Solidification of undercooled molten Ni-based alloys

The effect of undercooling on grain structure is investigated in pure nickel, Ni₇₅Cu₂₅, and DD3 singlecrystal superalloy by employing the method of molten salt denucleating combined with thermal cycling. Meanwhile, a comparison of factors that may be related to structure formation is performed and the difference in the refined structure between Ni₇₅Cu₂₅ alloy and DD3 single-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low and high undercoolings in Ni₇₅Cu₂₅ and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dendrite remelting driven by the chemical superheating produced in recalescence, and the second one at high undercoolings is due to the recrystallization process as a result of the high stress provided in the rapid solidification after high undercooling. Dislocation morphology evolution in as-solidified structure is also provided by the transmission electron microscopy (TEM) technique to further verify the recrystallization mechanism.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable δ phase formation in a wide composition range is confirmed.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

深过冷技术制备均质过偏晶合金及其形成机制的研究

采用熔融玻璃净化和循环过热相结合的方法使Ni 40 % (质量分数 )Pb合金获得 2 92K大过冷度 ,成功制备出大体积均质过偏晶合金。根据BCT模型和组织演化结果分析表明 :过冷粒状晶是在内应力的作用下 ,枝晶发生全面碎断 ,随后在枝晶段表面和应变能的驱动下使晶界移动发生再结晶的结果 ,即枝晶碎断 再结晶机制 ;试样基体上弥散分布的细密铅颗粒是由于快速凝固阶段溶质截留效应而形成的 ,少量较大尺寸铅颗粒的形成主要与慢速凝固阶段分布于枝晶骨架间残余富铅液相的聚合有关。     

深过冷铜镍合金微观组织演化规律研究

利用深过冷快速凝固技术使Cu65Ni35合金获得了不同的过冷度，最大过冷度达到284 K。对快速凝固组织拍摄金相图片，系统研究了Cu65Ni35合金微观组织形貌特征及其演化规律。结果表明，Cu65Ni35合金在大过冷度范围和小过冷度范围内均出现了晶粒细化现象。对具有典型晶粒细化特征的Cu65Ni35合金组织进行电子背散射衍射（EBSD）检测，发现大过冷度组织和小过冷度组织具有完全不同的晶粒取向，小过冷度下的组织发生晶粒细化是由枝晶重熔碎断致使的，而大过冷度下的组织发生晶粒细化却是由应力诱导再结晶完成的。     

Crystal multiplication in undercooled Cu + 2 Pct Sn alloy

The effects of undercooling (AT) from 10 to 175° on grain structure were observed in a Cu + 2 wt pct Sn alloy, in which grain refinement does not occur at large degrees of under-cooling. Quenching soon after recalescence retained transient grain structures not previ-ously reported in the literature. Crystal multiplication by dendrite fragmentation occur-red when undercooling below the liquidus lay in the range △T = 10 to 70°, and resulted in complete grain refinement in the range △T = 50 to 70°. Fragmentation affected primary, secondary and tertiary dendrite arms during and after recalescence. At △T = 70° a sharp transition occurred to a radiating fan-shaped structure of twin-related grains ori-ginating from a single point of nucleation, with no evidence of fragmentation. It is pro-posed that the transition results from a change in the free dendritic growth mode, the new shape being a wholly primary form without side-arms. The absence of fragmentation in this range (△T > 70°) suggests that self-buckling contributes to fragmentation in the other range (△T < 70°) and could be at least equal in importance to remelting. Formerly with the University of Queensland, Australia, and the National Research Council of Canada, Ottawa     

Phase Selection and Microstructure Evolution in Nonequilibrium Solidification of Fe40Ni40B20 Alloy

Phase selection and microstructure evolution in nonequilibrium solidification of ternary eutectic Fe₄₀Ni₄₀B₂₀ alloy have been studied. It is shown that γ-(Fe, Ni) and (Fe, Ni)₃B prevail in all the as-solidified samples. No metastable phase has been found in the deeply undercooled samples. This is explained as resulting from the size effect of undercooled solidification. At small and medium undercoolings, the dendrite γ-(Fe, Ni) appears as the leading phase. This is ascribed to the existence of the skewed coupled growth zone in FeNiB alloy. With increasing undercooling, the amount of dendrites first increases and then decreases, accompanied by a transition from regular eutectic to anomalous eutectic. The formation mechanisms of the anomalous eutectics are discussed. Two kinds of microstructure refinement are found with increasing undercooling in a natural or water cooling condition. However, for melts with the same undercooling, the as-solidified microstructure refines first, and then coarsens with an increasing cooling rate. The experimental results show that the nanostructure eutectic cell has been obtained in the case of Ga-In alloy bath cooling with an initial melt undercooling of approximately 50 K (50 °C).     

Dendritic growth of undercooled nickel-tin: Part II

High-speed optical temperature measurements were made of the solidification behavior of levitated metal samples within a transparent glass medium. Two undercooled Ni-Sn alloys were examined, one a hypoeutectic alloy and the other of eutectic composition. Recalescence times for the 9 mm diameter samples studied decreased with increasing undercooling from the order of 1.0 second at 50 K under-cooling to less than 10⁻³ second for undercoolings greater than 200 K. Both alloys recalesced smoothly to a maximum recalescence temperature at which the solid was at or near its equilibrium composition and equilibrium weight fraction. For the samples of hypoeutectic alloy that recalesced above the eutectic temperature, a second nucleation event occurred on cooling to the eutectic temperature. For samples which recalesced only to the eutectic temperature, no subsequent nucleation event was observed on cooling. It is inferred in this latter case that both the α and β phases were present at the end of recalescence. The thermal data obtained suggest a solidification model involving (1) dendrites of very fine structure growing into the melt at temperatures near the bulk undercooling temperature, (2) thickening of dendrite arms with rapid recalescence, and (3) continued, much slower recalescence accompanying dendrite ripening.     

Dendrite growth processes of silicon and germanium from highly undercooled melts

The crystal growth behavior of a semiconductor from a very highly undercooled melt is expected to be different from that of a metal. In the present experiment, highly pure undoped Si and Ge were undercooled by an electromagnetic levitation method, and their crystal growth velocities (V) were measured as a function of undercooling (ΔT). The value of V increased with ΔT, and V=26 m/s was observed at ΔT=260 K for Si. This result corresponds well with the predicted value based on the dendrite growth theory. The growth behaviors of Si and Ge were found to be thermally controlled in the measured range of undercooling. The microstructures of samples solidified from undercooled liquid were investigated, and the amount of dendrites immediately after recalescence increased with undercooling. The dendrite growth was also observed by a high-speed camera.     

Nonequilibrium Solidification Behavior of Co-Si Alloys Near the First Eutectic Point

Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co₃Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved.     

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.     

Phase selection in undercooled ti3sn melts

The solidification of undercooled Ti₃Sn melts was investigated using electromagnetic levitation and electrohydrodynamic atomization experiments followed by extensive microstructural char- acterization. The study was motivated by several reports on the kinetic preference for the body- centered cubic (bcc) phase over more closely packed disordered and ordered structures during competitive crystallization from undercooled melts. At low undercoolings, Ti₃Sn melts yield the equilibrium ordered hexagonal DO₁₉ structure, which is retained without change upon cool- ing. Undercoolings between ~100 and ~300 K yield primary dendrites with hexagonal sym- metry but a final microstructure which is clearly martensitic in origin. Two previously unknown metastable forms of Ti₃Sn were identified: an ordered base-centered orthorhombic derived from the α martensite and an ordered monoclinic phase related to the face-centered orthorhombic martensite observed in the Ti-V system. Both phases are believed to evolve from the solid state transformation of a high temperature β phase, but the dendritic structure clearly indicates the formation of a hexagonal phase different from DO₁₉,i.e., α. The latter forms in preference to β, which has a larger driving force in at least part of the undercooling regime studied. It is proposed that the primary α transforms to β as a consequence of recalescence, which subse- quently transforms martensitically and orders to yield the observed metastable forms of Ti₃Sn.     

Dendritic growth of undercooled nickel-tin: Part I

Solidification of undercooled Ni-25 wt pct Sn alloy was observed by high-speed cinematography and results compared with optical temperature measurements. Samples studied were rectangular in cross-section, and were encased in glass. Cinematographic measurements were carried out on samples undercooled from 68 to 146 K. These undercoolings compare with a temperature range of 199 K from the equilibrium liquidus to the extrapolated equilibrium solidus. At all undercoolings studied, the high-speed photography revealed that solidification during the period of recalescence took place with a dendrite-like front moving across the sample surface. Spacings of the apparent “dendrite” were on the order of millimeters. The growth front moved at measured velocities ranging from 0.07 meters per second at 68 K undercooling to 0.74 meters per second at 146 K undercooling. These velocities agree well with results of calculations according to the model for dendrite growth of Lipton, Kurz, and Trivedi. It is concluded that the coarse structure observed comprises an array of very much finer, solute-controlled dendrites.     

Undercooling-Induced macrosegregation in directional solidification

The accepted primary mechanism for causing macrosegregation in directional solidification (DS) is thermal and solutal convection in the liquid. This article demonstrates the effects of under-cooling and nucleation on macrosegregation and shows that undercooling, in some cases, can be the cause of end-to-end macrosegregation. Alloy ingots of Pb-Sn were directionally solidified upward and downward, with and without undercooling. A thermal gradient of about 5.1 K/cm and a cooling rate of 7.7 K/h were used. Crucibles of borosilicate glass, stainless steel with Cu bottoms, and fused silica were used. High undercoolings were achieved in the glass crucibles, and very low undercoolings were achieved in the steel/Cu crucible. During under-cooling, large, coarse Pb dendrites were found to be present. Large amounts of macrosegregation developed in the undercooled eutectic and hypoeutectic alloys. This segre-gation was found to be due to the nucleation and growth of primary Pb-rich dendrites, continued coarsening of Pb dendrites during undercooling of the interdendritic liquid, Sn enrichment of the liquid, and dendritic fragmentation and settling during and after recalescence. Eutectic ingots that solidified with no undercooling had no macrosegregation, because both Pb and Sn phases were effectively nucleated at the start of solidification, thus initiating the growth of solid of eutectic composition. It is thus shown that undercooling and single-phase nucleation can cause significant macrosegregation by increasing the amount of solute rejected into the liquid and by the movement of unattached dendrites and dendrite fragments, and that macrosegregation in excess of what would be expected due to diffusion transport is not necessarily caused by convection in the liquid.     

Mechanisms of the Origin of Fine and Non-Dendritic Grains at the Sonotrode–Liquid Metal Interface During Ultrasonic Solidification of Metals

Proposed grain refinement mechanisms during ultrasonic solidification have been explained in terms of refinement between cavitation enhanced nucleation and fragmentation of dendrites according to the casting conditions. Solidification studies also describe the activation of nucleation under pressure pulses after bubble implosion as an additional supporting mechanism for grain refinement. This study clarifies some overlooked concepts and proposes a plausible grain refinement mechanism explaining the role of cavitation in pure Zn and a Mg–6 wt pct Zn alloy. Equivalent grain size and grain density have been obtained in pure Zn and the Mg–6 wt pct Zn alloy (grain size distribution ranging from 40 to 200 µm) when UST was applied after the onset of solidification. These fine, non-dendritic grains originate from the cavitation zone beneath the sonotrode. Significant thermal undercooling surrounding the low superheat sonotrode in contact with the melt is responsible for the formation of a solidified layer (typically the thickness is equivalent to the average grain diameter) at the sonotrode–melt interface. High-frequency vibrations with or without cavitation at this interface assist the separation of these fine grains, which are then carried into the melt by acoustic streaming. A possible mechanism for the separation of fine grains produced from the cavitation zone is explained with the help of established concepts reported for the ultrasonic atomization process.

     

Microstructures of undercooled Germanium droplets

Small liquid Ge droplets (0.3–0.5 mm diameter) have been undercooled 150–415 ± 20°C below T_m in B₂O₃ flux before solidifying to the diamond cubic phase. A correlation was found between initial undercooling and final grain size. Droplets undercooled <300°C exhibited a coarse grain structure. At greater undercoolings, the grain size became progressively finer. This correlation may be subsidiary to the dependence of grain size on interfacial undercooling. Ge droplets lightly doped with Sn solidified dendritically for undercoolings greater than 250°C. Twinned dendrites have been observed at small undercoolings (~ 10°C) in other experiments. It appears that larger interfacial undercoolings are necessary to grow the twin-free dendrites which we have observed. The correlation between grain size and the presence of dendrites suggests that the grain refinement observed in Ge samples undercooled > 300°C stems from dendritic break-up during solidification.     

Theoretical treatment of the solidification of undercooled Fe-Cr-Ni melts

The solidification behavior of undercooled Fe-Cr-Ni melts is analyzed with respect to the competitive formation of body-centered cubic (bcc) phase (ferrite) and face-centered cubic (fcc) phase (austenite). The activation energies of homogeneous nucleation and growth velocities for both phases as functions of undercooling of the melt are calculated on the basis of current theories of nucleation and dendrite growth using data of thermodynamic properties available in the literature. As model systems for numerical calculations, the alloys Fe-18.5Cr-11Ni forming primary ferrite and Fe-18.5Cr-12.5Ni forming primary austenite under near-equilibrium solid-ification conditions are considered. Nucleation of the bcc phase is always promoted in the under-cooled primary ferrite alloy, whereas the barrier for bcc nucleation falls below that for fcc nucleation for large undercooling in primary austenite alloys. With rising undercooling, tran-sitions of the fastest growth mode were found from bcc to fcc and subsequently from fcc to bcc for the primary ferrite forming alloy and from fcc to bcc for the primary austenite forming alloy. The results of the calculations provide a basis for understanding contradictory experi-mental findings reported in the literature concerning phase selection in rapidly solidified stainless steel melts for different process conditions. Formerly Visiting Scientist at the Institut fur Raumsimulation     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

The growth rate of dendrites in undercooled tin

The dendrite growth velocity has been determined for tin in melts undercooled as much as 40°C (approximately twice the maximum undercooling reported previously). The results can be represented approximately asV = 0.8 (ΔT) ² WhereV is the growth velocity in mm s⁻¹ and ΔT is the undercooling in degrees centrigrade.