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The structural degradation and lamellar to rod transition in the Bi-Cd eutectic during unidirectional growth

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Abstract

The Bi-Cd eutectic system is a prototype quasi-regular eutectic in which the bismuth-rich phase has a volume fraction of 57%. It shows a high degree of regularity but, clearly, is not a normal (regular) eutectic. Microstructural observations of unidirectionally-grown specimens show that the minor cadmium-rich phase degrades at small temperature gradients and small growth rates. However, the structural refinement resulting from rapid freezing or chemical addition is found to be analogous to that of the F/NF eutectics. A lamellar → rod transition has been achieved at intermediate growth rates by adding 2.0 wt % Sn as a “modifier” to the eutectic alloy. However, this was accompanied by the bismuth phase showing cellular facets of the solid-liquid interface.

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Abbreviations

G L :

temperature gradient in the melt ahead of the solid/liquid interface (° C cm−1)

G S :

temperature gradient in the solid behind the solid-liquid interface (° C cm−1)

R :

growth rate of solid (cm sec−1)

S :

cooling rate (° C sec−1, ° C h−1)

K S :

thermal conductivity in the solid (W m−1 K−1)

K L :

thermal conductivity in the melt (W m−1 K−1)

L :

latent heat of fusion (J mol−1)

ΔT :

temperature difference, undercooling (° C)

K 1 :

constant in Equation 2

K 2 :

constant in Equation 3

D :

diffusion coefficient of solute in solid (m2 sec−1)

C :

solubility in solid (wt %, at %)

M :

molecular weight (g mol−1)

ρ :

density (g cm−3)

γ :

interfacial energy, surface tension (J mm−2)

R :

gas constant, 8.314J mol−1 K−1

r :

radius of curvature (μm)

T :

temperature (K)

t :

time (sec)

F:

faceted

NF:

non-faceted

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Savas, M.A., Smith, R.W. The structural degradation and lamellar to rod transition in the Bi-Cd eutectic during unidirectional growth. J Mater Sci 20, 881–888 (1985). https://doi.org/10.1007/BF00585730

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Keywords

  • Temperature Gradient
  • Bismuth
  • Structural Refinement
  • Small Temperature
  • Eutectic Alloy