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。     

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。     

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.     

负温度梯度熔体凝固过程中的形核与再辉行为

研究了63～292K热力学过冷度范围内,Cu-Ni单相合金的凝固组织演化规律,分析了负温度梯度熔体凝固过程中的形核与再辉行为.结果表明:①负温度梯度熔体凝固的冷却曲线上有较明显的形核特征;②在负温度梯度熔体凝固冷却曲线的快速再辉阶段,出现了明显的"二次再辉"特征,此"二次再辉"的本质有别于慢速凝固阶段的二次再辉,因此称之为"伪再辉".     

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.     

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.     

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。     

Cu-Cr合金初生相对共晶生长的影响机制研究

制备了定向凝固Cu-1.0%Cr亚共晶自生复合材料,研究了初生α相生长对共晶生长的影响机制,探讨了亚共晶合金中共晶的生长规律.研究结果表明,Cu-1.0%Cr合金定向凝固时,在初生α相间生长的共晶受到初生相生长的影响,在热场不定向和生长空间受限的双重作用下,共晶无定向地杂乱生长.初生α相的生长引起枝晶间液相溶质分布的变化,随着凝固速度的增大,初生α枝晶间液相溶质的浓度分布趋于平缓,成分趋近于CE.Cu-1.0%Cr合金在快速凝固条件下,初生α相生长改变了共晶的生长环境,致使形成非平衡凝固组织--离异共晶.     

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.     

Solidification of undercooled Ni-32.5 wt% Sn eutectic alloy

Bulk Ni-32.5 wt % Sn alloy was undercooled by purification. The solidification characteristics at different undercoolings and the various microstructures were studied. It was found that the two eutectic phases can grow in a matching form, to produce regular eutectic structures if the melt was undercooled by less than 10 K. If the melt was highly undercooled, the dendrite tip velocity of the Ni₃Sn phase was very much larger than that of Ni() phase during recalescence; as a result, the melt did not solidify in the regular eutectic morphology but in the dendrite cluster morphology with Ni₃Sn phase acting as the leading phase in the recalescence stage. The dendrite clusters subsequently developed into anomalous eutectic microstructures by ripening of Ni₃Sn dendrites and growth of Ni() phase between the Ni₃Sn arms. If the undercooling before nucleation was below 130 K, bulk melt of the original composition Ni-32.5 wt % Sn existed between the dendrite clusters after recalescence, which then solidified into regular eutectic microstructures. As a result, the structure of the sample consisted of regular eutectic zones and anomalous eutectic zones. If the undercooling before nucleation was above about 130 K, no bulk melts existed after recalescence, so the structures of the samples were completely anomalous eutectic.     

Verification of metastable phase diagrams byin situ measurements on undercooled melts

The electromagnetic levitation technique combined with pyrometric methods have been applied to verify the metastable phase diagram of Fe Cr Ni alloys byin situ observation of phase selection processes in undercooled melts. The temperatures of levitated drops prior to solidification were determined by a two-color pyrometer and the recalescence behavior of the undercooled melt was recorded from a high-speed photosensing device with a sampling rate of 1 MHz. Fe₆₉Cr₃₁ ,Ni, alloy melts with different Cr/Ni ratios were investigated for undercooling levels up to 320 K. The transition from single- to double-recalescence behavior was found beyond a critical undercooling level for primary austenitic alloys with Cr Ni < 1.5. For the first time, this gives direct evidence of metastable phase formation in this alloy system fromin situ observations. The dendrite growth velocity displayed a sudden drop at the critical undercooling level, confirming the metastable phase formation. The intermediate arrest temperature of the double-recalescence event fits well with the calculated metastable liquidas line derived from a subregular solution model for the Gibbs free energy.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Solidification of undercooled cadmium-zinc eutectic melts

Molten specimens of Cd-Zn eutectic alloy (26.50 at.% Zn) were undercooled by the glass-slag technique using glass-forming ZnCl₂ as the slag. The microstructure of the solidified samples, weighing about 20 g each, was found to be a function of the undercooling produced. At lower undercoolings the microstructure consisted of supersaturated primary cadmium dendrites co-existing with the eutectic, while at higher coolings duplex structures were observed. Possible causes for the differences in microstructure are discussed with due importance given to the recorded recalescence effects.     

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.     

Amorphous Formation in an Undercooled Binary Ni-Si Alloy under Slow Cooling Rate

High undercooling up to 392 K was achieved in eutectic Ni70.2Si29.8 alloy melt by using glass fluxing combined with cyclic superheating.A small quantity of amorphous phase was obtained in bulk eutectic Ni70.2Si29.8 alloy when undercooling exceeds 240 K under slow cooling conditions (about 1 K/s).The amorphous phase was confirmed by high-resolution transmission electron microscopy and differential scanning calorimetry.     

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.     

Cellular growth of lamellar eutectics in undercooled Ag–Cu alloy

Ag–Cu eutectic alloy was undercooled by the glass flux method and the solidification structure was investigated. It is revealed that when undercooling is not more than 70 K, the large difference in composition between two eutectic phases and very large thermal diffusion coefficient of the liquid result in cellular growth of the lamellar eutectics from the nucleation site. The variation in interface temperature during rapid solidification gives rise to a systematic change in microstructure within the sample. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectic rises, which indicates a gradually decreasing growth velocity of the primary eutectics. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with solute, and then partially remelted and ripened into anomalous eutectics. As undercooling increases, the area of the anomalous eutectics enlarges.     

Preparation of bulk crystallite alloys by rapid quenching of bulk undercooled melts

Rapid quenching of deeply bulk undercooled alloy melts (200–250?K) before recalescence was used to produce three-dimensional bulk crystallite alloys. It was found that the micro-grain sizes of the prepared bulk crystallite alloys decreased with increasing undercooling. Dense crystal defects such as dislocation networks and annealing twins could be observed in the microstructures of the crystallite alloys. Substantial recrystallisation could be observed when annealing these crystallite alloys. These new findings could expand and enrich not only the traditional methods of preparing three-dimensional bulk crystallite alloys but also the traditional technologies of recrystallisation.     

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。