深过冷Ni—Sn共晶合金的快速凝固机制

本文通过净化法使 Ni-32.5wt-%Sn 共晶合金液获得深过冷,对该合金液在不同过冷条件下的凝固机制和组织进行了研究。结果表明:当过冷度小于约10K 时,该合金液凝固生成 Ni_3Sn相和 Ni(α)相层片共晶。在深过冷条件下,由于 Ni_3Sn 枝晶的自由生长速度远大于 Ni(α)枝晶的自由生长速度,再辉过程中,Ni_3Sn 相和 Ni(α)相不能以匹配方式生长,而由 Ni_3Sn 相作为领先相以枝晶簇方式生长。再辉过程中形成的枝晶簇,其内部 Ni_3Sn 枝晶进一步熔断粗化及 Ni(α)相在Ni_3Sn 枝晶间形成生长,最后形成非规则共晶组织。当过冷度小于130K 时,再辉之后,枝晶簇间存留有较大体积的成分仍为 Ni-32.5wt-%Sn 的合金液,这部分合金液在共晶平台阶段以层片共晶方式凝固,所以试样内部的组织由非规则共晶区和层片共晶区组成。     

深过冷Sb-Sn过包晶合金的快速凝固

采用落管无容器处理技术研究了Sb74.7Sn25.3二元过包晶合金的快速凝固,获得的合金粒子直径D介于70～1080μm之间。理论计算表明,随着粒子直径的减小,过冷度和冷却速率均呈指数关系增大,最大过冷度为298K(0.36TL)。研究发现,在自由落体条件下,快速凝固组织由初生Sb固溶体相和包晶SbSn金属间化合物相组成,Sb固溶体相以非小平面和小平面两种生长方式长大。当过冷度增大时,释放的熔化潜热增多,初生相逐渐细化,非小平面初生Sb相由"粗大枝晶"向"碎断枝晶"转变,当D<400μm时,一次枝晶臂显著变短,二次枝晶间距明显减小;同时发生溶质截留现象,初生Sb固溶体相中溶质Sn的固溶度发生了显著拓展,由ΔT=32K时的7.86%(原子分数,下同)线性增大至ΔT=298K时的10.47%。     

深过冷Cu-Pb偏晶合金的快速凝固行为和凝固组织

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-25%Pb合金,Cu-37.4%Pb偏晶合金和过偏晶Cu-40%Pb(质量分数)合金过冷熔体凝固行为和凝固组织的演化规律,以及Cu-37.4%Pb偏晶合金的过冷度对磨损率的影响.研究表明:在过冷亚偏晶Cu 25%Pb合金熔体凝固过程中先形成α(Cu)初生相,随着过冷度的增大,凝固组织经历粗大枝晶重熔形成的细化枝晶向准球状晶粒演化的过程;在过冷Cu-37.4%Pb偏晶合金熔体凝固过程中初生相为L2相,当过冷度在20～150 K区间时,得到第二相S(Pb)弥散在α(Cu)枝晶间的凝固组织,并且在该过冷区间内随着过冷度的增加,材料的磨损率也逐渐降低;在过冷过偏晶Cu-40%Pb合金熔体凝固过程中初生相为L2相,在过冷度区间42～80 K时,得到以偏晶胞形式分布的凝固组织.     

基于三维LBM-CA模型模拟Al-4.7%Cu合金的枝晶形貌和成分分布

建立了三维格子玻尔兹曼方法(LBM)-元胞自动机(CA)耦合数值模型,并用该模型模拟研究了Al-4.7%Cu(质量分数)固溶体合金的凝固过程。该耦合模型采用元胞自动机方法模拟枝晶的生长,同时采用基于分子动力学理论的格子玻尔兹曼方法模拟合金凝固过程中的温度场、流场以及溶质场。模拟结果再现了合金凝固过程中的三维枝晶形貌变化以及溶质富集过程,并将三维流场因素考虑进去,定量研究了自然对流、过冷度对单枝晶形貌和成分分布的影响。研究表明,在纯扩散条件下,枝晶呈现对称的生长现象,模拟自由枝晶稳态生长的尖端速度、尖端半径和过冷度的关系与Lipton-Glicksman-Kurz(LGK)理论模型吻合得较好。在自然对流条件下,枝晶的生长形貌呈现不对称性,即枝晶生长在迎流方向上得到了促进,在顺流方向上受到了抑制。熔体过冷度对枝晶生长的影响较大,过冷度的增加导致枝晶生长加快,二次枝晶增多且呈现出粗化现象,枝晶尖端固液界面处的溶质浓度偏高,加重了溶质偏析。     

Simulation of dendrite growth of Fe–C alloy using phase field method

采用相场法模拟了Fe--0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长, 研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响, 获得了具有二次分枝的枝晶形貌, 再现了枝晶生长过程及枝晶臂之间的竞争生长. 模拟结果表明: 凝固过程中存在溶质富集和枝晶偏析, 枝晶主干溶质浓度最低, 枝晶臂之间的液相浓度最高. 随着过冷度的增大, 枝晶生长加快且分枝发达; 界面厚度直接影响枝晶的生长速度; 各向异性影响枝晶的形态; 晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

用相场方法模拟Fe-C合金枝晶生长

采用相场法模拟了Fe-0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长,研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响,获得了具有二次分枝的枝晶形貌,再现了枝晶生长过程及枝晶臂之间的竞争生长.模拟结果表明:凝固过程中存在溶质富集和枝晶偏析,枝晶主干溶质浓度最低,枝晶臂之间的液相浓度最高.随着过冷度的增大,枝晶生长加快且分枝发达;界面厚度直接影响枝晶的生长速度;各向异性影响枝晶的形态;晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.     

深过冷Ni80.3B19.7合金的再辉和非规则共晶的形成

采用熔融玻璃净化结合气体保护的方法,使Ni80 3B19 7过共晶合金获得了407 K的大过冷度,研究了其在不同过冷度下快速凝固过程中的再辉行为.结果表明,Ni80 3B19.7过共晶合金在0～112 K过冷度范围内无明显再辉,在112～323 K过冷度范围内,其再辉曲线表现为两个再辉峰,而在323～407 K过冷度范围内,其再辉曲线为一个再辉峰.初生固相含量的随着过冷度的增大而增大,导致一次再辉度随着过冷度的增大而增大.深过冷Ni80 3B19.7合金凝固组织中非规则共晶的形成,归因于共晶两相在快速凝固阶段以自由枝晶的形式进行的非耦合生长和再辉后的慢速凝固阶段两相枝晶所发生的形态上的转变.     

Cu-Cr合金初生相对共晶生长的影响机制研究

制备了定向凝固Cu-1.0%Cr亚共晶自生复合材料,研究了初生α相生长对共晶生长的影响机制,探讨了亚共晶合金中共晶的生长规律.研究结果表明,Cu-1.0%Cr合金定向凝固时,在初生α相间生长的共晶受到初生相生长的影响,在热场不定向和生长空间受限的双重作用下,共晶无定向地杂乱生长.初生α相的生长引起枝晶间液相溶质分布的变化,随着凝固速度的增大,初生α枝晶间液相溶质的浓度分布趋于平缓,成分趋近于CE.Cu-1.0%Cr合金在快速凝固条件下,初生α相生长改变了共晶的生长环境,致使形成非平衡凝固组织--离异共晶.     

强制对流作用下多枝晶生长的相场法研究

采用耦合溶质场、温度场和流场的相场模型,对Ni-Cu合金凝固过程中多枝晶生长进行模拟,研究了多枝晶生长形貌及温度场和溶质场分布.结果表明:熔体流动显著改变凝固前沿的传热和传质,从而影响枝晶生长.受过冷熔体冲刷,枝晶逆流侧前沿溶质浓度和温度低,枝晶臂尖端生长迅速;枝晶顺流侧前沿溶质浓度和温度高,枝晶臂尖端生长缓慢.在熔体...     

边界热通量作用下枝晶生长相场模型的有限元法模拟

基于Beckermann和Karma枝晶生长相场模型,建立耦合溶质场、温度场的相场模型,采用有限元法对控制方程进行求解,研究凝固过程Al-3.0%(质量分数)Cu合金在边界热通量作用下的枝晶生长行为。结果表明,边界热通量作用能够显著改变凝固前沿的传热和传质,影响枝晶生长形貌。在边界抽热条件下,枝晶前沿温度降低,实际过冷度增大,从而促进二次枝晶生长发育,界面前沿溶质扩散层薄,枝晶微观偏析严重。而边界加热条件下,枝晶前沿温度升高,实际过冷度减小,抑制枝晶生长发育,界面前沿扩散层厚,枝晶微观偏析减弱。     

深过冷DD3高温合金的两次细化机制

用复合熔盐净化与循环过热相结合的方法,获得了最大210K过冷度,研究了DD3高温合金过冷熔体凝固组织的演化规律,在所获得的过冷度范围内,凝固组织的形态发生两次晶粒细化,发生第一次细化的过冷度为30－70K,因枝晶熟化,重熔,高度发达的树枝晶转变为第一类粒状晶;发生第二次细化的过冷度超过153K,凝固组织因枝晶碎断和再结晶而志变为第二类粒状晶。     

Microstructure evolution in undercooled Co80Pd20 alloys

High undercooling has been achieved in Co₈₀Pd₂₀ melts by employing the method of molten glass denucleating combined with cyclic superheating, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–415 K, two kinds of grain refinements have been observed in the solidification microstructures. The three critical undercoolings are 72, 95, and 142 K, respectively. When undercooling is less than 72 K, the coarse dendritic morphology is formed, which is similar to the conventional as-cast microstructure. The first grain refinement occured in the range of undercooling, 72–95 K can be attributed to the breakup of dendrite-skeleton owing to remelting. When undercooling locates within 95–142 K, highly developed directional fine dendrite can be obtained because the severe solute trapping weakens the effect of solute diffusion during the dendrite growth. The second grain refinement occurred when undercooling exceeds the critical undercooling (∆T* = 142 K), the formation of fined equiaxed microstructure can be ascribed to the stress that originates from the extremely rapid solidification process, which resulted in the dendrite fragmentation finally.     

Modeling of a transition to diffusionless dendritic growth in rapid solidification of a binary alloy

Diffusionless growth of dendritic crystals results in microsegregation-free microstructures with an initial (nominal) chemical composition of solidifying systems. Normally, a transition from chemically partitioned growth to diffusionless solidification is accompanied by the morphological transition in crystal shape with the appearance of nonlinearity in the kinetic behavior of growing crystals. This phenomenon is discussed using a model of local non-equilibrium rapid solidification. Considering the transition from the solute diffusion-limited growth to purely thermally controlled growth of dendritic crystals, the model predicts the abrupt change of growth kinetics with the break points in the “dendrite tip velocity-undercooling” and “dendrite tip radius-undercooling” relationships. It is shown that the abrupt change of growth kinetics occurs with the ending of the transition to purely thermally controlled growth and the onset of diffusionless solidification. To predict the dendrite growth kinetics in a whole region of undercooling, numeric analysis shows that the model has to take into account both anisotropies of solid–liquid interfacial properties. These are anisotropy of surface energy and anisotropy of atomic kinetics of solidification.     

基于一种改进CA模型模拟双辊连续铸轧纯铝微观组织

基于枝晶生长的基本传输过程和元胞自动机(Cellular Automaton,简称CA)-有限元(Finite Element,简称FE)模型基本原理,建立了适应双辊连续铸轧纯铝薄带工艺特点的凝固过程形核和晶体生长的数学模型.模型耦合了宏观温度场和微观组织模拟计算,考虑了溶质扩散、曲率过冷和各向异性等重要因素的影响,定义了界面单元捕获规则,能够模拟凝固过程中枝晶生长的形态.应用本模型对双辊连续铸轧纯铝薄带凝固过程中等轴晶生长、等轴晶多晶粒生长及柱状晶生长、柱状晶向等轴晶演化进行模拟并与实验结果进行对比,模拟结果与实验结果吻合较好,验证了模型的正确性.     

Growth kinetics in levitated and quenched Nd-Fe-B alloys

We investigated the growth kinetics and the effect of quenching conditions on rapid solidification of undercooled Nd-Fe-B melts with compositions near the Nd-₂-Fe₁₄-B (2-14-1) phase. We prepared melt drops of various undercooling levels (up to 300 K below the liquidus temperature) were prepared by the electromagnetic levitation method and subsequently quenched them onto chill substrates. We measured the solidification kinetics of the undercooled melts in situ using a high-resolution Si photodiode. In accordance with the nucleation theory, the properitectic γ-Fe phase nucleates at first during the undercooling process. There were two different solidification routes, with the observed route depending on the undercooling level of the levitated melt prior to quenching. The peritectic reaction is favored in melts with high undercooling levels prior to quenching. Low previous undercooling levels lead to primary solidification of the 2-14-1 phase on quenching. The thickness of the homogeneous 2-14-1 phase zone, grown directly at the substrate side, depends strongly on the undercooling level prior to solidification. We estimated the growth velocity of the 2-14-1 phase from temperature-time-characteristics to be of the order of 1 mm/s. These investigations give rise to improved understanding about the high sensitivity of the microstructure of Nd-Fe-B alloys on different rapid solidification procedures     

Phase-field simulation for the evolution of solid/liquid interface front in directional solidification process

In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode, and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part I: Model description

Part I of this two-part investigation presents a volume-averaging multiphase solidification model that accounts for mixed columnar-equiaxed solidification, non-dendritic and dendritic crystal growth, nucleation of equiaxed grains, columnar primary dendrite tip tracking, melt flow, sedimentation of equiaxed crystals, and their influence on macrostructure and macrosegregation. Five distinct thermodynamic phases (phase regions) are defined: solid dendrites in equiaxed grains, the interdendritic melt between equiaxed dendrites, solid dendrites in columnar trunks, the interdendritic melt between trunk dendrites, and the extradendritic melt. These five phase regions are quantified by their volume fractions and characterized by their solute concentrations. The five phase regions are grouped into three hydrodynamic phases: equiaxed grains consisting of solid dendrites and interdendritic melt, columnar trunks consisting of solid dendrites and interdendritic melt, and extradendritic melt. The extradendritic melt is separated from the interdendritic melt with a grain envelope, whose profile connects the primary, secondary or tertiary dendrite tips to form a ‘natural’ enclosure of the equiaxed grains or columnar trunks. The envelope is further simplified as a volume-equivalent sphere for equiaxed grains, or as volume-equivalent cylinder for columnar trunks by use of morphological shape factors. Expansion of the envelopes during solidification is determined by dendrite growth kinetics, using the Kurz–Giovanola–Trivedi model for growth of columnar primary dendrite tips and the Lipton–Glicksman–Kurz model for growth of columnar secondary dendrite tips (radial growth of the columnar trunk) and equiaxed primary dendrite tips. The solidification of the interdendritic melt is driven by the supersaturation of the interdendritic melt and governed by the diffusion in the interdendritic melt region. Illustrative process simulations and model verifications are presented in Part II.     

Rapid Growth of TiNi intermetallic compound within undercooled Ti50Ni50 alloy under electrostatic levitation condition

The undercooling dependence of the solidification mechanism was systematically explored by the elec-trostatic levitation(ESL)facility.During the experiments,the maximum undercooling reached up to 406 K(0.26 TL)and the growth velocity of the primary TiNi phase was in-situ determined at various undercool-ings.At the initial increase of alloy undercooling,the value of growth velocity sluggishly rose followed by a power function.In this case,the primary TiNi phase preferentially developed as the equiaxed dendrite,then the remnant liquid participated as Ti2Ni and α-Ti phases on the grain boundary.Once the under-cooling exceeded the critical value of 350 K,the growth velocity of the primary phase displayed a sharply increase tendency.Meanwhile,the TEM results demonstrated that the precipitation of the intermetallic Ti2Ni compound was gradually restrained during the rapid solidification and the R-phase existing in the TiNi matrix at large undercooling implied that the martensitic transformation was incomplete.     

The dendrite growth kinetics of nickel-based alloys

The solidification velocity of several compositions of Ni–Ti and Ni–Sn alloys were measured as a function of undercooling by the direct imaging of levitated drops. During this investigation a plateau in the solidification velocity was observed at intermediate undercoolings as a direct result of the addition of tin and titanium to nickel. Past work has shown an additional solidification velocity plateau at high undercoolings can be attributed to residual oxygen solute. From these results, a logistic growth model for alloy solidification was developed that can describe the solidification velocity as a function of undercooling and reproduce the plateau behavior. Finally, it is shown that a logistic growth model is more accurate for describing the solidification of alloys than thermodynamic models based on the Ivanstov solution.