Rearrangement Of Dislocations Research Articles

Investigation on fatigue performance and residual stress release mechanism of split sleeve cold expansion holes of heterogeneous 7050-T7451 aluminum alloy

7050-T7451 aluminum alloy is widely applied in aircraft structural components due to good properties, these components majority connected with hole structures. However, stress concentration occurs around structural holes during application, which leads to fatigue cracking and ultimately resulting in low fatigue performance of these parts. The residual stress generated by cold expansion can provide sufficient strength to prevent fatigue crack initiation. However, this protective effect is lost when the residual stress is released during fatigue. Consequently, it is crucial to investigate the release of residual stress in hole structures during the fatigue process. In this work, the residual stress release process and microstructure evolution under to different cyclic loads were investigated. The back stress induced by the heterogeneous structure on residual stress release and the effect of heterogeneous structure on crack propagation were discussed. The findings indicate that the back stress generated by GNDs of heterogeneous structures can resist the applied load in early stages, delay residual stress release. As the cycle number increases, potential mechanisms for the release of residual stress include dislocation annihilation, dislocation rearrangement, and dislocation shearing. The heterogeneous grain structure with high tortuosity and the deflection and branching of cracks can reduce the driving force of crack growth.

Mechanical properties improvement of titanium alloy and its grain boundary dislocation evolution mechanism by novel electroshock treatment

Titanium alloys are extensively utilized in the aerospace industry due to their exceptional strength and resistance to corrosion. However, litmited performance and high dispersion has always existed for the traditional manufacturing process. A novel Electroshock Treatment (EST) procedure proposed by author's team can synergistically improve the mechanical properties and its consistency of titanium alloys under limited temperature rise, but the relevant mechanism is not yet clear. In present work, the effects of various EST conditions on the mechanical characteristics were investigated by uniaxial tensile testing, and the effect mechanism was revealed using multi-scale microstructure characterization of titanium alloys, such as SEM, EBSD and TEM. The uniaxial tensile test results show that, compared with the sample without EST, the average elongation after fracture improved by 12.5%, the strength-plastic product improved by 16.1%, and the consistency of UTS and elongation after fracture improved by 63.4% and 57.1%, respectively, with a slight increase of tensile strength (30 MPa) after appropriate treatment (current density of 0.93 × 108A/m2, and pulse duration of 300 ms). The multi-scale microscopic characterization reveals a more uniform distribution of stress concentration in TC11 titanium alloy following the appropriate EST process. Besides, the entanglement of dislocations is reduced with some dislocations being annihilated. Especially, the remaining dislocations undergoing orderly rearrangement at grain boundaries after EST. The homogenization of local lattice distortion distribution and orderly rearrangement of dislocations at grain boundaries are the primary factors contributing to the comprehensive improvement in the mechanical properties and consistency of TC11 Titanium alloy.

Microstructural decay of matrix and precipitates during rolling contact fatigue in a martensitic dual-hardening bearing steel

We investigate the microstructural degradation during rolling contact fatigue (RCF) in a martensitic dual-hardening bearing steel. The dual-hardening steel makes use of both carbide precipitation and intermetallic precipitation hardening. The microstructural degradation leading to fatigue failure is studied using electron microscopy, atom probe tomography, and synchrotron X-ray diffraction (SXRD). The initial microstructure of the steel consists of tempered martensite with a fine dispersion of secondary M7C3, and NiAl precipitates. During RCF testing at 2.2 GPa contact pressure, ferrite microbands develop and the partial dissolution of NiAl and M7C3 precipitates occur within the ferrite microbands. For the RCF testing at higher contact pressure of 2.8 GPa, nanosized ferrite grains develop in the ferrite microbands. The SXRD analysis reveals a decrease in dislocation density in the sub-surface region experiencing microstructural degradation. This is believed to be associated with the rearrangement of dislocations into low energy configuration cells. We conclude this manuscript by proposing a microstructure decay mechanism for martensitic dual-hardening bearing steel that provides insights in the fatigue initiation process.

Ultrafast recrystallization and grain refinement of 2A97 Al-Cu-Li alloy by electropulsing assisted annealing at lower temperature

In the current work, a pulsed current was applied on recrystallization annealing to promote the recrystallization of cold-rolled 2A97 Al-Cu-Li alloys. Besides, the effect of stored deformation energy on electropulsing assisted recrystallization was also investigated. The results indicated that, with the assistance of electropulsing, the complete recrystallization temperature of the 55 % cold-rolled alloys was decreased from 480 °C to 440 °C, and the holding time was significantly decreased from 30 min to 15 s. Moreover, the recrystallization promotion effect of electropulsing was enhanced with the increment of stored deformation energy. As the rolled reduction increased from 23 % to 55 %, the drop in complete recrystallization temperature induced by electropulsing rose from 20 °C to 40 °C. Additionally, the average grain size decreased from 46.8 μm to 22.6 μm, leading to an enhancement in the mechanical properties of the 2A97 alloy. The microstructure observation showed that the electropulsing assisted recrystallization was a process of nucleation and nuclei growth dominated by dislocation rearrangement, and the ultrafast recrystallization at lower temperature induced by electropulsing may be attributed to the reduction of the nucleation energy barrier and the promotion of dislocation rearrangement caused by the ‘local hot spot effect’ and ‘electromigration effect’.

Effects of thermal exposure on microstructure deformation and evolution of shot peened nickel-based single crystal superalloy

Microstructure deformation and thermally activated microstructural evolution of nickel-based single crystal (NBSC) superalloy subjected to shot peening (SP) were investigated. SP induced high-density dislocations and activated the octahedral slip system {111} 〈110〉 of NBSC superalloy. Under 0.25 mmA SP intensity, there appeared a severely deformed layer with a depth of 2.5–3.0 μm beneath the surface. High SP intensity resulted in rougher surface morphology, more dense slip bands, deeper cold work layer and larger mean geometrically necessary dislocation (GND) density. However, the deformed microstructure was thermodynamically unstable. Dislocation annihilation and rearrangement were the main thermally activated mechanism of microstructural evolution, and high temperature promoted dislocation movement and consumption. Under 0.25 mmA SP intensity after 24 h thermal exposure, GND density decreased from 4.86 × 1014 m−2 to 2.72 × 1014 m−2, and the depth of cold work layer decreased from 67 μm to 62 μm. In addition, SP treatment provided rapid diffusion channels for alloy elements, while thermal exposure further aggravated element diffusion.

High cycle fatigue response of grain refined EUROFER97

The present paper has investigated the high cycle fatigue behavior of EUROFER97 steel submitted to a novel process consisting of cold rolling with reduction ratio of 80 % followed by a heat treatment at 650 °C for 1 h. This process had already proved to be highly effective under static loads leading to a significant improvement in yield stress. The fatigue tests have been performed with 0.2 load ratio at room temperature. Results indicate that the fatigue strength of the steel submitted to the novel treatment is comparable to that of the standard EUROFER97. Moreover, the fatigued samples underwent a microstructural evolution consisting of grain size increase and texture change due to the stress-driven instability of grain boundaries, especially LAGBs. The collapse of some boundaries involves partial annihilation and re-arrangement of dislocations, and grain coalescence. As a consequence of such microstructural change the material softens with hardness variations up to 8 %.

Effect of isothermal tempering time and temperature on microstructure and hardness of 3Cr5Mo2SiVN hot-work die steel

In this work, the effect of isothermal tempering time and temperature on microstructure and hardness of 3Cr5Mo2SiVN hot-work die steel was studied. The results indicated that with the increase of isothermal tempering time and temperature, the hardness significantly decreased, which corresponded to the microstructural recovery including the transformation of high angle grain boundaries (HAGBs), the precipitation and coarsening of precipitates, the annihilation and rearrangement of dislocations, the formation and coarsening of sub-grains, and the coarsening and coalescence of laths. The driving force of grain boundary transformation was provided by the dislocation- and heat-induced boundary migration. The increased density of HAGBs was induced by the coalescence of low-angle lath boundaries and low-angle sub-boundaries that continuously absorbed dislocations. In addition, the formation and coarsening of sub-grains were the results of dislocation rearrangement with dislocation cells acting as the core. Moreover, the V-rich M(C, N), Fe-rich M3C, (Fe, Cr)-rich M7C3 and M23C6 formed by the desolvation of alloying elements exhibited an obvious coarsening, decreasing the pinning effect of dislocations and then resulting in the hardness loss. This study offered a systematically revelation into the softening mechanism of hot-work die steel.

Nanomechanical characterization of BiFeO3 ferroelectric ceramics

Ferroelectric perovskites are pivotal in diverse technological applications; however, a gap persists in our comprehension of their mechanical properties, emphasizing the need for additional exploration. In the present study, we report on the nanomechanical behavior of BiFeO3 ceramics in the force range between 200 μN and 2 mN, including the evolution of the hardness and the reduced Young’s modulus from (9.9 ± 0.4) GPa and (113.7 ± 6.9) GPa, respectively, with increasing force. A sequence of plastic-deformation mechanisms under the cube-corner probe was revealed through a first pop-in analysis in combination with a variety of electron microscopy techniques, starting with a homogeneous dislocation nucleation, multiplication and rearrangement leading to sub-grain formation, a phenomenon observed in metals but not in ceramics.

Dynamic precipitation and twinning-induced dynamic recrystallization in Mg–6Al–3Sn–1Zn alloy during multi-pass high-speed rolling

Ultrafine-grained microstructure was successfully obtained in Mg–6Al–3Sn–1Zn (ATZ631) alloy by multi-pass high-speed rolling (HSR) at a rolling speed of 1100 m/min and a rolling temperature of 400 °C. The grain refinement mechanism of ATZ631 alloy during HSR was investigated, which is attributed to mechanically fragmented grains and twinning-induced dynamic recrystallization (TDRX). During the initial stages of rolling, the initial coarse grains are refined by the intersection of twins. With increasing strain, the grain refinement mechanism is the typical dynamic recovery and TDRX. In general, twins are filled with a high density of dislocations and nano-sized precipitates at the dislocations, which can accelerate DRX by increasing the driving force or particle-stimulated nucleation (PSN) within the twins. Subsequently, the coarse grains are refined by forming fine DRXed grains via dislocation entanglement, dislocation rearrangement, and dislocation absorption. As the process proceeds, grain boundaries (GBs) migration is suppressed by the precipitates at the GBs of new DRXed grains. Thus, the growth of the DRXed grains can be inhibited. Finally, ultrafine-grained microstructure with an average size of ∼116 nm is obtained. Moreover, the new grains formed by TDRX exhibit random orientation during this process, indicating that the basal texture gets effectively weakened by increasing the fraction of DRXed grains. These findings bring forward the microstructural evolution during dynamic precipitation and DRX, thus providing a novel notion for improving microstructures and grain refinement in Mg alloys.

Hot Deformation Behavior and Microstructural Evolution of a TiB2/Al-Zn-Mg-Cu-Zr Composite.

In the present work, the hot deformation behavior and microstructural evolution of a TiB2/Al-Zn-Mg-Cu-Zr composite were studied. Hot compression tests were conducted within a temperature range of 370 °C to 490 °C and a strain rate of 0.001 s-1 to 10 s-1. We established the Arrhenius constitutive equation with Zener-Hollomon parameters and processing maps and discussed the microstructural evolution during hot deformation. The results indicated that the safe processing parameter region falls within 370 °C-490 °C and 0.001 s-1-0.025 s-1. The influence of the strain rate on the safe processing range is more dominant than that of deformation temperature, which is primarily attributed to TiB2. Dynamic softening is primarily governed by dynamic recovery (DRV). Small particles (η, Al3Zr) can pin dislocations, promoting the rearrangement and annihilation of dislocations and facilitating DRV. Higher temperatures and lower strain rates facilitated dynamic recrystallization (DRX). Continuous dynamic recrystallization (CDRX) occurs near high-angle grain boundaries induced by strain-induced boundary migration (SIBM). TiB2 and large second-phase particles generate high-density geometrically necessary dislocations (GNBs) during hot deformation, which serve as nucleation sites for discontinuous dynamic recrystallization (DDRX). This enhances dynamic softening and improves formability.

Mechanical properties and microstructure evolutions of deposited Ti–6Al–4V titanium alloy under short-term stress relaxation at elevated temperatures

This study investigates the stress relaxation behavior and associated microstructure evolution in laser melted deposited Ti–6Al–4V alloy under various temperature and pre-strain conditions for the first time. A set of stress relaxation and tensile tests has been performed to characterize the stress and strength evolution properties of the as-deposited alloy, and microstructural analysis has also carried out for mechanisms identification. It is found that the as-deposited alloy exhibits a lower threshold stress (less than 10 MPa), faster relaxation rate, and a more significant decrease in strength (9.9%∼15.8%) when compared with the same testing conditions of conventionally extruded alloy. Based on the microstructural evolutions, as well as the stress exponent analysis, the stress relaxation mechanisms have been identified: at low pre-strain and temperature (700 °C and 0.5%), diffusion creep plays the dominant role, while with the increasing pre-strains and temperatures, the mechanism transits to dislocation climb, and eventually to grain boundary sliding, at 750 °C and 10%. It is observed that the fine lamellae in the as-deposited alloy are susceptible to morphological changes during relaxation, including dislocation rearrangement, lamellae spheroidization and coarsening. These microstructural evolutions lead to the creep mechanism transitions, as well as the noticeable reduction in yield strength after relaxation. The results could provide insights into designing appropriate stress relaxation parameters for deposited alloys in hybrid additive manufacturing processes.

Elimination of Low-Angle Grain Boundary Networks in FeCrAl Alloys with the Electron Wind Force at a Low Temperature

Low-angle grain boundaries (LAGBs) accommodate residual stress through the rearrangement and accumulation of dislocations during cold rolling. This study presents an electron wind force-based annealing approach to recover cold-rolling induced residual stress in FeCrAl alloy below 100 °C in 1 min. This is significantly lower than conventional thermal annealing, which typically requires temperatures around 750 °C for about 1.5 h. A key feature of our approach is the athermal electron wind force effect, which promotes dislocation movement and stress relief at significantly lower temperatures. The electron backscattered diffraction (EBSD) analysis reveals that the concentration of low-angle grain boundaries (LAGBs) is reduced from 82.4% in the cold-rolled state to a mere 47.5% following electropulsing. This level of defect recovery even surpasses the pristine material’s initial state, which exhibited 54.8% LAGBs. This reduction in LAGB concentration was complemented by kernel average misorientation (KAM) maps and X-ray diffraction (XRD) Full Width at Half Maximum (FWHM) measurements, which further validated the microstructural enhancements. Nanoindentation tests revealed a slight increase in hardness despite the reduction in dislocation density, suggesting a balance between grain boundary refinement and dislocation dynamics. This proposed low-temperature technique, driven by athermal electron wind forces, presents a promising avenue for residual stress mitigation while minimizing undesirable thermal effects, paving the way for advancements in various material processing applications.

Short-time creep behavior of (TiB + TiC + Y2O3) reinforced titanium matrix composite in the range of 600 °C to 700 °C

A titanium matrix composite reinforced by TiB, TiC and Y2O3 was prepared by induction skull melting. The creep behavior of the composite was tested at the temperature range from 600 °C to 700 °C in short time to investigate the deformation mechanism and microstructure involution. The as-cast composite shows a typical basket-weave microstructure. At the stress of 150 MPa, the stress exponents of the composite are 3.48 at 600 °C and 5.33 at 700 °C, corresponding to the deformation mechanism mainly controlled by solute-dragging and dislocation climbing respectively. Reinforcements added hinder dislocations movement effectively, which improves the creep resistance of the composite. As the creep temperature rises, the solubility of β-Ti increases, accompanied by the precipitation of silicides with different morphologies. Silicides play a role in pinning dislocations and hindering interface from migration. The dislocation movement and rearrangement during creep lead to the formation of sub-grains.

Microstructural evolution and formability of Ti−6Al−4V alloy sheet during electropulsing-assisted bending

The electropulsing-assisted bending test was carried out on Ti−6Al−4V alloy sheet with the effective current density of 0−3.2 A/mm2 and duty ratio of 10%−70%. The results show that the bending forming displacement increased from 9.15 to 15.60 mm with increasing effective current density from 0 to 2.4 A/mm2. The crack changed to be shallow and small at the same time. Besides, the springback angle decreased as the effective current density increased at the bending displacement of 8 mm. Duty ratio also affected bending angle, where 30% duty ratio led to the lowest bending angle of 135.0°. The thermal stability temperature increased with increasing effective current density and varied non-monotonically with duty ratio. In addition, grain coarsening, {0001} texture strengthening and sub-grain development were observed during electropulsing-assisted bending. The electropulsing decreased dislocation density and accelerated dislocation rearrangement, the rearranged and piled-up dislocation inside α grains promoted sub-grain structure formation, which improved the formability. The contribution of athermal effect made the low-temperature formability of Ti−6Al−4V alloy comparable to that at high temperature.

Tensile properties and microstructural evolution of 17–4 PH stainless steel fabricated by laser hybrid additive manufacturing technology

Wire-based laser directed energy deposition (LDED) technology is an attractive and efficient additive manufacturing technology, which shows great potential in the process of manufacturing high-value engineering components. However, the thermal effect will lead to a large amount of residual tensile stress in the components, and the large crystal structures after solidification seriously affects the tensile performance. The pursuit of high mechanical properties in additive manufacturing components have prompted researchers to seek new strengthening process to produce high-strength components. Therefore, in this paper, laser shock peening (LSP) composite wire-based LDED were used for the additive manufacturing of 17–4 PH stainless steel under the specified experimental conditions, and the effects of LSP on microstructure evolution, microhardness, and tensile properties of wire-based LDED components were systematically studied. Results showed that uniform grains with an average size of 2.31 μm were generated on the top surface of specimen due to the ultra-high plastic strain induced by LSP shock wave. The size of hierarchical martensitic units shows varying degrees of reduction after LSP treatment, which is shown by the width decrease of packet, block and lath. Dynamic recrystallization caused by rearrangement and annihilation of high-density dislocations promoted the grain refinement. Meanwhile, the residual stress was completely converted from tensile state into compressive state with a maximum value of 425 MPa, and the microhardness was also enhanced to a peak value of 432 HV. In addition, we observed that the high-density dislocation environment created by LSP treatment promoted the further precipitation, and the aggregation phenomenon of precipitates was observed in the dense dislocation area. Furthermore, the tensile strength and elongation of LSP treated specimen were improved significantly, which was attributed to the combined effects of grain refinement, introduction of dislocation structures, precipitation strengthening and gradient residual compressive stress. In summary, laser hybrid additive manufacturing technology provides a new idea and method for the production of high-performance precipitation hardening stainless steel components.

Outstanding thermal stability of cold-rolled Al–Y alloy revealed using in-situ synchrotron X-ray diffraction and ex-situ microscopy

This paper describes the main results from an experimental investigation into the thermal stability of a cold-rolled Al–Y alloy. Tensile tests performed on specimens of the alloy and pure Al in annealed states reveal superior anti-softening properties of the former specimens relative to the latter counterparts. Origins of such thermal stability and underlying recovery and recrystallization kinetics are studied using electron microscopy and high energy X-ray diffraction (HEXRD) techniques. Microscopic observations reveal that recovery and recrystallization begin in the specimens of pure Al at a much lower temperature than in the specimens of Al–Y alloy. Onset of the recovery process in the Al–Y alloy is observed to begin with dislocation rearrangements in the β-Al3Y phases. The onset is followed by more substantial reduction in dislocation density, shrinkage of the β-Al3Y phases, and coarsening of the α-Al grains. The alloy contains stacking faults, which interestingly remain stable during the annealing treatments contributing to the thermal stability. The observed mechanisms are further confirmed by tracking the evolution of peaks under the in-situ synchrotron X-ray diffraction with heating. Onset of recovery was observed at 280 °C in both α-Al and β-Al3Y phases, while substantial shrinkage and spheroidization of β-Al3Y phases and decrease in dislocation density were detected at 400 °C causing loss of the alloy strength. Thermodynamic calculation showed that solubility of Y rapidly increases in α-Al at temperatures above 400 °C explaining the spheroidization and shrinkage of the β-Al3Y phases by diffusion of Y into the Al matrix.

Mechanism of self-healing behavior in rolled Cu-2Be flat product

In this research, we aimed to investigate the self-healing behavior of Cu-2Be alloy (UNS C17200), which is used as a non-magnetic high-strength material, under various loading conditions, both static and dynamic, including tension-annealing and fatigue tests. We observed crack filling in all heat-treated and tested samples, specifically those subjected to solution heat treatment at 800 °C, under-aged (UA) conditions, and aging conditions at 320 °C for 5 and 30 min.The latter two phenomena were triggered by the precipitation of γ (CuBe) and γ’ phases after straining and heat treatment. The synergistic effect of dynamic aging and dislocation recovery resulted in increased ductility in the pre-strained and UA sample, as evidenced by post-fracture elongation, compared to specimens elongated up to a certain pre-strained level followed by annealing and aging. In conclusion, uniaxial fatigue testing revealed that the lifetime of UA Cu-2Be samples surpasses that of annealed and fully aged ones due to the efficient preparation of self-healing effects brought about by the precipitation of intermediate and final age-hardening phases, dislocation rearrangement, and grain boundary reinforcement through beryllium atoms. To characterize the precipitated CuBe, we conducted XRD, XPS, and DIGE phase identification analyses, alongside relevant electron microscopy analyses. Additionally, we investigated the possibility of crack closure by integrating the findings from the positron annihilation lifetime spectroscopy and coincidence Doppler broadening spectroscopy outputs, thus validating the results of spectroscopy and diffraction.

The cyclic deformation behavior and microstructural evolution of 304L steel manufactured by selective laser melting under various temperatures

The low cycle fatigue (LCF) tests with strain-controlled are carried out to investigate the cyclic deformation behavior of 304L steel manufactured by selective laser melting (SLM 304L) at room temperature (RT), 300 °C, and 650 °C. According to the experimental results, the continuous cyclic softening behavior of SLM 304L is presented at different temperatures. Meanwhile, due to the external force during the fatigue test, the substructure morphology is changed from the elongated cellular substructure of the as-built SLM 304L to the cell substructure of the failure sample at different temperatures. Moreover, since the dislocation rearrangement process is a net decrease in dislocation density, the continuous cyclic softening behavior is presented. Besides, the dynamic recovery and recrystallization promote the cyclic softening behavior of SLM 304L at high temperatures. Additionally, when the multiplication and annihilation of dislocations research the equilibrium state, the dislocation density has no obvious change in the microstructure, leading to the steady state of the cyclic response curve. Finally, considering the influence of the crack closure effect, a new fatigue life prediction model is proposed and discussed.

Accelerating and decelerating effects of Gd content and peak-aging treatment on the grain growth of magnesium

This short communication concerns the dependence of thermal stability of fine grains in Mg alloy on the Gd content. The enhanced thermal stability via increasing Gd content can be reflected by the SRX mechanism and corresponding boundary migration ability. In the low Gd-alloyed Mg, continuous SRX dominates the annealing process via dislocation re-arrangement. The grain boundaries composed by dislocations can easily migrate, since the uniform Gd solute distribution provides limited delaying effect. In the high Gd-alloyed Mg, discontinuous SRX dominates the annealing process via consuming dislocations for grain nucleation. The grain boundaries can hardly migrate due to the strong delaying effect provided by the Gd-segregation along grain boundaries. But unexpectedly, peak-aging can deteriorate the decelerated effect of high Gd-alloying owing to the absent Gd-segregation. Consequently, an accelerated grain growth rate is attained, even faster than that in the low Gd-alloyed Mg.

Cyclic softening behavior of TC17 titanium alloy fabricated by laser directly energy deposition

In aero-engines, cyclic softening can cause premature fracture of components, which is regarded as a challenge for the security of titanium alloy parts in service. In this work, the cyclic deformation behavior of LDEDed and β-forging TC17 titanium alloys with different morphology of α phase was explored, cyclic softening mechanism was investigated by using transmission electron microscopy (TEM) observation of dislocation morphology. The experimental results demonstrate that cyclic softening occurs when the total strain range increases to 2.0 %. The LDEDed specimen has a smaller stress amplitude and a larger plastic strain during cyclic softening than wrought specimen. In the softened deformation microstructure of LDEDed specimen, the size of αp decreases with the increase of strain, but the size and shape of αp and αs in wrought specimen do not change much. The main dislocation forms that dominate the cyclic softening in the α phase and the β matrix are slip bands and dislocation tangles, respectively, and the slip band in the α phase cannot pass through the secondary phase in the β matrix for slip transfer. Under the same strain, the lower dislocation density in the LDEDed microstructure is due to the larger directional back stress produced by the forward loading, which causes the dislocation rearrangement and annihilation under the reverse loading.