Rapid solidification of undercooled Al-Cu-Si eutectic alloys

Under the conventional solidification condition, a liquid aluminium alloy can be hardly undercooled because of oxidation. In this work, rapid solidification of an undercooled liquid Al_80.4Cu_13.6Si₆ ternary eutectic alloy was realized by the glass fluxing method combined with recycled superheating. The relationship between superheating and undercooling was investigated at a certain cooling rate of the alloy melt. The maximum undercooling is 147 K (0.18T _E). The undercooled ternary eutectic is composed of α(Al) solid solution, (Si) semiconductor and θ(CuAl₂) intermetallic compound. In the (Al+Si+θ) ternary eutectic, (Si) faceted phase grows independently, while (Al) and θ non-faceted phases grow cooperatively in the lamellar mode. When undercooling is small, only (Al) solid solution forms as the leading phase. Once undercooling exceeds 73 K, (Si) phase nucleates firstly and grows as the primary phase. The alloy microstructure consists of primary (Al) dendrite, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic at small undercooling, while at large undercooling primary (Si) block, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic coexist. As undercooling increases, the volume fraction of primary (Al) dendrite decreases and that of primary (Si) block increases. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101, 50395105) and the Doctorate Foundation of Northwestern Polytechnical University (Grant No. CX200419)     

Peritectic solidification under high undercooling conditions

The solidification characteristics of highly undercooled Cu-7.77%Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 TL). It is found that the α(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases , which is lower than the thermodynamic calculation result owing to the actual non-adia-batic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite growth velocities of primary α(Co) phase are subsequently calculated as a function of undercooling on the basis of this model.     

Formation of ζ phase in Cu-Ge peritectic alloys

Rapid growth behavior of ζ phase has been investigated in the undercooling experiments of Cu-14%Ge, Cu-15%Ge, Cu-18.5%Ge and Cu-22%Ge alloys. Alloys of the four compositions obtain the maximum undercoolings of 202 K（0.17TL）, 245 K（0.20TL）, 223 K（0.20TL） and 176 K（0.17TL）, respectively. As the content of Ge increases, the microstructural transition of ＂a（Cu） dendrite ＋ ζ＂ peritectic phase → ζ＂ peritectic phase →, ζ dendrite ＋ （ε＋ζ） eutectic＂ takes place in the alloy at small undercooling, while the microstructural transition of ＂fragmented α （Cu）dendrite ＋ ζ peritectic phase →, ζ peritectic phase →ζ dendrite ＋ ε phase＂ happens in the alloy at large undercooling. EDS analysis of the Ge content in peritectic phase indicates that undercooling enlarges the solid solubility of ζ rdendrite, which leads to a decrease in the Ge content in ζ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ζ peritectic phase, the Ge content in ζ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.     

Rapid dendrite growth in quaternary Ni-based alloys

Dendritic growth is one of the most common micro-structural formation mechanisms during crystal growth. Its morphology provides the kinetics information of crystal growth. Therefore, it is valuable to perform the research on rapid dendrite growth in order…     

Rapid eutectic growth during free fall

Rapid eutectic growth of Sb-24%Cu alloy is realized in the drop tube during the free fall under the containerless condition. Based on the analysis of crystal nucleation and eutectic growth in the free fall condition, it is indicated that, with the increase of undercooling, microstructural transition of Sb-24%Cu eutectic alloy proceeds from lamellar to anomalous eutectic structure. Undercoolings of 0 –154 K have been obtained in experiment. The maximum undercooling exceeds to 0.19T_e. Calculated results exhibit that Cu₂Sb compound is the primary nucleation phase, and that the primary Sb dendrite will grow more rapidly than the eutectic structure when undercooling is larger than 40 K. The eutectic coupled zone around Sb-24%Cu eutectic alloy leads strongly to the Cu-rich side and covers a composition range from 23.0% to 32.7%Sb.     

Rapid solidification mechanism of Ag60Sb34Cu6 ternary alloy in drop tube

Ternary eutectic growth involves competitive nu-cleation and growth of three solids from one liquid. Thesolidification behavior of ternary eutectic alloy is morecomplex than that of binary eutectic alloy due to the addi-tion of the third component[1—4]. Up to now, most scientificinvestigations on ternary eutectic alloy focus on the influ-ence of changing the component or adding a fourth even afifth element on the performance of the alloy[5—8]. How-ever, the information on crystal growth char…     

Dendrite growth and micromechanical properties of rapidly solidified ternary Ni-Fe-Ti alloy

The rapid solidification of undercooled liquid Ni_(45)Fe_(40)Ti_(15)alloy was realized by glass fluxing technique.The microstructure of this alloy consists of primaryγ-(Fe,Ni)phase and a small amount of interdendritic pseudobinary eutectic.The primaryγ-(Fe,Ni)phase transferred from coarse dendrite to fragmented dendrite and the lamellar eutectic became fractured with the increase of undercooling.The growth velocity ofγ-(Fe,Ni)dendrite increased following a power relation with the rise of undercooling.The addition of solute Ti suppressed the rapid growth ofγ-(Fe,Ni)dendrite,as compared with the calculation results of Fe-Ni alloy based on LKT model.The microhardness values of the alloy and the primaryγ-(Fe,Ni)phase increased by 1.5 times owing to the microstructural refinement caused by the rapid dendrite growth.The difference was enlarged as undercooling increases,resulting from the enhanced hardening effects on the alloy from the increased grain boundaries and the second phase.     

Primary dendrite growth of Ni<Subscript>3</Subscript>Sn intermetallic compound during rapid solidification of undercooled Ni-Sn-Ge alloy

Liquid Ni-31.7%Sn-2.5%Ge alloy was highly undercooled by up to 238 K(0.17TL) with glass fluxing and drop tube techniques.The dendritic growth velocity of primary Ni3Sn compound shows a power-law relation to undercooling and achieves a maximum velocity of 380 mm/s.The addition of Ge reduces its growth velocity as compared with the binary Ni75Sn25 alloy.A structural transition from coarse dendrites into equiaxed grains occurs once undercooling exceeds a critical value of about 125 K,which is accompanied by both grain refinement and solute trapping.The Ni3Sn intermetallic compound behaves like a normal solid solution phase showing nonfaceted growth during rapid solidification.     

Rapid dendritic growth of Al-Ge hypoeutectic alloy in a drop tube

Abstract Dendritic growth in Al-45% Ge hypoeutectic alloy has been investigated during free fall in a 3 m drop tube. Calculationsindicate that the undercooling obtained for the falling Al-45% Ge droplets ranges from 13 K to 201 K. The maximum undercooling attains0. 27 T_L. With the increase of undercooling, the primary (Al) phase undergoes a "columnar dendrite to equiaxed dendrite" structural tran-sition. According to the current rapid dendritic growth theory, the growth of primary (Al) phase is always controlled by solute diffusion.     

An investigation on supercooling directional solidification process of Cu-Ni single phase alloy

Supercooling directional solidification (SDS) is put fotward by combination of melt supercooling and conventional solidification by application of supercooling inheritance. On the self-designed SDS equipment, SDS of Cu-Ni alloy was achieved successfully The results are as follows f (i) The primary arm spacing is about 30 μm, the growth of secondary arms are strongly suppressed. The primary arm spacing is nearly the same as LMC method (GL=25 K/mm, V=500 pm/s), the primary stems are straight, fine and completed. with an inclination angle of about 5.8° (ii) A semi-quantitative T-T model is brought fotward to describe the dendrite growth rate V vs. undercooling AT The prediction of T-T model agrees well with experimental results. The formation of fine equiaxed dendrites, transition region and dendrite region can be explained successfully by △T-V-x relation of T-T model.     

Catalytic nucleation of a meta-stabled phase in phase-seeded undercooled Fe75Ni25 melts

Undercooled Fe75Ni25 melts were phase-seeded by a high purity iron. It was found that above a critical undercooling,△Tc = 135 K, a metastable δ phase (b.c.c) solidifies from the iron-seeded melts instead of a thermodynamically stable γ phase (f.c.c). While undercooling of the melt (△T ) is below △Tc, solidification of the γ phase prevails in the iron-seeded melts. For the undercooled melts subjected to spontaneous nucleation, the γ phase always solidifies. After solidification, the as-solidified γ phase transforms completely to martensite; whereas the as-solidified metastable δ phase partially transforms to the γ phase, and then to martensite. The untransformed δ phase retains as a -ferrite particles in the microstructures. Based on the classical nucleation theory, catalytic nucleation of the metastable δ phase in the phase-seeded undercooledFe75Ni25 melts was analyzed. It was quantitatively demonstrated that when△T>△Tc, the formation of the δ phase can be ascribed to a better catalytic effect of the iron on its nucleation than that on the nucleation of the γphase.     

Rapid growth of primary dendrite in highly undercooled copper-antimany alloy

The droplets of Cu-11wt.%Sb hypoeutectic alloy have been rapidly solidified during containerless processing in a 3 m drop tube. The undercooling and cooling rates are estimated, and both play a dominant role in the dendritic growth of primary Cu phase. Undercoolings up to 200 K (0.16TL, where TL is the liquidus temperature) have been obtained in the experiment. With the increase of undercooling, the microstructural evolution of primary Cu phase proceeds from remelted dendrites to the equiaxed grains. A coarse dendritic grain microstructure can form in the undercooling range of 61～102 K and at cooling rates of 1.35×102～2.66×103 K/s. The segregationless solidification of Cu-11wt.%Sb hypoeutectic alloy occurs when undercooling is more than 176 K. The growth of primary Cu phase is mainly controlled by solute diffusion.     

旋转磁场及快速凝固对Pb-Bi合金凝固组织的影响

研究了不同转速的旋转磁场对Pb-Bi合金凝固组织的影响,对于Pb-52%Bi亚共晶,旋转磁场能碎断枝晶,细化晶粒;对于Pb-66%Bi过共晶,旋转磁场能消除比重偏析。另外,采用硅油净化法结合水淬使Pb-52%Bi亚共晶和Pb-60.9%Bi过共晶分别获得了47 K和66 K的较大过冷度,对于Pb-52%Bi亚共晶,金属间化合物ε相枝晶细化显著;对于Pb-60.9%Bi过共晶,组织中没出现初生相Bi,只有细密的共晶组织。对于Pb-52%Bi亚共晶在快速凝固的同时加旋转磁场,过冷度由47 K增大为55 K,ε相呈细小颗粒弥散分布。     

深过冷Cu-Ni-Fe三元合金自定向快速凝固

利用熔融玻璃净化结合循环过热,在２５～３０４Ｋ过冷度范围,分析了Ｃｕ－３９％Ｎｉ－６％Ｆｅ（ｗｔ％）三元合金凝固过程过冷组织的演化规律。确定了负温度梯度下实现自定向凝固的过冷度条件：下限为能够抑制快速凝固过程中形成的枝晶熟化的最低过冷度,上限为快速凝固过程中枝晶不发生准球状化转变的最高过冷度;就研究的合金而言,过冷度范围为１１０～１８０Ｋ。在定向凝固的过冷度范围内,无需人为控制固液界面前沿的温度梯度,而且,以点触发试样端部,可以获得单晶     

Rapid growth of FeAl intermetallic compound under high undercooling conditions

Fe-58at%Al alloy is undercooled up to 222K (0.15TL) with the drop tube technique.It is found that there exists a critical undercooling about 185 K,beyond which a “dendrite-equiaxed” growth morphology transition occurs in FeAl intermetallic compound. This transition is characterized by sharp decrease of its grain size.Once the undercooling exceeds 215 K, the peritectic transformation is suppressed completely and a fibrous structure is formed,which results from the cooperative growth of FeAl and FeAl2 compounds.     

Mechanical properties of directionally solidified lead-antimony alloys

The Pb-17wt% Sb alloy was directionally solidified under two solidification conditions: with different temperature gradients (G=0.93–3.67 K/mm) at a constant growth rate (V=17.50 μm/s) and with different growth rates (V=8.3–497 μm/s) at a constant temperature gradient (G=3.67 K/mm) in a Bridgman furnace. Microstructure parameters, such as primary dendrite arm spacing (λ₁), secondary dendrite arm spacing (λ₂), and dendrite tip radius (R), were measured. The microhardness (Hv) and ultimate tensile strength (σ) of the directional solidification samples were also measured. The influences of solidification and microstructure parameters on Hv and σ were investigated. The results obtained in this work were compared with similar experimental researches in literatures. It is shown that the Hv and σ values increase with the increase of G and V, but decrease with the increase of λ₁, λ₂, and R.     

Phase separation and microstructural characteristics of undercooled Cu-Pb immiscible alloy

Bulk samples of Cu-80%Pb hypermonotectic alloy were undercooled by up to 270 K (0.21 TL) with glass fluxing technique. The undercooling behavior and the final microstructure were investigated experimentally. It was found that the macrosegregation decreased with the increase of undercooling exponentially. When undercooling reached 270 K, the volume fraction of macrosegregation was reduced by one order of magnitude. Meanwhile, high undercooling brought about significant changes to the microstructural morphology of S(Cu) phase. At small undercoolings, S(Cu) phase grew in dendritic manner. As undercooling increased, S(Cu) dendrite transformed gradually to spherical shell. This morphology transition was ascribed to the concurrent action of the phase separation within miscibility gap and the subsequent solidification process of L2 (Pb) matrix. As an essential step to model the final microstructure, theoretical calculations related to the nucleation of L1 (Cu) droplets were carried out.     

Microstructural evolution of ternary Ag33Cu42Ge25 eutectic alloy inside ultrasonic field

Ultrasonic field with a frequency of 20 k Hz is introduced into the solidification process of ternary Ag33Cu42Ge25 eutectic alloy from the sample bottom to its top. The ultrasound stimulates the nucleation of alloy melt and prevents its bulk undercooling. At low ultrasound power of 250 W,the primary ε2phase in the whole alloy sample grows into non-faceted equiaxed grains, which differs to its faceted morphology of long strip under static condition. The pseudobinary(Ag t ε2) eutectic transits from dendrite shape grain composed of rod type eutectic to equiaxed chrysanthemus shape formed by lamellar structure. By contrast, the ultrasound produces no obvious variation in the morphology of ternary(Ag t Ge t ε2) eutectic except a coarsening effect. When ultrasound power rises to 500 W, divorced ternary(Ag t Ge t ε2) eutectic forms at the sample bottom. However, in the upper part, the ultrasonic energy weakens, and it only brings about prominent refining effect to primary ε2phase.The microstructural evolution mechanism is investigated on the cavitation, acoustic streaming and acoustic attenuation.     

Solidification characteristics of Pb-Sb hypereutectic alloy within ultrasonic field

The solidification of Pb-16%Sb hypereutectic alloy is investigated within ultrasonic field with a fre-quency of 15 kHz. It is found that the ultrasonic field promotes crystal nucleation and terminates the further bulk undercooling of the alloy melt. Theoretical analysis shows that the cavitation effect and the forced bulk vibration are the main factors that reduce the undercooling level. With the increase of ul-trasound intensity, the primary (Sb) phase experiences a growth mode transition from faceted to non-faceted branched growth, and the macrosegregation of primary (Sb) phase is gradually sup-pressed. In addition, the microstructures of Pb-Sb eutectic exhibit a conspicuous coarsening with in-creasing ultrasound intensity, and a structural transition of “lamellar eutectic—anomalous eutectic” occurs when ultrasound intensity rises up to 1.6 W/cm2. The ultrasonic field also changes the solute distribution adjacent to the solidification front, which lowers the Pb contents in primary (Sb) phase.     

Phase selection during solidification of undercooled Ni70.2Si29.8 eutectic alloy

High undercooling (about 392 K) was achieved in the bulk eutectic Ni_70.2Si_29.8 alloy melt through glass fluxing combined with cyclic superheating. It is found that the metastable phases Ni₃Si₂ and NiSi are obtained through slow post-solidification when undercooling exceeds 240 K. The metastable phases are confirmed by using the method of X-ray diffraction and differential scanning calorimetry (DSC). Based on the principle of the free energy minimum and the transient nucleation theory, the phase selection of melt is investigated with regard to the metastable phases formation in the bulk undercooled eutectic Ni_70.2Si_29.8 melts. The formation of metastable phases from undercooled Ni_70.2Si_29.8 melts is ascribed to competitive nucleation with the undercooling, i.e. high undercooling facilitates the preferential nucleation of metastable phases.