A study of the homogenization process of aluminium-zinc alloys
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Abstract
This paper is concerned with theoretical and experimental studies of the dendritic structure and its dissolution in aluminium-zinc alloys. The maximal dendritic microsegregation in the composition range of theα solid solution, as defined by the amount of non-equilibriumα+η eutectic, and the variation in zinc concentration across a model of dendritic grain have been calculated. The measurements of the above mentioned quantities were then performed using quantitative microscopy and X-ray microanalysis. The results obtained for the as-cast Al-Zn alloys confirmed the theoretically predicted data. In order to eliminate such micro-segregation an annealing treatment was applied. Specimens of Al 60wt% Zn alloy (0.383 mole fraction of zinc) showed a significant resistance to the homogenization treatment. This phenomenon can be explained theoretically on the basis of the calculated homogenization relaxation time, and the thermodynamical factor dμZn/dNZn, for several annealed alloys. Both of these quantities achieve their maximum in the composition range 0.30 to 0.40 mole fraction of zinc.
Keywords
Mole Fraction Composition Range Zinc Concentration Dendritic Structure Homogenization TreatmentNomenclature
- K
crystallization coefficient
- Nznx
the mole fraction of zinc in the liquid at a given stage of crystallization
- N(zn)x
the mole fraction of zinc in the solid formed on the dendrite surface at a given stage of crystallization
- nzn0
amount of zinc in one mole of alloy
- n(zn)0
amount of zince in a dendrite at a given stage of crystallization
- x
amount of dendritic constituent in one mole of solidified alloy
- c
constant calculated for the boundary valuex=0 and for the mole fraction of zinc in the initial liquid of the examined alloy
- \(\bar N_{({\text{Zn}})}^x \)
a mean content of zinc (in a mole fraction) at any arbitrary stage of solidification
- Nzn0
the total amount of zinc in the alloy (in a mole fraction)
- Dzn*,DAl*
the self-diffusion coefficients for zinc and aluminium
- \(\tilde D\)
the interdiffusion coefficient
- Nzn,NAl
the mole fraction of aluminium and zinc in alloyf the thermodynamic factor equal to 1 + (d ln γzn/d lnNzn)
- γzn
the activity coefficient of zinc
- τ
the relaxation time of the homogenization process
- l
the radius of the dendrite cell
- Jzn
flux of zinc diffusion in the solid solution
- μzn
the chemical potential of zinc
- M
the phenomenological coefficient of diffusion
- x1
the thickness of the considered diffusion layer
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References
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