On the selection of rod-type eutectic morphologies: Geometrical constraint and array orientation

Rod eutectic growth in succinonitrile–(d)camphor is investigated experimentally using directional solidification with specimen thicknesses ranging from 20 to 200 μm. Measurements in the projected plane are reconciled with a model for array distortion under the constraint imparted by the geometry, and the influence of geometry is examined with respect to the two-dimensional (2D) array basis vectors. A transition from 3D to quasi-3D growth at very low velocity is observed, where specimen thickness becomes comparable to eutectic spacing. The principal finding is a velocity-dependent but thickness-independent transition in array orientation in the “bulk” regime. The transition is correlated with the dominant unstable mode in the initial succinonitrile front, and we assert that this instability controls the array spacing along the slide wall direction. It is concluded that this type of analysis is required for meaningful interpretation of experimental results regarding rod eutectic growth, even in the apparent thick-slab or “bulk” regime.     

Microstructural prediction of cast A356 alloy as a function of cooling rate

In this paper, a relatively new approach is presented in order to predict the microstructure of A356 using finite element technique and artificial neural network. In the training and test modules of the neural network, different primary and secondary dendrite arm spacing obtained from finite element method were used as inputs and eutectic volume percentage, silicon volume percentage, silicon rod spacing, average length of silicon rods and silicon rod diameter were used as outputs. After the training set was prepared, the neural network was trained using different training algorithms, hidden layers and neurons number in hidden layers. The results of this research were also used to form analytical equations followed with solidification codes for SUT Cast software.     

Morphology evolution of Al9(Mn,Ni)2 eutectic phase in Al-4Ni-2Mn alloy during heat treatment at 350 °C

The morphology evolution of Al₉(Mn,Ni)₂ eutectic phase in Al-4Ni-2Mn alloy at 350 °C was examined. Two kinds of morphology evolution trends of Al₉(Mn,Ni)₂ eutectic phase were observed by using scanning electron microscopy and transmission electron microscopy. The ribbon-like Al₉(Mn,Ni)₂ eutectic phase taking up a small portion of the eutectic structures gradually changes from ribbon-like morphology to rod-like during the heat treatment. The rod-like Al₉(Mn,Ni)₂ eutectics, primary eutectic structure in the alloy, become unstable at the transverse sub-boundaries of the eutectic rod when the samples are heat treated at 350 °C, and then split up into short rods with the extension of heat treatment time. Also, the Vickers microhardness test was used to characterize the change of local mechanical properties. The hardness test results indicate that local morphology evolution of eutectic phase has no obvious effect on the local mechanical properties of the alloy. The microhardness of the eutectic area increases slightly when the heat treatment time is extended to 192 h or 360 h.

     

Stability of lamellar eutectic growth

The morphological stability of lamellar eutectic coupled growth is investigated by means of quantitative three-dimensional phase-field simulations, both for a generic eutectic alloy with a symmetric phase diagram and for the transparent organic alloy carbontetrabromide–hexachloroethane. The instabilities that limit stable steady-state growth at large spacings are identified and compared to the instabilities known in thin samples. For all alloys and compositions investigated, the first instability to occur is a zigzag instability, which can saturate or lead to the breakup of the lamellae into rods or labyrinth structures, depending on the initial spacing and the volume fractions. The simulation results are in good agreement with recent experimental observations.     

Effects of electromagnetic stirring on microstructures of solidified aluminum alloys

1　INTRODUCTIONPractically ,mechanicalstirringandelectromag neticstirring (EMS)arewidelyusedtogenerateforcedflowinsolidificationandcastingofmetalsandalloys,resultingin greatchangesofthesolidifiedstructures,segregationsand propertiesofthecast ings .Muchattentionhasbeenpaidtosolidificationofdendriticalloyssuperposedwithforcedflow ,andmanypreviousworkshavebeenfocusedontheeffectsoffluidflowonthestructuraltransitions,suchascolumnar equiaxedtransitions(CET)ormacro segre gationinthealloys[13] .…     

定向凝固共晶生长的元胞自动机数值模拟

本文在已有的二元初生相元胞自动机（CA）方法的基础上,针对二元共晶凝固过程提出了改进的元胞自动机（MCA）模型.该模型考虑成分过冷和曲率过冷对界面形态的影响,通过界面溶质浓度守恒来获得共晶α相和β相生长速率,模拟了层片的湮灭、分叉与稳态生长.为了验证模型的可靠性,对常见的CBr₄-C₂Cl₆共晶透明合金进行了模拟,研究了抽拉速率对共晶层片间距大小的影响,模拟结果与文献中的实验结果吻合良好;同时模拟了共晶层片间距调整过程的形貌演化以及层片振荡不稳定性现象.本文将MCA模型扩展到三维定向凝固过程中,研究了共晶形态的层棒状转变机制.     

Dynamic instabilities of rod-like eutectic growth patterns: A real-time study

We present a real-time experimental study of the rod-like growth patterns formed during directional solidification in a non-faceted transparent eutectic alloy, succinonitrile–(d)camphor. Slightly convex isotherms were used to slowly increase the pattern spacing λ from an appropriate starting value to the threshold spacing for rod elimination or rod-splitting instabilities allowing a quantitative determination of these thresholds as a function of the solidification rate V. We show that the threshold spacing for rod splitting obeys the general λ ～ V^?1/2 scaling law of eutectic growth, whereas the threshold spacing for rod elimination deviates from this law at low V, exhibiting the same overstability effect as previously reported for lamellar eutectic patterns. We demonstrate that topological defects (walls between hexagon domains) play an important role in rod-splitting processes. We also describe a spatially incoherent mode of oscillation that we observed in disordered rod-like patterns.     

Faceted solidification morphologies in low-growth-rate Al-Si eutectics

In this article, recent findings from the investigation of solidification morphologies resulting from the lowvelocity directional solidification of Al-Si eutectic alloys are discussed. The development of a primary array of angular silicon rods is examined, and the twinned structure of the faceted silicon phase is shown to be an essential contributor to the mechanisms of growth, branching, spacing selection, and the evolution of 〈100〉 Si texture within the two-phase microstructure. For more information, contact R.E. Napolitano, Iowa State University, Department of Materials Science and Engineering, 204A Wilhelm Hall, Ames, IA 50011; (515) 294-9101; fax (515) 294-4291; e-mail raplhn@iastate.edu.     

Onset of rod eutectic morphology in directional solidification

Abstract

Initial dynamics of morphological selection in a succinonitrile-(D)camphor organic transparent rod eutectic system is investigated experimentally using directional solidification with specimen thicknesses of 20 and 200 μm. The shape of the solid/liquid interface, the specimen thickness, the initial single-phase boundary thickness, and the grain boundaries are all observed to influence the onset of the eutectic morphology in a geometrically constrained system. Additionally, initiation of eutectic growth within a thin liquid layer at the specimen slide wall and lateral propagation of the array is observed, as suggested in a previous study by the present authors.     

Effect of microstructure modulation on mechanical properties of Ti-Fe-Sn ultrafine eutectic composites

A series of (Ti_70.5Fe_29.5)_100−xSn_x alloys with x=0, 1, 3, 5, 7 and 9 were fabricated by a suction casting into cylindrical rods with a 3 mm diameter and 50 mm length. Microstructural investigations of these alloys revealed that Sn addition was effective in modulating the phase selection, length-scale of eutectic spacing and morphology of the colony in ultrafine eutectic structures upon solidification. The spherical morphology of the ultrafine eutectic colony was effective in enhancing the plastic strain in the samples. On theother hand, the formation of the bimodal eutectic structure stemming from a large temperature difference between two eutectic temperatures had a strong influence on modulating the phase selection and length-scale of the ultrafine eutectic structures and on controlling the strength and plastic strain of the ultrafine eutectic composites.     

Experimental and numerical modeling of equiaxed solidification in metallic alloys

Nucleation undercoolings are measured for the primary dendritic structure and for the eutectic structure in aluminum–copper samples solidified during electromagnetic levitation. The recorded cooling curves are used to determine the heat extraction rate and the solidification times. Metallurgical characterizations consist of composition measurements using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometry and analyses of SEM images. The distribution maps drawn for the average copper composition, the volume fraction of the eutectic structure and the dendrite arm spacing reveal strong correlations. An equiaxed solidification model based on mass and heat balances is developed for the interpretation of the measurements. The model predicts the effect of diffusion in the liquid and in the solid, as well as the consequences of the recalescences occurring immediately after the nucleation of the primary dendritic structure and the eutectic structure. The nucleation undercooling of the eutectic structure is shown to be a key parameter for a quantitative prediction of the fraction of phases in the solidified samples.     

控制参数对Al—Al3Ni共晶合金间距的影响

在不同温度梯度及生长速度下测量了Ａｌ－Ａｌ３Ｎｉ共晶合金间距选择范围。实验结果表明,在一定生长速度下,共晶间距存在一容许范围。其宽度随生长速度的增加减小,在某一温梯度下,共晶间距与生长速度的关系满足λ＾ｎｕ＝常数ｎ＝１．７－２．０）的关系,温度梯度增加,其最大,最小及平均间距离略有减小。同时,Ａｌ－Ａｌ３Ｎｉ合金的间距选择范围与初始生长速度无关。     

Influence of samarium content on microstructure and mechanical properties of recycled die-cast YL112 aluminum alloys

The influence of Sm(Samarium) content on microstructure and mechanical properties of recycled die-cast YL112 aluminum alloys was investigated.The results show that many small Sm-rich particles form in the recycled die-cast YL112 alloys with Sm addition.At the same time,the secondary dendrite arm spacing in the YL112 alloys modified with Sm is smaller than that of the unmodified alloy.The eutectic Si of recycled diecast YL112-xSm alloys transforms from coarse acicular morphology to fine fibres,Mechanical properties of the investigated recycled die-cast YL112 aluminum alloys are enhanced with Sm addition,and a maximal ultimate tensile strength value(276 MPa) and elongation(3.76%) are achieved at a Sm content of 0.6wt.%.Due to the modification of eutectic Si by Sm,numerous tearing ridges and tiny dimples on the fractures of tensile samples are observed.     

Theory of rod eutectic growth under far-from-equilibrium conditions: Nanoscale spacing and transition to glass

The growth of eutectic under large undercooling conditions is important in obtaining nanoscale composite microstructures. Many glass-forming eutectic systems also exhibit a fine rod eutectic microstructure and often show a direct transition from eutectic to glass with increasing undercooling at the interface. A theoretical model of rod eutectic growth is developed in this paper, which quantitatively evaluates the system and growth parameters that will give rise to large undercooling at the interface. In addition to the diffusion and capillary undercooling, the model incorporates the effects of a sharp decrease in the diffusion coefficient that is exhibited by fragile glass-forming systems, the presence of highly nonlinear liquidus lines at large undercooling, and the effects of non-equilibrium at the interface. The results of the model are then discussed to obtain an insight into the system and growth parameters that are critical for obtaining a large undercooling at the eutectic interface, which is important in the design of nanoscale composite materials and in the selection of potential glass-forming systems.     

定向凝固NiAl-9Mo共晶合金的凝固组织特性

采用液态金属冷却法在恒定温度梯度GL=334 K/cm,大生长速率范围内（2~300μm/s）对Ni-45.5Al-9Mo （摩尔分数,%）共晶合金进行定向凝固制备。研究生长速率（v）对纤维间距（λ）、纤维直径（d）和纤维体积分数的影响。在实验中发现平界面和胞界面两类共晶生长界面。在平界面和胞界面组织中,生长速率（v）与纤维间距（λ）和纤维直径（d）的关系经回归分析分别为：λv1/2=5.90μm·μm1/2·s1/2和 dv1/2=2.18μm·μm1/2·s1/2。Mo纤维的体积分数可在一定的范围内随生长速率进行调整,这是由生长过程中界面前沿过冷度的增加及共晶组织中各组成相的生长特性引起的。     

变速定向生长条件下Pb－Sn共晶组织变化

利用改进后的定向凝固装置，研究了在阶跃增速和线性增、减速的变速生长条件下，Ｐｂ－Ｓｎ共晶合金定向凝固共晶组织的变化。实验结果表明：当共晶试样缓慢线性增速或减速时，定向生长共晶片间距的调节分别通过两相共晶片的逐步分叉或合并缓慢进行；当共晶试样增速和减速的加速度值相同时，实际共晶生长速度的变化过程呈对称的逆曲线。     

Fe-Ni-P-B共晶合金的深过冷凝固组织演化

采用玻璃熔覆法使Fe-Ni-P-B共晶合金在不同过冷度条件下凝固,研究了其组织随过冷度的演化规律.结果表明,随着过冷度的增加,凝固组织形态逐渐从棒状规则共晶向不规则的粒状共晶组织转化.当T＜35 K时,棒状规则共晶组织随过冷度增加而逐渐细化;当35 K＜ T＜150 K时,凝固组织由团状非规则共晶与棒状规则共晶构成,且随着过冷度增加非规则共晶逐渐增多,规则共晶组织减少,共晶间距增大;当T＞150K时,获得完全非规则共晶组织.应用Jackson-Hunt共晶生长模型和枝晶熔断理论,对Fe-Ni-P-B共晶合金凝固组织形成机制进行了分析讨论.     

定向凝固规则共晶生长相间距的选择

在Ｊａｃｋｓｏｎ和Ｈｕｎｔ共晶模型的基础上，通过准动力学分析考察了规则共晶生长相间的选择原理，给出了相间距存在的上，下限，发现下限对应于稳定性参数较小的相被淘汰的临界状态，上限对应于稳定性参数较大的相办面出现凹陷生成另一相的临界状态，共晶间的最可见分布对应于凝固系统相变自由参最低的状态。     

The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

A melt encasement (fluxing) method was used to undercool Ag–Cu alloy at its eutectic composition. The recalescence of the undercooled alloy was filmed at a high frame rate. For undercoolings <60 K, a microstructure consisting of mixed anomalous and lamellar eutectic is observed. Analysis of eutectic spacing in the lamellar eutectic reveals little dependence upon the undercooling of the bulk melt and is consistent with growth at an undercooling of 1.5 K. Depending upon undercooling, the progress of the recalescence front may be either continuous or spasmodic, wherein periods of rapid growth are separated by significant interludes in which growth totally arrests. Analysis of spot brightness profiles reveals that, during continuous growth, the recalescence is characteristic of the advancement of a planar, space-filling front, while a double recalescence occurs during spasmodic growth, the first of which is characteristic of the propagation of a dendritic, or non-space-filling, front. It is concluded that, during spasmodic growth, the propagation of two-phase, or eutectic, dendrites is observed, which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling.     

NONSTABLE GROWTH OF Al-Si EUTECTIC IN CONSTRAINED CONDITION

Nonstable growth of Al-Si eutectic in constrained condition has been investigated by means ofchange of the withdrawing rate in an unidirectional solidification system,In constantacceleration growth,a retarded effect of response of eutectic interflake spacing on the growthrate has been observed.The response mechanisms are cluster-branching and local terminationextension of Si-phase.In constant deceleration growth,the cluster-branching and termina-tion mechanisms of silicon still control adjustment of the interflake spacing,but the spacing isdetermined by the initial growth rate and does not respond to the decreasing growth rate.