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 dμ Zn/dN Zn, for several annealed alloys. Both of these quantities achieve their maximum in the composition range 0.30 to 0.40 mole fraction of zinc.
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Abbreviations
- K :
-
crystallization coefficient
- N xzn :
-
the mole fraction of zinc in the liquid at a given stage of crystallization
- N x(zn) :
-
the mole fraction of zinc in the solid formed on the dendrite surface at a given stage of crystallization
- n 0zn :
-
amount of zinc in one mole of alloy
- n 0(zn) :
-
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
- N 0zn :
-
the total amount of zinc in the alloy (in a mole fraction)
- D *zn ,D *Al :
-
the self-diffusion coefficients for zinc and aluminium
- \(\tilde D\) :
-
the interdiffusion coefficient
- N zn,N Al :
-
the mole fraction of aluminium and zinc in alloyf the thermodynamic factor equal to 1 + (d ln γzn/d lnN zn)
- γzn :
-
the activity coefficient of zinc
- τ:
-
the relaxation time of the homogenization process
- l :
-
the radius of the dendrite cell
- J zn :
-
flux of zinc diffusion in the solid solution
- μzn :
-
the chemical potential of zinc
- M :
-
the phenomenological coefficient of diffusion
- x 1 :
-
the thickness of the considered diffusion layer
References
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B. Dukiet-Zawadzka andR. Ciach, Proceedings of the IX Conference Pan-Stop Kraków (1976)1, 96.
P. G. Shewmon, ″Diffusion in Solids″ (McGraw-Hill, New York, 1963).
B. Dukiet-Zawadzka, Dissertation, Zpm Pan, Kraków, unpublished (1976).
J. E. Hilliard, B. L. Averback andM. Cohen,Acta Met. 9 (1959) 86.
K. Fitzner andK. Wiencek,Bull. Acad. Pol. Sci. Ser. Sci. Technol. 12 (1975) 31.
R. Ciach,Arch. Hutn. 19 (1974) 27.
A. Krupkowski,ibid. 12 (1967) 343.
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Ciach, R., Dukiet-Zawadzka, B. & Ciach, T.D. A study of the homogenization process of aluminium-zinc alloys. J Mater Sci 13, 2676–2686 (1978). https://doi.org/10.1007/BF00552698
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DOI: https://doi.org/10.1007/BF00552698