Interdiffusion and Self-diffusion in Nickel-rich Nickel-Tin Alloys

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The interdiffusion and intrinsic diffusion coefficients in Ni-rich nickel-tin alloys containing up to 7 at%Sn have been determined in the temperature range between 1223 and 1473 K by use of semi-infinite diffusion couples with the Kirkendall markers. The tracer diffusion coefficients of solvent and solute (or impurity) in pure Ni and the alloys at 1373 K have been determined by use of 63Ni and 113Sn. On the basis of the present diffusion data on 63Ni and 113 Sn in pure Ni and the value of intrinsic diffusion coefficient of Ni extrapolated to the infinite dilution of Sn, the vacancy flow factor has been estimated to be −0.86. From the linear enhancement factor of solvent diffusivity and the vacancy flow factor, an appropriate numerical set of the correlation factor and the jump frequency ratios in the five-frequency model has been evaluated. The result suggests highly correlated jumps of a Sn atom to a vacancy in Ni similar to those of Sn in Cu. On the basis of the interdiffusion and tracer diffusion data for the alloys, the thermodynamic factor in Darken’s relation has been calculated by Heumann’s method. The value ofthe thermodynamic factor is found to be less than unity in Ni-rich Ni-Sn alloys, suggesting that Sn atoms have a slight tendency to form clusters. The vacancy flow term in Darken’s relation as modified by Manning has been estimated to be larger than the value deduced from the random alloy model. This implies that the attraction between a Sn atom and a vacancy is highly strong. The concentration dependence of thermodynamic factor and vacancy flow term in Ni-rich Ni–Sn alloys has been shown to be similar to that in Ni-rich Cu–Sn alloys.