过冷过偏晶Cu-40wt%Pb合金凝固组织的演化

用熔融玻璃净化与循环过热相结合的方法,研究了Cu-40wt%Pb过偏晶合金过冷熔体凝固组织的演化规律.在过冷度40～296K的范围内,其凝固组织的形态有3次变化过程:第1次是在40～75K过冷度范围,经过液-液分离和偏晶反应形成了偏晶胞组织;第2次发生在75～196K过冷度范围,因枝晶熟化被抑制,由粗大的枝晶重熔形成的粒状晶转变为高度细化的细枝晶;第3次发生在196～296K过冷度区间,组织因细枝晶再结晶转变为均匀的准球状晶粒.     

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。     

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.     

Co-Si金属间化合物深过冷快速凝固及组织演化机理研究

目的 探究Co-Si金属间化合物在深过冷条件下产生的再辉现象及对应组织的演化阶段,并分析其形成机理。方法 为获得深过冷,结合了两种热力学方法：循环过热法及熔融玻璃净化法,实现了Co2Si金属间化合物（Co-33at%Si）、CoSi金属间化合物（Co-50at%Si）的深过冷快速凝固,并通过光学显微镜、扫描电子显微镜和XRD进行分析。结果 发现Co-33at%Si金属间化合物凝固过程中存在两次再辉现象,晶粒尺寸随着过冷度的增加而减小,ΔT=190 K时,二次枝晶间距大约为0.5 μm,层片间距为0.289 μm,而Co-50at%Si金属间化合物凝固过程仅有一次再辉现象,且随过冷度增加,晶粒尺寸减小。ΔT<50 K时,微观组织形貌为树枝晶;ΔT>90 K时,微观组织形貌为细小等轴晶。结论 Co-33at%Si中一次再辉对应于初始枝晶形成及细化过程,二次再辉对应于晶粒间残余液相二次凝固,即晶间细小树枝状共晶的形成。而Co-50at%Si中唯一一次再辉现象则对应枝晶向等轴晶的转变。     

快速凝固条件下Ni–Pb偏晶合金的组织演变机理研究

目的 研究过冷Ni–0.5%Pb(原子数分数)合金过冷组织的演化行为,阐明其组织演化和晶粒细化的基本机制。方法 采用熔融玻璃净化和循环过热方法制备出过冷度为0~255 K的试样,并结合枝晶生长的动力学–热力学模型,研究其深过冷快速凝固行为机制。结果 在0~255 K过冷度范围内,随着过冷度的增大,Ni–Pb偏晶合金的微观组织发生了2类晶粒细化现象,组织形态由粗大树枝晶向粒状等轴晶转变。结论 第1类粒状晶的形成是由于枝晶熟化和再辉重熔导致发达枝晶破碎,第2类粒状晶的形成是由于在应力和应变能的作用下,枝晶碎变和再结晶引起了晶粒细化。     

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。     

铸型涂层对熔体过冷度和凝固组织的影响

利用溶胶－凝胶方法,在铸型内表面玻璃涂层上制备ＳｉＯ２晶态和非晶态薄膜涂层,将深过冷的Ｃｕ７０Ｎｉ３０合金熔体浇入两种涂层铸型中,分别获得９０Ｋ和１９８Ｋ的过冷度。提出了用重合密度和重合原子中心偏离度分析涂层结构对合金熔体形核惰性影响的模型。用ＢＣＴ模型分析过冷熔体凝固过程中枝晶生长与过冷度的关系,结果表明,８０Ｋ为临界冷度,△Ｔ〈８０Ｋ时,枝晶生长受成分过冷控制;△Ｔ〉８０Ｋ时,受热过冷控制。在深过冷范围内,凝固组织为细密挺直枝晶。     

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.     

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.     

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.     

Cu—Ni—Fe合金在特殊涂层中的深过冷及其遗传性

以（８０％石英砂＋２０％石英玻璃粉）＋３％Ｈ３ＢＯ３（均为质量分数，下同）为坩埚涂层材料，烧结后获得厚３ｍｍ的表面光洁，无裂纹玻璃态涂层，在该涂层中熔炼Ｃｕ－３９Ｎｉ－６Ｆｅ和Ｎｉ－３０Ｃｕ－５Ｆｅ合金，分别获得了１９０Ｋ和２１０Ｋ的大过冷度，超过了晶粒骤然细化的临界过冷度△Ｔ２，以硅溶胶为粘结剂，石英玻璃粉为面涂层的铸型，浇铸后进行一次过热，使Ｎｉ－３０Ｃｕ－５Ｆｅ合金获得了１００过冷度，超过了     

Disorder trapping during the solidification of βNi<Subscript>3</Subscript>Ge from its deeply undercooled melt

A melt encasement (fluxing) technique has been used to solidify the congruently melting intermetallic βNi₃Ge from its deeply undercooled parent melt. High speed photography and a photo-diode technique have been used to measure the resulting growth velocity. The maximum undercooling achieved was 362 K, wherein a growth velocity of 3.55 m s⁻¹ was recorded. At an undercooling of 168 K, an abrupt increase in the gradient of the velocity-undercooling curve is observed and this is attributed to a transition from growth of the ordered L1₂ compound at low undercooling, to growth of the fully disordered compound at high undercooling. A change in the microstructure, from a distribution of very coarse (700 μm), randomly oriented grains to a fine-grained structure (15–20 μm) with a large number of low-angle grain boundaries is observed coincident with this transition in the velocity-undercooling curve, a grain refinement pattern that is different to that observed either in deeply undercooled solid solutions or other intermetallics. This transition is ascribed to a recovery and recrystallisation process in the disordered phase due to the low post-recalescence cooling rate.     

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.     

Containerless solidification of germanium by electromagnetic levitation and in a drop-tube

Containerless solidification of germanium has been realized by electromagnetic levitation and drop-tube processing, respectively. The effect of undercooling in the range of 40–426 K on the as-solidified structures of levitation melted Ge drops (∼8.4 mm diameter) was investigated. For undercoolings less than 300 K, the lamellar twins were grown, whereas a microstructural transition to equiaxed grains was observed at undercoolings ≥300 K. Further increasing the undercooling to 400 K, a significant reduction in grain size was achieved. In addition to a similar microstructural development among the particles solidified during free fall in a 8.5 m drop-tube, high-undercooling-induced single crystals were found for some droplets less than 200 μm in diameter. The results on the transition from twins to fine equiaxed grains are accounted for by theories of solidification kinetics and a dendrite break-up model. This revised version was published online in November 2006 with corrections to the Cover Date.     

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。     

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.