Kinds Of Microstructures Research Articles

Cryogenic tribological behavior of coarse, ultrafine grained and heterogeneous Fe-18Cr-8Ni austenitic stainless steel

Commonly used austenitic stainless steels (ASSs) have some limitation in sliding wear conditions due to their relatively low yield strength and hardness. To improve the wear resistance, three kinds of microstructure (coarse grain (CG), heterogeneous structure (HS), and ultrafine grain (UFG)) are prepared, to investigate the grain size on the dry sliding tribological behavior, as well as wear mechanisms of Fe-18Cr-8Ni ASSs at room temperature (RT) and cryogenic temperature (−120 °C). The results indicate that at RT the UFG specimen exhibits the lowest wear rate during the wear tests, where the wear mechanisms are mainly oxidation wear and abrasive wear. While, when tested at −120 °C, the CG specimen exhibits the best wear resistance compared with that of the other two specimens due to its superior plastic deformation ability and strain hardening ability. Moreover, the HS specimen exhibits the lowest coefficient of friction (CoF), which is due to the abrasive particles generated on the contact surface provide a certain level of friction reduction, while the wear rate increases as these particles serve as third-party abrasives, which further removing material.

Stacking faults stabilize oxygen vacancies at high temperatures to improve the thermoelectric performance of ZnO

Stabilizing the oxygen vacancies (Ovs) in ZnO at high temperatures is crucial for maintaining the high thermoelectric (TE) performance caused by Ovs. In this study, based on the complex interaction between different kinds of lattice defects, high-density stacking faults (SFs) were fabricated in ZnO with Ovs to effectively stabilize Ovs. The Ovs combined with SFs to significantly improve the electrical conductivity (σ) to 4649 S m−1 at room temperature, leading to an increased power factor (PF) of 280.4 μW m−1·K−2 at 500 °C. Meanwhile, the thermal conductivity was reduced to 3.12 W m−1·K−1 at 500 °C. Notably, the σ and PF were not weakened after five consecutive cycle tests up to 500 °C, suggesting the stabilization of Ovs by SFs at high temperatures. Consequently, the optimized ZT value reached 0.07 at 500 °C, an 18-fold enhancement over the initial sample. The study provides an effective strategy to achieve a simultaneous enhancement in TE performance and stability by modulating different kinds of microstructures.

Understanding and exploring anisotropy mechanism of mechanical properties for ferrous alloy under different cooling paths

To understand and illuminate the anisotropy mechanism of mechanical properties with respect to typical ferrous alloy, two different kinds of microstructures were fabricated by adopting thermo-mechanical controlled processing (TMCP) with identical rolling schedule and different cooling paths, named high coiling temperature process (HC) and low coiling temperature process (LC). The influences of microstructure together with distribution of crystallographic orientation on the anisotropy discipline were investigated, and the anisotropy mechanism were further explored on the basis of plastic deformation theory. Results indicated that the microstructure processed by HC was primarily composed of quasi-polygonal ferrite (QPF) and small fraction of bainitic ferrite (BF). While the LC-processed microstructure predominantly consisted of acicular ferrite (AF), BF as well as martensite-austenite island (M/A). The LC-processed alloy exhibited more obvious anisotropy performance indicated by tensile properties along different tensile directions compared to the HC-processed alloy. There was a phenomenon of uniform distribution of macro-texture and uneven distribution of micro-texture with respect to the both achieved microstructures, and the uneven distribution of micro-texture together with high density of dislocation for BF were responsible for the obvious anisotropy performance of the studied ferrous alloy.

MOF derived NiFe@C composites with controllable multi-dimensional microstructures for broadband microwave absorption

Metal-organic framework (MOF) derivatives exhibit great potential as microwave absorbing materials (MAMs) owing to their high porosity, abundant surface sites and tunable constituents. Herein, this paper proposed an efficient microstructure controllable strategy for synthesizing thin-thickness broadband MAMs named NiFe@C by regulating its material microstructure. NiFe-MOF-74 precursors were firstly prepared through solvothermal method. By controlling the supersaturation of precursors, three kinds of microstructures were obtained: 3-dimensional lotus-like nanorods self-assembled structure, 1-dimensional nanospheres/nanorods hybrid structure, and 0-dimensional nanospheres structure. Subsequently, NiFe@C derivatives were obtained by pyrolysis, inheriting the microstructures of NiFe-MOF-74. The lotus-like structure of NiFe@C achieves an effective absorption bandwidth (reflection loss < -10 dB) of 5.09 GHz (12.91–18.00 GHz) with thickness of only 1.6 mm, while the nanospheres structure of NiFe@C exhibits the maximum absorptance of 99.96% at 17.12 GHz with thickness of 2.3 mm. This work provides a facile direction for designing thin thickness MAMs with broadband microwave absorption capability.

Quantitative relationship between impact toughness and quasi-static tensile properties of Ti-652 titanium alloy

Opinions regarding the relationship between impact toughness and quasi-static properties of titanium alloys remain divided to this day. In this paper, quasi-static tensile property parameters were introduced into the classical model of elastic-plastic fracture toughness. Utilizing the absorbed energy during the elastic and plastic stages, as conveyed by the integral of the quasi-static tensile stress-strain curve, the description of elastic and plastic work within the fracture toughness model was provided. Additionally, for the first time, a quantitative model relating impact toughness to quasi-static tensile property parameters in high-strength titanium alloys was proposed and applied to the high-strength Ti-652 titanium alloy. Decoupling the elastic and plastic segments of stress-strain curve, it was found that plasticity contributes more to impact toughness than strength, especially for high-strength metallic materials. Comparing the mechanical properties of Ti-652 titanium alloy with 9 kinds of microstructure, M9 was found to have the highest impact toughness (53.713 J/cm2) which features a duplex microstructure and underwent more severe plastic deformation during the test of samples, leaving shear bands and “ripple-like” patterns on the quasi-static tensile and impact fractures. Besides, statistics of dislocation density assisted by TEM images demonstrate that the dislocation density (ρ̅GND=4.82 ×1014 m−2) of high-impact toughness specimens are apparently higher than that (ρ̅GND=3.86 ×1014 m−2) of low-impact toughness specimens. Severe plastic deformation increased the energy consumption during the fracture process, resulting in high impact toughness of specimen.

Enhancing shape precision in jet electrochemical additive micro-manufacturing process through confined electrolyte

This paper proposes a jet electrochemical additive micro-manufacturing (Jet-ECμAM) process with confined electrolyte to enhance the shape precision of micro structures. To achieve electrolyte confinement, a layer of dichloromethane is dispersed on the cathode workpiece. When the electrolyte is ejected out of nozzle and reaches onto the workpiece, it does not disperse freely on the workpiece but its extent of spread is restricted by dichloromethane due to their different densities and immiscibility. Consequently, the electrochemical reaction is confined to the processing zone and the distribution of current density in the confined zone is improved at the same time, which enhances the shape precision. To further analyze this method, a multi-physical simulation was performed to investigate the electrolyte flow characteristics and electric field distribution on the workpiece. The results showed that after electrolyte flow out of the nozzle, its extent of spread is restricted by dichloromethane leading to confinement within the processing zone on the workpiece. Moreover, the distance of radial spread of electrolyte is reduced from 241 μm to 133 μm with the inter-electrode gap (IEG) increased from 100 μm to 400 μm. On the other hand, there is an increased current density in the confined region, and the distribution of current density becomes uniform at the same time. Experiments were conducted to verify the proposed technique. The results indicate that the shape precision of a micro structure is significantly improved with the proposed method. Micro-pillars with vertical side walls were well deposited, and a reduction in diameter from 580 μm to 380 μm was observed with the increasing IEG. In addition, the surface quality improves when the current density increases from 30 A/dm2 to 70 A/dm2. At last, by controlling the IEG and the motion path of the nozzle, different kinds of micro structures were well deposited, demonstrating a flexible fabrication process based on Jet-ECμAM.

Deformation mechanism of highly textured Alnico magnets

Deformed Alnico magnets were prepared by hot isostatic pressing (HIP) spherical powders and hot rolling (HR), and the hot-rolled magnets were solution-treated at 1250 ℃, magnetic heat treated at 835 ℃, and three-stage aged at 600 – 500 ℃, respectively. The results show that Alnico spherical powder comprises 77.5% α phase, 8.6% α1 phase, and 13.9% α2 phase. The HIP magnet has spinodal decomposed during the preparation, and HR improved α content. The magnet has spinodal decomposed thoroughly after magnetic heat treatment (MHT) and aging heat treatment (AHT), composed of 62.1% α1 phase with a B2 structure and 37.9% Fe2NiAl-type α2 phase with the L21 structure. Moreover, the HIP and HR magnet preferentially produces the< 100 > texture, which is also enhanced along the< 211 > directions. The preferred deformation of the α1 phase is carried out under the synergistic action of the α2 phase. The HIP deformation mechanism is that composition diffusion and structural transformation are carried out in different stages, resulting in four kinds of two-phase structures. The rolling deformation mechanism is that the grains parallel or nearly parallel to the rolling direction are directly rolled into lath-like ones, and four kinds of microstructures are formed in the grains. Compared to the HIP magnet, the MHT and AHT promote the formation of the fibrous structure, and the maximum aspect ratio of α1 phase reaches ∼35:1. The coercivity of the heat-treated magnet increased by 160.9%, and the remanence and magnetic energy product also increased simultaneously.

Effect of solution annealing temperature on microstructural evolution and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys

Recent studies have shown that NbC precipitates in high-entropy alloys (HEAs) are strong obstacles for dislocation motion, which can substantially contribute to the mechanical performance over a wide temperature range. However, there are limited reports discussing the exceptional adjustability of microstructures and mechanical properties using thermomechanical processing (TMP). Therefore, the microstructures and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys (HEAs) tuned via TMP were systematically investigated. Homogenized HEAs with ∼0.3 at% C and Nb were solution annealed at 1100, 1200 and 1300 °C, followed by cold-rolling plus annealing. The microstructures and mechanical properties of the HEAs were systematically characterized using scanning electron microscopy, transmission electron microscopy and uniaxial tensile tests. The results indicated that NbC distribution evolved from eutectic-like primary NbC clusters surrounded by disorderly distributed secondary NbC to dispersive nano-sized secondary NbC as the solution annealing temperature increased above the NbC solvus temperature. Two kinds of microstructures were correspondingly produced: 1) samples solution annealed at 1100 and 1200 °C consisted of fine grain lamellae embedded with NbC precipitates and coarse grain lamellae without precipitates, which exhibited a combination of high strength (UTS ∼740 MPa) and good ductility (elongation ∼60%); and 2) samples solution annealed at 1300 °C obtained a homogeneous fine grain microstructure, which demonstrated excellent synergy of high strength (UTS ∼760 MPa) and exceptional ductility (elongation ∼63%). The enhanced strength of samples solution annealed at 1100 and 1200 °C originated primarily from the high back stress strengthening of lamellar microstructure, while precipitation strengthening caused by dispersed nanosized NbC precipitates and grain boundary strengthening provided extra strengthening for the sample annealed at 1300 °C. This work provides a new strategy for tailoring the microstructure and mechanical properties of NbC-reinforced HEAs via adjusting the solution annealing temperature of TMP.

Effect of a homogeneous recrystallized microstructure and a bimodal microstructure on mechanical properties in Mg-5Zn-0.6Zr alloys

For typical Mg-Zn-Zr alloys, exhilaratingly high strength of a yield strength (YS) higher than 300 MPa can hardly be attained by traditional rolling. In this paper, we compare the mechanical properties and strengthening mechanisms of the Mg-5Zn-0.6Zr alloys having a homogeneous dynamical recrystallized microstructure and a bimodal microstructure with high-density nano substructures. The Mg-5Zn-0.6Zr alloy with the bimodal microstructure (rolled at 150 °C with a thickness reduction of 60%) exhibits a YS of 332 MPa, an ultimate tensile strength (UTS) of 360 MPa, and an elongation of 5%. The high strength is attributed to the microstructure with high-density nano substructures, high-density nano (Mg, Zr)Zn2 precipitates, ultrafine recrystallized grains, and strong basal texture. In comparison, the Mg-5Zn-0.6Zr alloy with homogeneous microstructure (rolled at 200 °C with a thickness reduction of 70%) exhibits a YS of 209 MPa, an UTS of 317 MPa, and an elongation of 17%, which contains coarser recrystallized grains, coarser precipitates, weaker texture, and lower density of dislocations, further resulting in low strength. The difference between the strengthening mechanism in two kinds of microstructure is discussed in detail. The results facilitate the preparation of wrought Magnesium alloy with high strength by reasonable microstructure construction.

Effects of WC Particles on the Microstructure of IN718/WC Composite Coatings Fabricated by Laser Cladding: A Two-Dimensional Phase-Field Study

During the process of laser cladding IN718/WC composite coatings, the dissolution and metallurgical reaction of WC particles significantly influence the microstructures of the coatings. However, the limited experimental methods restrict direct observation and prevent a deep understanding of this complicated process. Therefore, a novel numerical model for the solidification process of IN718/WC composite coatings was proposed. The model was established with the coupled multi-phase-field model and lattice Boltzmann method. Different kinds of microstructure around WC particles were simulated by the model and verified by experiments. Subsequently, microhardness and wear tests were carried out to investigate the improved mechanical properties of IN718 coatings reinforced by WC particles. The results show that the cellular alloy reaction layer, IN718 grains, and eutectic structure are formed, in turn, around WC particles. The convection in the laser molten pool can induce double-tail-like or spindle-like WC convection diffusion bands. The hardness of these bands is higher than that of the IN718 matrix. More importantly, WC convection diffusion bands can inhibit the growth of columnar crystals, because the dissolved WC can decrease the freezing temperature of the melt. Finally, mechanical property tests show that WC particles increase the hardness of the coating and significantly improve its wear resistance.

The effect of kink-like defects on the twin boundaries of nanotwinned Ta under nanoindentation

To figure out the effect of kink-like defects on twin boundaries, BCC Ta with three different kinds of microstructures: single crystal (SC), perfect twin boundaries (PTB) and defective twin boundaries (DTB), are studied under nanoindentation. Both indenter load and hardness in PTB are enhanced by twin boundaries but reduced in DTB due to intrinsic kink-like defects. According to atomic surface morphology, the relative sink-in depth in DTB is lower than that in PTB but higher than that in SC. By comparing atomic images of prismatic loops in three different stages, it can be found that kink-like defects in DTB perform two opposite functions: acceleration and delay. In initial plastic stage, defective twin boundaries can accelerate pinching-off event and nucleation. In transmission stage, dislocation backward emission is advanced by defective twin boundaries. In obstruction stage, the symmetry of coherent twin boundaries will be broken and asymmetrical shear loops are formed, which will delay prismatic loops moving across twin boundaries. Dislocations of 12〈111〉 are the main factor of nanoindentation in [1¯1¯2] direction. Kink-like defects will lead to less drops in DTB on dislocation densities than that in PTB. These findings are helpful to reveal the influence of intrinsic kink-like defects in BCC nanotwinned materials.

The pH-dependent phase separation kinetics of type-A gelatin and dextran with different initial microstructures

The phase separation kinetics of type A gelatin and dextran (GE/DE) with different microstructures were investigated to get information on the relation between technofunctional properties and structures using static multiple light scattering (S-MLS), fluorescence microscopy, and macroscopic observations. GE/DE mixtures of 5.0 wt%/2.0 wt% and 5.0 wt%/3.0 wt% at pH = 5.50–9.00 were selected as the typical samples with the DE-in-GE and bicontinuous microstructures, respectively. Macrophase separation was found to occur in both the two mixtures at pH = 5.50–9.00 with the upper phase rich in GE and the lower phase rich in DE. However, the effect of pH on the phase separation rates of the two mixtures was different: pH-independent in DE-in-GE mixtures, while pH-dependent in bicontinuous mixtures. Specifically, the order of phase separation rates for the bicontinuous mixtures at pH = 5.50–9.00 was pH5.50 > pH6.00 > pH9.00 > pH7.00 ≈ pH8.00. Changes in the electrostatic interaction or steric exclusion effect among GE (DE) droplets at pH = 5.50–9.00 were used to explain the different phase separation kinetics of GE/DE mixtures with GE-in-DE, DE-in-GE, and bicontinuous microstructures. This work characterized the phase separation processes of GE/DE mixtures with three kinds of microstructure and provided the parameters for the phase separation kinetics at the macro-level.

Structure and properties of laser cladding CoCrNi multicomponent alloy coating used in rain gauge

Rain gauge is an important measuring instrument, which is widely used for rainfall measurement under various conditions. However, since the rain gauge is used in outdoor exposed environment, its shell is easily corroded by rain, especially by acid rainwater, which will affect the service life. In order to improve the corrosion resistance and prolong its service life, CoCrNi medium entropy alloy (MEA) coating was prepared on the surface of rain gauge's shell by laser cladding. The microstructure and properties of the CoCrNi coating were studied comprehensively. The results show that the coating and the substrate form a metallurgical bond. The microstructure of CoCrNi MEA coating is mainly composed of two kinds of microstructures, and mainly containing face centered cubic (FCC) structure. The coatings have excellent corrosion resistance in acid rainwater with different pH. The corrosion current density of the laser cladding CoCrNi MEA coatings are lower than that of 304 stainless steel (SS), reduced by 2 orders of magnitude, the corrosion potential increased in varying degrees, with an increase range of 0.03∼0.47 V. The pitting corrosion resistance of the coating is excellent, with the increase of pH, the pitting resistance is enhanced. The corrosion resistance of the coating is related to the phase structure, uniformity and compactness of the microstructure as well as the corrosion resistant elements in the coating. The microhardness of the coating is 703.6 HV, the specific wear rate of the alloy is 1.4×10−4 mm3/Nm, and the friction coefficient is 0.47. Experiments show that the coating have excellent corrosion resistance and mechanical properties for the rain gauge's shell, which provides a guarantee for prolonging its service life.

On the origin of grain refinement and twin boundaries in as-fabricated austenitic stainless steels produced by laser powder bed fusion

316 L austenitic stainless steel samples have been fabricated by laser powder bed fusion using two powder batches of slightly different compositions but keeping the same processing parameters. Compared to the first powder batch, the second powder batch led to a finer grain structure, a more random texture, and an extremely high density of Σ3 twin boundaries. A review of the literature shows that these two kinds of microstructures had been observed in previous works, but separately and not connected to changes in composition. Several hypotheses are reviewed to explain these microstructure differences. We conclude that local liquid ordering, also described as icosahedral short range order (ISRO) can have an effect on the microstructure formation during solidification. ISRO in the liquid is thought to be stronger in the second powder batch. Finding ways to enhance the influence of the ISRO in the liquid on the microstructure formation can be seen as a promising strategy to design alloys for additive manufacturing.

Microstructures of Graphene applied as Electrode Materials in Lithium-ion Batteries

With the dramatic increase in the consumption of electric energy, lithium-ion batteries (LIBs) become one of the most important devices for energy storage. Among various types of electrode materials in LIBs, graphene is one of the potential alternations due to the outstanding conductivity and ability of ions storage. But the limitation of capacity and stability brings challenges when applying graphene in LIBs directly. Recently, the microstructure of graphene is focused more to improve the performances of LIBs furtherly. Several kinds of microstructure in graphene have been designed including three-dimensional (3D) microstructure, core-shell microstructure, sandwich microstructure, and porous microstructure. This review aims to summarize the designs of these microstructures, conclude the effect of each microstructure and provide suggestions for future development.

Effect of metallurgical variables on hydride precipitation behavior in Zr-2.5Nb micro pressure tubes

The effect of manufacturing routes (heat treatment done by a conventional β-quenching (BQ), or slow cooling (SC) prior to hot extrusion), and location along the tube (front end (FE) and back end (BE) emerging from the press) on hydride precipitation behavior in Zr-2.5Nb micro pressure tubes were evaluated. After being hydrided to 100 wppm H, stepped micro pressure tube samples with three kinds of microstructure (BQ-BE, SC-FE, SC-BE) were pressurized to obtain nominal hoop stresses ranging from 65 MPa to 350 MPa. All the samples were heated to 350°C to dissolve all of the hydrogen, followed by cooling under stress to allow hydride reorientation to occur. Almost complete reorientation was observed in the BQ-BE sample at stress levels of above 100MPa, while the SC-FE and BE samples showed stronger resistance to reorientation. After subsequent reheating to 350°C for 1 hour or 24 hours and cooling without stress, little memory effect was found for the re-orientated hydrides in the BQ-BE sample. However, a clear memory effect was observed in the SC-FE and SC-BE sample in the range of angles where hydrides were naturally present.

An Inertial Impact Piezoelectric Actuator Designed by the Asymmetric Friction Principle and Achieved by Laser Texturing of the Driving Feet

An asymmetric friction principle is newly proposed for the design of inertial impact piezoelectric actuators. There are two ways to achieve asymmetric frictions: either by tuning the positive pressure or by tuning the friction coefficient. Compared with tuning the positive pressure by an asymmetric structure, the structural parameters can be reduced by employing a symmetric structure and tuning the friction coefficient. In this study, an asymmetric friction inertial impact actuator was developed using a symmetric compliant mechanism (SCM), and the asymmetric frictions were realized by laser texturing of the driving feet at one end of the SCM. Four kinds of microstructures were initially fabricated on the driving feet, and their friction properties were experimentally tested. Accordingly, two kinds of microstructures, namely Ta and Tb microstructures, were selected. Output characteristics of the actuator with these two microstructures were measured and comparatively analyzed. The experimental results indicate that the actuator could achieve stable step motion, and the output characteristics were affected by the fabricated microstructure, as it determined the friction coefficient. The actuator with the Tb microstructure achieved a maximum speed of 2.523 mm/s, a resolution of 188 nm, a vertical loading capacity of 2 N and a horizontal loading capacity of 0.6 N, whereas the actuator with the Ta microstructure had a higher resolution of 74 nm. This study provides a novel idea for the design of asymmetric friction inertial impact actuators by tuning the friction coefficient.

Effect of fibroblasts on promoting the sprout and migration of endothelial cells in three-dimensional pre-vascularized microstructures

To construct three-dimensional (3D) pre-vascularized microstructures and explore the promoting effect of human fibroblasts (HFs) on the sprout and migration of human umbilical vein endothelial cells (HUVECs) in 3D co-culture system. HUVECs and HFs were cultured and the 3rd to 5th generation cells were selected for subsequent experiments. In 2D co-culture system, HFs were stained with PKH26 and the cell density was fixed, which co-cultured with HUVECs in different ratios (1∶4, 1∶1, 4∶1) and inoculation methods (HUVECs inoculation at 48 hours after HFs, direct mixed inoculation). Then the formation of vascular like structures was observed under fluorescence microscope. In 3D co-culture system, HUVECs and HFs were labeled with green fluorescent protein and red fluorescent protein by lentivirus transfection, respectively. They were inoculated on porous micro-carriers followed by dynamically culturing in rotating bottles to prepare HF, HUVEC, HF-EC, or HF&EC microstructures. The cell growth in microstructures was testing by low permeability crystal violet staining. Subsequently, the microstructures were embedded in fibrin gel and the cell growth and adhesion in HF and HUVEC microstructures were observed by laser confocal microscopy. Laser confocal microscope were also used to observe the sprouts of 4 kinds of microstructures, as well as the cell composition, the number and length of sprouts from HF-EC and HF&EC microstructures. HFs conditioned medium was prepared to observe its effect on sprouts of HUVEC microstructures with DMEM as control group. In 2D co-culture system, HFs pre-culturing was helpful to the formation and stability of vascular like structures, and the best effect was when the ratio of two kinds of cells was 1∶1. In 3D co-culture system, it was found that the cells grew well on micro-carriers and had the ability of pre-vascularization. HUVEC microstructures did not sprout, but HF, HF-EC, and HF&EC microstructures could which indicated a good vascularization ability. The HF-EC microstructures were superior to HF&EC microstructures in terms of sprouts length and number ( P<0.05). The tubes sprouting from co-cultured group were composed of HFs and HUVECs, and HF microstructures migration preceded HUVEC microstructures always, and their migration trajectories were the same. HUVEC microstructures could sprout when cultured in HFs conditioned media. HF-HUVEC pre-vascularized microstructures can be prepared by pre-culturing HFs before HUVECs and with the cell ratio at 1∶1 in a rotating bottle. In 3D co-culture system, HFs can promote and guide the sprout of HUVECs.

Prediction of the overall mechanical response of gradient nano-grained materials based on grain size distribution profile

Inspired by the gradient structure of biological materials, various gradient nano-grained (GNG) materials, inside which grain size spans several orders of magnitude, have been manufactured in recent years. This kind of material often exhibits extra hardening ability and ultimately achieves synergetic strength and ductility. The extra hardening of GNG materials is closely related to the strain gradient formed during deformation. However, the analytical relation between the structural gradient, strain gradient is still unexplored. Besides, the previous works focused on only one type of GNG material, and the understanding of the effect of different kinds of microstructures is limited. This paper explores the analytical relations between the strain gradient and the average grain size, as well as the grain size gradient in GNG materials. The overall mechanical responses of GNG interstitial-free (IF) steel and GNG Ni are well predicted. The results verify that the extra hardening of GNG materials is dependent on their unique microstructures. The strength and ductility of GNG materials can be improved by adjusting the composition and distribution of coarse grains and nano-grains.

Level-Set Modeling of Grain Growth in 316L Stainless Steel under Different Assumptions Regarding Grain Boundary Properties.

Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values, and histograms. Two kinds of microstructures are considered: some are generated statistically from EBSD maps, and the others are generated by the immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, which are, respectively, defined as a function of the disorientation and both the GB misorientation and the GB inclination. In terms of mean grain size value and grain size distribution (GSD), both formulations provide similar responses. However, the anisotropic formulation better respects the experimental disorientation distribution function (DDF) and predicts more realistic grain morphologies. It was also found that the heterogeneous GB mobility described with a sigmoidal function only affects the DDF and the morphology of grains. Thus, a slower evolution of twin boundaries (TBs) is perceived.