Effect of sintering temperature on the phase composition and microhardness of WC-8 wt % Co cemented carbide

The effect of sintering temperature (800–1600°C) on the phase composition, density, and microhardness of WC-8 wt % Co cemented carbide has been studied using x-ray diffraction, scanning electron microscopy, optical microscopy, and density measurements. The results indicate that, during sintering of the starting powder mixture, containing not only WC and Co but also the lower carbide W₂C and free carbon, W₂C reacts with cobalt metal to form Co₃W. At sintering temperatures from 900 to 1200°C, the reaction intermediate is the ternary carbide phase Co₆W₆C. During sintering at 1300°C, this phase reacts with carbon to form Co₃W₃C. Sintering at 1000°C and higher temperatures is accompanied by the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co(WC). The density and microhardness of the sintered samples have been measured as functions of sintering temperature, and the optimal sintering temperature has been determined.     

Effect of WC nanoparticle size on the sintering temperature,density, and microhardness of WC-8 wt % Co alloys

Using X-ray diffraction, scanning electron microscopy, and density measurements, we have studied the effect of WC particle size (20 to 150 nm) on the optimal sintering temperature of the WC-8 wt % Co alloy and the effect of sintering temperature (800–1600°C) on its phase composition, density, and microhardness. The results indicate that, during sintering of the starting powder mixture, the first to form is the ternary carbide phase Co₆W₆C. At sintering temperatures of 1100°C and above, this phase reacts with carbon to form Co₃W₃C. Sintering above 1000°C leads to the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co〈WC〉, along with the ternary carbide phases. The density and microhardness of the alloy have been measured as functions of sintering temperature. The use of WC nanopowder has been shown to reduce the optimal sintering temperature of the WC-Co alloy by about 100°C.     

Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures

The high temperature behavior of SrSO₄, SrCO₃ and Al₂O₃ mixtures was studied. A mixture of 1:1 mole of SrSO₄ and mechanically activated SrCO₃ was mixed and characterized using thermal gravimetric analysis. Some samples were uniaxially pressed and sintered at 1100, 1200 and 1300 °C for 8 h and then analyzed using X-ray diffraction and scanning electron microscopy. Additionally, a mixture of SrSO₄:SrCO₃:Al₂O₃ was uniaxially pressed and sintered at 1500 °C. The decomposition temperature of SrCO₃ was decreased 18° by milling for 180 min. Samples sintered at 1300 °C showed a microstructure free of porosity. X-ray diffraction analysis showed the presence of SrO and SrSO₄ after sintering at 1100, 1200 and 1300 °C. The mixture containing alumina showed the formation of a strontium aluminum oxide sulfate compound in addition to strontium aluminate.     

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl₂ in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ?1000°C, however, PtAl₂ became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni₃Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni₃Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.     

Effect of Sintering Temperature on the Microstructure,Electrical, and Magnetic Properties of Bi 0 . 8 5 Eu 0 . 1 5 FeO 3 Ceramics

Polycrystalline Bi _0.85Eu _0.15FeO ₃ ceramics were synthesized by solid-state reaction method with rapid liquid phase sintering process at various sintering temperatures. The dependence of structural, microstructural, electrical, and magnetic properties on sintering temperature was systematically investigated. X-ray diffraction measurements reveal that single perovskite phase is developed in Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 850 and 870^° C, while secondary phases can be detected in the samples sintered at 890^° C due to the volatilization of Bi ³⁺ ions, and the crystallinity increases with increasing sintering temperature from 850 to 890 °C. The scanning electron microscopy investigation has suggested that the grain size increases with increasing sintering temperature from 855 t o 890^° C; while the pore size decreases with increasing sintering temperature from 850 to 870^° C and then increases with a further increase of sintering temperature. The electrical and magnetic measurements show that the leakage current, dielectric constant, dielectric loss, and magnetic properties are strongly dependent on the sintering temperature. The Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 870^° C have the lower leakage current, higher dielectric constant, and lower dielectric loss. The room temperature magnetization increases with increasing sintering temperature from 850 to 890 °C. The possible reason for all the above observations was discussed.     

Fractographic study of sintered nanophase TiO2

Fractography of sintered nanophase and commercial coarser TiO₂ powders was carried out using scanning electron microscopy. Compared with material sintered from commercial TiO₂ powder, nanophase TiO₂ sintered at 1400°C contains smaller voids and the sintering temperature for completely transgranular fracture is 200°C lower. The cracking of nanophase samples during sintering is also discussed.     

Effects of alumina surrounding in sintering process on ZnO–Bi2O3varistors doped with CoO

Nonlinear current–voltage properties of zinc oxide-bismuth oxide varistors doped with cobalt oxide were investigated. The composition of ZnO doped with 0.5 mol% Bi₂O₃ and 0.75 mol%CoO was prepared and sintered in two different surroundings, zinc oxide–bismuth oxide–cobalt oxide powder and alumina powder. Microstructures and element tracing were studied using scanning electron microscopy with X-ray microanalysis, electron probe microanalysis and X-ray diffraction. Results clearly showed that the alumina surrounding led to bismuth loss from the green body and degraded the nonlinear property of varistors. At sintering temperatures higher than 1150 °C, the alumina-surrounding sintered specimens became ohmic and the electrical resistivity of 200 Ω cm was found in the specimen sintered at 1200 °C.     

Effect of MnO<Subscript>2</Subscript> on chemical interaction between CaO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have studied the effect of manganese dioxide (5–20 wt %) on the formation of calcium monoaluminate in the CaO-Al₂O₃ system during solid-state sintering of five oxide mixtures corresponding to the known calcium aluminates at temperatures from 1100 to 1400°C. X-ray diffraction examination indicated the formation of calcium manganates. The mixture of the calcium manganates melts at 1330°C, promoting the sintering process and the reaction between the calcium oxide and alumina and raising the yield of calcium monoaluminate as the dominant phase. The calcium manganates are shown to be nonreactive with the forming calcium aluminates. The X-ray diffraction results are supported by scanning electron microscopy data for a number of samples.     

Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder

Stainless steel (316) foams of varying porosities have been made through powder metallurgy route using NH₄HCO₃ as a space holder. Green compacts of stainless steel powder with NH₄HCO₃ were sintered at two different temperatures: 1100 °C and 1200 °C. At higher sintering temperatures, neighboring stainless steel powders fused together to form polycrystalline grain structure with iron–chromium intermetallic phases segregated along the grain boundaries. Whereas, the fusion of neighboring stainless steel powders was limited around the particle–particle contact only when the green compacts were sintered at 1100 °C, which resulted in a larger amount of microporosities in the cell wall. These foams exhibited strain hardening behavior in the plateau region under compressive loading. The yield stress and the flow stress (at lower strain levels) of foams, sintered at 1100 °C were higher. But, the reverse is true for the flow stress at higher strain levels. The exponents and the coefficients of the power law relationships varied with sintering temperature and strain levels.     

Sintering of biphasic calcium phosphates

Biphasic calcium phosphate (BCP) discs were fabricated and then sintered using two different sintering programs to establish whether the phases present could be controlled at low and high sintering temperatures. X-ray diffraction (XRD) was used to establish the phases present after sintering and scanning electron microscopy (SEM) determined the microstructure. Sintering program 1 involved a simple heating and cooling schedule and temperatures of 1100, 1250, 1275 and 1300°C. It produced samples containing an additional alpha-tricalcium phosphate (α-TCP) phase at temperatures above 1100°C. The original ratio of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) could not be maintained above this temperature. Sintering program 2 combined the heating and cooling schedules of the first program with a 900°C hold stage to allow α-TCP to β-TCP conversion to take place. At temperatures of 1250 and 1275°C, this program was successful in completely removing the α-TCP phase and preserving the HA:β-TCP ratio. The SEM results show that the surface morphology of the discs was not greatly affected by choice of sintering program.     

Yb3+ and Er3+ co-doped Y2Ce2O7 nanoparticles: synthesis and spectroscopic properties

Yb³⁺ and Er³⁺co-doped Y₂Ce₂O₇ nanoparticles sintered at different temperatures were prepared by homogeneous co-precipitation method. The products were characterized by X-ray powder diffraction (XRD), energy-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The results indicated that the particle sizes and morphologies of the samples were heavily influenced by the sintering temperature. As temperature increased, the particle sizes became gradually larger and more agglomerate. The emissions including green and red upconversion emissions were investigated under 980 nm excitation. The emission intensities of the samples also depended on the sintering temperature. Two photon processes were mainly responsible for green and red upconversion emissions.     

Effect of sintering temperature and heating mode on consolidation of Al–7Zn–2·5Mg–1Cu aluminum alloy

Densification behaviour, phase transformation, microstructural evolution and hardness values of microwave sintered Al?C7Zn?C2·5Mg?C1Cu (7775) aluminum alloy were investigated and compared with conventionally sintered samples. Microwave sintering was performed in 2·45?GHz multimode microwave furnace at temperatures ranging from 570?C630 °C. Microwave sintering at a heating rate of as high as 22°C/min resulted in ~55% reduction of processing time as compared to conventional sintering. A lower sintered density observed in the case of microwave processed samples was attributed to the inhomogeneity in microstructure and phase distribution. The X-ray diffraction results of conventionally sintered samples showed the presence of MgZn₂, Mg₂Zn₁₁ and CuMgAl₂, while only MgZn₂ and CuMgAl₂ phases were found in the case of microwave sintered samples and in lesser amount. Higher hardness and high standard deviation values were noticed for microwave sintered samples as compared to conventional counterparts.     

Solid state phase transformations in Ti–Al–Ru system

Abstract

An investigation including the use of transmission electron microscopy has been carried out into the solid state transformations in Ti–Al–Ru alloys in the composition range 17–37 at.-%Al and 1–5 at.-%Ru. Ruthenium is shown to be a strong β stabiliser: a 5 at.-%Ru addition produced complete or substantially complete retention of metastable β on quenching from 1250°C (within the stable β range). Three types of martensite can result by quenching from the β field, depending on the composition: a hexagonal martensite α′ in α + α₂ alloys with 1 at.-%Ru, an ordered hexagonal (DO₁₉) in alloys close to Ti₃Al stoichiometry with 1 at.-%Ru; an orthorhombic martensite in α + α₂ alloys with 2 at.-%Ru. On heating martensite or retained β in the range 1100–770°C, various transformations occur depending on composition and temperature, e.g. tempering of martensite to α₂ + β or α₂ + γ + G, or reversion of martensite to β followed by precipitation of one or more of the phases α, α₂ and γ.

MST/1029     

Synthesis and characterization of nanocrystalline ferroelectric Sr0.8Bi2.2Ta2O9 by conventional and microwave sintering: A comparative study

In recent years mechanical activation technique has been utilized to synthesize the nanocrystalline form of compounds resulting in enhancement in the properties. Also, microwave sintering is being preferred over conventional sintering due to rapid processing and uniform temperature distribution throughout the specimen. In the present work, nanocrystalline non-stoichiometric strontium bismuth tantalate (SBT) of the composition Sr_0.8Bi_2.2Ta₂O₉ ferroelectric ceramics were synthesized by microwave sintering process (with sintering temperatures of 1000 °C and 1100 °C) and conventional solid state reaction process (with sintering temperature of 1100 °C) with an objective of comparing the properties of the synthesized specimens by the two processes. X-ray diffraction analysis shows the formation of single phase layered perovskite structure formation by both the processes. Scanning electron microscopy reveals the formation of a finer granular microstructure in the specimen synthesized by microwave sintering compared to that in the specimen prepared by conventional sintering. The specimen prepared by microwave sintering process exhibits improved electrical properties with higher dielectric constant, higher piezoelectric and pyroelectric coefficients and lower dielectric loss.     

Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.

     

Characterisation of two Al–Fe based high temperature alloy powders

Abstract

Aluminium alloys containing additions of iron and cerium are among the alloys being developed as potential replacements for titanium based alloys for moderately high temperature applications. Development of these alloys is possible using rapid solidification technology, which results in a very fine distribution of dispersoids in the aluminium matrix. The microstructures of two rapidly solidified high temperature alloy powders of composition (wt-%) Al–6·7Fe–5·9Ce (alloy A) and Al–6·2Fe–5·9Ce–1·63Si (alloy B) have been characterised using transmission electron microscopy and the results are explained on the basis of some of the major solidification parameters, such as nucleation undercooling and recalescence. It was observed that most of the powder particles in the +10 to ?20 μm size range contained both microcellular and cellular regions, which could be explained in terms of an initial large undercooling followed by recalescence. The decomposition of the powder microstructure after exposing the powders to temperatures of 350, 420, and 500°C for 1 h was investigated using transmission electron microscopy. This work was complemented by phase identification studies using X-ray diffraction. The equilibrium precipitates Al₁₃Fe₄, Al₈Fe₂Si, and Al₃FeSi were detected in the powder microstructure of alloy B, whereas Al₁₃Fe₄ precipitates were detected in alloy A after high temperature exposure (500°C).

MST/1571     

Effect of temperature on X-ray, IR and magnetic properties of nickel ferrite prepared by oxalate co-precipitation method

Polycrystalline Nickel ferrite was synthesized by oxalate co-precipitation method at different sintering temperatures (700, 800, 900, 1000 and 1100°C) and characterized by X-ray diffraction and far IR absorption techniques. The lattice parameter (a), A-site and B-site radii (r _A , r _B ) were computed. XRD shows the formation of single phase cubic spinel structure. The crystallite sizes are calculated by Scherrer formula. The crystallite size is found to be increasing with sintering temperature. Porosity and Bond lengths on A-site and B-site were calculated and found to be minimum for the sample sintered at 800°C. The Far IR absorption bands are observed around 600 cm^?1 and 400 cm^?1 on the tetrahedral and octahedral sites respectively. Magnetization parameters such as saturation magnetization (Ms), magnetic moment (n_B) were calculated and the results are discussed with the help of existing theories. Saturation magnetization was found to be 47.85 emu/gm for the sample sintered at 1100 °C. Physical densities are obtained by Archimedes principle and are found to be 97.36% of their corresponding X-ray densities.     

Preparation and characterization of precursor powders for yttria-doped tetragonal zirconia polycrystals (Y-TZP) and Y-TZP-Al2O3 composites

Precursor powders for the preparation of tetragonal 2.5 mol% Y₂O₃-ZrO₂ containing 0 to 30 wt% Al₂O₃ were made by coprecipitation. The behaviour of this powder during calcination from room temperature to 1200° C was studied using differential thermal analysis. X-ray diffraction and transmission electron microscopy methods, and measurements of surface area. The uncalcined powder was essentially amorphous. On heating alumina-free powder, zirconia crystallized at 485° C: for increasing alumina content, zirconia crystallized from an amorphous aluminous matrix at increasing temperatures (850° C for 20 wt% Al₂O₃), while the crystallite size decreased and the surface area of the powder increased. The zirconia first crystallized as cubic, but transformed to the tetragonal form near 1100° C. The alumina crystallized as corundum at 1200° C. No monoclinic zirconia could be detected when calcined aluminous material was cooled to room temperature. The sintering behaviour of the calcined powder is discussed.     

Preparation of NixTiy compound via Ni plating followed by thermal treatment of Ti powder

In this research, Ni_xTi_y compound was prepared by thermal treatment of Ni-plated Ti powder. For this purpose, Ti powder was plated in an electroless Ni bath for various times (120, 225, 300, and 720?min). Hydrazine hydrate was used as a reductant for the deposition of pure Ni on the Ti particles. The plated powder (225?min) was heat treated under argon atmosphere to achieve Ni_xTi_y powder. Finally, the heated/plated powder was pressed by CIP followed by sintering at 980°C for prepare the Ni_xTi_y bulk sample. The plated powders as well as sintered one were characterized using scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, X-ray diffraction and differential scanning calorimetric. The NiTi₂, NiTi, and Ni₃Ti phases were detected in the XRD patterns of heated/plated Ti powder. According to DSC data, the heated/plated Ti powder showed reversible martensitic transformation at temperature range of ?38.0°C to +38.1°C, while sintered/heated/plated Ti powder displayed reversible transformation at temperature range of 16.0°C–15.4°C.