Annealing-induced hardening of laminated structured nickel fabricated by electrodeposition

Abstract

Annealing of deformed metals is considered as a process which necessarily leads to softening, due to the annihilation of lattice defects. However, in laminated materials, annealing at moderate temperatures may induce hardening. In this study, a laminated structure at two sides of the sample is produced by ultrasonic introduced intermittently for 15 min, during the electrodeposition process. For comparison, a sandwich structure is prepared under ultrasonic without interruption at two sides of the sample. All the samples are annealed for 30 min at 373 K, 473 K and 573 K. The common softening after annealing is observed for the sample without laminated structure in the two sides. However, for the other sample that possesses laminated structure in the same area at the two sides, the strength and ductility increase rather than decrease after annealing. The sample that possesses laminated structure in the same area at the two sides annealed at 373 K shows an evident change, which increases in σ 0.2 from 299 to 353 MPa, in σ UTS from 477 to 533 MPa, and in ε ue from 7.6% to 9.5%. According to the XRD results, annealing-induced hardening is not attributable to occurring phase transformation. Detailed microstructural TEM results demonstrate that the annealing-induced hardening is attributed to the surface laminated interfaces and high-density growth twins in interfacial transition zones. In addition, the detwinning occurred during subsequent tensile deformation, also plays a crucial role.