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)     

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 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.     

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.     

High undercooling and rapid dendritic growth of Cu-Sb alloy in drop tube

Droplets of Cu-20%Sb hypoeutectic alloy has been rapidly solidified in drop tube within the containerless condition. With the decrease of droplet diameter, undercooling increases and the microstructures of primary copper dendrite refines. Undercooling up to 207 K (0.17 T _L) is obtained in experiment. Theoretic analysis indicated that because of the broad temperature range of solidification, the rapid growth of primary copper dendrite is controlled by the solutal diffusion. Judging from the calculation of T₀ curve in the phase diagram, it is shown that the critical undercooling of segregationless solidification is δT ₀ = 474 K. At the maximum undercooling of 207 K, the growth velocity of primary copper phase exceeds to 37 mm/s, and the distinct solute trapping occurs.     

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.     

Microstructural characteristics and electrical resistivity of rapidly solidified Co-Sn alloys

The rapid solidification behavior of Co-Sn alloys was investigated by melt spinning method.The growth morphology of αCo phase in Co-20% Sn hypoeutectic alloy changes senistively with cooling rate.A layer of columnar αCo dendrite forms near the roller side at low colling rates.This region becomes small and disappears as the cooling rate increases and a kind of very fine homogeneous microstructure characterized by the distribution of equiaxed αCo dendrites in γCo3Sn matrix is subsequently produced.For Co-34.2% Sn eutectic alloy,anomalous eutectic forms within the whole range of cooling rates.The increase of cooling rate has two obvious effects on both alloys:one is the microstructure refinement,and the other is that it produces more crystal defects to intensify the seattering of free electrons,leading to a remarkable increase of electrical resistivity,Under the condition that the grain boundary reflection coefficient γ approaches 1,the resistivity of rapidly solidified Co-Sn alloys can be predicted theoretically.     

Entropy as a selection rule for crystal growth in undercooled binary eutectic melts

A solution entropy model was developed for the undercooled binary eutectic alloy systems. As an extension of Taylor and Fidler et al.’s model, the present model considered the change of phase composition with the increase of undercooling. Furthermore, the sub-regular solution model and the interaction parameter (I _AB ) were also introduced. In this paper, the extended model is used to calculate the solution entropy for binary eutectic phases under the undercooled condition, and the application scope of the model is also extended. Not only the growth manner of eutectic phases, but also the transition of morphologies may be predicted and explained by calculating the solution entropy of eutectic phases under the non-equilibrium condition with the developed model. Experimental results show that the developed model is valid for the undercooled Ni-Si and Ni-Sn eutectic alloy systems. Supported by the National Natural Science Foundation of China (Grant No. 50395103) and Doctorate Foundation of Northwestern Polytechnical University (Grant No. CX200506)     

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     

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.     

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.     

Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu (wt%, signed as 3C), fabricated by traditional cast (TC) and spray forming (SF) processes, were investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that Al₇Cu₂Fe phase can be formed and transformed in TC- and SF-3C alloys between 802–813 K and 800–815 K, respectively. The transformation from β-Al₅FeSi to δ-Al₄FeSi₂ phase via peritectic reaction can occur at around 858–870 K and 876–890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature of δ-Al₄FeSi₂ phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation of δ-Al₄FeSi₂→β-Al₅FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.     

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.     

Core-shell microstructure formed in the ternary Fe-Co-Cu peritectic alloy droplet

The metastable liquid phase separation occurs in the ternary Cu₅₀Fe_37.5Co_12.5 peritectic alloy droplets during free fall. The separated alloy melt rapidly solidifies and evolves core-shell microstructure composed of L₁(Cu) and L₂(Fe,Co) phases. Based on the determination of the phase transition temperature, the core-shell microstructure evolution, the interfacial energy, the temperature gradient and the Marangoni migration are analyzed. The interfacial energy of the separated liquid phase increases with the decrease of the temperature. The temperature gradient changes from large to small along the radius direction from inside to outside in the alloy droplet. The Marangoni force (F _M) acting on the micro-droplet of L₂(Fe,Co) phase increases with the increase of the size of the L₂(Fe,Co) phase, and decreases with the increase of undercooling. Driven by F _M, the micro-droplet of L₂(Fe,Co) phase migrates from outside to inside in the alloy droplet, collides and coagulates each other during migration, and then forms different types of core-shell microstructures. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101, 50395105) and NPU Youth Scientific and Technological Innovation Foundation (Grant No. W016223)     

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.     

Fe-C二元合金包晶相变的相场法模拟

以Fe-0.79 mol%C二元合金为例,采用多相场法,模拟研究了包晶相变过程中不同过冷度下奥氏体的演化过程,讨论了过冷度对奥氏体生长速度与形貌的影响.结果表明:包晶反应过程中,在界面迁移率与溶质富集的影响下,靠近三相节点处的部分δ相重熔,导致γ相向δ相内生长;当液相中的球状δ相表面发生包晶相变时,γ相作为一个外壳包围着δ相生长,最后在δ相周围形成一层γ相的核壳;随着过冷度的增加,包晶相变的驱动力增加,γ相的生长速度加快,γ相包围δ相的量越多。     

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…     

Effects of aging on the microstructure of a Cu-Al-Ni-Mn shape memory alloy

The influence of aging on the microstructure and mechanical properties of Cu-11.6wt%Al-3.9wt%Ni-2.5wt%Mn shape memory alloy (SMA) was studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometer, and differential scanning calorimeter (DSC). Experimental results show that bainite, γ_2, and α phase precipitates occur with the aging effect in the alloy. After aging at 300dgC, the bainitic precipitates appear at the early stages of aging, while the precipitates of γ₂ phase are observed for a longer aging time. When the aging temperature increases, the bainite gradually evolves into γ₂ phase and equilibrium α phase (bcc) precipitates from the remaining parent phase. Thus, the bainite, γ₂, and α phases appear, while the martensite phase disappears progressively in the alloy. The bainitic precipitates decrease the reverse transformation temperature while the γ₂ phase precipitates increase these temperatures with a decrease of solute content in the retained parent phase. On the other hand, these precipitations cause an increasing in hardness of the alloy.     

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.