅱ不锈钢

利用新近研制的双相流冲刷腐蚀激光多普勒实验装置,通过失重测试、表面形貌观察、局部流速流态测试和典型部位的电化学测试,研究了突护管条件下不锈钢ＡＩＳＩ３１６Ｌ在单相及含砂人工海水介质中的冲刷腐蚀机理。结果表明：冲刷腐蚀峰值发生在距突扩位置约３．０Ｄ的部位（Ｄ为粗管直径）,与切向流速扰动度的峰值位置相同。在３．０Ｄ和５．５Ｄ的部位,冲刷作用是主要的,而在回流区（１．５Ｄ）,则为氧扩散控制,不同部位的冲     

半固态金属组织的形成模型及模拟

分析了半固态金属制备过程中搅拌状态下晶粒生长的浓度场及温度场，将凝固速度与液体流动速度作为晶粒生长形态的影响因素,建立了液体运动状态下的晶粒生长形态模型，并且模拟了在不同半固态制备条件下晶粒的生长形态．理论模拟与实验结果相符合.     

扩散界面场变量模型模拟晶粒长大过程

采用扩散界面场变量模型模拟二维晶粒长大的动力学过程，利用该模型可以避免常规方法中由于格点离散化带来的晶粒长大各向异性，并通过建立晶界能与梯度能量系数间的关系模拟得到了异常晶粒长大过程的微观组织演化图模拟结果得出的晶粒形态、动力学和拓扑学结构特征等与已有的实验结果和理论分析一致．     

Rapid Crystallization of Levitated and Undercooled Semiconducting Material Melts

Using a CO₂ laser-equipped electromagnetic levitator, we carried out the containerless crystallization of Si and Ge. From the point of interface morphologies, the relation between growth velocities and undercoolings was classified into three regions. In regions I and II, although the morphologies of growing crystals are different: plate-like needle crystals in region I and facetted dendrite at region II, the growth velocities in these two regions are fundamentally scaled by the thermal diffusivities and the temperature increase caused by the release of the latent heat. This result means that the growth velocity can be expressed by the product of the thermal diffusivity and the growth kinetics. An analysis of the dendrite morphologies revealed that the kinetics of crystal growth in regions I and II represent two-dimensional nucleation at the reentrant corner formed at the edge of the two parallel twins. In region III, thermal diffusion-controlled interface attachment kinetics control as described by a modified Wilson?CFrenkel model.     

等离子弧焊中电弧反翘现象的数值模拟

根据流体的质量、动量、能量守恒方程,建立了穿孔等离子弧焊接过程中的等离子电弧三维数学模型,用磁矢量法求解磁场问题.模型包括了一部分喷嘴和钨阴极,小孔也被包含进模型中.利用ANSYS有限元分析软件求解模型,得到等离子电弧的温度分布,以研究等离子弧焊中电弧反翘现象.结果表明,等离子电弧反翘随小孔尺寸的增大而减弱,电弧尾焰随小孔尺寸的增大而增强;而适当的增加焊接速度以使小孔轴线与电弧轴线之间形成一定的偏差是形成等离子电弧反翘现象的必要条件;焊接电流主要是通过改变小孔尺寸而对电弧反翘产生影响.     

《110》和《112》取向Tb-Dy-Fe超磁致伸缩合金的定向生长

采用区熔定向凝固方法,在30-720 mm/h生长速度范围内,均能获得〈110〉轴向择优取向生长的Tb-Dy-Fe合金;凝固形态随生长速度提高出现从平面晶到胞状晶、树枝晶转变;低速生长(30 mm/h)的样品磁致伸缩性能没有压力效应,高速生长(≥120 mm/h)的样品磁致伸缩性能具有明显的压力效应.低生长速度时〈110〉取向样品按双{111}系孪晶生长,高生长速度时按单{111}系孪晶生长,由此计算样品的磁致伸缩性能与实验结果基本吻合在更高生长速度下(900 mm/h)获得沿〈112〉轴向高度择优取向的样品,其磁致伸缩性能与高生长速度〈110〉取向样品性能相当,具有明显的压力效应.     

A three-dimensional cellular automaton model of dendrite growth with stochastic orientation during the solidification in the molten pool of binary alloy

A three-dimensional (3D) cellular automaton model is developed for the prediction of dendrite growth with stochastic orientation during solidification process in the molten pool of binary alloy. An angle-information transfer method is proposed for improving cellular automaton technique to simulate the growth of the dendrites whose preferred growth direction owns stochastic misorientation with respect to the direction of the coordinate system. Dendrite morphologies and solute distributions of single dendrite growth and multi-dendrite growth are able to be obtained by the simulation using present model. The model is also employed to study the difference between two-dimensional simulation and 3D simulation on solute segregation and dendritic growth. Using the established model, 3D multi-columnar dendrites with stochastic crystallographic orientations can be obtained efficiently, and the competitive growth and impinging of dendrites can be reproduced in practice. The simulation results agree well with the experimental results.     

A phase field model for spontaneous grain refinement in deeply undercooled metallic melts

The origin of spontaneous grain refinement in deeply undercooled metallic melts has been of enduring interest within the solidification literature. Here we present the results of phase field simulations of dendritic growth into pure undercooled melts, at growth velocities up to 35 m s⁻¹. We find that, at low growth velocities, dendrite morphologies are broadly self-similar with increasing growth velocity. However, above ≈15 m s⁻¹ the initiation of side-branching moves closer to the dendrite tip with increasing growth velocity. This appears to be related to the level of kinetic undercooling at the tip. Once side-branch initiation begins to occur within 1–2 radii of the tip, profound morphological changes occur, leading to severe thinning of the dendrite trunk and ultimately repeated multiple tip-splitting. This process can be invoked to explain many of the observed features of spontaneous grain refinement in deeply undercooled metallic melts.     

INTERFACE MORPHOLOGIES AND MICROSTRUCTURES DURING DENDRITE-TO-CELL TRANSITION AT HIGH GROWTH RATE

The interface morphologies and microstruetures of the directionally solidified Ni-5wt-% Cualloy during dendrite-to-cell transition at high growth rates have been investigated with anewly developed apparatus for unidirectional solidification with the temperature gradient atthe solid/liquid interface higher than 1000 K/cm.The results show that in the vicinity ofdendrite-to-cell transition point,the well developed sidebranches become shrivelled with theincrease of growth rate and disappear at the dendrite-to-cell transition,and the primaryspacing decreases simultaneously.Moreover,the length of mushy zone decreases greatly dur-ing the dendrite-to-cell transition.Cells obtained at high growth rates have very similarmorphologies to those at low growth rates,but with much smaller cell spacings andunsmoothed cell walls which may be attributed to the different stability conditions of the cellwalls at low and high growth rates respectively.     

Synthesis and characterization of Sm3+-doped CeO2 powders

Sm3 -doped CeO2 (denoted as Ce1-xSmxO2) powders with different morphologies were successfully synthesized via a precursor-growth-calcination approach, in which precursor was first synthesized by a hydrothermal method and Ce1-xSmxO2 powders were finally obtained through a calcination process. The products were characterized with X-ray diffractometry(XRD), field emission scanning electron microscopy(FE-SEM) and fluorescence spectroscopy. The results reveal that the Ce1-xSmxO2 powders obtained by calcining the precursors prepared in the absence and presence of poly(vinyl pyrrolidone) (PVP) exhibit bundle- and sphere-like morphology, respectively. The possible growth process was proposed by preparing a series of intermediate morphologies during the shape evolution of CeO2 based on the SEM image observation. It is also found that the luminescence intensity of bundle-like Ce1-xSmxO2 is enhanced in comparison with that of sphere-like one due to its special morphology.     

Mathematical model of determination of die bearing length in design of aluminum profile extrusion die

Based on the finite element simulation of profile extrusion process, the effect of local extrusion ratio, die bearing area and the distance between extrusion cylindrical center and local die orfice center on mental flow velocity was investigated. The laws of deformed metal flow on profile extrusion process were obtained. The smaller the local extrusion ratio, the faster the metal flow velocity； the smaller the area of die bearing, the faster the metal flow velocity； the smaller the distance of position of local die orifice(the closer the distance of position of local die orifice from extrusion cylindrical axis), the faster the metal flow velocity. The effect of main parameters of die structure on metal flow velocity was integrated and the mathematical model of determination of die bearing length in design of aluminum profile extrusion die was proposed. The calculated results with proposed model were well compared withthe experimental results. The proposed model can be applied to determine die bearing length in design of aluminum profile extrusion die.     

超声振动辅助激光熔化沉积316L不锈钢的微观组织演变研究

在激光熔化沉积316L不锈钢的过程中耦合不同功率的超声振动,研究了超声功率对晶粒形态尺寸、凝固组织形成机制以及晶粒生长特性的影响。研究表明,激光熔化沉积过程中316L不锈钢晶粒发生定向凝固外延生长,形成粗大的柱状晶。施加超声会将沿[100]方向外延生长的粗大原生柱状晶打碎,产生细化晶粒的效果；沿柱状晶外延生长方向传递的超声振动增大了晶粒内沿长轴方向的累积取向差,提高了平均位错密度。施加超声有助于加强熔池流体的对流,降低沿沉积方向的温度梯度,使得垂直生长的柱状晶更快转变为八字形柱状晶；同时提高合金凝固冷速,使得柱状晶宽度以及晶粒内部一次枝晶列间距减小,实现宏观晶粒尺寸与微观枝晶间距的细化。