Dislocation Recovery Research Articles

Effects of Ca contents on microstructures and mechanical properties of friction stir processed Mg alloys

Ca alloying is beneficial for improving the uniform elongation for friction stir processed (FSPed) Mg-4Al-2Sn-xCa (0, 0.5, 1.0 and 1.5 wt.%) (AT42-xCa) alloys. Coarse CaMgSn and Al2Ca particles in as-cast AT42-xCa alloys lead to a sharp decrease in elongation. Friction stir processing (FSP) is effective in improving strength and elongation by refining grain structure along with CaMgSn and Al2Ca particles, and inducing grain orientations with high Schmid factors for basal slip. Consequently, FSPed AT42-0.5Ca alloy exhibits simultaneously enhanced ultimate tensile strength (∼224 MPa) and elongation (∼30.9%). Compared to the Ca-free alloy, 0.5 wt.% Ca alloying increases the heat input and promotes the dislocation recovery, reducing residual dislocations. Nano-sized CaMgSn precipitates in grain interiors inhibit the dynamic recovery of dislocations during tensile, facilitating the work-hardening ability. Besides, high Schmid factors facilitate basal dislocation slips. Therefore, high EL is primarily due to the dislocation-free fine grain (∼2.7 μm), refined CaMgSn particles (micron-sized particles (∼1.3 μm) and nano-sized precipitates (∼55 nm)) and high Schmid factors (∼0.446) for basal slips.

Microstructural and mechanical properties evolution for 9Cr[sbnd]Si heat resistant steel deposited metals under different heat treatment states

To develop the matched filler metal for the structural materials in Generation IV lead-cooled fast reactor (LFR), the microstructural evolution and mechanical property variation of 9CrSi heat resistant steels deposited metals were investigated both for the as-welded and as-aged (at 550 °C) weld metals. The experimental results indicate two precipitates in the as-welded deposited metals: the bigger M23C6 and the smaller MX. Si facilitates the nucleation of M23C6. The chain-like precipitates (consisting of M23C6) form at the boundaries of prior austenite and martensitic laths in high-Si specimens. They deteriorate not only the strength but also the impact energy. Meanwhile, a few δ ferrites form in high-Si deposited metal deteriorating the impact energy. Precipitates tend to nucleate at the interface between the δ ferrite and the martensite after aging. New types of precipitates (Mo-rich M6X and Laves) with rapid coarsening rate form in as-aged high-Si deposited metals as time increases. Coarse precipitates deteriorate the pinning effect, which causes the recovery of laths and dislocations. Subgrain pinned by M23C6 forms in deposited metals with post-weld heat treatment (PWHT) during aging. The structure hinders the formation of M6X and facilitates the formation of nano-sized MX. Therefore, the mechanical properties of deposited metals with PWHT are stable during long-term (up to 10,000 h) aging treatments.

Effect of pulse current on the creep ageing behavior of pre-strained 2195 Al-Li alloy

Pre-strained aluminum lithium (Al-Li) alloy plates have ultra-high strength and comprehensive performance after aging treatments, and are widely used in the aerospace industry. The initial increase in dislocation density caused by these pre-strains will significantly affect the evolution of deformation, microstructure, and mechanical properties during the pulse current assisted creep ageing forming process. Therefore, the effect of pulse current on the creep ageing behavior of 2195 Al-Li alloy with different pre-strains (0%, 4%, and 8%) was investigated. As the pulse current increases within the first 1h creep aging process of the various pre-strain samples, the 1h creep strain significantly increases and the subsequent steady-state creep rate slightly reduce. Moreover, the larger the pre-strain, the higher the total creep strain. Pulse current promotes the movement and recovery of initial dislocations, and promotes the fine formation of T1 phase and suppresses the nucleation of θ' and phase, thereby improving mechanical properties. Compared with the conventional creep ageing process, the pulse current acting on pre-strained 2195 Al-Li alloy significantly increased the peak aged strengthen and shorten the peak aging time. Meanwhile, the peak time platform period has also been significantly extended. It is evident that the application of pulse current assisted creep ageing forming process to pre-strained 2195 Al-Li alloy plates can significantly improve formability and mechanical properties, enlarges the forming process window and realize collaborative manufacturing of Al-Li alloy component shape and performance.

Oxidation micromechanics and contribution during the low-cycle fatigue failure of a Ni-based single crystal superalloy

Understanding the comprehensive factors of oxidation behaviors and revealing their contribution to fatigue failure is of great importance. The low-cycle fatigue (LCF) coupled oxidation behavior of a novel low-cost third generation Ni-based single crystal superalloy with only 3 wt% of Re at 800 ℃ and 1000 ℃ was studied in the present work. The results indicated that oxidation-induced crack initiation was specifically responsible for the drastic reduction in LCF life at high temperatures, as opposed to the unobvious creep deterioration and the inferior fatigue damage via crack propagation in slow model-I. However, it was found that the preferential spallation of the weakly bonded (Ni, Co)O layer was beneficial in suppressing crack initiation. In terms of crack propagation, the proceeding oxidation of nearby fatigue cracks could boost dislocation recovery process, which thereby improved the toughness of the surrounding substrate to decelerate crack propagation. Noteworthy, a specific asymmetric oxidation phenomenon in single crystal alloy was identified for its required cyclic crystal rotation in one single grain. Importantly, guidance for chemical composition optimization was proposed that future efforts should be made to reduce oxidation damage during fatigue deformation by lowering oxidation kinetics and increasing the bonding strength of oxide layers.

The effect of multi-field coupling on dynamic recrystallization behavior in IN718 alloy

The dynamic recrystallization behavior of IN718 alloy is studied under multi-field coupling based on deformation difficulty at high temperature. The results show that dynamic recrystallization of IN718 alloy occur at 750°C under multi-field. At the same temperature, the resistance of IN718 alloy decreases and the degree of dynamic recrystallization increases with the increase of pulse current frequency. The dynamic recrystallization mechanism of alloy under multi-field coupling is discontinuous mechanism of grain boundary arch nucleation. The dynamic recrystallization ability is significantly improved under the action of multi-field coupling, thus the deformation resistance is reduced. The reason is that the dislocation movement intensifies under the action of multi-field coupling, which promotes dislocation proliferation and recovery, and then promotes dynamic recrystallization.

Microstructural Evolution of Tantalum During Deformation and Subsequent Annealing

Microstructure-aware models are necessary to predict the behavior of material based on process knowledge or to extrapolate mechanical properties of materials to environmental conditions which are not easily reproduced in the laboratory, e.g., nuclear reactor environments. Elemental Ta provides a relatively simple BCC system in which to develop a microstructural understanding of deformation processes which can then be applied to more complicated BCC alloys. In situ neutron diffraction during compressive deformation and subsequent heat treatment have been used to monitor the evolution of microstructural features in Ta throughout simulated processing steps. Crystallographic texture and dislocation density are determined as a function of first plastic strain, then temperature. Lattice strains are determined and attributed to stresses at macroscopic, grain and dislocation length scales. The increase of the dislocation density through deformation and subsequent recovery during heat treatment is monitored through the changing diffraction line profile. Also, randomization of the texture is used as a signature of recrystallization. The recovery of dislocations through annihilation is not observed to depend on the initial dislocation density in the range studied here. In contrast, recrystallization is observed to depend strongly on the initially dislocation density.

Thermal ratcheting of uranium simulated with a thermo-elasto-visco-plastic self-consistent polycrystal model

Uranium polycrystal aggregates accumulate large plastic deformation during thermal cycling in a process known as thermal ratcheting. This process is induced by highly anisotropic single crystal properties, texture, and a complex coupling between thermal, elastic, and plastic mechanisms of deformation. In this study, a thermo-elasto-visco-plastic polycrystal model is developed and applied to the prediction of thermal ratcheting in uranium. The model is based on the evolution of dislocation density on individual slip systems and accounts for static recovery effects. The model captures the plastic relaxation of intergranular stresses induced by thermal cycling and leads to quantitative understanding of the coupling between thermal, elastic and plastic mechanisms, and the important role played by static dislocation recovery in uranium. Results are compared with thermal ratcheting measurements.

Recovery of dislocation cell structures in 316L stainless steel manufactured by selective laser melting

Understanding the recovery mechanism associated with the dislocation cell structure in 316L austenitic stainless steel produced by selective laser melting (SLM), as well as the role played by the accompanying segregation network, is crucial for tailoring the microstructures and mechanical properties of SLM-prepared components. In the present work, the evolution of the dislocation cells was investigated during isothermal annealing, and a recovery mechanism for the dislocation cell structure was proposed based on a combination of microstructural observations and recovery kinetics analysis. The results show that the high-density dislocations at the cell boundaries annihilate during prolonged annealing at 800 °C, resulting in the decomposition of the dislocation cells. The recovery kinetics analysis reveals that SLM-prepared 316L exhibits a lower recovery rate and requires significantly higher apparent activation energy during annealing compared to conventional 316L deformed by cold rolling. The segregation network plays an important role during the dislocation recovery process, which limits the dislocation reaction occurring within the cell boundaries at the early stage of recovery, impedes the dislocation motion after the decomposition of dislocation cells, and leads to a strong temperature dependence of recovery kinetics in SLM-prepared 316L.

Cross slip based dynamic recovery during plane strain compression of aluminium and its role in preferential nucleation of the cube-oriented recrystallized grains

Understanding nucleation of the cube-oriented grains during static recrystallization of medium to high stacking fault energy (SFE) FCC metals is both scientifically and industrially important. Dynamic recovery due to weak Hirth junctions is considered as one of the plausible explanations for the nucleation advantage of the cube orientation. However, influence of cross slip based dynamic recovery on the nucleation propensity of the cube orientation has not been studied in detail. For this purpose, we explicitly incorporated stress dependent dynamic recovery of screw dislocations into a dislocation density based crystal plasticity model. The model is utilized for studying influence of cross slip based local dynamic recovery during deformation and recrystallization nucleation of aluminium. We show that dynamic recovery influences local dislocation density difference and disorientation with the neighbouring locations for the main rolling texture components. Next, we analysed nucleation propensity of these texture components using a stochastic nucleation model. Simulation results show that without explicit incorporation of the dynamic recovery of screw dislocations, the Goss orientation shows the highest nucleation propensity. On the other hand, incorporation of the cross slip based dynamic recovery enhances nucleation propensity of the cube component. Therefore, cross slip based dynamic recovery is one of the important mechanisms for nucleation of the cube-oriented recrystallized grains. These results provide new insights into deformation and recrystallization nucleation of medium to high SFE FCC metals.

Enhanced mobility of dislocation network nodes and its effect on dislocation multiplication and strain hardening

Understanding plastic deformation of crystals in terms of the fundamental physics of dislocations has remained a grand challenge in materials science for decades. To overcome this, the Discrete Dislocation Dynamics (DDD) method has been developed, but its lack of atomistic resolution leaves open the possibility that certain key mechanisms may be overlooked. By comparing large-scale Molecular Dynamics (MD) with DDD simulations performed under identical conditions we uncover significant discrepancies in the predicted strength and microstructure evolution in BCC crystals under high-strain rate conditions. These are traced to unexpected behaviors of dislocation network nodes forming at dislocation intersections, that can move in ways not previously anticipated as revealed by MD. Once these newfound freedoms of nodal motion are incorporated, DDD simulations begin to closely match plastic evolution observed in MD. This additional mechanism of motion whereby non-screw dislocations can change their glide plane profoundly affects fundamental processes of dislocation multiplication, recovery and storage that define strength of metals.

Microstructural analysis of flash-healed Vickers-indented microcracks near positive/negative electrodes on the (001) surface of cubic zirconia single crystals under direct current electric fields

Flash healing is a technique that uses power spikes to rapidly repair microcracks on ceramic surfaces. In this study, the microstructures of flash-healed Vickers-indented microcracks near the positive/negative electrodes under a DC electric field were examined by scanning transmission electron microscopy. The microstructural morphology near the positive electrode was almost identical to that of the microcracks located between both electrodes, as previously reported. However, a decrease in the degree of healing was observed near the negative electrode. This was attributed to the insufficient recovery of dislocations generated at the indentation state, which should have occurred in the initial stage of healing. The delay in dislocation recovery prevented contact between the inner surfaces of the microcracks and is possibly related to the increase in oxygen vacancies near the negative electrode.

Investigation of size effect on flow stress of Cu-3wt.%Ag-0.5 wt%Zr thin sheets at room and cryogenic temperature based on geometrically necessary dislocation evolution behavior

The influence of the grain sizes ranging from 23.4 to 69.2 μm on mechanical properties of Cu-3wt.%Ag-0.5 wt%Zr thin sheets was investigated by conducting uniaxial tensile experiments at room temperature (RT 298 K) and cryogenic temperature (CT 77 K). At RT, the strength and ductility of the sheet decreased as the grain size increased, and the size effect on flow stress was observed when the ratio of thickness to grain size (t/d) was less than 4.5. At CT, the sheet exhibited excellent strength and ducility, accompanied by a reduction in the size effect on flow stress. GND distribution maps showed that the average GND density of grains in the sample surface (surface grains) was lower than that of grains in the core of the sample (core grains). As the distance from the grain boundary increased, the GND density decreased. A constitutive model was developed for Cu-3wt.%Ag-0.5 wt%Zr thin sheets based on the dislocation accumulation and recovery process, which was able to predict the effects of grain size and deformation temperature on the flow stress and GND evolution in Cu-3wt.%Ag-0.5 wt%Zr thin sheets. This constitutive model indicated that CT not only inhibited the GND recovery process but likewise promoted the GND accumulation process. The established constitutive model revealed that the ratio of SSD density to GND density increased with increasing strain. The total dislocation density and GND density of samples deformed at CT was higher than those of samples deformed at RT.

Dislocation Hardening in a New Manufacturing Route of Ferritic Oxide Dispersion-Strengthened Fe-14Cr Cladding Tube.

The microstructure evolution associated with the cold forming sequence of an Fe-14Cr-1W-0.3Ti-0.3Y2O3 grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material, initially hot extruded at 1100 °C and then shaped into cladding tube geometry via HPTR cold pilgering, shows a high microstructure stability that affects stress release heat treatment efficiency. Each step of the process was analyzed to better understand the microstructure stability of the material. Despite high levels of stored energy, heat treatments, up to 1350 °C, do not allow for recrystallization of the material. The Vickers hardness shows significant variations along the manufacturing steps. Thanks to a combination of EBSD and X-ray diffraction measurements, this study gives a new insight into the contribution of statistically stored dislocation (SSD) recovery on the hardness evolution during an ODS steel cold forming sequence. SSD density, close to 4.1015 m-2 after cold rolling, drops by only an order of magnitude during heat treatment, while geometrically necessary dislocation (GND) density, close to 1.1015 m-2, remains stable. Hardness decrease during heat treatments appears to be controlled only by the evolution of SSD.

The microstructure effects on irradiation response of ferritic – martensitic steels

Microstructural optimization to achieve greater mechanical strength has been one of the focuses in ferritic–martensitic steels development. However, these optimized microstructures’ effects on the radiation response are not well known. In this work, two ferritic–martensitic steels (9Cr-NbMo and 9Cr-Ta) underwent neutron irradiation in the High Flux Isotope Reactor, and their room-temperature post-irradiation tensile properties and microstructure evolutions were investigated and compared. These two steels exhibit similar pre-irradiation tensile behavior, and their yield strengths are higher than that of other ferritic–martensitic steels by about 200–250 MPa. Microstructural characterization on pre-irradiated materials reveals a smaller grain size in 9Cr-Ta (2.8 ± 0.3 μm in 9Cr-Ta versus 4.3 ± 0.5 μm in 9Cr-NbMo) but higher dislocation density and precipitate density in 9Cr-NbMo. As is common for ferritic–martensitic steels at low irradiation temperatures (less than about 0.45Tm), irradiation-induced hardening at 400 °C was observed for both alloys. Irradiation at 490 °C causes the two alloys to exhibit different tensile behavior: 9Cr-Ta softens by 208 MPa in yield stress, whereas 9Cr-NbMo maintains strength. Microstructural characterizations were performed, including precipitate growth, dislocation, and defect formation. Using the barrier hardening model for microstructure–property correlation, the softening in irradiated 9Cr-Ta is primarily attributed to the significant dislocation recovery, while the strength lost from the slight dislocation recovery in 9Cr-NbMo was compensated by the additional strength from the irradiation-induced cavities. The microstructure effect (primarily precipitate, dislocation and boundary) on the radiation response is discussed herein.

Temperature-dependent electroplasticity in the Invar 36 alloy

Electroplasticity of the Invar 36 alloy is investigated based on comparisions between warm and electrically-assisted (EA) uniaxial tensions experiments at different temperatures (100–600 °C) and corresponding microstructural characterizations. Under warm deformation (WD), the flow stress decreases with the increase in temperature, which is directly attributed to dynamic recovery and dynamic recrystallization. With the introduction of high electric current during deformation, it shows a temperature-dependent electroplasticity phenomenon. No difference could be observed between WD and EA samples below a critical temperature of 500 °C, above which, an obvious softening behavior takes place in EA samples. This transition in softening respective to deformation temperature clearly demonstrated that it correlates with the intrinsic current-induced dislocation annihilation and recovery rather than the traditional theory of the Joule heating effect. Our findings shed more light on the utilization of EA softening effect on the development of novel forming routines, especially for materials with limited deformability.

Effect of Post-heat Treatment on Microstructure and Mechanical Properties of Laser Cladded High Co-Ni Secondary Hardening Steel Coating

The effect of post-heat treatment on the microstructure and microhardness of laser cladded high Co-Ni secondary hardening steel coating was investigated. The microstructures were analyzed using a SEM equipped with an EDS, and the microhardness was measured with a Vickers indenter. Decomposition of the retained austenite in the coating occurred during the post-heat treatment. As the temperature increased from 200 °C to 600 °C, the quantity of the retained austenite at the boundaries decreased significantly, while that of the needle-shaped M3C cementite and M2C carbides increased. The M2C carbides evidently coarsened when the temperature was higher than 500 °C. The microhardness of high Co-Ni steel coating increased as the temperature of post-heat treatment increased from 200 °C to 400 °C because the fine-scale M2C carbides were coherent with the matrix and increased distinctly in this temperature range. It decreased sharply when the temperature further increased from 500 °C to 600 °C due to both the incoherency of the coarsened M2C carbides and the recovery of dislocations in the carbon-supersaturated matrix.

Evaluation of metallurgical risk factors in post-test, advanced 9% Cr creep strength enhanced ferritic (CSEF) steel

ABSTRACT About 9 wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coal-fired applications for working temperatures of 600 ~ 650°C. This family of materials possesses a martensitic microstructure stabilised by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9 wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinders the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9 wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterisation approaches. Utilising focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Ta-containing phases from the early stages of creep.

Study on the stress relaxation with synchronous ageing of near α titanium alloy

The stress relaxation with synchronous ageing of TA15 titanium alloy was studied under a plastic pre-loading condition. The evolutions of microstructure and room temperature properties were characterized. Results show that the stress can be eliminated by 85 %, and the ultimate tensile strength can be increased by 5 % compared to the as-received sheet at room temperature. The plastic pre-loading results in dislocation entanglements in primary α (αp) and precipitation of partial secondary α (αs). The main stress relaxation mechanisms in the early stage are the precipitation and growth of αs, stress-induced twinning and the dynamic recovery in αp. The main stress relaxation mechanisms in the late stage are the globularization of αs and dislocation recovery in αp. The strength improvement after stress relaxation is mainly due to the formation of αs.

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.

Microstructural and mechanical properties of Al–Mg–Si–Cu joint fabricated by rotary friction welding

The correlation between the microstructural evolution and mechanical properties in different regions of Al–Mg–Si–Cu joint fabricated by rotary friction welding (RFW) has undergone a rigorous quantitative investigation using Vickers hardness evaluation, uniaxial tensile tests, electron backscatterred diffraction, X-ray diffractometry and transmission electron microscopy. The results of the investigation indicate that premature yielding and localized stress concentration inevitably occurred at the weakest heat affected zone (HAZ) region, remarkably compromising the mechanical properties of the joint. TEM observations revealed the presence of the rod-like Q′ phase and lath-like C phase within the BM and the pre-β″ phase in the weld zone, while no precipitate was identified in the HAZ on either sides. Although partial dynamic recrystallization occurred in HAZ reduced the grain size, extensive dislocation recovery and precipitate dissolving were two factors to be responsible for the lowest strength of the HAZ. On the contrary, fully recrystallization, severe plastic deformation and reprecipitation synthetically contributed to the strength recovery in the weld zone. Eventually, a model based on microstructural evolution that can quantitatively describe the observed strength difference in different regions is developed, suggesting that the major strength contributor is the dislocations strengthening, then the precipitation strengthening.