Showing papers on "Microstructure published in 2019"

Additive manufacturing of ultrafine-grained high-strength titanium alloys

Abstract: Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste1. Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys1. (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable2.) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains3. In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium-copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium-copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries.

Trace doping of multiple elements enables stable battery cycling of LiCoO2 at 4.6 V

Abstract: LiCoO2 is a dominant cathode material for lithium-ion (Li-ion) batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltages. However, practical adoption of high-voltage charging is hindered by LiCoO2’s structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO2 at 4.6 V (versus Li/Li+) through trace Ti–Mg–Al co-doping. Using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we report the incorporation of Mg and Al into the LiCoO2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. We also show that, even in trace amounts, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltages. These dopants contribute through different mechanisms and synergistically promote the cycle stability of LiCoO2 at 4.6 V. LiCoO2 is a widely used cathode material in Li-ion batteries for applications such as portable electronics. Here, the authors report multiple-element doping to enable stable cycling of LiCoO2 at high voltages that are not yet accessible with commercial Li-ion batteries.

Pea-like Fe/Fe3C Nanoparticles Embedded in Nitrogen-Doped Carbon Nanotubes with Tunable Dielectric/Magnetic Loss and Efficient Electromagnetic Absorption

Abstract: One-dimensional microstructure has been regarded as one of the most desirable configurations for magnetic carbon-based microwave absorbing materials (MAMs). Herein, pea-like Fe/Fe3C nanoparticles embedded in nitrogen-doped carbon nanotubes (Fe/Fe3C@NCNTs) are successfully prepared through a direct pyrolysis of the mixture of FeCl3·6H2O and melamine under inert atmosphere. The chemical composition and microstructural feature of these Fe/Fe3C@NCNTs composites are highly dependent on the pyrolysis temperature. As a result, their electromagnetic properties can be also manipulated, where dielectric loss gradually decreases with the increasing pyrolysis temperature and magnetic loss presents a reverse variation trend. When the pyrolysis temperature reaches 600 °C, the as-obtained composite, Fe/Fe3C@NCNTs-600 can perform a maximum reflection loss of −46.0 dB at 3.6 GHz with a thickness of 4.97 mm and a qualified bandwidth of 14.8 GHz with the integrated thickness from 1.00 to 5.00 mm. It is very interesting that...

Enhanced antiferroelectric phase stability in La-doped AgNbO3: perspectives from the microstructure to energy storage properties

Abstract: La-doped AgNbO3 lead-free ceramics were fabricated by conventional solid-state reaction, and the phase stability and energy storage properties were investigated. The temperature- and electric field-dependent dielectric constants show that the antiferroelectric (AFE) phase stability is enhanced via the La doping. Neutron diffraction was performed to obtain insights into the structural evolution with composition and temperature, where the local structural variation is found to involve subtle ion displacement as well as oxygen octahedral tilting, leading to the disruption of the long-range interactions, which is responsible for the enhanced AFE phase stability. As expected, the enhanced AFE phase stability, together with the improved breakdown strength, gives rise to a high energy density of 4.4 J cm−3 and an improved efficiency of 70%, which are achieved in 2 mol% La-doped AgNbO3 ceramics. Our research opens a new way to tailor the macroscopic properties by tuning the microstructure of AgNbO3-based materials.

Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy

Abstract: We describe here a comprehensive study on the effect of cellular structure and melt pool boundary (MPB) condition on the mechanical properties, deformation and failure behavior of AlSi10Mg alloy processed by selective laser melting (SLM). The morphology of melt pool (MP) on the load bearing face of tensile samples was significantly different with build directions. It resulted in different mechanical properties of the samples with different build directions. Furthermore, the microstructure analysis revealed that the MP in the SLM AlSi10Mg alloy mainly consisted of columnar α-Al grains which were made of ultra-fine elongated cellular structure. Electron back-scatter diffraction (EBSD) analysis revealed that the long axis of cellular structure and columnar grains were parallel to , which resulted in fiber texture in SLM AlSi10Mg alloy. However, Schmid factor calculation demonstrated that the anisotropy of mechanical properties of the SLM AlSi10Mg alloy built with different direction was mainly dependent on the distribution of MPB on the load bearing face, and not texture. The defects including pores, residual stress and heat affected zone (HAZ) located at MPB made it the weakest part in the SLM AlSi10Mg. The sample built along horizontal direction exhibited good combination of strength and plasticity and is attributed to the lowest fraction of MPBs that withstand load during tensile. MPB had strong influence on the mechanical properties and failure behavior of SLM AlSi10Mg built with different directions.

Effect of heat treatment on microstructure and mechanical properties of 316L steel synthesized by selective laser melting

Abstract: The influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by SLM has been investigated and the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains.

Influence of AlN particles on microstructure, mechanical and tribological behaviour in AA6351 aluminum alloy

Abstract: In the current trend, the hard ceramic particles reinforced aluminum matrix composites (AMCs) is extensively being exploited as a composite which shall be utilized for various engineering applications. In the present research, the Al-Si-Mg (AA6351) composite incorporated with aluminium nitride (AlN) filler were prepared via novel and low cost melt stirring process. This study was conducted using AA6351 aluminum alloy in conjunction with AlN particles whose percentages of incorporation were 4%, 8%, 12%, 16% and 20 wt.% in the ascending. The stir casted composites and the base alloy were characterized via X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). EDAX and XRD plots prove the occurrence of AlN filler contents in the synthesized AMCs. SEM studies exhibit the even dispersion of the reinforcement particles in the Al matrix. The effects of AlN contents on the mechanical characteristics of AA6351 matrix composites were examined. The dry sliding wear characteristics of the prepared composites was tested employing pin on disc machine. The mechanical and wear behavior of the AMCs had shown a great enhancement by the incorporation of AlN particles into AA6351 matrix alloy. The test outcomes discovered that Al/20 wt.% AlN composites had revealed superior wear resistance, hardness, yield strength and tensile strength than the AA6351 base alloy

Wire and arc additive manufacturing of HSLA steel: Effect of thermal cycles on microstructure and mechanical properties

Abstract: Wire and arc additive manufacturing (WAAM) is a viable technique for the manufacture of large and complex dedicated parts used in structural applications. High-strength low-alloy (HSLA) steels are well-known for their applications in the tool and die industries and as power-plant components. The microstructure and mechanical properties of the as-built parts are investigated, and are correlated with the thermal cycles involved in the process. The heat input is found to affect the cooling rates, interlayer temperatures, and residence times in the 800–500 °C interval when measured using an infrared camera. The microstructural characterization performed by scanning electron microscopy reveals that the microstructural constituents of the sample remain unchanged. i.e., the same microstructural constituents—ferrite, bainite, martensite, and retained austenite are present for all heat inputs. Electron backscattered diffraction analysis shows that no preferential texture has been developed in the samples. Because of the homogeneity in the microstructural features of the as-built parts, the mechanical properties of the as-built parts are found to be nearly isotropic. Mechanical testing of samples shows excellent ductility and high mechanical strength. This is the first study elucidating on the effect of thermal cycles on the microstructure and mechanical properties during WAAM of HSLA steel.