Formation Of Elongated Grains Research Articles

Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids

Stereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO2, CaCO3, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO2 content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO2. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO3 content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties.

Synergistic and competitive mechanisms of plastic anisotropy in high-efficiency laser melting deposited TC11 alloy

The layer-by-layer processing features of laser melting deposited (LMD) TC11 titanium alloy inevitably lead to microstructural inhomogeneity, and the grain morphology associated with micro-orientations induces plastic anisotropy, which causes uneven deformation and further performance deterioration. Understanding the main causes of these characteristics and how to distinguish their relative roles in inducing plastic anisotropy is critical. In this work, a high-efficiency LMD process is developed. Then, the individual factors causing the formation of elongated grains, microstructural inhomogeneity and micro-orientations are isolated to investigate the synergistic and competitive mechanisms of plastic anisotropy for high-efficiency LMD TC11 titanium alloy before and after various heat treatments. Initially, the TC11 titanium alloy is prepared by the LMD technique under high laser power and then heat-treated via three methods of double annealing in the α+β phase zone. Subsequently, the macro- and microscopic morphology, microstructure, and micro-orientations are characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The individual effects of grain shape, α grain boundary (αGB) and micro-orientations on tensile plastic anisotropy are systematically and thoroughly investigated via a hybrid method consisting of experimental and crystal plasticity finite element simulation methods. By introducing a redefined evaluation indicator of dislocation plugging coefficients, elongated grains are found to be the main source affecting the tensile strength in different loading directions. In addition, with increasing primary annealing temperature, the breakage degree of αGB parallel to the building direction (BD) is significantly higher than that perpendicular to the BD, which leads to an increase in tensile strength and elongation. Moreover, the preferred <0 0 0 1> orientation leads to different tensile strengths perpendicular and parallel to the BD, which is caused by the transformation between easy-to-slip and hard-to-slip orientations. The micro-orientations mainly affect the yield strength and elongation, while the tensile strength and elongation are simultaneously controlled by the grain morphology and αGB.

Structure and properties of rapidly solidifing foils Sn – 14 at.% In – 6.5 at.% Zn

The results of studies of the effect of ultra-high cooling rates of the melt equal to 10 5 K/s on the phase composition, microstructure, grain structure and mechanical properties of the Sn – 14 at.% In – 6.5 at.% Zn alloy are presented. To prepare the samples, the rapid quenching from the melt technique was used. A drop of melt was injected onto the inner surface of a rapidly rotating copper cylinder and solidifing in the form of a foil with a thickness of 30–90 μm. Investigations of the phase composition, carried out by the method of Х-ray diffraction analysis, made it possible to establish that the foil consists of a solid solution of zinc in the γ -phase (Sn4In) and zinc. Observations of the microstructure of the foil using scanning electron microscopy showed that the decomposition of a supersaturated solid solution proceeds at room temperature with the release of dispersed zinc particles. The character of the grain structure and texture of the foil is studied by the electron backscatter diffraction technique. A mechanism of the formation of elongated grains is proposed, which consists in the fact that at a high solidification rate, comparable to the rate of movement of the melt over the surface of the mold, grain growth can occur not only in the direction opposite to the direction of heat removal, but also in the direction of movement of the spreading. The formation of the preferred growth of grains, in which the most closely-packed plane (0001) is parallel to the foil surface, provides the maximum rate of decrease in the enthalpy of the alloy during crystallization. The features of the influence of the microstructure and grain structure on the mechanical properties of the foil are revealed. The microhardness of the rapidly solidifing Sn – 14 at.% In – 6.5 at.% Zn alloy is 105 MPa. The stress–strain curve of the Sn – 14 at.% In – 6.5 at.% Zn foil, obtained at room temperature, has a shape specific for the stress–strain curve of metals at high temperatures, which is due to the low melting point of the γ-phase.

The influence of different additives on microstructure and mechanical properties of aluminum titanate ceramics

The aluminum titanate (AT: Al2TiO5) ceramics with different additives, including SiO2, MgO, Fe2O3 and their mixture, were prepared by reaction sintering at 1500 °C in air for 2 h. The influence of additives on the microstructure and flexibility of AT ceramics is studied. Research illustrates that additives, MgO, Fe2O3 and their mixture, can be dissolved in AT ceramics to promote the formation of elongated grains and grain boundary microcracks. The elongated grains can interlock together and restrain crack propagation while microcracks can provide gaps for grains to move, leading to obvious flexibility of sample. Especially, the AT ceramics doped with MgO and Fe2O3 (ATFM) show the optimum flexibility with a deflection about 0.55 mm and strength up to 13.85 MPa. While the additive of SiO2 can reduce the size of grains and microcracks, resulting in high mechanical strength but poor flexibility. The successful fabrication of such AT flexible ceramics will provide potential opportunity for anti-vibration and refractory application.

Microstructure Evolution, Mechanical Properties and Deformation Behavior of an Additively Manufactured Maraging Steel.

In this work, the microstructure and mechanical properties of an additively manufactured X3NiCoMoTi18-9-5 maraging steel were determined. Optical and electron microscopies revealed the formation of melt pool boundaries and epitaxial grain growth with cellular dendritic structures after the laser powder bed fusion (LPBF) process. The cooling rate is estimated to be around 106 °C/s during solidification, which eliminates the nucleation of any precipitates. However, it allows the formation of austenite with a volume fraction of about 5% and dendritic structures with primary arm spacing of 0.41 ± 0.23 µm. The electron backscatter diffraction analysis showed the formation of elongated grains with significant low-angle grain boundaries (LAGBs). Then, a solutionizing treatment was applied to the as-printed samples to dissolve all the secondary phases, followed by aging treatment. The reverted austenite was evident after heat treatment, which transformed into martensite after tensile testing. The critical plastic stresses for this transformation were determined using the double differentiation method. The tensile strength of the alloy increased from 1214 MPa to 2106 MPa after the aging process due to the formation of eta phase. The experimental data were complemented with thermodynamic and mechanical properties simulations, which showed a discrepancy of less than 3%.

Anisotropic grain growth in iron-carbon films at high electric current densities

We investigate the effect of direct electric current (DC) on grain growth in 100 nm thick iron-carbon films with carbon concentrations between 0.7 to 4.4 at%. The application of DC-current during annealing at 550 °C confirms the expected transport of carbon in the direction of the electric current and the unexpected formation of elongated, abnormally large carbide and ferrite grains along the current direction in the carbon-rich regions. The formation of elongated grains is explained by electromigration-induced carbon flux divergences that result from the carbide precipitates. This presents a possible scenario for controlling microstructure evolution in iron by using DC electric currents. Changes for alternating current (AC) pulses had been observed before.

Anisotropic Grain Growth in Iron-Carbon Films at High Electric Current Densities

We investigate the effect of direct electric current (DC) on grain growth in 110 nm thick iron-carbon films with carbon concentrations at 2.7 at%. The application of DC-current during annealing at 550°C confirms the expected transport of carbon in the direction of the electric current and the unexpected formation of elongated, abnormally large carbide and ferrite grains along the current direction in the carbon-rich regions. The formation of elongated grains is explained by electromigration-induced carbon flux divergences that result from the carbide precipitates. This presents a possible scenario for controlling microstructure evolution in iron by using DC electric currents.

Columnar fine grained structure and chemical composition features of austenitic stainless steels 316L and 321 produced by the laser beam powder bed fusion

One of the promising ways of producing metal parts with the required properties and geometry is the technology of fusing powder layers by the using of laser beam. In the present work, the specific structure of samples obtained by the laser powder layer bed fusion is investigated. A comparative analysis of the structure of austenitic steels 316L and 321 with slightly different chemical composition are carried out. The epitaxial mechanism of the formation of elongated grains whose dimensions in the building direction is often several times higher than the powder layer thickness is considered. The reason for the formation of the droplet-shaped boundaries between the fusion layers is explained. The formation of ultrafine columnar grain substructure (the characteristic dimensions of the elements are less than 1 mcm) for both of this steels is explained

Revealing the heterogeneous nucleation behavior of equiaxed grains of inoculated Al alloys during directional solidification

An in-situ study on the directional solidification of an inoculated Al-20 wt%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out using a microfocus X-radiography set-up. The influences of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also promotes preferential growth of dendrite arms along the temperature gradient direction, and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is revealed that the propagation velocity of the nucleation front in directional solidification castings is approximately equal to the ratio between cooling rate Ṫ and temperature gradient G. Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’ (INZ) and ‘active nucleation zone’ (ANZ). The model has been applied to simulate the present in-situ solidification experiments. A good agreement was achieved between the predicted grain number density and the experimental measurements, showing the importance of including the temperature gradient effect on heterogeneous nucleation. Furthermore, the present model also has the capability to predict the temperature gradient necessary for the transition from equiaxed to columnar grain growth.

Thermal shock behavior of rare earth modified alumina ceramic composites

AbstractAlumina matrix ceramic composites toughened by AlTiC master alloys, diopside and rare earths were fabricated by hot-pressing and their thermal shock behavior was investigated and compared with that of monolithic alumina. Results showed that the critical thermal shock temperature (ΔT) of monolithic alumina was 400 °C. However, it decreased to 300 °C for alumina incorporating only AlTiC master alloys, and increased with further addition of diopside and rare earths. Improvement of thermal shock resistance was obtained for alumina ceramic composites containing 9.5 wt.% AlTiC master alloys and 0.5 wt.% rare earth additions, which was mainly attributed to the formation of elongated grains in the composites.

Grain refinement via formation and subdivision of microbands and thin laths structures in cold-rolled hafnium

The microstructural evolution and mechanical properties of pure hafnium subjected to cold rolling were investigated in this paper. Structural characterization indicates that slip dominated the plastic deformation and the microstructural evolution follows the sequence: original coarse grains – formation of microbands – formation of thin laths in microbands and formation of elongated grains in locally formed shear bands – the longitudinal splitting and transverse breakdown of laths to elongated segments – formation of nano-grains through further transverse breakdown of elongated segments. Hardness results indicate that strain hardening occurred during the whole rolling process, which can be attributed to dislocation activities and grain refinement effect.

Anomalous magnetic behavior below 10 K in YCrO3 nanoparticles obtained under droplet confinement

Nanoparticles of multiferroic YCrO3 synthesized using the droplet confinement of miniemulsions show unusual features in the magnetic properties at low temperatures, which have not been reported before. Below 10 K, there is a sudden increase in the magnetization, and the nature of M–H hysteresis loops changes appreciably. The hysteresis loop shows two contributions, one similar to ferromagnetic and another similar to that expected from antiferromagnetic systems. This behavior can be understood by the formation of elongated grains or mesocrystals. It is remarkable that YCrO3 behaves quite differently from other multiferroic chromates such as ACrO3 (A = In, Sc, Sm).

Effect of rare-earth oxides on properties of silicon nitride obtained by normal sintering and sinter-HIP

This paper presented the microstructure, mechanical properties and oxidation behavior of silicon nitride obtained by both conventional sintering and sintering followed by hot isostatic pressing (HIP). Silicon nitride with additives such as 5 wt.% Al2O3 and 5 wt.% Ln2O3 (Ln= La, La concentrate, Gd or La+Gd) were studied. The results revealed that Gd2O3 additions increased the formation of elongated grains of β-Si3N4, the fracture toughness and oxidation resistance. La2O3 additions led to higher densification and hardness values, while addition of La2O3 concentrate promoted the formation of materials with intermediate properties, compared to the other studied compositions. Hot isostatic pressing increased the hardness but decreased the fracture toughness of the material, mainly because it allowed residual pores to close and also reduced the average aspect ratio of β-Si3N4 grains.

Grain Refinement in Commercial Purity Titanium Sheets by Constrained Groove Pressing

Grain refinement studies in titanium have gained significant interest owing to its importance as a biomaterial. Severe plastic deformation techniques have been widely applied for grain refinement in metals and are capable of producing ultra fine and nano sized microstructures. In this study, repetitive shear deformations by constrained groove pressing have been applied to commercial purity titanium sheets of 2 mm thickness at a warm working temperature of 300oC. Microstructure studies reveal the formation of elongated grains with widths of the order of 0.5μm from an initial grain size of 40μm in the annealed condition. An increase in strength is also observed.

Characterization of adiabatic shear bands in AM60B magnesium alloy under ballistic impact

Adiabatic shear bands in Mg alloy under ballistic impact at a velocity of 0.5 km.s{sup -1} were characterized by means of optical microscope, scanning electron microscope, transmission electron microscope and indenter technique. The results show that adiabatic shear bands were formed around the impacted crater, and the deformed and transformed bands were distinguished by etching colors in metallographic observation. TEM observation shows that the deformed bands were composed of the elongated grains and high density dislocations, while the transformed bands composed of the ultrafine and equiaxed grains were confirmed. In initial stage, the severe localized plastic deformation led to the formation of elongated grains in the deformed bands. With localized strain increasing, the severe localized deformation assisted with the plastic temperature rising led to the severe deformation grains evolved into the ultrafine and equiaxed grains, while the deformed bands were developed into transformed bands. The formation of the ultrafine and equiaxed grains in the transformed bands should be attributed to the twinning-induced rotational dynamic recrystallization mechanism. High microhardness in the bands was obtained because of the strain hardening, grain refining and content concentration. - Research Highlights: {yields} Deformed and transformed bands are found in Mg alloy under ballistic impact. {yields}more » The microstructures in the deformed and transformed bands are characterized. {yields} The evolution process of the microstructure in the bands is discussed.« less

Boron-doped mullite derived from single-phase gels

Mullite with low dielectric constant and high transparency in infrared and microwave range has potential applications in communication industry. To improve the above properties of mullite, boron-doped mullite single-phase gels with a constant molar ratio of Al/Si = 3/1 and various B/Al ratios (B/Al = 0–0.4/3) were prepared in this study by slow hydrolysis of aluminum nitrate, boric acid and tetraethoxysilane. It was found that boron reduces the mullite formation temperature and suppresses spinel formation. The cell unit lattice parameters and cell volume in boron-doped mullite generally decrease with the increase of boron amount. The SEM observation shows that a small amount of boron reduces the grain sizes of mullite sintered bodies while a large amount of boron facilitates the formation of elongated grains and the amorphous glass phase. Boron decreases the transmittance of mullite ceramic and produces additional intensive absorption bond at 3.9 μm and also reduces the dielectric constants in the frequent range of 1 M–1 GHz.

Fabrication and characterisation of ultrafine-grained Al-5vol%Al&lt;SUB align=right&gt;2O&lt;SUB align=right&gt;3 nanocomposite

Nanocrystalline Al-5vol%Al2O3 nanocomposite was synthesised by mechanical milling of a mixture containing nanometric alumina with an average particle size of 35 nm. Morphology of as-synthesised powder was investigated by SEM while crystallite size of Al matrix was determined by XRD analysis. The results confirmed formation of nanocrystalline Al matrix induced by severe plastic deformation during mechanical milling. Nanocomposite bars were produced by hot powder extrusion route. TEM investigation of as-extruded nanocomposite revealed formation of elongated grains along the extrusion direction decorated by alumina nanoparticles. Tensile and compressive properties of as-extruded nanocomposite were measured and compared with those of monolithic Al. The results show a significant increase in the strength of Al due to ultrafine-grained structure coupled with the nanoparticles induced strengthening mechanisms.

Recent Advances in Microstructural Tailoring of Silicon Nitride Ceramics and the Effects on Thermal Conductivity and Fracture Properties

Tailoring the microstructure and the composition of silicon nitride ceramics can have profound effects on their properties. Here it is shown that the grain growth behavior, in particular its anisotropy, is a function of the specific additives, which allow one to tune the microstructure from one consisting of more equiaxed grains to one with very elongated grains. Recent studies are discussed that provide an understanding of the atomic level processes by which these additives influence grain shapes. Next the microstructural (and compositional) parameters are discussed that can be used to modify the thermal conductivity, as well as fracture toughness of silicon nitride ceramics. As a result of the open 〈0001〉 channels in β-Si₃N₄, the c-axis conductivity can be exceptionally high. Thus, the formation of elongated c-axis grains, particularly when aligned can result in conductivity values approaching those of AlN ceramics. In addition, the controlled formation of elongated grains can also be used to significantly enhance the fracture toughness. At the same time, both properties are shown to be affected by the composition of the densification additives. Utilizing such understanding, one will be able to tailor the ceramics to achieve the properties desired for specific applications.

Study on Phase Transformation and Microstructure Development of Yb α-SiAlON Ceramics Prepared by Pressureless Sintering

In this paper, single-phase Yb α-SiAlON ceramics were prepared by pressureless sintering and the densification, phase transformation as well as microstructure development were studied. It was demonstrated that the phase transformation could be completed at 1700 °C and elevated temperature was beneficial to grain growth. Additionally, effect of the addition of seed crystals prepared by combustion synthesis was discussed. From the experimental results, it was found that the added seed crystals facilitated the phase transformation and improved the microstructure development, resulting in the formation of elongated grains.

Microstructure development in hot-pressed silicon carbide: effects of aluminum, boron, and carbon additives

SiC was hot-pressed with aluminum, boron, and carbon additives. The Al content was modified either to obtain SiC samples containing a continuous Al gradient, or to vary the average Al content. In both cases, it was found that increasing Al content resulted in decreased number density but dramatically enhanced aspect ratio of the elongated SiC grains. Amorphous-to-crystalline transformation was also confirmed in grain boundary films when Al content exceeded 4 wt%. In addition, increasing Al decreased the degree of cubic-to-hexagonal SiC phase transformation. Similar processing and characterization were done with modified boron and carbon average contents. The systematic experiments demonstrated that boron and carbon promoted formation of elongated grains through promoting 3C-to-4H SiC phase transformation during hot pressing. The experiments also clarified the mechanical property responses to microstructural modification. Tailoring of the SiC microstructure to suit different applications would be possible.