Undercooled Alloys Research Articles

Microstructure Study of Undercooled Hypereutectic Cu<sub>95</sub>Cr<sub>5</sub> Alloys

Microstructure of Cr-rich phase in undercooled hypereutectic Cu95Cr5 alloys was studied by using arc melting, electromagntic levitation melting and splat quenching. The results showed that the alloys generally have a microstructure consisting of a fine Cr-rich dendrites in a Cu-rich matrix. Even there existed strong electromagnetic stirring during solidification process, the nonhomogeneous Cr-rich dendrites were observed in electromagnetic levitation melted alloys. However, fine homogenous primary Cr-rich particles or equiaxed Cr-rich grains distributed in the Cu-rich matrix in splat quenched alloys.

Modeling grain refinement for undercooled single-phase solid-solution alloys

A grain-refinement model for rapid solidification of undercooled single-phase solid-solution alloys was developed in which a combination of the model of dendrite fragmentation with the model of overall solidification kinetics allows dendrite fragmentation to occur along with the solidification process (e.g. during mushy zone solidification). Compared with Karma’s model, the dendrite break-up time as well as the time for the interdendritic liquid to be solidified completely can be predicted. Thus, the model can predict, in the absence of experimental data (e.g. the plateau duration), the microstructure transition. The model was applied to describe the rapid solidification of undercooled Ni–15 at.% Cu alloy, and a good agreement between the model predictions and the experimental results was obtained. On this basis, the effects of capillarity and back-diffusion on dendrite fragmentation during mushy zone solidification, as well as the effects of back-diffusion and cooling rate on the prediction of the microstructure transition, were found.

Metastable Phase in Undercooled Fe-Co Alloy

Metastable phase was observed in as-solidified microstructure of undercooled Fe-Co alloy, provided that the initial undercooling (T) of the melt exceeds the critical value. On this basis, the forming mechanism and stability of metastable phase were investigated. The forming mechanism of metastable phase in undercooled Fe-Co alloy was concluded as cooperative effect of “competitive nucleation—remelting—extensive growth—incomplete solid-state transformation” in this work. With the increase of annealing time, the number and dimension of metastable phase decreased at the same time. Moreover, metastable phase was transformed to stable phase completely when annealing time increased to three hours.

Thermophysical Properties of Undercooled Alloys: An Overview of the Molecular Simulation Approaches

We review the studies on the thermophysical properties of undercooled metals and alloys by molecular simulations in recent years. The simulation methods of melting temperature, enthalpy, specific heat, surface tension, diffusion coefficient and viscosity are introduced and the simulated results are summarized. By comparing the experimental results and various theoretical models, the temperature and the composition dependences of the thermophysical properties in undercooled regime are discussed.

Phase-field simulation of Non-Isothermal dendritic growth of NiCu alloy

Phase-field model is used and The dendrite growth of a Ni-40.83%Cu binaryalloy under were simulated by the model coupled with solute field and temperature field.The effects of solidification latent heat on the growth of equiaxed dendrite, distribution of solute field and temperature field in undercooled liquid alloy were analyzed.The results indicate that the dendritic has well-developed secondary arms as undercooling degree increases.Correspondently, the solute Peclet number and the tip speed increases, the tip radius decreases, and the solute segregation in solid-liquid interface increases. The results agree well with Ivantsov theory.

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.

Effect of high undercooling on Co–Ni–Ga ferromagnetic shape memory alloys

Co 45Ni 25Ga 30 ferromagnetic shape memory alloys were treated by glass fluxing combined with superheating cycling. The effect of high undercooling on solidified microstructure and transformation temperatures was investigated. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery. During the following annealing process, these sub-grains can gradually recrystallize and grow up with an amount of γ′ phase precipitated. Meanwhile, transformation temperatures of undercooled alloys are greatly elevated compared with those of as cast, which results from the large internal stress, and gradually reduce with the heating time.

Formation mechanism of the primary faceted phase and complex eutectic structure within an undercooled Ag–Cu–Ge alloy

The solidified microstructure of bulk undercooled Ag40Cu30Ge30 alloy consists of three parts: primary (Ge) phase, the complex structure of (Ag + Ge) and (Ag + e 2) pseudobinary eutectics, and (Ag + Ge + e 2) ternary eutectic. In comparison, the pseudobinary eutectic no longer appears in an alloy droplet solidified in a drop tube. Once the undercooling exceeds 225 K and the cooling rate is greater than 2×103 K s−1, the microstructure of the solidified droplet is totally composed of anomalous ternary eutectic. In both cases, the primary (Ge) phase exhibits various faceted growth morphologies at different undercoolings, such as columnar block, long dendrite, equiaxed dendrite and rod-like crystal. Some refined side branches grow from the equiaxed (Ge) dendritic branches composed of {111} twins, which is ascribed to the rapid epitaxial growth of (Ag + Ge) pseudobinary eutectic from the (Ge) dendritic branches. Moreover, both the primary (Ge) phase and the (Ge) phase in the (Ag + Ge) pseudobinary eutectic are effective heterogeneous nuclei for the (Ag+e 2) pseudobinary eutectic. As undercooling increases, the (Ge) phase in the (Ag + Ge+e 2) ternary eutectic transforms from faceted to non-faceted phase, while the independent nucleation and growth of the (Ag) and e 2 phases in the ternary eutectic displaces their previous cooperative growth. These growth kinetics transitions result in the formation of anomalous ternary eutectic.

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.

Microstructure and Phase Selection of Undercooled Single-Phase Fe-Co Alloy

Fe-Co single-phase alloy melts with different Co contents were undercooled using fluxing method. The maximum undercooling DT = 457K (relative undercooling DT/Tm=0.259) was achieved in this work. At low undercooling (DT), single-phased microstructure was observed, but metastable bcc phase emerged in the as-solidified microstructure once DT exceeded a critical value, DTcrit. In the presence of classical nucleation theory, phase selection in the undercooled Fe-Co melt was investigated, and the theoretical calculation was coincided with the experimental result.

Stress induced deformation in the solidification of undercooled Co80Pd20 alloys

Substantial undercooling ΔT up to 415K was achieved for Co80Pd20 melt applying molten glass denucleation combined with cyclic superheating. The as-solidified structure as function of ΔT was described concisely. On this basis, dense-regular fault (DRF) ribbons were detected provided if a critical undercooling is surpassed. An integrated analysis of the DRF formation with a model calculation [16] for shrinkage-stress developed in the coherent dendrite network upon rapid solidification was then performed. This confirms that the as-formed DRF originates from a stress-induced deformation, which also plays an important role in understanding the grain refinement occurring upon rapid solidification of undercooled melts.

Thermodynamic analysis of the Al–Cu–Zr bulk metallic glass system

As an essential constitution of either Cu- or Zr-based bulk metallic glass alloys, the Al–Cu–Zr ternary system has aroused a great attention for decades. Since the three constituent binary systems, Al–Cu, Al–Zr and Cu–Zr, have been optimized previously, the focus of this work was only given to the Al–Cu–Zr ternary system. Based on the experimental enthalpies of mixing of ternary liquid and undercooled liquid alloys as well as the evaluated isothermal sections, the Al–Cu–Zr ternary system has been assessed using the CALPHAD method. Most of the calculated results show good agreement with the experimental thermodynamic data and the reported phase diagrams. By employing the driving force criterion with the present thermodynamic description, the observed glass-forming abilities in the Al–Cu–Zr system can be accounted for satisfactorily.

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. II. Model application

An overall solidification kinetic model was applied to undercooled Ni–15 at.% Cu alloy. A good agreement between the model predictions and the measured cooling curves was obtained by adopting a “phenomenological” heat boundary condition. Applying numerical calculations, it was demonstrated that solute is uniformly distributed at the purely thermal-controlled growth stage; a transition from non-equilibrium to near-equilibrium solidification occurs at the mainly thermal-controlled growth stage only if sufficiently high initial undercooling is available; and a transition from recalescence to post-recalescence occurs at the solutal-controlled growth stage wherein the current model reduces to Scheil’s equation. The volume fraction solidified during recalescence is the same as the ratio of the initial undercooling to the hypercooling limit, and solidification should end generally at a temperature between the prediction of Scheil’s equation and that of the Lever rule if back diffusion is considered.

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. I. Model derivation

Departing from the volume-averaging method, the equiaxed solidification model was extended to describe the overall solidification kinetics of undercooled single-phase solid-solution alloys. In this model, a single grain, whose size is given assuming site saturation, is divided into three phases, i.e. the solid dendrite, the inter-dendritic liquid and the extra-dendritic liquid. The non-equilibrium solute diffusion in the inter-dendritic liquid and the extra-dendritic liquid, as well as the heat diffusion in the extra-dendritic liquid, is considered. The growth kinetics of the solid/liquid interface is given by the solute or heat balance, where a maximal growth velocity criterion is applied to determine the transition from thermal-controlled growth to solutal-controlled growth. A dendrite growth model, in which the nonlinear liquidus and solidus, the non-equilibrium interface kinetics, and the non-equilibrium solute diffusion in liquid are considered, is applied to describe the growth kinetics of the grain envelope. On this basis, the solidification path is described.

Mechanical properties of an undercooled aluminium alloy Al-0.6Mg-0.7Si

Compression tests of an undercooled aluminium alloy Al-0.6Mg-0.7Si have been performed in a quenching and deformation dilatometer. Samples have been solution annealed and quenched in the dilatometer with varying quenching rates (0.1 K/min to 1000 K/min) and varying quenching temperatures. Immediately after quenching, compression tests on quenching temperature have been performed in the dilatometer. The results have been correlated with the precipitation behaviour of the undercooled aluminium alloy Al-0.6Mg-0.7Si. Stress-strain-curves of quenching rates higher than the critical cooling rate for precipitation differ from those of quenching rates lower than the critical cooling rate. Further, stress-strain-curves of temperatures above and during precipitation differ from those of temperatures below precipitation.

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.

Description of Recalescence in Bulk Undercooled Alloys

Assuming phenomenological thermal flow balance and solute conservation, a numerical model was proposed for recalescence in bulk undercooled alloy. During the calculation, the evolution of residual melt temperature and concentration is solved as function of the transformed solid fraction. After quantitative thermodynamic calculations, a transition from non-equilibrium to equilibrium process can be described by the change of liquid/solid Gibbs energy difference. The proposed model has been applied to Cu-Ni solid solution and Fe-B hypereutectic alloy. The predicted post-recalescence temperature TR agrees well with the experimental results.

Macroscopic liquid phase separation of Fe–Sn immiscible alloy investigated by both experiment and simulation

The macroscopic phase separation of undercooled liquid Fe50Sn50 immiscible alloy was investigated by both drop tube experiment and the modified Model H. Core-shell structures with two, three, and five layers were formed during experiment. The concentric shells comprised of Fe-rich phase and Sn-rich phase distribute periodically in the solidified droplet, and the Sn-rich phase is always located at the sample surface owing to its smaller surface tension. The simulated results of modified Model H reproduce core-shell structures with the same layers, which justifies the prediction of this model. If the fluidity parameter Cf and solute concentration ϕ satisfy the physical conditions of both Cf<1 and 0<ϕ<0.45 or Cf≥1, the phase separation morphology exhibits the double-layer core-shell structure, whereas the multilayer core-shell structure is expected to form under the conditions of both Cf<1 and 0.45≤ϕ<0.5. The forming ability of core-shell structure for various immiscible alloys can be predicted accordingly.

Sluggish dendrite growth in substantially undercooled liquid Fe–Sb alloy

Dendrite growth within substantially undercooled liquid alloys is usually characterised by a very high growth velocity, even over 100 m/s. Here we report that dendrite growth becomes quite sluggish in a highly undercooled liquid Fe–8.5wt.%Sb alloy because it has a broad solidification temperature range of 412 K. The measured growth velocity of the dendritic α-Fe phase increases up to a maximum of only 1.38 m/s at 301 K undercooling. If this liquid alloy is undercooled further by 390 K (0.22T L), the dendrite growth velocity decreases gradually. A structural morphology transition from a coarse dendrite to an equiaxed grain takes place once undercooling exceeds 73 K. Energy dispersive spectroscopy analyses of the solute distribution reveal that solute trapping depends mainly on the actual dendrite growth velocity rather than the degree of undercooling.

Entropy as a selection rule for crystal growth in undercooled binary eutectic melts

A solution entropy model was developed for the undercooled binary eutectic alloy systems. As an extension of Taylor and Fidler et al.’s model, the present model considered the change of phase composition with the increase of undercooling. Furthermore, the sub-regular solution model and the interaction parameter (I AB ) were also introduced. In this paper, the extended model is used to calculate the solution entropy for binary eutectic phases under the undercooled condition, and the application scope of the model is also extended. Not only the growth manner of eutectic phases, but also the transition of morphologies may be predicted and explained by calculating the solution entropy of eutectic phases under the non-equilibrium condition with the developed model. Experimental results show that the developed model is valid for the undercooled Ni-Si and Ni-Sn eutectic alloy systems.