Unidirectional solidification of Al–Cu eutectic with the accelerated crucible rotation technique

https://doi.org/10.1016/S0022-0248(98)00735-0Get rights and content

Abstract

The accelerated crucible rotation technique (ACRT) has been applied to the unidirectional solidification of Al–Cu eutectic to reveal the effect of forced convection on the solidification microstructures by a systematic experimental investigation combining different rotation methods (the maximum rotation rate Ωmax=100–400 rpm) with various growth velocities (V0=5–60 μm/s). The results can be concluded as follows: (1) Forced convection introduced by ACRT dramatically changes the eutectic microstructures. When the fluid flow is weaker (Re<270), a great number of lamellar faults occur in the eutectic; on the other hand, when the flow is stronger (Re>500), fluctuation structure forms in the eutectic; (2) the lamellar spacing is not uniformly distributed in the radial direction of the solidified sample with ACRT, and the maximum spacing occurs near the crucible wall while the minimum one at the center of the crucible. The average lamellar spacing obtained by ACRT is smaller than that without ACRT and decreases upon increasing the convection intensity (i.e. increasing Ωmax); (3) whether or not a normal Bridgman process is performed before ACRT shows little influence on the growth of the nonfacet–nonfacet eutectic.

Introduction

The accelerated crucible rotation technique (ACRT), first proposed in the 1970s [1], has been a novel method to produce large and high-quality single crystals of functional materials 2, 3, 4. During the ACRT crystal growth process, the crucible rotates with varying speed in a given method and forced convection is produced in the front of solid–liquid interface. The fluid flow suppresses the adverse effects of constitutional supercooling [1], permits the realization of stable planar interface crystal growth at higher critical velocity than that in normal Bridgman process [5]and improves the crystallinity [6]. Therefore, ACRT has been widely applied to Czochralski (CACRT) and Bridgman (ACRT-B) crystal growth processes.

In order to search for a new technique of in situ preparation of composites and improve the understanding upon the effect of forced convection on solidification microstructures, several works 7, 8, 9on the application of ACRT to the eutectic alloys have been done during the last two decades. The experimental studies of Popov and Wilcox [7]on Pb–Sn eutectic showed that the stirring by ACRT increased the spiral rate of the microstructure and reduced the length of eutectic zone cooperatively, but had no influence on the lamellar spacing. In the unidirectional solidification of Bi–MnBi with ACRT, Eisa and Wilcox [8]found that the coarseness of MnBi quasiregular fibers was caused by the accelerated crucible rotation at a growth velocity of 9 mm/h. Jie [9]applied ACRT to Al–Si eutectic growth and found that the initial condition before ACRT influenced the growth morphology of Si phase. To this typical nonfacet–facet eutectic, if the crucible rotated at the beginning of the unidirectional solidification, irregular eutectic with short bars and chunks of silicon would form; but if a normal Bridgman process was performed before the crucible rotation, a more satisfactory directional needle-like silicon structure would grow up.

On the other hand, the studies on the application of ACRT to eutectic growth process mostly concentrated on the conditions of low growth velocities and low convection intensities (i.e. smaller Ωmax). There is also no reported evidence upon the influence of the initial conditions on nonfacet–nonfacet eutectic growth process. With the interest concentrated on the effect of the forced convection on the eutectic structures and lamellar spacing, the application of ACRT to Al–Cu eutectic, a widely studied nonfacet–nonfacet eutectic, has been investigated in which different initial conditions, growth velocities and crucible rotation methods have been systematically carried out.

Section snippets

Experimental methods

The experiments were carried out with the ACRT Crystal Growth Equipment described in details elsewhere [9]. The equipment consists of a crucible rotation system, a furnace drawing system, and a thermal field controlling system which can work independently.

The Al–Cu eutectic alloy was melted in the vacuum furnace with 99.99% Al and 99.99% Cu, and cast into bars (ID 8 mm×120 mm long) for further experiments. The graphite crucible with the inner diameter of 8 mm was used for unidirectional

Microstructures by the normal Bridgman method

During the normal Bridgman process (without crucible rotation), the morphology of the eutectic consists of regular lamellae as shown in Fig. 2 and the interface is nearly planar. The lamellar spacing decreases as the growth velocity increases. Regression analysis shows that the relation between growth velocity and lamellar spacing satisfies V0λ2=88.4 μm3/s, which is in good agreement with the theoretical prediction given by Jackson and Hunt [10].

Eutectic microstructures

The fluid flow exerted by the crucible rotation

Transportation condition and formation of fluctuation structure

There are three types of forced convection exerted by the crucible rotation, namely, spiral-shear flow, Couette flow and Ekman flow [11]. Spiral-shear flow can improve the uniformity of solute and temperature distribution cooperatively. Couette flow can produce more complete mixing in liquid during spin-down process. As for crystal growth process, Ekman flow is of much importance than the former two because it directly acts on S/L interface so as to influence the solidification microstructure.

Conclusions

  • 1.

    In the normal Bridgman process, the lamellar spacing and growth velocity of Al–Cu eutectic satisfies V0λ2=88.4μm3/s, which is in good agreement with that predicted by Jackson and Hunt's model.

  • 2.

    When the crucible rotates at a lower rate, a great number of lamellar faults are formed in the eutectic structure; when the rotation rate is larger, fluctuation eutectic structure occurs.

  • 3.

    The lamellar spacing is no longer uniformly distributed in the radial direction of the solidified sample with ACRT. The

References (17)

  • H.J Scheel

    J. Crystal Growth

    (1972)
  • P Capper et al.

    J. Crystal Growth

    (1988)
  • Ph Buffat et al.

    J. Crystal Growth

    (1986)
  • A Horowitz et al.

    J. Crystal Growth

    (1983)
  • P Capper et al.

    J. Crystal Growth

    (1984)
  • G Eisa et al.

    J. Crystal Growth

    (1986)
  • J.C Brice et al.

    Prog. Crystal Growth and Charact.

    (1986)
  • V.M Masalov et al.

    J. Crystal Growth

    (1992)
There are more references available in the full text version of this article.

Cited by (7)

  • Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo-Si-B alloy

    2014, Intermetallics
    Citation Excerpt :

    The present work introduces an alternative processing route using cold pressed powder mixtures which are consolidated by directional solidification via a crucible-free zone melting technique (ZM). While there is an abundance of literature on directional solidification for various alloy systems [26–30], ZM of multiphase Mo–Si–B materials is a rather new approach. Mason et al. [31] investigated a directionally solidified eutectic composition consisting of MoSi2–Mo5Si3.

  • 1 on the eutectic spacings, undercoolings and microhardness of directional solidified bismuth-lead eutectic alloy

    2013, Current Applied Physics
    Citation Excerpt :

    The value of λ2V = 104.5 μm3/s is very close to value of 101.80 μm3/s obtained by Kaya et al. [59] for Al–Ni eutectic system. The value of λ2V = 104.5 μm3/s is fairly smaller than the value of 156 μm3/s obtained by Çadırlı et al. [23] for Al–Cu eutectic system and also the value of 104.5 μm3/s is slightly higher than the values of 88.4 μm3/s, 69.05 μm3/s and 61.43 μm3/s obtained by Ma et al. [60] for Al–Cu eutectic system, Engin et al. [58] for Zn–Al eutectic system and Cadırlı et al. [61] for Sn–Cu eutectic system, respectively. Fig. 4 shows the minimum undercooling (ΔT) of the solidifying interface.

  • Effect of accelerated crucible rotation on the segregation of impurities in vertical Bridgman growth of multi-crystalline silicon

    2011, Journal of Crystal Growth
    Citation Excerpt :

    In this technique the fluid in the crucible is accelerated (spin-up) and decelerated (spin-down) periodically, generating Ekman boundary layers at solid surfaces [2] which induce a flow in radial direction. ACRT has been widely applied to improve the crystal quality by either improving the mixing of the liquid or achieving more diffusion controlled transport conditions in the liquid [3–8]. In a few numerical studies [9–17] the effect of ACRT on melt convection and segregation in crystal growth systems has been studied.

View all citing articles on Scopus
View full text