Experimental results on grain boundary diffusion and segregation in bi-crystalline, tri-crystalline, and poly-crystalline metals and alloys are critically examined with an emphasis on solute diffusion (Ag, Bi, Ge, Ni, Co, Fe) in the same high-purity Cu. The problems of grain boundary width and segregation-induced structure modifications will be addressed.
The temperature-induced transitions of the grain boundary structure as revealed by the radiotracer diffusion measurements for specific low sigma interfaces like Σ5 and Σ17 are discussed [1, 2]. Critic temperatures, which correspond simultaneously to specific "kinks" in the temperature dependences of the grain boundary diffusivity and a disappearance of the grain boundary diffusion anisotropy, are found and related to temperature-induced phase transitions .
The state-of-the-art in grain boundary diffusion in intermetallic compounds is reviewed, too, including interfaces in ordered multi-component and disordered high-entropy alloys.
 H. Edelhoff, S. I. Prokofjev, S.V. Divinski, Scr. Mater. 64, 374-377 (2011)
 S.V. Divinski, H. Edelhoff, S. Prokofjev, Phys. Rev. B 85 (2012) 144104
 T. Frolov, S.V. Divinski, M. Asta, Y. Mishin, Phys Rev Lett 110 (2013) 255502.