A mechanism of grain growth-assisted intergranular fatigue fracture in electrodeposited nanocrystalline nickel–phosphorus alloy

Quantitative investigation of the grain growth and resultant change in grain boundary microstructure during high-cycle fatigue was performed to understand intergranular fatigue fracture in electrodeposited nanocrystalline Ni – 2.0 mass% P alloys by using FE-SEM/EBSD technique. Pre-fatigued specimens had an average grain size of 45 nm, a sharp {0 0 1} texture and a high fraction of low-angle boundaries and of twin, or Σ3 coincidence site lattice (CSL) boundaries. The considerable grain growth occurred due to the migration of low-angle boundaries induced by shear stress during cyclic deformation. The misorientation angle of those low-angle boundaries increased covering the whole surface of fatigue-fractured specimen. A certain fraction of low-angle boundaries was transformed into high-angle random boundaries resultant from grain growth during high-cycle fatigue. Those random boundaries which surrounded the grown {0 0 1}-grains were aligned along shear bands at almost 45° to the stress axis, and formed the diamond-shaped grain configuration, as reported in the literature on high temperature fatigue. The reported increase of the fatigue limit by nanocrystallization is likely reduced due to the cyclic stress-induced grain growth associated with the migration of low-angle boundaries composing nanograin cluster. Moreover, the random boundaries transformed from low-angle boundaries can be preferential sites for crack nucleation and propagation at the positions of initially formed shear bands during fatigue.