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-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.     

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。     

变质ZA43合金中稀土的存在形式

通过电子探针附带扫描背散射电子成像(SEM)观察分析了ZA43合金的显微结构和相组成.利用电子探针对合金中各相的元素分布进行了分析.研究了变质ZA43合金铸态组织中含0.15%混合稀土的存在形式.实验结果表明,稀土元素主要分布于合金组织的晶界上,并以稀土金属化合物的形式存在.在枝晶中心和枝晶边缘没有发现稀土元素.稀土在α相的边缘含量较低,在η相中未见存在.ZA43合金经稀土调质,细化了合金晶粒.     

深过冷Ni—Sn共晶合金的快速凝固机制

本文通过净化法使 Ni-32.5wt-%Sn 共晶合金液获得深过冷,对该合金液在不同过冷条件下的凝固机制和组织进行了研究。结果表明:当过冷度小于约10K 时,该合金液凝固生成 Ni_3Sn相和 Ni(α)相层片共晶。在深过冷条件下,由于 Ni_3Sn 枝晶的自由生长速度远大于 Ni(α)枝晶的自由生长速度,再辉过程中,Ni_3Sn 相和 Ni(α)相不能以匹配方式生长,而由 Ni_3Sn 相作为领先相以枝晶簇方式生长。再辉过程中形成的枝晶簇,其内部 Ni_3Sn 枝晶进一步熔断粗化及 Ni(α)相在Ni_3Sn 枝晶间形成生长,最后形成非规则共晶组织。当过冷度小于130K 时,再辉之后,枝晶簇间存留有较大体积的成分仍为 Ni-32.5wt-%Sn 的合金液,这部分合金液在共晶平台阶段以层片共晶方式凝固,所以试样内部的组织由非规则共晶区和层片共晶区组成。     

深过冷Ni-Cu-Si合金熔体中的快速枝晶生长

为了揭示多元合金中的枝晶生长规律,采用电磁悬浮技术实现了Ni-10%Cu-10%Si三元合金的深过冷与快速凝固,实验中合金熔体获得的最大过冷度为236K。对合金快速凝固过程中初生相-αNi的枝晶生长速度测定结果表明,其与过冷度之间存在幂函数关系:V=1.6×10-13ΔT5.7。当ΔT较小时,随着ΔT的增加V增加缓慢,当ΔT较大时,随着ΔT的增加V迅速增加。对比分析表明,溶质Si对-αNi枝晶的生长影响显著,而溶质Cu则几乎没有影响。随着过冷度的增加,未发现-αNi相的微观形态从枝晶向等轴晶转变,但-αNi晶粒尺寸均随着过冷度的增加而急剧细化。     

大过冷Al-Mn-Si合金的凝固

本文采用玻璃包裹法加热冷却循环处理成分(wt-%)为 Al-25.44Mn-17.15 Si 的三元合金,得到了大过冷度的合金熔体。实验表明,三元合金大过冷凝固时有三次相析出。X 光衍射和透射电镜分析发现,合金中存在着五次对称准晶相及少量的 T 相、α-Al 和 Si 相。     

微量镨对Al-5Mg组织及室温和高温力学性能的影响

通过显微组织分析、室温及高温力学性能测试、XRD和SEM分析等方法,研究了稀土Pr对Al_5 Mg合金的显微组织、凝固区间和室/高温力学性能的影响。实验结果表明,Al_5Mg合金中加入稀土Pr使合金凝固区间变窄,细化了合金组织;Pr的加入净化了合金组织,减少了合金组织杂质缺陷,提高了合金性能;Pr的加入对合金有固溶和强化相强化作用,β-Al_(11)Pr_3强化相具有较高的高温稳定性能,提高了合金的高温性能。     

纯Al及Al-Mn合金中球状稀土相的形成

本文测定了含稀土纯 Al 及 Al-Mn 合金中球状稀土相的形貌,组成及分布。结果表明,适量稀土可使 Al 中多角形块状化合物球化,在轧制过程中球状稀土相不变形,不聚集,均匀分布在晶内。稀土改变了 Al 基体中杂质元素的微区分布规律,导致 Fe,Si 等元素由晶界向球状相迁移,晶界获得净化。分析了球状稀土相形成及晶界净化的原因。     

连续冷却快速凝固过程中相组成的计算模型

基于平面流铸快速凝固过程的牛顿冷却方式假定和过冷熔体中的枝晶生长理论，提出了凝固相重量分数计算的物理模型和数学计算方法。该模型可对快速凝固过程中不同相的过冷度，晶体的生长速度以及最终的相组成进行半定量的计算和预测。Ni31.5Al68.5合金条带的快速凝固过程的计算结果和试验测定数据有较好的吻合。     

Analysis on fluid permeability of dendritic mushy zone during peritectic solidification in a temperature gradient

Compared with the growing applications of peritectic alloys,none research on the fluid permeability K of dendritic network during peritectic solidification has been reported before.The fluid permeability K of dendritic network in the mushy zone during directional solidification of Sn-Ni peritectic alloy was investigated in this study.Examination on the experimental results demonstrates that both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G-T) effects have obvious influences on the morphology of dendritic network during directional solidification.This is realized through different stages of liquid diffusion within dendritic mushy zone by these effects during directional solidification.The TGZM effect is demonstrated to play a more important role as compared with the G-T effect during directional solidification.Besides,it is shown that the evolution of dendrite network is more complex during peritectic solidification due to the involvement of the peritectic phase.Through the specific surface Sv,analytical expression based on the Carman-Kozeny model was proposed to analyze the fluid permeability of dendritic mushy zone in directionally solidified peritectic alloys.In addition,it is interesting to find a rise in permeability K after peritectic reaction in both theoretical predication and experimental results,which is different from that in other alloys.The theoretical predictions show that this rise in fluid permeability K after pedtectic reaction is caused by the remelting/resolidification process on dendritic structure by the TGZM and G-T effects during peritectic solidification.     

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     

Comments on ‘Solidification modes and microstructure of Fe–Cr alloys solidified at different undercoolings’

The critical growth velocity for a planar solidification front in undercooled alloy melts is discussed on the basis of the absolute stability theory to reexamine the interpretation of a current analysis of the solidification modes in undercooled bulk Fe–Cr alloys contributed by Xuezhi Zhang. Theoretically, it is possible to produce a planar front in the solidification of undercooled bulk melts. But practically, it is imopssible for the undercooled bulk Fe–Cr melts to produce a planar front in the solidification. The dendritic growth theories as well as the calculations due to Zhang have been analyzed.     

The effect of microstructural evolution on hardening behavior of type 17-4PH stainless steel in long-term aging at 350 °C

The analysed Al₃Ni₂–Al₃Ni multi-layer on a Ni-substrate was solidified in an apparatus for diffusion soldering under vacuum. The δ–ζ multi-layer on the (Γ1 + Fe)-substrate was solidified in an apparatus for hot dip galvanizing.

Isothermal solidification was applied to produce both coatings. The devices constructed allowed the solidification process to be arrested after a predetermined time. The period of time from the beginning of solidification to the applied arrest was introduced into the definition of a back-diffusion parameter, . The back-diffusion parameter was employed to work out a model for solute redistribution. Experimental solute redistribution resulted from undercooled peritectic reactions accompanying the solidification of a multi-layer on a given substrate. The Al-solute and Zn-solute redistributions after the back-diffusion occurring during solidification with the presence of undercooled peritectic reactions were measured to identify the phases appearing. A thermodynamic theorem of the maximum driving force was applied to determine the sequence of phase appearance in contact with the liquid.

The experimental sequence of phases appearing was tested with the thermodynamic predictions. The model for solute redistribution was verified through a fitting of the theoretical solute redistribution profile with experimental data. These data were obtained by means of electron microscopy techniques in both the multi-layers coated onto metallic Ni and (Γ1 + Fe) substrates. The experimental sequences of the peritectic phases appearance in both multi-layers were also confirmed by the model of solute redistribution.     

New initiating mechanism of coupled growth in directionally solidified Fe-Ni peritectic system in diffusive regime

A new initiating phenomenon of peritectic coupled growth was observed in directionally solidified Fe-Ni alloys under diffusion-limited growth condition. Although it is generally accepted that peritectic coupled growth is initiated by island banding, the present experimental results show that two phases coupled growth could also be initiated by planar primary phase in samples of different compositions. Two models were proposed to explain the new initiating mechanism of peritectic coupled growth. It was found that due to the solute pile-up at the planar primary phase interface, the liquid at the planar primary phase interface will be suitable for nucleation of the peritectic phase or constitutionally undercooled with respect to the growing primary phase. The simultaneous growth of the nucleated peritectic phase with the primary phase or the cellular primary phases with the intercellular peritectic phase can induce the formation of coupled growth.     

Metastable phase diagrams of Cu-based alloy systems with a miscibility gap in undercooled state

Some Cu-based alloy systems with a large positive enthalpy of mixing display a eutectic or peritectic phase diagram under equilibrium conditions, but show a metastable liquid miscibility gap in the undercooled state. When the melt is undercooled below certain temperature beyond the critical liquid-phase separation temperature, it separates into two liquids with different compositions. The compositions of the two liquids change successively upon the metastable phase diagram before solidification occurs. The shape and position of the metastable miscibility gap are dependent of the alloy components and their interaction features. This study reviews the metastable phase diagrams of Cu-based alloy systems, which are derived from experiments and thermodynamic calculations.     

Diffusion-induced abnormal tertiary dendrite during peritectic solidification in a temperature gradient

1.Introduction Dendritic structure are commonly encountered during solidification [1-4], especially in systems freezing with relatively low entropies of transformation [1,5].Generally speaking, the dendritic structure is composed of primary dendrite stem, secondary branch and even tertiary and higher order dendrite arms [1].Different kinds of models have been proposed for describing the growth of primary dendrite [1-5] and secondary dendrite arms [1,6,7].In recent decades, dendritic growth of many industrial important peritectic alloys featured by peritectic reaction L+α→β have been witnessed [8-10].The dendritic growth is more complex during peritectic solidification due to the involvement of peritectic β phase.     

Ternary equilibria in bismuth-indium-lead alloys

The liquidus surface is characterized by three binary equilibria. One binary extends from the Pb-Bi peritectic to the Pb-In peritectic. The other two extend from In-Bi eutectics, merge at 50 a/o Bi and 30 a/o Pb, and end at the Bi-Pb eutectic. Based on analysis of ternary liquidus contours and vertical sections, it is suggested that solidification for high lead and very high indium alloys occurs from two-phase equilibria. Solidification from all other alloys occurs from three-phase equilibria. Four-phase solidification does not occur in this system.     

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。