An enhanced fracture toughness of bulk Cu sample processed by dynamic plastic deformation with a composite microstructure of nanoscale grains and embedded nanoscale twin bundles was reported. The improvement in fracture toughness was mainly due to the embedded twin bundles which generate elongated deep fracture dimples that consume much higher fracture energy.

Enhanced Fracture Toughness of Bulk Nanocrystalline Cu with Embedded Nanoscale Twins. E.W.Qin, L.Lu, N.R.Tao, K.Lu: Scripta Materialia, 2009, 60[7], 539-42