Regarding fracture plane with minimum toughness and fracture anisotropy associated with elongated grain boundaries in ultra-fine-grained Cu

Abstract This study is mainly focused on the relationship between the microstructure and fracture toughness of ultrafine-grained Cu produced by equal channel angular pressing. Crack initiation toughness (KIC) and crack growth toughness (TR) are quantitatively evaluated along different crack planes to identify the crack plane with minimum toughness. The results show that grain elongation plane (GEP), making an angle of 29° with the extrusion direction, exhibits the lowest fracture toughness (KIC = 54 MPa m1/2 and TR = 6), while the crack extending normal to GEP confronts the greatest fracture resistance. Both KIC and TR increase monotonically with the inclination angle between the crack plane and the GEP. The preferential micro-void nucleation and growth along the deformation-induced elongated grain boundaries lead to the formation of finely dimpled fracture facets on the fracture surface and favorable crack extension. The presence of distributed regions with elongated grains efficiently contributes to enhancement of fracture toughness by promoting microscopic crack deflections and crack path tortuosity.