The fatigue thresholds have been a topic of extensive research over the years mainly from the experimental viewpoint. From the theoretical side, there has been only handful of investigations (Chowdhury,P., H. Sehitoglu, R. Rateick, International Journal of Fatigue, 68, 277-301, 2014). It is well known that the microstructure affects the fatigue crack growth thresholds. In many cases, the fatigue threshold is not unique and depends on the crack tip environment, ie. the presence of twins, their distance with respect to the crack tip and the local friction stress (resistance to flow) as influenced by the interfaces. We devise an atomistic-continuum model for determining the irreversibility of displacements at crack tips, hence fatigue crack growth rates, in metallic alloys. We show examples of our calculations on NiCo alloys and conduct experiments showing the role of microstructure on fatigue threshold behavior.