Single Slip Research Articles

Tensile behavior and deformation mechanism for Ti–22Al–25Nb alloy with lamellar O microstructures

In this research, the tensile behavior and deformation mechanism of two microstructures for Ti–22Al–25Nb alloy are studied by in-situ tensile test and electron backscatter diffraction (EBSD) experiment. Due to the higher B2 phase content and the long and continuous slip line, the solution-treated microstructure has better plasticity. For the aged microstructure, the slip line is short and scattered due to the obstruction of acicular O phase, which leads to the higher strength. In addition, the slip mode of B2 phase includes single system slip, double system slip and cross slip. {110}<111> slip and {112}<111> slip for B2 phase can be activated at room temperature. Meanwhile, the potential slip systems of O phase are summarized. The slip mode of O phase includes single system slip and double system slip, and the former is the most important one. All three types of slip systems (prism ‘a’ slip, basal ‘a’ slip, and pyramidal ‘c+a’ slip) for O phase can be activated at room temperature. Moreover, there is a good slip transmission between B2 and O phases. B2 phase slip can induce single system slip and double system slip of O phase. The Burgers orientation relation (BOR) between B2 and O phases is conducive to the slip transmission.

Crystallographic Orientation Dependence of Mechanical Responses of FeCrAl Micropillars

Iron-chromium-aluminum (FeCrAl) alloys are used in automobile exhaust gas purifying systems and nuclear reactors due to its superior high-temperature oxidation and excellent corrosion resistance. Single-phase FeCrAl alloys with a body centered cubic structure plastically deform through dislocation slips at room temperature. Here, we investigated the orientation dependence of mechanical responses of FeCrAl alloy through testing single-crystal and bi-crystal micropillars in a scanning electron microscopy at room temperature. Single-crystal micropillars were fabricated with specific orientations which favor the activity of single slip system or two slip systems or multiple slip systems. The strain hardening rate and flow strength increase with increasing the number of activated slip system in micropillars. Bi-crystal micropillars with respect to the continuity of slip systems across grain boundary were fabricated to study the effect of grain boundary on slip transmission. The high geometrical compatibility factor corresponds to a high flow strength and strain hardening rate. Experimental results provide insight into understanding mechanical response of FeCrAl alloy and developing the mechanisms-based constitutive laws for FeCrAl polycrystalline aggregates.

Dislocation-grain boundary interaction in metallic materials: Competition between dislocation transmission and dislocation source activation

Dislocation transmission across grain boundary (GB) and activation of dislocation source in adjacent grains are considered common features of dislocation-GB interactions in regulating mechanical properties and behaviors of metallic materials and structures. However, it is currently unclear which of dislocation transmission and dislocation source activation plays the dominant role in regulating dislocation-GB interaction. To study the competition between dislocation transmission and dislocation source activation, a theoretical framework is established based on a dislocation pile-up model. It is found that both dislocation transmission and dislocation source activation exhibit strong dependence on the GB misorientation angle. The critical transmission stress derived based on an energy method also depends on the grain size in a scaling form similar to the Hall-Petch relation. The outgoing slip systems during dislocation transmission are successfully predicted and agree with experimental observations. Regarding the dislocation source activation, the critical activation stress is investigated by examining the local stress fields generated by single and multiple slip dislocation pile-ups. It is found that compared with single slip dislocation pile-up, the result of multiple slip dislocation pile-ups displays lower critical activation stresses and exhibits feature of sensitivity to loading direction, interpreting the reported results of dislocation source activation in polycrystals. Further comparison between dislocation transmission and dislocation source activation for ⟨100⟩ tilt GBs indicates that the dislocation transmission prevails at low angle GB, while dislocation source activation is prone to occur at high angle GB. Our theoretical studies quantify the dislocation-GB interaction induced by dislocation pile-ups, shed light on the competition between dislocation transmission and dislocation source activation, and might provide essential guidance for high-performance metallic material design.

A novel double slip loads friction damper to control the seismic response of structures

In this study, the performance of a proposed friction damper with two slip loads in controlling the seismic response of steel moment resisting buildings structures under moderate and intense earthquake excitations is investigated. The proposed friction damper initially operates with a smaller slip load and could shift to the higher slip load after a certain amount of slippage when subjected to intense ground motions. In this regard, the hysteresis behavior of the proposed double slip loads (DSL) friction damper is modeled and after necessary verifications is imported to the material library of OpenSees software to be used for parametric study. To evaluate the performance of the proposed DSL damper, three steel moment resisting frame (SMRF) models with 4, 8 and 14 stories are considered that are designed according to the AISC-LRFD and ASCE7-16 codes. Using a suite of earthquake records, nonlinear time history analyses (N-THA) are carried out to determine the optimal slip loads as well as the optimal amount of the slippage in between to minimize the seismic response of the considered models. The smaller slip load is optimally determined by performing N-THA on the structural models equipped with a single slip load friction damper using the earthquake records that are scaled to the design basis earthquake (DBE) level. Then, utilizing the obtained first slip load, N-THA are repeated for the models equipped with DSL friction dampers using the same earthquake records that are now scaled to the maximum credible earthquake (MCE) level to optimally determine the second slip load as well as the amount of the slippage between two slip loads. The genetic algorithm is employed to find the optimal slip loads and the amount of slippage in between. Two scenarios for height-wise variation of the DSL parameters, i.e, the slip loads and the extent of slippage, of the SMRF models are considered. In the first scenario as a practical approach, an upward decreasing pattern of slip loads along the height of the buildings is considered. In second scenario a simple search is performed to optimally identify those parameters for all floors. Furthermore, the performance of the structural models with DSL dampers are compared with those equipped with conventional single slip load systems. The results clearly demonstrate the efficiency of the proposed DSL dampers in effectively reducing the seismic response of the structures in terms of story drifts, residual drifts, absolute floor accelerations, and base shear forces in comparison to the conventional single slip load friction dampers.

In-situ investigation on tensile deformation and fracture behaviors of a new metastable β titanium alloy

The tensile deformation and fracture behaviors of a new metastable β titanium alloy (Ti–5Cr–4Al–4Zr–3Mo–2W-0.8Fe) with single β phase are investigated by in-situ tensile test under scanning electron microscopy. With the increase of deformation degree, in addition to the transition from single slip to multiple slip, the stress induced martensite (SIM) and mechanical twins will also occur to coordinate the overall deformation of the alloy, leading to further work hardening. The slip system activation, slip transfer and grain rotation are closely related to the crystallographic orientation, which can be evaluated by Schmid factor, geometric compatibility factor and misorientation. The dislocation pile-up leads to serious stress concentration and inhomogeneous deformation appeared in the areas near grain boundary, dislocation line and shear band, and the microvoids are easy to nucleate and grow in the above areas and then coalescence into microcracks. The primary crack formed by microcrack extension propagates along the activated slip system in the grain, and deflects as it passes through the grain boundary to coordinate the slip system in the adjacent grain, resulting in the overall crack propagation path being zigzag. Considering the damage prone location and crack propagation path, it can be concluded that the fracture mechanism of the alloy belongs to the intergranular and transgranular mixture.

System reliability analysis of soil slopes through constrained optimization

As a slope may have numerous potential slip surfaces, its failure probability as a system is usually different from that of a single slip surface. Nevertheless, the system failure probability of a slope is often governed by several representative slip surfaces. How to identify the representative slip surfaces is important for system reliability analysis of soil slopes. Previous studies on representative slip surface identification mainly focus on circular slip surfaces within the framework of limit equilibrium methods; there are also relevant researches, albeit limited, within the framework of finite element/finite difference analyses based on the shear strength reduction method. By viewing the task of representative slip surface identification as a multidesign point identification problem, a barrier-based optimization method is suggested in this paper to identify representative slip surfaces of arbitrary shape based on the shear strength reduction method. A multiple starting point strategy is suggested to enhance its efficiency. For the three slopes examined in this paper, the method suggested is capable of identifying representative slip surfaces efficiently without prior assumption on their shapes.

A Quantitative In Situ SEM Bending Method for Stress Relaxation of Microscale Materials at Room Temperature

Although the time-dependent deformation behaviors of microscale materials have been investigated through experiments with uniaxial loading conditions, the influence of the strain gradient has not been clearly clarified due to the lack of appropriate testing methods. In the current study, to investigate the stress relaxation behavior of microscale single-crystal copper (Cu) at room temperature, a quantitative in situ SEM bending experiment is presented using microcantilever specimens of single-crystal Cu. The microcantilever specimens were fabricated using a focused ion beam, and a tungsten (W) layer was deposited onto the front surface to eliminate the error induced by the penetration of the stiff indenter into the metallic specimen. The yield stress of microscale single-crystal Cu is determined to be 445 MPa by a monotonic loading test, showing an apparent size effect, and no strain hardening is observed due to single-slip deformation. On the other hand, the stress relaxation behavior of the microscale single-crystal Cu consists of both a continuous stress relaxation and an abrupt stress decrease due to a strain burst. The activated volume in each dwell stage is obtained by thermodynamics theory and is found to be mainly related to the abrupt stress decrease. The value of the activated volume indicates that the continuous stress drops in the 1st and 2nd dwell stages are attributed to the evolution of dislocation structures by the single slip on system B4, while the dislocation pile-up near the neutral plane leads to the dominance of cross slip on the stress relaxation behavior in the bending plateau. The proposed microcantilever bending experiment is applicable to explore the time-dependent deformation behavior of small-scale materials.

The significant size effect on nucleation and propagation of crack during tensile deformation of copper foil: Free surface roughening and crystallography study

Tensile tests were conducted on copper foils with 80 μm in thickness to elucidate the effect of grain size on nucleation and growth of crack. Experimental results indicated that samples with larger grain size had smaller fracture stress and strain when the ratio of sample thickness to grain size (T/D) was below 10. This is attributed to strain localization during the early stage of tensile deformation. Confocal laser scanning microscopy (CLSM) indicated an increase in free surface roughening with increase in grain size during tensile straining. The local depression trough and thinning of sample caused by surface roughening were precursors to crack propagation. (EBSD) studies of crack in sample with T/D value of 1.8, it was observed that in the vicinity of the crack tip, a high density of dense and deep slip traces were present. The slip process gradually changed from single slip to multiple slip and then to cross slip. Both grain boundary and annealed twin boundary acted as obstacles to the movement of dislocations and led to strain localization. Furthermore, during tensile deformation, the crack propagation direction was not always perpendicular to the loading direction. A change in the direction of crack propagation occurred at the junction of grain boundaries and entered into the adjacent grain. Large angle grain boundaries provided greater resistance to the propagation of crack.

What the Single Bamboo Slip Found in Mawangdui Tomb M2 Tells Us about Text and Ritual in Early China

Abstract A single bamboo slip was found at Mawangdui tomb M2 inside the passageway leading to the pit where Li Cang (d. ca. 186 BCE), the Marquis of Dai and Prime Minister of Changsha, was buried. Though almost entirely unnoticed in previous scholarship, the M2 slip has much to tell us about the overlapping textual, ritual, administrative, and funerary practices of early Western Han China. I offer a description of the slip, translations of its contents, a consideration of how it was used at the tomb site, and an analysis of what its archaeological context tells us about the use of talismans in Western Han burials. Specifically, I show that the slip originally formed part of a multi-piece tomb inventory manuscript, and that it was removed and ritually deposited inside the passageway in order to protect the tomb from robbers and malevolent spirits.

Key-block based analytical stability method for discontinuous rock slope subjected to toppling failure

Abstract This study introduces a simplified semi-distinct element algorithm for discontinuous rock slopes under toppling instability assessment based on block theory. The presented algorithm is developed to investigate block, flexural and block-flexural types of toppling failures which have been coded in the Python high-level programming language. In order to investigate toppling instabilities for different modes, the three modelling steps, namely, geometrical, behavioural and mechanical simulations were conducted to appoint high-order calculation trending loops. These loops were based on the analytical description of modified second-order reliability method with first-order efficiency to determine the factor of safety values for discontinuous rock slopes. Each type of toppling failure was described based on main assumptions that were modified by geometric qualifications such as the analytical procedure, single block slip, and the Voronoi diagram algorithm for the stability analysis of the three different failure types. According to the stability assessment results of the studied slopes which were verified by the distinct numerical method via UDEC software, it has been clarified that the results obtained from the algorithm and the numerical analysis were in an appropriate trend. The used algorithm is superior in pinpointing the critical sliding zones, safety factor estimation and progressive failure analyses as compared to the numerical analyses. Moreover, the block theory based algorithm has higher processing capability and speed with respect to the numerical method.

Assessment of Internal Stresses Using Dislocation Dipole Heights in Cyclically Deformed [001] Copper Single Crystals

There have been a number of studies on dipole separations in cyclically deformed FCC single crystals in single slip while there are no such studies in multiple slip. The dipole heights provide insight into the presence of long-range internal stresses (LRIS). In this study, we investigated how LRIS compare with the single slip studies through measuring the dislocation of dipole heights. [001] oriented copper single crystals were cyclically deformed in strain-control to saturation at ambient temperature. Transmission electron microscopy (TEM) confirms a labyrinth dislocation microstructure with high dislocation density walls and low dislocation density channels. The maximum dipole heights under the saturation stress were approximately independent of location, being nearly equal in the walls and within the channels. This, by itself, supports a uniform stress across the microstructure and low long-range internal stresses. The maximum value for dipole heights suggests dipole strengths (local stresses) that are about a factor of 2.4 higher than the applied stress based on the usual athermal equations. Considering the small “extra” stress that may be provided by tripoles or small dislocation pile-ups, a nearly homogenous stress distribution with only small internal stresses may be present, which is consistent with the observation of uniform dipole height across the heterogeneous dislocation microstructure. This observation that the stress state appears to be homogenous and higher than the applied stress has also been reported in the case of cyclically deformed metals in single slip.

A molecular dynamics study of effects of crystal orientation, size scale, and strain rate on penetration mechanisms of monocrystalline copper subjected to impact from a nickel penetrator at very high strain rates

This paper presents a systematic computational study to investigate the effects of crystal orientation, strain rate (impact velocity), and size (thickness) on plasticity and damage behavior of copper single crystals during the penetration process at the atomistic scale. For the penetration analysis, copper single crystals with different crystal orientations and thicknesses were impacted and penetrated by a cylindrical nickel penetrator at different initial velocities. Modified embedded atom method potentials were used to develop atomistic models, and over 250 molecular dynamics simulations were performed to fully reveal the effects of influence parameters on plasticity and damage behavior of the copper single crystals. The results show that the copper single crystal with an octal slip orientation exhibits the greatest strength and penetration resistance, while the copper crystal with the single slip orientation exhibits the lowest strength and resistance. The results further show that the strength and penetration resistance of the target increase as the thickness of the copper single crystals increases. Furthermore, as the impact velocity increases, damage and fragmentation increase. Conclusions drawn from this computational study are consistent with macroscale plasticity theories of metals and reaffirm the conclusions drawn by other researchers in previous experimental studies.

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45 μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.

Multiscale modelling of precipitation hardening in Al–Cu alloys: Dislocation dynamics simulations and experimental validation

The mechanisms of dislocation/precipitate interactions were analyzed in an Al–Cu alloy containing a homogeneous dispersion of θ′ precipitates by means of discrete dislocation dynamics simulations. The simulations were carried out within the framework of the discrete-continuous method and the precipitates were assumed to be impenetrable by dislocations. The main parameters that determine the dislocation/precipitate interactions (elastic mismatch, stress-free transformation strains, dislocation mobility and cross-slip rate) were obtained from atomistic simulations, while the size, shape, spatial distribution and volume fraction of the precipitates were obtained from transmission electron microscopy. The predictions of the critical resolved shear stress (including the contribution of solid solution) were in agreement with the experimental results obtained by means of compression tests in micropillars of the Al–Cu alloy oriented for single slip. The simulations revealed that the most important contribution to the precipitation hardening of the alloy was provided by the stress-free transformation strains followed by the solution hardening and the Orowan mechanism due to the bow-out of the dislocations around the precipitates.

Effect of Sample Size and Crystal Orientation on the Fatigue Behaviour of Single Crystalline Microbeams.

Beam deflection experiments were used to systematically examine size effects on the low cyclic fatigue (LCF) deformation behaviour of micro-sized bending beams of copper (Cu) single crystals oriented for single slip, critical and coplanar double slip. We present cyclic hardening curves and fatigue surface roughness, as well as dislocations structures of the micro-sized beams with sizes between 1 and 15 µm. A clear crystal orientation and size effect on the cyclic hardening curves, surface roughness, and the dislocation microstructures were observed. Based on the experimental results, the fatigue damage in single slip orientations clearly decreased with decreasing the sample size, however, below a critical size regime, the surface damage suddenly increases. Additionally, samples with sizes larger than 5 µm clearly revealed, besides PSBs-like structures, the emergence of kink bands leading to larger surface roughness in comparison to the smaller ones. Fatigue surface damages in microcrystals oriented for critical double slip became more prevalent compared to single slip orientations. Quantitatively, the correlation of the fatigue surface damage was also demonstrated with the formation of PSBs-like structures.

Experimental Reactivation of Shear‐fractured Berea and Boise Sandstones by Brine or Liquid CO2 Injection at Depth

AbstractInjection‐driven reactivation of fractured/faulted reservoirs is a key concern in the design and safe operation of water/CO2 injection projects. Here, a new laboratory testing protocol is devised by which the reactivation conditions are quantified, providing pivotal data for effective risk assessment/management. We simulate a field injection operation in the laboratory on prefaulted Berea and Boise sandstones subjected to stress/pressure conditions prevalent at 2 km depth. The protocol consists of two key stages: (i) drained normal faulting: triaxial shear failure induced by increase of the overburden stress; and (ii) injection‐driven reactivation of the faulted rock by pore pressure increase. Injection is conducted with either brine or liquid CO2. The data show that at 2 km depth, (i) shear fracturing and subsequent faulting occurs when the differential stress reaches 36–46 MPa for Boise, or 96–98 MPa for Berea, and (ii) subsequent reactivation triggers when the pore fluid over‐pressure reaches 4–5 MPa, regardless of the injected fluid or specific sandstone tested. Spatiotemporal monitoring of the concomitant microseismic activity proves effective in time‐lapse imaging the structural changes leading to the triaxial faulting of the intact rock, or to the reactivation of the prefaulted rock. Microseismic imaging suggests that (i) initial shear faulting results in a single slip surface, consistent with brittle (Berea) or semibrittle (Boise) failure; (ii) upon injection‐induced reactivation at 2 km depth, the preexisting fault ruptures and slips first, before a new and steeper fracture nucleates, progressively propagates, then slips; and (iii) dynamic slip transfer occurs from the preexisting to the newly formed fault.

Effect of microstructural evolution on the cyclic softening of a 10% Cr martensitic steel under low cycle fatigue at 600 °C

Microstructure evolution and softening during interrupted low cycle fatigue (LCF) tests at 600 °C and constant strain amplitudes of ±0.2% and ±0.6% corresponding to dominance of the elastic and plastic strain components, respectively, was examined in a 10% Cr steel. Cyclic softening attributed to changes in the strengthening from dislocations and lath structure is more pronounced during the first half-life. The Burgers vector analysis of dislocation reveals the role of the single and multiple dislocation slip in retention and transformation of lath structure at cycling with low and high strain amplitude, respectively.

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

Abstract. New Zealand's Alpine Fault is a large, plate-bounding strike-slip fault, which ruptures in large (Mw&gt;8) earthquakes. We conducted field and laboratory analyses of fault rocks to assess its fault zone architecture. Results reveal that the Alpine Fault Zone has a complex geometry, comprising an anastomosing network of multiple slip planes that have accommodated different amounts of displacement. This contrasts with the previous perception of the Alpine Fault Zone, which assumes a single principal slip zone accommodated all displacement. This interpretation is supported by results of drilling projects and geophysical investigations. Furthermore, observations presented here show that the young, largely unconsolidated sediments that constitute the footwall at shallow depths have a significant influence on fault gouge rheological properties and structure.

Influences of feed rate and wall thickness reduction on the microstructures of thin-walled Hastelloy C-276 cylindrical parts during staggered spinning

Staggered spinning is an advanced method to manufacture thin-walled cylindrical parts. The microstructure evolution, which dominantly influences the overall performance, is very complex during the staggered spinning of Ni-based cylinder. In this work, the influences of feed rate and wall thickness reduction on the microstructures of a thin-walled Hastelloy C-276 cylinder during staggered spinning are investigated. It is found that the deformation of parts is highly inhomogeneous, and the microstructures are sensitive to the feed rate and wall thickness reduction during the staggered spinning. When the wall thickness reduction is small, the single slip is responsible for the deformation of grains. The deformation mechanism changes from single slip to cross slip with the increase of wall thickness reduction. Moreover, the uniform deformation of thin-walled Hastelloy C-276 cylinder can be obtained when the feed rate is about 0.8 mm/r or the wall thickness reduction is about 44.1%. These findings provide guidance for controlling the microstructures of thin-walled cylindrical parts during staggered spinning.

On interface conditions on a material singular surface

The presence of grain boundaries significantly influences the overall mechanical behavior of materials with an underlying crystalline microstructure. They act as an obstacle against the movement of dislocations and, thus, significantly contribute to size effects as for example the Hall–Petch effect. Hence, the thermodynamically consistent modeling of the behavior of the plastic slip at a grain boundary is of utmost interest. To this end, balance equations at a grain boundary are derived from an extended energy balance by means of invariance considerations. The grain boundary is considered as a material singular surface with own internal and kinetic energy as well as energy supply. Consequently, the balances at the grain boundary depend on its mean curvature. The framework presented is applied to a small strain slip gradient crystal plasticity theory, regarding single slip. Accounting for the derived balance equations, thermodynamically consistent flow rules for the plastic slip at the grain boundary are obtained by exploitation of the Coleman–Noll procedure. In this context, a classification of flow rules for the plastic slip at the grain boundary is provided. Finally, the distribution of the plastic slip is presented regarding a three-phase laminate material with two elastoplastic phases and one elastic phase. The two elastoplastic phases represent the grains of a bicrystal. A grain boundary effect is discussed which is based on a variation of the internal length scale in one of the two elastoplastic phases.