Reduction Of Dislocation Density Research Articles

Strain-Controlled Thermal-Mechanical Fatigue Behavior and Microstructural Evolution Mechanism of the Novel Cr-Mo-V Hot-Work Die Steel.

In response to the intensifying competition in the mold market and the increasingly stringent specifications of die forgings, the existing 55NiCrMoV7 (MES 1 steel) material can no longer meet the elevated demands of customers. Consequently, this study systematically optimizes the alloy composition of MES 1 steel by precisely adjusting the molybdenum (Mo) and vanadium (V) contents. The primary objective is to significantly enhance the microstructure and thermal-mechanical fatigue performance of the steel, thereby developing a high-performance, long-life hot working die steel designated as MES 2 steel. The thermal-mechanical fatigue (TMF) tests of two test steels were conducted in reverse mechanical strain control at 0.6% and 1.0% strain levels by a TMF servo-hydraulic testing system (MTS). The microstructures of the two steels were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results indicate that throughout the entire thermomechanical fatigue cycle, both steels exhibit initial hardening during the low-temperature half-cycle (tension half-cycle) and subsequent continuous softening during the high-temperature half-cycle (compression half-cycle). Furthermore, under the same strain condition, the cumulative cyclic softening damage of MES 1 steel is more pronounced than that of the newly developed MES 2 steel. The number, width, and length of cracks in MES 2 steel are smaller than those in MES 1 steel, and the thermomechanical fatigue life of MES 2 steel is significantly longer than that of MES 1 steel. The microstructures show that the main precipitate phase in MES 1 steel is Cr-dominated rod-shaped carbide. It presents obvious coarsening and is prone to inducing stress concentration, thus facilitating crack initiation and propagation. The precipitate phase in MES 2 steel is mainly MC carbide containing Mo and V. It has a high thermal activation energy and is dispersed in the matrix in the form of particles, pinning dislocations and grain boundaries. This effectively delays the reduction in dislocation density and grain growth, thus contributing positively to the improvement in thermomechanical fatigue performance.

Temperature-Dependent Mechanical Properties of CoCrFeNi Medium-Entropy Alloy Produced by Laser-Directed Energy Deposition

The temperature dependence of the mechanical properties of the CoCrFeNi medium-entropy alloy (MEA) manufactured by laser-directed energy deposition (L-DED) and additionally annealed at 1200 °C for 24 h was studied. The microstructure of the as-deposited alloy was represented by a single-phase face-centered cubic structure with coarse columnar grains and a high density of dislocation. Annealing resulted in the development of recrystallization and a reduction in dislocation density. The CoCrFeNi alloy produced by L-DED demonstrated mechanical properties comparable with those of the fine-grained equiatomic CoCrFeMnNi alloy, produced by casting followed by thermomechanical processing. Namely, as-deposited CoCrFeNi had a yield strength (YS) and ultimate tensile strength (UTS) of YS = 370 MPa and UTS = 610 MPa at room temperature, and YS = 565 MPa and UTS = 965 MPa at cryogenic temperature, along with a ductility of ~60%. Annealing resulted in a decrease in strength to YS = 180/350 MPa at 293/77 K. A quantitative analysis of various strengthening mechanisms showed that some strength increment of the as-deposited alloy was ensured by the high dislocation density formed during L-DED.

Microstructural, Mechanical, and Tribological Properties of Selective Laser Melted Inconel 718 Alloy: The Influences of Heat Treatment

The present study investigates the microstructural, mechanical, and tribological properties of Inconel 718 alloy produced by selective laser melting (SLM) in relation to heat treatment. The SLM-processed samples received a two-step heat treatment: solutionizing at 1065 °C for 1 h, followed by double aging at 720 °C for 8 h and 620 °C for 6 h. The as-built sample exhibited a grain structure mostly characterized by fine Laves phases, while the hardening phases γ′ ((Ni3 (Al, Ti)) and γ″ (Ni3Nb) precipitated during the heat treatment. Following heat treatment, a transformation in crystallographic texture and dislocation density occurred, yielding a random texture and reduced dislocation density, particularly in the XZ direction, attributed to the formation of new grains via recrystallization in the microstructure. The grain size in the XY plane decreased following heat treatment, whereas the texture in the <001> direction remained unaffected. The heat-treated samples had significantly higher tensile strength (1330 MPa vs. 960 MPa) and hardness (530 HV vs. 340 HV) relative to the as-built samples. The wear resistance of heat-treated samples surpassed that of the as-built sample due to enhanced mechanical properties resulting from the fine and dispersed γ′ and γ″ precipitates in the microstructure with heat treatment.

Ultra‐Thin Cubic Ti3Al Buffer/Template Layer Achieving Giant Polarization of Epitaxial Pb(Zr0.40Ti0.60)O3 Film

AbstractInterface strain significantly affects polarization in ferroelectric devices. In this paper, an innovative strategy is proposed to substantially enhance ferroelectric polarization using an ultra‐thin “soft‐buffer‐layer” (SBL), which reduces dislocation density and increases the tetragonality of ferroelectric thin films because of small elastic constants of Ti3Al along x and y axes. And in this work, it is demonstrated that PbZr0.4Ti0.6O3 (PZT) thin films are perfect c‐axis‐oriented epitaxial structures on (001) SrTiO3 (STO) substrates. The PZT capacitor with Ti3Al buffer layer exhibits a remarkable remnant polarization, reaching up to 131.93 µC cm−2 at an applied voltage of 5.00 V. Surprisingly, under the clamping effect of SrRuO3 and STO, Ti3Al films exhibit cubic structure and facilitate matching to the PZT film. It is proposed that the introduction of the Ti3Al buffer layer notably improves the tetragonality (c/a) ratio from 1.002 to 1.024, and significantly enhancing the polarization. There is no doubt that this adjustment reduces dislocation density and decreases stress influence from STO substrate. Given these advantages, the SBL method presents a compelling option for epitaxial PZT films, and also for enhancing the physical properties of other functional thin film materials.

GaAs Solar Cells Grown Directly on V-Groove Si Substrates.

The direct epitaxial growth of high-quality III-V semiconductors on Si is a challenging materials science problem with a number of applications in optoelectronic devices, such as solar cells and on-chip lasers. We report the reduction of dislocation density in GaAs solar cells grown directly on nanopatterned V-groove Si substrates by metal-organic vapor-phase epitaxy. Starting from a template of GaP on V-groove Si, we achieved a low threading dislocation density (TDD) of 3 × 106 cm-2 in the GaAs by performing thermal cycle annealing of the GaAs followed by growth of InGaAs dislocation filter layers. This approach eliminates the need for a metamorphic buffer to directly integrate low-TDD GaAs on Si. We used these low-TDD GaAs/V-groove Si templates to grow GaAs double heterostructures that had a minority carrier lifetime of 5.7 ns, as measured by time-resolved photoluminescence, a value consistent with the material quality associated with a 20%+ efficient GaAs solar cell. However, front-junction GaAs solar cells grown on these low-TDD substrates produced a conversion efficiency of only 6.6% without an antireflection coating. Electron channeling contrast imaging measurements on this cell showed a high density of misfit dislocations at the interface between the AlInP/GaInP window layer and the GaAs absorber and between the GaAs absorber and the GaInP back surface field (BSF), likely causing a high surface recombination velocity and thus poor performance. We showed that we could reduce (and in the case of the BSF, eliminate) these dislocations by employing an AlGaAs-based window layer and BSF. Compared to GaInP, AlGaAs has dislocation glide properties that are more similar to those of GaAs, resulting in more even threading dislocation glide between layers. AlGaAs passivation improved the external quantum efficiency and open-circuit voltage of the devices, but the overall device performance was still low at an efficiency of 7.7% without an antireflection coating, likely due to cracking in the devices. This work demonstrates a route to high material quality in GaAs grown directly on Si that can be used for the production of III-V/Si optoelectronic devices.

Assessing the combined effects of chemical and mechanical parameters on silar-grown nanostructured ZnO thin films

In the literature, a comprehensive assessment of the combined impacts of chemical and mechanical parameters on the properties of thin films grown by SILAR is missing. In this work, ZnO film formation is investigated under variable precursor concentration, pH, withdrawal speed and number of cycles. Interestingly, the produced ZnO films displayed remarkable aspect ratio and morphological variability, ranging from the commonly obtained nanograins shape towards hexagonal nanorods, flower- like rods and nanoneedles, which to our knowledge have not yet been achieved by using single step SILAR process. More particularly, low concentration and intermediate pH and withdrawal rates were favorable for nanorods formation. In addition, increasing the withdrawal speed from 26 to 30 cm·min-1 resulted in a thinner film with improved rod uniformity and reduced crystallite size. This is the first study on the impact of substrate withdrawal speed on SILAR films. Among all studied parameters, the number of cycles was particularly useful for tuning film thickness, while preserving its target shape. In addition, the films grown under a higher number of cycles showed improved film crystallinity and rod orientation with reduced dislocation density, microstrain and bandgap energy. In our conditions, the most suitable combination of parameters required for exhibiting optimized nanorod-shaped coating are: a concentration of 0.07 M, pH of 10.5, speed of 30 cm·min-1 and 40 cycles. In this case, XRD, XPS, Raman and FTIR spectra displayed typical features of hexagonal Wurtzite structure of ZnO with no impurities within the film surface, whereas AFM measured a thickness of 1.4 μm with 243 nm surface roughness.

Effect of Plate Thickness on the Laser-Welded Microstructure and Mechanical Properties of 22MnB5 Hot-Forming Steel.

To meet the development and application needs of advanced high-strength steel, the laser welding of 22MnB5 hot-forming steel plates with thicknesses of 2 mm, 3 mm, and 4 mm was studied in this paper. Mechanical testing revealed that as plate thickness increased, the tensile strength of welded joints decreased from 1489 MPa to 1357 MPa and 1275 MPa, equating to 96%, 91%, and 88% of the corresponding base metal strength, respectively. The heat-affected zone exhibited the lowest mechanical properties. Microstructural characterization showed that with increasing plate thickness, martensite grains in the welded joints grew larger, transitioning from fine acicular to larger island structures. Concurrently, dislocation density in the welded joints decreased gradually. Furthermore, microstructural changes in the heat-affected zone were more pronounced than those in the fusion zone. The larger grain size and reduced dislocation density softened the joint structure, which consequently decreased the strength and hardness of the welded joint. Laser-welded joints of three thicknesses can exceed 85% of the corresponding base metal strength, demonstrating strong industrial application potential.

Influence of full-cycle heat treatment on microstructure and mechanical properties of CLF-1 steel

Based on the manufacturing requirements of tritium breeding blankets for fusion reactors, it is valuable to study the microstructure evolution and mechanical properties of CLF-1 steel, a candidate structural material, during the full-cycle heat treatment from profile to component. The full-cycle heat treatment includes isothermal annealing and multiple quenching and tempering treatments. Results indicate that multiple quenching and tempering treatments refine the size of prior austenite grains and reduce dislocation density and the width of lath martensite. With the increase in the number of quenching and tempering treatments, the CLF-1 steels exhibit a slight decrease in tensile strength while the impact toughness noticeably improves. Research has found that the effect of isothermal annealing treatment includes an increase in the size of the prior austenite grains and precipitates, as well as a change in the matrix from martensite to ferrite. As a result, the tensile strength and impact energy decrease sharply, while the ductility improves after isothermal annealing treatment. Subsequent quenching and tempering treatment can recover the microstructure and mechanical properties.

Effects of Short-Term Annealing on the Thermal Stability and Microstructural Evolution of Oxygen-Free Copper Processed by High-Pressure Torsion.

This study explores the impact of short-term annealing on the thermal stability and mechanical properties of oxygen-free copper subjected to high-pressure torsion (HPT). Copper samples were deformed through HPT with varying numbers of turns at room temperature and subsequently subjected to short-term annealing at temperatures of 398 K and 423 K. Microstructural analysis revealed that annealing led to grain growth and a reduction in dislocation density, with samples processed with fewer HPT turns exhibiting more significant grain coarsening. The microhardness measurements indicated a reduction in hardness after annealing, particularly at the edges of the discs, suggesting recrystallization. Samples processed with 10 HPT turns demonstrated higher thermal stability and less grain growth compared to 1/2-turn samples. The findings suggest that post-HPT short-term annealing can be used to tailor the balance between strength and ductility in oxygen-free copper, enhancing its suitability for industrial applications.

The Effect of Cyclic Heat Treatment on the Microstructure and Mechanical Properties of 18CrNiMo7-6 Gear Steel.

To avoid grain coarsening resulting from high-temperature carburizing, the effects of cyclic quenching and tempering on the microstructure and mechanical properties of 18CrNiMo7-6 gear steel were investigated. Three groups of samples were compared, which went through 0/1/3 times of quenching-tempering cycles after initial pseudo-carburizing. The variations in grain size, hardness, tensile strength, and toughness were systematically assessed using a series of experimental techniques. The experimental results indicate that the austenite grain size decreases from 14.8 μm to 5.0 μm as the number of cycles increases, accompanied by improved grain uniformity, which is beneficial to fine-grain strengthening mechanisms. During the phase transition, defects in the original martensite structure are transferred to the newly formed austenite, with the energy stored during the martensitic-to-austenitic transformation driving the grain refinement process. However, after several cycles of quenching and tempering, the release of some residual stresses and dislocations reduces the driving force for recrystallization, limiting further grain refinement. Although the strength decreased slightly after three cycles due to a reduction in dislocation density, toughness increased to a maximum of 172 J/cm2, primarily due to the enhancement of grain refinement and grain boundary density, which effectively hindered crack propagation. This study confirms the efficacy of cyclic heat treatment in refining grain structure and improving both strength and toughness, thereby contributing valuable insights to the research and development of high-performance gear steels.

Threading Dislocation Density Reduction in SiGe Strain Relaxed Buffer Layers through a Preexisting Dislocation Reservoir

Silicon germanium strain relaxed buffer layers (SRB) enable the integration of strained Si or strained Ge layers that can be used for improving CMOS devices with enhanced carrier mobilities, optoelectronics or emerging applications like qubit devices. Due to the lattice mismatch between Si and Ge, the formation of misfit dislocations during strain relaxation is inevitable. Misfit dislocations are accompanied by threading dislocations that penetrate the surface and possibly affect the device performance. A reduction of threading dislocation densities (TDD) with for example graded buffer layers [1] therefore has been subject of extensive research in the past and remains an active field today.In an earlier work we showed, that by using a backside deposition process [2],[3] - initially intended for controlling the wafer bowing - preexisting dislocations that are introduced at the edge of the wafer can fundamentally change the relaxation process of SiGe buffer layers.[4] Instead of rapid nucleation and multiplication of dislocations at single sites that lead to the formation of misfit dislocation bundles and pile-ups through blocking, the dislocations in the reservoir at the wafer edge can start gliding immediately upon reaching the Matthews and Blakeslee critical thickness [5] and form an evenly distributed dislocation network without pile-ups (Fig. 1a). With this method, the threading dislocation density of low strain layers (2% Ge) can be reduced by one order of magnitude.In this work, we analyze the influence of the backside approach on the crystal quality of strain relaxed buffer layers with higher Ge concentrations more relevant for industrial applications. SiGe buffer layers were grown using a high temperature (>820 °C), atmospheric CVD process using SiH2Cl2 and GeCl4 as precursor gases on commercial polished 300 mm Si wafers and substrates that received a backside deposition prior to the front side buffer growth. For strain relaxed buffers with constant composition the largest relative TDD reduction is observed at low Ge concentrations (one order of magnitude at 2% Ge). The TDD reduction becomes more and more ineffective towards higher Ge concentrations (Fig. 1b) which is attributed to increased spontaneous nucleation of misfit dislocation half-loops. At 10% Ge there is no significant improvement observed. However, when moving from constant composition buffers to graded buffers to 25% or 70% Ge, the TDD is still reduced by approximately 1 × 105 cm-2 to 1.5 × 105 cm-2 and 5 × 105 cm-2, respectively (Fig. 1c). This is attributed to the removal of misfit dislocation bundles and pile-ups, that would block gliding dislocations from efficiently relaxing strain by trapping them and therefore increasing the need for nucleation of additional loops that increase the TDD. The absence of dislocation pile-ups also improves the surface quality (visual haze) of the SRB (Fig. 1d,e) as inhomogeneities in the dislocation network caused by misfit dislocation bundles that would lead to deep troughs in the crosshatch pattern are reduced. This is also reflected in the FWHM from rocking curves measured by HRXRD which is reduced by 60% for SRB with high Ge concentrations indicating a vastly improved crystal quality.In conclusion, this work highlights, that the drastically changed relaxation mechanism in the early stage of buffer growth by introducing a dislocation reservoir at the edge of the wafer has lasting consequences for the crystal quality of SRB even at high Ge concentrations. The prevention of dislocation bundle formation leads to a reduction of pile-up density, TDD, surface roughness and an overall improved crystal quality. The presented approach opens up the opportunity for a further reduction of the TDD that is necessary for future industrial applications of SiGe SRB.[1] E. A. Fitzgerald, Y.-H. Xie, M. L. Green, D. Brasen, A. R. Kortan, J. Michel, Y.-J. Mii, and B. E. Weir, Appl. Phys. Lett. 59, 811 (1991).[2] P. Storck and M. Vorderwestner, US2009236696, (2009).[3] G. Kozlowski, O. Fursenko, P. Zaumseil, T. Schroeder, M. Vorderwestner, and P. Storck, ECS Transactions 50, 613 (2013).[4] L. Becker, P. Storck, T. Schulz, M. H. Zoellner, L. Di Gaspare, F. Rovaris, A. Marzegalli, F. Montalenti, M. de Seta, G. Capellini, G. Schwalb, T. Schroeder, and M. Albrecht, J. Appl. Phys. 128, 215305 (2020).[5] J. W. Matthews and A. E. Blakeslee, Journal of Crystal Growth 29, 273 (1975). Figure 1

Temperature-driven evolution of hematite (α-Fe2O3) nanoparticles: A study on structural, morphological and magnetic properties

This report explores the impact of annealing temperature on the structural and magnetic properties of hematite (α-Fe2O3) nanoparticles synthesized via the sol-gel auto-combustion method. The samples were annealed at 500 °C, 700 °C, and 900 °C, and characterized using XRD, FE-SEM, FT-IR, and VSM techniques. XRD analysis revealed an increase in crystallite size from 28.35 nm to 47.65 nm with temperature, accompanied by changes in lattice parameters, cell volume, and a reduction in dislocation density. Grain size distribution, observed via FE-SEM, showed a growth trend corresponding to the annealing temperatures, with sizes ranging from 114.8 nm to 167.8 nm. Energy dispersive X-ray spectroscopy (EDAX) verifies the elemental composition. FT-IR analysis revealed characteristic Fe-O vibrational bands between 430 and 525 cm⁻1, corresponding to Fe-O deformation in octahedral and tetrahedral sites. Subtle shifts in these bands with increasing temperature reflect structural changes in the nanoparticles. Magnetic characterization exhibited a decrease in saturation magnetization (MS) from 2.96 emu/g at 500 °C to 0.74 emu/g at 900 °C, while coercivity (HC) increased from 643.2 Oe to 1161.1 Oe, reflecting the influence of temperature on the magnetic domain structure. The observed relationship between the structural evolution and magnetic behavior underscores the potential of annealing to optimize α-Fe₂O₃ nanoparticles for use in magnetic storage, environmental remediation and multifunctional biomedical applications.

Exploring popped-out phenomena in large-scale silicon ingot growth

Silicon wafers are essential for the semiconductor industry, providing the foundation for most integrated circuits. As demand for microelectronics grows, larger silicon wafers, have become crucial for increasing chip production and reducing manufacturing costs. However, the crystal separation phase during Czochralski (CZ) ingot growth is particularly challenging for larger ingots, often resulting in defects due to premature detachment (“popped-out” tails). This study investigates the popping-out stage of 18-inch ingots under varying heater power and ingot pull-out speed conditions. Photoluminescence (PL) imaging and a convolutional neural network (CNN) were used to analyze dislocation density in the silicon wafers. Results show that increasing the pull-out speed after detachment can significantly increase dislocation density, while pausing the ingot near the melt surface minimizes dislocations. Additionally, increasing heater power after detachment reduces dislocation density. The optimal condition for minimizing dislocation was found when heater power was doubled, and the ingot was paused near the melt surface for 30 min.

Microstructure and property alterations and toughening mechanism of cast TC4 alloy by solid state treatment with low-density DC electric field

In this paper, the impact of low-density DC electric field treatment (LDCT) on the microstructure and mechanical properties of cast TC4 titanium alloy (CTC4) was investigated. The findings indicate that the driving force for the current provided by LDCT enhances the texture of CTC4. When the current density is sufficiently high, the induced recrystallization effect weakens the texture, leading to an isotropic state. The polygonization of grain boundaries in the LDCT state CTC4 has been identified, along with the emergence of a secondary α phase due to recrystallization. Additionally, this study highlights the phenomenon of preferential nucleation and growth occurring at the grain boundaries. LDCT induces a grain refinement effect in the CTC4 material, whereby the average grain size of the LDCT3 sample decreases from 31.05 μm in the untreated sample to 28.93 μm. The observed reduction in dislocation density in the LDCT states is believed to result from the diffusion of impurity atoms and vacancies. The observation of dislocation plugging at the inner grain boundary of the LDCT3 sample suggests that the electron wind promotes dislocation slip toward the grain boundary. Furthermore, the simultaneous enhancement of strength and toughness in the LDCT state of the CTC4 alloy has been achieved. The LDCT3 sample demonstrates superior performance, exhibiting noteworthy increases of 7.7%, 22.0%, and 10.9% in tensile strength, elongation, and fracture energy, respectively, compared to the untreated sample. The significant role of non-thermal effects in LDCT and the mechanism of atomic diffusion induced by this effect are discussed. The origins of dislocation motion and phase transition are elucidated from the perspective of atomic diffusion.

A novel coalesced quantum dot buffer approach to mitigate large lattice mismatch in III–V epitaxy

III–V compound semiconductors, such as indium arsenide (InAs), are crucial in the fields of electronics and optoelectronics due to their unique properties, including a narrow bandgap, high electron mobility, low effective electron mass, and outstanding optoelectronic performance. However, the lattice constant mismatch of 7.2% between InAs and gallium arsenide (GaAs) leads to a high dislocation density in the InAs epilayer, typically around 109 cm−2 or higher when InAs is directly grown on GaAs (001). A common traditional approach to reduce material defect density involves using a compositionally graded buffer structure, which requires a thickness of several micrometers. This paper presents an innovative method for growing device-quality InAs epilayers on GaAs (001) substrates through an InAs coalesced quantum dot buffer (CQDB) layer. Remarkably, the InAs CQDB method requires a thickness of only several tens of nanometers, leading to significant improvements in film quality. Utilizing the InAs CQDB layer allows for a substantial reduction in dislocation density from 1.8 × 109 to 3.6 × 107 cm−2. The findings of this study provide a crucial research foundation for addressing lattice mismatch challenges in epitaxial growth, particularly for those investigating methods to integrate different materials in advanced device applications.

The Annealing Temperature Effect Enhances the Structural and Optical Parameters of Bi2Te3 Thermally Evaporated Thin Films

The current work investigates the influence of the annealing temperature (373, 473 K) on the structural and optical properties of Bi2Te3 thin films produced by the thermal evaporation technique. The crystal structure of samples is examined using an X-ray diffraction, indicating that Bi2Te3 thin films have a Rhombohedral structure with space group R-3m. The crystallite size (D) increased from 10.85 to 14.46 nm by increasing the annealing temperature. Additionally, increasing the annealing temperature reduces the micro-strain and the dislocation density from (3.33 × 10−3 to 2.5 × 10−3) and (8.49 × 1015 to 4.78 × 1015) Lines / m 2 , respectively. These micro-strain and dislocation density reductions refer to the crystallinity enhancement of sample Bi2Te3 thin films. The transmittance and reflectance spectra were measured using an Fourier transform infrared spectrometer. All samples reveal direct and indirect transition types and both direct and indirect optical energy gap decreased with increased annealing temperature from (0.325 to 0.29 eV) and (0.24 to 0.274 eV), respectively. From transmittance and reflectance data, the refractive index is calculated. These results reveal the refractive index decreasing as temperature annealing increased from (2.45–1.79) at 3250 nm. Other optical parameters such as absorption index, dielectric constant, energy loss function, and optical conductivity were estimated.

Improving the mechanical properties and corrosion resistance of a highly alloyed Al-Zn-Mg-Cu alloy through Pre-Treatment at 350°C prior to multi-stage solid solution treatment

This study proposed a new solution treatment process for optimizing the microstructure and properties of Al-Zn-Mg-Cu alloy, by introducing a pre-treatment process that can achieve sufficient recovery and trace recrystallization of the alloy, namely, 350 ° C pre-treatment (PT-350 ° C), prior to traditional solution treatment. Research has found that during the PT-350°C/12h process, a large amount of nanoscale η(Mg(Zn,Cu)2) phases in the extruded state dissolved, diffused, re-precipitated at the larger η(Mg(Zn,Cu)2) phases, and form a large particles, with a size of about 0.74 μm, mainly distributed at the grain boundaries of the matrix. In the subsequent multi-stage solid solution treatment (MST) process, while fully dissolving, they also had a certain inhibitory effect on the growth of matrix grains. More importantly, the alloy underwent sufficient recovery and slight recrystallization during the PT-350°C/12h, significantly reducing dislocation density, suppressing the degree of recrystallization, and refining the alloy grains of the alloy during MST process. The intragranular aging precipitate changed from the η '+ η phase of MST to the GP zone + η' phase of PT-350 °C/12h+MST, with an increase in content and a decrease in size. The distribution of grain boundary precipitates (GBPs) transformed from continuous to discontinuous. The yield strength, ultimate tensile strength, and elongation increased from 740 MPa, 755 MPa, and 2.9% to 778 MPa, 792 MPa, and 5.8%, respectively. The corrosion resistance of the alloy also had significantly improved, in 3.5 wt.%NaCl solution, the corrosion current density decreased from 4.02 × 10-6 A·cm-2 to 8.33 × 10-7 A·cm-2.

Investigation of the Softening Resistance Behavior and Its Mechanism in Cu-Ni-Si Alloys with Discontinuous Precipitates.

In this study, isothermal annealing experiments were conducted on copper-nickel-silicon alloys containing continuous precipitates (CP) and discontinuous precipitates (DP) to investigate the effects of different types of precipitate phases on the microstructural evolution and softening temperature during annealing, as well as to analyze the differences in softening mechanisms. The experimental results revealed that the softening temperature of the CP alloy, subjected to 75% cold deformation, was 505 °C. In contrast, the DP alloy achieved softening temperatures of 575 °C and 515 °C after 75% and 97.5% cold deformation, respectively. This indicates that the DP alloy exhibits significantly superior softening resistance compared to the CP alloy, attributed to the distinct softening mechanisms of the two alloys. In the CP alloy, softening is primarily influenced by factors such as the coarsening of the precipitate phase, the occurrence of recrystallization, and the reduction in dislocation density. In the DP alloy, the balling phenomenon of the DP phase is more pronounced, and its unique microstructure exerts a stronger hindrance to dislocation and grain boundary motion. This hindrance effect reduces the extent of recrystallization and results in a smaller decrease in dislocation density. In summary, the DP alloy, due to its unique microstructure and softening mechanisms, demonstrates better softening resistance, providing higher durability and stability for high-temperature applications.

Study of the effect of SMAW welding speed on the microstructure and crystal stability of X70 steel using X-ray diffraction

This study investigates the impact of manual electric arc welding (SMAW) speed on the microstructure of X70 steel. Welding was performed at two distinct speeds: S1 = 20 mm/min and S2 = 35 mm/min, at the COSIDER BISKRA workshop in Algeria. X-ray diffraction (XRD) analysis was conducted on the welded zones to examine crystal orientation, size, dislocation density, and structural stability across different welding areas, including the base metal (MB), heat-affected zones (HAZ1 and HAZ2), and weld zones (WZ1 and WZ2). The XRD results revealed that key crystal peaks, such as {110} and {200}, retained their positions at both welding speeds, though variations in {110} peak intensity were noted. The lower welding speed (S1) resulted in a higher heat input, which enhanced crystal stability, increased crystal size, and reduced dislocation density. On the other hand, the higher speed (S2) produced lower heat input, leading to smaller crystal sizes, increased residual stresses, and a greater number of defects due to rapid cooling. These findings underscore the critical influence of welding speed on heat input, cooling rates, and their consequent effects on microstructure. The study highlights the importance of optimizing welding parameters to balance microstructural integrity, mechanical performance, and production efficiency.

Effect of Tempering Temperature on the Aqueous Corrosion Resistance of 9Cr Series Heat-Resistant Steel

In this investigation, the aqueous corrosion resistance of 9Cr series heat-resistant steel during tempering was investigated. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectrometer (EDS) were used to analyze the effect of tempering temperature on the microstructure and precipitation behavior of precipitates. The heat-resisting steel was heated to 1150 °C for 1 h, and then tempered at different temperatures between 680 °C and 760 °C for 2 h. The microstructure of the heat-resistant steel after tempering was composed of lath-tempered martensite and fine precipitates. The hardness decreased with increasing tempering temperature, ranging from HBW 261 to HBW 193. The aqueous corrosion resistance improved as the tempering temperatures increased from 680 °C to 720 °C but deteriorated at higher temperatures, such as 760 °C, which was obtained by an electrochemical corrosion performance test. The aqueous corrosion resistance was affected by the decrease in dislocation density and the decrease in Cr solution in the tempered martensite. With the increase in the tempering temperature, the aqueous corrosion potential first increases and then decreases, the self-corrosion current density first decreases and then increases, and the polarization resistance first increases and then decreases. Furthermore, the increase in corrosion resistance is attributed to the reduction in dislocation density and chromium depletion in the martensitic structure as the tempering temperature approaches 720 °C. This paper reveals the effect of tempering temperature on the corrosion resistance of 9Cr series heat-resistant steel, which is a further exploration of a known phenomenon.