深过冷定向凝固工艺过程的研究

利用过冷度的遗传性，提出了将合金熔体深过冷与传统定向凝固相结合的深过冷定向凝固技术，并在自制的实验装置上，对ＳＤＳ技术进行了探索性研究，实现了Ｃｕ与ｗ（Ｎｉ）为５．０％合金的深过冷和深过冷定向凝固。     

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.     

热力学深过冷熔体凝固组织非平衡效应的研究进展

热力学深过冷熔体的凝固是远离平衡的快速凝固,其凝固机制和凝固组织表现出与传统凝固不同的特点。热力学深过冷熔体凝固的非平衡效应主要表现在晶粒尺寸细化、亚稳相析出、调幅(Spinodal)分解、形成块体非晶等几个方面。在这些方面进行的探索与研究对研发新型高性能材料具有重要的理论意义和实用价值。近年来相关的研究发展较快。对在晶粒细化机制、亚稳相析出机制、调幅分解现象以及大块非晶的形成等方面研究的新进展进行了综述和分析。     

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。     

深过冷Cu—30Ni全金单向凝固组织的力学性能

研究了过冷０－２１０Ｋ的Ｃｕ－３０Ｎｉ合金的组织演化规律。在１０５－１５５Ｋ的过冷范围内实现了自由生长枝晶的单向凝固，获得了单向凝固的单昌组织，深过冷熔体的微观净化和单向快速凝固，有效地去除了合金名的微细夹杂物，减少了宏观偏析和枝晶偏析，显著改善了材料的均匀性，在拉应国作用下材料从沿晶断裂转变为穿晶断裂。     

深过冷熔体激发快速定向凝固

阐述了深过冷的方法，深过冷的遗传性，深过冷度的理论极限以及深过冷熔体激发发快速定向凝固的试验方法及其凝固过程，给出了深过冷熔体（Ｃｕ－５ｗｔ％）激发快速定向凝固与铸态试验激发快速定向凝固的初步试验结果。     

深过冷Fe85B15亚共晶合金晶粒的细化机制

采用熔融玻璃与循环过热相结合的方法使Fe85 B15 合金熔体获得了320K的大过冷度,并在共晶组织中获得了晶粒尺寸为100～200nm的共晶相.理论分析和实验结果表明,合金熔体的大过冷为晶粒细化提供了驱动力,而形核率和晶粒生长速度随过冷度的变化决定了晶粒的细化.实验结果为通过深过冷快速凝固技术制备块体纳米晶材料提供了新的思路和重要的实验依据.     

熔体深过冷过共晶铝硅合金的凝固组织研究

本工作采用熔体急冷装置对过共晶铝硅熔体进行深过冷处理,采用光学显微镜、扫描电子显微镜和X射线衍射仪等手段,研究了硅含量和熔炼工艺对熔体深过冷过共晶铝硅合金凝固组织的影响。研究结果表明,合金在800℃熔炼,保温时间为30 min时,熔体深过冷处理可抑制Al-(14～18) Si合金熔体在凝固过程中初晶硅的析出。当Al-18Si合金在800℃熔炼,保温时间超过30 min时,深过冷Al-18Si合金熔体在室温金属模型中凝固时可完全抑制初晶硅的析出,获得无初晶硅的凝固组织。     

快速凝固过程的高速摄影研究

以特制的无机盐玻璃70%Na_2SiO_3+17.7%Na_2B_4O_7+12.3%B_2O_3作为净化剂,去除液态Ni-32.5%Sn 共晶合金中的异质晶核,使过冷度达302K(0.215T_E)。采用高速摄影及快速红外测温技术研究了深过冷熔体的快速凝固行为。发现尽管过冷度超过了以往认为的均质形核临界过冷度0.2T_m,但是 Ni-32.5%Sn 共晶合金仍然优先发生界面异质形核,再辉过程中瞬时凝固速度最大可达784mm/s。     

深过冷Cu-30Ni合金单向凝固组织的力学性能

研究了过冷0～210K的Cu－30Ni（原子百分数）合金的组织演化规律．在105～155K的过冷范围内实现了自由生长枝晶的单向凝固，获得了单向凝固的单晶组织.深过冷熔体的微观净化和单向快速凝固，有效地去除了合金中的微细夹杂物，减少了宏观偏析和校晶偏析，显著改善了材料的均匀性，在拉应力作用下材料从沿晶断裂转变为穿晶断裂与常规铸态组织相比，其延伸率、极限抗拉强度和0．2％屈服强度分别提高到原组织的25倍、3倍和1．3倍     

Growth kinetics in levitated and quenched Nd-Fe-B alloys

We investigated the growth kinetics and the effect of quenching conditions on rapid solidification of undercooled Nd-Fe-B melts with compositions near the Nd-₂-Fe₁₄-B (2-14-1) phase. We prepared melt drops of various undercooling levels (up to 300 K below the liquidus temperature) were prepared by the electromagnetic levitation method and subsequently quenched them onto chill substrates. We measured the solidification kinetics of the undercooled melts in situ using a high-resolution Si photodiode. In accordance with the nucleation theory, the properitectic γ-Fe phase nucleates at first during the undercooling process. There were two different solidification routes, with the observed route depending on the undercooling level of the levitated melt prior to quenching. The peritectic reaction is favored in melts with high undercooling levels prior to quenching. Low previous undercooling levels lead to primary solidification of the 2-14-1 phase on quenching. The thickness of the homogeneous 2-14-1 phase zone, grown directly at the substrate side, depends strongly on the undercooling level prior to solidification. We estimated the growth velocity of the 2-14-1 phase from temperature-time-characteristics to be of the order of 1 mm/s. These investigations give rise to improved understanding about the high sensitivity of the microstructure of Nd-Fe-B alloys on different rapid solidification procedures     

Numerical simulation of rapid solidification of a spherical sample on a metallic substrate

Since the exact analytical solutions for rapid solidification process are available only for special boundary conditions, numerical techniques have to be applied for more general boundary conditions. In this paper we will describe a finite difference method for simulation of rapid solidification that is based on control volume methodology and interface-tracking technique. Heat transfer computer study will be realized for solidification with and without melt undercooling at the interface. Such numerical method will be applied for thermal history analysis of solidifying nickel on copper substrate.     

Dynamics of solidification and development of morphology of Cu‐base alloys with a metastable miscibility gap in the range of the undercooled melt

The Cu‐Co system shows a metastable miscibility gap in the range of the undercooled melt. In this work the method of electromagnetic levitation (EML) and drop tube experiments have been used to examine the metastable state of Cu‐Co alloys. The experiments show that both methods allow deep undercooling of the melt into the range of the miscibility gap. Due to the deep undercooling the velocity of the solidification front is very high and the actual microstructure is frozen in. The process of demixing can be observed and the binodal has been determined with high precision. The microstructure of samples processed in the electromagnetic levitation shows an influence of the electromagnetic stirring due to the induction of electric currents into the melt. Drop tube experiments, which lead to a rapid solidification under reduced gravity conditions, in contrary result in a homogeneous distribution of spherical particles of the minority phase. For this reason space experiments under microgravity conditions in the TEMPUS facility are under consideration. In these experiments the stirring effect would be greatly reduced compared to the EML.     

Non-equilibrium effects on solid transition of solidification microstructure of deeply undercooled alloys

Integrated effects of undercooling and solute drag on recrystallisation mechanism of rapid solidification microstructure were investigated in highly undercooled Ni-Cu alloys. The equiaxed grained microstructures were prepared by fluxing method and subsequent quenching. Annealing the microstructures of the as-quenched alloys, substantial recrystallisation growth was observed. Applying solute trapping model of undercooled melt and solute drag model of solid-state transformation, it can be inferred that the microstructural evolution was dominated by nucleation and growth of recrystallisation process which is strongly dependent on the initial undercooling of the alloy melt and the solute drag force of the solid-state transition process.     

Mechanism for metastable phase formation in undercooled Ti–Al peritectic alloys

Melt undercooling technique has been proved to be a powerful tool to investigate the formation of metastable phase in rapid solidification processing. Here, the recently observed phase evolution behaviour of the near equal atomic percent Ti–Al alloys was analyzed, as a function of melt undercooling, by thermodynamic and kinetic calculation. It was found that the formation of metastable phase is lightly predicted by the chemical Gibbs energy difference among competing phase but strongly controlled by kinetic effects arising from the competition of nucleation. The transient nucleation theory, with a consideration of incubation time, was proved to be an effective tool to explain the metastable phase formation in the undercooled near equal atomic percent Ti–Al alloys.     

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.     

Metastable phase formation in undercooled Fe-Co melts under terrestrial and parabolic flight conditions

Soft magnetic Fe-Co alloys display primary fcc phase solidification for>19,5 at% Co in conventional near-equilibrium solidification processes. Undercooled Fe-Co melt drops within the composition range of 30 to 50 at% Co have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolution Siphotodiode. Melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterize the solidification sequence and microstructure. The transition from stable fcc phase to metastable bcc primary phase solidification has been observed after reaching a critical undercooling level. The critical undercooling increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. Under microgravity conditions, a much smaller critical undercooling and an increased life time of the metastable bcc phase were obtained. This result was validated with TEM investigations. The appearance of Fe-O particles gives an indirect hint for an intermediate fcc phase formation from the metastable bcc phase at elevated temperature.