It is disclosed a method for strengthen the grain boundaries of an article (1) made from a Ni based superalloy while the article (1) is in the solid state and containing at least one grain boundary. A surface diffusion process is applied to the article (1) to enrich the at least one grain boundary with grain boundary strengthening elements of one or a combination of boron, hafnium, zirconium without forming brittle precipitates like borides or carbides.