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Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

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Abstract

A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB2, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB2 particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.

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Abbreviations

a r :

activity for a particle with radius r

a∞ :

activity for a particle with radius ∞

ε TiTi :

self-interaction coefficient Ti-Ti

ε BTi :

interaction coefficient between B and Ti in liquid aluminum

fTi :

fugacity

γ l Ti :

activity coefficient for Ti in liquid Al

γ sTi :

activity coefficient for Ti in solid Al

γ oTi :

activity coefficient for Ti, whose standard state is pure Ti and whose reference state is the infinitely dilute solution of liquid Al

ΔHf :

latent heat of fusion (J/mol)

K′Ti :

thermodynamic distribution coefficient of Ti between liquid and solid

KTi :

effective distribution coefficient of Ti between liquid and solid

K 0 :

distribution coefficient for Ti between liquid and solid when a positive interaction between Ti atoms is assumed

μr :

chemical potential of a particle with radiusr (J/mol)

μ∞:

chemical potential of a particle with radius ∞ (J/mol)

r:

radius (m)

R:

gas constant (J/mol · K)

σ:

solid/liquid interfacial energy (J/m2)

T l :

liquidus temperature in the system Al-Ti

T f :

melting point for pure aluminum (933.52 K)

ΔT:

change in freezing point of α-Al caused by the presence of Ti

v:

molar volume of TiAl3 = 3.3 · 10-5 m3/mol

X s Ti :

molar fraction Ti in solid solution with Al

X l Ti :

molar fraction Ti dissolved in liquid Al

X l B :

molar fraction B dissolved in liquid Al

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Authors and Affiliations

  1. Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden

    Mats Johnsson (Graduate Student) & Lennart Backerud (Professor)

  2. Metalworking Technology Inc., 15904, Johnstown, PA

    Geoffrey K. Sigworth (Principal Engineer)

Authors
  1. Mats Johnsson
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  2. Lennart Backerud
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  3. Geoffrey K. Sigworth
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Johnsson, M., Backerud, L. & Sigworth, G.K. Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys. Metall Trans A 24, 481–491 (1993). https://doi.org/10.1007/BF02657335

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  • DOI: https://doi.org/10.1007/BF02657335

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