Check Smithells Metals Reference Book, 8th Ed. (2004).
For Au-Ni, Smithells Metals Reference Book page 13-47 in the 7th Edn. (1992), gives a table with diffusion data for various compositions. Some data from J. E. Reynolds et al., Acta Met., vol. 5, p. 29+ (1957) rest from B. Khomka, Nukleonika, vol. 8, p.185+ (1963) [maybe check Chem. Abstracts].
Lots of newer data; just google. E.g.,
"...the diffusion coefficients of Ni in Au to be 5.3 × 10-16 cm2/s at 500 °C, 4.0 × 10-17 cm2/s at 400 °C, 2.5 × 10-18 cm2/s at 300 °C, and 1.2 × 10-19 cm2/s at 200 °C. An activation energy of 0.87 eV was calculated. The present diffusion data differ significantly from the corresponding values extracted by some other investigators and the reasons for this disagreement were discussed."
from
'Determination of diffusion coefficients in Au/Ni thin films by Auger electron spectroscopy,'
Abdul-Lettif, Ahmed M., Physica Status Solidi (A), Applied Research, vol. 201, Issue 9, pp. 2063-2066 (July 2004).