深过冷Ni-Si共晶合金晶粒细化机制

采用熔融玻璃净化和循环过热相结合的方法,在Ni-Si共晶合金中取得了最大330K的大过冷度.研究了大块过冷Ni-Si共晶合金晶粒细化,发现在某一关键过冷度(△T*=184K)以上,合金晶粒尺寸明显细化.实验证实在过冷Ni-Si共晶合金中枝晶重熔和碎断是晶粒细化的主要原因.为了更好地分析其晶粒细化机制,基于枝晶生长LKT-BCT模型完成了相关的计算.     

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.     

熔体过热对Sb-Bi合金凝固组织的影响

以Sb-4.6%Bi合金为研究对象,在限定其它因素保持不变的情况下,考察了熔体过热温度对凝固过程的影响。,实验结果发现,随着熔体过热温度的提高,合金形过冷度增大,凝固组织显著细化,研究表明,熔体过冷倾向是熔体结构状态的一个必然反映,熔体结构状态随温度发生变化是导致合金结晶过冷度发生明显的变化的原因,随着过热温度的提高,Sb-4.6%Bi合金晶粒显著细化,其原因是经过热处理的合金熔体在较大的过冷度下凝固。     

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.     

深过冷DD3高温合金的两次细化机制

用复合熔盐净化与循环过热相结合的方法,获得了最大210K过冷度,研究了DD3高温合金过冷熔体凝固组织的演化规律,在所获得的过冷度范围内,凝固组织的形态发生两次晶粒细化,发生第一次细化的过冷度为30－70K,因枝晶熟化,重熔,高度发达的树枝晶转变为第一类粒状晶;发生第二次细化的过冷度超过153K,凝固组织因枝晶碎断和再结晶而志变为第二类粒状晶。     

复合熔盐净化剂优化对GH4169高温合金过冷的影响

采用复合熔盐净化法对GH4169高温合金的深过冷进行研究,通过正交实验优化出GH4169高温合金深过冷的最优净化剂及工艺参数,从而使GH4169高温合金获得了250K的大过冷度,同时探讨了复合熔盐净化剂及工艺参数作用机理;通过对不同过冷度下凝固组织的观察发现,随着过冷度的增加,合金组织明显细化,并发生两次晶粒转变,由树枝晶向粒状晶转变。当过冷度超过250K后,晶粒平均尺寸达到5.5μm。     

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     

深过冷合金中的晶粒细化

深过冷熔体在一个临界过冷度下凝固,在很窄的过冷区间内晶粒尺寸将急剧减小,称之为晶粒细化.从研究过冷Ni基合金的晶粒细化现象入手,对迄今为止关于产生晶粒细化的各种机制做了较为系统的归纳和分析.并在此基础上,提出了存在的问题和今后的发展方向.     

深过冷Cu-Pb偏晶合金的快速凝固行为和凝固组织

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-25%Pb合金,Cu-37.4%Pb偏晶合金和过偏晶Cu-40%Pb(质量分数)合金过冷熔体凝固行为和凝固组织的演化规律,以及Cu-37.4%Pb偏晶合金的过冷度对磨损率的影响.研究表明:在过冷亚偏晶Cu 25%Pb合金熔体凝固过程中先形成α(Cu)初生相,随着过冷度的增大,凝固组织经历粗大枝晶重熔形成的细化枝晶向准球状晶粒演化的过程;在过冷Cu-37.4%Pb偏晶合金熔体凝固过程中初生相为L2相,当过冷度在20～150 K区间时,得到第二相S(Pb)弥散在α(Cu)枝晶间的凝固组织,并且在该过冷区间内随着过冷度的增加,材料的磨损率也逐渐降低;在过冷过偏晶Cu-40%Pb合金熔体凝固过程中初生相为L2相,在过冷度区间42～80 K时,得到以偏晶胞形式分布的凝固组织.     

过热处理细化亚共晶Al-7%Si合金的定量研究

在实验的基础上,运用定量金相和数理统计的方法,定量研究了过热处理细化亚共晶Al-7%Si合金的规律.通过对这些规律进行回归分析,得到相应的回归方程.方程分析表明,仅通过熔体过热处理这一因素,理论上能细化晶粒最小到20.0944μm.这一数据对熔体处理的工艺制定和熔体结构的研究有重要的意义.     

Effects of initial undercooling on microstructure formation and recrystallisation of undercooled melts

The critical undercoolings for the two grain refinement events and the onset of recrystallisation event are determined by detailed analysis of the microstructure evolution of bulk undercooled Ni–20?at.-%Cu alloy melts. The first grain refinement event occurred in the low undercooling range was explained by dendrite remelting. The second grain refinement event occurred in the high undercooling range was due to the combined effects of dendrite remelting stress-induced dendrite breakup during recalescence and recrystallisation during the near-equilibrium solidification stage after recalescence. The micro-stress induced by the solidification contraction during recalescence in the so called ‘first mushy zone’ would lead to distortion and breakup of primary dendrites. The stress-induced broken-up dendrites have sufficient driving force for recrystallisation.     

Study of grain refinement and eutectic transformation of undercooled IN718 superalloy

Abstract

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ～250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.     

Grain refinement in solidification of highly undercooled eutectic Ni–Si alloy

Substantial undercooling up to 550 K (0.386T_E, with T_E as the melting point) was achieved in eutectic Ni_78.6Si_21.4 alloy melt using glass fluxing combined with cyclic superheating. Accordingly, a particular refined microcrystalline morphology is obtained in the as-solidified structure. The physical mechanism of the grain refinement subjected to high undercooling is interpreted in terms of the classical nucleation theory and LKT/BCT model. It was concluded that the above refinement can be ascribed to the substantially increased nucleation rate under high undercooling.     

A Model for Evaluation of Grain Sizes of Aluminum Alloys with Grain Refinement Additions

Based on the assumption that the nucleation substrates are activated by constitutional undercooling generated by an adjacent grain growth and solute distribution during the initial solidification, a model for calculation of the grain size of aluminum alloys with the grain refinement is developed, where the nucleation is dominated by two parameters, i.e. growth restriction factor Q and the undercooling parameter P. The growth restriction factor Q is proportional to the initial rate of constitutional undercooling development and can be used directly as a criterion of the grain refinement in the alloys with strong potential nucleation particles. The undercooling parameter P can be regarded as the maximum of constitutional undercooling △Tc. For weak potential nucleation particles, the use of RGS would be more accurate. The experimental data of the grain refinement of pure aluminum and AlSi7 alloys are coincident predicted results with the model.     

Refining Effect of Boron on Hypoeutectic Al-Si Alloys

1. IlltroductionRefining treatmellt on hypoeutectic Al-St alloys isinevitably carried out because of the coarse dendriticgrain of a-Al. The grain refiner commonly used in theAl industry are nowadays usually master alloys of Tiplus B. It was Cibula in 194911], who clearly idelltiliedthe effectiveness of B in grain refining. The Chinesepublication first reported that Al-B master alloys isa powerful refiner better than Al-Ti or Al--Ti-B majster alloys[2]. Extensive theoretical and experime…     

The solidification process of highly undercooled bulk Cu-O melts

The effect of undercooling on grain structure was investigated in pure copper and alloys up to Cu 0.39wt% 0 (eutectic composition), in which grain refinement does not occur at any degree of bath undercooled when the oxygen content is less than 300 p.p.m. Grain refinement occurs in these alloys when the oxygen content exceeds about 300 p.p.m. and the undercooling prior to nucleation exceeds 100 K without quenching. Fragmentation affects primary, secondary and tertiary dendrite arms during and after recalescence. Quenching after recalescence at various solidification times retains transient grain structures. When the sample, which should have achieved complete grain refinement by furnace cooling, is quenched immediately after nucleation, the structure shows a trace of radiating fan-shaped grains originating from a single point of nucleation.     

Microstructure evolution in undercooled Co80Pd20 alloys

High undercooling has been achieved in Co₈₀Pd₂₀ melts by employing the method of molten glass denucleating combined with cyclic superheating, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–415 K, two kinds of grain refinements have been observed in the solidification microstructures. The three critical undercoolings are 72, 95, and 142 K, respectively. When undercooling is less than 72 K, the coarse dendritic morphology is formed, which is similar to the conventional as-cast microstructure. The first grain refinement occured in the range of undercooling, 72–95 K can be attributed to the breakup of dendrite-skeleton owing to remelting. When undercooling locates within 95–142 K, highly developed directional fine dendrite can be obtained because the severe solute trapping weakens the effect of solute diffusion during the dendrite growth. The second grain refinement occurred when undercooling exceeds the critical undercooling (∆T* = 142 K), the formation of fined equiaxed microstructure can be ascribed to the stress that originates from the extremely rapid solidification process, which resulted in the dendrite fragmentation finally.     

Phase selection in containerless solidification of FeSi2 during free fall in drop tube

The melts of the Fe-66.7%Si alloy ejected into a drop tube was solidified during its free fall. The spherical samples collected at the bottom of the drop tube were classified into several groups according to their diameters from 300 μm to 1000 μm. The microstructures of the samples were examined and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential thermal analysis (DTA). In addition to the grain refinement of the constituent phases, it was observed that the primary phase changed from the equilibrium FeSi (ε) phase to the metastable Fe₂Si₅ (α) phase and then returned to the ε phase again with decrease in the sample diameter. This result indicates that the microstructure of the sample solidified from the melt during free fall is controlled by the phase competition between α and ε depending on the degree of the undercooling. If it is assumed that the phase having the highest growth rate is selected as the primary phase, the dendrite growth model proposed by Boettinger, Coriell and Trivedi predicts the changes of the primary phase from ε to α and then to ε with increase in the undercooling. This means that the metastable eutectic point as a function of undercooling is expressed by the curve just like a character, C, in the Fe-Si binary phase diagram.     

Microstructural evolution and growth velocity-undercooling relationships in the systems Cu, Cu-O and Cu-Sn at high undercooling

A melt encasement (fluxing) technique has been used to systematically study the velocity-undercooling relationship in samples of Cu and Cu-O and Cu-3 wt% Sn at undercoolings up to 250 K. In pure Cu the solidification velocity increased smoothly with undercooling up to a maximum of 97 m s^-1. No evidence of grain refinement was found in any of the as-solidified samples. However, in Cu doped with >200 ppm O we found that samples undercooled by more than 190 K had a grain refined microstructure and that this corresponded with a clear discontinuity in the velocity-undercooling curve. Microstructural evidence in these samples is indicative of dendritic fragmentation having occurred. In Cu-Sn grain refinement was observed at the highest undercoolings (greater than 190 K in Cu-3 wt% Sn) but without the spherical substructure seen to accompany grain refinement in Cu-O alloys. Microstructural analysis using light microscopy, texture analysis and microhardness measurements reveals that recrystallisation accompanies the grain refinement at high undercoolings. Furthermore, at undercoolings between 110 K and 190 K, a high density of subgrains are seen within the microstructure which indicate the occurrence of recovery, a phenomenon previously unreported in samples solidified from highly undercooled melts.