Interactions of persistent slip bands with a grain boundary on the common primary slip plane in a copper bicrystal

In this letter, the dislocation patterns on the common primary slip plane in a fatigued \[1-34]-\[182-7] copper bicrystal with a Sigma=19b grain boundary (GB) have been investigated using the electron channelling contrast technique in a scanning electron microscope. The results show that the two-phase dislocation structure, such as veins and persistent slip band (PSB) walls, embedded within veins, can be clearly seen on the common primary slip plane. In particular, the interactions of PSBs with the GB are clearly revealed. It is found that there are three kinds of interaction mode between the GB and the dislocations during cyclic deformation, and those are discussed. It is suggested that the dislocations carried by PSBs cannot transfer through the GB continuously even though the bicrystal has a common primary slip plane and its surface slip bands are continuous across the GB.     

Application of electron channelling contrast to the investigation of strain localization effects in cyclically deformed fcc crystals

The dislocation substructure and the glide activity in cyclically deformed nickel single crystals have been studied using the channelling contrast of backscattered electrons in a scanning electron microscope. Dislocation arrangements which arise in the saturation region after cyclic loading at room temperature and at one elevated temperature are considered. The electron channelling contrast technique is shown to be a useful instrument for the qualitative and quantitative characterization of the dislocation pattern on a macroscopic and a mesoscopic scale. The correlation between the specific dislocation structure in persistent slip bands PSBs and the localized glide activity of PSBs and PSB macrobands are considered.     

Temperature dependence of the low-cycle fatigue behaviour of a Cu-SiO2 bicrystal with a [011], 18° twist boundary

The temperature dependence of the low-cycle fatigue behaviour of a Cu bicrystal containing dispersed SiO₂ particles and having a [011], 18° twist boundary has been studied. Failure occurred at shorter times with increasing temperature and stress amplitude. Crack nucleation took place at the particles' surfaces on the grain boundary where slip lines intersected. The crack tended to propagate along primary slip lines and this tendency became stronger as the temperature was increased.     

Survey of plateau behaviour in the cyclic stress-strain curve of copper single crystals

Experimental results relating to the plateau behaviour in the cyclic stress-strain (CSS) curve of copper single crystals located on different sides of the stereographic triangle are summarized. Unlike the situation for single-sliporiented crystals, the crystallographic orientation has a strong effect on the plateau behaviour in the CSS curves of double- and multiple-slip-oriented crystals. The existence or non-existence of a plateau in the CSS curves, as well as the corresponding plateau stress amplitude, depend not only on the modes and intensities of dislocation interactions among slip systems operating in the crystals but also on the slip deformation characteristics associated with crystal orientations. The plateau region in CSS curve disappears only when multiple slip plays a determining role during cyclic deformation.     

Non-dissociation of Lomer–Cottrell dislocations and 〈110〉{001} slip in silicon

Abstract

Two unexpected features have been revealed by transmission electron microscopy investigations of deformed silicon bicrystals: (a) slip of a/2 〈110〉 dislocations on {001} planes and (b) non-dissociation of Lomer–Cottrell dislocations formed by intersecting slip dislocations.     

Grain boundary networks in high-performance,heteroepitaxial, YBCO films on polycrystalline,cube-textured metals

Grain boundaries (GBs) in high-temperature superconductors suppress the critical current density (J _c) dramatically, with the J _c decreasing exponentially with GB angle, especially when GB misorientation exceeds 4°. To reduce the number of high-angle GBs, fabrication of biaxially textured, superconducting wires via heteroepitaxial growth on cube-textured metals has been widely investigated worldwide. Such wires exhibit very high J _c in applied magnetic fields despite having a majority of GBs with total misorientations in the range 4–8°. Here, we show that GB networks in these wires have numerous GBs with out-of-plane misorientations greater than 4° but few boundaries having in-plane misorientations greater than 4°. Transport measurements on bicrystal GBs show that GBs with out-of-plane tilts between 4° and 8° are well linked. Together, these results explain the high performance of superconducting films on cube-textured metals.     

Influence of crystallographic orientation on cyclic strain-hardening behaviour of copper single crystals

The cyclic strain-hardening behaviour of copper single crystals with various slip orientations is considered systematically. It is shown that the crystallographic orientation has a strong effect on the cyclic hardening behaviours of double- and multiple-slip-oriented copper single crystals. The initial cyclic hardening of differently oriented copper single crystals is mainly dependent on the modes and intensities of dislocation interactions between slip systems operating in the crystal, as well as on the possibility of cross-slip. A distinctive strain burst phenomenon has been frequently observed in the very early stage of cyclic hardening for critical double-slip-oriented crystals. A secondary cyclic hardening stage occurs readily late in cyclic deformation of coplanar doubleslip-oriented crystals.     

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.     

Dislocation arrangements and crystallographic characterization of deformation band in fatigued copper single crystal

This letter reveals the dislocation arrangements and crystallographic characterization of deformation bands (denoted DBII) in a copper single crystal fatigued at a high strain amplitude gamma_pl = 8 x 10^-3. The results show that the surface deformation morphology of the crystal displays the following features. (1) Primary slip bands (SBs) were formed after 2 x 10⁴ cycles and these carried a relatively homogeneous and small plastic strain. (2) Secondary slip bands did not operate during cyclic deformation. (3) Deformation bands (DBs) with a width of 50 mum were homogeneously distributed over the whole surface of the crystal and were perpendicular to the SBs. (4) Dislocation patterns within the SBs often consisted of irregular structures, which did not show a persistent feature. The results indicate that these SBs are not typical persistent slip bands (PSBs). (5) Within the DBII, the microstructure can be classified into two types. One type consists of regular 100% ladder-like parallel PSBs. The other type is full of dislocation walls parallel to DB direction, which have not been reported previously. By crystallographic analysis of the DBII, it is shown that the habit plane of the DBII should correspond to the (101) plane. Based on the observations above, it is suggested that the formation of DBII should be attributed to the local regularization of dislocation walls within primary slip bands.     

Segregation of phosphorus atoms to grain boundaries in ferritic steels under neutron irradiation

Segregation of solute atoms such as P to grain boundaries (GBs) in reactor pressure vessel steels can cause intergranular embrittlement. We study the segregation kinetics at low temperatures, where the thermal effects are negligible and the solute atoms are fully adhesive to GBs. An analytical dependence of the GB coverage of segregant atoms on the irradiation dose is derived.     

The fatigue limits of copper single crystals cyclically deformed at constant plastic strain amplitudes

Based on systematic surface observations, experimental measurements on the fatigue limits of differently oriented copper single crystals cyclically deformed at constant plastic strain amplitudes are summarized, and an orientation dependence of the fatigue limit is sketched. It was found that the fatigue limits of crystals depend mainly upon the low-strain-amplitude dislocation structures, which are characteristic of the particular crystal orientations. When multiple-slip deformation is the main mode of deformation and ultimately produces stable lowenergy dislocation structures such as labyrinth and cell structures, the fatigue limit can be improved notably.     

On the atomic structure of an asymmetrical near Σ = 27 grain boundary in copper

The atomic structure of an asymmetrical near Σ = 27 {525} tilt grain boundary (GB) in copper is determined by coupling high-resolution transmission electron microscopy and molecular dynamics simulation. The average GB plane is parallel to {414} in crystal (1) and {343} in crystal (2). The detailed GB structure shows that it is composed of facets always parallel to {101} and {111} in crystals (1) and (2), respectively. The atomic structure of one facet is described using the structural units model. Each facet is displaced with respect to its neighbours by a pure step, giving rise to the asymmetry of the GB plane orientation. The energy of this asymmetrical GB is significantly lower than that of both the {525} symmetrical and the {11,1,11}/{111} asymmetrical Σ = 27 GBs. One GB region displays another atomic structure with a dislocation that accounts for the misfit between interatomic distances in the {414} and {343} GB planes.     

Anomalous ductile-brittle fracture behaviour in fcc crystals

The fracture behaviour of fcc crystals has been investigated by numerically simulating the dynamical failure of a three-dimensional notched crystal using molecular dynamics, simple interatomic potentials for a rare-gas solid and system sizes of about ten million atoms. We find that the solid fails by brittle cleavage for cracking on a {110} face growing in a [110]direction and by ductile plasticity for cracking on a {111} face growing in a [110]direction. Comparison of equilibrium surface energies on the crack face and Schmid factors on the primary slip systems indicates that the classical theories of fracture give predictions in contradiction with the simulation results. A hyperelasticity model is proposed to explain this discrepancy. This anomalous fracture behaviour of fcc crystals has a profound implication on general modelling of dynamic failure of solids.     

Derivation of strain hardening in cyclic loading from that in tensile loading

The micromechanical model of extrusion formation in a single crystal under stress-controlled loading in high-cycle fatigue is reviewed. In the present letter, the microstress and strain fields of a single crystal under a high-amplitude fatigue are calculated by the boundary element method. Based on Schmidt's law, the resolved shear stress to activate or to cause the continuation of slip in a slip system is taken to depend on the amount of slip in that system. The cyclic strain hardening is calculated from the experimental tensile stress-strain curve. This calculated cyclic strain hardening is compared with the experimental cyclic hardening. Good agreement is shown.     

Shape effect related to crystallographic orientation of deformation behaviour in copper single crystals

The deformation behaviour of pure copper single crystals has been investigated by scanning electron microscopy and synchrotron radiation using the in situ reflection Laue method. Two types of sample with the same orientation of tensile axes, but with different crystallographic orientations in the directions of the width and thickness of the samples, have been studied. They showed different characteristics of deformation behaviour, such as the activated slip systems, the movement of the tensile axis, and the mode of fracture.     

On the length scale of cyclic strain localization in fine-grained copper films

Fine-grained copper films on a flexible substrate were cyclically deformed under constant strain range control. It was found that cyclic dislocation plasticity through individual dislocation glide is still dominant at the submicrometer scale, while the ability of irreversible slip of dislocations gradually decreases and the damage was changed from extrusion-induced localization to cracking along grain boundary. Statistical evaluation of the mean spacing between slip bands and/or lines leads to a critical scale (～28 nm) below which dislocation-controlled cyclic strain localization would be shut down.     

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.     

Threading dislocation with b=c+2a in 4H-SiC as determined by LACBED

The Burgers vectors of the so-called threading screw dislocations (a total of 28 dislocations) in 4H-SiC were determined by large-angle convergent-beam electron diffraction. A new type of TSD, that is, b = c + 2a dislocation, was identified. Thus, all of the four types of TSD predicted by Onda et al. [Phil. Mag. Lett. 93 (2013) p.591] were identified.     

Cross-slip in GaAs and InP at elevated temperatures

Slip lines on GaAs and InP crystals plastically deformed up to the lower yield point at temperatures around 700 K are compared with similar observations on Si. The slip lines in GaAs and InP differ from those in Si in two aspects, namely the primary slip lines are wavy and the non-primary slip lines are long and straight. The former are similar to those found in crystals deformed at low temperatures (below 400 K), being common among III-V compounds, and suggests a glide motion of undissociated screw dislocations. The operation processes of the non-primary slips are discussed with an X-ray topographic analysis of GaAs.     

Structure of twins in GaAs nanowires grown by the vapour–liquid–solid process

Semiconducting crystalline materials that are poor conductors of heat are important as thermoelectric materials and for technological applications involving thermal management. A combination of neutron scattering, lowtemperature ultrasonic attenuation and thermal conductivity measurements are reported on single crystals of the semiconductors Sr₈Ga₁₆Ge₃₀ and Ba₈Ga₁₆Ge₃₀. Taken together, these measurements suggest specific structural features that result in a crystal with the lowest possible thermal conductivity, namely that of a glass with the same chemical composition. Weakly bound atoms that 'rattle' within oversized atomic cages in a crystal result in a low thermal conductivity, but the present data show that both 'rattling' atoms and tunnelling states are necessary to produce a true glass-like thermal conductivity.