Dislocation Glide Research Articles

Atomistic and continuum study of interactions between super-screw dislocations and coherent twin boundaries in [formula omitted]-structured [formula omitted

Informed by previous experimental observations, this study employed a combination of molecular dynamics simulations and dislocation continuum theory to investigate the interplay of super-screw dislocations and coherent twin boundary (CTB) in Ni3Al. The results reveal multiple interaction mechanisms depending on both the applied stress and the pathway for dislocation gliding. A continuum model framework has been developed to quantitatively evaluate the critical shear stress necessary for the CTB to accommodate dislocations along different pathway with the effects of anti-phase boundary (APB) and Complex Stacking fault (CSF) considered. The critical shear stress exhibits a significant inverse dependence on the quantity of dislocations, rendering it unsuitable as a measure of twin boundary strength. Instead, the resistant force of the CTB against all gliding dislocations is suggested as a more appropriate metric for quantifying its strength. This enables a direct comparison of the twin boundary strength between Ni and Ni3Al containing different amounts of Shockley dislocations, thereby extending its applicability to a wider range of materials. Our work offers new mechanistic insights critical for understanding and quantitative analysis of the interplay of super-dislocations and micro twining in nickel-based superalloys.

Insights into the influence of Ni4Ti3 precipitates and martensite transformation on the glide of a[100] dislocation in austenitic NiTi alloys: an atomistic simulation study

Dislocation-dominated plastic deformation in austenitic NiTi alloys is highly detrimental and it could be suppressed by precipitation of Ni4Ti3. Additionally, at a certain temperature range, not only Ni4Ti3 precipitate but also the B19’ martensite produced from stress-induced martensitic transformation may prevent dislocation glide in austenitic NiTi alloys. In order to shed further light on this issue, four atomic configurations were constructed using an edge dislocation (a [100]) and two Ni4Ti3 precipitate variants (precipitate-V1 and precipitate-V2) to investigate dislocation glide in austenite containing Ni4Ti3 precipitate under shear at temperatures of 500 K or 700 K. As a result, all configurations at 500 K undergo the shear-induced martensitic transformation, producing B19’ martensite that impedes dislocation glide. And at 700 K, dislocation was pinned by precipitate or distortion zones caused by the superposition of strain field of precipitate and dislocation, without martensitic transformation. Furthermore, it was proven that precipitate-V1 impedes dislocation glide more strongly than precipitate-V2 at both temperatures of 500K and 700 K. It was attributed to the different in spatial relative location between the precipitate and dislocation and explained by the Orowan dislocation-precipitate bypass stress. Finally, three strategies for achieving high shear fatigue resistance in NiTi shape memory alloys were proposed based on these findings.

Development of low rare-earth containing magnesium alloy with high strength-ductility synergy by engineering nano-spaced lamellae

Conventional Mg-RE-based alloys containing a high content of RE solutes can generate high strength, but there is often low ductility. Herein, we reported a low RE alloying Mg–5Er-1Nd-1Zn-0.2Zr extrusion alloy with only 6 wt% RE addition, showing a high strength-ductility synergy, such as a yield strength of ∼380 MPa and an elongation of ∼12.5 %), which is superior to the majority of traditional Mg-RE-based wrought alloys with at least 12 wt% RE addition. Nano-spaced lamellae was formed in the α-Mg matrix during extrusion, such as stacking faults (SFs) evolved in dynamically recrystallized (DRXed) grains and the 14H long-period stacking-ordered (LPSO) phase developed in unDRXed grains as a result of dynamical solute partitioning during extrusion. And this alloy characterizes a high proportion (∼70 %) of elongated unDRXed grains with a strong basal texture. Thus, high strength is mostly related to SFs/LPSO lamellae strengthening and texture hardening, instead of the commonly accepted refinement strengthening and precipitation hardening in those of high RE containing Mg alloys. While high ductility is heavily dependent on SFs/LPSO lamellae to trigger strain hardening and <c + a> dislocations glide, independent of fine grains as in traditional Mg alloys with high elongation. The findings give an insight into the development of high-performance Mg wrought alloys with a high strength-ductility synergy.

Effects of hydrothermal alteration on shear localization and weakening in the mantle lithosphere

Hydrothermal alteration occurs in upper-mantle fault zones and significantly modifies the rheological properties of ultramafic rocks. To understand how hydrothermal alteration affects shear localization and the strength of the fault zones, we conducted simple-shear deformation experiments on water-saturated harzburgite (70 wt% olivine +30 wt% orthopyroxene) gouges sandwiched between three types of shear pistons (tungsten, yttria-stabilized zirconia, and corundum) using two deformation-DIA apparatuses at confining pressures of 1–4 GPa, temperatures of 500–580 °C, average shear strain rates of 6.1 × 10−6 s−1 to 1.0 × 10−4 s−1, and water contents of 4–30 wt%. The harzburgitic gouges with water contents of 10 and 30 wt% exhibited steady-state sliding or strain-hardening behavior, and the shear strength of the latter sample was less than one-third of that of the former sample, possibly reflecting the difference in pore fluid pressure. Olivine/orthopyroxene grains exhibit distributed microfracturing and grain rotation towards P foliation, indicating the operation of cataclastic flow. Shear displacement is also accommodated by the development of serpentine- or talc-bearing multiple shear zones including B, R1, and/or Y shears, where intense cataclasis-related grain size reduction and dissolution/precipitation occur. For experimental runs using tungsten and zirconia shear pistons, the serpentine or talc blades that wrap finer-grained and dissolved olivine/orthopyroxene grains form an interconnected network, indicative of frictional sliding or dislocation glide on their basal planes as a dominant deformation mechanism. Raman spectra of the serpentine minerals indicate that they represent antigorite with low crystallinity. These findings suggest that when deep lithospheric fault zones undergo hydrothermal alteration, deformation is controlled by competition between cataclastic flow in olivine/orthopyroxene matrix and slip or glide of antigorite or talc in semi-brittle fault zones, the predominance of which depends on the amount of the hydrous phyllosilicates and the magnitude of pore fluid pressure.

Alloy strengthening toward improving mechanical and tribological performances of CuxNi100−x/Ta nano multilayer materials

Alloy strengthening via introducing Ni atoms into Cu monolayer of Cu/Ta nano multilayer materials (NMMs) is used to enhance mechanical and tribological properties. Molecular dynamics simulations indicate that the introduction of solid-solution Ni atoms suppresses the dislocation multiplication and glide, leading to higher hardness, better tribological performances, and stronger deformation resistance for CuxNi100−x/Ta NMMs. The deformation resistance in nanoindentation is mainly determined by hardness and interlayer interface, where the interface acts as dislocation barrier and emission source to restrain main compression stress within about three monolayers. The resistance against friction-induced deformation relies on the comprehensive properties of strengthening monolayers and alloying interfaces, which induces further removal of Ta atoms. The atom-scale insights on the NMMs strengthening and the corresponding deformation mechanism are helpful for designing new wear-resistant materials.

Mechanism of thermally-activated prismatic slip in Mg

Prismatic slip of the screw <a> dislocation in magnesium at temperatures ≳ 150 K is understood to be governed by double-cross-slip of the stable basal screw through the unstable prism screw and back to the basal screw, with the activation energy controlled by the formation energy of two basal-basal kinks. However, atomistic studies of the double-kink process predict activation energies roughly twice those derived experimentally. Here, a new mechanism of prism glide is proposed, analyzed theoretically, and demonstrated qualitatively and quantitatively via direct molecular dynamics (MD) simulations. The new mechanism is intrinsically 3d, and involves the nucleation of a single kink at the junction where a 3d prismatic dislocation loop transitions from the basal screw segment to non-screw prismatic character. The relevant kink energies are calculated using recently-developed Neural-Network Potentials (NNPs) for Mg that show good agreement versus DFT for basal and prism <a> dislocations, enabling a parameter-free analytic model for the activation barrier. Direct MD simulations show both operation of the precise proposed mechanism and a stress-dependent activation barrier that agrees reasonably with the analytic model. Predictions of dislocation velocity compare very well with in-situ TEM data, and macroscopic strength versus temperature can also be understood. Overall, the new intrinsically 3d mechanism for dislocation glide due to single kink nucleation rather than double-kink nucleation explains key features of the prismatic slip in Mg and may have broader applicability in other metals where kink nucleation processes control thermally-activated flow.

Experimental and computational analysis of stacking fault energy in B-doped Fe50−XMn30Co10Cr10BX multi-principal elements alloys

The effect of boron doping in Fe50−XMn30Co10Cr10BX multi-component alloys on the resulting stacking fault energy has been experimentally and computationally assessed to understand the alloys' deformation mechanisms from structural and thermodynamic perspectives. Firstly, the fcc and hcp phases were identified together with stacking faults along their (110) planes using high-resolution transmission electron microscopy. At the same time, they were theoretically predicted through thermodynamic CALPHAD and ab initio calculations. The average stacking fault energy for the boron-free alloy was 23.9 ± 2.4 mJ/m2, suggesting that the deformation mechanisms relate to dislocation slip and deformation twinning. The average stacking fault energy for the highest boron content (5.4 at%) was 50.1 ± 14.1 mJ/m2, indicating dislocation glide as the possible deformation mechanism. The boron content in the solid solution was modelled. The modelling suggested that the presence of Cr-B, Mn-B and Fe-B bonds points towards forming (Cr,Fe)2B borides, which was experimentally confirmed. The borides, fcc phase stability, and the boron in solid solution contribute to increased stacking fault energy, preventing the motion of Shockley partial dislocations and influencing the ɛ-hcp martensitic transformation.

Pivotal role of retained austenite as a low temperature creep controlling mechanism in a martensitic spring steel

The rising demand for electric vehicles urges automotive suppliers to push their limits on the sag resistance of suspension coil springs. However, the mechanism contributing to sag loss in a coil spring or low temperature creep (LTC) in a spring steel is still unclear. Furthermore, the LTC rate-controlling mechanism remains elusive. In the current study, an attempt has been made to unfold the LTC controlling mechanism in a SAE 9254 steel grade. A combined analysis by means of (i) a mechanism-based exhaustion creep model (ECM) and (ii) advanced microstructural characterization prior to and post LTC suggests that dislocation glide, mainly localized in the metastable γ phase, is considered as one LTC contributing mechanism in martempered SAE 9254. Furthermore, stress-assisted martensitic transformation (SAMT) is considered as additional LTC contributing mechanism in SAE 9254 up to the temperature T≤323K. Beyond that temperature strain-induced martensitic transformation (SIMT) takes the place of SAMT. Our approach includes that the LTC rate-controlling mechanisms are stress-assisted recovery (SAR), SAMT, and SIMT especially at elevated temperature T.

Dislocation-mediated migration of the α/β interfaces in titanium

Interphase boundaries are essential in the deformation and phase transformations in titanium (Ti) alloys. While static structures of semicoherent α/β interfaces in various Ti alloys have been carefully examined, their migration behavior at atomic scales is far less clear. In this study, we employed molecular dynamics simulations to investigate the migration of the semicoherent α/β interface in pure Ti. The interface migration behavior shows a shear-coupled feature with the interface dislocation glide and a macroscopic shear. The simulation reveals that both the glide direction of the dislocations with respect to the interface and the dislocation spacing strongly influence the migration rate, and the low-index glide plane of the interface dislocation plays a minor role. The dependence of interface mobility on temperatures confirms the critical role of thermal activation during the interface migration, especially for activating the interface dislocation glide. Furthermore, the shear-coupled interface migration driven by element partition is simulated using a newly developed Ti-Mo potential, consistent with the displacive-diffusional features previously observed in the surface precipitates. The simulated interface migration mode is validated by comparing it with the crystallography features of surface precipitates in a Ti-Cr alloy. The interface energy and mobility obtained from simulations further explain why the distinctive crystallographic features of the surface precipitates observed experimentally are favored over other candidate interfaces. The present study has explored an approach for systematically examining thermodynamic and kinetic factors governing the development of phase transformation crystallography at different temperatures and chemical driving forces.

Anisotropic strain relaxation in epitaxially constrained α -(Al,Ga)2O3 thin films on a-plane Al2O3

Binary α-Ga2O3 and ternary α-(AlxGa1−x)2O3 thin films with x = 0.29 and 0.54 were deposited on a-plane Al2O3 substrates via plasma-assisted molecular beam epitaxy. The strain relaxation dynamics along three orthogonal crystal directions was investigated ex situ and in situ by synchrotron-based high-resolution x-ray diffraction. A pronounced in-plane anisotropy was observed as strain is preferably built up across [1¯1.0] whereas along [00.1] it is effectively relieved. The particular behavior depends on the Al content with almost immediate plastic relaxation for x = 0 and 0.29. For x = 0.54, strain relaxation is elastic within the first monolayers. A dislocation glide on the r-plane slip system has been identified as the dominating plastic relaxation mechanism as layers grow thicker.

Structures and energies of computed silicon (001) small angle mixed grain boundaries as a function of three macroscopic characters

Small angle grain boundaries comprised of dislocation structures have unique impacts on the performances of crystalline materials. In this atomistic study, we report structure-property correlations of Si (001) small angle mixed grain boundaries (SAMGBs) across three macroscopic GB characters (tilt character, twist character, and an implicit rotation character between them). Firstly, for the computed SAMGB energies spanning the three-dimensional GB character space, a revised Read-Shockley relationship is proposed and fitted, which hints that the energy trends follow the Read-Shockley formalism from the insight of GB decomposition. GB structural transitions from dislocation to amorphous structures are also given as a function of dislocation core radii, allowing the estimation of small angle GB proportion. Then, the proportion, topology and structural signatures of different SAMGB types defined from the ratio between the tilt and twist angles are presented. By extracting the transformation of metastable SAMGB phases, the formation process of these (001) SAMGB structures is characterized as the energetically favorable dislocation glide and reaction, from which the dislocation density function is derived. The relevant results about SAMGB energies and structures are validated and supported by theoretical calculations and experimental observations, respectively.

Modeling the material behavior of heterogeneous materials considering a two‐scale FE–FFT‐based simulation framework

AbstractIn various engineering applications, components subjected to high mechanical or multi‐physical loads such as thermal, thermo‐mechanical, or electro‐thermal loads are predominantly made of metals or composites. These materials are characterized by polycrystalline or multi‐phase microstructures which determine the overall material response. However, since the microscopic material behavior is directly related to the distribution, size, and morphology of the individual grains or phase constituents, precise predictions of the macroscopic material response require simulations of the microstructural behavior. Hence, this work considers a two‐scale finite element (FE) and fast Fourier transform (FFT)‐based simulation framework, which is an efficient alternative to the classical method for the simulation of periodic unit cells. Assuming scale separation, the macroscopic and microscopic boundary value problems are first solved individually and subsequently linked by performing a scale transition in terms of averaging the macroscopic quantities over the corresponding local fields. While the macroscopic solution is computed by means of the finite element method, the microscopic boundary value problem, which is embedded as a periodic unit cell in each macroscopic integration point, is solved using an FFT‐based simulation method. In general, these microscopic simulations allow to model grain‐scale phenomena such as martensitic phase transformation or plastic deformation caused by dislocation glide which can be observed in polycrystalline materials. In this work, we present a two‐scale FE–FFT‐based model to capture the martensitic phase transformations in shape memory alloys. In order to demonstrate the applicability of the model, a numerical example is given.

Effect of Severe Shot Peening on Mechanical Properties and Fatigue Resistance of Wire Arc Additive Manufactured AISI 316L

The wire arc additive manufacturing (WAAM) technology has become very popular recently. However, the properties of printed pieces are not yet examined properly. In this paper the effect of severe shot peening (SSP) on mechanical properties of wire arc additive manufactured AISI 316L is investigated. The effect of SSP on the surface hardness of the WAAM 316L is investigated by performing microhardness measurements. Changes to the surface microstructure caused by SSP are evaluated in the EBSD investigation. The effects of SSP on tensile and fatigue strength are investigated experimentally. The EBSD analysis showed that there has been remelting of each printed layer due to the heat input from the next printed layer, and heavy epitaxial grain growth was present in the microstructure. This led to coarse columnar grain structure. Investigation of deformed SSP surface layer indicated that the main deformation type was either conventional dislocation glide or twinning. This meant that no martensite formation was present on the surface. The SSP increased the surface hardness of WAAM printed 316L by 225% and the hardened layer was 0.4 mm thick. The SSP improved the yield strength of WAAM 316L by 34%. The SSP significantly improved WAAM 316L fatigue resistance in both low-cycle and high-cycle regime.

Modelling thermal and irradiation creep with crystal plasticity theory and self-consistent method

In order to help comprehend the underlying deformation mechanisms related to irradiation creep of metallic polycrystalline materials, the crystal plasticity theory and viscoplastic self-consistent method are combined in this work. In the developed constitutive laws for single crystals, the influence of irradiation-induced defects on the creep strain rate has been effectively addressed by considering dislocation climb and glide. For the former, dislocation climb with irradiation effect can be enhanced through the absorption of point defects by dislocations, where the steady-state concentration of point defects is affected by the defect sinks including dislocations and defect clusters. As a comparison, dislocation glide may be retarded through the impediment of mobile dislocations by irradiation-induced defects. More importantly, the effect of dislocation static recovery has been incorporated into the dislocation evolution law, which is noticed to play a deterministic role for the annihilation of dislocations during the long-term creep process. In addition, the viscoplastic self-consistent method is taken as a cross-scale way to predict the irradiation creep properties of polycrystals. To validate the developed creep model, experimental data of both single crystalline and polycrystalline nickel has been considered under both thermal and irradiation creep. A good agreement between the theoretical results and experimental data is achieved, which offers a solid basis to further analyze the macroscopic irradiation creep deformation from the aspect of microstructure evolution.

Origin of residual strain in heteroepitaxial films

Heterogeneous integration of diverse materials structures is critical to the scaling of electronic and photonic integrated circuits. For a model system of Ge-on-Si, we experimentally examine the roles of lattice misfit and thermal expansion misfit in determining the residual strain in as-grown and annealed heteroepitaxial films. We present data for Ge-on-Si growth from 400 to 730 °C followed by heat treatment from 500–900 °C. We show that strain fluctuations of 5.02% enable misfit dislocation formation, and we propose a comprehensive model for the conversion of compressive misfit strain to tensile elastic strain. The model is expressed in terms of three regimes: (1) misfit control for the low temperature growth regime at 400 °C; (2) point defect control via annealing in the point defect recovery regime at 500–650 °C; and (3) thermal expansion control for growth or anneal at T &amp;gt; 650 °C in the dislocation recovery regime. Growth from 400 to 730 °C exhibits near complete misfit strain relief by misfit dislocations leaving a consistent residual compressive strain of 0.09%. Growth at 400 °C followed by post growth heat treatment at 600 °C results in vertical threading dislocation density reduction via a point defect-mediated climb mechanism that gives minimal strain relief. Anneal above 650 °C promotes strain relief by dislocation glide. Temperature excursions at T &amp;gt; 730 °C followed by cooling to room temperature yield plastic strain in the Ge film that cannot be further relieved by thermal expansion misfit accommodation. Growth at 400–730 °C retains a residual compressive strain that represents the nucleation threshold for misfit dislocations.

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of &lt;110&gt; {111} dislocations. Then dislocations cut into γ′ phases as individual &lt;110&gt; super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The &lt;110&gt; super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance.

Creep behavior of Ti–6Al–4V fabricated by electron beam directed energy deposition

In this study, the creep behavior of Ti–6Al–4V fabricated by electron beam directed energy deposition was investigated. The results showed that the creep behavior was controlled by a combination of dislocation gliding and climbing. The stress index was rationalized to 4.1–4.4 by introducing threshold stress. The fracture behavior was influenced by the combination of hole aggregation, crack growth and reduction of the external cross-sectional area due to necking. The smaller steady-state creep rate of the transverse samples over the vertical samples resulted from more α phases with difficult-to-deform orientation in the transverse samples. The β annealing treatment resulted in significant creep strengthening due to the formation of a fine Widmannstätten structure that impeded dislocation movement at high temperature. EB-DED had a high potential to enhance the creep performance of Ti64.

Resonance with surface waves induces forbidden velocity bands in dislocation glide

We use molecular dynamics simulations to demonstrate a pronounced influence of free surfaces on the mobility of dislocations at near-sonic velocities. By correlating instantaneous velocities v and resolved stresses τ, we show that the mobility relations of both edge and screw characters are composed of multiple ranges of stable uniform motion separated by bands of forbidden velocity. The lower limiting velocity for uniform motion of both edge and screw dislocations is the Rayleigh wave speed, cR. When forced to propagate at an average velocity greater than cR, dislocations exhibit cyclic, intermittent jumps in instantaneous velocity between cR and higher velocity branches of the mobility relation. Bands of forbidden velocity may be calculated directly from the velocity dependence of dislocation drag functions, which contain peaks near cR and—in the edge case—one other characteristic surface wave speed, which we associate with shear-horizontal (SH) waves. We propose that peaks in the drag function are due to resonance of free surface vibrations forced by nearby moving dislocations.

Deformation behaviour of a cast Fe–19Al–5Cr-0.04Zr alloy at room and intermediate temperatures

The mechanical properties of a Fe–19Al (at.%) alloy containing 5 at.% Cr as the main alloying element are presented and discussed. While the alloy behaves brittle at room temperature and 100 °C, tensile stress-strain curves at temperatures between 200 °C and 500 °C show strain hardening recovery effects similarly to those of fcc twinning induced plasticity (TWIP) steels. It is supposed that a combination of dislocation glide and formation of internal boundaries in the submicron range leads to similar strain hardening stages at intermediate temperatures (200 °C–500 °C) in the FeAl alloy as usually observed in TWIP steels at room temperature. The maximum strain hardening rate after recovery reaches very high values of about G/19 at 400 °C. Low-cycle fatigue (LCF) tests at 400 °C and 500 °C up to strain amplitude values of 0.6% and stress-relaxation tests do not reveal high back stresses or pseudoelasticity in the FeAl alloy. This indicates that the deformation structures responsible for the strain hardening rate recovery remain stable upon stress relief.

Optoelectronic and structural characterization of trapezoidal defects in 4H-SiC epilayers and the effect on MOSFET reliability

To this day, trapezoidal defects are found in clusters and high counts in wafers representing the industry standard in terms of material quality being produced. This study sheds light on the nature, origin, behavior, and impact of this defect on device yield and reliability. Trapezoidal defects in 4H-SiC epitaxial layers were investigated by photoluminescence (PL) imaging, scanning electron microscopy (SEM), cathodoluminescence spectrum imaging (CLSI), SEM electron beam induced current (EBIC) imaging, and by transmission electron microscopy (TEM) observation. The bar-shaped stacking faults were identified by the PL and CL measurements with a peak emission wavelength of 420 and 450 nm. An optoelectronic behavioral study based on the recombination enhanced dislocation glide mechanism revealed how expanding dislocations and stacking faults interact with each other. Combining the luminescence and microscopy results, the nature of the stacking faults was identified as being a combination of Shockley-type and Frank-type stacking faults. The TEM analysis showed that these defects originate from the substrate and the stacking sequences of some of the faults were determined as (…2, 4, 2…) and (…2, 3, 2…) in the Zhdanov's notation by high-resolution TEM. The origin of this defect is speculated based on our results and previous reports. The EBIC imaging showed that the high density of SFs in these towers is a strong site of carrier recombination, which presumably has an impact on the transfer characteristics of SiC devices. Furthermore, these defects have shown to impact metal oxide semiconductor field effect transistors electrical performance via an increase in the on-state resistance depending on the coverage percentage of the tower of defects in the active area of the device.