Undercooled Alloy Melts Research Articles

Transformation kinetics of the metastable bcc phase during rapid solidification of undercooled Fe-Co alloy melts

Solidification of metastable δ-phase (bcc) and subsequent transformation into the stable γ-phase (fcc) in undercooled Fe-Co alloy melts have been investigated on electromagnetically levitated samples. Rapid solidification is monitored by a high-speed video camera, which enables the direct observation of the solidification front. Metastable phase formation is revealed by a two-step recalescence event showing primary solidification of metastable bcc phase and subsequent formation of stable fcc phase within the mushy-zone of primary bcc phase. A decisive parameter describing the kinetics of phase transformation is the delay time between nucleation of the metastable and the stable phase. The delay time obtained from high-speed video recordings strongly decreases with rising undercooling prior to solidification. A model is proposed, which yields an explanation for the origin of the undercooling dependency. It is based on the classical nucleation theory combined with an estimation of the evolution of the number of heterogeneous nucleation sites within the expanding mushy-zone during dendritic growth of the primary phase. Thus, the delay time is not only determined by nucleation kinetics but it also depends on both the growth velocity v and the morphology of the dendritic network of the primary metastable phase, which both depend on undercooling ΔT. Using the relation v(ΔT) for the metastable bcc phase calculated by current models for dendritic growth and verified by measured growth velocities, the experimental data as a function of undercooling are well described by the calculated delay times in a wide range of compositions between 30 and 65 at.% Co.

Statistical learning for evaluation of crystal growth in low-melting alloy droplets with application to quasicrystal-forming Ti–Zr–Ni alloys

The evaluation of the crystal growth in undercooled alloy melts is essential for investigations of their solidification behavior. Low-melting alloy systems usually exhibit little self-illumination and cinematography is not often feasible. A time-temperature data based statistical learning approach is presented to evaluate the crystal growth in an undercooled droplet, where a spatial geometry-informed, errors-in-variables probabilistic model is developed based on the prediction of the dendrite growth paths. The preferable growth mechanism and growth velocities can be evaluated from statistical model selection and parameter estimation. Based only on the pyrometry data of two Ti–Zr–Ni alloys, a bulk growth mechanism is justified and the growth velocities in both the stable C14 phase and metastable icosahedral phase are able to be measured and statistically interpreted.

Correlation between Solidified Microstructure Evolution and Undercooling of Au-12 Wt.%Ge Eutectic Alloy

Highly undercooled solidification experiments were carried out by melt purification combined with cyclic superheating method on Au-12 wt.%Ge eutectic alloy. The solidification structures of Au-12 wt.%Ge eutectic alloy under different undercoolings were also analyzed by using the scanning electron microscope (SEM). The experimental results revealed that the maximum undercooling could reach up to 102 K. The microstructure analysis showed that the coarse bulk eutectic existed in the solidification structure when the undercooling was less than 34 K. When the undercooling was larger than 34 K and less than 56 K, the solidification structure transformed into cellular eutectic. The coarse primary (α-Au) phase precipitated from the undercooled alloy melt when the undercooling was larger than 56 K. The volume fraction of the primary (α-Au) phase gradually increased with the increase of undercooling. In this paper, a method to regulate the solidification structure of Au-12 wt.%Ge eutectic alloy is proposed, which provides a new way to improve the solidification structure and has important guiding significance for the processing and forming process of Au-12 wt.%Ge eutectic alloy.

Microstructure evolution mechanisms of undercooled Ni80Cu20 alloys

AbstractMicrostructure evolution mechanisms of undercooled Ni80Cu20 alloys were systematically studied, and two types of grain refinement were observed in the as-solidified microstructures. The first type of grain refinement appeared in a low undercooling range (i.e., 50 K ≤ ΔT ≤ 90 K). The remelting of primary dendrites during or after rapid solidification was due to liquid/solid interfacial tension and chemical superheating. The second type of grain refinement occurred in a high undercooling range (i.e., ΔT ≥ 165 K), in which the liquid phase pressure drop due to solidification contraction induced the flow of liquid in the inter-dendrite region and produced stress accumulation in the dendrites. At the slow solidification stage, i.e., post-recalescence period, recrystallization was observed. In addition, highly undercooled Ni80Cu20 alloy melts were quenched before rapid solidification by Ga–In liquid alloy, and partially recrystallized microstructures were obtained. Using electron backscattering diffraction and transmission electron microscopy techniques, the occurrence of recrystallization was verified experimentally.

Effect of thermal anisotropy on binary alloy dendrite growth

A numerical model to study the effect of thermal anisotropy on binary alloy dendrite growth is presented. The model is based on the volume averaged enthalpy method with explicit surface tension anisotropy for crystal orientation. Thermal anisotropy is incorporated using anisotropic thermal conductivity in the energy equation. This is done by splitting the anisotropic conductivity into two parts, equivalent isotropic conductivity and anisotropic departure source term, enabling the use of a conventional isotropic solver to model anisotropic heat transfer. The proposed model is applied to study the effect of thermal anisotropy ratio on tip velocity, aspect ratio and equivalent radius of an equiaxed grain growing in an undercooled binary alloy melt. It is found that the thermal energy stored in the grain during solidification plays an important role in interface evolution, and thus anisotropic conductivity in the solid affects the grain morphology. There is a consistent increase in the aspect ratio of grains with increase in thermal anisotropy ratio, although the grain volume remains almost invariant. Due to unequal growth rates of the perpendicular arms, severe distortion of the solid crystal is seen at higher thermal anisotropy ratios. The model is further extended to study the growth of multiple dendrites in order to simulate microstructure evolution with thermal anisotropy. It is observed that thermal anisotropy significantly affects the grain morphology at low grain density but has a smaller influence at high grain density as compared to other governing factors such as solute transport.

Grain size gradient naturally prepared through recrystallization in rapidly solidified undercooled alloy melts

A grain size gradient was naturally prepared through recrystallization in rapidly solidified Ni80Cu20 alloys quenched before recalescence. The fine grains on the surface of quenched specimens were partially recrystallized. However, the coarse grains in the deeper layers of the quenched alloys were fully recrystallized. It is proposed that the grain size gradient is due to recrystallization in the microstructures of the quenched alloys. Annealing the microstructure on the surface of the quenched specimen led to crystal growth via recrystallization. These experimental results provided strong evidence for a grain refinement mechanism in undercooled solidification of alloys.

Remelting of Primary Solid in Rapid Solidification of Deeply Undercooled Alloy Melts

Remelting of Primary Solid in Rapid Solidification of Deeply Undercooled Alloy Melts

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

ABSTRACTRapid 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.

Nonequilibrium Solidification, Grain Refinements, and Recrystallization of Deeply Undercooled Ni-20 At. Pct Cu Alloys: Effects of Remelting and Stress

Grain refinement phenomena during the microstructural evolution upon nonequilibrium solidification of deeply undercooled Ni-20 at. pct Cu melts were systematically investigated. The dendrite growth in the bulk undercooled melts was captured by a high-speed camera. The first kind of grain refinement occurring in the low undercooling regimes was explained by a current grain refinement model. Besides, for the dendrite melting mechanism, the stress originating from the solidification contraction and thermal strain in the FMZ during rapid solidification could be a main mechanism causing the second kind of grain refinement above the critical undercooling. This internal stress led to the distortion and breakup of the primary dendrites and was semiquantitatively described by a corrected stress accumulation model. It was found that the stress-induced recrystallization could make the primary microstructures refine substantially after recalescence. A new method, i.e., rapidly quenching the deeply undercooled alloy melts before recalescence, was developed in the present work to produce crystalline alloys, which were still in the cold-worked state and, thus, had the driven force for recrystallization.

Bent dendrite growth in undercooled Fe-B alloy melts

Dendritic growth is the main solidification mode in alloy casting. In order to control dendrite growth for materials design from the melt it is important to fully understand the influence of process conditions. This study stands as an experimental note observing bent dendrite growth in Fe-B alloys and suggesting possible explanations as induced by fluid flow, thermal, and concentrational diffusion or impurities. Electromagnetic levitation technique (EML) is used for containerless processing of undercooled melts under 1g and reduced gravity conditions in parabolic flight. Further investigations are needed to find a suitable explanation for the observed bent dendrite growth behaviour.

Dendrite Growth Kinetics in Undercooled Melts of Intermetallic Compounds

Solidification needs an undercooling to drive the solidification front. If large undercoolings are achieved, metastable solid materials are solidified from the undercooled melt. Containerless processing provides the conditions to achieve large undercoolings since heterogeneous nucleation on container walls is completely avoided. In the present contribution both electromagnetic and electrostatic levitation are applied. The velocity of rapidly advancing dendrites is measured as a function of undercooling by a High-Speed-Camera. The dendrite growth dynamics is investigated in undercooled melts of intermetallic compounds. The Al50Ni50 alloy is studied with respect to disorder trapping that leads to a disordered superlattice structure if the melt is undercooled beyond a critical undercooling. Disorder trapping is evidenced by in situ energy dispersive diffraction using synchrotron radiation of high intensity to record full diffraction pattern on levitated samples within a short time interval. Experiments on Ni2B using different processing techniques of varying the level of convection reveal convection-induced faceting of rapidly growing dendrites. Eventually, the growth velocity is measured in an undercooled melt of glass forming Cu50Zr50 alloy. A maximum in the growth velocity–undercooling relation is proved. This is understood by the fact that the temperature dependent diffusion coefficient counteracts the thermodynamic driving force for rapid growth if the temperature of the undercooled melt is approaching the temperature regime above the glass transition temperature. The analysis of this result allows for determining the activation energy of atomic attachment kinetics at the solid–liquid interface that is comparable to the activation energy of atomic diffusion as determined by independent measurements of the atomic diffusion in undercooled Cu50Zr50 alloy melt.

Remelting-induced anomalous eutectic formation during solidification of deeply undercooled eutectic alloy melts

Anomalous eutectics are argued to form due to the remelting of the primary solid during solidification of deeply undercooled eutectic alloy melts. As an indicator of the tendency for anomalous eutectic formation, the remelted fraction of the primary eutectic was analyzed systematically based on the eutectic dendrite growth theory. For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes, the primary eutectic is highly supersaturated with solute and more prone to remelting. When the eutectic composition is set to different values, (e.g. the eutectic point is closer to one phase of the eutectic), the two eutectic phases under rapid growth change their compositions simultaneously, but their remelted fractions during temperature recalescence do not vary significantly. Three representative binary eutectic alloys Ag–39.9at.%Cu, Ni–19.6at.%P and Pd–16.0at.%P – their eutectic products are solid solution–solid solution, solid solution–stoichiometric intermetallic compound and stoichiometric intermetallic compound–stoichiometric intermetallic compound, respectively, were solidified at large undercooling. Anomalous eutectics were observed in the first two eutectic alloys whose eutectic structure consists of at least one solid solution phase, whereby the stoichiometric intermetallic compound phase remained highly oriented whereas the solid solution phase had a near random distribution of orientations. For the Pd–16.0at.%P alloy, however, the primary eutectic retained its original morphology as the final solidification structure regardless of the degree of undercooling. All these experimental results validate the argument that anomalous eutectics result from the remelting of the primary solid.

Measurement of Growth Kinetics of Deeply Undercooled Zr<sub>50</sub>Cu<sub>50</sub> Melt by Electromagnetic Levitation Technique

The crystal growth kinetics in the deeply undercooled Zr50Cu50 melt was studied with the electromagnetic levitation technique. The maximum growth rate umax was, for the first time, demonstrated in undercooled metal and alloy melts, and this allowed us to construct the complete profile of crystal growth kinetics in the whole undercooling region of Zr50Cu50 melt. In deep undercooling region near glass transition, the diffusion-controlled crystal growth mechanism accounted for the growth kinetics, while at lower undercooling the profile around umax remarkably differed from what the classical growth theory covers.

Peri‐eutectic transition during crystallization of undercooled ternary alloy melt

AbstractPeri‐eutectic transition was investigated in terms of the dependence of crystallization behavior on undercooling in Cu–Ag–Ge ternary alloy composed of (Ag), ζ(Cu5Ge), and ε2(Cu3Ge) phases. (Ag + ε2) pseudobinary eutectic is the product of both peri‐eutectic and subsequent pseudobinary eutectic transitions. Nanoscale structure of anomalous (Ag + ε2) pseudobinary eutectic forms at the maximum undercooling, which is ascribed to the independent nucleation and growth from the liquid. The decrease of (Ag + ε2) pseudobinary eutectic at a high undercooling is due to that the phase transition happening at the beginning solidification stage proceeds more thoroughly at a higher undercooling.

Parameter determination of critical nucleation frequency in solidification of undercooled metallic melts

A simple method was proposed to calculate the essential parameters correlated with the critical nucleation frequency of undercooled metals and alloy melt. Numerical results show that the calculation accuracy from this method can be improved using the experimental data either with high undercooling or with low undercooling range (the difference of undercoolings between two solidification events). The calculations of the interfacial energy for high undercooling of silver and of the catalytic factor f(θ) for high undercooling of Al, Cu and Al–30 wt-%Cu alloy indicate that the results are consistent with the experimental measurements and with the results of Jian’s model [Metall. Trans. A, 2001, 32A, 391–395]. In addition, by analysing the differential scanning calorimetry data of pure Sn subjected to different cooling rates, similar values of catalytic factor f(θ) are obtained. This further indicates the validity of the current method.

The effect of far-field uniform flow on the particle growth in an undercooled alloy melt

The present paper studies the mechanism of interface formation in the particle growth in an undercooled alloy melt with a far-field uniform flow. Using the method of matched asymptotic expansion, we obtain an approximate solution for particle growth and determine the pattern formation of crystal–melt interface. The analytical results show that the far-field uniform flow has significant effect on particle growth and the mechanism of the pattern formation of crystal–melt interface is similar to that of the convective pure melt. The drag effect of the far-field uniform flow results in the higher local growth rate in the upstream front, where the flow is incoming and the lower local growth rate in the downstream rear, where the flow is outgoing. Compared with the situation in the pure melt, the solute in the undercooled alloy melt further facilitates the growth of the particle. The critical radius for instability decreases as the far-field uniform flow increases.

Grain refinement in highly undercooled solidification of Ni85Cu15 alloy melt: Direct evidence for recrystallization mechanism

The grain refinement occurring upon rapid solidification of undercooled Ni85Cu15 alloy melts has been studied. Applying theoretical calculations for stress accumulation in dendritic skeletons, the grain refinement occurring with ΔT >∼180 K is ascribed to plastic deformation of dendrites and subsequent recrystallization. This has been evidenced directly using high-resolution transmission electron microscopy observation of the as-solidified granular crystals.

Solidification of undercooled eutectic alloys containing a third element

A theoretical model for lamellar eutectic growth in undercooled eutectic alloy melts with a third element has been developed; it was assumed that no other phase forms. It is found that the tip radius of eutectic dendrite decreases with increasing content of the third element so as to diffuse away its enriched atoms ahead of the solidifying interface easily. A critical content exists below which the eutectic growth is accelerated within a certain undercooling range due to the third element addition, but slowed down otherwise. When more of the third element is contained, the growth velocity is always reduced. However, the eutectic lamellar spacing varies with the content in an opposite way. The influence of the third element addition is enhanced when the difference in distribution coefficients of the third element in two eutectic phases becomes greater.

The steady-state solution of dendritic growth from the undercooled binary alloy melt with the far field flow

The steady-state dendritic growth from the undercooled binary alloy melt with the far field flow is considered. By neglecting the interface energy, interface kinetics and buoyancy effects in the system, we obtaine the steady-state solution for the case of the large Schmidt number, in terms of the multiple variable expansion method. The changes of the temperature and concentration fields, the morphology of the interface, the normalization parameter and the Peclet number of the system induced by uniform external flow are derived. The results show that, compared with the system of dendritic growth from undercooled pure melt, the convective flow in the system of growth from undercooled binary alloy has stronger effects on the morphology of the interface. Nevertheless, the shape of the interface still remains nearly a paraboloid.

Energy model for the Zr-based metallic glass alloy melt with clusters

An energy model for the melt of bulk metallic glass (BMG) with clusters was established, the Gibbs free energy and interfacial energy for the Zr-Al-Ni ternary alloy melt with Zr2Ni clusters were calculated, and the effects of the clusters on the Gibbs free energy, interfacial energy and nucleation rate were analyzed. The results showed that the existence of the clusters in the Zr-Al-Ni ternary alloy melt enables the Gibbs free energy to decrease in the composition range where bulk metallic glass forms easily, makes the interfacial energy increase and changes the distribution of the interfacial energy with the alloy composition. Because of the clusters in the melt, the Gibbs free energy of the Zr66Al8Ni26 alloy melt decreases about 0.3–1 kJ/mol and the interfacial energy between the melt and crystal nucleus increases about 0.016 J/m2. The nucleation rate of the undercooled Zr66Al8Ni26 alloy melt decreases evidently under the influence of the clusters on Gibbs free energy and the interfacial energy, and the maximum of the nucleation rate in the melt with the Zr2Ni clusters is only about 107 mol−1·s−1.