Recalescence Behavior Research Articles

Recalescence Behavior, Solidification Characteristics and Microstructure Transformation of Rapidly Solidified Undercooled Cu-based Alloys

Recalescence Behavior, Solidification Characteristics and Microstructure Transformation of Rapidly Solidified Undercooled Cu-based Alloys

Microstructure evolution and grain refinement mechanism of rapidly solidified single-phase copper based alloys

• The maximum undercoolings were achieved respectively by using the deep undercooling technology. • The recalescence behavior and microstructure refinement were systematically studied. • The relationship between solute composition and solidification process was established. • High-angle grain boundaries and twin boundaries exist in the microstructure at high undercooling. The Cu65Ni35, Cu60Ni40 and Cu55Ni45 alloys were undercooled by fluxing method, and the rapid solidification structure with different undercoolings were also obtained. At the same time, the interface migration process during rapid solidification was photographed by high-speed photography, and the relationship between the morphological characteristics of solidification front and undercooling was analyzed. The microstructures of the three alloys were observed by metallographic microscope, and the microstructure characteristics and evolution law were systematically studied. It was found that two grain refinement events occurred in the low undercooling range and high undercooling range, respectively. The EBSD test of grain refined microstructures showed that the microstructure in the low undercooling range has a high proportion of low-angle grain boundaries and high strength textures. However, there were a large proportion of high-angle grain boundaries and a high proportion of twin grain boundaries and more randomly oriented grains in the microstructure in the high undercooling range. The TEM test of the Cu55Ni45 alloy with the maximum undercooling of 284 K showed that there were high-density dislocation networks and stacking faults in the grains. Finally, the evolution relationship between microstructure hardness and undercooling was systematically studied. It was found that the microhardness of the three alloys decreased sharply near the critical undercooling. Combined with EBSD, TEM and microhardness analysis, it was confirmed that the grain refinement under low undercooling was caused by dendrite remelting, while the grain refinement under high undercooling was caused by stress-induced recrystallization.

Widmannstätten laths in Ti48Al2Cr2Nb alloy by undercooled solidification

Widmannstätten laths in Ti48Al2Cr2Nb alloy were obtained by containerless electromagnetic levitation during undercooled solidification rather than heat treatments in most cases. The nucleation of γWidmannstätten is confirmed by the thermal history of the second recalescence behaviour. High dislocation density, stacking faults and dislocation slip are observed by transmission electron microscopy. By quantitative analysis of the transformation energy, the driving force for γWidmannstätten in the solid state phase transformation is calculated, compared with that of heat treatments. Meanwhile, the proper undercoolings (ΔT=257–300K), post-solidification cooling rates (11.1–29.1K/s), the fine lamellar colony sizes (306–467μm) and proper undercoolings in the solid state phase transformation are in favour of promoting the formation of Widmannstätten laths at moderate undercoolings.

Design of process diagnostics for excimer laser irradiation of oxide thin films

The authors have developed an in situ excimer laser irradiation system that can monitor the temperature field of oxide thin films through thermal radiation produced upon a pulsed ultraviolet laser irradiation. Thermal emission signals from a Si single crystal obtained using with this system agreed well with a numerical simulation result, validating the time constant of the measurement system. The time-dependent transient temperature profile of tin-doped indium oxide thin films, which was converted from thermal emission signals, clearly showed a recalescence behavior in its cooling curve. The transmittance data of the laser-irradiated indium oxide thin films suggested that the transmittance monitoring at near-infrared and ultraviolet–visible regions could be useful in monitoring the dopant levels and laser processing conditions, respectively.

Solidification behaviors of undercooled Ni-P alloys

In order to gain an insight into the primary phase selection and solidification structure formation while undercooled eutectic alloys solidify, Ni100-xPx (x=18, 19, 19.6, 20, 21, atomic percent) alloy melts are undercooled to different temperatures below the equilibrium liquidus. The recalescence behavior associated with rapid solidification is monitored by a high-speed infrared pyrometer, and the solidification structure is analyzed systematically. When α-Ni/Ni3P regular eutectic forms as the primary phase during rapid solidification, single recalescence event takes place. In the resulting anomalous eutectic, fine granular grains of α-Ni are distributed uniformly in the Ni3P matrix. When the primary phase is one of the eutectic phases, however, there is a second recalescence event following the first one, resulting from nucleation of the other phase in the remaining liquid and the subsequent rapid eutectic growth. In this case, there are two types of granular grains whose sizes are significantly different from those in the anomalous eutectics. Finally, a coupled growth zone of eutectics is determined. At large undercoolings, α-Ni rather than Ni3P solidifies as the primary phase even in the Ni-P hyper-eutectic alloys due to its rapider growth kinetics.

Comparisons of grain refinement and recalescence behavior during the rapid solidification of undercooled Cu–Co and Cu–Ni alloys

Applying the melt-fluxing method, rapid solidifications of undercooled Cu 50Co 50 immiscible and Cu 70Ni 30 solid solution alloys were performed to investigate the grain refinement regularity at moderate undercooling. Due to the essential distinction of physical properties between these alloys, grain refinement phenomenon was detected in Cu 50Co 50 alloy with a tendency of liquid separation, while dendrite structure appears for Cu 70Ni 30 alloy. The association of microstructure morphology to non-equilibrium solidification process was constructed with the description of recalescence degree as a function of undercooling. Quantitative thermodynamic calculation for the undercooled liquid of the researched systems was carried out to elucidate the influence of liquid separation on the variation of Gibbs free energy for the subsequent rapid solidification, which gives a better understanding of the as-observed experimental results.

Use of recalescence behavior analysis for the prediction of grain refinement in undercooled Cu–Ni alloy

Considering non-equilibrium solidification and its influence on subsequent near-equilibrium solidification together, the description of recalescence behavior in bulk undercooled Cu70Ni30 alloy was adopted to predict the corresponding microstructure transition. The thermal plateau time for near-equilibrium solidification can be deduced directly with the calculation of non-equilibrium solid fraction formed in recalescence. On the basis of quantitative description for recalescence behavior, the non-equilibrium solid fraction, residual liquid fraction, dendrite broken-up time, and thermal plateau time can be determined as functions of initial undercooling. Then, a simple and accurate application of dendrite fragmentation model was performed as the grain refinement at both low- and high-undercooling originates from dendrite breakup. The predicted undercooling regions for the double grain refinement agree well with the experimental observation. Moreover, the change of grain morphology for the second grain refinement can be ascribed to the occurrence of recrystallization produced by the enhanced residual stress upon highly undercooled solidification.

Numerical description for the recalescence of bulk-undercooled Cu 70Ni 30 alloy

Assuming thermal balance and solute conservation, a numerical model has been proposed to describe the recalescence behavior of bulk-undercooled Cu–Ni melts. Applying a finite-difference scheme, the transformed solid fraction upon recalescence is given as a function of the liquid temperature, while the average liquid concentration can be tracked by calculation of the liquid/solid (L/S) Gibbs energy difference, in combination with a dendrite growth model. Accordingly, a transition from non-equilibrium to equilibrium process has been described from the evolution of L/S Gibbs energy difference. Applying the present model, the experimentally observed maximum recalescence temperature can be well predicted.

Discussion on nucleation mechanism of undercooled Ni 80.3B 19.7 melts

Ni 80.3B 19.7 alloy was undercooled using the glass-fluxing method. Applying high speed video (HSV) and metallographic methods, the recalescence behavior and as-solidified microstructure were observed. In combination with the analysis of the experimental results and the calculation on the basis of time dependent nucleation theory, the nucleation mechanism in undercooled Ni 80.3B 19.7 melt was investigated. Owing to the time required to form the critical nucleus is much smaller than the time required for solid/liquid (S/L) interface to touch the nucleus, the critical nucleuses ahead the S/L interface can form and develop into the nucleation events, which make the copious nucleation ahead the S/L interface be operative. The calculation is in quantitative agreement with the experimental results.

Non-equilibrium solidification of concentrated Fe–Ge alloys

The solidification of concentrated alloys containing ordered compounds is less well understood. These alloys often exhibit complex phase change like peritectic reaction during liquid to solid transformation. The Fe-rich part of Fe–Ge binary alloy system consists of several critical points and ordered–disorder transitions and can be used as a model system to study the effect of departure from equilibrium on the solidification microstructure. In order to understand the phase selection and morphological transitions, undercooling and recalescence behaviour; growth rate of the solidifying phases and microstructure need to be explored. In the present paper, we summarise the results obtained in several iron rich alloy compositions (Fe–(14–25) at.% Ge) using techniques of melt quenching, levitation and laser resolidification. These results provide insight to the current theories of dendritic growth and reveals possibility of a new pathway for phase evolution in peritectic alloys at high undercooling involving a massive transformation.

Recalescence behaviour of binary Ti–Al and ternary Ti–Al–Nb undercooled melts

The recalescence behaviour of undercooled melts of binary Ti 55Al 45, Ti 50Al 50, Ti 45Al 55, and ternary Ti 45Al 50Nb 5, Ti 45Al 45Nb 10, Ti 40Al 50Nb 10 alloys have been studied by a containerless electromagnetic levitation technique in a He/H 2 atmosphere in order to reveal phase formation and microstructure evolution processes. The time–temperature traces of the pyrometer exhibit steep single recalescence events for undercooling values of ∼300 K below the liquidus temperature. Dendrite growth velocities of the order of 10–20 m/s for highly undercooled ternary Ti–Al–Nb melts are consistent with a primarily formed bcc β-phase, which is promoted by Nb addition. For ternary Ti 45Al 50Nb 5 and Ti 40Al 50Nb 10 alloy compositions, a second recalescence near 1300 °C was observed, which can be attributed to a solid state transformation. The microstructure of levitated melt drops subsequently quenched onto a chill substrate shows more evidence for the predicted phase transformations.

Marangoni flow of molten silicon: The effects of oxygen partial pressure

Instability of the Marangoni flow of molten silicon is discussed. Time-dependent behavior of azimuthal wave numbers (m) for flow instability was investigated based on temperature oscillation measurements using a θ-t analysis, and appearance ratio of each azimuthal wave number is analyzed. The relationship between frequency and azimuthal wave numbers is deduced from an analysis of appearance ratios at a decomposed frequency band using a wavelet analysis. Oxygen partial pressure at the melt surface can be predicted using the Ratto-Ricci-Arato model. Anomalous recalescence behavior and surface oxidation of levitated molten silicon are shown. Growth striation is induced in silicon crystals grown even under high oxygen-partial pressure.

A Simple Model for Solidification of Undercooled Metallic Samples

A simple model for reproducing temperature recalescence behaviour in spherical undercooled liquid metallic samples, undergoing crystallization transformations, is presented. The model is applied to constant heat extraction rate, uniform but time dependent temperature distribution inside the sample (even after the start of crystallization), a classical temperature dependent rate of nucleation (including contributions from different specific heats for different phases and also a catalytic factor to model the possibility of heterogeneous distributed impurities) and the solidified grain interface velocity is taken proportional to the temperature undercooling. Different assumptions are considered for the sample transformed fraction as function of the extended volume of nuclei, like the classical Kolmogoroff, Johnson–Mehl, Avrami one (corresponding to random distribution of nuclei), the Austin–Rickett one (corresponding to some kind of clusterized distribution) and also an empirical one corresponding to some ordering in the distribution of nuclei. As an example of application, a published experimental temperature curve for a zirconium sample in the electromagnetic containerless facility TEMPUS, during the 2nd International Microgravity Laboratory Mission in 1994, is modeled. Some thermo-physical parameters of interest for Zr are discussed.

Phase selection in the containerless solidification of undercooled CaO · 6Al 2O 3 melts

The CaO · 6Al 2O 3 melts were solidified on an aero-acoustic levitator under a containerless processing condition at various undercoolings. A high-speed video was operated to monitor the recalescence behavior, from which the growth velocity as a function of melt undercooling was determined. The microstructures were observed and the crystalline phases were identified using the X-ray diffraction technique, indicting that the Al 2O 3 was solidified when the melt temperature was higher than the peritectic temperature, T p. When the melt was undercooled below T p, the CaO · 6Al 2O 3 (CA 6) peritectic phase was crystallized directly from the undercooled melts. With respect to the direct formation of the peritectic phase, further analysis from the viewpoints of competitive nucleation indicated that the minimum free energy principle may be applied to elucidate the nucleation of CA 6 phase. In terms of the competitive growth behavior, the interface attachment kinetics for Al 2O 3 and CA 6 phases are calculated by using the classical BCT model indicating that although the Al 2O 3 phase doped by CaO has about four times larger interface kinetic coefficient than that of the CA 6 peritectic phase, the growth kinetics of Al 2O 3 in the melt with the CaO · 6Al 2O 3 chemical composition is not sufficiently high to replace the CA 6 phase as the primary phase. Therefore, once CA 6 is nucleated, it can develop into a macro crystal as the primary phase. The competitive nucleation and growth behavior in the CA 6 system is different from those in other well-studied peritectic alloys and the present investigation on the phase formation will be an essential supplement to the phase selection theory.

Phase Selection in Undercooled Y<SUB>3</SUB>Al<SUB>5</SUB>O<SUB>12</SUB> Melt

Two kinds of solidification paths from Y3Al5O12 melt has been reported; one is stable Y3Al5O12 (YAG) garnet, the other is metastable YAlO3 perovskite (YAP) and subsequent YAP+Al2O3 eutectic. The reason for this, however, has been puzzled. The effect of cooling rate on this phase selection was addressed under containerless condition using an aero-acoustic levitator. A high-speed video camera (HSV) enabled us to directly observe the recalescence behavior. As the cooling rate increased from 15 to 350 K/s, the solidification of a metastable YAP and YAP+Al2O3 eutectic, a monophasic YAG, and an amorphous phase were successively obtained. At around the critical cooling rate of approximately 50 K/s for the formation of YAP and YAG, simultaneous recalescence of YAP and YAG was observed by HSV, and the sample obtained contained both the metastable YAP and stable YAG. The nucleation rate of YAG corresponds with that of YAP at the critical cooling rate and the growth velocity of YAP, which first nucleated in the undercooled melt, was slow enough for YAG to nucleate in the remaining undercooled melt, resulting the simultaneous recalescence. In general, the metastable phase nucleates at the higher cooling rate than the stable phase. However, in this system, the higher nucleation barrier of YAG than that of YAP led to the nucleation of YAG at the higher cooling rate.

Containerless solidification of highly undercooled Al 2O 3–ZrO 2 eutectic melts on an aero-acoustic levitator

The Al 2O 3–ZrO 2 eutectic oxide was levitated in an aero-acoustic levitator and melted by a continuous wave CO 2 laser beam and then solidified under a containerless condition. A high-speed video was operated to image the in situ recalescence behavior at different undercoolings. Based on the recalescence frames and ambient microstructures, copious nucleation was proposed to take place when the sample was solidified either spontaneously at the maximum undercooling or externally seeded at lower undercoolings. The recalescence front was not the reflection of the solid/liquid interface but induced by the simultaneous thermal release of concentrated crystallizing eutectic colonies. The occurrence of copious nucleation should be mainly ascribed to a chain-like successive nucleation reaction in undercooled melts. The temperature distribution inside a spherical sample was considered when applying dimensionless variables of the Biot number and Fourier number so as to elucidate the abnormal eutectic formation near the sample center. Further analysis from the viewpoint of thermal boundary layer indicated that there was a strong thermal interaction among crystallizing eutectic colonies in the flat thermal gradient region near the central part of a sample, which made it impossible to establish a steady thermal flux and thus generating abnormal eutectics after the coarsening process was taken into account.

Microstructure and phase selection in containerless processing of Fe–Ni droplets

Fe–Ni droplets (Ni content 0–30 at.%) were containerlessly processed by electromagnetic levitation, in a drop-tube or by atomization. The droplet diameter varied from 10 μm to 7 mm. The competition between the formation of b.c.c. and c.c.p. phases is elucidated by studies of the recalescence behaviour and the dendrite growth velocities as a function of undercooling. For levitation the results are summarized in a phase-selection map. For droplets produced in the spray methods, a selection map was constructed which relates the predominant phases and microstructures (coarse-dendritic or grain-refined) to the composition and the droplet size. Links are established between the final microstructures, the crystal phases and the processing conditions. Thermodynamic modelling (CALPHAD) and an analysis of dendrite growth velocities are used to analyse the nucleation and growth behaviour.

Numerical modelling of the solidification of ductile iron

Numerical calculations are presented describing the solidification of a ductile iron based on the Stefanescu macroscopic heat transfer-microscopic solidification kinetic model but using a different kinetic model than that used by Stefanescu. The results show that the kinetic model used influences the recalescence behaviour predicted by the modelling. Cooling curves calculated with the present model show reasonable agreement with experimentally measured cooling curves for four different cooling rates.

Phase selection, growth, and interface kinetics in undercooled Fe-Ni melt droplets

An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT< ΔT * (primary growth of the stable ccp phase) and two recalescence steps for Δ> ΔT * (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT * strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.

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.