DIFFUSION OF HF AND NB IN ZR-19-PERCENT-NB

Diffusion of Hf and Nb in large-grained bcc Zr-19%Nb has been studied. Diffusion coefficients of Hf, D(Hf), were measured in the range 620-1 173 K and D(Nb) was measured at 920 and 1167 K. The HT diffusion profi les were determined by SIMS and the Nb profiles by microtome sectionin g and radio-tracer counting. The Hi data show a smooth, temperature-de pendent behaviour through the monotectoid temperature, 875 K, and may be characterised by D approximate to 10(-9) x exp(-1.4(eV/kT)m(2)/s. D (Nb) tends to be lower than the corresponding values for D(Hf). Overal l, diffusion of Hf and Nb are characteristic of diffusion in bcc Zr. S urface hold-up (oxide film) at low temperatures was overcome by using ion-implanted Hf diffusion sources. The results are compared with earl ier work and discussed in terms of diffusion mechanisms and the beta-p hase transformation of commercial Zr-2.5Nb.