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.     

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)     

深过冷Ni-P共晶合金凝固动力学分析

根据经典的形核和生长理论，通过实验分析了深过冷Ni-P共晶合金的凝固行为．实验发现，深过冷Ni-P共晶合金的凝固组织由粒状的共晶团和棒状的规则共晶所组成．随过冷度的增加，共晶团的组织逐渐细化，同时深过冷熔体晶核生长速率很大，异质形核在凝固过程中起控制性的作用．深过冷熔体的单点形核和长大现象作为特例用以描述界面的生长速率对其凝固组织的影响。     

Dynamic solidification of Sn-38.1% Pb eutectic alloy within ultrasonic field

The dynamic solidification of Sn-38.1% Pb eutectic alloy within an ultrasonic field is investigated at a frequency of 35 kHz.As the sample height H is reduced,the effect of ultrasound on macrosegregation becomes more prominent,and the volume fraction of spherical eutectic cells increases correspondingly.When H equals the wavelength λ in liquid alloy,the introduction of ultrasound enlarges the distribution region of the primary (Sn) phase,but reduces the domains of the Sn-Pb eutectic and primary (Pb) phases.Meanwhile,a "dendritic-equiaxed" structural transition occurs in the primary (Sn) phase,and its grain size is significantly reduced within the ultrasonic field.Once H decreases to λ/2 and λ/4,the ultrasonic field promotes crystal nucleation and suppresses further undercooling of the bulk liquid alloy.Theoretical analyses indicate that the local high pressure induced by the cavitation effect and the stirring effect due to acoustic streaming are the main factors dominating the eutectic growth mechanism during dynamic solidification.     

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…     

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.     

Recalescence behavior and solidification structure of the undercooled Fe82B17Si1 eutectic alloy

By cyclic superheating incorporated with glass fluxing denucleation method the Fes2Bl7Si1 eutectic al-loy was undercooled up to △ T = 342 K. The relations between recalescence behavior and solidification structures weresystematically studied in the undercooling range of 6-342 K. Two critical undercoolings were observed: mixed eutecticwas the unique growth morphology when the undercooling was less than △T1 = 63 K; but the microstructure transformedto complete undercooled anomalous eutectic when the undercooling was greater than △T2 = 164 K. The two eutecticphases α(Fe,Si) and Fe2B conformed to the non-reciprocal nucleation effect. The boundary of the coupled zone of α(Fe, Si)-Fe2 B system shifted toward the Fe2 B side, and intersected the eutectic composition line at △ T = 154 K and△x T= 264 K, whose valley was at about △ T = 207 K.     

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)     

Rapid solidification of acoustically levitated Al-Cu-Si eutectic alloy under laser irradiation

Al-27%Cu-5.3%Si ternary eutectic alloy was melted using a YAG laser and then solidified while being acoustically levitated. A maximum undercooling to 195 K (0.24 TL) was achieved with a cooling rate of 76 K/s. The solidification microstructure was composed of (Al+θ+Si) ternary eutectics and (Al+θ) pseudobinary eutectics. During acoustic levitation, the (Al+θ+Si) ternary eutectics are refined and the (Al+θ) pseudobinary eutectics have morphological diversity. On the surface of the alloys, surface oscillations and acoustic streaming promote the nucleation of the three eutectic phases and expedite the cooling process. This results in the refinement of the ternary eutectic microstructure. During experiments, the reflector decreases with increasing alloy temperature, and the levitation distance always exceeds the resonant distance. Because of the acoustic radiation pressure, the melted alloy was flattened, and deformation increases with increasing sound pressure. The maximum aspect ratio achieved was 6.64, corresponding to a sound pressure of 1.8×104 Pa.     

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.     

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.     

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.     

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tin) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.     

Phase-selection and metastable phase formation in highly undercooled eutectic Ni_(78.6)Si_(21.4)alloys

A large undercooling (250 K) was achieved in eutectic Ni78.6 Si21.4 melt by the combination of molten-glass denucleation and cyclic superheating. The metastable phase formation process in the bulk undercooled eutectic Ni78.6 Si21.4 melts was investigated. With the increase of undercooling, different metastable phases form in eutectic Ni78.6 Si21.4 melts and part of these metastable phases can be kept at room temperature through slow post-solidification. Under large undercooling, the metastable phases β2-Ni3Si, Ni31Si12 and Ni3Si2 were identified. Especially, the Ni3Si2 phase was obtained in eutectic Ni78.6 Si21.4 alloy for the first time. Based on the principle of free energy minimum and transient nucleation theory, the solidification behavior of melts was analyzed with regard to the metastable phase formation when the melts were in highly undercooled state.     

Rapid monotecticsolidincation during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and a-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.     

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.     

固液反应球磨制备Al-Cu-Co三元金属间化合物

利用固液反应球磨技术制备了Al-Cu-Co三元合金．分别采用Co球球磨Al-33．2％Cu(此文中的百分比为质量分数),Al-54％Cu(A12Cu)和Al-70％Cu(A1QJ)二元合金熔体,在923K和973K球磨Al-33．2％Cu熔体12h后生成Al65Co15Cu20粉末;在923K和1023K球磨24h后生成Al69Co25Cu6粉末,在893K和993K分别球磨Al-54％Cu(A12Cu)合金熔体12h和24h后均生成Al65Co15Cu2n粉末;在1123K球磨Al-70％Cu合金熔体24h后生成Al65Co15Cu20粉末．采用Al-Cu-Co固液反应球磨得到的金属间化合物粉末为纳米粒子．同时,对Al-Cu-Co三元合金相形成的规律进行了研究,对固液反应球磨机理进行了探讨．在固液反应球磨过程中,三元合金产物的元素摩尔比接近于二元母合金中的元素摩尔比;三元合金产物成分中固相第三组元的成分含量与二元母合金熔体成分有很大关系;提高反应球磨温度、延长球磨时间有利于三元合金产物的形成;延长球磨时间,形成的三元合金产物中磨球的成分增加;反应球磨温度超出二元母合金熔点越高,球磨反应越容易进行．     

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.     

Surface tension and specific heat of liquid Ni<Subscript>70.2</Subscript>Si<Subscript>29.8</Subscript> alloy

The thermophysical properties of liquid alloys in me-tastable state are of importance for liquid-solid phase transformation under nonequilibrium conditions[1―10]. Sur-face tension and specific heat, two of the most important thermophysical properties, have significant influences on the process of crystal nucleation and growth. Furthermore, it is necessary to obtain these data so as to perform quan-titative research on rapid solidification[3―10]. The tradi-tional measurement methods, however,…     

Effects of Sb and heat treatment on the microstructure of Al-15.5wt%Mg2Si alloy

The modification effects of alloying element Sb and heat treatment on Al-15.5wt%Mg₂Si alloy were investigated by Olympus microscopy (OM), scanning electron microscopy and energy disperse spectroscopy (SEM-EDS), and X-ray diffraction (XRD). It is found that Sb plays a significant role in shaping primary Mg₂Si phase and eutectic Mg₂Si phase in Al-15.5wt%Mg₂Si alloy. The Sb addition of about 1.0wt% makes the resultant alloy show the finest primary Mg₂Si phase and the eutectic Mg₂Si phase with well distribution. But further increasing the Sb content decreases the amount of primary Mg₂Si phase, and some segregated phases appear at regions between the grains. In addition, heat treatment can modify the microstructural feature of Sb-modified Al-15.5wt%Mg₂Si alloy in terms of obviously shortening the nodulizing time of primary Mg₂Si phase and eutectic Mg₂Si phase.