Gas entrapment and evolution in prealloyed aluminium powders

Part of a comprehensive research programme involving different aspects of degassing of powder metallurgy (P/M) aluminium alloys carried out in the P/M Group of the Delft University of Technology, is reported. The fundamental aspects of moisture and gas evolution during degassing of a porous billet are described in a semi-quantitative manner using a kinetic approach. During degassing of Al-20Si-X P/M alloys, at temperatures up to 550 °C, the partial pressures of moisture and hydrogen were within the range 10^–4 to 10^–7 mbar. The thermodynamics of gas desorption is mainly influenced by temperature which is the critical degassing parameter. It appears that the diffusion of aluminium through the oxide layer can explain, to a large extent, the kinetics of degassing of aluminium powders. A shift in the release of moisture and hydrogen towards higher temperatures is due to the presence of MgO in the surface layer, compared to the situation when only Al₂O₃ builds the oxide film. Thermodynamical data indicate that the reaction of magnesium with water vapour proceeds more intensely than that between aluminium and water vapour.     

Investigation in the sintering of Y2O3 powders in the temperature range 1000 to 1400° C

This paper is devoted to a study of the sintering of two Y₂O₃ powders in the temperature range where only minor densification occurs. Two powders have been examined; one powder, Y₂O₃-A, was obtained by decomposition of hydroxide, because earlier examinations showed [11] that use of this powder resulted in the highest densities of samples in the sintering temperature range from 1300 to 1900° C. The second powder, Y₂O₃-D, was purchased externally. In order to ensure that the pores in the Y₂O₃-A compacts closed as late as possible, the heating rates up to the appropriate temperatures (1000 to 1400° C) were varied in the range 0.013 to 6° C sec^–1. The results obtained show that the heating rate in this temperature range, for the powder obtained by decomposition of hydroxide, is of primary importance in the densification of the material, and that cessation of shrinkage was not observed in the period of 240 min.     

Photoluminescence from terbium doped silica-titania prepared by a sol-gel method

Terbium doped (0.5 at.%) TiO₂-SiO₂ (30%/70%) was prepared by a sol-gel method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the powder calcined at two different temperatures. At a low temperature of 550 °C an amorphous phase was obtained, but at a higher temperature of 1000 °C, the anatase TiO₂ phase was crystallized in the amorphous SiO₂ phase. Green photoluminescence from ultraviolet excitation was detected after heating to either temperature, but the amorphous sample heated to 550 °C exhibited a higher intensity. X-ray diffraction and photoluminescence excitation data are discussed to explain these observations.     

Oxidation sizing of iron and iron-neodymium-boron powders

A powder sizing test developed for use on WC powders has been extended for use on iron and iron-neodymium-boron powders. In this test the particle size is derived from the rate of oxidation, because finer powders oxidize quicker. The rate of oxidation is monitored in a thermogravimetric analyser, where the powders are subjected to a controlled heating rate from room temperature to 1100 °C. If the constants from the Arrhenius law are known the powder size can be determined by comparing experimental oxidation plots with theoretical curves. For the sizing of a commercial spherical iron powder, the oxidation technique compared favourably with direct sizing using scanning electron microscopy and image analysis. The values for the activation energy of 125 kJ mol^–1 determined in this study agree with previous studies. Validation of the sizing technique on a hydrogen-decrepitated stoichiometric Nd₂Fe₁₄B powder proved difficult because it was not possible to determine a definitive size distribution independently. Metallography of partially oxidized samples showed that the process is two-stage, at low temperatures the neodymium oxidizes, and above 400 °C the powder behaves as pure iron. Theoretical curves based on one oxidation process with an activation energy of 100 kJ mol^–1 gave the best fit to the experimental curves.     

Formation of ternary carbide Fe3Mo3C by mechanical activation and subsequent heat treatment

Ternary carbide, Fe₃Mo₃C was prepared from the powder mixture of Fe/Mo/C = 1/1/1 which was ground for 3 h in a planetary ball mill and subsequently heated at a temperature as low as 700°C, its amount increased with heating temperature. In contrast, when the 1 h-ground and unground samples were heated at 700–1000°C, Mo₂C formed. From the results obtained about the effect of mixing ratio, grinding time and heating temperature of Fe/Mo/C samples on the formation of Fe₃Mo₃C, it was found that the formation of Fe₃Mo₃C strongly depends on the mixing homogeneity and the activated state of the particles of Fe, Mo and C components induced by mechanical grinding. Fe₃Mo₃C obtained belongs to a hard magnet, having saturation magnetization of 0.4 emu g^–1, remanence of 0.13 emu g^–1 and coercivity of 200 Oe.     

Relationship between degassing conditions and tensile properties of Al-20Si-X P/M products

Results are reported which show the effect of different degassing modes on the properties of the Al-20Si-3Cu-1 Mg powder. The paper complements previous papers [1–3] concerning the conventional and modified degassing of the same powder. This research was mainly directed to study the influence of temperature on the tensile properties, ultimate tensile strength, σ_UTS, and elongation, ɛ, of extrudates obtained of Al-20Si-3Cu-1Mg compacts non-degassed, conventionally degassed, and treated by a modified process, namely degassing assisted by flushing with a depurative gas such as argon or nitrogen. The processing of the Al-20Si-3Cu-1Mg P/M powder must include a degassing step which significantly improves the tensile properties, at room and elevated temperatures, of the products of compacted powder with respect to those of the products whose compacts were non-degassed. It is apparent that degassing assisted by flushing with argon or nitrogen gives products with higher tensile properties than those of the products conventionally degassed under optimal conditions of temperature and time and much higher than those of the non-degassed products. The tensile results are in agreement with the theoretical approach to the gas entrapment and evolution of the aluminium powders presented in previous papers.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Reaction characteristics of La0.84Sr0.16CrO3 formation

We studied the kinetics of La_0.84Sr_0.16CrO₃ formation from a precursor consisting of La and Sr chromium oxides and carbonates made by spray roasting. Pure LaCrO₃ becomes cubic at temperatures exceeding 1900 °C. Strontium doping lowers the transition temperature, for example, that of La_0.84Sr_0.16CrO₃ is 1700 °C. This transition is gradual and occurs over a 700 °C range upon heating and cooling. Low temperature (LT) air calcination (450 °C) of the precursor yields a mixture of LaCrO₄ and SrCrO₄, which following 20 h of heating at 1440 °C produces a homogeneous powder. Secondary electron images of this precursor reveal dense spheres with 95% of the theoretical density of La_0.84Sr_0.16CrO₃. High temperature (HT) calcination (800 °C) yields a mixture of LaCrO₃ and SrCrO₄, which following 40 h of heating at 1500 °C produces a uniform product. The LT and HT calcination causes oxygen loss.     

Phase transformation reactions in rapidly solidified Al-6.5Fe-1.5V alloys

Phase transformation reactions, occurring during heating of as-atomised Al-6.5Fe-1.5V powders, extrusion of the powders, and heating of the as-extruded alloys produced from the powders, have been studied by DSC, XRD and TEM. The DSC studies of the as-atomised powders revealed several phase transformation reactions. The solid solution in zone A decomposed to form metastable phases at 360°C. These metastable phases further transformed to form equilibrium phases at 500°C. The microquasi-crystalline icosahedral (MI) phase particles present in zone A and zone B transformed to equilibrium phases at 500°C. The globular clusters of microquasi-crystalline icosahedral (GCMI) phase particles in zone C transformed polymorphously to icosahedral (I) phase particles at 450°C. These reactions were believed to occur during extrusion of the powders. During heating of the as-extruded alloys produced from coarse powder particles, I phase transformed polymorphously to hexagonal phase at 550°C. The hexagonal phase decomposed to monoclinic Al₄₅(V, Fe)₇ and Al₁₃Fe₄ phases upon heating for longer times.     

Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO₂-CeO₂, metastable t-ZrO₂ solid solutions containing up to 30 mol% CeO₂ crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce₃ZrO₈ solid solutions are formed in as-prepared powders between 30–75 mol % CeO₂. The variation of the lattice parameters of both solid solutions is determined as a function of CeO₂ content. The value of the lattice parameter of pure Ce₃ZrO₈ (cubic) is a = 0.5342 nm. Detailed characterization of the Ce₃ZrO₈ powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m²g^–1 until the heating temperature is above 1000°C.     

The sintering of non-stoichiometric UO2 under non-isothermal conditions

The sintering kinetics of non-stoichiometric uranium dioxide powders have been studied in the temperature range 700 to 950° C. The results of the relative linear shrinkage during the stepwise heating of samples, were analysed as a function of sintering temperature and time. It has been shown that it is impossible to explain the exceptionally large shrinkage of UO_2+x compacts in the temperature range 0.3 to 0.4 T _m by means of a single sintering mechanism.     

Characteristics of aerosol-synthesized AlN particles

Particulate characteristics were investigated for AlN synthesized by a thermochemical reaction of aluminium aerosol with ammonia gas. The product powders had a decreasing specific surface area between 15.7 and 36.7 m²g^–1, with increasing reaction temperature from 1100–1500 °C, and the powders with large surface area were strongly hygroscopic. Although the powders were severely aggregated and had a small amount of unreacted aluminium, light milling and post heat treatment made them ultrafine and completely converted. When sintered with 0.5% yttrium at 1900 °C, full densification and high thermal conductivity of about 130 Wm^–1 K^–1 were obtained.     

Preparation and performance of MAX phase Ti3AlC2 by in-situ reaction of Ti-Al-C system

In order to clarify the effects of the proportion of raw powders, heating temperature and holding time on the purity and properties of MAX phase Ti₃AlC₂, powder mixtures of Ti, Al and C powder with different ratio were prepared by planetary ball mill and heated under different conditions by spark plasma sintering. The microstructures and phase structures of the as-synthesized samples were characterized, the correlation between the mechanical properties and microstructure and fracture mechanism were investigated systematically. The results show that with the proportion of raw powders Ti:Al:C = 3:1.2:2, the sample heated at 1300 °C for 60 min has the highest purity 97.23 wt% of MAX phase. It has a compact and uniform lath-like structure with the length-thickness ratios of 3–5 and excellent comprehensive mechanical properties: the Vickers hardness, bending strength and fracture toughness are 5.26 GPa, 500 MPa and 7.35 MPa∙m^1/2, respectively. The experimental results show that among the three factors, the proportion of raw powders has the greatest influence on purity of Ti₃AlC₂ phase, followed by heating temperature and holding time. The fracture morphologies of the tested samples demonstrate that under the action of external force, extrusion and kink occurred in the layered structures of Ti₃AlC₂ phase. These two forms of energy dissipation lead to the bending strength and fracture toughness of Ti₃AlC₂ are higher than that of traditional ceramics.     

Oxidation behaviour of reaction-bonded alumina compacts using an Al88Si12 alloy precursor

The oxidation behaviour of attrition-milled Al₈₈Si₁₂/Al₂O₃ powder mixtures was investigated for the formation of mullite/Al₂O₃ composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5°C min^–1 to temperatures between 450 and 1400°C. Oxidation occurred rapidly between ca. 400 and 750°C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850°C) While the incorporation of ZrO₂ improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO₂ to form phases which prevented the formation of mullite at the temperatures used in the experiments.     

Novel synthesis of TiN fine powders by nitridation with ammonium chloride

TiN fine powders were prepared by the reaction of TiH₂ with ammonium chloride at various temperatures under a flow of N₂/H₂ mixed gas. In a temperature range of 500-800°C, the powder samples obtained had a particle size of 3-20 nm, and a specific surface area of 30-60 m²/g. The particle size increased with the increase of the reaction temperature. The lattice parameters and the chemical analysis data showed that the TiN powder had the stoichiometric composition. The TiN powder prepared at temperatures of 600-800°C showed superconductivity with a transition temperature of 4.0-4.5 K.     

Aluminium titanate formation by the gas-phase hydropyrolysis method

A novel process of aluminium titanate formation by thermal decomposition of an aqueous nitric-hydrofluoric solution containing stoichiometric amounts of aluminium and titanium is described. The solution is decomposed by spraying it into a heated reactor with subsequent oxide formation and recycling of the corresponding nitric and hydrofluoric acids by means of an absorption system. The oxides formed were mostly amorphous, in a fine dispersed, homogeneously distributed form of highly reactive state. X-ray (RDA) analysis showed crystalline forms of TiO₂ (anatase), complex titanium oxides, such as Ti₃O₅, Al₂TiO₅ (tialite) and AlO(OH) (boehmite). Complete formation of tialite from this oxide mixture was studied by thermogravimetry, differential thermogravimetry and differential thermal analysis. RDA of compounds formed after the successive and stepwise heating of this powder in a muffle helped in the understanding of tialite formation. The reaction was found to be time dependent and to be completed at 1400° C. Decomposition of the tialite formed into Al₂O₃ (corundum) and TiO₂ (rutile) was observed in the temperature range 900 to 1070° C.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

The effects of hydroxide gel drying on the characteristics of co-precipitated zirconia-hafnia powders

Mixed zirconia-hafnia (Hf_0.25Zr_0.75O₂) powders of fine particle size and narrow particle-size distribution can be prepared via co-precipitation routes using mixed zirconium and hafnium salts as the starting materials. The characteristics of the resultant zirconia-hafnia powders are dependent strongly on the dehydration route by which the co-precipitated hydroxide gels are dried. Zirconium-hafnium hydroxide gels are formed when zirconium and hafnium oxynitrates are co-precipitated in an ammonia solution of pH 10.5. The co-precipitated hydrous gels were dried by three very different routes including organic solvent dehydration, microwave drying, and conventional infrared heating lamp drying. The dried hydroxides were then calcined at various temperatures in the temperature range 550–1150 °C, followed by ball milling to remove large soft-particle agglomerates. The resultant zirconia-hafnia powders were characterized for crystallite size, particle size, particle-size distribution, particle morphology, and the degree of powder agglomeration, using experimental techniques such as X-ray diffraction, BET surface area, differential thermal analysis, thermo-gravimetric analysis, sedigraph, scanning and transmission electron microscopy. Hard particle aggregates, which cannot be effectively eliminated using ball milling, occur in the zirconia-hafnia powders processed via either the microwave drying or conventional infrared heating lamp drying routes. In contrast, the organic solvent dehydration route resulted in an almost aggregate-free powder of fine crystallite and particle sizes. Therefore, the zirconia-hafnia powder processed via the organic solvent dehydration route exhibited high sinterability on sintering at 1300 °C.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.