Study of toughening mechanisms through the observations of crack propagation in nanostructured and layered metallic sheet

A layered and nanostructured (LN) 304 SS sheet was produced by combination of surface mechanical attrition treatment (SMAT) with warm co-rolling. The microstructure of LN sheet is characterized by a periodic distribution of nanocrystalline layers and micron-grained layers with a graded transition of grain size. Tensile test results show that exceptional properties of high yield strength and large elongation to fracture are achieved. A multiple interlaminar cracking was observed by scanning electron microscopy, which is induced by repeated crack initiation and propagation. The toughening mechanisms of the LN sheet are proposed to be controlling the crack propagation path by several strategies. The main cracks initiating at interface defects are arrested by large compressive residual stress, deflected by weak interface bonding and blunted by the graded grain size distribution.

► A nanostructured and layered steel exhibits high strength and large ductility.
► The excellent combination originates from a multiple interlaminar cracking.
► The initiation and propagation of cracks are controlled by three aspects.
► The cracks are deflected by interface and arrested by compressive residual stress.
► Finally, the cracks are blunted by the graded grain size distribution.