Measurement of Diffusion Profile of Boron in α Iron by Secondary-ion Mass Spectrometry and Determination of Its Diffusion Coefficient

We report an attempt to determine the diffusion coefficient of B in a Iron by measuring the penetration profile by means of secondary-ion mass spectrometry (SIMS). Pure iron plates of grain-size of 1 to 3 mm were prepared, and thin films of Fe-B alloy (200 nm) and alumina (50 nm) were deposited on the surface as a B source and a capping layer, respectively. The samples were subjected to diffusion annealing at 700 degrees C, 800 degrees C, and 900 degrees C for certain periods of time, and the intensity of secondary ions of B was measured as a function of depth by SIMS. The mesa method was employed, in which a groove is prepared first around the target area by sputtering, and then the depth profile of B through the inner pillar was obtained. The concentration profiles thus obtained were analysed with the thin-film solution, the error-function solution, and also using Hall's method, depending on the form of the profile. The diffusion coefficient was of the order of 10(-18) m(2) s(-1 )in all the cases, which is seven to eight orders of magnitude smaller than those evaluated from deboronising experiments in the 1950 s, but is close to recent theoretical prediction for substitutional diffusion.