Hot-pressed Samples Research Articles

An exploration on microstructural observation and mechanical behavior of nanocrystallite Al 7017 alloy via mechanical alloying and uniaxial hot pressing

The present research work investigates the detailed microstructural observation and mechanical behavior of nanocrystallite Al 7017 alloy prepared by Mechanical Alloying (MA) for a different milling times (0, 5, 10, 15, and 20 h)followed by uniaxial hot-pressing. The milled powders were characterized by X-ray diffraction analysis (XRD), High-Resolution Scanning Electron Microscope (HR-SEM), High-Resolution Transmission Electron Microscope (HR-TEM), and Particle Size Analyzer (PSA). The characterization of milled powder samples confirms the formation of nanocrystallite Al 7017 alloy through MA. X-ray peak broadening analysis was used to establish the various structural parameters such as crystal size, lattice strain, dislocation density, lattice parameter, and precise lattice parameter of milled powder and hot–pressed sample through Scherrer Equation and different models of Williamson–Hall methods [Uniform Deformation Model (UDM), Uniform Stress Deformation Model (USDM), and Uniform Energy Density Deformation Model (UDEDM)]. Additionally, a particle size analyzer was carried out to find the powder particle size and its distribution for different milling times. Powder morphology was characterized to determine the morphological change from flake type to an equiaxed spherical particle with a uniform dispersion of powder particle. Furthermore, it is used to analyze the development of nanograin MgZn2 precipitate during mechanical alloying. This work deliberates the influence of milling time on the mechanical properties (hardness, density, and compressive strength) of hot-pressed Al 7017 alloys. Finally, the Finite Element Analysis (FEA) was performed on hot-pressed samples to analyze the effective stress distribution and the results of the compression test are compared with the experimental analysis.

Vacuum Hot Pressed Novel 21-4N Valve Steel Strengthened by Y-Ti-O Through High-Energy Ball Milling

In recent years, the selection of engine valves materials is based on high temperature performance, longer maintenance life, prominent creep and corrosion and oxidation resistance. Typical engine valve includes materials like 21-2N and titanium alloys. In order to develop a more suitable material to enhance its performance, new nanodispersion strengthened austenitic stainless steels were developed with and without the addition of Y2O3, Ti and MnN from ferro-alloys (ferro-manganese, ferro-nickel and ferro-chrome) to obtain Fe-21Cr-4Ni-9Mn-0.5C-0.4Si as the required composition through powder metallurgy route. These pre-alloyed powders in proper proportion were milled mechanically within a planetary ball mill to obtain the required composition and nanocrystalline structure. The milled powders were subsequently vacuum hot pressed at 1200 °C and 60 MPa by maintaining it at a vacuum level of 10−3 mbar for a period of 2 h. Vacuum hot pressed samples were subjected to densification and hardness studies, and microstructural analysis and characterization studies. Density of vacuum hot pressed samples was 97.9% when compared to its theoretical density. With the addition of Y2O3 and Ti as minor alloying elements, very fine austenitic grain structure was formed with the formation of Y-Ti-O complex oxides. The presence of chromium-rich precipitates was revealed during SEM–EDS analysis at the grain boundaries which was attributed due to lower cooling rate of austenitic stainless steel. TEM analysis revealed homogeneous distribution of Y-Ti-O complex oxides within the metal matrix. A marginal increase in hot pressed density was reported with the inclusion of Y2O3 and Ti in the austenitic matrix along with 28% increase in hardness values.

Improvement in mechanical and magnetocaloric properties of hot-pressed La(Fe,Si)13/La70Co30 composites by grain boundary engineering

Magnetocaloric composites La(Fe,Si)13/La70Co30 were fabricated by hot-pressing (HP) followed by annealing. With increasing La70Co30 binder content up to 16 wt%, the maximum compressive strength (σbc)max improved dramatically from 63 to 352 MPa. However, the maximum magnetic entropy change (–ΔSM)max decreased from 10.0 to 5.6 Jkg–1K−1 because of the presence of non-magnetic La70Co30. After annealing at 1323 K for 4 h the (σbc)max of HP + Annealed composites with up to 12 wt% La70Co30 doubled compared to HP samples. On the other hand, for a La70Co30 content of 16 wt%, the (σbc)max of HP + Annealed composites decreased slightly compared to that of HP samples, due to reduced α-Fe content and much greater 1:13 phase content in HP + Annealed samples. Remarkably, the (–ΔSM)max of HP + Annealed composites improved from 4.8 to 9.2 Jkg–1K−1 (under 2 T), due to 1:13 phase recombination, after annealing at 1323 K for 4 h.

Effect of milling ball size on the densification and optical properties of transparent Y2O3 ceramics

In this study, we report highly transparent Y2O3 ceramics fabricated by hot-pressing only at 1500 °C without a HIP treatment, featuring in-line transmittance levels of 77% and 84% at a wavelength of 400 and 1100 nm, respectively with the grain size suppressed to 710 nm. The effect of the ball size during the grinding of Y2O3 powders on the correlation between the thus-prepared Y2O3 powders and the optical properties of the hot-pressed samples is demonstrated for the first time. With a decrease in the diameter of the ZrO2 balls from 5 mm to 1 mm, the milling efficiency was enhanced and admirable transparency of Y2O3 was attained at a short milling time. However, several micron-sized pores remained in the transparent specimens prepared with 1 mm balls, originating from the inhomogeneously packed region of the green body. Finally, the 2 mm was found to be optimum for obtaining a fine-grained and pore-free microstructure with the best in-line transmittance of Y2O3 ceramics.

Sintering and mechanical properties of lithium disilicate glass-ceramics prepared by sol-gel method

The aim of this study was to investigate the sintering and mechanical properties of lithium disilicate (Li2Si2O5) glass-ceramics prepared by sol-gel method. The results showed that Li2Si2O5 phase started to crystallize in the gel-derived powders around 500 °C. The glass-ceramics powders with a main phase of Li2Si2O5 were obtained after a calcination at 600 °C. Microstructure and flexural strength of pressureless sintered and hot-pressed glass-ceramics were further evaluated, respectively. The Li2Si2O5 glass-ceramic after sintering at 900 °C exhibited the highest flexural strength of 137.1 MPa with an interlocked microstructure. While, the hot-pressed samples could achieve higher densification and much higher flexural strength compared with the pressureless sintered samples, and a flexural strength of 277.0 MPa was obtained after sintering at 950 °C under a pressure of 16 MPa. The results suggest that Li2Si2O5 glass-ceramics fabricated by sol-gel method have a good application prospect for dental restorations.

Effect of Hot Rolling on the Microstructure and Properties of Nanostructured 15Cr ODS Alloys with Al and Zr Addition.

Oxide dispersion strengthened (ODS) alloys with Al and Zr addition have excellent radiation tolerance, high-temperature strength, and corrosion resistance. The 15Cr-Al-Zr-ODS alloys are processed by mechanical alloying (MA), hot isostatic pressing (HIP), subsequent hot rolling to large strains of 70%, and further annealing. The effect of hot rolling on the microstructure, and the properties of nanostructured 15Cr ODS alloys with Al and Zr addition, were investigated. The microstructure after hot rolling and annealing showed obvious anisotropy. The cubic texture (φ1 = 0°, Φ = 0°, φ2 = 0°) {0 0 1} <1 0 0> and brass-R texture (φ1 = 0°, Φ = 55°, φ2 = 45°) {1 1 1} <1 1 0> were observed. The similar size distribution of precipitates was obtained for the comparison of the hot rolling samples with the hot isostatic pressed samples, which can be attributed to excellent thermal stability. After hot rolling, the alloy showed higher yield strength but did not lose too much plasticity.

Preparation and Thermoelectric Properties of Microcrystalline Lead Telluride

We have worked out conditions for the preparation of microcrystalline n-type lead telluride-based materials doped with lead iodide and investigated their microstructure and thermoelectric properties. The materials were prepared by hot-pressing powders produced by grinding an ingot to a particle size on the order of hundreds of microns in a planetary mill and to a particle size under hundreds of nanometers (mechanical activation) and by melt spinning. Fracture surfaces of the hot-pressed samples were examined on an optical and a scanning electron microscope. All of the samples had a nonuniform microstructure, with both small and larger grains present. In the samples prepared from the powders produced by mechanical activation, nanograins were detected. We have measured the Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples at room temperature and in the range 300–800 K and evaluated their lattice thermal conductivity and thermoelectric figure of merit, ZT. Their lattice thermal conductivity was shown to decrease with decreasing grain size. The highest thermoelectric figure of merit, (ZT)max = 1.32 at 630 K, was offered by the materials produced from the mechanically activated powder.

XRD and SEM Analyses of Bulk Ga-Doped Li&lt;sub&gt;7&lt;/sub&gt;La&lt;sub&gt;3&lt;/sub&gt;Zr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;12 &lt;/sub&gt;Li- Ion Conducting Solid Electrolyte Prepared via Hot-Pressing Method

In this study, bulk lithium-ion conducting solid electrolyte of Ga-doped Li7La3Zr2O12 (Li7-3XGaxLa3Zr2O12) where x = 0.1 (Ga-LLZO) was prepared via hot pressing at 500 °C. Precursor powder for hot-pressing was prepared using conventional solid state reaction method. Planetary ball milling was employed to investigate the particle size effect on the structure and densification of hot-pressed samples. XRD patterns of the bulk hot-pressed sample revealed a crystalline phase of which the major peaks observed can be indexed to a cubic LLZO structure; however, a major impurity phase of La2Zr2O7 was observed for the ball-milled sample. Thermogravimetric and differential thermal analysis showed about 12% weight loss below 900 °C which may have affected the observed hot-pressing structure. Although lower density measurement and an impurity phase of La2Zr2O7 were observed for the ball-milled sample, ball-milling also resulted to a more homogeneous and finer particle size as shown by SEM images results.

Delayed radioluminescence of some heterostructured organic scintillators

The processes leading to the appearance of phosphorescence and delayed fluorescence in organic molecular materials compete. This paper compares the ratio of the intensities of delayed fluorescence and phosphorescence with the pulse shape discrimination (PSD) capability of organic scintillators. The paper considers hot-pressed polycrystals and composite samples containing single-crystal organic grains as the heterostructured organic scintillators.The single crystals and heterostructured scintillators contained trans-stilbene, p-terphenyl, anthracene, and phenanthrene. Samples based on trans-stilbene and p-terphenyl at 77 K showed an intense delayed fluorescence, which appeared against the background of weak phosphorescence. It indicates the active formation of delayed fluorescence in these materials. We did not observe a similar effect for samples based on anthracene and phenanthrene. At the same time, it was the samples containing trans-stilbene and p-terphenyl grains that showed the best PSD capability at room temperature. Single-crystal and heterostructured materials containing the same scintillation substance demonstrate a close PSD capability. A comparative analysis of the intensities of delayed fluorescence and phosphorescence at low temperatures can be a method of searching for new materials that are promising for PSD problems.

Ultra-low temperature fabrication of copper carbon fibre composites by hydrothermal sintering for heat sinks with enhanced thermal efficiency

Fabrication of dense Cu/40CF metal matrix composites for heat sink applications by hydrothermal sintering at low temperature (P = 250 MPa; T = 265 °C, tsinter = 60 min; water quantity = 5 wt%) is reported. Study of various sintering parameters and their influence on densification are investigated. Comparative thermal and microstructural properties of hydrothermally sintered samples with the conventionally used uniaxial hot-pressing samples on Cu/40CF material show considerable improvements, namely increase of thermal conductivity (TC) and decrease of coefficient of thermal expansion (CTE). Hydrothermal sintering yields, through a dissolution/precipitation mechanism, quasi-isotropic thermal properties (TC = 300 W/mK and CTE = 8 10−6/K).

Influence on mechanical properties of hot pressed, solution treated and age hardened 21-4N ODS alloy developed through pre-alloyed powders

In the current study, mechanical properties of hot pressed, solution treated and aged austenitic stainless steels (21-4N) for were evaluated by varying the percentage of yttria addition. The milled powders were vacuum hot pressed at 1,200°C with a pressure level of 56 MPa with a holding period of 2 hrs under the vacuum level of 10-2 torr. Hot pressed samples were solution treated at 1,200°C for a period of 2 hrs and immediately subjected to water quenching to dissolve more grain boundary carbides. Further, to achieve the desired hardness samples were aged at 750°C held for 4 hrs followed by air cooling. Optical micrographs and SEM-EDS studies reveal austenitic grain with grain boundary carbides in the hot pressed condition. After solution treatment these precipitates disappears which was confirmed through SEM-EDS. Austenitic stainless steel containing 0.3 wt% yttria has attained higher tensile strength of 598 MPa in the age hardened condition.

Effect of Organosilane Coupling Agents on Thermal, Rheological and Mechanical Properties of Silicate-Filled Epoxy Molding Compound.

Global industries strive towards the production of materials with superior mechanical characteristics, and their development remains a big challenges. One of the more interesting materials that exhibit these properties are silicate-filled epoxy molding compounds (EMCs). A good interaction between silicate filler and epoxy matrix is generally needed to achieve advantageous mechanical properties, as well as the desirable rheological behavior of EMCs. Understanding the influence of different organosilane coupling agents on the rheological and mechanical properties of EMCs is essential in the development and optimization of the manufacturing process. For this matter, a mixture of calcium silicate and aluminosilicate was treated by using organosilane coupling agents with different chemical structures and thus treated silicates were applied as fillers in the EMCs. The thermal behavior of the organosilane-modified, silicate-filled EMCs was studied by using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). Flow-curing behavior (torque rheometer) and spiral flow length measurement (EMMI) were used to monitor the rheological properties and reactivity of the EMCs. The results showed that 3-glycidyloxypropyltrimethoxysilane- and 3-aminopropyltriethoxysilane-treated filler had a greater influence on the tensile strength of hot-pressed test samples, while 3-aminopropyltriethoxysilane and a blend of primary and secondary aminosilanes had a more significant impact on the rheological behavior of the material.

Thermoelectric transport and microstructural study of Cu doped Mg2Si0.4Sn0.6

The Mg2X (X = Si and Sn) based alloys which are composed of abundant, non-toxic and inexpensive elements are promising mid-temperature thermoelectric (TE) materials. Herein, we report synthesis of Mg2Si0.4Sn0.6 by high energy ball milling followed by hot pressing for thermoelectric application. The use of Cu as a dopant, appears to provide p-type conductivity but experimentally, the system has been found to generate n-type conduction. The XRD investigations revealed successful attainment of a near to single phase alloy with only minute presence of elemental Si. Electrical conductivity of the order of 104 S/m and charge carrier concentration of the order of 1019 cm−3 have been obtained through ZEM and Hall coefficient measuring equipment respectively. LFA provides thermal conductivity in the range of 4.4 – 3.0 W/mK. In addition to the thermoelectric measurements, the elemental mapping of the hot-pressed sample has been carefully explored.

Effects of Filament Extrusion, 3D Printing and Hot-Pressing on Electrical and Tensile Properties of Poly(Lactic) Acid Composites Filled with Carbon Nanotubes and Graphene.

In this study, the effects of three processing stages: filament extrusion, 3D printing (FDM), and hot-pressing are investigated on electrical conductivity and tensile mechanical properties of poly(lactic) acid (PLA) composites filled with 6 wt.% of multiwall carbon nanotubes(MWCNTs), graphene nanoplatelets (GNPs), and combined fillers. The filaments show several decades’ higher electrical conductivity and 50–150% higher values of tensile characteristics, compared to the 3D printed and the hot-pressed samples due to the preferential orientation of nanoparticles during filament extrusion. Similar tensile properties and slightly higher electrical conductivity are found for the hot-pressed compared to the 3D printed samples, due to the reduction of interparticle distances, and consequently, the reduced tunneling resistances in the percolated network by hot pressing. Three structural types are observed in nanocomposite filaments depending on the distribution and interactions of fillers, such as segregated network, homogeneous network, and aggregated structure. The type of structural organization of MWCNTs, GNPs, and combined fillers in the matrix polymer is found determinant for the electrical and tensile properties. The crystallinity of the 3D printed samples is higher compared to the filament and hot-pressed samples, but this structural feature has a slight effect on the electrical and tensile properties. The results help in understanding the influence of processing on the properties of the final products based on PLA composites.

The influence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)

In Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower.

Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites

ZrB2–SiC–graphite composites with 0–35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 °C under the uniaxial pressure of 40 MPa for 1 h. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr3C2 and B4C phases besides the main existing ZrB2, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB2 grains. The addition of graphite into the ZrB2–SiC composites improved the sintering process and caused a fine-grained microstructure.

High-Throughput Preparation of Antibacterial Polymers from Natural Product Derivatives via the Hantzsch Reaction.

SummaryThe Hantzsch and free-radical polymerization reactions were combined in a one-pot high-throughput (HTP) system to simultaneously prepare 30 unique polymers in parallel. Six aldehydes derived from natural products were used as the starting materials to rapidly prepare the library of 30 poly(1,4-dihydropyridines). From this library, HTP evaluation methods led to the identification of an antibacterial polymer. Mechanistic studies revealed that the dihydropyridine group in the polymer side-chain structure plays an important role in resisting bacterial attachment to the polymer surface, thus leading to the antibacterial function of this polymer. This research demonstrates the value of multicomponent reactions (MCRs) in interdisciplinary fields by discovering functional polymers for possible practical applications. It also provides insights to further developing new functional polymers using MCRs and HTP methods with important implications in organic chemistry, polymer chemistry, and materials science.

Processing and Characterization of Extremely Hard and Strong Cu-(0-15 wt pct)Al Alloys

The present work investigates the microstructure development and mechanical properties of mechanically alloyed and hot-pressed copper (Cu)-X wt pct aluminum (Al) (X = 0, 3, 5, 10, 15) alloys. The morphology of the ball-milled Cu-Al powders changed from coarse flaky structure to small hard agglomerates with the addition of Al. It was observed that the density of Cu-Al samples varied between ~ 95 and 98 pct of theoretical density (ρth) after hot pressing (Temperature: 500 °C, Pressure: 500 MPa, Time: 30 min). The crystallite size of Cu-Al samples decreased for both the milled powders and hot-pressed samples. The XRD and SEM-EDS analyses of the hot-pressed samples confirmed the presence of α-Cu solid solution phases for the Cu alloyed with Al up to 5 wt pct. On the other hand, further addition of Al to Cu leads to the formation of both intermetallic compound (Cu9Al4) and solid solution phase. The nano-indentation tests indicated a significant increase in hardness (2.4 to 7.9 GPa) and elastic modulus (121.1 to 177.4 GPa) of Cu-Al alloys. The Cu-Al alloys were measured with very high compressive strength (813.8 to 1120.2 MPa) and the compressive strain varied in the range of 29.81 to 5.81 pct.

Preparation, thermal stability and electrical transport properties of vaesite, NiS2

Vaesite, a nickel chalcogenide with NiS2 formula, has been synthetized and studied by theoretical and experimental methods. NiS2 was prepared by solid-state reaction under vacuum and densified by hot-pressing, at different consolidation conditions. Dense single-phase pellets (relative densities &gt;94%) were obtained, without significant lattice distortions for different hot-pressing conditions. The thermal stability of NiS2 was studied by thermogravimetric analysis. Both as-synthetized and hot-pressed NiS2 have a single phase nature, although some hot-pressed samples had traces of the sulfur deficient phase, Ni1-xS (&lt;1%vol), due to the strong desulfurization at T &gt; 340 °C. The electronic band structure and density of states were calculated by Density Functional Theory (DFT), indicating a metallic behavior. However, the electronic transport measurements showed p-type semiconductivity for bulk NiS2, verifying its characteristic behavior has a Mott insulator. The consolidation conditions strongly influence the electronic properties, with the best room-temperature Seebeck coefficient, electrical resistivity and power factor being 182 µVK−1, 2,257 µΩ m and 14.1 µWK−2 m−1, respectively, pointing this compound as a good starting point for a new family of thermoelectric materials.

Microstructure and mechanical stability of Bi doped Mg2Si0.4Sn0.6 thermoelectric material

Bi doped Mg2Si0.4Sn0.6 had been synthesised in a high energy ball mill followed by compaction using a sintering hot press. The structural and compositional characterization of sintered mass indicated the formation of a highly densified single-phase product. The microstructure of the hot-pressed samples had been critically assessed. Thermoelectric properties were measured between room temperature and 723 K. A decrease in electrical conductivity was found with the increase in temperature but the Seebeck coefficient showed a reverse trend justifying the attainment of degenerate semiconducting behaviour. Meanwhile, the lattice thermal conductivity was subdued to 1.5 W/mK at 623 K. However, the highest zT value of 0.8 was achieved at 723 K. Moreover, the detailed X-ray photoelectron spectroscopic analysis was carried for the determination of binding energy of the constituent elements in the experimental alloy; it also provided the correct estimation of atomic percentage of the concerned elements. The Raman spectrum revealed a shift in F2g peak with respect to that of Mg2Sn and Mg2Si in correspondence with the composition of the synthesised alloy. The synthesised alloy showed micro and nano hardness of 3.7 and 4.03 GPa respectively, which implies that good mechanical strength could be achieved in the synthesised alloy.