Submicron Grain Research Articles

Surface microstructure and property modifications in AISI 304 stainless steel induced by pseudospark pulsed electron beam treatments

In this paper, the modifications of microstructure, mechanical properties and corrosion behaviors of AISI 304 austenite stainless steel treated by pseudospark pulsed electron beam (PSPEB) with different number of pulses were investigated. PSPEB is a high frequency superfine pulsed electron beam characterized by the rapid current growth of~1012 A/s, high power density of~109 W/cm2, short-duration pulse of102~103ns and self-focusing beam diameter of0.5~6 mm. The results of EDS analysis indicated that the precipitated ferrite phase dissolved into the matrix, forming the homogenous composition in the modified layer. Microstructure observations revealed that the homogenous submicron grains with slip bands in different directions were distributed fully in the modified layer completely replacing the original coarse grains (more than 10 μm) after 5000 pulses PSPEB treatment. Simultaneously, the multiple abundant microstructures including dislocations, sub-boundaries and nanotwins with the twin lamella thickness no more than 8 nm were induced into the modified layer. The microhardness of AISI 304 stainless steel was increased with the number of PSPEB pulses, which was mainly attributed to the grain refinement and plastic deformation strengthening in the modified layer. Potentiodynamic polarization and electrochemical impendence spectrometry (EIS) tests of AISI 304 stainless steel before and after PSPEB treatment in the 5 wt% NaCl solution showed that the 5000 pulses treated specimen exhibited the highest corrosion potential and polarization resistance due to the homogeneous alloying element distribution in the modified layer.

Texture analyses and microstructural evolution in monolithic U-Mo nuclear fuel

This work describes the microstructural evolution of prototypical monolithic U-Mo fuel plates analyzed via scanning electron microscopy and electron backscattered diffraction (EBSD). The understanding of the microstructural and textural evolution of nuclear fuel from as-fabricated to post-irradiation is important in assessing changes in material properties during irradiation. In our work it was observed that the typical fabrication techniques applied in U-10Mo monolithic fuel plates develop features associated with a cold-rolled body-centered cubic (bcc) texture and development of α and γ fiber (parallel to the rolling and normal direction). After irradiation, a loss of the fabrication-induced preferred orientation was observed with an increased spread of grain-boundary misorientation as burnup increases. Grain subdivision was observed in the irradiated samples with the formation of submicron grains (200–500 nm). Evidence for polygonization as the mechanism leading to grain subdivision was detected. This has been observed for the first time for U-Mo monolithic fuel via EBSD and has been associated to formation of low-angle grain boundaries (<15 degrees) at the site of the submicron grains. Such analyses of the microstructural and textural evolution of fuel (from fabrication to after irradiation) have the potential to help develop and validate microstructural physics-based models and provide a key feedback loop to further understand the interplay between fabrication processes and fuel performance.

Synthesis of mesoporous silica with ordered nanostructural morphology from natural quartz sand

Silica with ordered nanostructured morphology characteristic of mesoporous molecular sieves with cubic packing of cylindrical pores and a three-dimensional porous structure is obtained from natural quartz sand. It was shown by X-ray diffraction, scanning electron microscopy, and low-temperature nitrogen adsorption-desorption that the obtained silica material is thermally stable, amorphous, consists of submicron grains, has a high specific surface area of 1396 m2/g, a pore volume of 0.780 cm3/g and an average diameter of 2.2 nm with a narrow size distribution of mesopores.

High-strength GWZ1031K alloy with gradient structure induced by surface mechanical attrition treatment

This work investigated the surface mechanical attrition treatment (SMAT) on the microstructure and mechanical properties of the Mg-10Gd-3Y-1Zn-0.4Zr (GWZ1031K, wt%) alloy. Before SMAT, the studied alloy with bimodal-grained structure was prepared by extrusion and aging. The results showed that the traditional bimodal-grained structure transformed into gradient structure including two characteristic areas after SMAT: (a) the severely deformed layer containing nanograins and sub-micron grains; (b) the deformed coarse grained layer, where the banded structure is distorted in this region. The gradient structure induced by SMAT can improve the mechanical properties of GWZ1031K alloy obviously. After SMAT for 10 min, the micro-hardness of the surface increases from 117 HV to 137 HV because of the grain refinement. The alloy also exhibited superior mechanical properties at room temperature, the yield strength (YS) is 427 MPa, the ultimate tensile strength (UTS) is 562 MPa and the elongation is 4.1%. The findings indicate that SMAT is a suitable method for solving structural optimization problems and achieving high strength in Mg-RE alloys.

Studies of ferroelectric and magnetic phase transitions in Pb(Fe0.5Nb0.5)1-xCexO3 multiferroic ceramics

Multiferroic Pb(Fe0.5Nb0.5)1-xCexO3 (x = 0.025, 0.05, 0.075, 0.10) ceramic compositions were prepared by solid state synthesis and usual sintering. XRD studies have shown a formation of the perovskite structure in all the compositions studied. However a small amount of pyrochlore phase was detected in Ce-doped compositions with x ≥ 0.075. Scanning electron microscopy studies and elemental analysis have shown that at the boundaries of the large grains of nearly pure PbFe0.5Nb0.5O3 there exist a lot of small submicron grains enriched by Ce. Concentration dependencies of the unit-cell parameter and temperatures of ferroelectric and magnetic phase transitions also imply that solubility of Ce in PFN does not exceed ≈ 2 at.%.

Thermal stability of nanocrystalline and ultrafine-grained titanium created by cryomechanical fragmentation

Using X-ray diffraction analysis methods, the authors have studied the effect of stepwise isothermal annealing in the temperature range 150–670 C on the deformation microstructure parameters of cryodeformed VT1-0 titanium with micron, submicron, and nanoscale grain sizes. The structural states were obtained by rolling at liquid nitrogen temperature. This study discusses the influence of dislocations and twins as elements of deformation microstructure on the thermal stability of crystallite sizes (coherent scattering regions) and the magnitude of microdeformations. The dependences of microhardness on annealing temperature have been determined for samples in different initial structural states.

Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys (x = 0, 0.5, 1.0) fabricated by the powder metallurgy process

Light and strong AlxCrNbVMo (x = 0, 0.5, and 1.0) refractory high-entropy alloys (RHEAs) were designed and fabricated via a the powder metallurgical process. The microstructure of the AlxCrNbVMo alloys consisted of a single BCC crystalline structure with a sub-micron grain size of 2−3 μm, and small amounts (< 4 vol.%) of fine oxide dispersoids. This homogeneous microstructure, without chemical segregation or micropores was achieved via high-energy ball milling and spark-plasma sintering. The alloys exhibited superior mechanical properties at 25 and 1000℃ compared to those of other RHEAs. Here, CrNbVMo alloy showed a yield strength of 2743 MPa at room temperature. Surprisingly, the yield strength of the CrNbVMo alloy at 1000℃ was 1513 MPa. The specific yield strength of the CrNbVMo alloy was increased by 27 % and 87 % at 25 and 1000℃, respectively, compared to the AlMo0.5NbTa0.5TiZr RHEA, which exhibited so far the highest specific yield strength among the cast RHEAs. The addition of Al to CrNbVMo alloy was advantageous in reducing its reduce density to below 8.0 g/cm3, while the elastic modulus decreased due to the much lower elastic modulus of Al compared to that of the CrNbVMo alloy. Quantitative analysis of the strengthening contributions, showed that the solid solution strengthening, arising from a large misfit effect due to the size and modulus, and the high shear modulus of matrix, was revealed to predominant strengthening mechanism, accounting for over 50 % of the yield strength of the AlxCrNbVMo RHEAs.

Thermoelectric Transport Properties of n-Type Sb-doped (Hf,Zr,Ti)NiSn Half-Heusler Alloys Prepared by Temperature-Regulated Melt Spinning and Spark Plasma Sintering

Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf0.35Zr0.35Ti0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf0.35Zr0.35Ti0.3NiSn1−xSbx (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m−1 K−1 is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf0.35Zr0.35Ti0.3NiSn0.99Sb0.01.

Processing and Properties of Medium-Mn TRIP Steel to Obtain a Two-Stage TRIP Behavior

Two medium-Mn steels of nominal composition 0.15C-2.0Si-10.5Mn-(0.75, 1.5) Al-0.04Nb-balFe (wt pct) were processed to produce sub-micron grain sizes of 0.65 and 0.80 μm. Mechanical testing was performed in three successive conditions: hot rolled, intercritically annealed, and cold rolled with subsequent intercritical annealing. Intercritical annealing was performed at 923 K for 20 hours. Electron-backscattered diffraction and X-ray diffraction were utilized to characterize the microstructure, consisting of α-ferrite, α-martensite, e-martensite, and γ-austenite. Microstructural constituents were tracked during tensile deformation and it was found that both steels exhibited two-stage TRIP with γ-austenite martensitically transforming first to e-martensite and as strain increased e-martensite transformed to α-martensite.

Multipass Friction Stir Processing of Steel/SiC Nanocomposite: Assessment of Microstructure and Tribological Properties

A surface layer of steel/SiC nanocomposite was fabricated by multiple passes of friction stir processing (FSP) on a low-carbon steel. The characterization methods such as optical microscopy, field emission scanning electron microscopy, microhardness, and wear test were performed to obtain the microstructure and mechanical properties of the produced layers. The results revealed that the dispersion of nano-SiC particles is directly related to the FSP passes and that the superior scattering of the nanosized SiC particles is achieved at higher FSP passes. Nano-composite surface layers demonstrated an average microhardness value of ~ 424 HV, which is ~ 3.4 times higher than that of the as-received substrate. Moreover, ultra-fine submicron grains were formed by FSP due to the role of SiC particles as obstacles to grain growth. The produced surface-layer nanocomposite exhibited an outstanding enhancement in wear resistance in comparison with the FSPed sample without reinforcing particles.

Grain Size Dependence of Brightness Phase Curves of the Lunar Surface

AbstractThe lunar opposition effect is the increase of brightness on the lunar surface when the phase angle approaches 0°, which is caused mainly by shadow‐hiding and coherent backscattering. However, the contribution of coherent backscattering is not yet well established. This study analyzes the contribution of coherent backscattering to the opposition effect using the correlation between the contribution of increased brightness of the theoretical coherent backscattering and the grain size of the reflecting surface. We analyze the correlation between the two using data from the Lunar Reconnaissance Orbiter Wide Angle Camera and median grain size d calculated by linear polarization observations. The size of effective scatterers of coherent backscattering is similar to the observation wavelength. In this case, the effective scatterer size for coherent backscattering is 0.6 μm. The results of our analyses show that there is no correlation between d and reflectance phase curve in the phase angle range from 0° to 6°. In addition, we analyze the distribution of grain size of lunar samples collected during the Apollo and Luna missions. We find an inversely proportional correlation between mean grain sizes and submicron grain content in the lunar samples. Therefore, d seems to indicate the content of effective scatterer. The strength of the coherent backscattering opposition effect seems to be affected by the content of the submicron grains. Thus, the lack of correlation between d and the opposition effect seems to imply minimal contribution of coherent backscattering to the lunar opposition effect.

Effect of Sulfuric Acid Baking and Caustic Digestion on Enhancing the Recovery of Rare Earth Elements from a Refractory Ore

To improve the recovery of rare earth elements (REEs) from a refractory ore, this study investigated two different chemical decomposition methods, namely sulfuric acid baking and caustic digestion, with their respective leaching processes. The studied lateritic ore contained goethite (FeOOH) as a major constituent with REEs scattered around and forming submicron grains of phosphate minerals, such as apatite and monazite. Therefore, despite the substantially high content of REEs (3.4% total rare earth oxide), the normal acidic leaching efficiency of REEs reached only 60–70%. By introducing sulfuric acid baking and caustic digestion, the REE-leaching efficiency was significantly improved. After sulfuric acid baking at 2.0 acid/solid ratio and 200 °C for 2 h, the leaching efficiency reached 97–100% in the subsequent water-leaching. When the ore was digested with a solid/liquid ratio of 100 g/L in a 30 wt% NaOH solution at 115 °C and 300 rpm for 3 h, the REE-leaching efficiency of 99–100% was attained at 80 °C using a 3.0 M HCl solution. The correlation between the REE and the Fe-leaching was determined. The improvements in REE-leaching in both methods were mostly attributed to the mineral phase and crystallinity changes of Fe-bearing minerals due to the ore pretreatments. Such findings were also supported by X-ray diffraction and scanning electron microscopy analyses.

Effects of combined use of inoculation and modification heat treatment on microstructure, damping and mechanical properties of Zn–Al eutectoid alloy

Inoculation and modification heat treatments were conducted on the Zn–Al eutectoid alloy (ZA22) with the aim of grain refinement. Evolution law of microstructure, and changes of damping and mechanical properties of resultant specimens were also studied. It was found that inoculation could refine the primary α phase and effectively improved the damping arising from the reverse eutectoid transformation, as well as the tensile strength and elongation. After modification heat treatments, all original phases and structures disappeared completely. The newly formed ZA22 alloy only consisted of sub-micron granular eutectoid structure. The significantly increased density of interfaces and the change in α/η interface microstructure resulted in remarkable improvement of damping over the whole temperature range. The damping of all ZA22 alloys showed obvious strain amplitude dependence in two strain regions, which has been ascribed to the depinning of sliding interfaces and the dynamic recrystallization due to microplasticity at the surface of the specimens, respectively. Sub-micron grains and uniform composition of granular eutectoid structure could effectively promote the sliding of grain boundaries, which leaded to the appearance of room temperature superplasticity.

Enhanced Mechanical Properties of a Gradient Nanostructured Medium Manganese Steel and Its Grain Refinement Mechanism

As the third generation of advanced high strength steel (AHSS), medium manganese steel (MMS) has been widely emphasized by scholars around the world. Presently, we applied sliding friction treatment (SFT) of severe plastic deformation (SPD) on the surface of MMS to form surface gradient nanostructures, the formation mechanism of microstructure and the corresponding mechanical behavior was studied. The results show that the deformation layer can be divided into nano-grain (NG), submicron grain (SMG) and coarse grain (CG) in terms of grain size. It has been demonstrated that in the CG layer and a part of SMG layer, new fine grains can be formed through discontinuous dynamic recrystallization (DDR) mechanism, while continuous dynamic recrystallization (CDR) is a favorable nucleation mechanism for the new formed small grains in the SMG layer and the NG layer. The SFT process increases microhardness sharply in the surface region. Compared with conventional MMS, it is apparent that the yield strength (YS) and the ultimate tensile strength (UTS) of gradient medium manganese steel specimens have been greatly improved, while the elongation does not decrease significantly. Fracture surface analysis demonstrates that the fracture morphology of different layers can be generally characterized by different fracture mechanisms, i.e., cleavage, quasi-cleavage and dimple.

Expanded linear polarization response and excellent energy-storage properties in (Bi0.5Na0.5)TiO3-KNbO3 relaxor antiferroelectrics with medium permittivity

Dielectric materials with both high recoverable energy-storage density Wrec and efficiency η have attracted a lot of attention in recent years. Permittivity plays a crucial role in simultaneously achieving high polarization strength and large applied electric fields for any kinds of dielectric materials, among which antiferroelectric ceramics exhibit giant potentials in energy storage. Adjustment of permittivity has seemed to be more in linear dielectrics, but much less cared about in polar dielectrics so far for high-power energy-storage applications. In this work, a novel lead-free solid solution of (1-x)(Bi0.5Na0.5)TiO3-xKNbO3 was reported to show excellent comprehensive energy-storage performances of Wrec of ~5.2 J/cm3, large η of ~88%, fast discharge rate of t0.9 < 200 ns and outstanding temperature and frequency stability at x = 0.16. The X-ray, Raman spectra and scanning electron microscopy and so on demonstrate that it was basically ascribed to the formation of relaxor antiferroelectric phases with temperature-stable dielectric response and expanded linear polarization response, as well as enhanced dielectric breakdown strength induced by sub-micron grains. Particularly, selection of an appropriately medium permittivity value of about one thousand enables the studied AFE composition bring its superiority into full play in energy-storage properties, as demonstrated by a comparison between Wrec values of most previously-reported lead-free ceramics and theoretically predicted ones for linear dielectrics. These findings achieved in current work would provide an important guidance for compositionally designing high-performance energy-storage dielectrics in terms of modifying dielectric properties and polarization responses, particularly for relaxor antiferroelectrics with linear-like characteristics.

Effects of alumina sols on the sintering of α-alumina ceramics

The effects of two kinds of alumina sols on the densification behavior of sub-micron grain sized α-alumina ceramics have been investigated. Composition of the sol-derived gels was investigated by energy dispersive spectra and Fourier transform infrared spectra. Structural evolution of the gels at different temperatures was characterized by a combination of X-ray diffraction and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. Results showed that the gel containing chlorine and carbon transformed to α-alumina at about 950 °C, significantly lower than the other gel which transformed at about 1050 °C. Density measurements and scanning electron microscopy analyses were used to investigate the sintering of alumina ceramics with or without alumina sols. It was found that the alumina sols had profound effects on the densification of alumina ceramics. The ceramic displayed the best densification behavior when the sol containing chlorine and carbon was added.

DSC Cycling Effects on Phase Transformation Temperatures of Micron and Submicron Grain Ni50.8Ti49.2 Microwires

The effect of thermal cycling parameters on the phase transformation temperatures of micron and submicron grain size recrystallized Ni–Ti microwires was investigated. The suppression of martensitic transformation by thermal cycling was found to enhance when combined with room temperature aging between the cycles and enhances even more when aged at elevated temperature of 100 °C. While aging at room temperature alone has no clear effect on the martensitic transformation, elevated temperature aging at 100 °C alone suppresses the martensitic transformation. All aforementioned effects were found to be stronger in large grain samples than in small grain samples. Martensitic transformation suppression in all cases was in line with the formation of Ni4Ti3 precursors in the form of 〈111〉B2 Ni clusters as concluded from the observed diffuse intensity in the electron diffraction patterns revealing short-range ordering enhancement. Performing thermal cycling in some different temperature ranges to separate the effect of martensitic transformation and high temperature range of DSC cycling revealed that both high temperature- and martensitic transformation-included cycles enhance the short-range ordering.

Correlation between microstructural heterogeneity and energy product in hot deformed Nd-Fe-B magnets

We systematically investigated the local crystallographic texture, magnetic domain structure, and the associated magnetic reversal process of different regions, namely fine-grain (FG), coarse-grain (CG), and large-grain (LG) regions in hot-deformed Nd-Fe-B magnets. The FG region possesses the strongest (00 l) texture and plays a dominant role on the magnetic properties. In contrast, the CG region with partially aligned sub-micron grains and the LG region with randomly aligned micro-sized grains exhibit much weaker (00 l) texture, where the reversal magnetic domains primarily nucleate, deteriorating the remanence and the squareness of the demagnetization curve seriously. This finding highlights a new strategy to achieve high energy density in hot-deformed Nd-Fe-B magnets, i.e. maximizing the volume fraction of highly (00 l) textured FG regions and reducing the undesirable CG and LG regions. Our further experiment enhancing the maximum energy product of the magnet from 42.6 to 51.1 MGOe through optimizing the microstructure homogeneity unambiguously demonstrates the general applicability of the strategy.

The effect of submicron grain size on thermal stability and mechanical properties of high‐entropy carbide ceramics

Abstract(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two‐step sintering process. Both X‐ray and neutron diffractions confirmed the formation of single‐phase with rock salt structure in the as‐fabricated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine‐grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse‐grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine‐grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine‐grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.

Mg-Based Materials with Quasiamorphous Phase Produced by Vertical Twin-Roll Casting Process

Metallic materials with micron grains, submicron grains, or amorphous structures have attracted great interest in recent decades owing to their excellent mechanical properties and corrosion resistance. Compared with traditional forming processes, rapid solidification technology has shown great superiority and potential in the preparation of materials in such structures. In this study, fine-grained quasiamorphous Mg-based alloy strips fabricated by a twin-roll strip casting process were explored using simulation and experimental methods. The concept of critical casting speed was proposed to reflect the optimum casting conditions. The product of critical casting speed and strip thickness was used to evaluate the cooling capacity of the casting system. Based on simulation results, a twin-roll strip-casting experiment was performed on a Mg-rare earth alloy. A novel puddle-like microstructure of the as-cast alloy strip was obtained. Tensile testing results showed that the novel strip exhibited improved ductility.