The acceleration of dissociated dislocations to transonic and supersonic velocities in copper fcc crystals is investigated using molecular dynamics simulations. A thin and long system with a single stationary dislocation is constructed to study the dislocation acceleration process in anisotropic materials, which have two transverse, νT1 and νT2 , and one longitudinal acoustic velocity, νL along the dislocation glide direction. Copper is used as the representative anisotropic material and the embedded atom model is used to calculate interatomic forces. The common neighborhood parameter and local stresses are used to monitor the position and structure of the dislocations. Initial stationary dislocations on the (111) plane, aligned along the direction are accelerated in the direction in two different ways. By using the shear stress of a homogeneously sheared simulation box, and by using a strain-rate deformation obtained by shearing the top and bottom atomic layers in opposite directions by a given velocity. Results show that different levels of stress make dislocations accelerate and move in three distinguished regimes: i) in a plateau of velocities close to 1.6 km/s in the subsonic regime just below νT1; ii) in a narrow range of velocities around 2.6 km/s in the first transonic regime between νT1 and νT2; iii) in the second transonic regime, above νT2 but below νL, with increasing velocities for increasing stresses. Supersonic dislocations moving above νL are generated but their motion is transient. To be generated, they require high stresses above the shear strength which trigger spontaneous nucleation of dislocation dipoles throughout the system. The stacking fault width fluctuates around 35 Å in the subsonic regime but decline subsequently with velocity and fluctuates around 13 Å in the second transonic regime. In the first transonic regime however the stacking fault anomalously increases with velocity. Both the decreasing stacking fault width for fast dislocations and the plateau of velocities in the first transonic regime, indicating the existence of a radiation-free state, are in agreement with theoretical predictions.

The magnitude and distribution of elastic strain for a nickel alloy 600 (A600) sample that had been subjected to uniaxial tensile stress were measured by micro Laue diffraction (MLD) and neutron diffraction techniques. For a sample that had been dimensionally strained by 1%, both MLD and neutron diffraction data indicated that the global residual elastic strain was on the order of 10−4, however the micro-diffraction data indicated considerable grain-to-grain variability amongst individual components of the residual strain tensor. A more precise comparison was done by finding those grains in the MLD map that had appropriate <hkl> oriented in the specific directions matching those used in the neutron measurements and the strains were found to agree within the uncertainty. Large variations in strain values across the grains were noted during the MLD measurements which are reflected in the uncertainties. This is a possible explanation for the large uncertainty in the average strains measured from multiple grains during neutron diffraction.

High-temperature constant-force indentation creep tests of 200 seconds duration were performed on an annealed gold specimen at 473K to 773K, to investigate the dependence of the micro-/nano-indentation deformation kinetics upon indentation stress, temperature and time. The indent stress displayed a clear indentation size effect at 473 K. An analysis of the measured indentation creep rate, and its dependence upon temperature and stress, indicate that the strength of the deformation rate limiting obstacles increases with temperature. This is consistent with the expected temperature dependent evolution of the dislocation cell structure whose boundaries become the primary obstacles to dislocation glide.

Molecular dynamics (MD) simulations are reported for buckling and interfacial friction of single- and multi-wall carbon nanotubes (CNT) with interwall sp3 bonds imprinted on copper substrates. A small perturbation of mechanical vibration is applied to the systems in the nanoimprinting. The imprinting capabilities of multi-wall CNTs are much better than single-wall CNTs. While the single-wall CNT is insensitive to the vibration, the indentation force and buckling of multi-wall CNTs with interwall sp3 bonding is dependent on the amplitude of the perturbation, providing a way to controlling the interfacial friction. There is an optimal amplitude, at which the buckling and friction force of the CNTs are minimized in nanoimprinting.

Degradation of carbon nanotubes (CNTs) significantly affects CNT's electron emission capability and limits their applications as electron emitters. In this study, CNTs were placed in controlled environment and their degradations were studied. Two types of CNT degradations, thermal and chemical degradations were identified and their effects on electron emission were studied. It was found that CNTs placed in low vacuum environment at high temperature were subject to oxidation that led to severe degradation of CNTs and their electron emission ability, whereas, CNTs treated at the same high temperatures in an ultrahigh vacuum environment only suffered from morphological changes that had minor impact on electron emission. The CNT samples used in this study were grown on tungsten substrates via plasma enhanced chemical vapor deposition (PE-CVD). The surface morphology changes of the CNT emitters were examined using scanning electron microscopy (SEM). The field emission properties of the CNTs were measured and correlated to the morphology changes.

Nanoindentation tests are widely used in recent years to characterize the mechanical properties of viscoelastic-plastic materials like polymers and biomaterials at the micro or nano-scale using the analysis method proposed by Oliver & Pharr (OP). However, recent studies revealed that the mechanical properties of viscoelastic-plastic (polymeric) materials determined using the OP method does not lead to a correct evaluation of Young's modulus. A systematic experimental study is performed with different indenter geometries like spherical and Berkovich geometries using various polymers in order to identify the limitations of the OP method.

Switched phase change material in Phase Change Random Access Memory (PCRAM) is confined within a solid surrounding. As a result of mechanical properties and microstructure differences between the crystalline and the amorphous phases, strains and stresses are generated and may degrade the performance of PCRAM devices. This paper investigated the crystallization-induced stress in phase change Ge2Sb2Te5 (GST) nano film. The electric-thermal and thermo-mechanical simulation results show that the increases of both of the Young's modulus and Coefficient of Thermal Expansion (CTE) are responsible for the stress generation upon crystallization. The XRD studies correlate the strains and stresses with the lattice deformation in crystalline GST films.