Grain Structure at Crack Path in Fatigued Nano-Crystalline Ni

Abstract

The generally acceptable mechanisms of fatigued crack growth in nanocrystalline (NC) materials were based on empirical studies and molecular dynamics (MD) simulations of pure Ni films [1,2]. MD simulation suggested that fatigued crack growth is due mainly to crack-tip blunting, nanovoid formation, subsequent decohesion at grain boundaries [2] while Meirom et al. suggested a crack propagation rate dependent and dislocation-slip mechanism [3]. However, these studies were based on two-dimensional thin films. The lack of 3-dimensional constraint and a high volume fraction of the surface, 2-D thin-film samples may not represent the mechanical behavior of bulk polycrystalline materials [4]. Furthermore, the investigations of fatigued crack structures in NC materials have been limited to surface studies using SEM; the lack of knowledge on the inner grain structure at the crack path impedes the further understanding of fatigued crack growth mechanism in NC materials. A crack propagation study of sufficient thick, full condensed 3-dimensional fatigued NC Ni with the emphasis of grain structure at crack path using TEM was thus undertaken.Bulk NC Ni-99.9% sheets (0.5 mm in thickness with average grain size of 29 nm) were fabricated using a pulsed electrodeposition technique (Integran Technologies Inc., Canada). Testing specimens (37.5mm x 3.81mm x 0.5 mm) were machined from the as-deposited sheets by electrical discharged machining (EDM) (Misubishi DWC90C with 0.25 mm brass wire), followed by mechanical polishing along the longitudinal direction to flatten the surface (final thickness = 0.43 mm) and remove transverse scratches that may serve as crack precursors. Tests were conducted with a three-point bend fatigue system under deflection-controlled conditions. After examining the crack/fracture surfaces with a SEM, crack/fracture surfaces were protected by electrodepositing a thick Ni layer to prevent possible artifacts before/during preparing TEM specimens.