Non-uniform Grain Size Distribution Research Articles

Variations of thermal properties with water content and density of residual and basaltic sands from Chile

Renewable energy has become a relevant alternative to solve energy and environmental problems worldwide. Characterization of geothermal resources is fundamental for an efficient and sustainable extraction of heat. In this study, thermal conductivity, λ , volumetric heat capacity, C , and thermal diffusivity, α , of two soils from Chile, namely Bío Bío sand and Maicillo residual soil, were analysed. Bío Bío sand is a uniform and clean sand that mainly consists of basaltic particles, whereas Maicillo soil is a silty and clayey sand with a high quartz content. Samples were tested with a thermal needle probe at varying water contents and densities. Measurements to obtain λ , C and α were undertaken. These results allowed the patterns and relationships between geotechnical and heat transfer parameters to be analysed. It was found that dry soil conditions led to the lowest values of λ , while saturated soil conditions led to the highest values. Moreover, Maicillo soil has higher capabilities to transfer heat than Bío Bío sand, and can reach up to 50% greater λ for dense soil and saturated conditions. This is due to its quarzitic and clayey mineralogy and non-uniform grain-size distribution. The results from this research represent an important contribution to industrial applications.

The Influence of Short-Term High Temperature Environment on the Non-Uniform Distribution of Ferrite Grain Size in 40 mm-thick Q345 Steel

In order to reveal the non-uniform distribution of grain size in thick direction for engineering heavy plate, microstructure of 40 mm-thick Q345 steel was observed and measured under different short-term high temperature environments formed by fire. Moreover, the influence of the short-term high temperature environment was revealed on the distribution of ferrite grain size in the Q345 steel. Under different fire service environments, there was a log-normal distribution relationship between the distribution parameter Nf (number of ferrite grains) and df (average grain diameter), as well as ρAf (area fraction density) and df, at different positions along the thickness direction. However, the statistical results are greatly affected by the length of the statistical interval. When df is about 4 to 6 times the length of the statistical interval, the statistical accuracy is higher. By using nonlinear fitting method, multiple non-uniform distribution empirical models including Nf-df empirical formulas and ρAf-df empirical formulas were established at different positions along thick direction under various fire environments. Furthermore, the interrelationships between fire temperature T and Nf , T and ρAf , fire duration t and Nf , t and ρAf were revealed, respectively.

Influence of Nonuniform Microstructure on High‐Cycle Fatigue Properties of the Electron‐Beam‐Welded Joint of High‐Strength Steel

The relatively weak fatigue property of electron beam (EB)‐welded joints is one of the key issues restricting their wide application. Herein, the impact of accelerating voltage, specifically the process‐induced nonuniform microstructure, is investigated on the high‐cycle fatigue properties of 15CrMnMoVA steel EB‐welded joints. It is indicated in the results that increasing the accelerating voltage from 120 to 150 kV (150‐specimen) results in a longer fatigue life and an 11.8% improvement in fatigue limit for the welded joint with the locking bottom structure. However, even after removing the locking bottom structure, the fatigue life of the 150‐specimen is still inferior to that of the base metal. The shorter fatigue life is primarily due to stress concentration and premature fatigue failure caused by the nonuniform distribution of grain size and crystallographic orientation across the EB‐welded joint. Thus, further optimization of the welding process is necessary to improve the fatigue reliability of EB‐welded joints by obtaining a more uniform microstructure.

Reducing surface roughening in microscale laser dynamic flexible bulging by using laser pre-shocking

Roughening of the free surface in micro-forming results in the local necking and subsequent fracture, which seriously affects the formability of sheet metals. In this work, laser pre-shocking (LPS) was introduced into microscale laser dynamic flexible bulging (μLDFB) to reduce surface roughening in terms of reducing the surface roughness and refining the internal grains. In LPS experiments, when the laser energy is 447 mJ, the rebound of the copper foils can be negligible. LPS can be expected to suppress the surface roughening by reducing the initial roughness R0, increasing the N value and decreasing the rate of surface roughening. The copper foils subjected by 447 mJ LPS were used for the μLDFB experiments. The 50 μm T2 copper foils with four different grain sizes were used in μLDFB under different laser pulse energies. When laser pulse energy ranges from 325 mJ to 575 mJ, compared with μLDFB without LPS, the surface roughness on the bottom surface of the bulging sample is decreased. And the nano-hardness, elastic modulus and forming uniformity are significantly improved. In μLDFB experiments without LPS, for fine-grained samples, the cracks initiate in the bulging transition zone under 725 mJ laser energy due to surface roughening. As laser energy increases to 1050 mJ, the cracks initiate in the mold entrance zone due to the strong stress concentration. For coarse-grained samples, cracks initiate under 575 mJ laser energy, and the propagations of the cracks are difficult to predict due to non-uniform distribution of grain size and the difference of grain orientation. The experimental results suggest that LPS can reduce the possibility of the premature fracture caused by the local necking, and alleviate the uncertainty in the fracture mode.

Grain size heterogeneity controls strengthening to weakening of granite over high-temperature treatment

Deep geological repository is usually constructed in a typical crystalline rock , granite . Therefore, comprehensive understanding on the thermo-mechanical properties of granite exposed to high temperature is crucial to repository design and risk assessment. Granite experiences a transition from strengthening to weakening as temperature increases; however, the critical temperature at which this transition occurs is still poorly known. Here we examined the physical and mechanical variations of granite specimens of three different grain grades. The specimens sourced from the same origin are heated up to 800 °C. Grain size distribution, thermal-induced cracks, and crack evolution under uniaxial compression of the heated specimens were examined by XRF (X-ray fluorescence spectrometric analysis, with a scanning capacity of 50 mm at a spatial resolution of 20 μm), SEM (Scanning electron microscope) and AE (Acoustic emission), respectively. We found that grain size heterogeneity quantified by the coefficient of variation (CV) modulates the critical temperature magnitude, and the granite specimen with a higher CV exhibits a lower critical temperature. The cracks reducing the granite strength are induced by grain mismatch due to inconsistent thermal expansion. Nonuniform distribution of grain size enhances the mismatch and broadens the favorable path for crack propagation and coalescence . Thermally-induced microcracks appear earlier in the more heterogeneous granite, which propagate and coalesce to form visible cracks earlier, leading to an earlier strength degradation. We also proposed an empirical formulation to predict the strength of granite with different grain size distribution and thermal environment . Our work is crucial for the site location and design of the deep geological repository since establishment of many construction parameters heavily depends on laboratory-determined properties of granite. • Luhui granite specimens of various grain size grades after four high-temperature treatments are experimented. • Grain size heterogeneity controls the critical temperature at which a transition from strengthening to weakening occurs. • We propose an equation considering grain size distribution to predict the strength of granite exposed to high temperature.

Characteristics of the pollen morphology and viability of Bougainvillea (Nyctaginaceae)

Pollen grains of 92 Bougainvillea genotypes were examined using a light microscope (LM) and scanning electron microscope (SEM) to identify taxonomic characteristics and the relationship between pollen morphology and germination capacity. The shapes of fresh pollen grains of Bougainvillea genotypes are spheroidal to prolate-spheroidal or suboblate after infolding. The sexine of Bougainvillea pollen grains consists of columellae and a reticulate cristatum, with or without microechini. The distribution pattern of the tectum microechini is uniform on the pollen grains within each genotype, yet it is variable among the different genotypes. Pollen with a smooth reticulate cristatum was observed in the genotypes of B. peruviana, while pollen with muri covering the cristatum were found in the genotypes of B. glabra and B. spectabilis. The pollen grains of each Bougainvillea germplasm have a wide range of diameters. The mean diameter of the pollen grains of the diploid genotypes ranged from 22.2 to 32.1 μm, while that of the polyploid genotypes ranged from 26.7 to 40.4 μm. The germinated pollen grains were observed in the diploid and tetraploid genotypes, the pollen germination rate of which was commonly less than 20 %. The size of germinated pollen grains was significantly larger than that of the silent ones, which indicated that the nonuniform distribution of grain size is a key reason for the low germination capacity of Bougainvillea pollen.

In situ investigation of annealing effect on thermophysical properties of single carbon nanocoil

In reported high-temperature annealing of carbon nanocoils (CNCs), the samples studied before and after annealing are different ones. This significantly hinders annealing effect understanding due to unknown and remarkable sample-to-sample structure difference. Here using the transient electro-thermal technique (TET) and current-induced annealing, we report the first time in situ investigation of annealing effect on the thermophysical properties for the same individual CNC. Our dynamic annealing track uncovers an electrical resistance relation with annealing time as R∼−Rsln(t). The reaction rate (Rs) shows a normal distribution against the annealing power/temperature, proposing that the activation energy for structure reconstruction in CNCs follows a normal distribution. After annealing at 5–35€μA, the average thermal diffusivity (α) and electrical conductivity (σ) of CNCs show respective 50–160% and 100–170% increase. Normative linear relation between α and σ is discovered, which proposes axial-direction parallel structure in CNCs. The nonuniform temperature distribution along the sample during annealing creates different annealing levels and provides a great advantage to study the relation between structure and thermophysical properties. Our micro-scale Raman characterization reveals the nonuniform distribution of grain size along the length direction of CNCs after annealing and finds a rapid grain size increase from 4.0 to 7.8 nm near the sample's middle point. The middle point of the sample has the highest temperature rise (Tc), largest thermal conductivity (κ) increase, and the most dramatic structure improvement. Its κ shows a rapid improvement (8.7-fold maximum change) from 1200 K to 1800 K. A linear relation between κ−1 and Tc is observed and is attributed to the change of grain size during annealing. Using the concept of thermal reffusivity (Θ=1/α), a 1-fold increase of average grain size and a 197 K decrease of Debye temperature of CNCs after annealing are uncovered.

A comprehensive parametric study of grain-based models for rock failure process simulation

Grain and contact are the two key components in 3DEC-GBMs (grain-based models) because they control the micro-mechanical behavior and consequently the macro-mechanical behavior of rock. Three types of grains – rigid, elastic, and breakable grains – are considered in this study to explore the influence of grain properties on the mechanical behavior of rock. Equivalent 2D plane strain problems are solved using 3DEC to reduce computation time. A non-uniform distribution of grain size is used to generate numerical models using Neper, a software package for polycrystal generation and meshing. A comprehensive parametric study of properties of contact and grain of the three types of grains is conducted and calibration procedures are suggested for numerical modeling. The modeling results indicate that both the contact and the grain properties affect the macroscopic mechanical behavior of synthetic rock specimens. All the three types of grains produce reasonable results for pre-peak deformation (E and υ) and macroscopic strength (UCS, σt, C and ϕ) parameters. Each grain type has advantages and disadvantages in numerical modeling. Rigid grains cannot produce volumetric strain properly and a near-zero residual friction angle of contact is needed to capture post-peak deformation behavior due to the grain interlocking effect. Elastic grains also need a very low residual friction angle of contact and brittle rock deformation behavior cannot be captured. The complex calibration procedure and long computation time are the main issues of breakable grains, although it simulates well the volumetric strain, does not require a very low contact residual friction angle, and captures brittle rock behaviors better than other two types of grains. The findings from this study are useful for rock failure process simulation using the 3DEC-GBM modeling approach.

Drag Effects of Solute and Second Phase Distributions on the Grain Growth Kinetics of Pre-Extruded Mg-6Zn Alloy

In order to control the grain size during hot forming, grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated. Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator. The mean grain size Dg of each annealed specimen was measured by the quantitative metallography technique. The grain growth kinetics of the Mg-6Zn alloy annealed at 473–623 K was obtained as D g 4 − D g 0 4 = 2.25 × 10 11 exp ( − 95450 / R T ) t by the least square linear regression method. The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration. Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc. This indicates that second phase pinning effect plays an important role in microstructure refinement.

3D grain structure modelling of intergranular fracture in forged Haynes 282

In this paper, 3D grain structure finite element models are used to simulate the anisotropic tensile ductility of Ni-based superalloy Haynes 282. The anisotropic ductility is believed to be caused by a nonuniform distribution of both grain size and large carbides, which influences the intergranular fracture of the material. To simulate this anisotropic ductility, grain structure models with nonuniform grain size distribution are generated by Voronoi tessellation. Additionally, the mechanical behaviour of the models is governed by crystal plasticity and the intergranular fracture is simulated by cohesive elements embedded in the grain boundaries. The paper investigates how the shape and orientation of regions with smaller grains in the grain structure model influence the cracking behaviour. Furthermore, the study also focuses on modelling the embrittling impact of large carbides in the grain boundaries. Results from the simulations qualitatively correspond to experimental results, which supports the assumption that the nonuniform grain size distribution and the large carbides cause the anisotropic tensile ductility seen in forged Haynes 282.

Study on exit temperature evolution during extrusion for large-scale thick-walled Inconel 625 pipe by FE simulation

The extruded large-scale thick-walled pipes made from Inconel 625 alloy are widely used in many fields such as in the oil and chemical industry, thermal power generation, nuclear power plant, aerospace, and defense industry. However, the nonuniform distribution of grain size along the thickness and length directions of the extruded pipe always appears, which has a great influence on the mechanical property and uniformity of the pipe. The inhomogeneous distribution of the exit temperature in the entire extrusion process is the decisive factor leading to the grain nonuniformity of the extruded pipe. Therefore, it is necessary but difficult to reveal the evolution of the exit temperature under some key extrusion parameters in order to effectively control the exit temperature uniformity. In this paper, a thermomechanical coupled finite element (TMC-FE) model, which can precisely predict the hot deformation behavior of the extrusion process for large-scale thick-walled Inconel 625 pipe, has been firstly developed based on the DEFORM-2D platform. And then, the influence of key extrusion parameters, namely extrusion speed (V), initial billet temperature (T b), extrusion ratio (λ), and friction (μ), on the exit temperature was revealed by comprehensive simulations. In addition, using the standard deviation of exit temperature (SDexit) as the measure indicator of the exit temperature uniformity, we also disclosed the influence of the key process parameters on the exit temperature uniformity in detail. The results show that with the increase of V, T b, and μ, the uniformity of exit temperature in the entire process firstly becomes better and then gets worse. The larger λ can result in a more uniform exit temperature. In consideration of the obtained results, within the feasible window of the key extrusion parameters defined in this paper, the conditions of T b = 1,100–1,150 °C, V = 125–150 mm/s, μ = 0.015–0.02, and a larger extrusion ratio (λ) are suggested for the extrusion of large-scale thick-walled Inconel 625 pipe in order to get more uniform exit temperature.

Crystallization kinetics of amorphous Nd3.6Pr5.4Fe83Co3B5 and preparation of α-Fe/Nd2Fe14B nanocomposite magnets by controlled melt-solidification technique

The crystallization kinetics of amorphous Nd3.6Pr5.4Fe83Co3B5 and the preparation of α-Fe/Nd2Fe14 B nanocomposite magnets by controlled melt-solidification of Nd3.6Pr5.4Fe83Co3B5 was investigated by employing DTA, XRD, and TEM. The results show that a metastable intermediate phase Nd8Fe27B24 prior to α-Fe and Nd2Fe14B phases is crystallized as the amorphous Nd3.6Pr5.4Fe83Co3B5 is heated to 1 223 K. The crystallization activation energy of α-Fe and Nd8Fe27B24 phases is larger at the beginning stage of crystallization, and then it decreases with crystallized fraction x for the former and has little change when x is below 70% for the latter, which essentially results in an α-Fe/Nd2Fe14B microstructure with a relatively coarse grain size about 20–60 nm and a non-uniform distribution of grain size in the annealed alloy. The α-Fe/Nd2Fe14B nanocomposite magnets with a small average grain size about 14 nm and a quite uniform grain size distribution were prepared by controlled melt-solidification of Nd3.6Pr5.4Fe83Co3B5 at a wheel speed of 20 m · s−1 during melt-spinning. The magnets show a high maximum energy product of (BH)max=194 kJ · m−3, which is nearly twice of that of the nanocomposite magnets made by annealing the amorphous Nd3.6Pr5.4Fe83Co3B5 precursor alloy.

Structure of plasma-sprayed oxides in the MgO-Al2O3-SiO2 system

Cordierite (2MgO · 2Al2O3-SiO2), forsterite (2MgO · SiO2) and spinel (MgO · Al2O3) were rapidly solidified into a thick film through plasma spraying. The crystal structure of the assprayed deposits was examined using X-ray diffractometry and transmission electron microscopy, and shows that as-sprayed cordierite is amorphous, as-sprayed spinel is a crystalline phase, and as-sprayed forsterite is a mixture of amorphous and crystalline phases. Moreover, transmission electron microscopy reveals uniform amorphous cordierite, a nonuniform grainsize distribution of crystalline spinel and a twinned texture of as-sprayed forsterite.