Dislocation Mechanisms for Shock-Induced Hot Spots

Void nucleation in astroloy: theory and experiments

Stress corrosion cracking (SCC) of single-crystal type 321 (UNS S32100) stainless steel (SS) was investigated in 42% boiling magnesium chloride (MgCl2) solution. Tests were performed under Mode II loading to study the SCC mechanism. Maximum normal stress and maximum shear stress were located at different positions on the notch surface in the Mode II notch specimens. Results showed SCC initiated at the maximum normal stress position. No SCC occurred at the maximum shear stress position. SCC initiated on the (111) plane and propagated on the normal stress plane. Slip bands appeared around the maximum normal stress position but not in the area around the main crack. Many microcracks ~ 2 μm long were observed on the slip bands. Their direction was inclined ~ 35° to the slip bands instead of along them. Nucleation resulted from interaction of dislocation pileup stress and local anodic dissolution. Multiple microcracknucleations increased stress around crack tips of the main and microcracks. Highly concentrated local stress from dislocation pileups increased the anodic dissolution rate locally.

Rate theory investigation of influence of cascade cluster formation and solute trapping on point defect agglomeration and extended defect evolution

Point and small cluster defects in magnesium aluminate spinel have been studiedfrom a first principles viewpoint. Typical point defects that occur during collisioncascade simulations are cation anti-site defects, which have a small formationenergy and are very stable, O and Mg split interstitials and vacancies. IsolatedAl interstitials were found to be energetically unfavourable but could occur aspart of a split Mg–Al pair or as a three atom–three vacancy Al ‘ring’ defect,previously observed in collision cascades using empirical potentials. The structure andenergetics of the defects were investigated using density functional theory (DFT)and the results compared to simulations using empirical fixed charge potentials.Each point defect was studied in a variety of supercell sizes in order to ensureconvergence. It was found that empirical potential simulations significantly overestimateformation energies, but that the type and relative stability of the defects are wellpredicted by the empirical potentials both for point defects and small defect clusters.

Spinel LiMn2O4 has attracted wide attention due to its advantages of a high-voltage plateau, good capacity, environmental friendliness, and low cost. Due to different experimental synthesis methods and conditions, there are many intrinsic point defects in LiMn2O4. By means of first-principles calculations based on a reasonable magnetic configuration, we studied the formation energies, local structures, and charge compensation mechanism of intrinsic point defects in LiMn2O4. The formation energies of defects under the assumed O-rich equilibrium conditions were examined. It was found that O, Li, and Mn vacancies, Mn and Li antisites, and Li interstitial could appear in the lattice at some equilibrium conditions, but Mn interstitial is hard to form. The charge was compensated mainly by adjusting the oxidation state of Mn around the defect, except for the defects at the 8a Wyckoff site. The binding energies between point defects were calculated to shed light on the clustering of point defects. Furthermore, the diffusion of Li ions around the defects was discussed. Cation antisites led to a decrease of the Li diffusion barrier but O vacancy caused an increase of the barrier. This study provides theoretical support for understanding point defects in spinel LiMn2O4.

A study on the mobility of [formula omitted] edge dislocation in low-carbon α-Fe and its interactions with damage cascade: On picosecond time scale using molecular dynamics simulations

Temperature dependent He-enhanced damage and strain in He-implanted AlN

Interaction between transition metals and point defect clusters in silicon characterized by photoluminescence

Line, point and surface defect morphology of graded, relaxed GeSi alloys on Si substrates

Computer simulation of displacement cascades in tungsten with modified F–S type potential

Tribological properties of diamond-like carbon and related materials

Anomalies in precipitation hardening process of 7075 aluminum alloy extruded by KOBO method

Investigation on wall shear stress measurement in supersonic flows with shock waves using shear-sensitive liquid crystal coating

We have carried out a series of lateral epitaxial overgrowths (LEO) of GaN through thin oxide windows by the hydride vapor phase epitaxy (HVPE) technique at different growth temperatures. High lateral growth rate at 1100°C allows coalescing of neighboring islands into a continuous and flat film, while the lower lateral growth rate at 1050°C produces triangular-shaped ridges over the growth windows. In either case, threading dislocations bend into laterally grown regions to relax the shear stress developed in the film during growth. In regions close to the mask edge, where the shear stress is highest, dislocations interact and multiply into arrays of edge dislocations lying parallel to the growth window. This multiplication and pileup of dislocations cause a large-angle tilting of the laterally grown regions. The tilt angle is high (∼8 degrees) when the growth is at 1050°C and becomes smaller (3-5 degrees) at 1100°C. At the coalescence of growth facets, a tilt-type grain boundary is formed. During the high-temperature lateral growth, the tensile stress in the GaN seed layer and the thermal stress from the mask layer both contribute to a high shear stress at the growth facets. Finite element stress simulations suggest that this shear stress may be sufficient to cause the observed excessive dislocation activities and tilting of LEO regions at high growth temperatures.

In this paper, the probability of fatigue cracking along different kinds of grain boundary (GB) and persistent slip bands (PSBs) is considered in the light of data obtained by cyclic deformation of copper bicrystals and columnar crystals. It is found that, in copper bicrystals, fatigue cracks always nucleate and propagate along large-angle GBs, irrespective of whether the GB is perpendicular, parallel or inclined to the stress axis. On the contrary, for columnar copper crystals containing small-angle GBs, PSB-matrix interfaces become the preferential sites for initiation of fatigue crack; fatigue cracking along the small-angle GBs was never observed. For a special [1-34]/[182-7] copper bicrystal with a Sigma = 19b GB and a common primary slip plane, GB cracking also results in fatigue failure. Based on the results above, the interactions of dislocations carried by PSBs with GBs, including 'pile-up of dislocations', 'passing through of dislocations' and 'partial passing-through dislocations', are discussed. It is suggested that the probability of fatigue cracking in fatigued copper crystals increases in the order of small-angle GBs, PSBs and large-angle GBs.