Diffusion Mechanism of Atoms in Ordered Alloys at Low Temperature

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The knowledge of diffusion phenomena of atoms is essential to develop new materials for modern technologies and then to use safely mechanical and electrical equipments made from those highly designed materials. The sufficient resolution of secondary iron mass spectroscopy has proved to be available for determining the small diffusion coefficients and the isotope effects of many atoms down to relatively ll"ow temperature. Our research clarified the small diffusivities and the isotope effects and the diffusion mechanisms of several solute atoms in the ordered alloys of B2, DO_3 and L1_0 types at low temperatures, by using the profit of secondary ion mass spectroscopy. The main results obtained are as follows. The temperature dependences-of diffusivities of nickel, vanadium, titanium and boron in theFe-Al alloy were not explained by a simple Arrhenius relation. The diffusivities of vanadium, titanium and boron were controlled by the change of B2 order, while the diffusivity of nickel was determined by the changes of DO_3 order as well as B2 order. The isotope effect of boron was about 0.2 in the disordered state and the ordered states of B2 and DO_3 types in the Fe-Al alloy. The diffusion of boron proceeded through highly correlated mechanism in the Fe-Al alloy. The diffusivity of boron in the ordered state of the CuAu alloy was not smaller than that extrapolated from the disordered state. The isotope effect of boron in the CuAu alloy was 0.1 in the disordered state and 0.8 in the ordered state of L1_0 type. Correlation for the diffusion of boron was stronger in the disordered state than in the ordered state.