Separation of size-dependent strengthening contributions in fine-grained Dual Phase steels by nanoindentation

Dual Phase steels with different grain sizes ranging from 0.7 to 4 μm were processed by swaging followed by intercritical annealing. Nanoindentation tests were conducted on both ferrite and martensite phases. By changing the indentation depth with respect to the size of the microstructure, separation can be made between the various contributions to strengthening: nanoindentation size effect, grain size effect, natural hardening of the ferrite and reinforcement by the martensite. A simple model that takes into account these various contributions supports the interpretation of the experimental results and sheds light on the complex relationships existing between the different length scales. The methodology of using hardness–indentation depth evolution curves to separate strengthening contributions is relatively general and can be transferred to many other heterogeneous materials involving similar or different hardening mechanisms with or without associated size effects.