Dislocation Recovery Research Articles

Yield strength modelling via precipitation/recovery kinetics in rapidly solidified thin-strip cast AA6005 AlMgSi sheets subjected to cold rolling and direct aging

A comprehensive and experimentally verified yield strength model of AA6005 Al–Mg–Si sheets rapidly solidified via a novel Thin Strip (TS) casting route is developed and presented. The cast sheets were cold-rolled and directly aged via a heat-treatment process (termed TSH) that effectively utilizes the increased solute supersaturation of matrix due to the high casting cooling rates experienced. The TSH conditions were optimized based on the hardness and tensile data, with the peak aging conditions found to be 160°C-4 hrs, 180°C-1 hr and 200°C-15 min. The TEM analysis revealed that, due to the presence of prior cold work, β′ precipitates formed during TSH, i.e., as opposed to the β" precipitates that form upon a typical T6 aging treatment. Also, while the total precipitate volume fraction was comparable among the three peak aging conditions, the sample aged at 200 °C exhibited a lower peak aging strength (300 MPa vs. ∼315 MPa in the other two conditions), i.e., due to a higher dislocation recovery rate and therefore a lower strengthening contribution from the remaining cold work. Interestingly, at each temperature, aging beyond the peak aging time still resulted in an increase in the precipitate volume fraction. The concurrent drop in the yield strength could be explained by the offsetting effect of the recovery of the remaining cold work. Therefore, thanks to the sufficiently high solute supersaturation of the matrix during TSH, together with the remaining cold work (as well as the formation of β′), peak aging at 180 °C is achieved after only 30 min to 1 h, i.e., as opposed to generally 4–6 h of aging needed for a traditional T6 aging. This gives the low-cost, less energy-intensive TS-cast sheets also a significantly faster response to the paint-baking process (typically performed at near 180 °C for around 30 min), thus allowing for a higher strength achieved via a typical automotive body fabrication route. Finally, a robust model of precipitation/recovery kinetics, incorporating precipitate aspect ratio evolution, was developed to simulate the yield strength during TSH. The model predicts a peak-aging time of ∼100 min, aligning well with the observed 60–120 min interval measured from the tensile test results.

Revealing the Thermal Damage Mechanism of an Al–Zr–Er Alloy Wire by Quantitative Calculation: The Contribution of Dislocations and Nano‐Precipitates

The thermal damage mechanism of Al wires for long‐distance overhead transmission under extreme service conditions is an important scientific issue that restricts the design and development of high‐performance Al wires. In this study, the thermal damage mechanism of the Al–Zr–Er alloy wires with a good combination of ultimate tensile strength in 170.0 MPa and electrical conductivity in 61.42% IACS is investigated via a simulated high‐temperature service experiment. In the results, it is shown that strength evolution exhibits a negative exponential pattern mainly attributed to the partial dislocation recovery in the cold‐drawn Al wires based on the quantitative calculation. However, the nanoscale Al3(Zr, Er) precipitates exhibit good thermal stability. The strength residual rate of the Al–Zr–Er alloy wires exceeds 90% after the simulated high‐temperature treatment at 280 °C, which can be attributed to the strongly pinning effect of nanoscale Al3(Zr, Er) precipitates on the dislocations and the grain boundaries, finally leading to the larger Zener drag force as compared with the driving force for recrystallization, thereby achieving good heat resistance of Al–Zr–Er alloy wires.

An atomistic study on the strain rate and temperature dependences of the plastic deformation Cu–Au core–shell nanowires: On the role of dislocations

Abstract Recently, Cu–Au core–shell nanowires have been extensively used as conductors, nanocatalysts, and aerospace instruments due to their excellent thermal and electrical conductivity. In experimental studies, various methods have been presented for producing, characterizing, and strengthening these structures. However, the mechanical behavior and plastic deformation mechanisms of these materials have not been investigated at the atomic scale. Consequently, in the present study, we carried out uniaxial tensile tests on Cu–Au nanowires at various tension rates and temperatures by means of the molecular dynamics approach. The Cu–Au interface was found to be the main site for nucleation of perfect dislocations, Shockley partials, and stacking faults due to the stress concentration and high potential energy arising from the atomic mismatch between shell and core layers. It was observed that an increase in the strain rate from 108 to 1,011 s−1 shortened the time required for the nucleation of dislocations, decreasing the dislocation density. This emphasizes that dislocation nucleation and slip mechanisms are time-dependent. Moreover, it was found that the interaction of Shockley partials can lead to the creation of lock dislocations, such as Hirth, Frank, and Stair-rod dislocations, imposing obstacles for the slip of other dislocations. However, as the tension temperature rose from 300 to 600 K, opposite-sign dislocations removed each other due to thermally activated mechanisms such as dislocation climb and dislocation recovery. Furthermore, the combination of Shockley partial dislocations decreased the stacking fault density, facilitating the plastic deformation of these structures. The yield strength and elastic modulus of the samples increased with the strain rate and substantially decreased as the temperature rose.

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 > 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 > 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.

Exceptional thermal stability of additively manufactured CoCrFeMnNi high-entropy alloy with cellular dislocation structures

CoCrFeMnNi high-entropy alloy (HEA) was additively manufactured (AM) by laser powder-bed fusion (L-PBF). The AM CoCrFeMnNi has prominent cellular dislocation structures with a small number of Mn-rich oxides. The thermal stability of the AM CoCrFeMnNi was investigated by isochronal annealing treatment at various temperatures from 400 to 1300 °C for 1 h. Microstructural analysis shows slow dislocation recovery, retarded recrystallization process, and precipitation of additional Cr-Mn based oxides during thermal annealing, resulting in exceptional thermal stability and retained high hardness at elevated temperatures. By thermodynamic calculations, a low stored energy of 1.31 MJ/m3 and a high activation energy of 353 kJ/mol for recrystallization were estimated for the AM CoCrFeMnNi. The exceptional thermal stability of the AM CoCrFeMnNi HEA is mechanistically attributed to the low crystallographic misorientations across the dislocation cell walls, sluggish atomic diffusion, and the pinning effects of the oxide nanoprecipitates.

Numerical Analysis of Precipitation Kinetics in Cu-Co-P Alloy Based on Dislocation Recovery Model and N Model

Numerical Analysis of Precipitation Kinetics in Cu-Co-P Alloy Based on Dislocation Recovery Model and N Model

Effects of annealing temperature on microstructure and hardness of cold rolled high-purity V-4Cr-xTi alloys

The microstructural evolution and Vickers hardness variations upon annealing in the temperature range of 873 – 1273 K of cold rolled high-purity V-4Cr-(0∼4)Ti alloys are investigated. Results show that partial recrystallization occurs at 1073 K, recrystallization is complete at 1273 K, and Ti addition results in grain refinement. Grains become more isotropic after annealing at 1273 K. Besides, precipitates are observed after annealing at 973 K and above when Ti concentration is 1 wt% and higher. The size of precipitates increases with increasing Ti concentration and annealing temperature. Furthermore, Vickers hardness of cold rolled V-4Cr-xTi alloy drops to a minimum with increasing annealing temperature to 1173 K due to recovery of dislocations and recrystallization process. With annealing at 1273 K, hardness does not vary for V-4Cr-0Ti alloy, whereas it slightly increases for V-4Cr-(1∼4)Ti alloys, which is presumably attributed to partial dissolution of precipitates into the matrix. Additionally, purification by further reducing the high-activation impurities and interstitial impurities dose not evidently affect the recrystallization behavior and hardness of V-4Cr-4Ti alloy after recrystallization, however, the low-activation property is expected to be enhanced.

Evolution of the microstructure and mechanical properties of a V-containing microalloyed steel during coiling

The microstructural evolution during coiling and its effects on the mechanical properties of a vanadium microalloyed steel were investigated. Samples were held at 500 °C for 80 s (S1), 600 s (S2), 3600 s (S3), and 18000 s (S4) to capture the various stages of microstructure evolution. Experimental results demonstrated that nano precipitates (<10 nm) nucleated during holding at 500 °C were heterogeneously distributed. These precipitates mainly contained V and N were more commonly found in areas with a high value of the Kernel Average Misorientation (KAM) because these areas have a larger number of dislocations which act as nucleation sites for the precipitates. The tensile flow curves of the samples show extended elastoplastic regions that are related to the heterogeneity of the microstructure as well as the presence of microscopic residual stresses. The precipitation strengthening effect is not very large and is offset by softening which is caused by the aging of bainite and the associated recovery of dislocations.

Advancing the mechanical integrity and fragmentation behavior of reactive projectiles

A multivariant statistical approach was used to identify treatment conditions that improve the survivability of structural reactive material (SRM) projectiles upon launch and enhance energy release upon impact. The study included both mechanical testing of projectiles as well as their reactive characterization. The projectiles were launched in a high-velocity impact-ignition testing system and impacted an anvil for vented chamber calorimetry. This study examined a link between ultimate compressive stress and combustion performance. Two treatments were applied to consolidated aluminum projectiles including annealing and addition of silica (SiO2) inclusions. Results showed annealing at moderate temperatures resulted in intact SRM projectiles upon launch. Adding small concentrations (1–2 wt. %) of SiO2 to the SRM promoted fragmentation and combustion performance upon impact. Compared to the untreated projectiles, annealing with SiO2 inclusion processing treatments improved the energy conversion efficiency from 37–84% (for untreated projectiles) up to 54–98%. Increasing interparticle dislocation recovery by annealing while balancing inclusions promoting fragmentation upon impact was the key to optimizing combustion performance for SRM ballistic impact applications.

Simultaneous improvement of strength and damping capacity of Mg-Mn alloy by tailoring bimodal grain structure

A high damping capacity and a high strength is paradox in magnesium alloys needed to be addressed immediately. In this work, adding Mn into Mg matrix, and combined with low extruded temperature, alloys with bimodal grain structure that is made up of fine recrystallized grains and coarse unrecrystallized grains were obtained. The fine recrystallized grains are conducive to the activation of grain boundary slip, leading to alloy exhibits good plasticity. A large number of parallel dislocations exist in unrecrystallized grains, due to the dislocation recovery and dynamic recrystallization rate are evidently suppressed at low extruded temperature and the Mn precipitates impede the movement of dislocations, which cause a high work hardening rate in bimodal grain structure. This phenomenon not only brings about a high yield strength, but also improves damping capacity of Mg-Mn alloys at room temperature. Because the unrecrystallized grains initial to recrystallize at high temperature, the P2 internal friction peak rises and moves toward the low temperature. When the extruded temperature is 220 °C, the comprehensive properties of the Mg-1Mn alloy outperform the high-damping Mg-0.6Zr alloy, and the yield strength, plasticity and damping capacity at ε = 1 × 10−3 are 169 MPa, 32.5% and 0.042, respectively. Tailoring bimodal grain structure is an effective strategy to ensure strength, plasticity and damping capacity of the Mg alloy at the same time.

Understanding the effect of deformation combined with heat treatment on age hardening of Al–Zn–Mg–Cu alloy AA7075

Al–Zn–Mg–Cu alloys obtain their exceptional high strength predominately relying on precipitation. Deformation can influence the evolution of precipitation and may potentially provide additional strength through work hardening. In this study, the coupling of dislocation and precipitation strengthening to achieve unique property combinations with strengths greater than those due to heat treatment alone was investigated on pre-aged AA7075 subject to a pre-age, deform, post-age cycle. The evolution of precipitates and mechanical properties were characterised to understand the contribution of each mechanism to the final strength. Small angle X-ray scattering (SAXS) was used to monitor the precipitate evolution during post-ageing, with the support from transmission electron microscopy (TEM) and isothermal calorimetry. The results revealed that uni-axial deformation to 10% strain had negligible effects on pre-existing precipitates but introduced sufficient dislocations to enable significant work hardening, which resulted in strengths greater than those obtained with a T6 heat treatment but with a sacrifice in further strain to failure. Post-ageing at 120 °C was found being effective on partially recovering dislocations and promoting precipitation strengthening. A simple model was applied to demonstrate that the time required to reach peak strength is determined by the competing effects of the enhanced kinetics of precipitation and dislocation recovery. The acceleration effect of prior deformation on the growth of precipitation during post-ageing is consistent with the expected effect of dislocations acting as the fast diffusion paths.

Microstructure – Toughness relationships in 316L stainless steel produced by laser powder bed fusion

The impact toughness and fracture toughness properties at room temperature of 316L austenitic stainless steel manufactured by laser powder bed fusion were determined and compared. The effect of the grain size and morphology, of the dislocation structures and of the oxide nanoparticles on the toughness behavior was systematically investigated. In all cases, ductile fracture resulted from decohesion at the matrix/oxide interfaces. After a stress-relief heat treatment, Charpy absorbed energy and fracture toughness at crack initiation were 170–250 J/cm2 and 350–550 kJ/m2, respectively. Dislocation recovery has no significant influence on the toughness properties. Recrystallization decreased the impact toughness and fracture toughness by up to 25% and 50% respectively, due to synergistic effects between recrystallization and oxide particle coarsening, resulting in coarse oxides located along recrystallized grain boundaries that facilitated ductile cracking. Whatever the as-built microstructure and the heat treatment, fracture and impact toughness obeyed a linear correlation with an accuracy of ±20%. They were also linearly correlated for each of the crack initiation and crack propagation stages.

Reversible Cyclic Softening in HSLA Steels at Low Homologous Temperatures

Critical mechanical components employed in energy applications require certification of their mechanical response under monotonic or cyclic loading at various constant temperatures. However, components often experience combined load and temperature changes, which are more complex and time-consuming to evaluate. This work presents previously unreported reversible cyclic softening in low-alloy steel undergoing loading sequences between 293 K and 453 K (0.15 to 0.25 of the homologous temperature). Mechanical cycling at 80 pct of the yield stress results in a similar cyclic response for all temperatures, while cycling at 90 pct of the yield stress doubles the cyclic strain when preceded by cycling at 453 K. The softening can be reversed by maintaining the material at elevated temperatures and cannot be explained by dislocation recovery mechanisms or changes in the microstructure. Instead, serrated deformation at specific deformation levels suggests that Dynamic Strain Aging (DSA) contributes to softening, which is further supported by a theoretical analysis of interstitials and dislocations mean free paths at various temperatures.

Effect of pre-tensile treatments on the mechanical properties and deformation mechanism of a novel Ni-based superalloy

Stacking fault energy can be reduced by adding Co to Ni-based superalloys. In this work, a novel Ni-based superalloy was developed with 20 wt% Co content to reduce the stacking fault energy. After pre-tensile treatments, the alloy formed substructures with numerous dislocations, slip bands, and stacking faults. When the deformation reached 6%, stacking faults in different directions intersected, forming Lomer-Cottrell locks (L-C locks) at the intersections. Mechanical properties were tested at 760 °C. Compared to the standard heat-treated alloy, the yield strength of the alloy with 3% pre-tensile deformation increased by 16.2%, while the stress rupture life maintained 64.2% at 760 °C/480 MPa. The yield strength of the alloy with 6% pre-tensile deformation increased by 29.8%, and the stress rupture life maintained 74.1% at 760 °C/480 MPa. During the tensile process, the densities of stacking faults and deformation twins in the pre-tensile alloys increased, leading to increased hindrance to dislocations and consequently an increase in yield strength. The grain boundaries of the pre-tensile alloy have high dislocation density, and the dislocation recovery and recrystallization softening mechanism occurred during the stress rupture process, resulting in a lower stress rupture life than that of standard heat-treated alloy. As the deformation was increased to 6%, the simultaneous presence of M23C6 and stacking faults enhanced the pinning of dislocations and grain boundaries, improving the stress rupture life of the pre-tensile alloy.

A study of microstructural factors governing strength and ductility of titanium alloys fabricated by powder bed fusion additive manufacturing

This work aims to reveal the general patterns of the strength and ductility of titanium alloys fabricated by using powder bed fusion (PBF) additive manufacturing (AM). The dislocation-based constitutive modeling and reported experimental data are coupled for a comprehensive evaluation. The analytical solutions describing the plastic instability are proposed and validated. A set of single expressions, which are variation-independent of the material type (CP-Ti, Ti64), PBF-methods and heat-post-processing (HPP) conditions, are obtained using the semi-analytical method. It is found that during the plastic deformation, the mean-free-path for the dislocation storage is the intrinsic length L that is represented by the α/α’ phase width. The uniform elongation corresponding to the onset of necking is primarily controlled by the dislocation recovery factor. The increasing of the dislocation density at the onset-state of necking is mainly determined by the initial dislocation contribution to the flow stress. It results in a linear correlation between the true tensile strength and yield strength.

Nondestructive Ultrasonic Evaluation of Ferritic CrW Steel upon Long-term Aging

The microstructural evolution of heat-resistant ferritic CrW steel during long-term aging was characterized by ultrasonic wave velocity measurement. Long-term aging was conducted at 700°C for 4000 h. The ultrasonic wave velocity increased monotonically upon long-term aging. The ultrasonic wave velocity increased rapidly during the initial 1000 h of aging and moderately thereafter. The MSUB23/SUBCSUB6/SUB carbide and Fe2W Laves phases underwent coarsening through the Ostwald ripening mechanism, causing a decrease in Cr and W solute atom concentration of the iron matrix during 4000-h aging. The dislocation density also decreased because of the heat treatment. The width of martensite lath increased owing to dislocation recovery at the lath boundaries. The ultrasonic wave velocity depends on the Young’s modulus of the material, which may influence microstructural features such as dislocation substructure, precipitates, solute atom concentration, and martensite lath boundaries. This study utilizes ultrasonic wave velocity to quantify the damage level and microstructural changes during the long-term aging of ferritic CrW steel.br/

Evaluation of Arthroscopic Bankart Repair and Open Latarjet Technique for Treatment of Recurrent Shoulder Dislocation

Background: Arthroscopic Bankart repair and open Latarjet procedures are the most frequent surgical treatments for shoulder instability and recurrent shoulder dislocation. Objectives: Therefore, we performed research to assess the clinical outcomes and recurrence rate of instability between both Arthroscopic Bankart repair and open Latarjet techniques. Methods: A cross-sectional study was performed at Lahore General Hospital, Lahore from 2021-23, comprising 74 patients, managed surgically for the condition of recurrent shoulder dislocation. The patients were allocated into two groups viz Group A and B comprising 37 patients in each, who were managed through Arthroscopic Bankart repair and open Latarjet techniques, respectively. Post-operative complications, re-occurrence and success rate were measured in terms of SSV scores. Results: Average age of patients in Arthroscopic Bankart group was 28.34+5.31 years while in open Latarjet group; it was 29.09+6.19 years. 14 (37.83%) patients of Arthroscopic Bankart repair group revealed recurrent shoulder dislocation, while no such complaint was found in the open Latarjet technique. Twenty-seven patients of Arthroscopic Bankart group showed satisfaction with the procedure and 33 out of 37 patients (89.18%) were satisfied with open Latarjet technique. Patients in open Latarjet technique group showed successful recovery of shoulder dislocation and their SSV was 87.91%, while the patients in Arthroscopic Bankart repair group showed less SSV score of 59.76%. Conclusion: Open Latarjet group patients had a greater rate of functional satisfaction, while the arthroscopic Bankart repair group showed a trend for more recurrence. Open Latarjet operation had a higher success rate and patients had a better rate of return to previous top-level sports than Arthroscopic Bankart technique. Keywords: Latarjet; Re-occurrence; Shoulder dislocation; Sports medicine; Surgical complications.

Heterogeneity of deformation, shear band formation and work hardening behavior of as-printed AlSi10Mg via laser powder bed fusion

The deformation heterogeneities and micro-to macro-structural evolution of shear bands were thoroughly characterized and assessed in correlation with the work hardening behavior of laser powder bed fused as-built AlSi10Mg using compression testing at 78 and 293 K in a wide range of constant strain rates. Using the Kocks-Mecking model, work hardening behavior was correlated with the evolved microstructure and rate-controlling deformation mechanisms. It was found that initial work hardening (athermal hardening rate) is dependent on the strain rate at both 78 and 293 K, where it increases by a transition from shearing to looping of nano-precipitates in the 10−2-10−1 s−1 strain rate range. Stages II (hardening stage) and Stage III (dynamic recovery stage) of work hardening were also observed and their dependence on temperature and strain rate was assessed. The former includes the dislocation-obstacle interactions resulting in work hardening, whereas the latter consists of recovery processes where density of dislocations decreases. Shear banding was reported for higher strain rates manifesting in micro- and then evolving into macro-shear bands with deformation, which led to the increase in the dislocation recovery rate in Stage III of work hardening. Compressive work hardening mechanisms were discussed for both 78 and 293 K temperatures.

Enhanced mechanical properties by laser powder bed fusion using cost-effective hydride-dehydride titanium powders

The high cost of powder feedstock has emerged as a critical issue in additive manufacturing of titanium (Ti). In this work, pure Ti parts were successfully produced via laser powder bed fusion (LPBF) by employing cost-effective hydride-dehydride (HDH) powders modified through gas-soild fluidization. The microstructures and tensile properties were comprehensively investigated as a function of scanning speed. The pure Ti parts made under the optimized processing parameter exhibited an ultimate tensile strength of 826.1 MPa with a fracture strain of 22.3% at room temperature, superior to those made by the expensive atomized powders. The fatigue performance of the LPBF-made pure Ti using the HDH powders is comparable to those made by rolling. The excellent mechanical performance can be attributed to the effects of grain refining, texture evolution, dislocation recovery, and cyclic softening by dislocation cells and tangles generated. This work provides an effectively low-cost route to fabricate pure Ti by LPBF with excellent mechanical properties, which also serves as a useful guidance on the processing window setup using the HDH powder.

Electroplasticity in the Al0.6CoCrFeNiMn high entropy alloy subjected to electrically-assisted uniaxial tension

Electrically assisted deformation (EAD) was adopted in this work to overcome the shortcomings such as poor formability and easy cracking in the processing of dual-phase the Al0.6CoCrFeNiMn high entropy alloy (HEA) at room temperature. Electroplasticity of the Al0.6CoCrFeNiMn HEA was studied systematically using electrically assisted uniaxial tension. The results showed that pulse current caused the temperature gradient along the tensile direction and the temperatures of the samples increased with the current density. The flow stress decreased, and the elongation increased with increasing current density during the EAD. When the current density was 30 A mm–2, the total elongation of the samples could be increased by 50% compared to that with no pulse. Pulse current can reduce local stress concentration and postpone microcracks initiation in the body-centered cubic (BCC) phases, and hence can effectively inhibit cracks and ruptures. The dislocation tangles were opened by pulse current, and the dislocation recovery was enhanced at a high current density. Compared with dilute solid solution alloys, the lattice distortion effect, the high fraction of the BCC phases, and the dislocations in HEAs can lead to the enhancement of the local Joule heating, which accelerated dislocation slip and dislocation annihilation. This study confirms that EAD can effectively improve the formability of HEAs and provides theoretical guidance and an experimental basis for forming HEAs components.