Atomistic simulation of thermally activated glide of dislocations in Fe---Ni---Cr---N austenite

Thermally activated glide of edge and screw dislocations with the Burgers vector b = a/2110 in Fe---Ni---Cr austenite without and with nitrogen has been analyzed using the conjugate gradient method to minimize the potential energy of the crystal (subject to appropriate constraints) and the embedded-atom method to quantify the atomic interactions. The results of the analyses were compared with their experimental counterparts. In austenite without nitrogen, the variation of the crystal energy with the dislocation position gives rise to a Peierls frictional stress. However, the magnitude of this stress is relatively low and both edge and screw dislocations can readily overcome it by the assistance of thermal activation. The presence of nitrogen, on the other hand, causes the core of a screw dislocation to dissociate onto two {111} planes containing the dislocation line. This renders the screw dislocation sessile. Such changes in the core structure of an edge dislocation were not observed. Instead, the interaction of nitrogen with edge dislocations was found to give rise to both an athermal (long-range) and a thermal (short-range) component of the frictional stress. A detailed analysis of the interactions between the nitrogen atoms and the edge dislocations suggests that the athermal component of the frictional stress observed in the present work is most likely responsible for the experimentally observed athermal flow stress in Fe---Ni---Cr polycrystalline austenite. On the other hand, with a possible exception of very low temperature, the contribution of the thermal frictional stress associated with the interaction of edge dislocations with nitrogen to the strength of Fe---Ni---Cr polycrystalline austenite is small.     

A 2xxx aluminum alloy crept at medium temperature: role of thermal activation on dislocation mechanisms

Fuselage skin of the future supersonic civil transport aircraft requires a material resistant to long-term creep at temperatures ranging from 100 to 130 °C. A candidate is the 2650 aluminum alloy which presents such properties at relatively high temperatures (130 °C=0.43T_melting). From an accelerated creep test conducted at 150 °C under a 280 MPa load and subsequent observations of the dislocation microstructures by transmission electron microscopy, the role of thermal activation on dislocation mechanisms is analyzed. It appears that thermal activation favors cross-slip activity and allows dislocations to glide in non-close-packed planes, namely the {0 0 1} planes. This is the first time that evidence of primary {0 0 1} glide is reported. Associated to a strain-assisted decrease of the precipitate density, thermal activation softens the material and seems to contribute to the acceleration of the strain rate (tertiary stage) by facilitating bypassing of precipitates and the production of mobile dislocations. The different creep stages are explained in terms of individual dislocation mechanisms and not by referring to the evolution of dislocation substructures.     

Dislocation structures on the plane of shear in compressed double-notch copper single crystal specimens

Dislocation structures in the zone of shear of double-notch copper single crystals, generated by a shear stress 20 MPa acting in the specimen axis ([110] direction) at 773 K were investigated by means of transmission electron microscopy. Dislocation configurations in different crystal planes of shear, and (001), were compared. The role of the 110{111} slip systems in the process of shear is assessed and possible indications of a direct glide of dislocations with Burgers vector b=a/2[110] in the mentioned compact and non-compact planes are presented.     

Low frequency fatigue crack growth behavior of a470 class 8 rotor steel at 538°C (1000°F)

Fatigue crack growth rate data were developed at various frequencies and hold times at maximum load for A470 Class 8 steel at 538°C (1000°F) by using an accelerated test method which involves alternating test frequency and temperature. These data were consistent with fatigue crack growth rate data obtained from the same material and developed according to the ASTM specification E-647-T78. This result suggests that there is no transient effect associated with the alternating test frequency and temperature and that the accelerated testing procedure can be used to expedite the development of elevated temperature fatigue crack growth rate data at very low frequencies and long hold times. At 538°C (1000°F) fatigue crack growth properties with hold time developed from both 1T-CT and multiple-edge-craek tension specimens fall in the same scatter band on the da/dN vs ΔK plot. This result indicates the applicability of ΔK to characterize the fatigue crack growth behavior with hold time at elevated temperature. Also, the model proposed by Saxena et al. was found to successfully predict the fatigue crack growth rate properties with 28 min hold time of the A470 Class 8 rotor steel at 538°C (1000°F).     

Bearing strength of carbon epoxy laminates under static and dynamic loading

An experimental study has been carried out to investigate both the static and dynamic bearing strengths of a pinned-joint carbon epoxy composite plate with [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking configurations. The static and dynamic experiments have been carried out according to the ASTM D953 standards and ASTM STP 749, respectively [ASTM D 953-D, Standard Test method for Bearing Strength of Plastics, ASTM Designation. 342; Joining of Composite Materials, ASTM STP 749, American Society for Testing and Materials (1981) 131]. The ratio of the edge distance to the pin diameter (E/D), and that of the width to the pin diameter (W/D) of the specimens were varied to obtain the static bearing strength and the S–N fatigue curve. The experiments show that the static bearing strengths reach their upper limit when E/D and W/D ratios are equal to or greater than 4 for both [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking sequences. The fatigue strength, on the other hand, reduces by up to 65% as E/D and W/D ratios increase for both stacking configurations.     

Transmission electron microscopy observations of misfit dislocations in GaAsP epitaxial films

Transmission electron microscopy observations have been made of misfit dislocation structures in GaAsP epitaxial films in foils both parallel to (1) the interface between the epitaxial film and the substrate and (2) the {1 1 1} glide planes. These observations support a near surface source mechanism of dislocation multiplication for relief of the epilayer misfit. It is also suggested that the recently observed surface reconstruction in the III-V compounds might allow for an easier nucleation of dislocations at the surface than hitherto thought. Furthermore, an efficient Lomer dislocation has been observed forming from two 60° glide dislocations thus supporting the hypothesis that all dislocations found in these foils, including the sessile Lomer type, originate from a glide process.     

Determination of the fracture toughness of fabric reinforced composites by the J-integral approach

The fracture behaviour of a glass-fabric-reinforced epoxy composite has been investigated experimentally. Load-displacement curves for single-edge-notched specimens were obtained on an MTS system and the J-integral evaluated through its energy rate interpretation. J_c, the critical value of the J-integral, obtained directly for a₀/w > 0·4 and that obtained through an extrapolation procedure for a₀/w < 0·4 compare quite well. J_c appears to be independent of crack length for specimen widths between 15 and 45 mm. J_c for ±45° specimens is less than half that for 0/90 specimens.     

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von –184 bis 600°C

Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm³ was investigated in tension tests in the temperature range of –184 and 600°C. Supplementary compression tests were carried out at 20°C. Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores. After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed. The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes.     

Transmission electron microscopy structural characterisation of GaN layers grown on (0001) sapphire

GaN layers grown by MOCVD or by MBE on (0001) sapphire have been characterised by transmission electron microscopy (TEM). We make a review of the different crystallographic structures found in theses GaN layers. We comment shortly on the nitridation of the sapphire and the structure of the buffer layer (BL). We point out that the roughness of the BL can be an important parameter for releasing the residual strain of the GaN layer. We compute the Keating energies of the main inversion domain boundaries (IDBs) and translation domains boundaries (TDBs) observed in some GaN layers. The observed structures correspond to the lowest energy models. Perfect dislocations have Burgers vectors equal to a, a+c and c. The dislocation lines are generally parallel to the c-axis. a-Edge dislocations are generally not dissociated and we propose an atomic model for them. Screw dislocations with a Burgers vector equal to c, can ‘open and close' during growth leaving holes (the so-called nanopipes) in the structure.     

About thaumasite formation in Portland-limestone cement pastes and mortars––effect of heat treatment at 95 °C and storage at 5 °C

Owing to the presence of finely divided calcite, mortars and concretes made with Portland-limestone cements are particularly susceptible to damaging thaumasite formation during sulfate attack at lower temperatures. This work reports the results of investigations on mortars made according to DIN/EN 196 and pastes (w/c ratio of 0.5) with CEM I 42,5 R, as well as with mixtures of cement with limestone filler. Some of the samples were heat-treated at 95 °C. The length changes and resonant frequencies of the samples were measured during long-term water-storage at 20 and 5 °C. There was no evidence from X-ray diffraction data of thaumasite formation in the samples. Only for pastes containing 30 wt.% limestone filler were small areas found by SEM and X-ray microanalysis whose chemical analysis matched thaumasite or a thaumasite–ettringite solid solution.     

Low ductility at intermediate temperature of Ni–base superalloy M963

Tensile behavior of a cast Ni–base superalloy M963 under solution treatment and age treatment was studied in the temperature range from 20 to 1100 °C. Extensive TEM investigations were performed after tensile test to fracture. Furthermore, the fracture surfaces were studied in the SEM. The yield and tensile strengths under the two conditions initially increase with temperature and reach a peak at around 800 °C. Beyond this temperature, a sharp decrease of both yield and tensile strengths was observed. A ductility minimum was observed at 800 °C under solution treatment and disappeared under age treatment. With the increment of temperature, the following sequence of deformed substructure features was observed: dislocation pairs → connected slip bands within matrix channel under solution treatment and homogeneous interface dislocations under age treatment → homogeneous dislocation network within matrix channel. The fracture surface observation indicated that localized slip which leading to glide plane decohesion caused the poor ductility of M963 alloy.     

Behavior of notched and unnotched [0/±30/±60/90]s GFR/EPOXY composites under static and fatigue loads

The main objective of the present paper is to study the bending behavior of notched and unnotched angle-ply, [0/±30/±60/90]_s, glass fiber reinforced epoxy (GFRE) composites under static and fatigue loads. Static and fatigue bending properties have been determined for notched and unnotched angle-ply specimens. For this purpose different circular notch sizes (2, 4.5, 7, 9 mm) were drilled at the specimen center. Constant-deflection bending fatigue tests were performed at zero mean stress and 25 Hz. A 15% reduction of the initial applied moment was taken as a failure criterion. S–N diagrams for notched GFRE specimens have been constructed based on gross and net cross-section area. The results show that the ultimate bending strength of notched GFRE specimens decreased linearly with increasing notch diameter. Based on gross-section the fatigue life increases with decreasing notch size and the longer fatigue life was for the unnotched specimens. On the other hand, the S–N diagrams based on net-section indicate the insensitivity of angle-ply composites to the notch size. This is considered to be a peculiar phenomenon to composite materials. The results also show that the S–N diagrams have not any fatigue limit rigorous within 10⁷ cycles.     

CO-DEPOSITION OF FULLERENES AND SULFUR FROM DILUTED SOLUTION

A new method to synthesize fullerene and sulfur compounds has been developed. Using this method, C₆₀S₁₆ and C₇₀S₁₆ compounds were grown from dilute fullerene and sulfur toluene solution. Their atomic structures were analyzed by x-ray diffraction with the single crystal. The C₆₀S₁₆ crystal is C-centered monoclinic structure of a=2.0874 nm, b=2.1139 nm, c=1.05690 nm and β=111.93°, and the C₇₀S₁₆ has a primitive monoclinic, P2₁/c, with lattice parameters of a=1.5271 nm, b=1.49971 nm, c=2.18024 nm and β=109.791°. In this compound, the structure of fullerenes is maintained and sulfur atoms form S₈ rings placed around the fullerenes.     

Strengthening via deformation twinning in a nickel alloy

Nanograins and nanotwins are produced in specimens using one processing technique to allow direct comparison in their nanohardnesses. It is shown that the hardness of nanotwins can be close to the lower end of the hardness of nanograins. The resistance of nanotwins to dislocation movement is explained based on elastic interactions between the incident 60° dislocation and the product dislocations. The latter includes one Shockley partial at the twin boundary and one 60° dislocation in the twinned region. The analysis indicates that a resolved shear stress of at least 1.24 GPa is required for a 60° dislocation to pass across a twin boundary in the nickel alloy investigated. It is this high level of the required shear stress coupled with a limited number of dislocations that can be present between two adjacent twin boundaries that provides nanotwins with high resistance to dislocation movement. The model proposed is corroborated by the detailed analysis of high-resolution transmission electron microscopy.     

Thermal and mechanical fatigue analysis of CFRP laminates

[0°/90°]_s and [±45°]_s CFRP laminated plates were analysed using a finite element formulation for their fatigue behaviour. A fatigue criterion which is based on the laminate interlaminar stresses and the basic lamina fatigue parameters was used. Thermal effects were included in the formulation. In particular, initial thermal stresses resulting from the curing of the laminate were also included in the analysis. The results showed that both laminates had predicted S-N behaviour similar to that from experiments of past investigators. Also, the fatigue behaviour for the [±45°]_s laminate between room temperature and the curing temperature were found to be the same. However, in the case of the [0°/90°]_s laminate the fatigue strength at high temperatures was found to be lower than that at low temperatures.     

Growth mechanism of stress corrosion cracking in high strength steel

Stress corrosion crack growth rates are measured at sveral stress intensity levels for low-tempered 4340 steel in 0.1N H₂SO₄ solution. The characteristics of the growth rates are divided into three regions of stress intensity factors: Region I near K_1SCC; Region III near unstable fracture toughness, K_1SC; and Region II, which lies between the two. K_1SCC is the value of K at which no crack growth can be detected after 240 hr.

In order to explain these experimental results, the crack initiation analysis reported in a previous paper is extended to the growth rates. A detached crack initiates and grows at the tip of an already existing crack. When the detached crack reaches the tip of the main crack, the process repeats as a new existing crack.

A relationship between crack growth rate, v, and stress intensity factor, K, is obtained as a function of b/a and a = b + d, where b is the distance from the tip of the main crack to the detached crack, and d is the ydrogen atom saturated domain.

The experimental data are in good agreement with the theoretical values in Region II when a = 0.02 mm, b/a = 0.8, c₁/c₀ = 2.8 for 200°C tempered specimens and a = 0.015 mm, b/a = 0.7, c₁/c₀ = 3.0, ρ_b = 0.055 mm for 400°C tempered specimens, where ρ_b is a fictitious notch radius. The plateau part in Region II for 400°C tempered specimens is also successfully explained by the present theory. For Region III, the value of b/a will be almost equal to 1 because v → ∞ for b/a → 1. On the other hand, for Region I, b/a will be zero, since the value of v becomes negligibly small and no crack growth is observable.     

Atomistic simulation of misfit dislocation in metal/oxide interfaces

In this work, we use a Chen–Möbius inversion method to get the interatomic potentials for metal/oxide interfaces, and then study the misfit dislocation in a series of interfaces, including Au/MgO, Rh/MgO and Ni/MgO. The calculation shows that dislocation line always prefers at the first monolayer of metal side, with metal on top of Mg at the dislocation core, and metal on top of O at the interface coherent area. Also, the Burgers vector for these interfaces is determined at two cases. For Rh/MgO and Ni/MgO, it keeps the value of . But for Au/MgO, it changes from to a[1 0 0] as the number of monolayers in metal side increases. This work shows a theoretical understanding of misfit dislocations in metal/oxide interfaces, from dislocation structure, density to Burgers vector orderly, and gives some hints to experiments.     

Dislocations studies in β-silicon nitride

Some preliminary results of dislocation analysis and associated glide systems in as-grown as well as in superplastically deformed β silicon nitride are presented. Transmission electron microscopy observations using the weak-beam technique, are reported. [0 0 0 1] {1 0 1 0} and 1/3〈1 2 1 0〉 {1 0 1 1} glide systems have been characterized. In the basal plane, a superposition of two hexagonal networks built with screw dipoles has been observed. Both a sequence extended node–constricted node and partial dislocations have been identified in these networks which clearly evinces dislocation dissociation in β-silicon nitride following the reaction 1/3〈2 1 1 0〉→1/3 〈1 0 1 0〉+1/3 〈1 1 0 0〉. This revised version was published online in November 2006 with corrections to the Cover Date.     

Dislocation nucleation and propagation during thin film deposition under compression

In this paper, we study the nucleation of dislocations and their subsequent propagation, during thin film deposition, using the three-dimensional (3D) molecular dynamics (MD) method. Aiming to reveal the generic mechanisms, the case of tungsten on a substrate of the same material is investigated. The substrate is under uniaxial compression along the [1 1 1] direction, with the thermodynamically favored surface being horizontal. The simulation results indicate that the nucleation starts with a surface step where an atom is squeezed to the layer above, generating a half-dislocation loop at the surface. It may then either propagate into the film or become the bottom of a sessile dislocation loop. In the first case, the dislocation loop, having a Burgers vector on a (1 0 1) glide plane, propagates along the direction on the surface, and extends to about two atomic layers along the [1 1 1] direction. In the second case, the missing layer propagates along the [1 0 0] direction on the surface, extending to about four atomic layers along the [1 1 1] direction. In this case, the sessile dislocation has a Burgers vector on the plane (0 1 1).     

Mechanics and mechanisms of delamination in a poly(ether sulphone)—Fibre composite

The interlaminar fracture behaviour of AS4/PES (poly(ether sulphone)) composite has been investigated in Mode I, Mode II and for fixed Mode I to Mode II ratios of 0·84, 1·33 and 2·13. The data obtained from these tests have been analysed using several different analytical approaches. The results obtained show that in Mode I the interlaminar crack growth in double cantilever beam (DCB) specimens is accompanied by fibre bridging behind the crack tip and by splitting at the crack tip, and in Mode II by the formation of a damage zone at the crack tip. These failure mechanisms are shown to increase the value of the interlaminar fracture energy considerably as the crack propagates through the composite, i.e. a rising ‘R-curve’ is measured. It is shown also that the value of the interlaminar fracture energy at crack initiation in Mode I, G_CI (init), increases as the length of the initial precrack is increased. The lowest G_IC (init) value obtained for the poly(ether sulphone) (PES) composite in this study is 0·8 kJm⁻², and this value was ascertained from a specimen with the precrack being grown by about 2 mm ahead of the initial crack (a₀ = 23 mm, a_p = 25 mm). The typical Mode II steady-state propagation energy, G_IIC (s/s-prop), value obtained for the specimens was about 2·0 kJm⁻². The length of the initial precrack had no significant effect on the G_IIC (init) and G_I/IIC (init) values. The Mode II tests gave values of G_IIC (init) = 1·25 kJm⁻² and of G_IIC (s/s-prop) = 1·85 kJm⁻². Finally, the failure loci for the PES composite have been constructed and theoretical expressions to describe these data considered.