Increase In Dislocation Density Research Articles

Thermoelastic properties of non-metamict zircon

Abstract The thermoelastic properties and thermal expansion of natural single-crystal zircon without detectable radiation damage were determined in the temperature range between 100 and 1650 K on five samples from Cambodia, Sri Lanka, and Tanzania. Up to about 1500 K, the results are in excellent agreement and fully reversible. The average adiabatic elastic stiffness coefficients at room temperature in GPa are c11 = 426.8(6), c12 = 68.1(4), c13 = 152.0(4), c33 = 490.8(9), c44 = 113.1(2), and c66 = 49.0(1). Polycrystalline averaging for the aggregate bulk and shear modulus (Voigt-Reuss-Hill average) yields K = 229.3(4) GPa and G = 109.3(1) GPa. From 293 to 1573 K, the elastic stiffnesses soften almost linearly with the temperature coefficients in MPa/K: dc11/dT = –45.36(7), dc12/dT = –2.03(7), dc13/dT = –9.59(7), dc33/dT = –42.85(12), dc44/dT = –9.60(3), and dc66/dT = –2.19(1). Below room temperature, the behavior of the cij is nonlinear, with decreasing absolute values of the temperature coefficients. Above about 1500 K, a temperature- and time-dependent irreversible elastic stiffening accompanied by an increasing ultrasound dissipation was observed in all samples. These anomalies are interpreted to be the consequence of increased dislocation densities.

Boron nitride nanosheets as precursors for 3D nano-configurations in titanium matrix composites: Interfacial evolution and mechanical behavior

Boron nitride nanosheets (BNNSs) have tremendous potential as reinforcements for titanium (Ti) due to their excellent strengthening and toughening efficiency from in-situ generated 3D nano-configurations. However, the effect of interfacial evolution on the mechanical properties of the BNNSs/Ti composite remains unexplored. To this end, in this study, the interfacial structure was regulated utilizing the time-dependent reaction of BNNSs/Ti system. Its effects on mechanical behavior were studied by combining experiments and molecular dynamic simulations. BNNSs were gradually converted to 3D nano-configurations over time and finally consumed to form coarsened TiB whiskers in the TC4 matrix. The composite with 3D nano-configurations exhibited a tensile strength of 1140.5 MPa and excellent toughness of 42.9 MJ/m3. Strength improvement stems primarily from nano-interlocking, which increases dislocation density under load and improves the interfacial shear strength. The considerable toughness results from the enhanced crack resistance brought by the 3D nano-configurations.

Strain-rate-dependent plasticity of Ta-Cu nanocomposites for therapeutic implants

Recently, Ta/Cu nanocomposites have been widely used in therapeutic medical devices due to their excellent bioactivity and biocompatibility, antimicrobial property, and outstanding corrosion and wear resistance. Since mechanical yielding and any other deformation in the patient's body during treatment are unacceptable in medicine, the characterization of the mechanical behavior of these nanomaterials is of great importance. We focus on the microstructural evolution of Ta/Cu nanocomposite samples under uniaxial tensile loading conditions at different strain rates using a series of molecular dynamics simulations and compare to the reference case of pure Ta. The results show that the increase in dislocation density at lower strain rates leads to the significant weakening of the mechanical properties. The strain rate-dependent plastic deformation mechanism of the samples can be divided into three main categories: phase transitions at the extreme strain rates, dislocation slip/twinning at lower strain rates for coarse-grained samples, and grain-boundary based activities for the finer-grained samples. Finally, we demonstrate that the load transfer from the Ta matrix to the Cu nanoparticles via the interfacial region can significantly affect the plastic deformation of the matrix in all nanocomposite samples. These results will prove useful for the design of therapeutic implants based on Ta/Cu nanocomposites.

Simultaneous strength and ductility enhancement of wire-arc directed energy deposited Al–Cu alloy by interlayer laser shock peening

Wire-arc directed energy deposition (wire-arc DED), recognized for its ability to produce large-scale parts, has gained considerable attention. However, a critical issue with this method is the high prevalence of internal porosity defects found in the manufactured aluminum components, adversely impacting their mechanical properties. For the first time, this study introduces in-situ interlayer Laser Shock Peening (LSP) during wire-arc DED of 2319 aluminum alloy. The thickness of each deposited layer was meticulously regulated within the 0.7–1.3 mm range utilizing a spiral-path oscillation mode. Following this process, LSP was applied to the top surface of each layer. Compared to the as-deposited samples, interlayer LSP-treated samples showed a significant decrease in pore numbers by 73.9% and a reduction in the total area by 87.4%.Furthermore, the LSP-treated samples displayed improved mechanical properties with increases in ultimate tensile strength, yield strength, and elongation by 20.1%, 19.1%, and 27.3%, respectively. The primary impact of LSP on the microstructure is the generation of high-density dislocations, providing a driving force for grain refinement during subsequent layer heat input. With the combined effects of heat input and dislocation density, samples treated with LSP form a tight metallurgical bound around the closed pores. This process of effective defect elimination and an increased dislocation density between the layers results in a simultaneous improvement in strength and plasticity.

Influence of scanning speed on the microstructure, nanoindentation characteristics and tribological behavior of novel maraging steel coatings by laser cladding

This paper utilized a novel high-Mo maraging steel powder to prepare maraging steel coatings through laser cladding. The study investigated how scanning speed affects the microstructure, phase formation, nanoindentation properties, and wear properties of the resulting laser-clad maraging steel coatings. The experimental results reveal that the maraging steel coating consists of martensite and austenite phases. Moreover, diffraction peaks of Fe2Mo phase were observed at low scanning speeds. As the scanning speed increases, there is a gradual reduction in the austenite content, accompanied by a decrease in the Fe2Mo phase content. Moreover, both martensite and austenite exhibit an increase in lattice distortion. EBSD analysis demonstrates a reduce in grain size as the scanning speed increases, accompanied by the emergence of LAGBs and an increase in dislocation density, as indicated by the rising KAM value. TEM analysis confirms the presence of a Mo-rich precipitated phase, characterized by a dense hexagonal structure consisting of the Fe2Mo intermetallic compound. The Fe2Mo phase displays high hardness, thereby significantly enhancing the second precipitation strengthening effect of the coating. The nanoindentation characteristics of the S2 coating are exceptional, displaying remarkable values for nanohardness, HU, H/E ratio, and H3/E2 ratio. Specifically, the S2 coating exhibits a nanohardness of 4.843 ± 0.104 GPa, accompanied by an HU value of 0.2073 GPa. Additionally, the H/E ratio and H3/E2 ratio are reported as 0.0214 and 0.002218, respectively. Demonstrates excellent mechanical properties of S2 coating. Furthermore, the S2 coating demonstrates outstanding performance in terms of wear resistance and deformation resistance, exhibiting the lowest friction coefficient and specific wear rate, measuring at 0.438 and 1.381 × 10−5 mm3/N•m, respectively. The observed wear mechanisms in the S2 coating include adhesive wear, abrasive wear and oxidative wear. Notably, lower scanning speeds promote the precipitation of Fe2Mo particles, which play a supportive role in the wear process by weakening the tendency of oxidative wear. Overall, the remarkable properties exhibited by the S2 coating, such as nanohardness, wear resistance, and deformation resistance, position it as a promising candidate for various technological applications.

Synergistic effect of Gamma-Irradiation on morphological, structural, and optical properties of Carbon-coated Nickel Cobalt Oxide/rGO nanocomposites

Radiation exposure induces defects in the crystal structure, leading to alterations in the structural, optical, and morphological properties of metal oxides. These irradiated metal oxides have found extensive use in various household applications. In this particular investigation, carbon-coated nickel cobalt oxide/rGO nanocomposites were synthesized through ultrasonication. Subsequently, these nanocomposites were subjected to three different doses of gamma radiation using a 60Co source. Both un-irradiated and irradiated samples were subjected to characterization using FTIR, SEM, XRD, and UV-Vis spectroscopy. The FTIR analysis confirmed the presence of a spinel-type structure in NiCo2O4. SEM analysis revealed the presence of crumbled sheets of rGO, and the irradiation process caused a transformation of these stacked sheets into thinner sheets. XRD analysis indicated a decrease in crystalline size and an increase in dislocation density with higher doses of gamma irradiation. UV-Vis spectroscopy demonstrated a reduction in both direct and indirect optical band gaps as a result of gamma irradiation. Furthermore, it was observed that Urbach energy and refractive index increased with gamma irradiation. Based on these findings, it can be concluded that gamma irradiation has a significant impact on the morphological, structural, and optical properties of NiCo2O4/rGO@C nanocomposites.

Strengthening effect in laser metal deposited Ti6Al4V alloy via layer-by-layer ultrasonic impact treatment

The present work investigates the influence of incorporation of the layer-by-layer ultrasonic impact treatment (UIT) into the laser metal deposition (LMD) processing of Ti6Al4V alloy. The microstructure and mechanical properties of the samples with/without the assistance of UIT are comparatively studied. The curved shape boundaries of α/α′ phase with inconsistent misorientation angles and the ultra-fine α phase are observed in the UIT-assisted LMD (ULMD) sample. The α/α′ phase in the ULMD sample is characterized by high-density dislocations, larger lattice strain and distortion in comparison with that in the LMD sample. An ultimate tensile strength of 1386 MPa is obtained in the ULMD sample that exceeds that of the LMD sample by 373 MPa and is superior to the values reported in majority of the previous works. Crystallographic orientation change, increased dislocation density and α/α′ phase lattice distortion, and formation of ultra-fine α grains are found to influence the properties. The present work provides an in-depth understanding of the mechanisms/factors influencing the mechanical strengths of additive-manufactured titanium alloys.

Mechanical property analysis and tribological response optimization of SiC and MoS2 reinforced hybrid aluminum functionally graded composite through Taguchi's DOE

Pistons and cylinder liners in automotive engines often experience wear, requiring mechanical and tribological characteristics that are not attainable with conventional metals and alloys. This study explores the viability of an Al11Si2CuFe functionally graded composite, produced through centrifugal casting, reinforced with 5 wt% SiC and 3 wt% MoS2 for these applications. Microstructural and tribo-mechanical characteristics were investigated, and wear properties were statistically analyzed using Taguchi's method, considering parameters such as sliding distance (750–1750 m), load (10−30N), and sliding velocity (1–3 m/s). Microstructural analysis revealed a gradient reinforcing particle distribution and particle clusters, alongside θ-Al2Cu phase formation confirmed through transmission electron microscopy. Mechanical property examination of the outer layers indicated an improvement of 17.3 and 27.34 % in tensile strength and micro-hardness, respectively, over the inner layers owing to the centrifugal force, increased dislocation density, and effective bridging effect between the reinforcing particles and matrix. Wear studies identified applied load as the influential parameter, with optimal wear occurring at 10 N, 750 m, and 2 m/s, as confirmed through analysis of variance studies. Worn surface analyses revealed abrasion, fatigue, and tribo-chemical wear regimes, characterized by the formation of wear tracks, particle pull-outs, delamination layers, mechanically mixed and oxide layers.

Dislocation density-mediated creep ageing behavior of an Al-Cu-Li alloy

Creep aging is well-known to be a time-dependent, coupled process of deformation and precipitation strengthening for age-hardening alloys. Its existing mechanisms are mainly attributed to those interactions between atomic diffusion and dislocation motion. However, an understanding of the relationship between dislocation density and a special multistage creep behavior, i.e., double steady creep feature, is still far limited. Here we investigate the effect of various dislocation density levels on such an abnormal multistage creep of an Al-Cu-Li alloy. We find that the increased dislocation densities enable an apparent time decrease (from 6.2 h to 0.8 h) of their first steady Ⅱ-stage. The yield strength of post-aged samples increases from 425.0 MPa to 580.0 MPa while the corresponding elongation decreases from 12.3% to 7.3% for the creep-aged samples #1 to #4. Microstructural results also reveal that a great difference in dislocation configuration, tailored by various density levels, results in varying creep processes of the Ⅱ-stage. This stage is closely related to the nucleation and early growth of T1 precipitates. Their number densities (maximum: 2.9 × 1019 m–3) and the average length (maximum: 21.3 nm) of T1 precipitates are much smaller than those of the stable peak-aged T1 phases, suggesting that creep Ⅱ-stage of all three creep-aged samples is dominant by the nucleation and initial growth of those T1 precipitates. This study provides valuable insights into the dislocation density-mediated creep deformation of an Al-Cu-Li alloy.

Understanding the Microstructure Evolution of 8Cr4Mo4V Steel under High-Dose-Rate Ion Implantation.

In this study, the effect of microstructure under various dose rates of plasma immersion ion implantation on 8Cr4Mo4V steel has been investigated for crystallite size, lattice strain and dislocation density. The phase composition and structure parameters including crystallite size, dislocation density and lattice strain have been investigated by X-ray diffraction (XRD) measurements and determined from Scherrer's equation and three different Williamson-Hall (W-H) methods. The obtained results reveal that a refined crystallite size, enlarged microstrain and increased dislocation density can be obtained for the 8Cr4Mo4V steel treated by different dose rates of ion implantation. Compared to the crystallite size (15.95 nm), microstrain (5.69 × 10-3) and dislocation density (8.48 × 1015) of the dose rate of 2.60 × 1017 ions/cm2·h, the finest grain size, the largest microstrain and the highest dislocation density of implanted samples can be achieved when the dose rate rises to 5.18 × 1017 ions/cm2·h, the effect of refining is 26.13%, and the increment of microstrain and dislocation density are 26.3% and 45.6%, respectively. Moreover, the Williamson-Hall plots are fitted linearly by taking βcosθ along the y-axis and 4sinθ or 4sinθ/Yhkl or 4sinθ(2/Yhkl)1/2 along the x-axis. In all of the W-H graphs, it can be observed that some of the implanted samples present a negative and a positive slope; a negative and a positive slope in the plot indicate the presence of compressive and tensile strain in the material.

On the Change in Hydrogen Diffusion and Trapping Behaviour of Pearlitic Rail Steel at Different Stages of Production.

To avoid hydrogen flaking in rail production, it is of crucial importance to understand the differences in hydrogen diffusion and trapping between different production steps. Therefore, as-cast unfinished material was compared with two finished rails, hot-rolled and head-hardened, using electron backscattered diffraction (EBSD), electrochemical permeation, and thermal desorption spectroscopy (TDS). A significant increase in dislocation density was in the head-hardened rail compared with the other material states. This leads to an effective hydrogen diffusion coefficient of 5.8 × 10-7 cm2/s which is lower by a factor of four than the diffusion coefficients examined in the other states. Thermal desorption spectroscopy analyses show a clear difference between unfinished and finished rail materials. While a peak in activation energy between 32 and 38 kJ/mol is present at all states, only as-cast unfinished material shows a second peak with an activation energy of 47 kJ/mol, which is related to microvoids. The results show that in the investigated material, the effect of increasing dislocation density has a stronger influence on the effective diffusion coefficient than the presence of a second active trapping site.

Mechanical characteristics and deformation behavior of Al polycrystal reinforced with SiC particles

Molecular dynamics models are established to investigate the influence of silicon carbide (SiC) particle size and strain rate on the mechanical characteristics and deformation behavior of aluminum (Al)/SiC composite. The results show that the ultimate strength of the Al/SiC composite increases from 0.788 to 0.910 GPa as the SiC reinforcement particle size increases from 10 to 30 Å, due to load transfer effects and barriers to dislocation motion. Plastic deformation of Al/SiC composite decreases with increasing SiC particle size, manifested by a lower proportion of atoms subjected to high shear strain for larger SiC particles. The SiC particle size also influences the change in crystal structure during deformation, as shown by the larger particles better supporting the Al matrix in minimizing the dislocation movement. The study shows that the grain boundary and Al/SiC interface play a significant role in the mechanical properties of the composite by impeding the evolution of dislocation and stacking fault. Moreover, the higher strain rates lead to increased ultimate strength and flow stress due to a greater rate of deformation and energy storage. The distribution of shear strain in the composite shows concentrated deformation at the grain boundary, and the large shear-strain region decreases as the strain rate increases. Additionally, higher strain rate leads to more formation of hexagonal-close-packed and amorphous structures, increased dislocation density, and material failure.

Enhancing corrosion resistance of selective laser melting Al/SiC composites via substrate preheating

The microstructure and corrosion resistance of Al/SiC composites fabricated by selective laser melting (SLM) with substrate preheating are studied for the first time in this study. The results show that the defects such as cracks and voids is decreased, which is beneficial for reducing the localized corrosion. In addition, the preheating treatment leads to the increase of dislocation density and the decrease of high-angle grain boundaries, which is beneficial to improve the corrosion resistance. Therefore, it is concluded that substrate preheating is conducive to enhancing corrosion resistance of SLM Al/SiC composites.

Effects of Zr addition on the microstructures, mechanical properties and thermal stability of Cu-Ta alloys

In this work, Cu-1.4 wt%Ta-Zr alloys with different Zr content (0, 0.1, 0.2 and 0.5 wt%) were fabricated by mechanical alloying combined with spark plasm sintering. The effect of Zr addition on the microstructures, mechanical properties and thermal stability of Cu-Ta alloy were characterized. Cu-Ta alloys composes of ultrafine equiaxed grains along with the uniformly dispersion of semi-coherent Ta nanoclusters and incoherent Ta coarse particles. As Zr is introduced into Cu-Ta alloy, the Ta precipitates were effectively refined, especially for the reduction of Ta coarse particles. The significant refinement effect of the grains is realized when Zr content reaches 0.2 wt%, and high-proportioned nano-sized grains are facilitated to form, along with the activation of a lot of twins and the formation of intermetallic composed of Cu and Zr. The dislocation density and strength increase remarkably with the increase of Zr content, and the highest strength (499 MPa for yield strength and 600 MPa for ultimate tensile strength) is achieved as the Zr content is up to 0.2 wt%. Compared with Cu-Ta alloy (382 MPa for yield strength and 560 MPa for ultimate tensile strength), the significant increment of 31% for the yield strength is successfully realized by Cu-Ta-0.2 wt%Zr alloy. Cu-Ta-0.2 wt%Zr alloys exhibit the excellent thermal stability, and the hardness keeps stable even the sample was annealed at 700 °C for 300 h, showing superior microstructural stability in comparison with Cu-Ta alloy (∼4.5% reduction of the hardness after 300 h at 700 °C). The softening temperature of Cu-Ta-0.2 wt%Zr alloy is up to 1028 °C (1022 °C for Cu-Ta alloy) along with higher hardness than that of Cu-Ta alloy at the same annealing temperature. This study provides a new way to fabricate dispersion strengthened Cu alloys with high strength, exceptional thermal stability and softening resistance, which have great potential application value in the field of future fusion reactor.

Improved damping behavior of squeeze-cast AZ91-Ca-Sb magnesium alloy with nano-SiC particles additions

The damping behavior of the SiCnp reinforced AZ91 + 2.0Ca+ 0.3Sb (wt%) alloy in the temperature range of 25–350 °C and at frequencies of 1, 5, and 10 Hz has been evaluated. All the AZ91 + 2.0Ca+ 0.3Sb+xSiCnp [x = 0.5, 1.0, and 2.0 (wt%)] nanocomposites exhibit higher storage modulus, loss modulus, and damping capacity than the alloy. The storage modulus decreases with an increase in temperature and frequency, whereas the loss modulus and damping capacity increase with an increase in temperature and decrease with frequency. The nanocomposites display a higher dislocation density and activation energy than the alloy, and the values are the maximum in the nanocomposite with 2.0SiCnp. The higher damping response of the nanocomposites is due to the presence of nano-SiC particles, higher storage and loss moduli, higher activation energy, and increased dislocation density. The Granato-Lucke mechanism, grain boundary or viscous damping, and dislocation damping are the operative damping mechanisms in the alloy and all the nanocomposites.

Enhanced fatigue resistance and fatigue-induced substructures in an additively manufactured CoCrNi medium-entropy alloy treated by ultrasonic surface rolling process

There is a significant need to elucidate the underlying mechanisms of cyclic plastic damage mechanism for additively manufactured materials and develop effective surface modification techniques to improve their fatigue life. This study investigates the efficacy of ultrasonic surface rolling process (USRP) technology in the creation of a ∼300 μm gradient nanotwinned structure on the surface of additively manufactured CoCrNi medium-entropy alloy (AM-MEA), which results in a beneficial result that yield strength and 107-cycle fatigue endurance limit are significantly improved, achieving the increment of 192.1 MPa and ∼130 MPa, respectively. The superior fatigue property is attributed to multiple factors that suppress crack initiation from sample surfaces jointly, including the presence of a gradient nanotwinned layer and the reduction in irregular defects located both on and beneath the surface. The cyclic plastic deformation behavior of AM-MEA samples with and without USRP under both high and low stress levels was studied in-depth through multiscale characterization techniques. When exposed to cyclic loading at a low stress level of 480 MPa, the fatigue damages of both samples were dominated by accumulation of statistical stored dislocations (SSDs) and persistent Lüders bands. There is no significant difference in the increase in dislocation density between both samples. However, under cyclic loading at a high stress level (660 MPa), the fatigue damage of the AM-MEA sample primarily originated from the accumulation of deformation nanotwins, stacking faults, geometrically necessary dislocations and SSDs. Conversely, the fatigue damage observed in the AM-MEA sample with USRP at the same stress level was dominant by an increase in stacking faults and SSDs. Notably, this increase in total dislocation density was visibly lower than that observed in the AM-MEA sample, which is ascribe to the stable gradient layer providing enhanced hetero-deformation induced stress for the core region in the AM-MEA sample with USRP at high stress level.

Effect of Fe-based metallic glass on microstructure and properties of Hastelloy X manufactured by laser powder bed fusion

Hastelloy X is widely used in aerospace because of its excellent high-temperature performance. Laser powder bed fusion (LPBF) can be used to manufacture Hastelloy X and its reinforced materials with more complex structures and better performance. In this study, Hastelloy X reinforced materials added Fe-based metallic glass powder was manufactured using LPBF, and four sequences of 0, 3, 5, and 7 wt%. The introduction of Fe-based metallic glass powder has exhibited notable improvements in the mechanical properties at room and high temperatures. At room temperature (RT), the 5 wt% reinforced material exhibited a larger increase in hardness, with a yield strength and ultimate tensile strength of 856Mpa and 1081 MPa, and the reduction in elongation was only 9.6%. At high temperatures, introducing Fe-based metallic glass enhanced the yield strength and ultimate tensile strength, and the elongation was greatly improved at 800 ℃. The enhancements are mainly due to the grain size refinement caused by the introduction of Fe-based metallic glass powder, which increases dislocation density and grain boundary strengthening. In addition, as the content of Fe-based metallic glass increased, the corrosion resistance of the samples was also improved due to increased Cr and W content and the refinement of grains. This study provides a new research direction for improving the properties of Hastelloy X.

Mechanical Properties and Strain Hardening Behavior of Few‐Layered Graphene Nanosheets‐Reinforced Powder Metallurgy Nickel‐Based Superalloy Metal Matrix Composites

Herein, few‐layered graphene nanosheets (GNS) with approximately 3, 6, or 9 layers are used to reinforce high‐performance nickel‐based superalloy metal matrix composite. A powder metallurgy method comprising solution‐mixing, hot isostatic pressing, and thermal processing is used to prepare GNS‐FGH96 composites with different numbers of GNS layers as well as referential FGH96 matrix materials. Compared with those of unreinforced FGH96, the mechanical properties of the GNS‐FGH96 composites are enhanced, specifically, the ≈6 layers GNS‐FGH96 composite exhibits an ultimate tensile strength of 1660 MPa and a yield strength of 1229 MPa, which are 9.79% and 6.87% higher, respectively, than those without the addition of GNS. Furthermore, the ≈6 layers GNS‐FGH96 composite exhibits the highest elongation‐at‐fracture of 29.9%. The as‐prepared GNS‐FGH96 composites show a good balance of strength and ductility owing to the increased dislocation density between the FGH96 matrix and GNS reinforcement interface area, as well as the high structural integrity of the GNS. Thus, this study provides a novel approach for designing and creating high‐performance graphene‐reinforced FGH96 metal matrix composites that exhibit exceptional strength and toughness.

Microstructure evolution during the heavy warm rolling of a nickel aluminum bronze

Nickel aluminum bronze (NAB) alloys quenched after rolling to different reduction ratios were used to investigate the microstructure evolution during the heavy warm rolling (HWR) process. The results demonstrate that the average grain size continuously declines as rolling progresses while the dislocation density steadily increases. As a result of Widmanstatten and eutectoid transformation, the β phases are almost entirely transformed into α phases. Simultaneously, different types of k phases also rapidly precipitate from the matrix. After HWR, the microstructure comprises coarse-grained Widmanstatten α phases, ultrafine-grained eutectoid structures, and many types of k phases dispersedly distributed on the matrix. There is no noticeable texture due to the formation of numerous equiaxed α grains generated by eutectoid transformation. As the reduction ratio increases, the increasing dislocation lines gradually become entangled, eventually forming many dislocation cells. The precise statistics of the precipitates' size, position, and area fraction manifest that the newly generated k phases during rolling are primarily distributed in the region of eutectoid structures, and their size is typically below 50 nm. The improvement of the mechanical properties is attributed to the increase of dislocation density and nano precipitates as well as the continuous microstructure refinement. Based on the accurate characterization of microstructures, the phase transformation kinetics during rolling influenced by combined thermo-mechanical effects are holistically described.

Uniaxial ratcheting behavior and microstructure evolution of 316H stainless steel under the random cyclic loads

In this study, the Computational Fluid Dynamics (CFD)-Finite Element Method (CFD-FEM) were used to simulate the transient stress response of the structure surface under thermal striping environment, and a uniaxial random cyclic loading (RCL) method was used to reveal the mechanical properties and initial microstructure evolution of 316H austenitic stainless steel (SS) under random stress induced by thermal striping. The experimental results showed that 316H exhibited a lower saturation ratcheting strain rate at 334.44 MPa, along with stronger cyclic hardening effects and resistance to plastic accumulation at loading rate of 100 Hz. Random loading tests with different cycles were performed at σpeak=334.44MPa and fRCL=100Hz, and ratcheted specimens were characterized to investigate the effect of the micro-mechanism on cyclic softening/hardening. The geometrically necessary dislocation (GND) density increases and then decreases with cyclic loading, which is mainly attributed to the deformation twin (DT) and multi-dislocation slip mechanisms. The deformation mechanism during the ratcheting cycle from 1000 to 3000 cycles is mainly dominated by the twinning reduction, where the dislocation migration accelerated the transformation from dislocation walls to subgrain boundaries; the twinning increase dominates from 3000 to 9000 cycles, where the subgrain boundary grew into boundaries with high angle misorientations.