Hetero-deformation induced strengthening and toughening of pure iron with inverse and multi-gradient structures

Here an inverse gradient structure (IGS) is processed by induction heat treatment on ultrafine-grained substrate, and a multi-gradient structure (MGS) is gained by surface mechanically peening treatment on IGS material. The grain size distributions of coarse-ultrafine and nanostructure-coarse-ultrafine gradient from surface to interior are identified in IGS and MGS materials, respectively. Gradient structures maintain reasonable elongation and at the same time display 3-4 times higher yield strength as compared with homogeneous coarse-grained counterpart, indicating improved strength-ductility synergy. Moreover, it is found that the fracture of gradient structures depends largely on the high internal stress caused by the heterogeneous deformation of layers.