Papers by Keyword: Undercooling

Abstract: 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.

Abstract: This paper investigated the effect of trace addition of Al and Mg on the grain refinements of Cu₆Sn₅ in Sn-3wt%Ag-5wt%Cu high temperature solder alloys. Furthermore, the effect of Al and Mg addition on the Sn/Ag₃Sn eutectic were also investigated. It was found that the addition of both Al and Mg successfully refined the Cu₆Sn₅ in Sn-3wt%Ag-5wt%Cu solder alloy. In addition, Al suppresses the formation of Ag₃Sn in the Sn/Ag₃Sn eutectic; while Mg promotes the formation of fine Sn/Ag₃Sn eutectic microstructure. The refinement of Cu₆Sn₅ is believed to be due to heterogeneous nucleation by Al and Mg rich intermetallic particles respectively. Effect of Al and Mg addition on the undercooling of the Sn/Ag₃Sn eutectic was found to be similar, both reducing undercooling effectively at a low addition rate of 0.025wt%. The addition of Al and Mg have mixed effect on the nucleation temperature of Cu₆Sn₅. It is found that the nucleation temperature of Cu₆Sn₅ is increased with 0.025wt% Al and 0.1wt% Mg addition to the unmodified alloy, while the nucleation temperature slowly decreases again as the trace element addition rate increases.

Abstract: The effect of Al-5Ti-1B grain refiner addition on solidification microstructure, especially the formation of the binary eutectic (Al)+Al₁₅(Fe,Mn)₃Si₂ was investigated in Al-5Mg-2Si-0.7Mn-1.1Fe (wt.%) alloy. The experimental results showed that the solidification microstructure were consisted of primary α-Al phase, inter dendritic binary eutectic (Al)+Al₁₅(Fe,Mn)₃Si₂, and ternary eutectic (Al+Mg₂Si+Al₁₅(Fe,Mn)₃Si₂). The heterogeneous nucleation of the primary (Al) phase in the alloys was greatly enhanced by the Al-5Ti-1B grain refiner, refining the (Al) grains significantly. The subsequent binary eutectic was nucleated on these refined (Al) phase. Consequently, the size of Al₁₅(Fe,Mn)₃Si₂ phase with a Chinese script morphology in the binary eutectic was greatly refined, without much change in the secondary branch spacing.

Abstract: In order to eliminating the banded structure in SAE8620H gear steel,an isothermal annealing process and its mechanism has been studied in this paper. Results showed that isothermal annealing process can effectively eliminate banded structure in SAE8620H gear steel.With isothermal temperature 640°C-a certain low temperature indeed high undercooling,and for high nucleation at this temperature resulting in the decrease of nucleation rate difference in all areas,banded structure level decreased to 1 level.When isothermal temperature decreased to 610°C and 590°C,although the undercooling is so high that various regions began to nucleate at the same time,the transition temperature is so low that alloy elements in the regions previously occupied by pearlite bands diffuse faster than other regions,thus proeutectoid ferrite grains in these regions grow faster,regenerating the banded structure .

Abstract: Solidification behaviors of hypoeutectic Ni-3.3wt% B and hypereutectic Ni-4.5wt% B alloys were investigated by using differential scanning calorimetry (DSC) technology. Experimental results showed that the solidification behaviors are similar for the two kinds of alloys with small sizes, i.e. three transformed peaks (corresponding to L→α-Ni, L→Ni-Ni₂₃B₆ and Ni₂₃B₆→Ni+Ni₃B, respectively) appear in the cooling curves, and three phases (primary phase α-Ni, rod eutectic and dot precipitates) appear in the microstructures, which are different from that of the alloys with large sizes. It can be attributed to the fact that the nucleation of α-Ni and Ni₂₃B₆ phases are easier than that of Ni₃B phase for small samples of hypoeutectic and hypereutectic Ni-Ni₃B alloys.

Abstract: The undercooled Fe₅₀Cu₅₀ alloy experiences a metastable liquid phase separation and separates into a Fe-rich zone and a Cu-rich zone within the gravity field. The growth characteristics of the Cu-rich zone were investigated by the glass fluxing method, and the achieved undercooling range was 20−261 K. The volume fraction of the Cu-rich zone decreases with the enhancement of the bulk undercooling. The microstructural morphologies of the Cu-rich zone are similar at all the undercooling conditions, that is, αFe dendrites and particles are distributed inside (Cu) phase matrix. The secondary dendritic arm spacing of αFe dendrites decreases with the increase in bulk undercooling. The growth mechanism of αFe dendrites was analyzed by using the LKT/BCT dendritic growth theory. The dendritic growth in the Cu-rich zone is mainly controlled by solute diffusion so that the dendritic growth velocity is only several millimeters per second. Besides, the calculated results indicate that there is only inconspicuous solute trapping during the solidification of Cu-rich zone.

Abstract: Liquid phase separation occurred in undercooled Fe-Cu alloys when the undercooling of the melt exceeds a critical value. 5at.% of Si was added into Cu50Fe50 alloy to investigate its effect on the liquid phase separation of Fe-Cu alloys in this paper. It indicated that the addition of Si could enhance the liquid-phase separation in Cu50Fe50 alloy. Additionally, with the increase of undercooling, second liquid-separation occurred in Fe-rich liquids, and many globular, tadpole and peanut-like morphology emerged in the Fe-rich region of Fe45Cu50Si5 alloy. Because of the mixing enthalpy of Fe-Si atomic pairs is more negative than that of Cu-Si atomic pairs; so Si is inclined to rich in Fe-rich region.

Abstract: Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ (Ni₃₁Si₁₂) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β₁-Ni₃Si and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-Ni₃Si would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase Ni₂₅Si₉ and β₁-Ni₃Si is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β₂/β₃) being suppressed from the melt.

Abstract: A melt encasement (fluxing) method has been used to undercool Ag-Cu alloy at its eutectic composition. The recalescence of the undercooled alloy has been filmed at high frame rate. At low undercooling lamellar eutectic is observed to grow, giving way to a mixed anomalous-lamellar structure at higher undercooling. In situ observation of the spot brightness reveals, as expected, that the lamellar eutectic grows via a planar front mechanism, while the anomalous eutectic grows via a more complex process characterised by a double recalescence. The first recalescence is non-space-filling (dendritic) in character and is followed shortly afterwards by a second recalescence which appears to be of the planar front type. Moreover, the first recalescence event appears to be to a temperature in excess of the equilibrium eutectic temperature. This is strongly suggestive that the anomalous eutectic morphology arises due to the growth and subsequent partial remelting of a dendritic morphology, probably a two-phase (eutectic) dendrite, followed by planar front growth of a lamellar eutectic into the residual liquid.

Abstract: An undercooled melt possesses an enhanced free enthalpy that enables to crystallize metastable solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Heterogeneous nucleation on container-walls is completely avoided leading to large undercooling with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Rapid dendrite growth is measured as a function of undercooling by a high-speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts.