Investigation on the phase separation in undercooled Cu–Fe melts

doi:10.1016/j.jnoncrysol.2011.09.009

Journal of Non-Crystalline Solids

Volume 358, Issue 2, 15 January 2012, Pages 196-199

https://doi.org/10.1016/j.jnoncrysol.2011.09.009 Get rights and content

Abstract

On the basis of the gravity difference and the temperature dependence of the interfacial energy of the separated phases, the amalgamation of the droplets of minor phase was affected by Stokes sedimentation and Marangoni motion during the liquid-phase separation. Moreover, the velocity of Stokes motion increased distinctly with the undercooling, while the velocity of Marangoni motion was slightly impacted. At the beginning of the liquid-phase separation, the droplets of minor phase were dominated by Marangoni motion when the droplets radius was small. As the droplets radius increased by combination, the effect of Stokes sedimentation strengthened gradually. Additionally, the coagulation process was mainly controlled by Stokes sedimentation when a critical radius r_d was far exceeded.

Highlights

► Liquid-phase separation was influenced by the gravity difference and the interfacial energy of the separated phases. ► The Stokes motion velocity increased with the undercooling, but the Marangoni motion velocity was slightly affected. ► At the beginning of the liquid-phase separation, the droplets of minor phase were controlled by Marangoni motion. ► As the droplets radius increased, the effect of Stokes motion strengthened and play an dominate role finally.

Introduction

Since Nakagawa [1] detected the liquid phase separation in Cu–Co and Cu–Fe alloys by magnetic methods in 1958, the investigations on liquid immiscible alloys have been one of the focuses in materials science during the past few decades. These systems, possessing a nearly flat liquidus and a positive deviation from the Raoult's law, all exhibit a definite thermodynamics tendency for liquid immiscibility as the undercooling or cooling rates exceed a critical value. If a homogeneous liquid is undercooled into the metastable immiscibility gap, it will decompose into two liquids, Co or Fe-rich (L1) phase, and Cu-rich (L2) phase.

Recently, the phase separation of Co–Cu alloys was investigated using drop tube technique and an immiscible gap was obtained [2], [3], [4], [5], [6], [7]. Yamauchi et al. reported the effect of undercooling on the as-solidified microstructures of Co–Cu alloys, and found that the final structure depended on the cooling rate after liquid separation [8]. Moreover, the solidification behavior of Co–Cu alloys was investigated by means of containerless process in electromagnetic levitation facility [9], [10], and the microstructure and the solidification dynamics of the Co-rich phase were fully discussed.

As a high strength and high conductive material [11], [12], [13], Cu–Fe alloy has become one of the hotspots in material science. He et al. [14], [15], [16], [17] investigated the rapid solidification of Cu–Fe alloy by means of the high-pressure gas atomization technique. Adopting the cyclic superheating and DSC analysis, solidification behavior of undercooled Cu₇₀Fe₃₀ alloy was studied [18]. However, researches on the Cu-based alloys with a metastable miscibility gap have focused mainly on determining the immiscible gap boundaries. Especially, the immiscible process and the dynamics of phase separation are not yet fully understood. Adopting the bulk undercooling technique, the solidification process and the dynamics of the liquid-phase separation in undercooled Cu–Fe melt were discussed herein.

Section snippets

Experiment procedures

High purity of iron (99.99%) and copper (99.999%) were taken in the required composition. The alloy samples were undercooled by fluxing purification and cyclic superheating technique. The thermal history of sample was monitored by an infrared pyrometer with 5 K relative accuracy and 1 ms response time. Detailed experimental procedure was available elsewhere [19]. The specimens were cross-sectioned and polished (without etching); the microstructures of Cu–Fe alloys were documented by scanning

Results

When ΔT = 46 K, dendrites were observed in the as-solidified microstructure of Cu₅₀Fe₅₀ alloy, as shown in Fig. 1(a and b). The dark dendritic phase was Fe-riched, while the interdendritic phase was Cu-riched, as analyzed by line scanning (Fig. 1d). With the increase of undercooling, many Cu-rich agglomerations were observed among dendritic phase (Fig. 1b). The phase-separated structure was found when undercooling of the melt increased to 65 K, which was defined as the liquid-phase separation

Discussion

At low undercooling, liquid–solid transformation took place in the melt and no evidence of phase separation was observed in the final microstructure. It indicated that the melts solidified in a near-equilibrium mode. Dendrites of γ(Fe) nucleated and grew first in the melt, followed by the peritectic reaction L + γ(Fe) → ε(Cu); with the decrease of the temperature, the eutectoid reaction γ(Fe) → α(Fe) + ε(Cu) occurred, so Cu element was rich at interdendritic. Once the melt was undercooled into the

Conclusions

When the undercooling of the melt is less than 65 K, the typical dendritic structure was observed in the final microstructure. The dendritic phase was Fe-riched, while the interdendritic phase was Cu-riched. However, the separated structure was obtained at ΔT_sep = 65 K. The degree of the phase-separation increased with the undercooling of the melt. Additionally, the second phase-separation occurred in the separated phases at larger undercooling.

Affected by Stokes sedimentation and Marangoni motion,

Acknowledgments

The author is grateful to the financial support of Natural Science Foundation of the Education Office of Jiangsu Province (09KJB430004).

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Investigation on the phase separation in undercooled Cu–Fe melts

Abstract

Highlights

Introduction

Section snippets

Experiment procedures

Results

Discussion

Conclusions

Acknowledgments

Acta Mater.

Physica B

J. Alloys Compd.

Scr. Mater.

Mater. Sci. Eng. A

Acta Mater.

Acta Metall. Mater.

Mater. Sci. Eng. A.

Acta Mater.

J. Alloys Compd.

Physica B