Reduction Of Dislocation Density Research Articles

Energy density to explain the ductility loss during electroplastic deformation of a dual phase steel

A systematic investigation is performed to quantify the change in ductility during a single pulse electric-assisted test in a commercial dual-phase (DP) steel. The tests were performed under different combinations of current density, pulse time, and prestrain. A dimensionless quantity, strain ratio, has been utilized to compare the ductility change across various test conditions. Unlike aluminum, the ductility was negatively affected by a set of input parameters. The mechanisms responsible for this negative change were analyzed by conducting fractography of the failed samples. X-ray diffraction and EBSD analysis indicate a reduction in dislocation density upon applying the electric pulse, which concurs with the fractography observations. It is found that the depletion of mobile dislocations due to electric pulse can lead to premature strain localization and adversely affect ductility.

Microstructural Evolution of a Re-Containing 10% Cr-3Co-3W Steel during Creep at Elevated Temperature

Ten percent Cr steels are considered to be prospective materials for the production of pipes, tubes, and blades in coal-fired power plants, which are able to operate within ultra-supercritical steam parameters. The microstructural evolution of a Re-containing 10% Cr-3Co-3W steel with low N and high B content during creep was investigated at different strains at 923 K and under an applied stress of 120 MPa using TEM and EBSD analyses. The studied steel had been previously normalized at 1323 K and tempered at 1043 K for 3 h. In the initial state, the tempered martensite lath structure with high dislocation density was stabilized by M23C6 carbides, NbX carbonitrides, and M6C carbides. At the end of the primary creep stage, the main microstructural change was found to be the precipitation of the fine Laves phase particles along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths. The remarkable microstructural degradation processes, such as the significant growth of martensitic laths, the reduction in dislocation density within the lath interiors, and the growth of the grain boundary Laves phase particles, occurred during the steady-state and tertiary creep stages. Moreover, during the steady-state creep stage, the precipitation of the V-rich phase was revealed. Softening was in accordance with the dramatic reduction in hardness during the transition from the primary creep stage to the steady-state creep stage. The reasons for the softening were considered to be due to the change in the strengthening mechanisms and the interactions of the grain boundary M23C6 carbides and Laves phase with the low-angle boundaries of the martensitic laths and free dislocations.

Compressive creep behavior of high Nb containing TiAl alloy: Dynamic recrystallization and phase transformation

In this work, compressive creep behavior of Ti-43Al-6Nb-1Cr-1.5V alloy fabricated by vacuum induction skull melting technology was investigated at 800-950 °C. The results show that compressive creep behavior of this alloy strongly depends on temperature. At 800 °C and 850 °C, this alloy exhibits conventional three-stage creep characteristics with a well-defined steady-state creep stage, and creep strain is small. However, at 900 °C and 950 °C, this alloy exhibits unconventional creep characteristics, and creep strain increases sharply. Microstructure observation proves that this is mainly due to the occurrence of dynamic recrystallization (DRX). During creep, continuous dynamic recrystallization and discontinuous dynamic recrystallization appear simultaneously, but the latter is more significant. At 800 °C and 850 °C, creep deformation mainly depends on dislocation motion, while at 900 °C and 950 °C, DRX also greatly contributes to creep deformation. During creep, recrystallized grains preferentially form at β-segregation and α-segregation, and rising temperature from 850 °C to 900 °C facilitates the sharp increase of DRX fraction. The occurrence of DRX reduces dislocation density, which greatly softens the alloy. Besides, more α2 grains generate at γ grain boundary and grain boundary/twin intersection during creep, and maintain Blackburn orientation relationship with γ grains. This is the result of stress-induced γ→α2 phase transformation.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

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.

Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint

Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint were investigated to reveal its softening mechanism. The results showed that a fine equiaxed grain structure (5.61 μm) was formed in the nugget zone (NZ), while the thermo-mechanically affected zone (TMAZ, 45.32 μm) and heat-affected zone (HAZ, 100.73 μm) maintained a fibrous grain structure. The fraction of low angle grain boundaries decreased from 75.6% of the base metal (BM) to 15.6% of the NZ. The annealing effect resulted in obvious reduction in dislocation density from 1.8×1014 m−2 of the BM to 4.5×1012 m−2 of the NZ. The average diameter size and volume fraction of Al3(Er, Zr) precipitates of the NZ, TMAZ and HAZ were close to those of the BM (13.7 nm and 0.13%). The NZ and TMAZ exhibited the lowest yield strength of about 201 MPa while the BM had the highest yield strength of about 295 MPa. The loss of the dislocation strengthening and substructure strengthening was the main reason for the decrease of yield strength from the BM to the NZ.

Grain Structure Evolution in 6013 Aluminum Alloy during High Heat-Input Friction-Stir Welding.

This work was undertaken to evaluate the influence of friction-stir welding (FSW) under a high-heat input condition on microstructural evolution. Given the extreme combination of deformation conditions associated with such an FSW regime (including the highest strain, temperature, and strain rate), it was expected to result in an unusual structural response. For this investigation, a commercial 6013 aluminum alloy was used as a program material, and FSW was conducted at a relatively high spindle rate of 1100 rpm and an extremely low feed rate of 13 mm/min; moreover, a Ti-6Al-4V backing plate was employed to reduce heat loss during welding. It was found that the high-heat-input FSW resulted in the formation of a pronounced fine-grained layer at the upper weld surface. This observation was attributed to the stirring action exerted by the shoulder of the FSW tool. Another important issue was the retardation of continuous recrystallization. This interesting phenomenon was explained in terms of a competition between recrystallization and recovery at high temperatures. Specifically, the activation of recovery should reduce dislocation density and thus retard the development of deformation-induced boundaries.

Acoustic field-assisted powder bed fusion of tungsten carbide-reinforced 316L stainless steel composites

Due to its excellent corrosion and oxidation resistance, good formability, and affordability, 316L stainless steel (SS) is widely used in industries such as biomedical, marine, chemical, and water treatment. However, to further enhance its mechanical properties, scholars have turned to reinforcing 316L SS with ceramic additives. Among ceramic additives, tungsten carbide (WC) is favored due to its good wettability ability to bond closely with iron-based materials. Despite these benefits, defects of cracks, pores, and balling commonly exist in WC-reinforced metal matrix composites due to the agglomeration of WC particles. In response to these problems, author propose a novel method that combines acoustic field (AF) with powder bed fusion (PBF) to process WC-reinforced 316L SS materials. This approach takes advantages of AF's ability to homogenize materials, smooth out thermal gradient, reduce cracks, and refine grains in liquid materials, while also utilizing PBF's capabilities to fabricate complex-shaped customized parts with high density and good quality. Experimental results have shown that adding an AF to the PBF can reduce dislocation density, mitigate stress, alleviate cracks, refine grain size, and alter grain shape, ultimately enhancing the tensile properties of WC-reinforced 316L SS composites. By adopting this method, a balance of strength and ductility can be achieved, with ultimate tensile strength and percentage elongation falling into a narrow but high range of 750–930 MPa and 44–55%, respectively.

Tailoring the microstructure and mechanical properties of laser metal-deposited Hastelloy X superalloy sheets via post heat-treatment

Nickel-based superalloys are subject to severe microsegregation and exhibit anisotropic properties during laser metal deposition (LMD), which restricts their wide application in the aerospace field. Thus, post heat-treatments must be investigated to further regulate the microstructure and mechanical properties of such components. The aim is to provide a comprehensive understanding of the effects of double-stage aging (DA) and solution plus double-stage aging (SDA) heat treatments on microstructure and mechanical properties of Hastelloy X superalloy sheets fabricated via LMD. The effects of DA and SDA heat treatments on the density, residual stress, phase composition, carbides precipitation behavior, dislocation structure, grain structure, and mechanical properties of the sheets were studied and in comparison with the as-deposited (AD) samples. The results showed that the DA heat treatment yielded a significant amount of lamellar and granular phases in the interdendrites and grain boundaries, whereas the anisotropic microstructure between the transverse and vertical planes was similar to that observed in the AD samples. These phases are mainly composed of Laves phase and carbides. However, the needle-like carbide phase was dispersedly precipitated within the grains and at grain boundaries, and exhibited full recrystallization and reduced dislocation density after SDA heat treatment. Consequently, the SDA samples presented superior strength-ductility balance to those of the AD and DA samples with irregularly shaped Laves phase and carbides owing to microstructural changes. For instance, the SDA heat treatment improved the ultimate tensile strength and fracture elongation of the vertical samples by up to 10.9% and 30.7%, respectively, whereas the microhardness decreased by 13.3% compared to the AD samples. The compressive residual stress of 98 MPa and tensile residual stress of 147 MPa were also observed on the surfaces of the SDA and AD samples, respectively. In addition, the SDA samples exhibited an almost isotropic tensile behavior in the vertical and horizontal directions as the epitaxially grown columnar grains transforms into equiaxed grains, as well as exhibited a prominent overall crystallographic texture. The effect of DA and SDA heat treatments on the mechanical properties were analyzed on the basis of stress–strain curves, hardening behavior, and multiple strengthening mechanisms.

Hydrogen-induced microstructure optimization and mechanical properties enhancement of forged (TiB + TiC)/Ti6Al4V

This study investigates the impact of hydrogen on the mechanical properties and microstructure of forged titanium matrix composites (TMCs) reinforced with 2.5 vol% TiB and 2.5 vol% TiC. Melt hydrogenation technology was used to fabricate the TMCs with different hydrogen contents, which were then forged at 880 °C. Microstructural analysis revealed that hydrogen accelerated atomic diffusion, resulting in an approximate network microstructure. Hydrogen also promoted dynamic recrystallization during forging, leading to grain refinement and reduced dislocation density. Room temperature tensile tests showed that hydrogen significantly improved the strength and ductility of the forged TMCs within a certain range of hydrogen content. The greatest improvement was observed in the forged TMCs with a hydrogen content of 7.87 × 10−2 wt%, which exhibited a 10.48% increase in yield strength and a 31.36% increase in ductility relative to those without hydrogen. The effects of hydrogenation on crack propagation mechanisms are also discussed in detail. Overall, these findings provide valuable insights into the strengthening and toughening of forged titanium matrix composites through hydrogenation.

Electromagnetic shocking induced fatigue improvement via tailoring the α-grain boundary in metastable β titanium alloy bolts

The fatigue performance of bolts directly determines the safety of aircraft. Here, we report a novel electromagnetic shocking (EMS) strategy to achieve the improvement of fatigue life in metastable β titanium alloy bolts. The width of α-grain boundary was obviously decreased, accompanied by the reduction of dislocation density inside the β-phase grain after EMS, which is also verified by the molecular dynamics simulation. Remarkably, the rated fatigue life increases from 42.6k to 50.1k and the electrical resistivity decreases from 2.95 to 2.51 μΩ•m after EMS. The refined α-grain boundary enables cyclic loading stress transfer and weakens the obstruction of dislocation motion, which makes it difficult for crack to extend through intracrystalline and thus slows down the expansion of fatigue cracks. The work suggests the “targeting” effect of electromagnetic energy on grain boundary and dislocation with higher resistance, which provides a new way for improving the fatigue performance of engineering parts fabricated by metastable β titanium alloy.

Significant annealing-induced hardening effect in nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 medium-entropy alloy by severe cold rolling

Due to the easy coarsening caused by poor thermal stability, the verified annealing-induced hardening in nanograined metals can only maintain at a relatively low-temperature range. In this study, a nanolaminated (CrCoNi)97.4Al0.8Ti1.8 medium-entropy alloy with an average lamellae thickness of ∼ 20 nm embedded by thinner nanotwins was fabricated by severe cold rolling to achieve superior thermal stability. Compared with the conventional nanotwinned CrCoNi with nanotwins inside ultra-fined grains, the hierarchical nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 exhibits a significant annealing-induced hardening effect, i.e., hardness increasing from ∼ 250 HV in the original specimen to ∼ 500 HV in the cold-rolled status and finally ∼ 630 HV after annealing at 600 °C for 1 h. Detailed microstructure characterizations reveal that the reduced dislocation density and formation of L12 ordered domain are mainly responsible for such hardening effect, which is facilitated by the effectively suppressed coarsening with annealing temperature, i.e., slow detwinning process and well-retained low-angle nanolamellar structure. The coarsening mechanisms from the cold-rolled nanolamellae to the fully recrystallized micro-equiaxed structures under the annealing temperatures ranging from 400 to 800 °C were also elucidated by atomic observations.

Temperature-Dependent Optical Behaviors and Demonstration of Carrier Localization in Polar and Semipolar AlGaN Multiple Quantum Wells

Semipolar AlGaN multiple quantum wells (MQWs) have unique advantages in deep ultraviolet light emitters due to the weak Quantum-Confined Stark Effect. However, their applications are hampered by the poor crystalline quality of semipolar AlGaN thin films. Different treatments were developed to improve the crystal quality of semipolar AlGaN, including a multistep in situ thermal annealing technique proposed by our group. In this work, temperature-dependent and time-resolved photoluminescence characterizations were performed to reveal the carrier localization in the MQW region. The degree of carrier localization in semipolar AlGaN MQWs grown on top of the in situ-annealed AlN is similar to that of conventional ex situ face-to-face annealing, both of which are significantly stronger than that of the c-plane counterpart. Moreover, MQWs on in situ-annealed AlN show drastically reduced dislocation densities, demonstrating its great potential for the future development of high-efficiency optoelectronic devices.

Precipitates evolution during isothermal aging and its effect on tensile properties for an AFA alloy containing W and B elements

A 20Cr-25Ni-2.5Al alumina-forming austenitic alloy containing W and B elements was aged at 923 K for 5000 h, and the microstructure and tensile properties with different aging time were investigated. NiAl, σ and Laves were observed at grain boundaries (GBs) successively. The matrix was covered by NiAl and Laves with the extension of aging. The evolution of precipitates during aging contributed to the variation of tensile properties. Precipitation of nanosized NbC carbides within grains and σ phase at GBs led to a rapid increase in strength and a decrease in elongation for 500 h aging sample. In the later stage of aging, the coarsening of NiAl and Laves phases, as well as the reduction in dislocation density caused a decline in strength. The coarsening of precipitates upon aging time follows the Ostwald ripening theory. Due to its lower diffusion rate in austenite compared to Mo, W may accelerate the growth of Laves at GBs. Boron was mainly enriched in Laves instead of NiAl, NbC and σ phases after high temperature aging. The addition of W and B improved the precipitation strengthening of Laves, increasing the high temperature tensile strength after long term thermal aging. The difference in tensile properties between room temperature and 923 K is due to the ductile–brittle transition of NiAl. No σ phase was observed within grains even after 5000 h aging and elemental chromium particles occurred around Laves due to boron hindering the growth of σ.

Improved creep resistance of 20Cr25NiNb heat resistant steels through grain boundary intermetallic precipitation strengthening

The creep performance of 20Cr25NiNb austenitic steel through the addition of W, Mo, and B elements was investigated. W and Mo enhanced high temperature creep resistance primarily through solid solution strengthening σss and precipitation strengthening σp by introducing Laves phase. Boron enrichment was predominantly observed in Laves phase at grain boundaries (GBs) rather than in σ and G phases. The significant improvement in creep life of B-bearing steel was primarily attributed to a remarkable increase in σp, as trace amounts of B greatly accelerated Laves precipitation at GBs and inhibited σ phase coarsening. The creep resistance decreased significantly after 5000 h thermal aging due to reduced dislocation density, grain coarsening, and decreased solid solution elements in austenite. Creep cracks mainly propagated along GBs, where coarse σ phase provided sites for creep cavity nucleation. The applied stress accelerated σ precipitation and coarsening, resulting in the absence of tertiary creep stage during 4700 h creep of B containing 2025WMoB steel.

Comparative study of the effects of conventional shot peening and ultrasonic shot peening on very high cycle fatigue properties of GH4169 superalloy

This study investigates the effects of two different shot peening techniques, conventional shot peening (CSP) and ultrasonic shot peening (USP), on the very high cycle fatigue (VHCF) properties of the GH4169 superalloy. The study evaluates and compares surface roughness, X-ray diffraction patterns, microhardness, and residual stress in specimens treated with CSP and USP. The VHCF tests reveal that both CSP and USP treatments improve the VHCF properties of the GH4169 superalloy. However, while the USP-treated specimens have deeper work hardening and compress residual stress layers, their VHCF lives are significantly shorter than those of the CSP-treated specimens. Additionally, the study finds that the stress triaxiality, which inhibits dislocations at the crack tip, is higher closer to the center of the specimen. The transmission electron microscopy (TEM) results further confirm that the reduced dislocation density and increased stress concentration in the plastic zone of the high stress triaxiality region promote fatigue short crack propagation and reduce the fatigue properties of the material.

Employing high-temperature-grown SrZrO3 buffer to enhance the electron mobility in La:BaSnO3-based heterostructures

We report a synthetic route to achieve high electron mobility at room temperature in epitaxial La:BaSnO3/SrZrO3 heterostructures prepared on several oxide substrates. Room-temperature mobilities of 157, 145, and 143 cm2 V−1 s−1 are achieved for heterostructures grown on DyScO3 (110), MgO (001), and TbScO3 (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO3 buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La:BaSnO3 active layer by molecular-beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below 1.0×1010 cm−2 for buffered films on DyScO3, MgO, and TbScO3 substrates. The present results provide a promising route toward achieving high mobility in buffered La:BaSnO3 films prepared on most, if not all, oxide substrates with large compressive or tensile lattice mismatches to the film.

Reduction of dislocation density in single crystal diamond by Ni-assisted selective etching and CVD regrowth

The development of large-area diamond films with low dislocation density plays a key role in diamond-based devices. In this study, we present a straightforward and reasonably priced Ni-assisted etching and re-growth method to decrease the dislocation density of diamond. By adjusting the annealing temperature, a high-density inverted pyramid and/or trapezoidal pit can be effectively exposed because the interaction between carbon and Ni metal. The Ni tends to dissolve the diamond surrounding the dislocation at high temperature while maintaining the surface's flatness. The precipitated graphite layer is removed using wet etching, and the step flow mode is then restored via the re-growth procedure. It shows that the dislocations located beneath the etching pits are partially annihilated during the lateral coalescence, resulting in a decrease in dislocation density.

Investigation of the microstructure and mechanical properties of borosilicate reinforced magnesium nano composites

In the current investigation, nano borosilicate of varying weight proportions was used to strengthen AZ91D magnesium alloy. The squeeze casting process was used to create the material, and its microstructure and mechanical characteristics were analyzed to determine its suitability for functional applications. Using SEM and ASTM standards, morphology and mechanical properties were analyzed. Morphological studies indicate that reinforcing particles are evenly distributed throughout the matrix alloy's regions. Morphological studies illustrates that the nBS was uniformly distributed in AZ91D alloy without forming residual pore. The hardness (21 %) and tensile properties (26 %) of synthesized nano composites was improved due to reduction in grain size and higher dislocation density in comparison with monolithic alloy. Owing to the strong interfacial bonding strength and the dispersion strengthening the compressive strength of magnesium nano composites increased (24 %). As a result of improvements in the load bearing capacities of the reinforcing particles as well as the formation of transfer layers between the matrix and the reinforcement particles wear resistance significantly improved (25 %).

Reduction of dislocation density in α-Ga2O3 epilayers via rapid growth at low temperatures by halide vapor phase epitaxy

We demonstrate that the dislocation density in α-Ga2O3 epilayers is markedly reduced via rapid growth at low temperatures by halide vapor-phase epitaxy. An α-Ga2O3 epilayer grown on (0001) sapphire at a high growth rate of 34 μm h−1 and a low temperature of 463 °C exhibited a dislocation density of 4 × 108 cm−2, which was approximately 1/100 of that in a conventional film. It is likely that the three-dimensional surface morphology developed during the growth enhanced the bending of the dislocations to increase the probability of pair annihilation. The combination of this technique with thick film growth and epitaxial lateral overgrowth resulted in a further low dislocation density of 1.1 × 107 cm−2.