The desirable properties of ultrafine grain (UFG) materials have prompted significant research efforts over the past decade. These materials have proven to be suitable for many industrial applications where conventional grain sized materials have limitations. Some properties of UFG materials which make them preferred over their conventional grain sized counterparts include high strength, corrosion resistance, and high shock resistance.

The understanding of mechanical behavior of UFG materials under cyclic loading still remains a challenge. Available data in the literature about UFG material subjected to cyclic loading is limited. As most engineering components experience complex stress-strain states, an understanding of multiaxial fatigue is critical in applications where reliability and optimum performance are required. The objective of this research, therefore, was to study the mechanical behavior of UFG nickel under multiaxial loading conditions.

In the first part of this research, the uniaxial fatigue behavior of UFG nickel synthesized by pulsed electrodeposition in a nickel sulfamate bath was studied. Bulk nickel cylinders, 10mm in diameter and 60mm long, were electroformed. The cylinders were machined into test specimens and cycled in fully reversed tension-compression at room temperature at different plastic strain amplitudes. The second part involved multiaxial deformation of thin-walled nickel tubes. The thin-walled UFG nickel tubes were produced by the same technique as that used in electroforming the UFG nickel solid cylinders. Thin-walled tubes were subjected to axial-torsional cyclic loading. For comparison purposes, conventional grain size (CG) nickel specimens were also tested under the same loading conditions as the UFG nickel specimens.

The UFG nickel shows high cyclic strength as compared to CG nickel under both uniaxial and multiaxial loading conditions. CG nickel shows higher effective saturation stress under nonproportional loading than proportional loading. The UFG nickel shows the opposite; higher effective saturation stress amplitudes under proportional loading than under nonproportional. Hysteresis loop shape analyses show that the reverse magnetostriction effect is reduced as a result of a decrease in grain size.

Transmission electron microscopy (TEM) images for the as-deposited UFG nickel show less evidence of dislocation structures. The post fatigued UFG nickel under uniaxial loading shows loose dislocations at low plastic strain amplitude and cellular structures at high plastic strain amplitudes. Similar dislocation structures are observed under multiaxial loading conditions. The CG nickel on the other hand, shows a vein matrix structure when subjected to both proportional and nonproportional loading at low effective plastic strain amplitude, ϵ_pa,eff = 1.0x10^-4. At ϵ _pa,eff = 1.0x10^-3 the CG nickel exhibits persistent slip bands, labyrinths, and dislocation veins under proportional loading and dislocation cells under nonproportional loading.