Increase In Dislocation Density Research Articles

Effect of WC content on the microstructure and wear resistance of laser cladding AlCoCrFeNiTi0.5 high-entropy alloy coatings

AlCoCrFeNiTi0.5+x(WC) high-entropy alloy (HEA) coatings with varying WC content (x = 0, 10, 20, and 30 wt%) were prepared by laser cladding. The results of the microstructure analysis showed that the AlCoCrFeNiTi0.5 coating consisted of body-centered cubic (BCC1) and BCC2 phases, while additional TiC and W2C phases formed within the coating with the WC content reaching 20 wt% mol. The addition of WC particles acted as a barrier to grain growth, preventing grains from growing larger. Additionally, the in-situ TiC phase was distributed at the grain boundaries and further inhibited the grains growth. The grain refinement and increase in dislocation density jointly improved the ability of the coating to resist plastic deformation. Due to the addition of the WC particles, the micro-hardness and wear resistance of the coatings improved considerably. The AlCoCrFeNiTi0.5 coating with 30 wt% WC exhibited the best performance. Its hardness increased by 35.8 % and wear rate decreased by 58.2 % compared with the AlCoCrFeNiTi0.5 coating.

Superior combination of strength and ductility in Fe-20Mn-0.6C steel processed by asymmetrical rolling

A Ce-alloyed twining-induced plasticity steel with the gradient structure was prepared through asymmetrical rolling. The primary role of Ce addition was to refine grains, and the asymmetrical rolling also effectively reduced grain sizes, and increased dislocation density. Under the combined effects of asymmetrical rolling and Ce addition, the steel exhibited superior combination of strength and ductility, with the yield strength of 1320 ± 5 MPa, ultimate tensile strength of 1775 ± 10 MPa and good elongation of 27 ± 1 %. Dislocation strengthening was the key factor of the increasing yield strength. As grain size decreased, the thicknesses of twins were significantly refined to several nanometers, accompanied with the activation of secondary twins. The promoted TWIP effect effectively enhanced the strain hardening ability of the steel.

Effect of hydrogen on the microstructural evolution and local mechanical properties of 430 ferritic stainless steel at high-temperature conditions

The present study investigated the impact of hydrogen on the microstructural evolution and mechanical properties of 430 ferritic stainless steel at high-temperature conditions. Hydrogen trap sites were analyzed at high temperatures using thermal desorption spectrometry through high-temperature hydrogen atmosphere charging and electrolytic hydrogen charging. The microstructural evolution was characterized using scanning electron microscopy and electron backscatter diffraction. The nanoindentation technique was employed to analyze the local mechanical properties of ferrite matrix, grain boundaries, and M23C6/matrix interfaces. Hydrogen trap sites in 430 ferritic stainless steel included ferrite crystal lattice, grain boundaries, intragranular dislocations, and M23C6/matrix interfaces where hydrogen could not intrude at room temperature. At high temperatures, hydrogen caused a decrease in grain size. Furthermore, at high temperatures, hydrogen altered the crystal orientation and texture of the ferrite matrix. There was an increase in the number of ferrite grains with (001) and (101) orientation increase and the formation of a new α-fiber texture. Hydrogen inhibited the triggering of the slip system in body centered cubic crystal lattice and induced an increase in dislocation density and internal stress. While hydrogen at high temperatures had a minimal impact on the matrix strength limit, it reduced the strength limit of the near grain boundaries and (Fe, Cr)23C6 precipitates. The influence of hydrogen on the microstructural evolution and mechanical properties of 430 ferritic stainless steel could be elucidated by the mechanisms of hydrogen enhanced decohesion, hydrogen-enhanced localized plasticity, and hydrogen enhanced specific crystal plane rotation suggested in the present study.

The influence of hydrogen on the recrystallization texture of a face centered cubic metal

The evolution of an alloy's microstructure in a hydrogen atmosphere at high temperatures is one of the important factors contributing to the degradation in the alloy's strength. The present study aimed to clarify the influence of hydrogen on the recrystallization texture of a face centered cubic (FCC) metal. As a model of FCC metal, commercially pure nickel (Ni) was cold-rolled and then annealed separately in vacuum and hydrogen atmospheres. Subsequently, the recrystallization textures were analyzed using EBSD and XRD. Hydrogen promoted the recrystallization of 〈001〉 − oriented deformed grains while hindering the recrystallization of 〈101〉 − oriented deformed grains. It increased the content of {011} 〈1−11〉 texture, promoted the decrease of R-Cube texture content, hindered the increase of Cube texture content， and hindered the reduction in contents of Brass, Copper, Goss, and S textures. In addition, it had an insignificant effect on {011}〈011〉 texture contents. Hydrogen demonstrated an anisotropic effect on the dislocation movement and density of grains with different orientations. Specifically, it impeded the dislocation movement within <101> − oriented deformed grains while facilitating the dislocation movement within <001> − oriented deformed grains. As a result, hydrogen induced an increase in dislocation density in <101> − oriented grains and a decrease in dislocation density in <001> − oriented grains.

Combination effects of laser heating and water cooling on the repair performance of Hastelloy C-276 by underwater laser direct metal deposition

The Ni-based Hastelloy C-276 was repaired by in-air and underwater laser direct metal deposition (in-air DMD and UDMD) with four gradient cooling rates (in-air DMD, in-air DMD with interlayer cooling, UDMD and UDMD with each layer re-covered by water). The microstructure and mechanical properties of repaired samples were systematically characterized with respect to the cooling rates. All samples presented good metallurgical bonding and columnar dendrites with epitaxial growth. The underwater environment had a more pronounced acceleration effect on the cooling rate compared to interlayer cooling. The rapid cooling induced intense thermal cycling, resulting in increased dislocation density and grain refinement with primary dendrite arm spacing (PDAS) diminishing from 5.30 to 3.21 μm. Accelerated cooling rate contributed to precipitation of massive M6C carbides. Carbides coarsened (sizes from 257 to 354 nm) and exhibited a reduced content as cooling rate slowed down due to intrinsic heat treatment (IHT). The residual water film in UDMD repair promoted oxide formation. Samples with increased cooling rates exhibited superior mechanical performance. The improved microhardness, tensile and impact properties were primarily attributed to the enhanced fine grain strengthening and Orowan strengthening facilitated via reduced grain size, increased dislocation density and carbide content. Quantities of large-size carbides were detrimental to tensile and impact properties as displayed in the in-air DMD sample with interlayer cooling (carbide size of 341 μm). The study could serve as a reference for in-situ restoration in deep-water environments.

Effect of final rolling temperature on the microstructure and properties of high-manganese austenitic low-temperature steel

The effect of final rolling temperature ranging from 962 to 696 °C on as-rolled high-Mn austenitic low-temperature steel was investigated using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The microstructure of the experimental steel remained austenite without suffering ferrite phase transformation, even when produced with a final rolling temperature as low as 696 °C. A decrease in the final rolling temperature of the steel was accompanied by a decrease in grain size from 20.2 to 6.0 μm, accompanied by an increase in dislocation density from 3.01 × 1014 to 10.9 × 1014 m−2. In addition, M23C6 carbides precipitated at the grain boundaries when the final rolling temperature was below 782 °C. Grain refinement and increased dislocation density significantly enhanced the strength of the material. However, an increase in the intragranular dislocation density compromised grain deformation ability. Moreover, the precipitation of M23C6 reduced the plasticity and toughness of the material. Considering the balance between yield strength and low-temperature toughness, the optimal final rolling temperature was 898 °C; this produced a material with a yield strength of approximately 507 MPa and a low-temperature impact energy absorption of approximately 143 J.

Microstructure and mechanical properties of hammer-forging assisted wire-arc directed energy deposition AZ91 alloy

ABSTRACT With the development of lightweight aerospace equipment, magnesium alloys are receiving increasing attention. Wire-arc directed energy deposition (Wire-arc DED) is a highly promising manufacturing method for magnesium alloy parts, but its development has been severely restricted by the problems of coarse grain size and low mechanical properties. To address these issues, a hammer-forging assisted Wire-arc DED technology for magnesium alloy AZ91 is proposed. The effects of interlayer hammer-forging and synchronous hammer-forging on macrostructure, microstructure and mechanical properties of the Wire-arc DED samples are compared, and the microstructure evolution and performance enhancement mechanism are discussed. The results show that the maximum plastic deformation caused by hammer forging reaches 11.7%. Hammer forging can significantly refine grains, and the average grain size decreases from 27.7 μm to 13.5 μm. Synchronous hammer-forging is better than interlayer hammer-forging in terms of performance enhancement, the UTS reaches 301.8 MPa, an increase of 10.9%, which is comparable to that of traditional forged parts, mainly attributed to the grain refinement and increased dislocation density.

Influence of synchronous-hammer-forging intervention temperature on the microstructure and properties of 316L components fabricated by laser directed energy deposition

Synchronous-hammer-forging has been proved to be an effective method to control the microstructure and properties of metal components in additive manufacturing, but the influence of the intervention temperature is still unclear. 316L stainless steel samples with different hammer-forging intervention temperature were prepared by laser directed energy deposition (LDED) technology. The results show that due to dynamic recrystallization and dislocation accumulation effect, the grain size of the component decreases first and then increases with the increase of hammer-forging temperature, while the dislocation density and tensile strength increase first and then decrease.

Micromechanism insights and constitutive modeling for deformation in a novel high manganese austenitic steel

We explore the mechanical behavior and microstructural evolution of a novel high manganese austenitic steel (HMAS), extensively applied in underground engineering and structural construction. Our investigation focuses on two variants of HMAS, namely HMAS-1.9 and HMAS-10, which exhibit distinct yield strengths and elongation behaviors. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to elucidate the evolution of microstructures during plastic deformation. Based on these experimental results, we developed a new phenomenological multiscale constitutive model that captures the mechanical behavior of HMAS, integrating micromechanisms and microstructural evolutions. Optimized with the MATLAB genetic algorithm toolbox, this model was successfully implemented in a uniaxial tensile finite element model (FEM) within ABAQUS, proving its efficacy in predicting both macroscopic flow stress and microstructural evolutions. Our findings highlight the important roles of initial grain size, dislocation density, and twin fraction in determining the mechanical properties of HMAS. Throughout the deformation process, both HMAS variants demonstrate similar plastic deformation behaviors, characterized by comparable rates of increase in dislocation density and twinning fraction, accompanied by a consistent trend of grain refinement. These findings not only deepen our understanding of the complex relationship between microstructure and mechanical properties of the HMAS steel, but also provide an effective multiscale constitutive modeling approach for predicting its deformation behavior.

Effect of strain rate on tensile characteristics and failure behavior of Al/steel welded joint producing by a laser-MIG composite heating source

Studies on dynamic tensile characteristics for Al/steel dissimilar materials are crucial to ensure the integrity and dependability of light-weight structures. In this study, the tensile characteristics and failure behaviors of Al/steel laser-MIG welded-brazed joints were studied. The dynamic fracture mechanisms of the joints are discussed. With the increase in strain rate, the fracture mode and tensile properties of Al/steel joints with reinforcement changed. The joints exhibited two failure modes of heat-affected zone failure at lower strain rates and interface failure at higher strain rates. When the strain rate was 1000 s−1, the increase in dislocation density in the weld reinforcement and diversion of cracks in the intermetallic compound layer significantly enhanced the joint’s properties. The tensile and yield strengths reached 222.6 and 166.6 MPa, 6.5 % and 17.9 % higher than those at a stain rate of 1 s−1, respectively. For Al/steel joints without reinforcement, as the strain rate increased, the crack gradually grew to the Al8Fe2Si phase, leading to a notable enhancement in the joint’s performance. At a strain rate of 500 s−1, the maximum strength of the joint reached 238.3 MPa, representing an increase of 58.9 % in comparison to the strain rate of 1 s−1.

Investigation of the microstructural behavior of Al–Mg–Si(X)–Mn aluminum alloys based on biaxial hot tensile tests

This study investigates the microstructural behavior of laboratory-produced Al–Mg–Si(X)–Mn aluminum alloys, focusing on the influence of varying Si content during biaxial hot tensile testing. Alloys with Si contents of 0.7%, 0.9%, and 1.3% were subjected to biaxial deformation at temperatures of 200 °C, 300 °C, and 400 °C. Using digital image correlation analysis, the impact of Si content on microstructural evolution under biaxial tensile loading was analyzed. Force–displacement analysis revealed a consistent inverse relationship between temperature and the maximum force required to initiate strain. At the temperature of 200 °C, the Al–Mg–Si(1.3)–Mn alloy required a maximum force of 1500 N, while at the temperature of 400 °C this force decreased to 900 N. The degree of anisotropy varied, with higher Si alloys exhibiting increased resistance to deformation in the transverse direction. In particular, the Al–Mg–Si(1.3)–Mn alloy showed pronounced strain anisotropy, with large major true strain φ1 values reaching up to 0.32 at 400 °C, compared to 0.26 at 300 °C and 0.2 at 200 °C. Microstructural analysis using electron backscatter diffraction (EBSD) and energy dispersive X-ray spectrometry (EDS) showed minimal changes at low temperatures, while increased dislocation density and grain boundary distortion were observed at elevated temperatures. The β-Mg2Si precipitates, influenced by Si content and temperature, significantly affected the mechanical properties. In the Al–Mg–Si(0.7)–Mn alloy, precipitates were predominantly 1–3 µm in diameter, whereas in the Al–Mg–Si(1.3)–Mn alloy, precipitates grew to 4–8 µm at higher Si content. These findings provide critical insights into the mechanical response and deformation mechanisms of aluminum alloys under biaxial tensile conditions, essential for optimizing material performance in engineering applications.Graphical abstract

Nanostructuring and Hardness Evolution in a Medium‐Mn Steel Processed by High‐Pressure Torsion Technique

Severe plastic deformation (SPD) is performed on a newly developed medium‐Mn steel with the composition of Fe–7.66Mn–2Ni–1Si–0.23C–0.05Nb (wt%) to achieve a nanocrystalline microstructure. The SPD process utilizes the high‐pressure torsion (HPT) technique, resulting in a nominal shear strain of approximately 36 000% after processing the disk for 10 turns. In X‐Ray diffraction line profile analysis, an increase in dislocation density to around 230 × 1014 m−2 is observed. In addition, under high strains, a face‐centered cubic (fcc) secondary phase emerges within the body‐centered cubic (bcc) matrix. In analytical transmission electron microscopy, using energy‐dispersive X‐Ray spectroscopy, it is indicated that the secondary‐phase particles are enriched in Al, Mn, and Si. As the strain imposed during HPT increases, the simultaneous rise in dislocation density and nanostructuring lead to material hardening. However, the partial phase transformation from bcc to fcc contributes to material softening. As a result of these two opposite effects, the hardness exhibits a non‐monotonic variation with the shear strain, displaying, for 10 turns of HPT, a lower hardness compared to fewer turns, despite the continuous increase in dislocation density and decrease in crystallite size.

A Comparative Analysis of a Microstructure and Properties for Monel K500 Hot-Rolled to a Round Bar and Wire Deposited on a Round Surface

Metal manufacturing processes based on deformation (forging, rolling) result in a fine grain structure with a complex dislocation substructure, which positively influence mechanical properties. Casting and additive manufacturing (powder- or wire-based) usually produce a coarse grain structure with a poorly developed dislocation substructure, which negatively affect mechanical properties. Heat treatment may alter phase balance and stimulate precipitation strengthening; however, precipitation kinetics depends on the dislocation substructure. In this paper, a comparative study of the microstructure and strength is presented for Monel K500 alloy containing 63 Ni, 30 Cu, 2.0 Mn, and 2.0 Fe (wt.%), and microalloyed with Al, Ti, and C hot-rolled to a round bar and deposited on a round surface using wire additive manufacturing (WAAM) technology. An increased dislocation density and number density of fine precipitates resulted in 8–25% higher hardness and 1.8–2.6 times higher compression yield stress in the hot-rolled alloy compared to these in the WAAM-produced alloy. However, due to a high work hardening rate, only 3–10% cold deformation was necessary to increase the strength of the WAAM alloy to this of the hot-rolled one. Age hardening heat treatment, through the intensification of the precipitation strengthening mechanism, reduced the value of cold deformation strain required to equalise the properties. Based on the obtained results, a new technology consisting of additive manufacturing, heat treatment, and cold deformation can be proposed. It can produce WAAM components with strength and hardness improved to the level of hot-rolled components, which is a significant development of additive manufacturing.

Accurate determination of active slip systems using a novel lattice rotation analysis: Quasi-in-situ EBSD/ECCI study on the homogeneous/heterogeneous deformation of polycrystalline zirconium

The accurate determination of slip systems is crucial for understanding deformation mechanisms of metals in particular, how slip/twining transfer or blocking occurs. This paper proposes a novel crystallographic lattice rotation analysis, which combines quasi-in-situ Electron Back Scatter Diffraction (EBSD) and Electron channeling contrast imaging (ECCI) techniques to precisely predict the active slip system with specific slip directions in polycrystalline Zr. Large data sets for hundreds of grains were quantitatively analyzed by this method to statistically gain deep insight on the deformation mechanism of Zr. Our results show that prismatic 〈a〉 slip is the primary slip system in most grains, and pyramidal 〈a〉 slip plays a secondary role in coordinating the local stress state of plastic deformation. The predicted slip direction is the same as the direction in which dislocation density increases. This method can serve as a complementary tool to the intragranular misorientation axis (IGMA) method, bridging the gap between macro-mechanical response and microstructural deformation mechanisms.

A crystal plasticity based strain rate dependent model across an ultra-wide range

Numerous studies have investigated the strain rate sensitive behaviors of materials, consistently reporting enhanced stress values and increased dislocation density with rising strain rates. Behind these phenomena lies the intrinsic nature of dislocation activity. In this context, we introduce an analysis method within a crystal-plasticity (CP) framework, incorporating molecular dynamics insights for a comprehensive range of strain rates (7.5 × 10−5/s to 5 × 107/s). This approach offers a refined understanding of strain rate sensitive behaviors, mainly influenced by dislocation movement laws and strain-rate-dependent saturation of dislocation density. We elucidate the impact of deformation loading conditions on Schmidt factors and active slip systems, which are also crucial for understanding variations in SRS. Ultimately, this study underscores the CP method's effectiveness in comprehensive SRS analysis, seamlessly integrating experimental observations with theoretical predictions for advanced material characterization.

Acoustic Assessment of Microstructural Deformation Mechanisms on a Cold Rolled Cu30Zn Brass.

The relationship between acoustic parameters and the microstructure of a Cu30Zn brass plate subjected to plastic deformation was evaluated. The plate, previously annealed at 550 °C for 30 min, was cold rolled to reductions ranging from 10% to 70%. Linear ultrasonic measurements were performed on each of the nine specimens, corresponding to the nine different reductions, using the pulse-echo method to record the times of flight of longitudinal waves along the thickness axis. Subsequently, acoustic measurements were conducted to determine the nonlinear parameter β through second harmonic generation. Microstructural analysis, carried out by X-ray diffraction, Vickers hardness testing, and optical microscopy, revealed an increase in deformation twins, reaching a maximum at 40% thickness reduction. At higher deformations, the microstructure showed the generation and proliferation of shear bands, coinciding with a decrease in the twinning structure and an increase in dislocation density. The longitudinal wave velocity exhibited a 0.9% decrease at 20% deformation, attributed to dislocations and initial twin formation, followed by a continuous increase up to 2% beyond this point, resulting from the combined effects of twinning and shear banding. The nonlinear parameter β displayed a notable maximum, approximately one order of magnitude greater than its original value, at 40% deformation. This peak correlates with a roughly tenfold increase in twinning fault probability at the same deformation level.

Influence of friction and back stresses evolution on cyclic softening of laser powder bed fusion Ti-6Al-4V ELI alloy

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V ELI enables the fabrication of complex and individualized load-bearing patient-specific implants (PSI). These PSI are subjected to cyclic loading, which necessitates their fatigue performance evaluation. In this work, the fatigue properties of LPBF Ti-6Al-4V ELI in heat-treated conditions were characterized by studying the low cycle fatigue (LCF) behavior and underlying deformation mechanism. The fully reversed strain-controlled fatigue tests were performed at various strain amplitudes at room temperature. The ratio of fatigue life of conventional wrought alloy and LPBF Ti-6Al-4V ELI at the lowest (0.8 %) and the highest (1.8 %) strain amplitudes were found to be approximately 2.7 and 4.5, respectively. Further, the softening response during fatigue loading was correlated to the variation in the back and friction stresses. The TEM observations revealed that the dislocation pileup along α/β interfaces caused an increase in dislocation density, which drives the increase in back stress. However, the ease of slip transmission at high strain amplitude (1.8 %) reduced the heterogeneity between α and β phases, causing a decrease in the back stress. The TEM observations further suggested that the dislocation annihilation and subsequent rise in dislocation-free zones in α-phase reduced the friction stress at all strain amplitudes, which resulted in cyclic softening of LPBF Ti-6Al-4V ELI.

Effects of pre-strain on hydrogen-induced stress corrosion cracking behavior of Q345R steel in hydrofluoric acid vapor environment

This study investigates microstructure, hydrogen diffusion properties, and stress corrosion cracking (SCC) behavior of Q345R steel subject to pre-applied strains of 1%, 3%, and 5%. The relationship between microstructure and SCC susceptibility in hydrofluoric acid (HF) vapor is examined. It is found that pre-strain cannot change the microstructure of Q345R steel but increases dislocation density. The increasing SCC sensitivity of Q345R steel in HF vapor is associated with the dislocation density induced by pre-strain. With increasing pre-strain, dislocation density and reversible hydrogen trap increase, and interaction between hydrogen and dislocation accelerates hydrogen-induced SCC. In addition, irreversible hydrogen traps caused by high pre-strain can severely aggravate hydrogen-induced SCC, resulting in high SCC vulnerability.

Design of Quenching and Tempering Process and Elucidation of the Relationship between Microstructure and Properties of 20Mn2SiNiMo Bainitic Wheel Steel

The rapid development of train transportation toward higher speed and heavier load requires superior wheel materials with high strength and toughness. Herein, a 20Mn2SiNiMo bainitic wheel steel is developed and bainitic wheels are produced. The quenching and tempering process of bainitic wheels is studied considering the bainitic ferrite content and stability of retained austenite (RA). The quenching process includes water spraying and air cooling. Proeutectoid ferrite can be suppressed during the water‐spraying process. During air cooling, the bainitic ferrite transformation is promoted and the stability of RA is improved with the extension of cooling time. Corresponding microstructure of as‐quenched steel at wheel rim is bainite with different morphologies, martensite, and RA. The evolution of RA at lower and higher tempering temperatures is dominated by carbon enrichment and carbon consumption, respectively. Yield strength of bainitic wheel steel mainly comes from body‐centered cubic phase (containing martensite and bainitic ferrite). The increase of dislocation density and the decrease of effective grain size are beneficial for yield strength improvement. Bainitic packets with various direction, deformation‐induced martensite transformation of RA, and plastic deformation all play an important role in enhancing the toughness of bainitic wheel steel.

Martensitic transformation induced strength-ductility synergy in additively manufactured maraging 250 steel by thermal history engineering

Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility. Here, we report an intermittent printing strategy to tailor the microstructure and mechanical properties of maraging 250 steel via tuning the thermal history during wire-arc directed energy deposition. By introducing a dwell time between adjacent layers, the maraging 250 steel is cooled below the martensite start temperature, triggering thermally-driven martensitic transformation during the printing process. Thermal cycling during subsequent layer deposition results in the formation of reverted austenite which shows a refined microstructure and induces elemental segregation between martensite and reverted austenite. The Ni enrichment in the austenite promotes stabilization of the reverted austenite upon cooling to room temperature. The reverted austenite is metastable during deformation, leading to strain-induced martensitic transformation under loading. Specifically, a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% reverted austenite, resulting in improved work hardening via martensitic transformation induced plasticity during deformation. Meanwhile, the higher cooling rate and refined prior austenite grains lead to substantially refined martensitic grains (by approximately fivefold) together with an increased dislocation density. With 3 min interlayer dwell time, the yield strength of the printed maraging 250 steel increases from 836 MPa to 990 MPa, and the uniform elongation is doubled from 3.2% to 6.5%. This intermittent deposition strategy demonstrates the potential to tune the microstructure of maraging steels for achieving strength-ductility synergy by engineering the thermal history during additive manufacturing.