Formation of excess atomic volume and its role in the processes of fracture of nickel single crystal

Molecular dynamics simulation of crack propagation peculiarities in a nickel single crystal under uniaxial tension along the cubic direction was carried out. It was found that at room temperature regions with excess atomic volume are formed near the tips of the opening crack. Subsequently nanopores are formed in these areas which then merge with the crack stimulating high-speed opening. It is shown that if dislocations begin to form at the crack tip in a region with an increased atomic volume the crack propagation velocity in this direction significantly decreases.

Large values of transverse magnetic reduction of nickel single crystal films

In this work we studied the anisotropy of the transverse magnetoresistance of single-crystal nickel films. The measurements were carried out on samples whose surface plane coincided with the [001] cavity. Studies of magnetoresistance in a monocrystalline nickel film showed tensile stresses acting on it from the side of magnesium oxide. The modification of the magnetization anisatropy of the film on the substrates as compared with the free sample is apparently associated with a change in the shape of the Fermi surface of the carriers.

Multiscale analysis of hydrogen-induced softening in f.c.c. nickel single crystals oriented for multiple-slips: elastic screening effect

Hydrogen-deformation interactions and their role in plasticity are well accepted as key features in understanding hydrogen embrittlement. In order to understand the nature of the hydrogen-induced softening process in f.c.c. metals, a substantial effort was made in this study to determine the effect of hydrogen on the tensile stress-strain behavior of nickel single crystal oriented for multiple-slips. It was clearly established that the hydrogen softening process was the result of a shielding of the elastic interactions at different scales. Hydrogen-induced softening was then formalized by a screening factor S of 0.8 ± 0.05 for 7 wppm of hydrogen, which can be incorporated into standard dislocation theory processes. The amplitude of softening suggests that the shielding process is mainly responsible for the stress softening through the formation of vacancy clusters, rather than a direct impact of hydrogen. This effect is expected to be of major importance when revisiting the impact of hydrogen on the processes causing damage to the structural alloys used in engineering.

Investigation of Grain Refinement Mechanism of Nickel Single Crystal during High Pressure Torsion by Crystal Plasticity Modeling

The excellent properties of ultra-fine grained (UFG) materials are relevant to substantial grain refinement and the corresponding induced small grains delineated by high-angle grain boundaries. The present study aims to understand the grain refinement mechanism by examining the nickel single crystal processed by high pressure torsion (HPT), a severe plastic deformation method to produce UFG materials based upon crystal plasticity finite element (CPFEM) simulations. The predicted grain maps by the developed CPFEM model are capable of capturing the prominent characteristics associated with grain refinement in HPT. The evolution of the orientation of structural elements and the rotations of crystal lattices during the HPT process of the detected differently oriented grains are extensively examined. It has been found that there are mainly two intrinsic origins of lattice rotation which cause the initial single crystal to subdivide. The correlation between the crystallographic orientation changes and lattice rotations with the grain fragmentation are analyzed and discussed in detail based on the theory of crystal plasticity.