Dislocation Arrangements Research Articles

Interpretation of the stress dip test as a means of characterizing backstress: Experiments and backstress-aided crystal plasticity modeling of polycrystalline tantalum

Backstresses, associated with certain dislocation arrangements and their inter-dislocation long-range stresses, are known to contribute significantly to deformation response of metals, including kinematic hardening, the Bauschinger effect (BE) and the Hall-Petch effect. Various methods have been employed to quantify these backstresses at the macro-scale. One of these approaches, which has received relatively little attention, is the stress dip test. The strain rate observed during a load dip and hold, after previous plastic deformation, can be positive or negative, depending upon the level at which the load is held, and the relative magnitudes of competing friction and backstresses. The most direct interpretation of previously reported tests indicates a surprisingly high level of backstress in common materials, and which is generally also higher than the value extracted from an unload-reload test. In this paper, stress dip tests are performed on pure polycrystalline tantalum, along with unload-reload tests. A plateau is seen in the strain rate observed during the stress dip test, which has not been previously reported. If the backstress is interpreted to correspond with the stress level associated with the middle point of the plateau, in line with the friction/backstress model of the unload-reload test, the resulting backstress obtained from both tests are very similar. A novel crystal plasticity model, incorporating backstress, reversible dislocations and non-Schmid effects, is used to help justify this new approach. The model predicts the observed plateau in strain rate, and provides a slip-level interpretation of the macroscopically observed backstress. The slip-level backstress (when considered as a fraction of the stress prior to the dip) is reasonably similar to the values interpreted from the dip test experiment. The ∼23% lower value in the simulation may be due to the lack of certain aspects of the actual physics in the model.

Fracture behaviors and microstructure evolution of an Al–Mg–Mn-Sc-Zr alloy at elevated and cryogenic temperature

The mechanical properties and fracture behavior of Al-5.8Mg-0.4Mn-0.25Sc-0.1Zr alloy were investigated across cryogenic temperatures (−196.15 °C ∼ −56.15 °C) and elevated temperature (room temperature ∼ 250 °C), with results indicating sensitivity of the alloy's mechanical properties to temperature. Firstly, the tensile test at elevated temperatures shows a decrease in strength and strain hardening exponent (n) due to thermal softening effects, while elongation increases and then decreases, peaking at 100 °C. Within the temperature range of −196.15 °C ∼ −56.15 °C, the yield stress (YS), ultimate tensile strength (UTS), and ductility tend to increase with the decreasing temperature gradually, which can be attributed to the decrease in lattice thermal vibration energy at low temperatures, resulting in an elevated external force requirement for dislocation motion. Meanwhile, throughout the entire high-temperature range, the alloy exhibits ductile fracture, with the number of dimples peaking at 100 °C. Differently, when the alloy is deformed at low temperatures, dislocations tend to accumulate within the grains, and with the grain boundaries acting as weak bonding areas between grains and experiencing significant stress concentration, the fracture mechanism gradually transitions from transgranular to intergranular fracture. Microstructural analysis at low temperatures shows minimal texture content changes, while at elevated temperatures, regular changes in texture components occur due to grain orientation rearrangement. Furthermore, high-resolution transmission electron microscopy was employed to investigate the evolution of precipitates at various temperatures, and the results shows that uniform and fine Al3(Sc1−x, Zrx) precipitates are distributed within the crystal, acting as effective dislocation pinning sites and leading to a uniform intragranular dislocation arrangement after tensile test at cryogenic temperature. Notably, after tensile test at higher temperatures, a few precipitates lose coherency with the matrix and started growing, which diminishing grain boundary pinning effect and promoting recrystallization.

A comparison of conventional and additively manufactured 316L under thermomechanical fatigue

The present study compares the thermomechanical fatigue (TMF) behaviour of 316L stainless steel manufactured conventionally by hot rolling and additively by laser powder bed fusion (L-PBF). Machined cylindrical specimens were tested under strain-controlled in-phase and out-of-phase TMF loading at a temperature range of 550–750 °C and total mechanical strain amplitudes of εamech = 0.2–0.6 %. While the conventional 316L significantly outperforms the L-PBF 316L under in-phase TMF, their lifetimes are comparable under out-of-phase TMF. Under in-phase TMF, creep damage in the form of intergranular crack networks occurs which is significantly more pronounced for the L-PBF 316L due to the higher amount of interfaces. Under out-of-phase TMF, the damage is mostly due to stress-assisted oxide cracking and the crack propagation is fatigue-dominated. TEM inspection revealed that L-PBF 316L exhibits a cellular dislocation structure in the initial state, which rearranges only slightly during cycling. For conventional 316L similar dislocation cells form during TMF cycling indicating that they represent a stable dislocation arrangement in 316L under TMF loading. This is evidenced by the rather stable cyclic stress response of the L-PBF material when compared to the conventional material.

Transmission-scanning electron microscopy of interface fracture of ferrite deformation twins

AbstractDeformation twins in the ferrite matrix of an age-hardened duplex stainless steel have been observed using on-axis transmission Kikuchi diffraction (TKD) in a scanning electron microscope. This provided details of the lattice misorientation and dislocation arrangement, including the dislocation-free zone at the twin tip. These observations provide evidence for the link between microcracking of the irregular twin/parent interface and relaxation of the residual strains that arise from twin growth, offering new insights into fracture mechanics in these materials. Graphical abstract

Kinking induced plasticity in a novel refractory high-entropy alloy

A novel Ti41V27Hf13Nb13Zr6 refractory high-entropy alloy (RHEA) with an excellent fracture elongation of 30 % at a high strength level of 986 MPa was successfully fabricated. We discovered that kinking induced plasticity, involving successive stages of dislocation arrangement, pre-kinking, and the coalescence of multiple kink bands. The soft character of kinking originated from the degraded dislocation density within the kink bands. As such, the interplay between strain hardening caused by dislocation accumulation and the softening effect due to kink bands led to a plateau-type engineering stress-strain curve to large strains. These findings provide a novel design pathway to achieve ductile RHEAs via deformation kinking.

Yield stress anomaly and creep of single crystal Ni-base superalloys – Role of particle size

In the present work we subject the single crystal Ni-base superalloy ERBO1 (CMSX 4 type) to constant strain rate (CSR) and creep testing at temperatures between 1023 and 1223 K. Three material states are considered which have similar particle volume fractions >60% but differ in γ′-particle sizes (material states S, M and L of particle sizes: 240, 390 and 540 nm). In constant strain rate testing, a yield stress anomaly is observed for all three material states, with a yield stress maximum at 1073 K. This increase of strength with increasing temperature is not observed during creep testing at significantly lower deformation rates in this low temperature high stress creep regime, where different elementary deformation mechanisms govern CSR and creep behavior. In contrast, in the low stress high temperature creep regime, stress/strain rate data pairs from CSR creep tests both show decreasing strength with increasing temperature. It is found that in both types of tests the material state M shows the highest strength (highest yield stress and lowest creep rate). This can be rationalized based on a scenario where both, γ-channel and γ′-particle dislocation activities are important. Diffraction contrast transmission electron microscopy is used to study the relevant elementary deformation processes. Details of dislocation arrangements are discussed with a special focus on the role of Kear Wilsdorf (KW) locks, γ′-particle shearing by superlattice stacking faults (extrinsic and intrinsic) and dislocation climb.

High-Temperature Tensile Behavior of an As-Cast Ni-W-Co-Ta Medium–Heavy Alloy

High-temperature tensile experiments with tensile rates ranging from 0.01 s−1 to 10 s−1 were carried out at various temperatures ranging from 1000 °C to 1250 °C with a Gleeble-3800 thermal simulation tester to evaluate the physical properties of an as-cast Ni–W–Co–Ta medium–heavy alloy. The microstructure evolution of the alloy during high-temperature stretching was characterized by metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results indicated the emergence of multiple slip lines and the parallel arrangement of dislocations in the grain of the alloy after high-temperature stretching, and typical characteristics of plane slipping were observed. The plasticity of the Ni–W–Co–Ta medium–heavy alloy increased, but its strength decreased with an increase in the deformation temperature. In contrast, an increase in the strain rate resulted in a noticeable increase in the strength and plasticity of the medium–heavy alloy. The experiments revealed that the maximum tensile strength of the as-cast Ni–W–Co–Ta medium–heavy alloy was 735 MPa (T = 1000 °C, ε˙ = 10 s−1). Additionally, the maximum reduction in area and elongation was 38.1% and 11.8% (T = 1250 °C, ε˙ = 10 s−1), respectively. The mode of fracture after high-temperature tensile deformation was brittle fracturing.

Multiscale evaluation of hydrogen-assisted mechanical degradation in grade 2 titanium

The mechanism of hydrogen embrittlement in grade 2 titanium was studied by a combined micro and nanoscale approach. Insights into the hydrogen related deformation and degradation mechanisms were acquired via a unique characterisation procedure, correlating the topological observations on the fracture surface after tensile testing to the dislocation arrangement underneath. Focused ion beam lift-out was employed to extract a subsurface slice perpendicular to the fracture surface. High resolution characterisation provided information about the dislocation configuration and active hydrogen-assisted degradation mechanism. A high hydrogen content forms a brittle titanium hydride phase, failing via hydride cleavage. At a moderate hydrogen quantity, hydrogen in solid solution is responsible for a ductility decrease via the hydrogen-enhanced localised plasticity mechanism.

Multiscale study of additively manufactured 316 L microstructure sensitivity to heat treatment over a wide temperature range

The main subject of this study is to investigate the correlations between the evolution of mechanical behavior and the multiscale microstructure of 316 L stainless steel obtained by laser powder bed fusion process (LPBF) after various post-manufacturing heat treatments across a wide temperature range. The microstructure of 316 L LPBF parts exhibits a hierarchical microstructure based on unique grain structures, chemical segregations, dislocation arrangements at the microscopic scale and fine nano-oxides at the nanoscopic scale. These microstructural elements play a crucial role in determining the material's mechanical and corrosion properties. To understand how different microstructural features contribute to the material's behavior, the researchers conduct post-manufacturing heat treatments to isolate and study these components. The results show that dislocation and/or micro-segregation networks significantly influence the high tensile properties of 316 L LPBF steel in its as-built state and after heat treatments above 900∘C. Despite their disappearance during heat treatments, the material maintains high tensile strength due to an increase in strain-hardening capabilities. The study also examines the impact of nano-oxides and the σ phase on the material's properties. The contribution of nano-oxides to yield strength diminishes with increasing temperature. Interestingly, the σ phase does not necessarily lead to a detrimental effect on failure elongation for 316 L LPBF steel. Overall, this research provides insights into the relationship between microstructure and mechanical properties for additively manufactured stainless steel. By understanding these relationships, it becomes possible to tailor the microstructure to achieve desired mechanical properties for specific applications and extend the use of 316 L LPBF to higher temperatures compared to conventional methods.

Study on the relationship between microstructure and properties of heavy-wall X70 pipeline steel with different processing directions

The neglect of microstructure and anisotropy in the design of subsea pipeline steel can seriously affect the pipe service life and lead to fracture failure. In this work, experimental and representative volume element (RVE) model results were used to analyze the effect of differences in the microstructural characteristics of heavy-wall ferritic/bainitic dual-phase (DP) ×70 pipeline steel in the transverse and rolling directions on the mechanical properties. The results showed that blocky bainite with an aspect ratio 1.9 in the transverse microstructure induced the ferrite to produce strain localization, decreasing the plasticity. Banded bainite with an aspect ratio of 4.4 in the rolling direction was more prone to activate the dislocation slip system, which enhanced deformation compatibility by limiting the ferrite deformation space through plastic deformation at low strain levels. The dislocation wall type of the rolling direction promoted the production of back stress (long-range internal stress) and reduced the strain gradient at the interface. The dislocation arrangement in the transverse direction was transformed from a mixed type to a dislocation wall type during cyclic loading, and the back stress was dynamically saturated by grain size. Higher forest hardening in the transverse direction increased strength and hardening rates, while higher back-stress hardening in the rolling direction promoted plasticity. The results reveal the relationship between microstructural characteristics difference and mechanical properties and back stress evolution to provide a theoretical basis for designing the heavy-wall ×70 pipeline steel.

The evolution of subsurface deformation and tribological degradation of a multiphase Fe-based hardfacing induced by sliding contact

Multiphase Fe-based hardfacing alloys, for example Tristelle 5183 Fe-21%Cr-10%Ni-7.5%Nb-5%Si-2%C in wt.%, are extensively used for tribological applications, including valves, bearings and drive mechanisms, where two surfaces are unavoidably subjected to loaded sliding contact within engineering systems. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterise, for the first time, the tribologically affected material induced by the self-mated sliding contact of HIPed Tristelle 5183. This provided novel insight into the deformation modes which permit the accumulation of the high levels of subsurface strain required for plasticity dominated (adhesive) wear in a commercial hardfacing. In the subsurface regions furthest from the sliding contact, plastic deformation is accommodated by deformation induced martensitic transformation to ϵ-martensite and α′-martensite, twinning, the generation of planar dislocation arrangements (generated by planar slip) and the generation of dislocation tangles. Closer to the sliding contact, the subsurface becomes unstable, and nanocrystallisation driven by grain boundary mediated deformation mechanisms and crystallographic slip completely engulf the near surface microstructure. It is postulated that nanocrystalisation within the subsurface is a needed in order to accommodate the extremely high strains required in order to permit tribological degradation via plasticity dominated wear. The extrusion of metallic slivers via plastic ratcheting generates ductile shear cracks governed by plastic strain, and the failure of these slivers generates plate/flake-like wear debris.

Influence of pre-existing configurations of dislocations on the initial pop-in load during nanoindentation in a CrCoNi medium-entropy alloy

ABSTRACT The origin and mechanisms responsible for incipient plasticity in metals are still poorly understood. Moreover, the reasons for the recently reported large scattering of the initial pop-in load remain unclear. Hence, this study addresses these issues through a combination of nanoindentation tests and electron channelling contrast imaging characterisation considering a CrCoNi medium-entropy alloy. Experimental findings were also supported by elastic calculations that consider both the indentation and dislocation stress fields. A wide scatter in the maximum shear stress underneath the indenter, as expected, was observed for the analysis based on dislocation density. As a consequence, the spatial arrangement of dislocations within the indented region or local dislocation configuration is introduced as a new parameter to overcome overly simple analysis based on the dislocation density. The maximum shear stress underneath the indenter increased from 6 GPa for dislocation closer to the indentation axis to 11 GPa at 600 nm for dislocation far away from it. Additionally, elastic calculations revealed that the response to the incoming nanoindenter was different for dislocations with different configurations. Thus, the complex interactions of stress fields due to configurations of dislocations and indentation account for the large scatter of the maximum shear stress beneath the indenter.

Experiment on evacuation behavior: Applying the sector grid model to a clear area

Researching personnel evacuation is crucial for preventing and reducing emergencies. In this study, the movement and retention laws relating to personnel in building bottlenecks are derived from research on safe evacuation from building bottlenecks and personnel evacuation tests. Accordingly, a fan-grid partitioning method is proposed for building bottlenecks. Because the personnel distribution in the area is fan-shaped, a dynamic grid-division method is proposed and applied to the safe evacuation simulation model. Additionally, a dislocation arrangement is utilized for the front and rear grids, and the experimental analysis reveals that the time step in the safety evacuation simulation. Moreover, a grid partition model is proposed, and the evacuation parameters of the evacuation area bottleneck are characterized via the human flow rate sequence. The Visual Basic language can be utilized in the safe evacuation simulation algorithm for building bottleneck, which has great significance in guiding research on the safe evacuation of personnel during fire hazards.

Dislocation Arrangements and Cyclic Microplasticity Surrounding Stress Concentration in a Ni‐Based Single‐Crystal Superalloy

Local cyclic plasticity near stress concentrations governs the fatigue crack initiation in cyclicly loaded Ni‐based single‐crystal superalloys, but has not been well studied and understood. The first of its kind transmission electron microscopy (TEM)‐based site‐specific study of plasticity in the crack initiation region in a notched single‐crystal superalloy subjected to fatigue testing at 800 °C, coupling it with microstructure‐based crystal plasticity modeling, is presented. Detailed TEM examinations show that local plasticity near the notch significantly differs from bulk plasticity, featuring high dislocation densities and distinctive arrangements of dislocation pairs within γ’ precipitates. It further shows that the increased local stresses alone are responsible for the increase in dislocation density and extensive γ’ shearing, but not solely for the distinctive arrangement of dislocation pairs seen in the notch vicinity, thus highlighting the considerable role played by the local variations in loading rates and stress state surrounding the notch. The results of this work provide new fundamental insights into the deformation micromechanisms leading to fatigue crack initiation in single‐crystal superalloys.

Fractional densities and character of dislocations in different slip modes from powder diffraction patterns

Irradiation induced dislocation loops and lattice dislocations produced by plastic deformation have very different dipole characters, dislocation contrasts and range of strain fields. To determine partial dislocation densities of the different dislocation types from X-ray or neutron diffraction patterns, all differences in dislocation properties should be considered. We have extended the convolutional multiple whole profile line profile analysis method to convolve different strain profiles with different effective outer cut-off radii, corresponding to different dislocation type, into a single size profile. The extended procedure is applied to determine the 〈a〉 loop and lattice dislocation densities in a neutron irradiated and tensile deformed Zr-2.5 %Nb alloy and in a proton or neutron irradiated Zircaloy-2 alloy. We show that when the dislocation densities are very different the effective outer cut-off radius of dislocations is a better descriptor of dislocation character than the dislocation arrangement parameter, which was used previously in line profile analysis. Our results show that the combination of line profile analysis and electron microscopy methods provides a detailed and comprehensive description of defect structures in deformed and irradiated materials.

Deformation and damage mechanisms during clockwise diamond and counter clockwise diamond thermomechanical fatigue in Timetal 834 alloy

In the present investigation, the thermomechanical fatigue (TMF) behavior of Timetal 834 alloy was studied under clockwise diamond (CWD) and counter clockwise diamond (CCD) loading conditions. These tests were conducted within 723 K ↔ 873 K temperature range at the strain amplitudes of Δεm/2 = ±0.6% and ±1.0%. The cyclic softening behavior observed under CWD-TMF condition at both strain amplitudes, is attributed to shearing of coherent Ti3Al precipitates. Contrarily, the alloy exhibits both cyclic hardening and softening responses under the CCD-TMF conditions. This additional hardening response is attributed to the occurrence of dislocation–dislocation interactions coupled with dynamic strain aging. The present study reveals that both phase angle and strain amplitude affect the TMF life. The alloy exhibited relatively lower fatigue life under CCD-TMF condition than under CWD-TMF condition, at both strain amplitudes. Transmission electron microscopy analyses of the post-deformed specimens reveal that the dislocation arrangements are similar in both loading conditions at Δεm/2 = ±0.6%, while being distinctively different at Δεm/2 = ±1.0%. At higher strain amplitude, the fatigue life is significantly reduced under CCD-TMF condition. Fractographic analyses indicate the involvement of oxidation damage during both the TMF loading conditions. However, its effect is more pronounced under CCD-TMF than under CWD-TMF condition.

Microstructural evolution investigation of irradiated oxide fuels by means of electron backscattered diffraction

In this paper, both irradiated UO2, together with a fresh UO2.001 sample from the same batch as a reference, and irradiated FR-MOX U0.55Pu0.45O1.99 (TRABANT 2/3) fuel were studied using Electron Back Scattered Diffraction (EBSD). Irradiation of both oxide fuels was carried out at the High Flux Reactor (HFR) Petten, the UO2 at a moderate maximum Linear Heat Rate (LHR) of 200 W/cm to a burn-up of 5.4 % FIMA (fission per initial metal atom) or 50 MWd/kgUO2, and the MOX at a high maximum Linear Heat Rate (LHR) of 374 W/cm to a burn-up of 1.7% FIMA. In the irradiated UO2, the grain growth was observed in the fuel centre. The irradiated MOX (U, Pu)O2 showed the typical FBR fuel microstructure zones, i.e. as-fabricated (at the fuel edge), equi-axed and columnar grains as observed from the fuel-clad region towards fuel central hole.The LAGB (Low Angle Grain Boundary), 3° < θ < 15°, as an initial state for the HBS structure was observed on both UO2 and MOX. This restructuring mechanism for both oxide fuels is proposed to follow the well-known polygonization mechanism in which an arrangement of dislocations at the grain boundaries occurs.

Delineating dislocation structures and residual stresses in additively manufactured alloys

Dislocation structures and residual stresses have tremendous impact over the properties of alloys produced by fusion-based additive manufacturing (AM) processes. Yet, their origin is still unclear. We quantify and map, for the first time, the spatial distribution of dislocations, including those classified as geometrically necessary dislocations (GND), lattice strains, and type-III residual stresses in body-centered cubic (BCC) Ti-42Nb alloy and face-centered cubic (FCC) stainless steel (SS) 316L using a combination of electron channeling contrast imaging and cross correlation-based electron backscatter diffraction. Our findings reveal distinct residual stresses, lattice strains, and dislocation arrangement along cell walls in both alloys. In particular, Ti-42Nb demonstrates no GND network in cell walls, while SS316L exhibits a high GND density. We show that the Ti-42Nb alloy primarily accommodates solidification strains and cooling stresses elastically while SS316L accommodates them by elasto-plastic deformation and builds GNDs. Our work establishes relationships between dislocations and residual stresses in BCC and FCC LPBF alloys and highlights that not all LPBF-manufactured materials contain a dense dislocation network in their microstructures. These findings can also be extended to other AM alloys, therefore enabling a better control over their properties and performance.

Performance of Sound Absorption and Noise Reduction for Dislocation Honeycomb Sandwich Panel With Built-in Coating

The effects of diatom mud coating on characteristic impedance is studied by using the double-thickness method. The parallel relationships among the concave arc column filled with diatom mud, honeycomb cells, the overall core layer, and micro-perforation are considered in using the acousto-electric analogy method. In the case of a series relationship of plates, the sound absorption coefficient of the honeycomb structure of the micro-perforated plate is calculated. The effects of diatom mud coating and dislocation arrangement on the sound absorption coefficient of sandwich panels are numerically simulated by COMSOL. To further verify the reliability of the sound absorption theoretical model, test specimens are made and the effects of dislocation arrangement and diatom mud coating on sound absorption performance are tested by using impedance tube equipment. The effect of dislocation arrangement and diatom mud coating on the sound absorption performance of low-frequency noise of sandwich panels is analysed from the parameters of sound absorption coefficient, resonance frequency, acoustic impedance, and fitting results.

The Deformation Behavior of Niobium Microalloyed Steel during Lüders Band Formation

In microalloyed steels, plastic instabilities often appear which have been found to be associated with changes in the microstructure. In this paper, research was carried out on the influence of the microstructure in different areas of the deformation zone during the formation of Lüders bands in niobium microalloyed steel. Thermography and digital image correlation during static tensile testing were used to research deformation behavior and the area before and during the formation of the Lüders band. Different local values of temperature changes, i.e., stress changes, and strains in the examined areas during the formation of the Lüders band were determined. The highest values of the temperature changes and strains during the formation of the Lüders band were measured in the area of the initial appearance of the Lüders band. In order to clarify the observed changes, a microstructural analysis, using X-ray diffraction, scanning electron and transmission electron microscopy methods, was used. It was established that the observed temperature, i.e., stress, and strain changes are related to changes in the microstructure. The analyses of changes in the microstructure, arrangement and interaction of dislocations with precipitates revealed significant changes in the movement of dislocations and their interaction with fine niobium-containing precipitates during the formation of Lüders bands. The influence of microstructural parameters on the Lüders band formation in niobium microalloyed steel has been established based on this research.