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.     

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.     

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.     

Strain localization in nickel single crystals cyclically deformed at 77 K

Nickel monocrystals oriented for single slip have been cyclically deformed at 77 K at plastic strain amplitude between 5 x 10^-4 and 1 x 10^-2 up to saturation of the stress amplitude. After unloading from maximum compression, the slip markings on the surface of the specimens were removed and the deformation continued for one half cycle in tension. As previously observed for room-temperature deformation, the plastic strain was found to be localized in narrow slip bands (SBs). Using atomic force microscopy at a given imposed strain amplitude, a wide spectrum of local plastic strains was found. The averaged resolved shear strain of the SBs was found to be independent of the imposed plastic strain amplitude and turned out to be a factor of three larger than the upper plateau strain limit of the cyclic stress-strain curve.     

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.     

Yielding and strain hardening in aluminium single-crystal foils subjected to tension and bending

A series of tensile and microbend tests were conducted on aluminium single-crystal foil specimens with different thicknesses ranging from 20 to 90?µm. Two different crystal orientations relative to the tensile direction were considered in the tests: one is an orientation that was excepted to activate at least four slip systems on four different slip planes, and the other is an orientation that was expected to activate only a single-slip system. In the tensile tests, typical size dependence of initial yield strength was observed. The microbend test method was extended to include a reversal of bending direction. Using the curves of bending moment normalized by the square of foil thickness versus surface strain, which were obtained from the reversed bending tests, the total amounts of strain hardening were divided into isotropic and kinematic hardening components. In the microbend tests, a pronounced size-dependent kinematic hardening behaviour was clearly observed; meanwhile, the amounts of isotropic hardening were very small, particularly for the single-slip orientation.     

The effect of electropulsing on dislocation structures in [233] coplanar double-slip-oriented fatigued copper single crystals

The effect of high-current-density electropulsing on dislocation structures in a coplanar double-slip-oriented copper single crystal that had previously been fatigued is reported. The results show that, after electropulsing, vein structures are transformed to cell structures with some dark regions. It is proposed that the thermal compressive stress caused by electropulsing activates a coplanar slip system and leads to strong dislocation interactions between primary and coplanar slip systems, thereby forming cell structures. Partial recrystallization may occur by electropulsing, leading to the appearance of some dark regions.     

Relationship between the fatigue cracking probability and the grain-boundary category

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.     

Effects of fatigue induced by repetitive movements and isometric tasks on reaction time

PurposeThe understanding of fatigue of the human motor system is important in the fields of ergonomics, sport, rehabilitation and neurology. In order to understand the interactions between fatigue and reaction time, we evaluated the effects of two different fatiguing tasks on reaction time.Methods83 healthy subjects were included in a case-control study with three arms where single and double choice reaction time tasks were performed before and after 2 min fatiguing task (an isometric task, a finger tapping task and at rest).ResultsAfter an isometric task, the right-fatigued hand was slower in the choice component of a double choice reaction time task (calculated as the individual difference between single and double choice reaction times); also, the subjects that felt more fatigued had slower choice reaction time respect to the baseline assessment. Moreover, in relationship to the performance decay after two minutes, finger tapping task produces more intense fatigability perception.ConclusionsWe confirmed that two minutes of isometric or repetitive tasks are enough to produce fatigue. The fatigue perception is more intense for finger tapping tasks in relation to the performance decay. We therefore confirmed that the two fatiguing tasks produced two different kind of fatigue demonstrating that with a very simple protocol it is possible to test subjects or patients to quantify different form of fatigue.     

Tension and compression of bulk Al-7.5 wt% Mg alloy

The mechanical behaviour of bulk ultrafine-grained Al-7.5 wt% Mg alloy consolidated from cryomilled powders has been investigated. The experimental data show that the alloy exhibits high strength, low strain hardening, serrated flow and relatively high ductility. In addition, the data indicate that the yield strength in tension is essentially equal to that in compression. The yield and flow strengths of the alloy are discussed in terms of strengthening processes that are related to grain size, the Orowan mechanism and solid-solution hardening. The serrations in the stress-strain curve are discussed in terms of dynamic strain ageing and deformation twinning.     

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.     

Formation of prismatic loop alignments in crystals deformed in single slip

In γ-TiAl deformed at room temperature in single slip, prismatic loops ar often organized in a staircase-like configuration, called strings whose generation by cross-slip is facilitated by the dislocation tendency to form screw locks as described recently by Grégori and Veyssière. The present letter is aimed at showing that strings may be encountered in virtually any crystal provided that the two impinging dislocations exhibit significantly different velocities. The crossslip annihilation mechanism of crossed dislocations considered by Tetelman is revisited and shown to evolve into a configuration significantly distinct from that originally predicted.     

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.     

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.     

Mechanical hysteresis due to microplasticity in alumina with microcracks

Abstract

Stress-strain hysteresis in alumina with microcracks has been investigated by a loading–unloading test in the microstrain range around 10 ^?4 While there remains a permanent strain after the initial loading, steady-state cyclic loading results in a single closed hysteresis loop with a symmetrical shape. Such a stabilized hysteresis loop is responsible for internal friction and can be attributed to the microplasticity associated with a forerunning phenomenon of fracture.     

Multiple phase transitions in CuO

Electrical resistivity, thermoelectric power, magnetic susceptibility and calorimetric measurements were carried out on a single crystal of CuO. In addition the two known antiferromagnetic transitions between 210K and 230K, a new phase transition around 160K was found in CuO. The experimental results show that this new transition is related to changes in the electronic state of CuO.     

Plasticity of GaAs(011) at room temperature under concentrated load

Indentations have been made on a (011) surface of GaAs single crystal at room temperature. The loading-unloading curves were compared with those obtained under the same experimental conditions on (001) surfaces. The indents formed were observed by transmission electron microscopy in plan view as well as in cross-sectional view in samples prepared by the focused-ion-beam technique. The gliding systems could be identified, providing a better understanding of the plastic flow under the indenter. The arrangements of dislocations generated by the indenter have been analysed and compared with results previously reported by Ning et al.     

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.     

Measurement of the relative strain ratios normal to the crystal planes in Cu thin films using grazing incidence X-ray diffraction

The relative strain normal to the crystal planes in Cu thin films has been experimentally measured using grazing incidence X-ray diffraction. This was performed by measuring the ratio of the strain exerted in the same specific (hkl) crystal plane parallel and perpendicular to the film surface. The strain ratios of the (1?1?1), (2?0?0), (2?2?0) and (3?0?0) planes were found to be –0.804, –1.324, –1.003, and –1.369, respectively. These values are within experimental error of those calculated theoretically under the assumptions of plane stress conditions.     

Planar faults in MoSi2 single crystals deformed at high temperatures

Abstract

Dislocation structure and planar faults have been examined in MoSi₂ single crystals deformed at high temperatures. Pure stacking faults were found in a crystal deformed at 900°C. The formation of the stacking fault is closely related to the phase stability of the C11_b and C40 ordered structures. Profuse stacking faults with increasing deformation temperature assist the ductility improvement of the MoSi₂ above about 1200°C. The critical resolved shear stress for {110}(331) and {013}(331) slip is determined in the temperature range 1000 to 1500°C.