Dynamic recrystallization of ferrite in a low-carbon steel

Plane strain compression tests were performed on a low-carbon steel from 550 °C to 700 °C (ferritephase range) at strain rates of 10 to 5 × 10⁻⁴ s⁻¹, and the deformation microstructure evolution was investigated by means of scanning electron microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results indicate that under the present deformation conditions, dynamic recrystallization of ferrite can occur in the low-carbon steel and lead to grain refinement. With increasing Zener-Hollomon parameter Z, the mechanism of this process changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization; the turning point is approximately at Z=1 × 10¹⁶ s⁻¹. The increase of parameter Z leads to the decrease of recrystallized grain size of ferrite under steady state of deformation, and can lead to the formation of ultrafine microstructures with average grain size of about 2 μm.     

Synthesis of nanocrystalline Zn-22 Pct Al using cryomilling

In the present investigation, the synthesis of nanocrystalline Zn-22 pct Al by ball milling was studied. The microstructural evolution during cryomilling and subsequent annealing was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Observations made during the cryomilling of the alloy reveal three findings. First, minimum average grain sizes of about 33 nm for the Al phase and 41 nm for the Zn phase are reached as cryomilling time increases to 16 hours. Second, the morphology of the powders changes from spherical (as-sprayed) to flaky (milled 8 hours) and then back to spherical again (milled 16 hours). Third, the microstructure transforms from two-phase coarse structure (0.8 μm, as-sprayed) to bimodal structure (milled 8 hours) and then to a uniform fine-grained structure (milled 16 hours). The minimum grain size characterized by the peak broadening of the XRD patterns is much larger than that reported in previous work on Al and Zn but agrees well with those predicted from the approximate linear relationship between the minimum grain size and the critical equilibrium distance between two edge dislocations in a pileup. The mechanism of grain size refinement is discussed at three different levels: macroscopic level (individual powders), mesoscopic level (individual small fragments), and microscopic level (individual grains). The excellent thermal stability of the milled powders during subsequent annealing has been attributed to three factors: the nature of the eutectoid structure, grain-boundary pining by dispersions, and microporosity in the particles.     

Grain growth of nanocrystalline Ni powders prepared by cryomilling

Grain growth of nanocrystalline Ni powders with an average grain size of ∼22 nm prepared by cryogenic mechanical milling (or cryomilling) was investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). A dispersion of NiO and Ni₃N particles with a size less than 5 nm was formed in the cryomilled powders. The Ni₃N particles decomposed at 773 K. It was found that at 0.56 homologous temperature (T/T _M ), Ni grains were retained at ∼150 nm even after long annealing times (e.g., 4 hours). For 0.45 to 0.62 T/T _M , the time exponent n deduced from D ^1/n −D ₀ ^1/n =kt was 0.16 to 0.32, tending toward 0.5 as T/T _M increased. The activation energy for grain growth in the Ni sample was determined to be 113 kJ/mol, which is close to the activation energy for grain boundary self-diffusion in polycrystalline Ni. The observed high grain size stability was attributed primarily to a grain boundary pinning mechanism arising from the NiO particles as well as impurity segregation.     

球磨时间对纳米晶Al-7Si-0.3Mg合金粉末微观组织及硬度的影响

以机械破碎Al-7Si-0.3Mg合金粉末为原料进行高能球磨, 对不同球磨时间的合金粉末进行金相观察、X射线衍射分析、透射电镜表征及显微硬度测试, 研究球磨时间对纳米晶Al-7Si-0.3Mg合金粉末的影响。结果发现, 高能球磨导致共晶硅颗粒从微米尺度细化到亚微米尺度, 颗粒形状从多面体转变成圆形, 颗粒内部有层错生成。随着球磨时间逐渐增加到60 h, 合金粉末平均颗粒尺寸从134μm逐渐下降到22μm, Al(Si, Mg)基体晶粒尺寸从438 nm降低到23 nm, 粉末显微硬度从HV 93增加到HV 289。粉末硬度的增加主要归功于球磨导致的晶粒细化(细晶强化作用), 此外, 球磨过程中硅颗粒的细化以及球磨引起的Mg、Si原子在基体内固溶度的增加也有利于粉末硬度的提高。     

Nanocrystalline Ni coatings strengthened with ultrafine particles

In this study, nanocrystalline Ni powders and thermally sprayed coatings, containing ultrafine AlN particles, were synthesized and characterized. The results indicated that the presence of AlN particles in the powders drastically decreased the dimension of agglomerates formed by cryomilling and increased the surface roughness of the agglomerates. The AlN phase was broken down into ultrafine particles of approximately 30 nm in size. These particles were dispersed in the Ni matrix and enhanced the development of a nanocrystalline structure in the Ni matrix during cryomilling. Selected-area diffraction patterns, obtained from transmission electron microscopy (TEM) and X-ray mapping with scanning electron microscopy (SEM), confirmed the presence of AlN particles in the coatings. The presence of AlN particles also led to an increase in the amount of NiO phase that was distributed in the coating, in the form of ultrafine, round particles. AlN particles increased the microhardness of the Ni coating by approximately 60 pct. Indentation-fracture results also indicated that the fine, dispersed AlN particles raised the apparent toughness of the Ni coating. The synthesized Ni coatings containing ultrafine AlN particles were characterized as equiaxed nanocrystalline grains with an average size of 24 nm, in which twins were observed. The increase in microhardness resulted from both grain refinement and the presence of ultrafine particles. The latter played the primary role in strengthening.     

Application of Thermohydrogen Processing for Formation of Ultrafine Equiaxed Grains in Near <Emphasis Type="Italic">α</Emphasis> Ti600 Alloy

In this article, the grain refinement of the near α Ti600 alloy by thermohydrogen processing (THP) is investigated. The THP processing includes hydrogenation, high-temperature deformation, and vacuum dehydrogenation. The microstructural evolution in each stage of THP processing was carefully investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The experimental results show that face-centered-cubic (fcc) hydride (δ phase) precipitates in the hydrogenated Ti600 alloy at the hydrogen contents of 0.31 wt pct and above. The critical role of hydrides in the processing of grain refinement is clearly demonstrated. The hydrides divided the α grains in hydrogenation treatment, promoted α-phase recrystallization, and facilitated the stabilization of finer α grain size in isothermal compression.     

用于YAG透明陶瓷的喷雾造粒改性粉体的制备

通过喷雾造粒方法对共沉淀合成的纳米粉体进行改性,制备出球形的纳米颗粒.用XRD对粉体进行物相分析;用TEM观察了改性前粉体的颗粒形状、尺寸大小和团聚情况;用SEM观察改性前后粉体的团聚体的颗粒形状、尺寸大小与分散性,以及陶瓷热腐蚀抛光后的表面形貌.结果表明:PVB添加质量分数1.0%为最优添加量;改性后粉体所制素坯的密度显著提高,从而影响陶瓷的致密度和晶界形貌;经真空烧结制备出相对密度达99.95%的无孔净晶界YAG透明陶瓷,陶瓷晶粒的平均尺寸为10μm左右,尺寸分布较均匀,晶界清晰,晶粒中与晶界处较干净,无杂质与气孔的存在.     

Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment

Nanocrystalline INCONEL 625 powders were fabricated via cryomilling (mechanical alloying under a liquid nitrogen environment), and their grain growth behavior during isothermal heat treatment was investigated in detail. The grain size after milling for 8 hours was approximately 22 nm, measured by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD). Along with this refined structure, the NiO and Cr₂O₃ oxide particles were distributed in the cryomilled material with average size of 3 nm. Following heat treatment at 800 °C, correspond to T/T _m = 0.65, for 4 hours, the grain size was approximately 240 nm, which represents an improved grain stability compared to that of conventional INCONEL 625 and cryomilled pure Ni. The improved grain stability of cryomilled INCONEL 625 is originated from a particle pinning effect by the oxide particles in addition to solute drag. The grain stability of the cryomilled powders at 900 °C was better than that at lower temperatures. This behavior was attributed to the formation of two types of secondary particles that precipitated at this temperature, which were identified as spherical NbC carbides and cylindrical-shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain-boundary pinning effect. Contribution of 30 pct Nb solute atoms in alloy on the forming precipitates on grain boundary, the grain growth will be restricted to approximately 200 nm, on the basis of a Zener mechanism. This calculation is in qualitative agreement with the experimental results. The observation that precipitation kinetics were accelerated over those of conventional INCONEL 625 was rationalized on the basis of the shortened diffusion paths and more nucleation sites available in the nanocrystalline materials.     

Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy

The microstructural evolution and grain refinement mechanisms of a Ti-10V-2Fe-3Al alloy, β-solution quenched and cold forged (CF) to strains of 0.1, 0.35, and 1.2 have been investigated using optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the stress-induced martensitic transformation became a predominant deformation mode in the metastable Ti-10V-2Fe-3Al alloy during cold forging. These martensites α″ repeatedly divided the original β parent phase into a large number of micron-sized blocks when the forging strain was 0.1. Shear bands were observed to traverse α″/β lamellae and resulted in a significant grain refinement of the β phase, as the forging strain increased to 0.35. The degree of grain refinement inside shear bands was higher than the outside. Nanocrystalline and amorphous structures were produced in local areas of the original β phase, when the forging strain rose to 1.2. This dramatic grain refinement in the metastable Ti-10V-2Fe-3Al alloy could be attributed to the stress-induced martensitic transformation promoting the initiation and growth of shear bands across α″/β lamellae. More dislocations were produced and accumulated inside grains to accommodate plastic deformation. The crystal structure was collapsed and an amorphous structure was formed as soon as the dislocation density was accumulated to a critical value of 10¹⁴/cm². Moreover, some of the reverse martensitic phase transformation, α″→β, was observed to contribute to grain refinement of Ti-10V-2Fe-3Al alloy as well.     

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T _mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al₂O₃ particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.     

Influence of cetyltrimethylammonium bromide and sodium lauryl sulfate on production of zinc powders by alkaline electrowinning

The effects of cetyltrimethylammonium bromide (CTABr) and sodium lauryl sulfate (SLS) on the current efficiency, cell voltage, voltammetric behavior, and deposit character during production of zinc powder by alkaline electrowinning were investigated. Surface morphologies, crystallographic orientations, and grain size distributions of zinc powders were examined by Scanning Electron Microscopy, X-ray Diffraction, and Malvern Laser Particle Size Analyzer, respectively. Maximum current efficiencies of 97.5 and 98.3% were obtained at 1 mg/L CTABr and 10 mg/L SLS, respectively. Voltammetric studies indicated that the surfactants polarized the cathode, which was reflected in powder character. The surfactants dramatically changed the surface morphology and decreased the grain size. Zinc powders with an average grain size of 34.7 μm were obtained in the presence of 10 mg L^?1 CTABr. At 0–10 mg L^?1 CTABr/SLS, zinc powders exhibited an orientation dominated by (101) (002) (100). 1 mg L^?1 SLS hadapositive effect on counteracting the influence of Sb.     

A comparative study on physicochemical properties of hydroxyapatite powders derived from natural and synthetic sources

Hydroxy apatite (HA) is effectively used as a bioimplant material because it closely resembles bone apatite and exhibits good biocompatibility. So, in this research, HA powders were produced by calcinations of natural bones including human, bovine, camel and horse bones, and also via sol-gel method. Powders characterizations of natural HA and Synthetic HA were studied by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM), combined with transmission electron microscopy (ТЕМ). In order to verify the biocompatibility of these HA powders, MTT assay was applied. The XRD results showed that the HA powders were successfully produced by using different sources. Also, it was obvious from XRF analysis that the main components of them were Ca and P. Furthermore, it was seen that the size of particles was in the nanometric scale and they showed agglomerates consisting of numerous nanocrystals. FTIR spectra of all samples proved the presence of various CO ₃ ^2- , PO ₄ ^3- and OH^– groups in the powders. In addition, the MTT assay revealed that the cells proliferations in the presence of horse and human HA nanopowders were stimulated.     

Recrystallization in Commercially Pure Aluminum

Recrystallization behavior in commercial aluminum with a purity of 99.4 pct was studied by techniques such as high voltage electron microscopy, 100 kV transmission electron microscopy, and light microscopy. Sample parameters were the initial grain size (290 and 24 microns) and the degree of deformation (5 to 30 pct reduction in thickness by cold-rolling). It was found that the original grain boundary region is the preferred site for nucleation. A few intragranular nuclei were, however, also observed. The effectiveness of the nucleation sites is enhanced by the presence of intermetallic particles (FeAl₃), which start to become operative when the degree of deformation is raised from 15 to 30 pct. The temperature of nucleation and of recrystallization decreases when the degree of deformation is increased and the initial grain size is decreased. The recrystallized grain size follows the same trend and it is observed that the refinement of the recrystallized grain size caused by an increasing degree of deformation and decreasing initial grain size is enhanced by the FeAl₃ particles (when the degree of deformation is raised from 15 to 30 pct). Finally, the structural and kinetic observations are discussed and compared with results from an earlier study¹ covering the recrystallization behavior of commercial aluminum of the same purity deformed at higher degrees of deformation (50 to 90 pct reduction in thickness by cold-rolling).     

Evaluation of Al-Ti-C Master Alloys as Grain Refiner for Aluminum and Its Alloys

Al-Ti-C master alloys have a great potential as efficient grain refiners for aluminum and its alloys. In the present work, the Al-Ti-C master alloys are synthesized via a relatively novel technique through the reaction of a compacted mixture of K₂TiF₆ and graphite with molten aluminum. The obtained alloys are examined using scanning electron microscopy (SEM), energy-dispersive spectroscopy, and X-ray diffraction (XRD) methods. The results indicate that the produced Al-Ti-C master alloys contain TiC and TiAl₃ particles within the aluminum matrix. Also, these alloys were evaluated using the KBI test mold. The results indicate that the produced Al-Ti-C master alloy is an efficient grain refiner for pure aluminum and its alloys compared with the Al-Ti-B one. The factors affecting the grain refinement of aluminum and its alloys are studied. The proper conditions for evaluating the efficiency of the produced Al-Ti-C master alloy to obtain a minimum grain size are as follows: temperature, 993 K (720 °C); holding time, 2 minutes; and (Ti/Al) weight ratio, 0.01 pct.     

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al₈₀Fe₁₀V₄Si₆ alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.     

In situ formation of FeAl–Al2O3 nanocomposite at different conditions of milling and subsequent annealing

Abstract

FeAl–Al₂O₃ nanocomposite powder was synthesised under different conditions of milling and annealing. The structure, morphology and microstructure of the milled powders were monitored by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively. Results showed that the formation of FeAl and Al₂O₃ took place in explosive mode during milling with the cup speed of 600 rev min^?1. However, at the cup speed of 500 rev min^?1, FeAl and Al₂O₃ were synthesised only during annealing. Formation of the FeAl and Al₂O₃ was completed after 120, 270 and 360 min of milling at the ball to powder weight ratios (BPRs) of 5∶1, 15∶1 and 10∶1 respectively. Maximum microhardness of 8·8 GPa was obtained in the 270 min milled sample with the BPR of 15∶1 and cup speed of 600 rev min^?1. Mean grain size of 30 nm was calculated in the annealed FeAl that was in consistent with TEM results.     

Synthesis and characterisation of Si3N4p/Cu nanocomposite powders by high energy ball milling

Abstract

The present work reported the preparation of Cu–25 wt-%Si₃N₄ nanocomposite powders via high energy ball milling (HEBM). The phases and morphologies of as-milled powders with various milling times were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that with increasing the milling time, the irregularly shaped Cu powder became flattened, and, subsequently, refined and near spherical. After 12 h milling, the particle size of Cu–Si₃N₄ composite powders was in the range of 200–300 nm, while the grain size of Si₃N₄ particulates, 10–25 nm, was well within a nanometre scale. A uniform distribution of the nanosized Si₃N₄ reinforcing phase throughout the Cu matrix was successfully obtained. A reasonable mechanism for the formation of Cu–Si₃N₄ nanocomposite powders during HEBM was also proposed.     

A sintering study on the β-spodumene-based glass ceramics prepared from gel-derived precursor powders with LiF additive

Beta-spodumene (Li₂O·Al₂O₃·4SiO₂, LAS) powders were prepared by a sol-gel process using Si(OC₂H₅)₄, Al(OC₄H₉)₃, and LiNO₃ as precursors and LiF as a sintering aid agent. Dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED) were utilized to study the sintering, phase transformation, microstructure, and properties of the β-spodumene glass-ceramics prepared from the gel-derived precursor powders with and without LiF additives. For the LAS precursor powders containing no LiF, the only crystalline phase obtained was β-spodumene. For the pellets containing less than 4 wt pct LiF and sintered at 1050 °C for 5 hours the crystalline phases were β-spodumene and β-eucryptite (Li₂O·Al₂O₃·2SiO₂). When the LiF content was 5 wt pct and the sintering process was carried out at 1050 °C for 5 hours, the crystalline phases were β-spodumene, β-eucryptite (triclinic), and eucryptite (rhombohedral (hex.)) phases. With the LiF additive increased from 0.5 to 4 wt pct and sintering at 1050 °C for 5 hours, the open porosity of the sintered bodies decrease from 30 to 2.1 pct. The grains size is about to 4 to 5 μm when pellect LAS compact contains LiF 3 wt pct as sintered at 1050 °C for 5 hours. The grains size grew to 8 to 25 μm with a remarkable discontinuous grain growth for pellet LAS compact contain LiF 5 wt pct sintered at 1050 °C for 5 hours. Relative densities greater than 90 pct could be obtained for the LAS precursor powders with LiF > 2 wt pct when sintered at 1050 °C for 5 hours. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 × 10⁻⁷ to 5.2 × 10⁻⁷/°C (25 °C to 900 °C) as the LiF addition increased from 0 to 5 wt pct.     

Effect of Annealing Time on Microstructure and Mechanical Properties of Cold-Rolled Niobium and Titanium Bearing Micro-alloyed Steel Strips

Effects of annealing time on microstructure of cold-rolled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy, scanning electron microscopy, electron back-scatter diffraction (EBSD) and transmission electron microscopy. The complete recrystallization annealing temperature of 670 °C and complete annealing time of 9 min were determined using Vickers-hardness testing and EBSD analysis. The ferrite microstructure with spheric cementite particles and nano-scale precipitates of Nb(C, N) in matrix was obtained. The kinetics of the ferrite grain growth is lowered due to ferrite grain boundaries pinned by the cementite particles, so the ferrite grain size of 5. 5 μm remains unchanged among the annealing time ranging from 9 to 30 min. In addition, the strength of tested steel also keeps unchanged with the increase of annealing time. The higher yield strength of approximately 420 MPa can be obtained by grain refinement and precipitation hardening and the higher elongation of approximately 40% and work-hardening exponent of approximately 0. 2 can be gained due to grain refinement and presence of cementite particles, indicating that the balance of strength, ductility and forming property is realized.