W.L. Wang

Liquid phase separation and rapid dendritic growth of highly undercooled ternary Fe62.5Cu27.5Sn10 alloy

The phase separation and dendritic growth characteristics of undercooled liquid Fe62.5Cu27.5Sn10 alloy have been investigated by glass fluxing and drop tube techniques. Three critical bulk undercoolings of microstructure evolution are experimentally determined as 7, 65, and 142 K. Equilibrium peritectic solidification proceeds in the small undercooling regime below 7 K. Metastable liquid phase separation takes place if bulk undercooling increases above 65 K. Remarkable macroscopic phase separation is induced providing that bulk undercooling overtakes the third threshold of 142 K. With the continuous increase of bulk undercooling, the solidified microstructure initially appears as well-branched dendrites, then displays microscale segregation morphology, and finally evolves into macrosegregation patterns. If alloy undercooling is smaller than 142 K, the dendritic growth velocity of γFe phase varies with undercooling according to a power function relationship. Once bulk undercooling exceeds 142 K, its dendri...

Thermodynamic properties and solidification kinetics of intermetallic Ni7Zr2 alloy investigated by electrostatic levitation technique and theoretical calculations

The thermodynamic properties, including the density, volume expansion coefficient, ratio of specific heat to emissivity of intermetallic Ni7Zr2 alloy, have been measured using the non-contact electrostatic levitation technique. These properties vary linearly with temperature at solid and liquid states, even down to the obtained maximum undercooling of 317 K. The enthalpy, glass transition, diffusion coefficient, shear viscosity, and surface tension were obtained by using molecular dynamics simulations. Ni7Zr2 has a relatively poor glass forming ability, and the glass transition temperature is determined as 1026 K. The inter-diffusivity of Ni7Zr2 alloy fitted by Vogel–Fulcher–Tammann law yields a fragility parameter of 8.49, which indicates the fragile nature of this alloy. Due to the competition of increased thermodynamic driving force and decreased atomic diffusion, the dendrite growth velocity of Ni7Zr2 compound exhibits double-exponential relationship to the undercooling. The maximum growth velocity is...

Macroscopic phase separation and primary FeSi compound growth within undercooled ternary Fe–Sn–Si monotectic alloy

Liquid ternary Fe37Sn32Si31 monotectic alloy in bulk form has been undercooled by 305 K (0.18T L) by the glass fluxing method. Macroscopic phase separation took place and led to the formation of a top Fe-rich zone and a bottom Sn-rich zone. The large degree of undercooling was found to facilitate the evolution of macrosegregation. Dendritic growth was the characteristic of the primary FeSi compound in an Fe-rich zone. The growth velocity was up to 0.42 m/s and the dendrite grains were efficiently refined by an increase in the level of undercooling. The solute trapping of Sn atoms could not occur in the FeSi compound within the undercooling range achieved here, resulting in the suppression of the dendritic growth velocity compared with that in binary Fe50Si50 alloy. The interdendritic microstructures mainly consisted of decomposed Fe5Si3 compound plus some α-Fe phase.

Metastable coupled-growth kinetics between primary and peritectic phases of undercooled hypoperitectic Fe54.5Ti45.5 alloy

The metastable coupled-growth kinetics between the primary Fe2Ti and peritectic FeTi phases of undercooled Fe54.5Ti45.5 alloy was systematically investigated by both electromagnetic levitation and drop tube techniques. Employing a high-speed camera, the rapid crystallization processes of levitated bulk alloy were recorded in the undercooling range of 34–187 K. In small undercooling regime below 143 K, peritectic solidification proceeded and the dependence of primary Fe2Ti dendritic growth velocity V on the bulk undercooling ΔT satisfied a power relation of V = 2.43 × 10−14 × ΔT7.72 (mm s−1). Once liquid undercooling increased beyond 143 K, the metastable coupled-growth was induced and the microstructure was characterized by the Fe2Ti rods embedded in FeTi phase. Furthermore, the coupled-growth velocity decreased linearly with the rise in undercooling according to V = 1.47 × 103-7.44ΔT (mm s−1). In drop tube experiment, peritectic solidification characteristics of small alloy droplets disappeared and the p...

Liquid phase separation and subsequent dendritic solidification of ternary Fe35Cu35Si30 alloy

Abstract Liquid Fe35Cu35Si30 alloy has achieved the maximum undercooling of 328 K (0.24TL) with glass fluxing method, and it displayed triple solidification mechanisms. A critical undercooling of 24 K was determined for metastable liquid phase separation. At lower undercoolings, α-Fe phase was the primary phase and the solidification microstructure appeared as homogeneous well-defined dendrites. When the undercooling exceeded 24 K, the sample segregated into Fe-rich and Cu-rich zones. In the Fe-rich zone, FeSi intermetallic compound was the primary phase within the undercooling regime below 230 K, while Fe5Si3 intermetallic compound replaced FeSi phase as the primary phase at larger undercoolings. The growth velocity of FeSi phase increased whereas that of Fe5Si3 phase decreased with increasing undercooling. For the Cu-rich zone, FeSi intermetallic compound was always the primary phase. Energy-dispersive spectrometry analyses showed that the average compositions of separated zones have deviated substantially from the original alloy composition.

A Videographic Study of Dynamic Phase Separation for Immiscible Solutions under Acoustic Levitation Condition

The transparent aqueous solutions of succinonitrile (SCN) provide an effective model system to simulate the phase separation process of various advanced materials. Here we report a real-time and in-situ study of phase separation dynamics for the SCN-15%H2O, SCN-48%H2O and SCN-70%H2O solutions implemented by high-speed CCD videography together with acoustic levitation technique. It is found that liquid phase separation induces an unsteady state of drop rotation under levitated conditions. The resultant centrifugal force plays the dominant role in the migration of secondary liquid globules. The most desirable homogeneously dispersive structures can only be derived from the earlier stage of phase separation, whereas three kinds of macrosegregation are always the finally stable structure patterns. The migration velocity of minor liquid phase displays the nonlinear feature owing to the variations of globule location and centrifugal force. The surface tensions and volume fractions of immiscible phases also show a conspicuous influence upon the evolution dynamics of separation morphology.

Rapid Solidification Processing of Ag-5%Bi Alloy under Microgravity Condition inside Drop Tube

Liquid Ag-5%Bi alloy has been rapidly solidified in the form of 84-1140 μm droplets under free fall condition. The results show that there are dendritic (Ag)1 phase, interdendritic (Ag)2 phase and (Bi) solid solution phase in this alloy respectively. The microstructure of (Ag)1 phase displays a conspicuous “dendritic-equiaxed” morphology transition with the decrease of droplet diameter, which is attributed to the fragmentation of dendrites induced by the recalescence of highly undercooled liquid alloy. The theoretical analysis with the LKT/BCT dendritic growth model predicts that the growth velocity of (Ag) dendrite increases with undercooling, and the kinetics transition from solute diffusion-controlled growth to thermal diffusion-controlled growth occurs at 98 K during solidification. Meanwhile, the remarkable solute trapping effect takes place according to theoretical calculated calculation.

An Experimental Study of Thermophysical Properties for Quinary High-Entropy NiFeCoCrCu/Al Alloys*

Two quinary high-entropy alloys (HEAs) with equiatomic concentrations formed by doping either Cu or Al elements into the quaternary NiFeCoCr alloy are produced by arc melting and spray casting techniques. Their entropy of fusion, thermal expansion coefficient and thermal diffusivity are experimentally investigated with differential scanning calorimetry, dilatometry and laser flash methods. The NiFeCoCrCu HEAs contain a face-centered cubic high-entropy phase plus a minor interdendritic (Cu) phase and display a lower entropy of fusion and the Vickers hardness. The NiFeCoCrAl HEAs consist of two body-centered cubic high-entropy phases with coarse dendritic structures and show higher entropy of fusion and the Vickers hardness. Both the thermal expansion coefficient and the thermal diffusivity of the former Cu-doped alloy are significantly larger than those of the latter Al-doped alloy. Although the temperature dependence of thermal diffusivity is similar for both HEAs, it is peculiar that the thermal expansion curve of the NiFeCoCrAl alloy exhibits an inflexion at temperatures of 860–912 K.

Liquid State Undercoolability and Crystal Growth Kinetics of Ternary Ni-Cu-Sn Alloys

The liquid state undercoolability and crystal growth kinetics of ternary Ni-5%Cu-5%Sn and Ni-10%Cu-10%Sn alloys are investigated by the glass fluxing method. In these two alloys, experimental maximum undercoolings of 304K (0.18TL) and 286 K (0.17TL) are achieved and the dendritic growth velocities attain 39.8 and 25.1 m/s, respectively. The transition of morphology from coarse dendrite into equiaxed structure occurs and the grain size of the α (Ni) phase decreases remarkably when the undercooling increases. Both the lattice constant and microhardness increase obviously with the enhancement of undercooling. The enrichment of Cu and Sn solute contents reduces the dendritic growth velocity, while enhances the lattice constant and microhardness of α (Ni) phase.