Approaching the refinement limit of nano laminated structures in cubic metals by severe plastic deformation

Grain size refinement stagnates at the balance between refinement by dislocation accumulation and coarsening by thermal recovery for generally equiaxed structures upon increase of plastic strain. It is curious if this stagnation occurs also for laminated structures. In this paper, three kinds of metal with nanolamellar (NL) structure were successfully prepared by the method of equal channel angular processing (ECAP) followed by liquid nitrogen rolling (LNR). Their lamellar spacings reaches 41 nm, 35 nm, and 29 nm for pure nickel, tantalum, and niobium respectively; And their tensile strength reached 1.6 GPa, 1.2 GPa and 1 GPa, respectively. The concept of physical refinement limit of the lamellar spacing obviating the effect thermal recovery was proposed and modeled. A model of lamellar spacing stagnation at the balance between geometrical refinement and thermal coarsening induced by the triple junction migration have been established. The experimentally achieved average lamellar spacing given above of the three metals at 95% LNR reduction subsequent to 8 passes of ECAP is above the modelled lamellar spacing at stagnation, indicating a potential for further refinement of the lamellar spacing for all the three metals at higher LNR reduction. At the same time, the theoretical derivation reveals that the triple junction proliferation caused by the inevitable shear band in the rolling process is the key governing the refinement limit of the nanolamellar structure.