Sintering and microstructure development in the system MgO–TiO2

The sintering and microstructure development of magnesia containing 0–10 wt% TiO2 at temperatures in the range 1300–1600°C have been investigated. The addition of TiO2 markedly promoted densification at relatively low temperature, and grain growth. Excess TiO2 over the solid solubility limit of TiO2 (0.3 wt%) reacted with magnesia to form inter- and intra-granular magnesium titanate (Mg2TiO4) above 1300°C. The grain size of MgO increased with increasing TiO2 content, and densification was mainly governed by MgO grain growth. © 1998 Kluwer Academic Publishers     

Austenitic grain size evolution and continuous cooling transformation diagrams in vanadium and titanium microalloyed steels

The evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization temperatures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest grain size was found in the steel with a Ti concentration = 0.019 wt%. Low heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temperatures above 1050 °C the influence of the reheating temperature on their hardness for cooling rates around 2 °C · s^–1 was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the titanium content, the greater the hardness of these steels, but only when the titanium percentages were higher than 0.020 wt%.     

20wt% SiO2/Al-Mg复合材料的界面反应及其微结构

采用粉末冶金法制备了20wt% SiO₂/Al-Mg复合材料。研究了SiO₂和基体元素Al,Mg反应机制,研究表明:在原SiO₂颗粒内,形成MgAl₂O₄,MgO,Mg₂Si和少量Al和Si。MgAl₂O₄呈不规则形状,而且MgAl₂O₄往往和Al相邻;MgO和Mg₂Si形成片层状共析体;经620℃烧结30min,SiO₂被完全反应掉。反应生成物Si多数被排到Al基体中;原Al-Mg基体中主要物相为:Al,Mg₂Si和Si,Mg₂Si颗粒的尺寸小于0.2μm。原Al-Mg基体中,单质Mg已不存在,Mg反应形成Mg₂Si。     

Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained 23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4m. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4m. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is 43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Production of stabilized and non-stabilized ZrO2 by carbothermic reduction of ZrSiO4

The kinetics of reduction of zircon by carbon have been investigated in the temperature range 1400–1650 °C. Volatilization of SiO permits production of zirconia of about 96% purity direct from zircon with 100% removal of the silicon content being achieved in less than 2 h at 1650 °C. Addition of MgO, Y₂O₃ or CaO stabilizers to zircon prior to the reduction permits direct production of partially stabilized zirconia from zircon by means of carbothermic reduction. The presence of such stabilizers has been found to produce a slight increase in the reduction of kinetics of zircon.     

Preparation of MgO-doped mullite by sol-gel method,powder characteristics and sintering

Mullite doped with MgO in quantities ranging from 0.01 to 1.5 wt% were prepared by the sol-gel method. The mullitization temperature decreases with increased MgO dopant content. The XRD patterns of the MgO-doped mullite calcined at temperatures up to 1600° C for durations ranging from 1 to 10 h did not show the presence of any other phase except mullite. The IR spectrum shows a broadening of the Al-O absorbance band at 1175cm^–1 with MgO content, indicating the solid solution of MgO. Sintering temperature decreases with increased MgO dopant content. The microstructure observed consisted of equiaxed grains. The TEM observation of the microstructure showed the presence of glassy pockets at the triple grain junctions. The thermal coefficient of expansion and dielectric constant were not changed up to 0.75wt% MgO dopant concentration. The three-point bend strength observed for 0.3wt% MgO-doped mullite at room temperature was 300 MPa and decreased below 200 MPa at 1400° C.     

Preparation of slaking resistant CaO aggregate from lightweight CaCO3 with oxide addition

CaO aggregate was sintered from reagent-grade lightweight CaCO₃ powder by the addition of 0-20% (molar ratio) MgO and ZrO₂, respectively. The results showed that the CaO derived from lightweight CaCO₃ was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO₂ was added, CaZrO₃ layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure.     

Development and bioactivity evaluation of bioglasses with low Na2O content based on the system Na2O–CaO–MgO–P2O5–SiO2

Osteoconductive bioglasses, free of K(2)O and Al(2)O(3) and with content of Na(2)O lower than 10?mol%, were designed based on the ratio (SiO(2)?+?MgO)/(P(2)O(5)?+?CaO?+?Na(2)O) in the system Na(2)O-CaO-MgO-P(2)O(5)-SiO(2). The developed glasses have shown a strong potential for the formation of hydroxycarbonated apatite (HCA) in vitro. The particles of HCA aggregates tend to be of finer size with increasing the ratio of (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) in the glass chemical composition indicating significant bioactivity. Critical size bone defects created in the femurs of albino adult female rats, and grafted with the glass particles for 12?weeks post implantation, were completely healed by filling with mineralized bone matrix without infection showing a strong potential for new bone formation in vivo. Osteoblasts and osteocytes were observed close to the surface of the granular implants with active areas of bone deposition, resorption and remodelling. The bioglass with lowest (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) ratio has shown the highest bioactivity while the bioglass with the highest (SiO(2)?+?MgO)/(CaO?+?P(2)O(5)?+?Na(2)O) has shown the lowest bioactivity. The newly formed bone in vivo has shown a similar structure to that of the original bone as indicated by the histology and microstructural results. In addition, Ca/P molar ratio of the newly formed bone was found to be (~1.67), which is similar to that of the original bone.     

The influence of temperature cycling on the creep properties of Nickel 201 and Inconel 600 in combustion gas

The effect of temperature cycling on the creep behaviour of Nickel 201 and Inconel 600 in combustion gas has been studied. Specimens were tested both at constant temperature, 900° C, and at 900° C interrupted by temperarature drops down to 510° C. The creep straining has been analysed with respect to a weighted time parameter which includes the creep contribution during the lower temperatures of each cycle. With respect to this compensated time parameter, the temperature variations were generally observed to result in a strong acceleration in creep. The effect seemed to increase with increasing frequency of temperature drops, increasing grain size and decreasing stress. Thus, at low stress levels, large-grained specimens of both alloys experienced an acceleration even inabsolute creep rate upon cycling. The grain size dependency indicates that the destructive effect of the cycles is caused by crack formation. Surface cracking associated with grain boundary oxidation seemed to be the dominant cracking mode. It is suggested that, during creep in oxidizing environments, repeated periods of cooling might strongly accelerate the growth of surface creep cracks due to the difference in thermal expansion between metals and oxides. This difference causes high tensile stresses to arise in the metal in front of the grain boundary oxides, and the stresses are assumed to be high enough to nucleate microcracks along the boundary.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

The effect of MgO additions on the kinetics of hot pressing in Al2O3

The kinetics of hot pressing of Al₂O₃ with and without MgO additives have been measured at 1475 and 1630° C and at 5 to 20 MPa using Al₂O₃ powders of different grain size and using different additive levels. Data obtained within the solid solution regime are interpreted in terms of diffusional creep processes. MgO additions accelerate densification within this regime; the consequent reduction in pore size, and hence in pore drag, can explain the function of MgO as sintering additive for Al₂O₃.     

Chemical stability of MgO/CaO/Cr2O3–Al2O3–B2O3–phosphate glasses in solid oxide fuel cell environment

The thermal properties, glass forming tendency and glass stability in solid oxide fuel cell (SOFC) relevant atmospheres have been determined for four different refractory MgO/CaO/Cr2O3–Al2O3–B2O3 phosphate based glasses. Also, the bonding ability and interaction as a function of temperature between the glasses and La0.8Ca0.22CrO3 interconnect material have been studied. The microstructural characterization and elemental analysis of reaction couples reveal that the bonding ability and interface interaction is closely related to the CaO content and the relative glass forming tendency. The reactions at 1000°C between the glass and the solid interconnect material for the high CaO or MgO containing compositions occur by a liquid phase or a vapour phase mechanism, respectively. The reaction at 1200°C in both cases occurs by a liquid phase mechanism. © 1998 Chapman & Hall     

Electrical conductivity/microstructural relationships in aged CaO and CaO + MgO partially-stabilized zirconia

Samples of CaO and CaO + MgO partially-stabilized zirconia (PSZ), solution treated at 1830° C and aged at 1400° C to produce a fine dispersion of tetragonal precipitates, were subjected to a.c. electrical measurements in the frequency range 10^–3 to 10⁶ Hz, at a temperature of 300° C. The component of resistivity due to the grain interiors, obtained from plots of complex resistivity, showed three stages of behaviour, attributed to equilibration, coarsening and finally transformation of a tetragonal precipitate into a more resistive monoclinic phase. A model of the electrical response of a dispersion of particles was used to estimate the conductivities of the constituent phases of the PSZ ceramic and to simulate the complex resistivity data obtained.     

动态高温X射线衍射法研究水镁石的热分解过程

用水镁石矿粉矿作原料,用差热-热重分析和动态高温X射线衍射法研究水镁石的热分解过程,以及在不同温度和保温时间下的相变机理。结果表明:400℃是Mg(OH)2向MgO转变的相变点,水镁石在450～800℃的升温区间,产物均为纳米级MgO粉体,其晶粒度由23 nm增加到176 nm。水镁石在900℃下煅烧3 h,Mg(OH)2已经完全分解为MgO,杂质主要存在形式是CaMgSiO4和Mg2SiO4,约占总质量的2%左右。     

Crystallization behavior of silica-calcium phosphate biocomposites: XRD and FTIR studies

Silica and calcium phosphates (CaP) are the most important ingredients in bioactive materials that bond to bone and enhance bone tissue formation. In this study, silica-calcium phosphate (SiO2-CaP) composites were developed by powder metallurgy method, using silica (SiO2) and anhydrous dicalcium phosphate (CaHPO4) powders (CaP) in the ratios (wt%): 20/80, 40/60, 60/40 and 80/20. The effects of temperature and chemical composition on crystallization and phase transformation of the SiO2-CaP composites were evaluated by XRD and FTIR. Thermal treatment of the starting material suggested that CaHPO4 transforms into: y-Ca2P2O7 at 800°C; p-Ca2P2O7 at 1000°C and a-Ca2P2O7 at 1200°C. On the other hand, P-quartz was the only detected phase after thermal treatment of silica in the temperature range 800-1200 °C. For all SiO2-CaP composites, SiO2 and CaP did not modify the crystallization behavior of each other when sintered in the temperature range 800-1000°C. However, at 1200°C, CaP promoted the transformation of p-quartz into a-cristobalite. Moreover, SiO2 stabilized p-Ca2P2O7. The modifications in the crystallization behavior were related to ion substitution and formation of solid solutions.     

Application of deformation mechanism maps to the study of high-temperature creep of a precipitate-free 25wt%Cr-20wt%Ni austenitic stainless steel

Deformation mechanism maps for a precipitate-free 25 wt% Cr-20 wt% Ni austenitic stainless steel are introduced with normalized stress, reciprocal of homologous temperature and normalized grain size as co-ordinate axes. The maps are plotted, using experimental results wherever possible rather than comparing constitutive strain-rate equations predicted by deformation mechanisms. The maps make it possible to systematically classify the complex creep behaviour of the stainless steel, so that transitions in creep behaviour with changes in stress, temperature and grain size are clearly revealed. It is indicated that, if the maps are prepared from the constitutive equations alone, they are not in agreement with the experimental results.     

Effect of mixing ratio on bactericidal action of MgO–CaO powders

MgO–CaO powders were prepared with different molar ratios (MgO/CaO) at 1400 °C for 2 h in air. By using the powder samples obtained, the change in bactericidal effect as a function of MgO–CaO composition was studied by colony count method. From the XRD measurements, it was found that CaO solid solution was formed by the replacement of Mg²⁺ ion with larger Ca²⁺ ion. However, no formation of MgO solid solution was observed. The average particle size and the specific surface area of the samples used in this study were about 0.2 m and 10.5m²g^–1, respectively. The pH values of physiological saline containing powder samples increased with the increase of CaO content, and the value reached 12.1 in sample with the molar ratio (MgO/CaO) of 0.25. From the results of bactericidal tests for Staphylococcus aureus, it was found that the bactericidal effect increased with the increase of CaO content in the samples.     

Effect of Laves phase strengthening on the mechanical properties of high Cr ferritic steels for solid oxide fuel cell interconnect application

Two types of high chromium ferritic steels envisaged as construction materials for SOFC interconnects, were investigated in respect to microstructure and creep in the proposed application temperature range from 700 to 800 °C. The steel compositions mainly differed in the amounts of the Laves phase forming elements Nb, W and Si. The steel containing these alloying additions exhibited substantially higher creep resistance in the temperature range 700-800 °C than the high purity steel. The Laves phase formation occurred trans- as well as intragranular whereby the extent and size of grain boundary precipitates increased with increasing exposure time. Especially at 800 °C the precipitates inside the grains virtually completely vanished after longer exposure times and only intergranular precipitates remained. This change in precipitate morphology resulted especially at 800 °C in a decrease of creep resistance with increasing exposure time, although the Laves phase containing steel still exhibited higher creep strength than the high purity steel.     

Effect of CuO–BaO mixture content on the grain growth of BaTiO3

The present work has been aimed at studying the effect of processing variables such as amount of additions, sintering temperature, and duration on the density, microstructure, and grain growth of BaTiO3/CuO–BaO ceramics. Grain growth of BaTiO3 during liquid-phase sintering is investigated for CuO–BaO (mole ratio, CuO/BaO=2.5) mixture additions from 0.5 wt% to 2 wt%. Sintering in air for 1 to 16 h at 950 °C to 1250 °C is studied. BaTiO3 with 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt% CuO–BaO mixture additions, the average grain growth exponents are 5.3, 4.3, 5.9, and 9.2 and the activation energies are 605±60, 348±19, 650±50, and 810±110 kJ mol-1, respectively. For all compositions, the grain growth exponents decrease with increasing sintering temperature. © 1998 Chapman & Hall