The effect of anisotropic microstructure on the crack growth and fatigue overload behaviour of ultrafine-grained nickel

Abstract

Changes in crack growth rate associated with overload events during fatigue are poorly understood, especially for materials with anisotropic microstructures. Here overload fatigue tests are reported for compact tension samples cut in two different orientations from high pressure torsion disc samples. During growth the crack planes reoriented either slightly, or significantly, to align with the elongated grain structure leading to low, and high, levels of mixed mode fatigue loading respectively. In both cases the ultrafine grained microstructure led to macroscopically flat crack faces. The fatigue crack growth rate was around 3.4 times slower for the case with the high mode II component than for the low. A 100% overload was then introduced and synchrotron X-ray diffraction and digital image correlation (DIC) were applied in-situ to map the bulk crack-tip elastic strain field (plane strain) and surface displacement field (plane stress) respectively prior to, during and after overload. The high mode II case displayed a larger degree of retardation after overload. Residual stress and plasticity-induced crack closure were found to be the primary causes for the retardation as the crack grows into the overload plastic zone. Significant crack face contact was observed for the high mode II case along with significant levels of compressive stress transferred across the crack faces at minimum load. Compared with conventional (coarse) grain Ni, the ultrafine grained Ni is less retarded by overload, because of its relatively flatter crack path and higher yield stress and thus less plasticity and residual stress induced closure.

Metal hybrids processed by high-pressure torsion: synthesis, microstructure, mechanical properties and developing trends

“…The small dimensions of conventional HPT samples (∼ 10 mm diameter and ∼ 1 mm thickness) limit the evaluation of their mechanical properties beyond microhardness using standard testing geometries and procedures. However, several works have reported stress-strain curves from uniaxial tensile testing [174][175][176][177][178][179][180], as well as stress vs. number of cycles to failure curves [181][182][183] or crack growth curves [184][185][186] from fatigue testing using specimens taken from HPT-processed pure metals and alloys. Global mechanical properties, including ultimate tensile strength (UTS), yield strength (YS), and elongation-to-failure (ε f ), have also been reported from different HPT-processed hybrids.…”

Influence of Microstructure on Synchrotron X-ray Diffraction Lattice Strain Measurement Uncertainty

“…A broad study of measurement uncertainty has been carried out on previously published experimental work [14][15][16][17][18][19], with SXRD data from a total of 8 experiments being systematically analyzed. These experiments ran across 7 synchrotron beamtime sessions and are detailed in Table 1.…”

“…For completeness, the process followed by pyXe is briefly described in detail in Section 2.2. The software has been used previously to analyze the data from a number of synchrotron experiments (e.g., see [14][15][16][17]24]) results of which were possible compared with widely used software such as Fit2D [23]. While the code was evaluated against other results to the best of authors' ability, it is difficult to carry out a full evaluation of the code without significant resources thus care should be taken when using it.…”