Multiple Slip Systems Research Articles

Deformation and metasomatism recorded by single-grain apatite petrochronology

Abstract The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.

The mechanism for an orientation dependence of grain boundary strengthening in pure titanium

The Hall-Petch slope k represents the magnitude of grain boundary strengthening. For the first time, a strong orientation dependence of the k for pure titanium (Ti) is reported in the present study, namely, a much lower k for the TD-tension of a Ti plate (188 MPa μm1/2) than the k values for the RD-tension of the Ti plate (358 MPa μm1/2), SD-tension (369 MPa μm1/2) and SD-compression (397 MPa μm1/2) of a Ti rod. Here, the RD and TD are respectively the rolling direction and transverse direction of the plate, and SD is the axial direction of the rod, while tension and compression stand for uniaxial tension and compression, respectively. It is found that the mechanisms reported previously cannot explain the orientation dependence of k experimentally observed in the present study. A new mechanism is proposed by combining crystal plasticity finite element modeling and the Eshelby model for the stress concentration at grain boundaries. The results indicate that an orientation-mediated deformation-transfer is the main contributor to this orientation effect on k. More specifically, for the RD-tension, SD-tension and SD-compression, a single slip system is predominant in most grains when plastic deformation-transfer occurs. In contrast, multiple slip systems are activated in the majority of grains in the TD-tension. The activation of additional slip systems remarkably reduces the stress concentration at grain boundaries, leading to a lower k. Afterward, the reasons for the orientation-mediated deformation transfer behavior are discussed.

Structural evolution during the cycling of NiTi shape memory alloys

Abstract The structural evolution in NiTi shape memory alloys subjected to pseudoelastic cycling is examined in the present study. Single crystals with [100] and [111] orientations were subjected to repeated compressive cycles and then studied by transmission electron microscopy (TEM). TEM observations were made at cycle numbers 1, 2, 5, 10, and 20 since the majority of degradation occurs during these initial cycle numbers. Under compression, single crystals with [111] orientations degraded much faster than crystals with [100] orientations. Under tension, single crystals with [100] orientations fractured in the elastic region, and crystals with [111] orientations showed considerable degradation as a function of cycling. Intermittent TEM observations on single crystals oriented along the [111] direction showed an increase in dislocation density on multiple active slip systems as a function of cycling. Single crystals oriented along the [100] orientation show a less dramatic increase in dislocation density as a function of cycling. TEM observations have revealed that dislocation structures formed near martensite plates have a similar periodicity as internal twin modes within the martensite. This observation implies that, although the interface between the martensite and parent phase is a low-energy boundary, the local disruptions due to internal twins create preferential nucleation sites for the formation of lattice defects.

Helium irradiation-induced ultrahigh hardening in niobium

In this work, using in-situ uniaxial tensile and compressive testing, microscopy, and theoretical analysis, we study the mechanism underlying the ultrahigh irradiation hardening in niobium (Nb). We show that irradiated Nb pillar exhibits a more than two-fold increase in the yield stress. With in-situ mechanical testing, we observe that He bubbles in Nb promote dislocation nucleation and multiple slip systems. The Nb pillars with 1.2 nm He bubbles fail by bubble coalescence and form a faceted fracture surface. In contrast, the Nb pillars with 8 nm He bubbles fail by bubble elongation and fragmentation. A theoretical analysis of the hardening contribution based solely on the density and size of He bubbles finds that it is less than one third of the experimentally observed hardening. To explain the large gap between the model and the experiment, we propose that the ultrahigh irradiation hardening originates from a large quantity of atomic-size, undetectable He-vacancy (He-V) complexes. The implanted He ions only account for less than 50% percent in the visible He bubbles, while most of them bind to vacancies to form stable He-V complexes that are distributed throughout the lattice. The strong interaction between dislocations and high density of He-V complexes is the chief source for the remarkable irradiation hardening observed in Nb.

Effect of β phase on deformation behavior of Ti alloys

Static tensile and creep tests were conducted on Ti-64 (α+β) and β single-phase Ti-15-3 to confirm the effect of the β phase for Ti alloys. Both tests were conducted in a scanning electron microscope (SEM), and strain distribution within the observation area was measured by a digital image correlation (DIC) method. With respect to Ti-64 (α+β), the influence of β phase was not clearly observed in the tensile and creep tests. The crystallographic orientation distribution of α phase strongly affected the deformation behavior and strain distribution. For the deformation behavior of Ti alloys with α+β phases, it is sufficient to consider only the crystallographic orientation distribution of the α phase. On the other hand, Ti-15-3 (β single-phase) formed slip zones along the direction of 45° to the loading line in both tensile and creep tests, which coincided with the macroscopic shear zones. This is primarily because β phase (BCC) has multiple slip systems and little plastic anisotropy. For the deformation behavior of β single-phase Ti alloys, there is little need to focus on the plastic anisotropy caused by the crystallographic orientation.

Effects of extrusion ratio and temperature on the microstructure and mechanical properties of Mg–Zn–Yb–Zr extrusion alloys

In this work, the effects of extrusion ratios (ER) and temperatures (ET) on the microstructure and mechanical properties of Mg–6.0 Zn–2.0 Yb–0.5 Zr (wt.%) extrusion alloys are investigated. The results reveal that prismatic and basal slips dominate the deformation of the low ER and ET extruded alloy, resulting in the generation of heterogeneous microstructure with an intensified basal texture. In contrast, non-basal slips robustly activate when hot extruded at the large ER and ET, promoting the development of the homogeneous-grained extrudates with a weakened “Rare-Earth” (RE) texture. A large ER effectively refines the microstructure and induces profuse nanoscale precipitates, corresponding to increased rotation angles and a wide spread of orientations in newborn grains, whereas an elevated ET coarsens the precipitates and promotes the preferential growth of grains with a certain crystallographic orientation thus developing the RE texture component. The studied extrusion alloys exhibit tunable ultimate tensile strength (301–370 MPa) and elongation (17.1–26.7%) depending on extrusion parameters. With the optimum extrusion condition (ER: 20:1 and ET: 320 °C), the alloy exhibits the best mechanical performance, which is primarily attributed to grain boundary strengthening, multiple slip systems activations, and stress concentration alleviation.

Modified Embedded‐Atom Method Potentials for the Plasticity and Fracture Behaviors of Unary HCP Metals

AbstractModified embedded‐atom method (MEAM) potentials have been widely used in molecular dynamics (MD) simulations to describe the interatomic interactions in metallic systems. However, conventional MEAM potentials are almost exclusively fit to a limited number of key single‐value properties, limiting the predictability of MD simulations, especially for complex hexagonal‐close‐packed (HCP) metals with multiple slip systems. Here, three MEAM potentials for unary HCP metals (Mg, Co, and Ti) are fit to conventional target properties, as well as continuous‐energy curves and their gradients obtained from density‐functional theory calculations. These targets include the lattice parameters, cohesive energy, elastic constants, and surface energies as well as the cohesive energy curve, basal plane decohesion energy curve, and generalized stacking fault energy curves for basal, prismatic, pyramidal I, and pyramidal II planes. These interatomic potentials demonstrate excellent reproduction of relevant properties and enable accurate MD simulations of volumetric and plastic deformations, as well as fracture in Mg, Co, and Ti HCP metals. Importantly, the interatomic potentials demonstrate better performance than all existing EAM/MEAM potentials readily available from the literature.

Glimmerite: A product of melt-rock interaction within a crustal-scale high-strain zone

The paradigm for hydrous high-strain zones that cut dry host rocks is for fluid-rock interaction to have involved aqueous fluids. However, the role of silicate melt is increasingly recognised. This contribution examines the formation of glimmerite (biotitite) bands during melt migration in the Gough Dam shear zone, a high-strain zone in central Australia that was active during the Alice Springs Orogeny (c. 450–300 Ma). The glimmerite bands cut and replace a range of quartzo-feldspathic protoliths, including granitic gneiss and quartzite. Melt that migrated through the high-strain zone is interpreted to have penetrated relict layers along a network of fractures, enhancing dissolution of the precursor rock and causing replacement by glimmerite crystallisation. Microstructures indicative of the former presence of melt in the high-strain zone include: pseudomorphs of former melt pockets of granitic composition; small dihedral angles of interstitial phases; elongate grain boundary melt pseudomorphs; neighbourhoods of grains connected in three dimensions; and localised static grain growth and recovery. Other microstructures indicative of melt-present deformation include randomly oriented neosome grains, and evidence of activation of multiple slip systems during deformation. The degree of quartzite modification to glimmerite is recorded by an increase in biotite mode, and correlated with higher Ti concentrations in biotite (higher apparent temperature) and changes to trace element and REE compositions. Melt-assisted coupled dissolution-precipitation reactions during melt flux are interpreted to partially reset Proterozoic monazite U-Pb ages inherited from the protolith (> 1630 Ma) to younger Palaeozoic ages, with a complex age pattern partially congruent with the Alice Springs Orogeny (apparent ages range from c. 606–371 Ma, with a dominant age peak at c. 451 Ma). We propose that the glimmerite formed during dynamic melt migration of an externally-derived hydrous peraluminous melt, driving reaction replacement of various felsic protoliths during this orogenic event.

Non-orthogonal computational grids for studying dislocation motion in phase field approaches

In this work, new non-orthogonal computational grids are implemented into a phase field model called Phase Field Dislocation Dynamics (PFDD). We demonstrate that the new non-orthogonal grid can accommodate multiple slip planes in either the face centered cubic (FCC) or body centered cubic (BCC) crystallographic systems. We show that they avoid numerical errors induced when modeling glide on inclined slip planes in an orthogonal grid. The Gibbs effect that arises in the orthogonal or rotated orthogonal grids is substantially diminished when a non-orthogonal grid is employed. A few test cases demonstrate the effectiveness of using non-orthogonal grids in solving systems with multiple non-planar slip systems.

Strain rate dependent deformation behavior of BCC-structured Ti29Zr24Nb23Hf24 high entropy alloy at elevated temperatures

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10−3 to 10 s−1, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10−3 s−1. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s−1, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening.

Microstructural evolution in pure copper during accumulative skin pass rolling: experimental and crystal plasticity numerical investigations

Surface mechanical treatments are known to introduce deformation heterogeneity, thus entailing a combination of excellent strength and ductility in metallic metals. Accumulative skin pass rolling (ASPR) was employed to introduce heterogeneous structure in pure copper, which significantly improved yield strength and maintained high ductility. The experimental observations of the ASPR processed copper showed that surface grains possessed a much larger magnitude of misorientations and geometrically necessary dislocations (GNDs) than the centre grains. Crystal plasticity finite element modelling was utilised to provide an in-depth understanding of microstructure evolution during ASPR. It was found that the surface grains accommodated higher shear strain and had higher slip resistance than the centre grains. The multiple active slip systems in the surface grains induce the lattice rotations and then the formation of intragranular misorientations accompanied by a high density of GNDs, while the single active slip system in the centre grains can accommodate the applied material rotation without the lattice rotation.

Experimental investigation on the mechanical properties and strain rate sensitivity of Mg–Al–Ca–Mn alloy under various strain rates

An Mg-0.8Al-0.1Ca-0.4Mn (AXM080104, wt%) alloy was developed with extraordinary extrudability. The mechanical properties and strain rate sensitivity of the alloy were evaluated under various strain rates. The as-extruded alloy exhibited a tensile 0.2% proof stress of 222 MPa and an ultimate tensile strength of 380 MPa at the strain rate of 500/s. The performance of the extruded AXM080104 is superior to that of the AZ31 alloy at the high strain rate. The deformation mechanism of the extruded AXM080104 was examined and analyzed. The high strength is attributed to the simultaneous activation of multiple slip systems, and then a large number of dislocations are packed and entangled at the grain boundaries to cause work hardening.

Anisotropic tensile properties and failure mechanism of laser metal deposited Ti–5Al–5Mo–5V–3Cr alloy before and after sub-transus heat-treatment

The tensile behaviour and failure mechanism of Ti-5553 (Ti–5Al–5Mo–5V–3Cr) components manufactured with Laser Metal Deposition (LMD) on forged and heat-treated Ti-5553 substrate were analysed. Two Ti-5553 blocks have been manufactured by LMD on substrate of the same alloy. One block has been analysed in the as-deposited condition, the other following a solution treatment and ageing (STA) heat-treatment. Tensile specimens were extracted parallel and perpendicular to the build direction to analyse the influence of anisotropic β grains on the tensile properties at room temperature. As-deposited material is characterised with long columnar β grains aligned with the build direction and no α-phase resolvable with Scanning Electron Microscopy (SEM). Tensile strength in the as-deposited condition is low, but ductility is good. A homogeneous microhardness distribution was noted throughout the build direction of the as-deposited material. The heat-treated block was characterised with the presence of grain boundary α, and both primary as well as secondary α laths. Tensile strength of this condition has improved, but the ductility varies significantly with respect to the tensile loading direction. A homogenous microhardness distribution was noted throughout the heat-treated deposited material and substrate. The fracture surface and microstructure in the plane parallel to the loading direction from fractured samples was analysed using SEM to identify critical locations for strain induced porosity and crack propagation. Failure mechanisms for both the as-deposited and heat-treated conditions have been proposed. In the as-deposited condition, failure occurs through microvoid nucleation and coalescence along slip bands and along the β/β grain boundaries. Slip bands of multiple slip systems frequently form at 45° to the loading direction and correspond to slip systems of the highest Schmid factors. In the heat-treated condition, failure occurs predominantly through easy crack propagation along the GBα/β interfaces. When the loading direction was parallel to the columnar β grain boundaries, the ductility was improved compared to specimens containing columnar β grain boundaries perpendicular to the loading direction.

Influence of macrozones on the fatigue cracking behavior and fracture mechanisms of rolled Ti–6Al–4V alloy

In this work, rotating bending fatigue, scanning electron microscope in-situ fatigue, pre-fatigue (and post-fatigue) electron backscatter diffraction tests were carried out for rolled Ti–6Al–4V alloys. The macrozone sizes and orientations of the materials were classified and the effect of macrozones on fatigue cracking behavior and fracture mechanisms was studied. The results indicated that intra-granular fracture was dominated in the process of crack propagation. Basal and prismatic slip systems were favored for the macrozones whose crack paths were straight and zigzag, respectively. Multiple active slip systems can also induce the fatigue crack deflections inside grains. Moreover, fatigue crack propagation rate, threshold stress intensity factor range (ΔKth) and fracture toughness (KIC) were measured for different kinds of macrozones. For the macrozones favorably orientated for prismatic slip, their crack propagation resistance and ΔKth were excellent. However, the KIC values for different macrozones were similar. Finally, the effect of macrozone orientations on fracture mechanisms (cleavage and plastic fracture) was discussed through a combination of Schmid factor, active slip systems and the Δθ angle (between ɑ-Ti phase (0001) plane normals (c-axes) and cyclic load directions). The Δθ is an adequate parameter to predict the fatigue fracture modes.

Strain hardening recovery mediated by coherent precipitates in lightweight steel

We investigated the effect of κ-carbide precipitates on the strain hardening behavior of aged Fe–Mn-Al-C alloys by microstructure analysis. The κ-carbides-strengthened Fe–Mn-Al-C alloys exhibited a superior strength-ductility balance enabled by the recovery of the strain hardening rate. To understand the relation between the κ-carbides and strain hardening recovery, dislocation gliding in the aged alloys during plastic deformation was analyzed through in situ tensile transmission electron microscopy (TEM). The in situ TEM results confirmed the particle shearing mechanism leads to planar dislocation gliding. During deformation of the 100 h-aged alloy, some gliding dislocations were strongly pinned by the large κ-carbide blocks and were prone to cross-slip, leading to the activation of multiple slip systems. The abrupt decline in the dislocation mean free path was attributed to the activation of multiple slip systems, resulting in the rapid saturation of the strain hardening recovery. It is concluded that the planar dislocation glide and sequential activation of slip systems are key to induce strain hardening recovery in polycrystalline metals. Thus, if a microstructure is designed such that dislocations glide in a planar manner, the strain hardening recovery could be utilized to obtain enhanced mechanical properties of the material.

Temperature-dependent fatigue response of a Fe44Mn36Co10Cr10 high entropy alloy: A coupled in-situ electron microscopy study and crystal plasticity simulation

The fatigue behavior of Fe44Mn36Co10Cr10 high entropy alloy (HEA) at room temperature and 300 °C was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD) and crystal plasticity simulation. The microstructure evolution and deformation process was observed by in-situ SEM fatigue experiment. The misorientation and geometrically necessary dislocation (GND) density was analyzed by EBSD. A crystal plasticity finite element (CPFE) modeling method based on the real grain morphology was proposed. The cumulative plastic strain, dominant slip system and dominant slip plane distributions were obtained. CPFE simulation shows multiple dominant slip systems being activated, which agrees with the multiple slip phenomena observed by in-situ SEM.

Electrodeposition of slanted nanotwinned Cu foils with high strength and ductility

The current study presents a new microstructure of nanotwinned copper (nt-Cu) with slanted columnar grains and its correlated mechanical properties. We deposited such nt-Cu foils via rotary electroplating. These unique nt-Cu foils possess higher tensile strength and ductility than that of straight columnar nt-Cu foils. An incline of slant degree results in the activation of multiple dislocation slip systems, where twin boundaries benefit from the highest strengthening effect of slanted grains. Thus, it leads to greater tensile strength and ductility of our foils compared to the <111>-oriented columnar nt-Cu. Our study presents a promising method to fabricate nt-Cu foils with excellent mechanical strength and ductility.

Enhanced resistance to fatigue crack propagation in metastable austenitic stainless steel by nanotwin bundles

Microstructure-sensitive fatigue crack growth in 304 metastable austenitic stainless steel containing nanotwin bundles was studied using a miniature compact tension specimen combined with post-fatigue metallographic examinations. The single-variant nanotwinned specimens exhibited high crack growth resistance compared to the coarse-grained specimen without nanotwin bundles, regardless of lamellar orientation. In the nanotwin bundles, martensite variants are formed with the habit plane parallel to the twin plane during crack propagation. Even when the crack propagates along the twin boundary, it tends to proceed by activation of multiple slip systems into the formed martensite phase, which inhibits brittle twin boundary separation. Post-fatigue transmission electron microscopy revealed that detwinning occurred ahead of the crack tip. Therefore, introducing nanotwin bundles into the metastable austenitic steel changes the course of damage accumulation through detwinning and martensite formation, enhancing the fatigue crack growth resistance.

Development of local plasticity around voids during tensile deformation

Voids can limit the life of engineering components. This motivates us to understand local plasticity around voids in a nickel base superalloy combining experiments and simulations. Single crystal samples were deformed in tension with in-situ high angular resolution electron back scatter diffraction to probe the heterogeneous local stress field under load; the reference stress is informed by crystal plasticity finite element simulations. This information is used to understand the activation of plastic deformation around the void. Our investigation indicates that while the resolved shear stress would indicate slip activity on multiple slip systems, slip is reduced to specific systems due to image forces and forest hardening. This study rationalizes the observed development of plastic deformation around the void, aiding in our understanding of component failure and engineering design.

Statistical aspects of grain-level strain evolution and reorientation during the heating and elastic-plastic loading of a Ni-base superalloy at elevated temperature

The advancement of microstructure-sensitive models for elevated temperature applications requires experimental data at the relevant length scales and usage environments. Towards this goal, we studied the evolution of grain-level strains in a Ni-based superalloy during heating and subsequent loading at elevated temperature. The specimen was heated using two halogen lamps in a furnace designed explicitly for the special load frame used for the synchrotron-based experiment. High energy X-ray diffraction was used to non-destructively characterize the initial 3D microstructure, confirm the temperature of the specimen, and monitor the evolution of grain-level strains, stresses, and orientations through the elastic-plastic transition. The distributions of grain-level normal strains were observed to broaden with elastic loading, whereas elastic-plastic loading led to development of local intergranular “hotspots”, or activation of plastic slip within select grains, that both broaden and skew the grain-level strain distributions. These grain-level strain distributions were fit to a generalized extreme value (GEV) distribution function. The GEV shape parameter is observed to change significantly at the onset of macroscopic plasticity. At the grain scale, more grain reorientation (plasticity) occurred for grains oriented for lower strength-to-stiffness ratios. During the onset of plastic deformation for LSHR at 660∘C load shedding from lower strength-to-stiffness-oriented grains to higher strength-to-stiffness-oriented grains was observed. Additionally, based on the grain-averaged reorientation axis, it was found that LSHR has a preference for activation of multiple slip systems at 660∘C. We conclude with a discussion on the critical need for statistical analysis of such mesoscale data obtained in situ at elevated temperature for the verification/validation of the next generation of microstructure-sensitive models.