Effects of melt temperature on the magnetic treated refinement of eutectic and primary phases in Al-Fe binary alloy melt by measuring thermopower

https://doi.org/10.1016/j.jcrysgro.2020.125653Get rights and content

Highlights

  • Measuring thermoelectric power provides a new insight into the phase evolution.

  • Thermoelectric power responds to nucleation and growth of phases.

  • Refinement efficiency of magnetic field over temperature ranges is discussed.

Abstract

Magnetic field is an effective strategy to refine microstructures, however, few researches focus on refinement efficiency over the different temperature ranges. In this paper, we develop the online thermopower measurement to provide the real-time and continuous check for the melt treated by alternating magnetic field over the selected temperature ranges. The effects of alternating magnetic field on eutectic and primary phases of Al-Fe binary alloy are discussed by both the thermopower and the solidification microstructures. Thermopower-temperature coefficient α = dS/dT turns from 0.1565 μV·K−2 to 0.0259 μV·K−2 at liquidus due to the solid particles precipitation during the direct cooling process without treatment. Relationships of thermopower and temperature are given to display the kinetics process. Thermopower increases with violent oscillation during the magnetic field. From the results of microstructures, magnetic field induces the promoted nucleation and decreased spacing of needle eutectic phases, as well as the refinement of bars and blocks primary phases, which strongly depends on the temperature ranges. Thermopower responds to nucleation and growth of eutectic and primary phases. This should be mainly attributed to the solid particles precipitation and the thermoelectric magnetic effects, respectively. Meanwhile, thermopower can be used to respond the microstructure evolution.

Introduction

Aluminum alloys have been widely applied in aerospace and automobile industries. Fe presents as one of the most common impurities in aluminum alloys. Because the Al-Fe intermetallics are in the form of needles, flakes and lath shapes, which dissever substrate and deteriorate the mechanical properties [1], [2], [3], [4], [5]. Meanwhile, some researchers found that the refined Al-Fe intermetallics can increase the strength and high temperature resistance of the aluminum alloys [6], [7], [8], [9]. Therefore, it is important to develop some methods to reduce the size and control the morphology and distribution of Al-Fe intermetallics. Magnetic field treatment is an effective way to refine the eutectic and primary phases.

Researchers have found the beneficial influences of magnetic field on the solidified microstructures. The magnetic field can decrease eutectic and dendrite arm spacing and refine cells/dendrites [10], [11], [12], as well as achieve an excellent transition and distribution of phases [13], [14]. The alternating magnetic field parameters impact the primary phases during the solidification process [15]. Furthermore, the efficiency of magnetic field is affected by the conditions of magnetic field (intensity, frequency and gradients of magnetic field) [14], [16], [17], [18], treatment temperature, treatment duration [19], cooling rate [20], and alloy composition [21], [22]. Among the above factors, the treatment temperature range mainly determines the efficiency. Because initial nucleation and following growth stages involve some mechanisms controlled by melt temperature.

Widely used investigations of metal melt are post-solidified examination methods, called the metallographic observation, due to the difficulty of metal melt observation at elevated temperature. In this work, online thermopower measurement provides the real-time and continuous check for the melt, as well as a new insight into the phase evolution treated by magnetic field. Electrical parameters depended on temperature have been theoretically and experimentally investigated by many researchers [23], [24]. Meanwhile, the electrical parameters of liquid metal have gradually drawn much attention to reveal the melt structure variation, even in the external fields [25], [26], [27], [28]. Liu et al [25] have investigated the ultrasonic irradiation refines and homogenizes the short ordered structures in the liquid alloys, based on the resistivity drop. Li et al [26], [28] have separately indicated the decreased Seebeck voltage as the growth speed increased and the refinement of primary dendrite in directional solidification by magnetic field. However, as one of the important electrical parameters, Seebeck voltage is detected during the directional solidification rather than the one under the magnetic field. It is noted that the relationships between electrical parameters and solidified microstructures help us understanding what happened to the melt under the magnetic field.

This work presents both the thermopower and the solidified microstructures of Al-Fe binary alloy melt treated by the alternating magnetic field over different temperature ranges, which derives from the nucleation and growth of eutectic and primary phases. The results reveal the mechanisms of magnetic field on refining eutectic and primary phases during the selected solidification stages.

Section snippets

Alloys preparation and treatment

As-cast billet was hypereutectic Al-3.66 wt% Fe binary alloy prepared by commercial pure aluminum and iron. The liquidus and eutectic temperature of Al-3.66 wt% Fe alloy was calculated to be 1001 K and 928 K suing the JMatePro software. Billet was cut into small blocks (5 × 10 × 100 mm3). Then some small blocks were placed into a corundum crucible and re-melted by resistance furnace to make sure the same composition for each experiment. Alloy was heated up to 1033 K and isothermal held for

Thermopower

Fig. 2(a) gives the thermopower during the direct cooling process. The curves are divided into three stages. Initial thermopower shows stable value during the isothermal holding in the absence of magnetic field (denoted I). Thermopower increases in the time range of 300–1900 s, which corresponds to the cooling ranging from 1033 K to 933 K. Thermopower increases near linearly with the decreasing melt temperature. Thermopower changes from three different contributions, namely electron, phonon and

Conclusion

Effects of alternating magnetic field on the Al-Fe alloy during cooling process are quantitatively investigated by measuring thermopower and observing the solidification microstructures. Refinement efficiencies are discussed by applying the magnetic field over the different temperature ranges, which correspond to the nucleation and growth of eutectic and primary phases.

The results indicate that thermopower increases near linearly with melt temperature during the direct cooling process.

CRediT authorship contribution statement

Qing Lan: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Qichi Le: Supervision, Funding acquisition, Project administration. Ruizhen Guo: Data curation, Validation. Jianfeng Zhang: Investigation, Methodology, Formal analysis.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Funding: This work is supported by the National Key Research and Development Program of China [2017YFB0305504]; the National Natural Science Foundation of China [51974082].

References (52)

  • W.D. Xuan et al.

    Effect of a high magnetic field on microstructures of Ni-based superalloy during directional solidification

    J. Alloys Compd.

    (2015)
  • L. Hou et al.

    Evolution of microstructure and microsegregation in Ni-Mn-Ga alloys directionally solidified under axial magnetic field

    J. Alloys Compd.

    (2018)
  • Y.H. Dong et al.

    Effect of high static magnetic field on the microstructure and mechanical properties of directionally solidified alloy 2024

    J. Alloys Compd.

    (2018)
  • R. Tao et al.

    Microstructures and properties of in situ ZrB2/AA6111 composites synthesized under a coupled magnetic and ultrasonic field

    J. Alloys Compd.

    (2018)
  • S.X. Tu et al.

    Enhancement of magnetostrictive performance of Tb0.27Dy0.73Fe1.95 by solidification in high magnetic field gradient

    J. Alloys Compd.

    (2018)
  • Y.X. Zhuang et al.

    Effect of high magnetic field on crystallization behavior of Fe83B10C6Cu1 amorphous alloy

    J. Alloys Compd.

    (2016)
  • C. Zhai et al.

    Thermodynamically analyzing the formation of Mg12Nd and Mg41Nd5 in Mg-Nd system under a static magnetic field

    J. Alloys Compd.

    (2019)
  • Z.H.I. Sun et al.

    Alignment of weakly magnetic metals during solidification in a strong magnetic field

    J. Alloys Compd.

    (2013)
  • A. Ben Abdellah et al.

    Spin-state dependence of electrical resistivity and thermoelectric power of molten Al–Mn alloys: experiment and theory

    J. Alloys Compd.

    (2016)
  • X. Liu et al.

    Electrical resistivity behaviors of liquid Pb–Sn binary alloy in the presence of ultrasonic field

    Ultrason.

    (2015)
  • Z.Y. Lu et al.

    Effect of a high magnetic field on the morphology of the primary dendrite in directionally solidified Pb–25at% Bi peritectic alloy

    Mater. Lett.

    (2015)
  • D.F. Du et al.

    Effect of a transverse magnetic field on solidification structure in directionally solidified Sn–Pb hypoeutectic alloys

    J. Cryst. Growth

    (2014)
  • K.Y. Kim et al.

    Thermoelectric conductivities at finite magnetic field and the Nernst effect

    J. High Energy Phys.

    (2015)
  • C.P. Jiang et al.

    Thermal stability of p–type polycrystalline Bi2Te3-based bulks for the application on thermoelectric power generation

    J. Alloys Compd.

    (2017)
  • Y.S. Lim et al.

    Enhanced thermoelectric properties and their controllability in p-type (BiSb)2Te3 compounds through simultaneous adjustment of charge and thermal transports by Cu incorporation

    J. Alloys Compd.

    (2016)
  • H. Matsushima et al.

    Effects of magnetic fields on iron electrodeposition

    Surf. Coat. Technol.

    (2004)
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