深过冷合金中的非平衡现象分析

深过冷技术是快速凝固技术中的一种，它能实现金属或合金在慢速冷却条件下发生快速凝固，这就为研究者研究合金快速凝固中的一些非平衡现象提供了一种新的途径，讨论了影响熔体过冷度的主要因素，介绍了获得熔体深过冷的几种主要方法，分析了深过冷熔体的非平衡现象产生的原因。     

融化和凝固温度对三水醋酸钠相变储热性能的影响

为了研究储热材料三水醋酸钠(含成核剂十二水磷酸氢二钠和增稠剂羧甲基纤维素)在实际贮能工程中应用的融化和凝固温度范围,通过不同水浴温度下的融化实验和不同冷却条件下的凝固实验,得到了储热材料的相变性能参数.实验结果表明,储热材料的过冷度随着加热水浴温度的升高有增大的趋势,水浴温度在83℃以内时其相变性能较稳定,超过这个温度,其储热性能就无法保证;冷却温度降低,其过冷度有增大的趋势,但只要是在常温条件下的水浴或空气浴中冷却,即使冷却温度变化较大,储热材料的相变性能仍比较稳定.     

深过冷Sb-Sn过包晶合金的快速凝固

采用落管无容器处理技术研究了Sb74.7Sn25.3二元过包晶合金的快速凝固,获得的合金粒子直径D介于70～1080μm之间。理论计算表明,随着粒子直径的减小,过冷度和冷却速率均呈指数关系增大,最大过冷度为298K(0.36TL)。研究发现,在自由落体条件下,快速凝固组织由初生Sb固溶体相和包晶SbSn金属间化合物相组成,Sb固溶体相以非小平面和小平面两种生长方式长大。当过冷度增大时,释放的熔化潜热增多,初生相逐渐细化,非小平面初生Sb相由"粗大枝晶"向"碎断枝晶"转变,当D<400μm时,一次枝晶臂显著变短,二次枝晶间距明显减小;同时发生溶质截留现象,初生Sb固溶体相中溶质Sn的固溶度发生了显著拓展,由ΔT=32K时的7.86%(原子分数,下同)线性增大至ΔT=298K时的10.47%。     

深过冷Ni80.3B19.7合金的再辉和非规则共晶的形成

采用熔融玻璃净化结合气体保护的方法,使Ni80 3B19 7过共晶合金获得了407 K的大过冷度,研究了其在不同过冷度下快速凝固过程中的再辉行为.结果表明,Ni80 3B19.7过共晶合金在0～112 K过冷度范围内无明显再辉,在112～323 K过冷度范围内,其再辉曲线表现为两个再辉峰,而在323～407 K过冷度范围内,其再辉曲线为一个再辉峰.初生固相含量的随着过冷度的增大而增大,导致一次再辉度随着过冷度的增大而增大.深过冷Ni80 3B19.7合金凝固组织中非规则共晶的形成,归因于共晶两相在快速凝固阶段以自由枝晶的形式进行的非耦合生长和再辉后的慢速凝固阶段两相枝晶所发生的形态上的转变.     

过冷度对Fe-Ni-P-B软磁合金凝固组织的影响

采用玻璃包覆法(fluxing)提纯和在不同温度下保温,获得了Fe40Ni40P14B6合金熔体的凝固组织,研究了过冷度对凝固组织的影响.结果表明,随着过冷度的增大,Fe40Ni40P14B6的凝固组织从亚共晶转变为共晶组织,晶粒尺寸明显减小.当过冷度超过某一临界值时,合金熔体发生Spinodal分解,形成网状结构的凝固组织并使晶粒显著细化,达到纳米尺度.在深过冷条件下,可获得块体纳米晶凝固组织.     

快速凝固Al-Si-Fe合金粉末工艺研究

快速凝固技术也称为急冷凝固技术,主要是通过提高金属凝固冷却速度的方法来增大凝固过冷度和凝固速度,从而获得传统铸件冷却速率下所不能获得的成分、相结构或显微结构。利用快速凝固技术制备Al-Si合金可显著改善合金组织,大幅度提高合金性能,使合金具有良好耐磨性、耐热性,以及高强、质轻及低热膨胀系数等特点。     

过冷Fe-Co合金的包晶凝固

采用熔融玻璃净化法使Fe-Co包晶合金实现了深过冷快速凝固。当熔体过冷度较小时,Fe-Co包晶合金的凝固组织为典型的包晶组织。借助电子探针分析和DTA差热分析,证实了非平衡条件下Fe-Co包晶合金凝固过程中发生了包晶反应和包晶转变。研究表明,深过冷Fe-Co包晶合金的非平衡凝固过程从理论上可以划分为4个阶段:初生δ相的形核与生长、包晶反应、包晶转变和γ相的外延生长。     

DP1180汽车用双相钢连续冷却转变过程中的相变规律

为研究DP1180汽车用双相钢连续冷却转变过程中的相变特性,对不同冷却速率下的DP1180双相钢试样进行了金相检验及显微硬度测试,绘制了该双相钢的连续冷却转变曲线,并分析了DP1180钢在连续冷却过程中的相变规律。结果表明:对于DP1180汽车用双相钢,其连续冷却转变曲线分为铁素体转变区、贝氏体转变区和马氏体转变区。当冷却速率小于0.5℃·s~(-1)时,主要发生铁素体-贝氏体转变;当冷却速率增大到1℃·s~(-1)时,显微组织中出现马氏体;随冷却速率逐渐增大,铁素体不断减少,当冷却速率达到10℃·s~(-1)时,组织变为马氏体+贝氏体;当冷却速率大于40℃·s~(-1)时,组织主要为马氏体。     

深过冷Ni-Sn合金非规则共晶的形成机制

采用熔融玻璃净化配合循环过热使Ni-32.5%Sn(质量分数)共晶合金实现了深过冷快速凝固.当过冷度大于某一临界值时,非规则共晶在凝固组织中出现.随着过冷度的提高,最终得到完全的非规则共晶组织.通过分析Ni-Sn共晶合金中各相形核、生长、以及枝晶熔断机制随过冷度的变化,解释了非规则共晶的形成机制.在深过冷条件下熔体中初生相率先形核并长入过冷熔体中,形成枝晶骨架,再辉重熔后次生相从残余熔体中析出并包围初生相,形成非规则共晶.     

Al-Y包晶合金非平衡凝固过程及组织特征

包晶凝固过程中小平面-小平面两相复相生长方式是凝固领域研究的一个热点。以初生相和包晶相都是严格计量比金属间化合物的Al-Y包晶合金作为研究对象,利用DSC热分析技术,严格控制冷却速度,获得不同凝固条件的非平衡凝固试样,研究了两相为小平面相的Al-Y包晶合金的凝固行为。发现小平面-小平面包晶系合金包晶凝固过程中,非平衡凝固特性及宏观偏析特点比非小平面包晶系更加明显。凝固特征温度与平衡相图偏差明显,包晶反应温度和包晶相直接凝固温度都远高于平衡相图给定的值,相对于相图是在"过热"条件下发生的,而固溶体型包晶合金一般是在"过冷"条件下发生的。包晶转变过程非常微弱,致使初生相残留量远高于平衡相图。即使对于过包晶成分的合金,其凝固组织中仍存在大量的共晶凝固组织,最终得到的凝固组织与平衡相图存在显著差异。     

Improved prediction of the grain size of aluminum alloys that includes the effect of cooling rate

A comprehensive study of the effect of cooling rate on the grain size of a range of grain refined wrought aluminum alloys was carried out under quiescent solidification conditions where nucleation occurs predominantly by a constitutional undercooling mechanism. Increasing the cooling rate reduced the grain size by increasing the number of particles that nucleate grains and by affecting the development of constitutional undercooling. Both effects are represented using simple analytical relationships. By coupling these results with earlier work, an empirical relationship is developed between grain size, density of nucleant particles, cooling rate, nucleant potency and alloy composition that allows prediction of grain size across a wide range of alloys and cooling rates.     

Crystallization of 3M/4X Mg-Si-Al-O-N melts

MgSiAlON 3M/4X liquids have been crystallized at slow cooling rates from the melt and isothermally at varying degrees of undercooling. Forsterite is always the primary crystallizing phase but the secondary phases vary with the degree of undercooling. A magnesium-substitutedβ′-sialon,β″, grows with forsterite via a coupled mechanism at a slow continous cooling rate. However,β″ is progressively replaced as the secondary phase by a petalite-type Mg-Si-Al-O-N phase and a fourth phase on isothermal crystallization at increasing degrees of undercooling. The compositions ofβ″ and Mg-petalite as analysed by energy-dispersive X-ray microanalysis are discussed.     

Parameter determination of critical nucleation frequency in solidification of undercooled metallic melts

Abstract

A simple method was proposed to calculate the essential parameters correlated with the critical nucleation frequency of undercooled metals and alloy melt. Numerical results show that the calculation accuracy from this method can be improved using the experimental data either with high undercooling or with low undercooling range (the difference of undercoolings between two solidification events). The calculations of the interfacial energy for high undercooling of silver and of the catalytic factor f(θ) for high undercooling of Al, Cu and Al–30 wt-%Cu alloy indicate that the results are consistent with the experimental measurements and with the results of Jian’s model [Metall. Trans. A, 2001, 32A, 391–395]. In addition, by analysing the differential scanning calorimetry data of pure Sn subjected to different cooling rates, similar values of catalytic factor f(θ) are obtained. This further indicates the validity of the current method.     

Modelling the Solidification Microstructure Formation of Cobalt Dendritic Alloys

In the present work a mathematical model has been developed to explain the microstructure characteristics obtained during the solidification process of dendritic cobalt alloys, under ordinary low cooling rate conditions. The model, taking into account physical aspects such as undercooling, cooling rate, solute diffusion, interfacial energy, and dendrite tip morphology, allowed results to explain the experimental microstructure changes observed when the processing conditions were varied. The mathematical model involved micro and macroscopic phenomena occurring during the solidification process of metallic alloys. The solutions of the governing equations were obtained applying a non-coupled scheme, which enables the possibility to simulate the solidification of complex geometry castings.     

Numerical study of heat transfer and solidification behavior of gas-atomized Fe-6.5%Si (mass fraction) droplets

During spray atomization process, the heat transfer and solidification of droplets play very important roles for the deposition quality. Due to the difficulties of experimental approach, a numerical model is developed, which integrates liquid undercooling, nucleation recalescence and post-recalescence growth to present the full solidification process of Fe-6.5%Si (mass fraction) droplet. The droplet velocity, temperature, cooling rate as well as solid fraction profiles are simulated for droplets with different sizes to demonstrate the critical role of the size effect during the solidification process of droplets. The relationship between the simulated cooling rate and the experimentally obtained secondary dendrite arm spacing is in excellent agreement with the well-established formula. The pre-constant and exponent values lie in the range of various rapid solidified Fe-based alloys reported, which indicates the validity of the numerical model.     

大体积Ni-Sn共晶合金的化学净化与过冷

将化学净化引入深过冷研究.采用熔融玻璃净化、循环过热与气氛化学净化结合的方法，使100gNi-Sn共晶合金在慢冷条件下重复实验12次均获得268K的大过冷度.该过冷度可保持30个过热循环周期不衰减气氛化学净化机制是以化学反应抑制合金液与熔融玻璃界面上金属氧化物质点的增加，使过冷熔体的异质形核率稳定，从而使过冷度稳定化学净化的主要控制参数是气氛反应温度.但它在达到某一值后，过冷度的变化不明显     

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field (HMF) on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis (DTA).It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heatingcooling rates.Moreover,the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc.Regardless of magnetism,the nucleation frequency under an HMF is higher than that without an HMF.Furthermore,the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle,calculated by the functional relationship between the cooling rate and undercooling.This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc.Additionally,the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.     

Comparison of microstructure and solidification behavior of rapid quenching with bulk undercooled superalloy

Microstructure and solidification behavior in rapid quenching (i.e., gas-atomized powders and melt-spun ribbons) of superalloy have been compared with bulk undercooled superalloy. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been discussed for both methods of solidification with respect to undercooling, nucleation, and recalescence as well as recrystallization during post-recalescence. Comparison of the observed microstructure and morphologies indicates that the melt-spun ribbons and the gas-atomized powders with cooling rate above 10⁴K sec^–1crystallize only after achieving a large degree of undercooling, which becomes higher and higher with the increase of cooling rate. Furthermore, it should be noted that, grain refinements, which play a decisive role in the undercooled as-solidified structure, however, result from different sources in the rapid quenching process.