Abstract
A solute diffusion model, aimed at predicting microstructure formation in metal castings, is proposed for dendritic solidification of alloys. The model accounts for the different length scales existing in a dendritic structure. This is accomplished by utilizing a multiphase approach, in which not only the various physical phases but also phases associated with different length scales are considered separately. The macroscopic conservation equations are derived for each phase using the volume averaging technique, with constitutive relations developed for the interfacial transfer terms. It is shown that the multiphase model can rigorously incorporate the growth of dendrite tips and coarsening of dendrite arms. In addition, the distinction of different length scales enables the inclusion of realistic descriptions of the dendrite topology and relations to key metallurgical parameters. Another novel aspect of the model is that a single set of conservation equations for solute diffusion is developed for both equiaxed and columnar dendritic solidification. Finally, illustrative calculations for equiaxed, columnar, and mixed columnar-equiaxed solidification are carried out to provide quantitative comparisons with previous studies, and a variety of fundamental phenomena such as recalescence, dendrite tip undercooling, and columnar-to-equiaxed transition (CET) are predicted.
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Abbreviations
- a :
-
constant, in the Appendix
- A :
-
interfacial area, m2
- As :
-
area of the solid/interdendritic liquid interface, m2
- Ae :
-
area of the dendrite envelope, m2
- b :
-
constant, in the Appendix
- c :
-
constant, in the Appendix
- C :
-
concentration of a chemical species, weight percent
- cp :
-
volumetric specific heat, J m-3 K-1
- d :
-
microscopic length scale, m
- ds :
-
mean characteristic length or diameter of the solid phase, m
- de :
-
mean characteristic diameter of the dendrite envelope, m
- D :
-
diffusion coefficient, m2 s-1
- Iv:
-
Ivantsov function
- j:
-
species diffusion flux, kg m-2 s-1
- J :
-
interfacial species transfer rate per unit of volume, kg m-3 s-1
- k :
-
constants in the nucleation law, see Figure 6
- l :
-
species diffusion length, m
- L :
-
macroscopic length scale, m
- ml :
-
liquidus line slope
- n :
-
nuclei density, m3
- n :
-
outwardly directed unit normal vector
- Pe:
-
envelope Peclet number, -wneRf/Dl
- Pet :
-
solutal Peclet number at the dendrite tip,V tRt/2Dl
- r :
-
radial coordinate
- R :
-
radius, m
- S :
-
interfacial area concentration, Ak/Vo, m-1
- Sv :
-
specific interfacial area, Ak/Vk, m-1 tet, time, s
- T :
-
temperature, K
- T :
-
cooling rate, Ks-1
- v:
-
velocity, m s-1
- Vk :
-
volume of phasek, m3
- Vo :
-
averaging volume, m3
- Vt :
-
dendrite tip velocity, m s-1
- w:
-
interface velocity, m s-1
- wn :
-
normal interface velocity, ms-1
- x:
-
position vector
- X :
-
phase function
- α:
-
diffusion Fourier number, 4Dstf 2λ2
- Β:
-
a factor
- г:
-
interfacial mass transfer rate due to interface movement (kg m-3 s-1) or Gibbs-Thomson coefficient (mK)
- δh:
-
volumetric latent heat of phase change, J m-3
- ε:
-
volume fraction
- κ:
-
partition coefficient
- λ:
-
dendrite arm spacing, m
- ρ:
-
density, kg m-3
- Φ:
-
shape factor
- Φ:
-
a field property
- ψ:
-
a field property
- Ω:
-
solutal supersaturation
- d :
-
interdendritic liquid
- e :
-
dendrite envelope
- f :
-
final dimension of the dendrite envelope
- j :
-
phasej
- k :
-
phasek
- ]kj :
-
pertinent to phasek on thek- j interface
- l :
-
extradendritic liquid
- m :
-
melting point of pure metal
- n :
-
normal direction
- o :
-
initial state
- r :
-
in the r-direction
- s :
-
solid
- t :
-
dendrite tip
- j :
-
due to species gradients
- г:
-
due to interface movement interfacial area—averaged
- *:
-
effective or dimensionless
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Wang, C.Y., Beckermann, C. A multiphase solute diffusion model for dendritic alloy solidification. Metall Trans A 24, 2787–2802 (1993). https://doi.org/10.1007/BF02659502
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DOI: https://doi.org/10.1007/BF02659502