Dislocation Multiplication Mechanism Research Articles

A steady-state irradiation creep and thermal creep model for zirconium alloys

In this work, a mechanistic steady-state creep model is developed to characterize the macroscopic strain rate of metallic materials affected by irradiation flux, testing temperature and applied stress. Thereinto, the steady-state strain rate is obtained by considering the density evolution of mobile dislocations, which involve the mechanisms of dislocation multiplication, dynamic annihilation and time-dependent dislocation annihilation. In order to effectively analyze the irradiation creep behavior in the low and high stress region, main attentions are focused on the influence of dislocation mobility on the process of time-dependent dislocation annihilation. At low stress, dislocation mobility is affected by dislocation climb and thermally activated glide. For the former, the absorption of irradiation-induced point defects can promote dislocation climb; whereas, thermally activated dislocation glide might be inhibited by the existing defect clusters. At high stress, the dominant deformation mechanism changes from dislocation climb to displacement cascade unpinning. For the latter, an explicit formula for dislocation mobility is deduced to address the influence of dislocation unpinning from the barriers on irradiation creep. To further verify the established model, thermal creep and irradiation creep data of zirconium alloys are considered to compare with the theoretical results. A good agreement is achieved over a wide range of temperature and stress indicating that the model can well describe the deformation behavior of steady-state creep. In addition, contribution of the dominant dislocation mobility components and dislocation density evolution components is compared under both thermal and irradiation creep, which can facilitate the comprehension of fundamental creep mechanisms of metallic materials.

Ductilizing Ti19Zr19Hf19Nb19TM5Be19 (TM = Fe, Co, Ni and Cu) high-entropy bulk metallic glass composites via in-situ precipitated refractory high-entropy alloy dendrites

In this work, a series of Ti19Zr19Hf19Nb19TM5Be19 (at.%, TM = Fe, Co, Ni and Cu) high-entropy bulk metallic glass composites (HE-BMGCs) were successfully developed to address the absence of tensile ductility in high-entropy bulk metallic glasses (HE-BMGs). It is shown that the mechanical properties of HE-BMGCs are jointly affected by the two constituent phases of refractory high-entropy alloy (RHEA) dendrites and HE-BMG matrix. The present composites show that the good tensile ductility as well as excellent work-hardening capability at ambient temperature. Based on the post-deformation microstructure and theoretical analyses, cross slips dominate the deformation mechanism of HE-BMGCs, and it is found that the dislocation multiplication mechanism composed of dislocation pinning and double cross-slip is very prevalent in current composites. Therein, the dislocation multiplication of RHEA dendrites facilitates the inhibition and retardation to the propagation of shear bands in the HE-BMG matrix, which is responsible for the good tensile ductility of HE-BMGCs. Additionally, the excellent work-hardening capability of composites is attribute to severe dislocation interactions caused by intrinsic local-regional multicomponent fluctuations and dislocation cross-slips in RHEA dendrites. Our research results not only aid in understanding the underlying deformation mechanism of HE-BMGCs, but also offer a novel perspective for designing the ductile high-entropy dual-phase alloys.

Analysis of strain hardening behavior of a high-Mn TWIP steel using electron microscopy and cyclic stress relaxation

Electron microscopy and cyclic stress relaxation experiments were employed to study the dislocation-obstacle interactions and activated hardening mechanisms, during tensile straining of a high-Mn TWIP steel specimen, and to estimate the contributions of hardening mechanisms to the strain hardening behavior. Electron contrast channeling and high-resolution electron backscattered diffraction images showed that deformation twins and their twin boundaries affect the dislocation evolutions inside both the matrix and twin lamellae. High-resolution transmission electron microscopy suggested that twin boundaries (TBs) can be passed by dislocations through the dislocation multiplication mechanism resulting in the nucleation of sessile dislocations and alternated stacking faults at TBs. Consequently, high dislocation density was developed behind the TBs resulting in enhancement of the strain hardening rate through the composite hardening effect. Various thermally activated parameters determined through cyclic stress relaxation tests indicated that dislocation evolution, developed by TBs in terms of the composite effect, was the prevalent rate-controlling mechanism, imparting higher strengthening than TBs in terms of the dynamic Hall–Petch effect. Finally, the high nucleation tendency of Shockley partial dislocations at TBs was interpreted from the variation of the stress exponent and independent plastic strain rate, and also from the general stacking fault energy point of view.

Mechanical tailoring of dislocation densities in SrTiO3 at room temperature

AbstractDislocation‐tuned functional properties such as electrical conductivity, thermal conductivity, and ferroelectric properties in oxides are attracting increasing research interest. A prerequisite for harvesting these functional properties in oxides requires successful introduction and control of dislocation density and arrangement without forming cracks, which is a great challenge due to their brittle nature. Here, we report a simple method to mechanically tailor the dislocation densities in single‐crystal perovskite SrTiO3. By using a millimeter‐sized Brinell indenter, dislocation densities from ∼1010 to ∼1013 m−2 are achieved by increasing the number of indenting cycles. Depending on tip radius and indenting load, large and crack‐free plastic zones over hundreds of micrometers are created. The dislocation multiplication mechanisms are discussed, and the work hardening in the plastic zone is evaluated by micro‐hardness measurement as a function of dislocation density. This simple approach opens many new opportunities in the area of dislocation‐tuned functional and mechanical studies.

A synchrotron X-ray topography study of crystallographic defects in ScAlMgO4 single crystals

ScAlMgO4 is a promising material that can be used to fabricate lattice-matched substrates for the epitaxial growth of GaN and InGaN. To improve its crystal quality, we used synchrotron X-ray topography to study the dislocations in a 50.8-mm-diameter ScAlMgO4 single crystal grown via the Czochralski method. Dislocation character was analyzed in terms of Burgers vectors by comparing dislocation contrasts recorded using six g-vectors that represented the m+c, a+c, and c-type crystal planes. The central area of the substrate, which corresponded to the portion grown vertically under the seed crystal, was characterized by millimeter-sized domains bound by mixed-type dislocation arrays with both in-plane and c-axis Burgers vectors. Basal plane dislocations (BPDs) typically formed an arrow-like shape with three branches. Growth striations with alternating high- and low-dislocation-density areas were observed in the diameter-expanding area and high-dislocation-density areas were covered with tangled BPD networks. The mechanisms of dislocation generation, multiplication, and reaction are discussed based on their line directions and dislocation character.

The gradual disappearance of yield plateau in Zr–Sn–Nb–Fe–Mo alloy by the trace addition of Cr and V

Trace chromium (Cr, 0.1 wt. %) and vanadium (V, 0.05 wt. %) were doped to the Zr–Sn–Nb–Fe–Mo alloy (denoted as Zir) to investigate the tensile properties at room temperature. A clear yield plateau was observed in Zir alloy, while the yield plateau was gradually disappeared by the addition of Cr to Zir alloy (denoted as Zir-0.1Cr) and by the combined addition of Cr and V to Zir alloy (denoted as Zir-0.1Cr-0.05 V). Microscopic characterization results revealed that doping of Cr and V retarded the re-crystallization process in Zir alloy. Firstly, the mean grain size of matrix was reduced from 1.9 μm to 1.7 μm and 1.5 μm by addition of 0.1% Cr and 0.1% Cr + 0.05% V to Zir alloy, respectively. The fraction of low angle grain boundaries (LAGBs) increased from 50.2% to 53.9% and 60.8% in Zir-0.1Cr and Zir-0.1Cr-0.05 V, respectively. Secondly, the volume fraction of second phase particles (SPPs) increased from 7.2% to 11.1% and 11.8% in Cr-containing and Cr + V-containing alloys, respectively. The mechanism of yield plateau in our alloy was explained by the impurity-dislocation interaction and dislocation multiplication mechanism, and the gradual disappearance of yield plateau behavior at the early stage of yielding could be attributed to the gradual increment of mobile dislocation density in Zir-0.1Cr and Zir-0.1Cr-0.05 V alloys. The tensile tests with different pre-strains proved that the gradual disappearance of yield plateau was caused by the increase of mobile dislocations with increasing pre-strain.

The elimination of the yield point phenomenon in a new zirconium alloy: Influence of degree of recrystallization on the tensile properties

The different yielding behaviors of annealed samples of Zr-Sn-Nb-Fe-Cu alloy and its dependence on microstructure have been studied. The yield drop was observed in samples with complete recrystallization, and this behavior gradually disappeared with the reduction of recrystallization degree. Meanwhile, as the strain rate increased, the yield point phenomenon (YPP) became less prominent in the stress-strain curves. It was assumed that solute-dislocation interaction mechanism and dislocation multiplication mechanism were responsible for this phenomenon. The unload-reload tests proved that the evolution of yielding behavior was caused by the increase of mobile dislocations with increasing pre-strain.

Microstructural characteristics and strengthening mechanisms in a polycrystalline Ni-based superalloy under deep cold rolling

Surface modification is an essential process route to improve the fatigue performance of aerospace components. Microstructural evolution in Ni-based superalloy Udimet720Li processed by deep cold rolling (DCR) was investigated experimentally using X-Ray diffraction, electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Deep cold rolling produces hardened surface due to a range of microstructural changes associated with grain refinement, low angle grain boundaries (LAGBs) formation, and pile-up of dislocations around γ' precipitates and across twin boundaries. The defect structures within the deformed subsurface comprised of equiaxed and elongated dislocation cells at grain boundaries, mutual interactions of slip bands, slip bands- γ' precipitate at grain boundaries and multi-variant modes of twinning. The plastic deformation is predominantly driven through slip and dislocation multiplication mechanism during DCR. Surface compressive residual stresses, FWHM, micro-hardness, the fraction of LAGBs and the depth of plastically strained region increased with DCR hydrostatic pressure. These fundamental understanding on process-microstructure-property could provide a deep insight into the fatigue crack initiation mechanism of surface modified Ni-based superalloys.

Discrete Dislocation Dynamics simulations of dislocation transport during sliding

Plastic deformation largely determines the properties of tribological contacts. To modify such contacts, it is thus necessary to understand the underlying mechanisms of dislocation motion and multiplication. In a sliding contact, dislocations do not only multiply, but also follow the moving tip — and are therefore ”transported” by the tip. Three dimensional Discrete Dislocation Dynamics simulations of sliding with a spherical tip are conducted to analyse dislocation transport in an fcc crystal, at stresses below those typically required to nucleate dislocations at a free surface. Sliding direction, glide plane orientation and Burgers vector orientation determine whether a dislocation can be transported or not, and how far the transport leads. A model is proposed to identify the glide systems, on which dislocations are transported.

Alternate approach for calculating hardness based on residual indentation depth: Comparison with experiments

It is well known that plastic deformation is a highly nonlinear dissipative irreversible phenomenon of considerable complexity. As a consequence, little progress has been made in modeling some well-known size-dependent properties of plastic deformation, for instance, calculating hardness as a function of indentation depth independently. Here, we devise a method of calculating hardness by calculating the residual indentation depth and then calculate the hardness as the ratio of the load to the residual imprint area. Recognizing the fact that dislocations are the basic defects controlling the plastic component of the indentation depth, we set up a system of coupled nonlinear time evolution equations for the mobile, forest, and geometrically necessary dislocation densities. Within our approach, we consider the geometrically necessary dislocations to be immobile since they contribute to additional hardness. The model includes dislocation multiplication, storage, and recovery mechanisms. The growth of the geometrically necessary dislocation density is controlled by the number of loops that can be activated under the contact area and the mean strain gradient. The equations are then coupled to the load rate equation. Our approach has the ability to adopt experimental parameters such as the indentation rates, the geometrical parameters defining the Berkovich indenter, including the nominal tip radius. The residual indentation depth is obtained by integrating the Orowan expression for the plastic strain rate, which is then used to calculate the hardness. Consistent with the experimental observations, the increasing hardness with decreasing indentation depth in our model arises from limited dislocation sources at small indentation depths and therefore avoids divergence in the limit of small depths reported in the Nix-Gao model. We demonstrate that for a range of parameter values that physically represent different materials, the model predicts the three characteristic features of hardness, namely, increase in the hardness with decreasing indentation depth, and the linear relation between the square of the hardness and the inverse of the indentation depth, for all but 150 nm, deviating for smaller depths. In addition, we also show that it is straightforward to obtain optimized parameter values that give good fit to the hardness data for polycrystalline cold worked copper and single crystals of silver.

Evolution of dislocation density in bainitic steel: Modeling and experiments

Bainitic steels with intensive lath boundaries demonstrate an excellent combination of strength and toughness. The bainite lath has an inhomogeneous distribution of dislocations across its lath thickness. The present work investigates quantitatively the effect of bainite lath boundaries with inhomogeneous dislocation distribution on the evolution of dislocation density in bainitic steels by both modeling and experimental works. For the first time, the Kocks-Mecking (K-M) model is extended by including two new terms to probe the boundary induced dislocation multiplication and annihilation mechanisms in bainitic steels. The initial dislocation density and the lath thickness of bainite are obtained by synchrotron X-ray diffraction analysis and microscopy observation, respectively. Based on the modeling and experimental works, it is found that the enrichment of dislocations at the lath boundary exhibits significant influence on multiplication as well as annihilation of dislocations during tensile deformation. The dislocation density at the bainite lath boundary is estimated by the present model as twice as the value in the bath interior, which is consistent with experimental observations.

Atomistic simulations on ductile-brittle transition in ⟨111⟩ BCC Fe nanowires

Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of ⟨111⟩ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10–1000 K employing a constant strain rate of 1 × 108 s−1. The results indicate that at low temperatures (10–375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2⟨111⟩ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450–1000 K. At 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2⟨111⟩ and immobile ⟨100⟩ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in ⟨111⟩ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transition increases with increasing nanowire size. The change in fracture behavior has been discussed in terms of the relative variations in yield and fracture stresses and change in slip behavior with respect to temperature. Further, the dislocation multiplication mechanism assisted by the kink nucleation from the nanowire surface observed at high temperatures has been presented.

Effect of pulsed magnetic treatment on the corrosion of titanium

ABSTRACTResults of corrosion tests of titanium in the initial state and after treatment using pulsed magnetic field are presented. It is shown that samples after treatment have better corrosion resistance due to the formation of denser and finer corrosion products with better adhesion to the substrate. Samples after treatment have more homogeneous microstructure due to a substantial increase of dislocations which are uniformly distributed. Mechanisms of dislocation multiplication and a model explaining the effect of the treatment on the corrosion are discussed.This paper is part of a themed issue on Materials in External Fields.

An investigation on the compressive strength enhancing mechanism of directionally solidified Ti-47Al-2Nb-2Cr-0.2Er alloy in case of cyclic loading

In the present study the microstructure evolutions and compressive strengths of the directionally solidified Ti-47Al-2Nb-2Cr-0.2Er alloy undergoing single and cyclic loadings are studied. It is found that bending in lamellar structure can be caused by compression and the extent of this bend exacerbates with the increase of compression strain. For the sample under cyclic-loading, the density of dislocation is improved, resulting in a higher compressive strength in comparison with the sample subjected to single loading with the same strain. Enhancements of Peierls stress, long-range elastic interactions between dislocations, and jog quantity all contribute to the higher strength of the cyclic-loading sample. Dislocation loops releasing from the semi-coherent interfaces under the applied stress are the main dislocation multiplication mechanism during the room temperature compression.

Dislocation multiplication mechanisms – Glissile junctions and their role on the plastic deformation at the microscale

Dislocation junctions are considered to control the hardening behavior of crystalline materials during plastic deformation. Here the influence of the glissile junction on the plastic deformation of microscale samples is investigated, based on discrete dislocation dynamics simulation results. It is found that with increasing dislocation density ρ, sample size d, which can be collapsed into a single dimensionless parameter dρ, and an increasing number of activated slip systems due to different global crystallographic orientations, the glissile junction forms frequently and can bow out easily, acting as an effective source. The resulting new dislocations are mobile and contribute to the macroscopic plastic deformation on the order of 30–60%. In the size regime from 0.5 to 2μm and dislocation densities in the range of 1012–1014m-2, the glissile junction is therefore an important source for generating mobile dislocation density. Furthermore a significant correlation between stress drops and activity of dislocations originating from glissile junctions is found. A rate formulation is proposed to include these findings in crystal plasticity or continuum dislocation density frameworks.

Mechanics and Dislocation Structures at the Micro-Scale: Insights on Dislocation Multiplication Mechanisms from Discrete Dislocation Dynamics Simulations

ABSTRACTThe plasticity of micro-pillar deformation has widely been studied by discrete dislocation dynamics simulations to explain the so-called size effect. In this study the role of glissile junctions forming during plastic deformation under various loading scenarios is in the center of interest. The activity of these naturally forming dislocation sources is followed in detail. Surprisingly these junctions are rather active sources and not just another obstacle as often assumed. Their relative contribution to the overall dislocation density for the simulated specimens reaches often values of 20% or even more. The formation of such a glissile junction is often correlated to stress drops or the end of a stress drop. It is therefore suggested – at least for the sample sizes considered – that this dislocation multiplication mechanism should be take into account in continuum models such as crystal plasticity of higher order dislocation continuum theories.

Mechanisms of dislocation multiplication at crack tips

Whether a stressed material fractures by brittle cleavage or ductile rupture is determined by its ability to convert elastic strain energy to plastic deformation through the generation and motion of dislocations. Although it is known that pre-existing dislocations play a crucial role in crack tip plasticity, the involved mechanisms are unclear. Here it is demonstrated by atomistic simulations that individual pre-existing dislocations may lead to the generation of large numbers of dislocations at the crack tip. The newly generated dislocations are usually of different types.The processes involved are fundamentally different for stationary cracks and propagating cracks. Whereas local crack front reorientation plays an important role in propagating cracks, the multiplication mechanism at stationary cracks is connected with cross-slip in the highly inhomogeneous stress field of the crack. Analysis of the forces acting on the dislocations allows to determine which dislocations multiply and the slip systems they activate. These results provide the necessary physical link between pre-existing dislocations and the generation of dislocations at crack tips.

Twinning dislocation multiplication at a coherent twin boundary

Using in situ nanoindentation in a high-resolution transmission electron microscope we have studied the interaction of glide dislocations with a Σ3 {1 1 1} coherent twin boundary (CTB) of growth type in Cu. These high-resolution observations indicate that, in addition to acting as barriers to slip transmission, CTB can react with a lattice dislocation to facilitate the multiplication of partial dislocations, resulting in translation of the CTB. On the basis of dislocation theory and molecular dynamics (MD) simulations we propose a dislocation multiplication mechanism by which successive dissociation reactions, starting with a single glide dislocation trapped at a CTB, can lead to a continuous source under certain applied stress states. No evidence of deformation twinning was noted in the nanotwinned lamellae in the indented foils. These findings provide insights into understanding the plastic deformation mechanisms, the migration of CTBs the high strength, and work hardening of highly twinned face-centered cubic metals.

Dislocation cross-slip in GaN single crystals under nanoindentation

The dislocation multiplication and movement mechanism in GaN single crystals has been studied using nanoindentation and cathodoluminescence. Dislocation loops can multiply and move from plane to plane by cross-slip, thus producing a wide plastic deformation in GaN during indentation. This mechanism is further supported by the remarkable movement of indentation induced dislocations during annealing. Furthermore, the so-called pop-in events, in which the indenter suddenly enters deeper into the material without the application of any additional force, can be better understood by considering the cross-slip mechanism.

Frank–Read sources and the yield of anisotropic cubic crystals

Frank–Read sources are among the most important mechanisms of dislocation multiplication, and their operation signals the onset of yield in crystals. We show that the critical stress required to initiate dislocation production falls dramatically at high-elastic anisotropy, irrespective of the mean shear modulus. We hence predict the yield stress of crystals to fall dramatically as their anisotropy increases. This behaviour is consistent with the severe plastic softening observed in α-iron and ferritic steels as the α − γ martensitic phase transition is approached, a temperature regime of crucial importance for structural steels designed for use in future nuclear applications.