Tsuyoshi Yamamoto1, Toshiyuki Takashima1 and Keizo Nishida2
1Department of Mechanical Engineering, Hokkaido Institute of Technology, Sapporo
For the purpose of studying interdiffusion in an α -solid solution of the Ni-Zn system, nickel test pieces were diffusion-annealed in an evacuated capsule containing the powdered 45 wt%Zn-Ni alloy (α +β 1 or α +β ) or the powdered 40 wt%Zn-Ni alloy (α ) as a Zn vapor source at 1073∼1323 K for 3.6∼705.6 ks.
In the case of the diffusion-annealing with the former 45 wt%Zn-Ni alloy as a Zn vapor source, the surface concentration of Zn in the test piece at 1073 K coincided approximately with the solubility limit reported in the phase diagram of the Ni-Zn system, while a significant deviation toward the higher Zn concentration in the solubility limit was found at other temperatures.
For the powdered 45 wt%Zn-Ni alloy as a vapor source, fine alumina markers placed on the surface of test pieces prior to diffusion still remained on the surface after annealing, so that it is considered that Zn atoms diffuse predominantly into the α -solid solution at the marker position. For the latter vapor source used at 1323 K, the markers were found inside the test pieces after annealing. In this case, the Zn concentration at the markers position was 36.3 at%Zn and the ratio of these intrinsic diffusion coefficients in this Zn concentration, (DZn/DNi), was about 21.
The interdiffusion coefficients (\TildeD) were dependent upon Zn concentration and were evaluated to be the orders of 10-17∼10-13 m2/s in this experimental temperature range. Log \TildeD increased linearly with increasing Zn concentration except the case annealed at 1073 K.
The activation energies for interdiffusion (\TildeQ) obtained from these data decreased linearly with increasing Zn concentration from 265 kJ/mol for 5 at%Zn to 208 kJ/mol for 30 at%Zn. The impurity diffusion coefficient (DZn*) at each temperature was obtained from extrapolating of \TildeD to 0 at%Zn and the activation energy for the impurity diffusion of Zn in Ni(QZn*) obtained from the Arrhenius plot of DZn* was evaluated to be 276 kJ/mol.
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