Easiest Slip System Research Articles

Natural fabrics of a hornblende-rich amphibolite: Implications for hornblende crystallographic preferred orientation and seismic anisotropy of the lower crust

The relation of seismic anisotropy to the deformation and flow of the lower crust relies on the intrinsic properties of the minerals in the crust and on the formation of crystallographic preferred orientation (CPO). Hornblende is an important constituent of the continental and oceanic lower crust and often exhibits a strong CPO. However, the formation of CPO in hornblende is not well understood, and it is difficult to uniquely relate its texture and anisotropy to conditions of metamorphism-deformation. We present a petrofabric analysis, using electron backscatter diffraction, of two hornblende-rich amphibolites from the Mamonia complex (Cyprus) with peak pressure and temperature of 0.6 GPa and 610 ± 40 °C. We distinguish between microstructures with varied strains and show that, while the common hornblende CPO—in which the [001] is aligned with lineation—is independent of strain, its symmetry does depend on strain: it is hexagonal (axial-[001]) under low strains but becomes orthorhombic ([001] and (100) parallel to the lineation and foliation normal of the deformation, respectively) under increasing strains. This transition is correlated with a decrease in the shape-preferred orientation and an increase in the intragrain misorientation, whose rotation is consistent with the easy slip system (100)[001] of hornblende. Thus, we interpret the hexagonal CPO to represent a metamorphic fabric (a texture formed under quasi-static conditions) and the orthorhombic CPO to represent a solid-state deformation fabric (a texture formed by strain accommodated via dislocation-mediated deformation). We consider the seismic anisotropy of the natural hornblende textures and compare two tectonic scenarios of lower crust deformation - crust thickening and channel flow with marked differences in the expected seismic anisotropy between the two scenarios. Taken together, the dataset presented here can be used as a new framework for understanding and interpreting natural textures and seismic anisotropy from the lower crust.

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

AbstractSeismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep.

Microstructure evolution of annealed 7075 aluminum alloy and its influence on room-temperature plasticity

Compared to hot forming, cold forming can manufacture components with higher accuracy and better mechanical properties. However, for some light-weight and high-strength materials, such as 7xxx series aluminum alloy, their room-temperature plasticity is poor, which limits the production of complex structural components of these materials. In this study, the microstructure evolution of annealed 7075 aluminum alloy and its influence on room-temperature plasticity were investigated. Firstly, the annealing experiments with different annealing temperature and soaking time were conducted, and the annealed microstructure evolution was analyzed. Secondly, the room-temperature tensile tests were conducted to evaluate the room-temperature plasticity. The relationship between annealed microstructure and plastic deformation behavior was studied. The new effect mechanisms of annealed microstructure on room-temperature plasticity was found. The results indicated that the annealed grains with high Schmid factor (˃0.4) adjacent to coarse granular-shaped secondary particles exhibited easy multiple slip system activation and smoothly transformed strain through the slip bands, which promoted the grain fragmentation. Concurrently, the secondary particles underwent rupture and separation. Owing to the low strain localization and multiple slip systems of the grain fragments, they can coordinately deform around the prior particle-matrix interfaces or between the separate particle pieces, which led to good room-temperature plasticity.

Effect of heat treatment on the tensile behavior of selective laser melted Ti-6Al-4V by in situ X-ray characterization

Selective Laser Melted Ti-6Al-4V (as-SLMed) exhibits decreased yield strength, increased work hardening, and increased ductility after heat treatment at 730 °C (HT-730) or 900 °C (HT-900) for 2 h. To understand the change of mechanical properties, in situ high energy X-ray diffraction (HEXRD) is used to examine the phase composition, load partitioning, slip system activity, and dislocation density evolution in all three specimens. The as-SLMed specimen is dominated by martensitic α΄. After heat treatment, α΄ partly or fully decomposes into α+β, reducing the yield strength. In HT-730, β precipitates with confined size show much higher lattice strain than the α΄/α matrix during deformation; in HT-900, the lattice strain difference is mostly eliminated. This is a key reason for the increased ductility in HT-900. From the anisotropic lattice strain development, basal slip is identified as the easiest slip system in α΄/α. Using an elasto-plastic self-consistent (EPSC) model, the critical resolved shear stress ratio between prismatic slip and basal slip (CRSSprism/CRSSbasal) is estimated to be 1.31 and 1.16 in the as-SLMed and the HT-900 specimens, respectively. The α phase in HT-900 is able to activate multiple slip systems and accumulate more dislocations during plastic deformation. This explains why HT-900 has better ductility and higher work hardening rate than the other two specimens.

Influence of plastic properties on the grain size effect on twinning in Ti and Mg

Deformation twinning is an inevitable mode of plastic deformation in hexagonal close packed (HCP) crystals and has been considered a hinderance to achieving formability and ductility. It has long been recognized that reductions in grain size can lower the propensity for twinning. In prior work, EBSD statistical analyses on big data sets of twins in pure Ti and Mg were carried out for the same twin type, loading conditions, initial texture, and range of grain sizes. Here, we revisit these data sets to investigate the role of material plastic properties on grain size effects on deformation twinning. We show that while reductions in grain size dramatically lower the number of twins per grain and the twin thickness in both metals, these effects are several times stronger in Ti than that for Mg. To rationalize this difference, full-field, 3D crystal plasticity calculations are performed. The analysis indicates that the stronger grain size effect in Ti arises due to the larger critical resolved shear stress (CRSS) of its easiest slip system, making Ti less accommodating of the twin shear than Mg. This result points to plastic properties as additional variables that can be tuned towards developing HCP materials for structural applications.

On the Origin of Radial Anisotropy Near Subducted Slabs in the Midmantle

AbstractRecent seismic studies indicate the presence of seismic anisotropy near subducted slabs in the transition zone and uppermost lower mantle (mid‐mantle). In this study, we investigate the origin of radial anisotropy in the mid‐mantle using 3‐D geodynamic subduction models combined with mantle fabric simulations. These calculations are compared with seismic tomography images to constrain the range of possible causes of the observed anisotropy. We consider three subduction scenarios: (i) slab stagnation at the bottom of the transition zone; (ii) slab trapped in the uppermost lower mantle; and (iii) slab penetration into the deep lower mantle. For each scenario, we consider a range of parameters, including several slip systems of bridgmanite and its grain‐boundary mobility. Modeling of lattice‐preferred orientation shows that the upper transition zone is characterized by fast‐SV radial anisotropy anomalies up to −1.5%. For the stagnating and trapped slab scenarios, the uppermost lower mantle is characterized by two fast‐SH radial anisotropy anomalies of ∼+2% beneath the slab's tip and hinge. On the other hand, the penetrating slab is associated with fast‐SH radial anisotropy anomalies of up to ∼+1.3% down to a depth of 2,000 km. Four possible easy slip systems of bridgmanite lead to a good consistency between the mantle modeling and the seismic tomography images: [100](010), [010](100), [001](100), and . The anisotropy anomalies obtained from shape‐preferred orientation calculations do not fit seismic tomography images in the mid‐mantle as well as lattice‐preferred orientation calculations, especially for slabs penetrating into the deep lower mantle.

The dislocation properties of B2-AlPr in {110} plane

The dislocation properties of intermetallic compound (IMC) B2-AlPr in {110} plane is studied in detail based on the truncated approximation method in this paper. The results suggest that the core width of screw dislocation and edge dislocation of Burgers vector is wider than that of . Meanwhile, their unstable-stacking-fault energy ($$ \gamma_{\text{us}} $$) of $$ \gamma_{\text{us}} \langle 110\rangle > \gamma_{\text{us}} \langle 001\rangle $$ indicates that the unstable-stacking-fault energy performs very important function on the core width of screw dislocation. The easiest slip system of B2-AlPr is {110}. The lattice misfit energy is always less than elastic strain energy except the dislocation angle with 54.7°. The phase position of misfit energy and elastic strain energy is always showing opposite phases. However, the misfit energy is greater than elastic strain energy for 54.7° dislocation, and shows the same phase. The misfit energy, elastic strain energy, total energy and the corresponding stress decrease with the increasing of dislocation angle.

Deformation and recrystallization mechanisms inferred from microstructures of naturally deformed rock salt from the diapiric stem and surface glaciers of a salt diapir in Southern Iran

Microstructures of three naturally deformed rocksalt samples, from the Deh Kuyeh salt fountain in South Iran, were studied by electron backscatter diffraction to constrain deformation and recrystallization processes. Two samples came from the upper salt fountain glaciers and one from the diapiric stem. Lattice distortion and low-angle boundaries in salt grains have intra-crystalline misorientations described by rotations around low-order crystallographic axes and are inferred to relate to dislocation creep and recovery. Misorientation analysis suggests that both easy and hard slip systems are responsible for grain distortions. Small grains have similar dimensions to subgrains, which we interpret to mean that subgrain rotation recrystallization (SGR) is important. Many boundaries are quite lobate, consistent with grain boundary migration (GBM) also being active. Bulges off of some large grains are related to the host by a tilt or twist boundary suggesting that GBM and SGR interacted to facilitate a bulge nucleation process. Idiomorphic grains occur, particularly in the upper fountain (surface glaciers) samples and are less deformed internally than neighboring grains. The grain and subgrain sizes in the sample from the diapiric stem are smaller than those in the upper salt fountain. Subgrain sizes suggest differential stresses were twice as large (2 MPa) during deformation in the diapiric stem compared to the upper fountain. The diapiric stem sample has greater intragranular distortion compared to the fountain samples. Furthermore, the diapiric stem sample has a continuous grain-boundary hierarchy, suggesting a dominance of SGR, whereas the upper fountain samples have different boundary hierarchy characteristics for boundary misorientations above and below ∼7° suggesting that GBM is limiting the SGR process: grains are consumed by GBM before boundaries with misorientations >7° developed by SGR. The increased importance of GBM relative to SGR in the upper fountain compared to the diapiric stem is consistent with the change in balance of the different recrystallization mechanisms expected as a function of lower stress magnitude in the upper fountain.

Electronic and atomic structures of edge and screw dislocations in rock salt structured ionic crystals

ABSTRACTFirst-principles calculations were performed to investigate electronic and atomic structures of edge and screw dislocations in NaCl and AgCl. 1/2<110> edge and 1/2<110> screw dislocations, which are the ones due to the easiest slip system of these crystals, were treated in a first-principles manner. It was found that the edge dislocation has extended core structures over several atomic planes in both NaCl and AgCl. In contrast, the screw dislocation in AgCl showed the different shear strain around the dislocation core from that in NaCl although the one has a similar atomic structure in the two crystals. Local electronic structures around the dislocation cores in AgCl exhibit larger bonding interactions between atomic orbitals than those in NaCl, indicating that covalent interactions are more significant at the dislocation cores of AgCl. From excess dislocation energies, it was found that the dislocations in AgCl are much more stable than those in NaCl. Especially, the screw dislocation of AgCl has much smaller core energy than that of the edge dislocations. This suggests that the screw dislocations can be easily formed and multiplied in AgCl, which may be closely related to the observed ductility of AgCl.

Proliferation of twinning in hexagonal close-packed metals: Application to magnesium

Plastic deformation of metallic alloys usually takes place through slip, but occasionally involves twinning. In particular, twinning is important in hexagonal close packed (HCP) materials where the easy slip systems are insufficient to accommodate arbitrary deformations. While deformation by slip mechanisms is reasonably well understood, comparatively less is known about deformation by twinning. Indeed, the identification of relevant twinning modes remains an art. In this paper, we develop a framework combining a fundamental kinematic definition of twins with large-scale atomistic calculations to predict twinning modes of crystalline materials. We apply this framework to magnesium where there are two accepted twin modes, tension and compression, but a number of anomalous observations. Remarkably, our framework shows that there is a very large number of twinning modes that are important in magnesium. Thus, in contrast to the traditional view that plastic deformation is kinematically partitioned between a few modes, our results suggest that deformation in HCP materials is the result of an energetic and kinetic competition between numerous possibilities. Consequently, our findings suggest that the commonly used models of deformation need to be extended in order to take into account a broader and richer variety of twin modes, which, in turn, opens up new avenues for improving the mechanical properties.

Strength of orthoenstatite single crystals at mantle pressure and temperature and comparison with olivine

Oriented single crystals of orthopyroxenes (OPx) were deformed in axisymmetric compression in the D-DIA at pressure and temperature in excess of 3 GPa and 1040 °C. Two crystal orientations were tested with the compression axis parallel to either [101]c crystallographic direction, to investigate [001](100) dislocation slip-system strength, or [011]c direction to investigate [001](010) slip-system strength. These slip systems are the most active in orthopyroxenes. Applied differential stresses and specimen strain rates were measured in situ by synchrotron X-ray diffraction and radiography. We used these data and comparison with previously reported low-pressure flow laws for protoenstatite and orthoenstatite to determine the power law parameters for the deformation of orthoenstatite crystals, which characterize OPx dislocation slip-system strengths. Applying these laws at reasonable mantle stresses along oceanic and continental geotherms indicates that OPx [001](100) slip system is weaker than OPx [001](010) slip system to ∼260 km depth where the strengths converge. It also indicates that both OPx slip systems are significantly stronger than olivine slip systems throughout the upper mantle, except in the upper most mantle, in the lithosphere, were OPx [001](100) slip system may be as weak or even weaker than olivine [100](010) easy slip system.

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe3C) and (Fe,Ni)3C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1 0 0] and [0 0 1] type, [1 0 0] being the most frequent. [1 0 0] dislocations are dissociated and glide in the (0 1 0) plane. [0 0 1] dislocations glide in (1 0 0) and (0 1 0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.

On the deformation mechanism of olivine single crystals at lithospheric temperatures: an electron tomography study

We present an electron tomography study of dislocations in deformed olivine single crystals, at ca . 0.5 T m , along two distinct orientations. The easiest slip systems are [001](100) and [001]{110}. Disorientating a single crystal away from easy glide conditions leads to massive cross-slip, which generates three-dimensional dislocations, and thus contributes to hardening. Fast motion of curved non-screw dislocation in those planes leaves long straight screw dislocations which bear lattice friction and control plastic strain. We have identified several hardening mechanisms. Non-screw [001] dislocations interact elastically to form dipoles. Recovery mechanisms leading to dipole annihilation are observed, but they are slow at those temperatures and produce numerous sessile loops. These loops represent obstacles for gliding dislocations. Interactions between dislocations and sessile loops produce sessile segments (super jogs), which efficiently impede dislocation motions.

Calculation of the influence of plastic deformation on the evolution of crack stress intensity factors in a body-centered cubic crystal

The evolution of stress intensity factors in a mixed loading mode has been calculated for different cleavage planes, the direction of the crack front, and the easy slip systems near the crack tip in a body-centered cubic (bcc) crystal. Calculations take into account the mutual influence of the plastic deformation and crack tip shape as well as T stress. The dependence of screening the crack tip by dislocations on the crack orientation, easy slip system, and fracture modes is found.

Bond deformation paths and electronic instabilities of ultraincompressible transition metal diborides: Case study ofOsB2andIrB2

The energetically most stable orthorhombic structure of ${\mathrm{OsB}}_{2}$ and ${\mathrm{IrB}}_{2}$ is dynamically stable for ${\mathrm{OsB}}_{2}$ but unstable for ${\mathrm{IrB}}_{2}$. Both diborides have substantially lower shear strength in their easy slip systems than their metal counterparts. This is attributed to an easy sliding facilitated by out-of-plane weakening of metallic Os-Os bonds in ${\mathrm{OsB}}_{2}$ and by an in-plane bond splitting instability in ${\mathrm{IrB}}_{2}$. A much higher shear resistance of Os-B and B-B bonds than Os-Os ones is found, suggesting that the strengthened Os-B and B-B bonds are responsible for hardness enhancement in ${\mathrm{OsB}}_{2}$. In contrast, an in-plane electronic instability in ${\mathrm{IrB}}_{2}$ limits its strength. The electronic structure of deformed diborides suggests that the electronic instabilities of $5d$ orbitals are their origin of different bond deformation paths. Neither ${\mathrm{IrB}}_{2}$ nor ${\mathrm{OsB}}_{2}$ can be intrinsically superhard.

Elasticity and dislocations in ice X under pressure

We determine the compressibility, the elasticity and the slip systems for ice X in the 100–350GPa pressure range from first-principles calculations. Our results show that ice X has a high elastic anisotropy, which increases by more than two orders of magnitude over this pressure range. We use the Peierls-Nabarro model to compute the dislocation structure for the most likely glide systems. We obtain that the easiest slip systems are 〈111〉{110} at low pressures, and 〈100〉{110} at higher pressure. This suggests that the mantle of large icy planets may have different rheological properties at shallow and at deep conditions. In contrast the mantle of smaller icy planets would be more homogeneous from a rheological point of view.

Protracted fabric evolution in olivine: Implications for the relationship among strain, crystallographic fabric, and seismic anisotropy

Crystallographic fabrics in olivine-rich rocks provide critical information on conditions and mechanisms of deformation as well as seismic properties of Earthʼs upper mantle. Previous interpretations of fabrics produced in laboratory experiments were complicated by uncertainty as to whether the steady-state fabric was attained. To examine the systematics of the evolution of olivine crystallographic fabrics at high strain, we conducted torsion experiments on olivine aggregates to shear strains of up to ∼20. Our results demonstrate that a steady-state fabric is not reached until a shear strain >10, a much higher value than previously thought necessary. Fabrics characterized by girdles of [010] and [001] axes or by clusters of [010] and [001] axes are both observed. Until now, these fabrics were associated with either two different deformation mechanisms or two different sets of deformation conditions. Here we establish that both fabrics are, in fact, part of the same evolutionary process. An eigenvalue analysis allows the fabric shape to be quantitatively correlated with the magnitude of shear strain. Misorientation analysis suggests that the observed fabric evolution results from the competition of the two easiest slip systems in olivine, (010)[100] and (001)[100]. Our results open up the possibility of using olivine crystallographic fabrics or seismic anisotropy to quantitatively evaluate strain histories in both field studies and geophysical investigations of upper-mantle rocks.

Micropillar compression deformation of single crystals of the intermetallic compound ζ-FeZn13

The deformation behavior of the ζ phase in the Fe–Zn system has been investigated by micropillar compression tests at room temperature with the use of single crystals with 13 different crystal orientations prepared by the focused ion beam method. Two different slip systems, {110}〈11¯2〉 and (100)[001], are observed to operate. The critical resolved shear stresses (CRSS) value for {110}〈11¯2〉 slip is more than three times smaller than that for (100)[001] slip. From the anisotropy in CRSS for these two slip systems, {110}〈11¯2〉 slip is predicted to operate for most crystal orientations, except for a narrow orientation region around [3¯05] where (100)[001] slip operates. The CRSS for {110}〈11¯2〉 slip shows an inverse power-law scaling against the specimen size with an exponent of −0.517. The bulk CRSS value for {110}〈11¯2〉 slip is estimated to be 62–76MPa by taking into account the specimen size effects of CRSS. The reasons why {110}〈11¯2〉 slip with a rather long Burgers vector (0.7700nm) is selected as the easiest slip system are discussed in terms of the nature of atomic bonding in the crystal structure, especially the rigid atomic bonding within an Fe-centered Zn12 icosahedron (for slip plane selection), and the energetic barrier height along the slip direction and the resultant possible dissociation schemes (for slip direction selection). Some implications are made on how the deformability of the ζ phase can be improved in the textured coating layer in galvannealed steels based on the results obtained.

Compression of Single-Crystal Micropillars of the ζ Intermetallic Phase in the Fe-Zn System

ABSTRACTThe deformation behaviour of the ζ (zeta) phase in the Fe-Zn system has been investigated via room-temperature compression tests of single-crystal micropillar specimens prepared by the focused ion beam method. Trace analysis of slip lines indicates that {110} slip occurs for the specimens investigated in the present study. Although the slip direction has not been uniquely determined, comparison of Schmid factors and yield stress values suggests that the slip direction might be &lt;1$\overline 1 $2&gt;, which is inconsistent with the easiest slip system {110}[001] predicted on the basis of the primitive Peierls-Nabarro model.

Dislocation creep accommodated by grain boundary sliding in dunite

To investigate the role of grain boundary sliding during dislocation creep of dunite, a series of deformation experiments were carried out under anhydrous conditions on fine-grained (∼15 μm) samples synthesized from powdered San Carlos olivine and powdered San Carlos olivine+1.5 vol.% MORB. Triaxial compressive creep tests were conducted at a temperature of 1 473 K and confining pressures of 200 and 400 MPa using a high-resolution, gas-medium deformation apparatus. Each sample was deformed at several levels of differential stress between 100 and 250 MPa to yield strain rates in the range of 10−6 to 10−4 s−1. Under these conditions, the dominant creep mechanism involves the motion of dislocations, largely on the easy slip system (010)[100], accommodated by grain boundary sliding (gbs). This grain size-sensitive creep regime is characterized by a stress exponent of n=3.4±0.2 and a grain size exponent of p=2.0±0.2. The activation volume for this gbs-accommodated dislocation creep regime is V*=(26±3)×10−6 m2·mol−1. Comparison of our flow law for gbs-accommodated dislocation creep with those for diffusion creep and for dislocation creep reveals that the present flow law is important for the flow of mantle rocks with grain sizes of 20 MPa. Hence, gbs-accommodated dislocation creep is likely to be an important deformation mechanism in deep-rooted, highly localized shear zones in the lithospheric upper mantle.