FAST DIFFUSION OF BORON IN THE INTERMETALLIC COMPOUND NI3AL

Boron diffusion in Ni3Al was measured between 773 and 1345 K by second ary ion mass spectrometry (SIMS) using the stable B-10 and B-11 isotop es and Ni3Al single crystals (75.9 at% Ni). The temperature dependence of the diffusion coefficient Dg follows a perfect linear. Arrhenius r elation with a frequency factor D-0 = 1.53 x 10(-4) m(2)/s and an acti vation enthalpy Q(B) = 227.6 kJ/mol. In comparison to Ni self-diffusio n in Ni3Al (Q(Ni) = 303.0 kJ/mol), which occurs via thermal vacancies, B diffusion is faster by a factor of 10(3)-10(4) which is compatible with an interstitial diffusion mechanism. The actual diffusion path of the B atoms is influenced by their preference to occupy mostly the Ni atom surrounded octahedral sites in the Lit-lattice. Assuming nearest neighbour jumps of B atoms between octahedral interstices, this disso lution behaviour may explain the observed fairly large activation enth alpy of interstitial atom motion in the ordered Ni3Al compound. (C) 19 97 Elsevier Science Ltd.