Obvious Grain Research Articles

Microstructure Evolution and Coarsening Mechanism of 7075 Semi-Solid Aluminum Alloy Pre-Deformed by ECAP Method

To investigate the influence of refined grains on the microstructure of 7075 aluminum alloy in semi-solid state, a new strain induced melting activation (SIMA) method was put forward containing two main stages: pre-deformation with equal channel angular pressing (ECAP) method and isothermally holding in the semi-solid temperature range. The breaking up and growth mechanisms of the grains and kinetics of equiaxed grains coarsening during the semi-solid holding were investigated. The results showed that the average grain size after ECAP extrusion decreased significantly, e.g., microstructure with average globular diameter less than 5μm was achieved after four-pass ECAP extrusion. Obvious grain coarsening had been found during isothermal holding in the semi-solid state and the roundness of the grains increased with the increasing holding time. The proper microstructure of 66.8μm in diameter and 1.22 in shape factor was obtained under proper soaking condition (at 590°C for 15 min). Two coarsening mechanisms, namely, coalescence in lower liquid fraction and Ostwald ripening in higher liquid fraction contributed to the grain growth process.

Jet electrodeposition of nanocrystalline nickel assisted by controllable friction

Abstract A controllable friction auxiliary method (CFAM) was developed to improve the quality of a coating surface prepared through flexible friction-aided jet electrodeposition (FFAJED) at a high current density. A rolling nozzle was also designed. This nozzle can provide controlled pressure constantly during deposition so that the proposed method can forcibly restrain the rapid growth of the coarse grains. The influence of different pressures and cathode scanning speeds on coating surface quality was discussed, and the optimum parameters of the method were determined. With the optimum parameters, the effect of different current densities on coating surface quality was studied. Results show that when the rolling pressure is 5 N and the scanning speed of the cathode is 860 mm/min, the surface quality of the coating is excellent. No obvious coarse grains, defects (e.g., pinhole or nodules), and apparent traces of friction were observed. The magnetic properties of Ni coating were improved too. The upper limit of current density in the preparation of Ni coating by CFAM reached 200 A/dm2, which is 1.54 times that of traditional jet electrodeposition (130 A/dm2) and 1.43 times that of FFAJED.

Intrinsic high cycle fatigue behavior of ultrafine grained pure Cu with stable structure

Ultrafine grained (UFG) materials have attracted considerable attention owing to their unique microstructure and mechanical properties. However, the easy formation of large-scale shear bands and severe grain coarsening during cyclic deformation gives rise to enormous difficulties when investigating the intrinsic fatigue behavior of UFG materials. Herein, we discuss the fabrication of an ideal model material, based on pure Cu, by friction stir processing (FSP), which exhibits equiaxed ultrafine grains, low dislocation density, and a high ratio of high-angle grain boundaries. This model material was used to investigate the intrinsic high cycle fatigue behavior of UFG material. It was found that an enhanced fatigue limit and fatigue ratio can be achieved by FSP Cu due to its uniform and stable UFG structure. Instead of traditional large-scale shear bands, protrusion was found to be the main surface damage morphology for FSP Cu during high cycle fatigue deformation, and no obvious grain coarsening was observed. Dislocation related activity also dominated, but was limited to the ultrafine grains without the formation of regular dislocation structures.

Microstructure and Mechanical Properties of CNT-Reinforced AZ91D Composites Fabricated by Ultrasonic Processing

Carbon nanotube (CNT)-reinforced AZ91D alloy composite was fabricated by ultrasonic processing. The microstructure and mechanical properties of the CNTs/AZ91D composites were investigated. Obvious grain refinement was achieved with the addition of 0.5 wt% CNTs. The SEM observation indicated that CNTs were distributed near the grain boundary or around the inter-grain β-Mg17Al12 phase. No evident reaction product was found at the interface between CNTs and AZ91D matrix. Compared to the monolithic AZ91D alloy, the yield strength, ultimate tensile strength, and elongation of the 0.5 wt% CNTs/AZ91D composite were improved significantly. However, the poor interface bonding between CNTs and AZ91D matrix restricted further improvement in mechanical properties.

High thermal stability of submicron grained Cu processed by asymmetrical rolling

In this paper, we report the microstructure evolutions of symmetrically and asymmetrically rolled Cu at different temperatures, and reveal an important role of asymmetry induced microstructure homogeneity and removal of local severely distorted structures on the enhancement of thermal stability. With the same total reduction of 90%, two kinds of submicron grained (SMG) Cu were processed by symmetrical and asymmetrical rolling respectively. The temperature dependent boundary spacing curves show the asymmetrically rolled Cu has significantly higher thermal stability, with the onset temperature of obvious grain coarsening being 40°C higher than that for symmetrical rolled Cu. The enhanced thermal stability of asymmetrically rolled Cu can be attributed to the retardation of nucleation mainly realized by the improvement in microstructure homogeneity and the removal of local severely distorted structures. Our results demonstrate the feasibility of using asymmetrical rolling to improve the thermal stability of bulk SMG metals.

SEM analysis on rock failure mechanism by supercritical CO2 jet impingement

Supercritical carbon dioxide (CO2) jet-assisted radial drilling is regarded as a potential alternative drilling method because supercritical CO2 jet has higher efficiency in rock-erosion efficiency than water jet. Although researchers have investigated how the rock-erosion performance by supercritical CO2 jet impingement correlates with the influencing parameters, there is little on the rock failure mechanism and change of pore structure. Scanning electron microscopy (SEM) analysis has been an important means in mineral identification, material damage detection, and aiding in water jet research. Therefore, rock-erosion experiments and SEM observations and analyses were carried out on different rocks including sandstone, marble and so forth. Secondary electron (SE) images show obvious grain shapes on the complex impinged surface of sandstone and marble by supercritical CO2 jet, and cleavages on the mineral grains, and cracks and holes between the grains in different sizes. It suggests that cementations in weaker rocks are firstly removed and then mineral grains broken, generating the intergranular and intragranular cracks and micro-fractures and fissures. Supercritical CO2 jet cannot bring obvious changes to hard limestone and shale at the same condition because the rock substances are tightly packed and filled with large cementing forces and ultra-low permeability and porosity, but can produce a few micro-cracks vertical to bedding planes. The features show that supercritical CO2 jet broke rock substances mainly in the brittle tensile failure mechanism and made the rock much easier to further break, accompanied with shear failure mechanism in particular locations of the perforation hole. In contrast, water jet impingement forms flat cutting sections in marble with continuous parallel breaking traces, indicating the shear failure mechanism. Low viscosity and high diffusivity of supercritical CO2 are primarily attributed to the rock tensile failure mechanism, they facilitate supercritical CO2 penetrates into deep into rock pores and produces static pressure on the inner wall and at the tips of the fractures, besides exerting jet impinging pressure. Moreover, the energizing effect caused by the phase change from supercritical to gas leads to the energizing effect and intensify rock breaking into more shattered pieces and, concurrently, improve general rock connectivity and porosity in the surface layers. As a consequence, supercritical CO2 jet appears to be more efficient and suitable than water jet in slim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability.

Effects of annealing process on magnetic properties and structures of Nd–Pr–Ce–Fe–B melt-spun powders

The effects of annealing process on magnetic properties and structures of Nd–Pr–Ce–Fe–B melt-spun powders have been investigated. The magnetic properties improve a lot when the annealing temperature is 590–650°C and the annealing time exceeds 1min. The magnetic properties is stable when the annealing time is 590–650°C. The powders contains obvious grains when the annealing time is only 1min, while the grains grow up obviously, leading to the decrease of Br and (BH)max, when the annealing time is more than 9min. The Hcj changes little for different annealing time. The cooling rate also affects the magnetic properties of powders with different Ce-content. Faster cooling rate is favorable to improve magnetic properties with low Ce-content powders, while high Ce-content powders need slower cooling rate.

Hot deformation behavior of uniform fine-grained GH4720Li alloy based on its processing map

The hot deformation behavior of uniform fine-grained GH4720Li alloy was studied in the temperature range from 1040 to 1130°C and the strain-rate range from 0.005 to 0.5 s−1 using hot compression testing. Processing maps were constructed on the basis of compression data and a dynamic materials model. Considerable flow softening associated with superplasticity was observed at strain rates of 0.01 s−1 or lower. According to the processing map and observations of the microstructure, the uniform fine-grained microstructure remains intact at 1100°C or lower because of easily activated dynamic recrystallization (DRX), whereas obvious grain growth is observed at 1130°C. Metallurgical instabilities in the form of non-uniform microstructures under higher and lower Zener–Hollomon parameters are induced by local plastic flow and primary γ′ local faster dissolution, respectively. The optimum processing conditions at all of the investigated strains are proposed as 1090–1130°C with 0.08–0.5 s−1 and 0.005–0.008 s−1 and 1040–1085°C with 0.005–0.06 s−1.

Preparation and Properties of W-15Cu Composite by Electroless Plating and Powder Metallurgy

Abstract W-15Cu composite samples were prepared by electroless plating (for composite powder) and powder metallurgy (for sintering samples). Starting W powder was activated by a chemical pretreatment first, and then the pretreated W powder was electroless plating to get Cu-cladded W powder, which was pressed into green compacts at different pressing pressures. Afterward, the composite was sintered into W-15Cu composite samples, and Cu-cladded W powder was characterized. The relative density, hardness, compressive strength and electrical conductivity of the composite samples were tested. The morphologies of the Cu-cladded W powder and the samples were investigated. Results show that the prepared powder has good compaction behavior with high purity, uniform particle size and a dense Cu cladding layer. W particles still remain sole ones and no obvious grain coarsening appears in the sintering samples, while 3D Cu network is all over the structure of the samples. The electrical conductivity and the compressive strength of the sample change with pressing pressure and sintering temperature.

Surface Plastic Deformation and Nanocrystallization Mechanism of Welded Joint of 16MnR Steel Treated by Ultrasonic Impact

The welded joint surfaces of 16MnR steel were treated using an ultrasonic impact machine. The effects of ultrasonic impact treating (UIT) on the plastic deformation and nanocrystallization mechanism of the welded joints of 16MnR steel were studied. The micro-structural features of the surface layer produced by UIT were observed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM), and micro-hardness measurements were performed. Experimental results showed that the thickness of the plastic deformation layer was approximately 80 μm. It was found that grains in the surfaces of the welded joints of 16MnR were greatly refined by UIT. Obvious grain refinement was observed, with resultant gain sizes less than 100nm. The micro-hardness of the treated surface layer of the welded joint was enhanced significantly compared to that of the un-treated sample. The micro-hardness on the treated surface of the welded joint was 62.3% higher than that of the un-treated surface.DOI: http://dx.doi.org/10.5755/j01.ms.21.4.9563

Development of High Performance Magnesium Matrix Nanocomposites Using Nano-SiC Particulates as Reinforcement

In the present study, magnesium-based composites with three different volume percentages of nano-sized SiC particulates (SiCp) reinforcement were fabricated using a simple and inexpensive technique followed by hot extrusion. Microstructural characterization of the materials revealed uniform distribution of nano-size SiCp and obvious grain refinement. The tensile test result indicates a remarkable improvement on the strength for the as-extruded SiCp/AZ31B nanocomposite, while the elongation to fracture was decreased by comparing with the AZ31B alloy. Although, compared with the as-extruded AZ31B alloy, the ductility of the SiCp-reinforced AZ31B nanocomposite is decreased, but the ductility of the present SiCp-reinforced AZ31B nanocomposite is far higher than that of the conventional micron or submicron SiCp-reinforced magnesium matrix composites. It is concluded that, compared with the larger sized (micron or submicron) particles, the addition of nano SiCp in the AZ31B alloy resulted in the best combination of the strength and ductility. An attempt is made in the present study to correlate the effect of presence of nano-SiCp as reinforcement and its increasing amount with the microstructural and mechanical properties of magnesium.

Effect of Carbide Distribution on the Grain Refinement in the Steel for Large‐Size Bearing Ring

AbstractThe effect of carbide distribution on the grain refinement in the steel for large‐size bearing ring is investigated. It is observed that Ni‐containing steel has an obvious grain refinement below 1050 °C, whereas its grain growth trend is steeper than that of Ni‐free steel above 1050 °C. No evident segregation of Ni is observed, whereas the pinning effect of carbides distributed along grain boundaries is advantageous for the grain refinement. It is found that Ni addition could affect the type of carbides and contribute to decreasing the size of carbides via accelerating the diffusion of carbon. A stronger pinning effect with smaller carbides distributed along grain boundaries could be obtained, which is beneficial to grain refinement with Ni addition in small amounts.

Research on Microstructure and Wear Resistance of (Fe,Cr)<sub>7</sub>C<sub>3</sub>/Fe Surface Gradient Composite

In this study, the (Fe,Cr)7C3 particles strengthened gradient composite was produced by in situ synthesis process with subsequent heat treatment from gray cast iron (HT300) and high purity chrome plate. The microstructure, phase composition and wear resistance of the composite were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and scratch tester. The results showed that the thickness of the gradient layer was about 758 μm after heat treatment at 900 °Cfor 4 h. And it can be divided into three areas depending on microstructure. The outermost layer which was ~60 μm of thickness, was the dense ceramic layer with high volume fraction of (Fe,Cr)7C3 ~90%. No obvious grain boundaries were observed. The subsurface layer was the particles dispersed layer, which was ~525 μm of thickness, with the volume fraction of (Fe,Cr)7C3 decreased to 70%. The lowermost layer was ferrite, with about 173 μm thickness. A good metallurgical bond generated between the composite layer and matrix. The depth and the width of surface scratch increased with the raising loads from 0 to 100 N, and the cracks mainly included micro-crack, tiny dens crack, mixture crack and through-wall crack. The (Fe,Cr)7C3 particles were broken and scraped when the load exceeded 80 N.

Microstructure and compression properties of in situ dual phase nanosized (TiB2–Ti5Si3)/TiAl matrix composites fabricated by combustion synthesis and hot press consolidation

The dual phase nanosized (TiB2–Ti5Si3)/TiAl composites were successfully fabricated using in situ method of combustion synthesis and hot press consolidation. The effects of proportions of the in situ dual phase nanoparticles (TiB2 and Ti5Si3) on microstructures and compression properties of the composites were investigated. Compared with the monophase TiB2 and Ti5Si3 particles, dual phase TiB2–Ti5Si3 particles exhibit more obvious grain refinement effect. The grain size of TiAl alloy was refined from ∼66 to ∼12 μm with the the generation of dual phase TiB2–Ti5Si3 particles. Moreover, the TiAl matrix composites reinforced with dual phase nanosized TiB2–Ti5Si3 particles exhibit better comprehensive compression properties. When TiB2:Ti5Si3 = 2:1[Σ(TiB2 + Ti5Si3) = 4 vol.-%], the TiAl matrix composite has a high true fraction strain of 26.1%, a true yield strength of 761 MPa and an ultimate compression strength of 1647 MPa.

Microstructure and properties of Al–0.70Fe–0.24Cu alloy conductor prepared by horizontal continuous casting and subsequent continuous extrusion forming

A novel process for manufacturing Al–0.70Fe–0.24Cu alloy conductor was proposed, which includes horizontal continuous casting and subsequent continuous extrusion forming (Conform). The mechanical properties, electrical conductivity and the compressed creep behaviour of the alloy were studied. The results indicate that the Conform process induces obvious grain refinement, strain-induced precipitation of Al7Cu2Fe phase and the transformation of crystal orientation distribution. The processed alloy has good comprehensive mechanical properties and electrical conductivity. Moreover, a better creep resistance under the conditions of 90 °C and 76 MPa is shown compared with pure Al and annealed copper, and the relationship between primary creep strain and time may comply with the logarithmic law. The enhanced properties are attributed to the grain refinement as well as the fine and homogeneously distributed thermally stable AlFe and Al7Cu2Fe precipitation phases.

Microstructure and tensile properties of SiC nanoparticles reinforced magnesium matrix composite prepared by multidirectional forging under decreasing temperature conditions

Multidirectional forging (MDF) under decreasing temperature conditions was adopted to process SiC nanoparticles reinforced magnesium matrix composite. The results showed that grains of the nanocomposite were effectively refined while the amount of precipitated phases was significantly increased after MDF under decreasing temperature conditions. The precipitated phases as well as dispersed SiC nanoparticles could hinder the growth of dynamic recrystallization. The yield strength and ultimate tensile strength of the nanocomposites were increased compared with that of as-cast nanocomposite. This improvement could be attributed to obvious grain refinement as well as Orowan strengthening effect introduced by the dispersed SiC nanoparticles and the precipitated phases. Microcrack could initiate within the SiC nanoparticle dense zones during tensile test at room temperature, resulting in decrease of the elongation.

Optical and morphological investigation of aluminium and nickel oxide composite films deposited by spray pyrolysis method as a basis of solar thermal absorber

Applications of alumina and nickel oxide in various fields specially in solar energy conversion technology encouraged us to study physical properties of such materials. Hence after the deposition of the thin films on glass substrate by spray pyrolysis, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–visible spectrophotometry, various physical properties were investigated. Different optical quantities such as optical band gap, refractive index, extinction coefficient, dielectric constants, volume and surface energy loss functions and optical conductivity were determined. Within this paper for different nickel to aluminium ratio (from 20/80 to 80/20 ratio) at specific substrate temperatures (300°C), decrease of optical transmittance with nickel content was notable. Using the transmittance data, other optical quantities were achieved by a numerical approximation method. We also observed an increase in the volume energy loss (VELF) more than the surface energy loss (SELF) and simultaneously a decrease trend prevailed according to nickel amount. On the basic of the XRD results, the amorphous phase changed by the presence of more nickel and according to SEM, more obvious nanosized spherical grains at higher nickel ratios can be observed.

Grain size effect of V-shaped multiple twins in annealed nanocrystalline Cu

V-shaped multiple twins have been observed in annealed nanocrystalline Cu by high-resolution transmission electron microscopy. The density of V-shaped multiple twins depends on the grain size remarkably. With decreasing grain size, the amount of V-shaped multiple twins increases initially and then decreases, reaching the maximum at a critical grain size of 40nm and exhibiting an obvious grain size dependence. The formation mechanism of V-shaped multiple twins is proposed based on cooperative grain rotation and grain boundary migration. However, the amount of V-shaped multiple twins decreases significantly below the critical grain size, which is attributed to the grain rotation-mediated single crystal growth.

生物多样性不同层次尺度效应及其耦合关系研究进展

Biodiversity can be studied at four levels: molecular,species,ecosystem and landscape. Previously,many researchers have focused on the analysis of biodiversity at a single level,and few have explored the relationships among the different levels of biodiversity or conducted an integrated analysis of the four levels. The research methodologies are relatively well established at each level of biodiversity,especially for species diversity and landscape diversity. However,few studies have put forward a research method that spans all four levels of biodiversity. Species diversity is always measured using field samples collected in an inventory survey,and landscape diversity is usually calculated using remote sensing techniques,a geographic positioning system,and a geographic information system technical analysis supplemented with field surveys. Additive partitioning of diversity has been used in regional biodiversity analysis to calculate α,β,and γdiversity. The α,β,and γ diversity indices are calculated by the Shannon-Wiener index,Simpson diversity index,and the total diversity at a regional scale. These indices indicate that spatial-scale effects exist in the analysis of four-level diversity.With increasing spatial scale,the values of α diversity and the Shannon-Wiener index increase sharply and then more gradually beyond some threshold,whereas β diversity increases almost constantly. The behavior of spatial-scale effects differs for different biological species. From this viewpoint,an objective assessment of regional biodiversity is not possible based only on species diversity estimated from a field survey in a restricted geographic area. In contrast to the wellestablished method of analyzing species biodiversity,the method of evaluating ecosystem diversity is still developing because it is based on various biogeographic classification systems that differ among ecological regions and countries,and which areoccupied by their own local biological communities. There is an obvious spatial-scale effect on α,β,and γ diversity in analysis of landscape diversity,and the effect may be highly significant at some spatial scales. There are obvious resolution grain effects and spatial-extent effects in landscape diversity analysis. The grain size can reflect the behavior of the dominant landscape class. This may be useful for landscape management. In remote sensing data the grain size and spectral heterogeneity are inherent,thus it is reasonable to analyze landscape diversity based on a variety of remotely sensed images.With regard to relationships among the four levels of biodiversity,habitat diversity has a close relationship with species diversity and,to some extent,rich landscape diversity is always accompanied by high species diversity,attributable to αindices or β indices. At some scales,this relationship may be statistically significant. Scale effects also exist in regional biodiversity in response to human disturbance. To monitor and assess the biodiversity status of a given area,an integrative analysis involving multiple levels of species,ecosystem and landscape diversity is suggested to elucidate the complicated relationships among these levels. Such research will be helpful to explore the coexisting regimes that contribute to regional biodiversity,and to establish a red list of key ecosystems as assessed by ecosystem functions or in other terms in China,as well as to develop regional biodiversity monitoring and assessment methods. We recommend that a multiple-level analysis of biodiversity should be conducted when assessing the biodiversity status of a region.

Microstructure and mechanical properties of reactive sputtered nanocrystalline (Ti,Al)N films

Nanocrystalline (Ti,Al)N thin films with different Al contents were prepared by alternatively sputtering Ti and Al targets in N2 gas atmosphere at 600°C and by mutual diffusion between Ti and Al atoms during film deposition. The microstructure and mechanical properties of the thin films were investigated by X-ray diffraction, field emission scanning electron microscopy and nanoindenter. The film surface was smooth and dense with an obvious column grain growth. The grain sizes of the deposited (Ti,Al)N films were approximately 11–12nm independent of the Al content. The preferential orientation of the (Ti,Al)N films varied from (200) to (111) plane and the lattice constant reduced with increasing Al content, associated with Ti atom substitution by smaller Al atoms during alternative deposition. The nanohardness of the (Ti,Al)N films remarkedly increased, the elastic modulus slightly decreased and thus H3/E*2 values greatly increased compared to the TiN film. The enhancement of nanocrystalline (Ti,Al)N films was interpreted in terms of solid solution hardening and preferential orientation effects.