Modeling of aluminum-silicon irregular eutectic growth by cellular automaton model

Due to the extensive application of Al-Si alloys in the automotive and aerospace industries as structural components, an understanding of their microstructural formation, such as dendrite and(Al+Si) eutectic, is of great importance to control the desirable microstructure, so as to modify the performance of castings. Since previous major themes of microstructural simulation are dendrite and regular eutectic growth, few efforts have been paid to simulate the irregular eutectic growth. Therefore, a multiphase cellular automaton(CA) model is developed and applied to simulate the time-dependent Al-Si irregular eutectic growth. Prior to model establishment, related experiments were carried out to investigate the influence of cooling rate and Sr modification on the growth of eutectic Si. This CA model incorporates several aspects, including growth algorithms and nucleation criterion, to achieve the competitive and cooperative growth mechanism for nonfaceted-faceted Al-Si irregular eutectic. The growth kinetics considers thermal undercooling, constitutional undercooling, and curvature undercooling, as well as the anisotropic characteristic of eutectic Si growth. The capturing rule takes into account the effects of modification on the silicon growth behaviors.The simulated results indicate that for unmodified alloy, the higher eutectic undercooling results in the higher eutectic growth velocity, and a more refined eutectic microstructure as well as narrower eutectic lamellar spacing. For modified alloy, the eutectic silicon tends to be obvious fibrous morphology and the morphology of eutectic Si is determined by both chemical modifier and cooling rate. The predicted microstructure of Al-7Si alloy under different solidification conditions shows that this proposed model can successfully reproduce both dendrite and eutectic microstructures.

A steady solution of the gasar eutectic growth in directional solidification

This paper presents the general mathematical model on gasar eutectic growth in directional solidification. Using multiple scale expansion and matching method, we obtain the global steady solution of gasar eutectic growth as the Peclet number ε≤1, where ε is defined as the ratio of half an inter-pore spacing and solutal diffusion length. We also give the interfacial shape and predict the porosity of gasar eutectic growth. Results show that porosity is mainly dependent on gas pressure above the metal melt, but independent of pulling velocity. Our predicted results are in agreement with experimental data.

EUTECTIC GROWTH OF CAST IRON UNDER RAPID SOLIDIFICATION

The eutectic growth of cast iron during rapid solidification has been studied with laser remelt- ing and deep etching technology.A new mode of mushroom eutectic growth was observed oth- er than the usual honeycomb and lamellar ones.The mushroom eutectic growth may finally form the lamellar dendritic structure.

Crystallographic evidences for twin-assisted eutectic growth in undercooled Ni-18.7 at.%Sn eutectic melts

Rapid solidification in undercooled Ni-18.7 at.%Sn eutectic melts was observed in-situ by a high-speed high-resolution camera and the microstructures were characterized in detail by electron backscattering diffraction.For the first time,the exact crystallographic orientation relations(ORs)between HCP-Ni_(3)Sn(α-Ni)subsets were analyzed.For HCP-Ni_(3)Sn,the{1121}<1126>and/or{1122}<1123>twin ORs(i.e.,HCP-Ni_(3)Sn twins)hold independently on undercooling,whereas forα-Ni,the{111}<112>twin OR is the case at low undercooling and would hold initially at intermediate and high undercooling.The roles of twinning and allotropy transformation(i.e.,FCC-Ni_(3)Sn→HCP-Ni_(3)Sn)were integrated to reveal the formation mechanism of HCP-Ni_(3)Sn twins,and a reversed OR transition analysis was carried out for rep-resentative samples from low to high undercooling.Consequently,novel twin-assisted eutectic growth was found to occur all along.On this basis,we showed that the single nucleation mode of Herlach is followed,and speculated that primary and secondary coupled eutectic dendrite growth and un-coupled growth ofα-Ni and FCC-Ni_(3)Sn might all be the origins of anomalous eutectics.This work would shed some lights on the long-time controversies about the nucleation mode and the formation mechanism of anomalous eutectics in undercooled eutectic alloys.

Effects of Strontium,Magnesium Addition,Temperature Gradient,and Growth Velocity on Al-Si Eutectic Growth in a Unidirectionally-solidified Al-13 wt%,Si Alloy

Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium （Sr） and magnesium （Mg） additions, growth velocities and temperature gradients. Macro- and micro- scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying AI-Si eutectic front and the final morphology of eutectic grains in the solidified A1-13 wt% Si alloys. By varying （tailoring） these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition （CET） of eutectic structure, which can be interpreted on the basis of Hunt＇s Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.

PHASE-FIELD MODELING OF ELASTIC EFFECTS IN EUTECTIC GROWTH WITH MISFIT STRESSES

Elastic interactions,arising from a difference of lattice spacing between two coherent phases in eutectic alloys with misfit stresses,can have an influence on microstructural pattern formation of eutectic colonies during solidification process.From a thermodynamic point of view the elastic energy contributes to the free energy of the phases and modifies their mutual stability.Therefore,the elastic stresses will have an effect on stability of lamellae,lamellae spacing and growth modes.In this paper,a phase-field model is employed to investigate the influence of elastic misfits in eutectic growth.The model reduces to the traditional sharp-interface model in a thin-interface limit,where the microscopic interface width is small but finite.An elastic model is designed,based on linear microelasticity theory,to incorporate the elastic energy in the phase-field model.Theoretical and numerical approaches,required to model elastic effects,are formulated and the stress distributions in eutectic solidification structures are evaluated.The twodimensional simulations are performed for directed eutectic growth and the simulation results for different values of the misfit stresses are illustrated.

Microstructural Evolution of Rapidly Solidified Co-Mo and Ni-Mo Eutectic Alloys

Droplets of Co-37.6 wt pct Mo and Ni-47.7 wt pct Mo eutectic alloys were rapidly solidified during containerless processing in a 3 m drop tube. A kind of anomalous eutectic appears in these two eutectic alloys when undercooling is beyond 56 and 61 K, respectively. The two eutectic phases in anomalous eutectic were observed to grow in dendrite manner. The formation of anomalous eutectic is ascribed to the cooperative dendrite growth of the two independently nucleated eutectic phases. Current dendrite and eutectic growth theories are applied to describe the observed processes.

Effect of Rare Earths on Solidification Microstructure and High Temperature Mechanical Property of Sn60-Pb40 Solder alloy

Trace rare earth elements were used in order to strengthen the Sn60 Pb40 solder alloy. The experimental results show that the high temperature tensile strength of near eutectic Sn60 Pb40 solder alloy is increased by about 70% after adding trace rare earth elements. Analysis shows that the high affinity between rare earth element and Sn leads to the variation of contact angle at the three phase junction of S/L interface during eutectic growth and further changes the Pb concentration at the S/L interface needed for coupled eutectic growth. Thus the eutectic microstructure can directly grow upon the primary Pb rich phase and the formation of coarse Sn rich halo is suppressed. Therefore homogeneous metallurgical microstructure can be obtained.

Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior.Herein,the microstructure trans-formation of faceted Al_(2)O_(3)/Er_(3)Al_(5)O_(12) thermal emission eutectic composite ceramics is explored over wide ranges of compositions(13.5 mol%-22.5 mol%Er_(2)O_(3))and solidification rates(2-200μm/s).Entirely cou-pled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%-22.5 mol%Er_(2)O_(3) at low solidification rates.The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion.In ad-dition,the mechanisms of irregular eutectic spacing selection and adjustment under different solidifi-cation rates are revealed based on Magnin-Kurz model.A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling.According to the well mi-crostructure tailoring,the flexural strength of Al_(2)O_(3)/Er_(3)Al_(5)O_(12) eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation.The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refin-ing microstructure.The as-designed Al_(2)O_(3)/Er_(3)Al_(5)O_(12) composites with microstructural tailoring have great potential as integrations of structural and functional materials.