Anisotropic fracture behavior of ultrafine-grained iron

Fracture toughness measurements with ultrafine-grained bcc iron produced by high pressure torsion (HPT) are reported. The measurements were performed with respect to three different crack plane orientations, which showed pronounced differences in fracture toughness as well as in the appearance of the fracture surfaces. The mechanical anisotropy was found to be a result of the elongated and aligned submicrocrystalline microstructure. This causes intergranular fracture for the crack plane orientation of lowest toughness, simultaneously favoring a higher fracture toughness for the other specimen orientations. Since this mechanical anisotropy led to one crack plane orientation with a limited fracture toughness, a strategy for increasing the fracture toughness of this orientation is also presented.