The effects of cold rolling thickness reduction and path on the microstructure and mechanical properties of NiCoCr alloy

Bulk NiCoCr alloy samples with different volume fraction of nano-twins and nano-grains were fabricated by cold rolling with various thickness reduction and strain path. Transmission electron microscopy observations indicate that the microstructure in the one-directional rolled sample with 50% thickness reduction is characterized by nano-twins and high density of dislocations. With increasing the rolling thickness reduction to 90%, the volume fraction of nano-twins decreases due to the activation of shear banding within the nano-twined lamellar area, which consequently leads to the formation of massive nano-grains with average sizes of 35 nm. The microstructure of the multi-directional rolled sample is mainly composed of nano-grains with main sizes of 27 nm, which is slightly smaller than that of one-directional rolled sample. Tensile test results show that the strength of the one-directional rolled sample with 50% thickness reduction is significantly enhanced with yield strength and ultimate tensile strength reaching 1 312 MPa and 1 396 MPa, respectively, which is much higher as compared to the coarse-grained alloy with solid solution heat treatment. The yield strength is further increased to 1 599 MPa with thickness reduction reaching 90%, and multiple-directional rolling gives rise to a higher yield strength of 1 705 MPa. Tensile testing results indicated that the grain refinement could significantly increase the strength, but deteriorate the ductility to some extent.