Fabrication of dense thin sheets of γ-TiAl by hot isostatic pressing of tape-cast monotapes

A method is described for the fabrication of dense thin sheets of γ titanium aluminide (γ-TiAl) by a powder metallurgy route involving hot isostatic pressing (HIP) of tape-cast monotapes. Gamma-TiAl powder (particle size <90 μm) was incorporated into a concentrated slurry by mixing with an organic binder in a solvent and the system was tape-cast to form sheets with a thickness of 400–600 μm. After insertion of the tape-cast sheet into a HIP can and binder removal in situ by thermal decomposition, HIP at 1100 °C under a pressure of 130 MPa produced dense sheets with a thickness of 250–400 μm. The free, dense sheets with a fine-grain microstructure were obtained by dissolution and oxidation of the HIP can. The carbon content of the fabricated sheets was 0.035 wt.%. Facile adaptation of the process to the production of γ-TiAl thin sheets with complex shapes is expected.     

Densification of a rapidly solidified nickel aluminide powder Part II Characterization of microstructure and mechanical properties

A Ni₃Al–X intermetallic compound prepared from a rapidly solidified powder was consolidated with hot isostatic pressing (HIP). Its microstructural development during the process has been examined. It involves a change from an inhomogeneous structure (a mixture of dendritic and equiaxed structures) to a uniformly distributed equiaxed structure and the formation of a disordered network phase from a metastably ordered matrix supersaturated with chromium, as a result of non-equilibrium solidification of the powder. The resulting microstructure of the consolidated material is mainly a function of HIP temperature. Mechanical properties of the HIP material at room and elevated temperatures have been determined. The results show that both the hardness and yield strength of the material decrease, while both the ultimate tensile strength and tensile elongation increase with rising HIP temperature up to 1250°C. Scanning electron microscope examination of the fracture surfaces of tested samples reveals a transition from interparticle fracture to transparticle fracture with increasing HIP temperature.     

A study on preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot isostatic pressing,and its characterization

Mo–0.6Ti–0.2Zr–0.1C alloy was prepared by mechanical alloying (MA) and subsequently consolidated by powder processing techniques. The pellets prepared from the fine size MA powder showed a high rate of densification during sintering in the temperature range of 1300–1500 °C. Close to theoretical density was attained by hot isostatic pressing (HIP) at 1250 °C and TEM studies revealed the uniform distribution of complex carbide precipitates (<100 nm) in the fine grain microstructure of the consolidated alloy. The alloy consolidated by HIP showed a high hardness of the order of 500 HK due to the presence of the carbides in the fine grain microstructure.     

Hot Isostatic Pressing of MgAl2O4 Spinel Infrared Windows

Optically transparent MgAl₂O₂ spinels have been produced by hot isostatic pressing (HIP). The spinel samples were prepared by hot pressing and subjected to capsule-free HIPing. The hot-pressed samples were made at 1500°C with a pressure from 14 to 41 MPa for 2 to 4 hours. In the HIP process, the pressure ranged from 14 to 207 MPa at 1500°C. The effect of hot isostatic pressure on the bulk density, microstructure and optical properties of the spinel sample were investigated here. The bulk density of the sample increased with HIP pressure and the sample HIPed at 207 MPa resulted in a bulk density of 3·576 g/cm³, about 99·94% of the theoretical density. A bimodal grain size distribution exists in samples HIPed at pressures ≤ 138 MPa. The extent of the abnormal grain growth decreased with pressure. The transparent spinel with uniform and fine grain size of 2 μn was obtained at 207 MPa. The transmission at short wavelength increased significantly with HIPing pressure. The transmittance of the sample HIPed at 207 MPa at a wavelength of 0·7 μn was 72%.     

Effect of pH on the Crystallization of Homogeneously Precipitated Nanosized PZT Powder

Nanosized lead zirconate titanate (PZT) powder with Zr:Ti ratio in the morphotropic phase boundary region was synthesized by homogeneous precipitation of metal ions. The powder precipitated at 90°C and at pH 6.7 resulted single-phase perovskite lead zirconate titanate powder when calcined at 550°C and above for 4 hours in air. The solution pH and the precipitation temperature strongly affect the composition of the calcined powder. The results obtained by structural characterization of homogeneously precipitated powder were compared with that obtained from the conventional precipitation method using ammonia in terms of crystallization, homogeneity, and microstructure. The homogeneously precipitated powder showed smaller particle size, minimum agglomeration and uniform shape on calcination and annealing. Powdered samples that precipitated by homogeneous precipitation crystallized directly to perovskite PZT, without any intermediate pyrochlore phase formation. In contrast, the NH₃ precipitated powder converted to perovskite PZT via metastable pyrochlore and it showed phase segregation upon annealing at higher temperatures. The reaction kinetics has been studied by X-ray diffraction, differential thermal analysis, and differential scanning calorimetry.     

Synthesis of (Bi0.5Na0.5)TiO3 nanocrystalline powders by stearic acid gel method

Bismuth sodium titanate (Na_0.5Bi_0.5)TiO₃ (NBT), is considered to be an excellent candidate for a key material of lead-free piezoelectric ceramics. In this study, we propose a stearic acid gel method for the preparation of nanocrystalline single phase NBT powder at relatively low treatment-temperature. Infrared (IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TG) and X-ray diffraction (XRD) were used to characterize the process of crystallization. The particle size and morphology of the calcined powders were examined by TEM. It shows that pure single phase NBT powders could be obtained at 700 °C for 1 h, and the particle size is about 20 nm. With an increase in the calcination temperature, crystallite size increased. The powders were further pressed into disk and sintered at 1120 °C for 2 h in air, and its density and microstructure were compared with traditionally prepared samples.     

Role of glycine-to-nitrate ratio in influencing the powder characteristics of La(Ca)CrO3

Ultrafine La(Ca)CrO₃ (LCC) powders were prepared through glycine-nitrate gel combustion process. The effect of glycine-to-nitrate ratio on batch size, particle size, nature of agglomeration and densification was studied. As-prepared powders when calcined at 700 °C resulted in LCC along with a small amount of CaCrO₄. The primary particle size obtained in case of stoichiometric and fuel-rich precursor was found to be 25–60 and 60–180 nm, respectively. It was found that the final powder had softer agglomerates with increasing glycine-to-nitrate ratio, which in turn improved the sintered density. The powder obtained through fuel-rich precursor could be sintered to ≈98% of its theoretical density at 1300 °C without applying any ball milling operation.     

Strength and toughness of beryllium-niobium intermetallic compounds

Bend and compression strengths, fracture toughness, and high-temperature microhardness of Be---Nb intermetallic compounds were measured at temperatures up to 1200 °C. Be₁₂Nb and Be₁₇Nb₂ materials exhibited brittle behavior at temperatures below 1100 °C in bending and below 800 °C in compression. Hot isostatically pressed (HIP) Be₁₂Nb had the highest low-temperature strengths (250 MPa in bending and 2750 MPa in compression) resulting from its greater fracture toughness (K_IC = 4 MPa m^1/2) compared with the other Be---Nb materials, vacuum hot pressed (BHP) Be₁₂Nb, and HIP Be₁₇Nb₂, which had . Measured strengths for the HIP Be₁₂Nb were more than twice that measured for the VHP Be₁₂Nb or for HIP Be₁₇Nb₂. The HIP Be₁₂Nb also exhibited good high-temperature mechanical properties, having a bend strength of 250 MPa at 1200 °C, compared with less than 100 MPa for the VHP Be₁₂Nb. However, intergranular embrittlement was observed at intermediate temperatures, reducing the HIP Be₁₂Nb bend strength and fracture toughness below those measured for the other materials. HIP Be₁₇Nb₂ exhibited poor low-temperature properties, but high-temperature bend strengths of 740 MPa at 1100 °C and 400 MPa at 1200 °C were measured. Strength in compression was similar for all materials above 800 °C, decreasing sharply to about 600 MPa at 1000 °C and to 200 MPa at 1200 °C. Microhardness and indentation creep tests also revealed similar high-temperature behavior among the materials. Power-law creep exponents ranging from 4.1 to 6.6 and activation energies of 220–290 kJ mol⁻¹ were measured for the beryllides, with the HIP Be₁₂Nb having the highest activation energy for creep.     

纳米SiC及Si3N4/SiC的高温等静压研究

采用高温等静压工艺，制备了纳米结构的单相ＳｉＣ及Ｓｉ３Ｎ４／ＳｉＣ复相陶瓷，并通过Ｘ射线衍射分析透射有高分辨电镜对其相组成及结构进行了表征。实验表明，在温度１８５０℃，压力２００ＭＰａ条件下保温１ｈ，要获得晶粒尺寸〈１００ｎｍ，结构均匀，致密的单相ＳｉＣ纳米结构陶瓷。     

Effect of γ′ precipitation during hot isostatic pressing on the mechanical property of a nickel-based superalloy

The effect of the precipitation of γ′ phase during hot isostatic pressing (HIPing) on the mechanical property of a nickel-based superalloy, GTD-111, was evaluated by conducting tensile and creep-rupture tests at 871 °C. In the 4-h two-step HIP process, the coupons were isostatically compressed (at 120 MPa) and heated to 1230 °C, well above the dissolution temperature of γ′ precipitates into the γ matrix, for the first 2 h, and cooled down to a temperature to induce the precipitation of γ′ phase and held for the last 2 h at 120 MPa or at ambient pressure. The precipitates were controlled in size by varying the temperature for the last half of the process. According to the result of the tensile test, the mechanical properties of the alloy were varied upon the microstructural evolution, and improved more than 40%, compared to those of the untreated ones. The precipitation of γ′ phase under high pressure further improved in the properties, suggesting that the precipitation of γ′ phase at high pressure provides an advantage for the rigidity of the structure. Based on these findings, a 6-h three-step HIP process was tried, and proved to be an effective substitute for the normal heat treatment, especially in terms of creep properties. This feature was mostly attributed to the homogenized microstructure of HIPed ones, as evidenced by the X-ray diffraction patterns.     

Infrared Transmission Curves of Pb0.1 Ca0.9La2S4 Pellets Densified by Pressureless Sintering

Highly sinterable submicron Pb_0.l Ca_0.9La₂S₄(PCLS) powders were prepared by sulfidizing calcium and lanthanum alkoxides al 500°C under CS, atmosphere for 8 hours and then in pure H₂S atmosphere at 600-800°C for 8 hours. After sintering the pellets were used as infrared transmitting window material of 8-14 μm wavelength. The CdS was added from 3 to 7 wt.% lo improve the sinterability by forming liquid phase during sintering. For sulfidization of lanthanum alkoxide, sulfide powder with LaS₂ phase was formed at 500°C, and a pure Th3P4 phase formed follow by 700°C heat treatment. A powder with β-La₂S₃phase formed at 800°C, and a pure Th₃ P₄phase formed follow by 900°C heat treatment. The powder with β-La₂S₃ phase was sintered to full density at 1350°C by adding 3 wt.% CdS. The PCLS powder with Th3P4 phase sintered to full density at 1400°C by also adding 3 wt.% CdS. The pellet exhibited 45% transmittanceat 13 μm when sintered from the powder with p-La₂S₃phase. The transmittance at 2.5 μm for the pellet sintered from the PCLS powder with Th₃P₄ type structure was 3 times higher than that from the p-La₂S₃ powder.     

Nickel Enriched Diffusion Layers on a NiAl Alloy

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. Formation of Ni₅Al₃, Ni₂Al, Ni₃Al phases which influence the extent of martensitic transformation in NiAl have been studied up to now with controversial results. We have investigated (using SEM and local elemental analyses) the microstructure of nickel enriched surface layers on a Al-79 wt.% Ni alloy. The layers were prepared by diffusion annealing and subsequently given two different heat treatments: at 930°C outside the Ni₅Al₃ region and at 500°C within the Ni₅Al₃ region of the phase diagram. In the specimen which was only diffusion annealed separate islands of Ni₅Al₃ phase elongated in the direction of the concentration gradient could be recognized within the nickel enriched surface layer. In the samples additionally annealed at 500°C, a well defined continuous layer of the Ni₅Al₃ phase situated 0.4 mm below the specimen surface was found. In the samples annealed at 930°C, isolated Ni₃Al precipitates were observed. Their number and size gradually increased with increasing nickel content.     

Densification of a rapidly solidified nickel aluminide powder-I application of hot-isostatic pressing diagrams

Densification of a gas-atomized, Cr-containing Ni₃Al-X intermetallic powder consolidated by hot isostatic pressing (HIP) was experimentally determined and also simulated using the Ashby model that describes mechanisms governing the deformation and consolidation during the process. The model has been applied to develop HIP diagrams, using best estimates or available values of input data on material properties so that the established HIP diagrams demonstrated a reasonable accuracy to allow prediction of the densification rates and controlling deformation mechanisms for this intermetallic alloy. It is believed that differences between the predicted and the experimentally determined densification of this material resulted mainly from the difficulty in estimating complex properties of this material which has an unusual property-temperature relationship and also from the deviation of the real particle size distribution from the monosized particle distribution used in the model.     

Microstructural stability of the directionally solidified γ′-base superalloy IC6

The microstructural stability of an Ni---Al---Mo directionally solidified γ′-base superalloy, IC6, was investigated and the effect on microstructure and tensile properties at room temperature and 760°C of high temperature aging at 900–1100°C for up to 200 h was studied. The experimental results show that the tensile strength decreases with aging time and that aging at 1000°C has the strongest effect on tensile properties. This is attributed to the precipitation of large amounts of the δ-NiMo phase. Microstructural changes during high temperature creep tests were also examined and are different from those observed during aging at the same temperature but without any applied stress.     

High Pressure Compaction and Sintering of Nano-Size γ- Al2O3 Powder

The effects of pressure on the compaction and subsequent sintering of nano-size Y- γ-Al₂O₃ powder were studied. Pressures up to 5 GPa were used to compact the powder in a WC piston/cylinder type die and also in a diamond anvil cell. The green body compacts obtained from both methods of compaction were pressureless sintered at temperatures between 1000°C to 1600°C. Results demonstrated that green body density was enhanced with increased compaction pressure. For compaction pressures less than 3 GPa, microstructures containing significant porosity developed at all sintering temperatures studied and is due to the development of a highly porous or vermicular structure during the y too phase transformation, occurring at temperatures between 1000°C and I I50°C. At compaction pressures greater than 3 GPa, however, the formation of the vermicular structure did not occur and near theoretical densities with grain size = 150 nm were obtained.     

Annealing effects on microstructure and mechanical properties of chromium oxide coatings

Reactive radio frequency magnetron sputter-deposited chromium oxide coatings were annealed at different temperatures and times. The influence of annealing temperature on the microstructure, surface morphology and mechanical properties was examined by X-ray diffraction, nanoindentation, pin-on-disc wear and scratch tests, respectively. X-ray results show that the chromium oxide sputtered at room temperature in low oxygen flux is primarily amorphous. Annealing below 400 °C did not cause much change, while annealing at higher temperature of 500 °C caused a significant change in microstructure and mechanical properties. Hardness increased from 12.3 GPa to 26 GPa, and the wearability improved with higher annealing temperature due to the formation of crystalline Cr₂O₃ phase, which occurs at 470 °C. Annealing time had little effect on mechanical properties and microstructure, although coating surface roughness increased with a longer annealing time. Coating adhesion was improved by annealing, due to residual stress relief and possible interfacial interdiffusion.     

Physicochemical properties of amphotericin B liposomes prepared by reverse-phase evaporation method

The physicochemical properties of phosphatidylcholine-cholesterol liposomes containing amphotericin B and prepared by reverse-phase evaporation method were studied. Uniformly dispersed liposomal suspensions were obtained by employing 3:1 ratio (by volume) of diethyl ether to normal saline, 5 min sonication time at 7°C, and evaporation of diethyl ether at 25°C. Microscopic examination showed that the prepared liposomes were spheroids with unilamellar, oligolamellar, or multilamellar structure. The liposomes containing amphotericin B 2.0 mol% of total lipid led to the highest percentage of drug entrapment. Liposomes with maximum entrapment efficiency were obtained from using 250 µmol of total lipid. The liposomal amphotericin B possessing the highest drug entrapment efficiency (approximately 95%) with particle size range of 1307-1451 nm was the one composed of 1:1 molar ratio of phosphatidylcholine to cholesterol.     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.     

Sol–gel thin film deposition and characterization of a new optically active compound: Er2Ti2O7

This paper reports on the first sol–gel thin film preparation of a new optically active compound: Er₂Ti₂O₇ (ETO). Optical, microstructural and spectroscopic properties of ETO films annealed in a temperature range 300–1000°C are studied. This work shows that the porosity and microstructure of ETO films depend closely on the heat-treatment temperature. Photoluminescence (PL) has been observed for films heat-treated at 600°C or more. The PL decay appears strongly influenced by quenching effects. For thin films treated at 600°C, quenching is essentially due to the presence of hydroxyl groups. After heat-treatment at 800°C or more, quenching can be explained by the high concentration of erbium atoms and by their distribution in the ETO lattice.     

Microstructure, mechanical properties and chemical degradation of brazed AISI 316 stainless steel/alumina systems

The main aims of the present study are simultaneously to relate the brazing parameters with: (i) the correspondent interfacial microstructure, (ii) the resultant mechanical properties and (iii) the electrochemical degradation behaviour of AISI 316 stainless steel/alumina brazed joints. Filler metals on such as Ag–26.5Cu–3Ti and Ag–34.5Cu–1.5Ti were used to produce the joints. Three different brazing temperatures (850, 900 and 950 °C), keeping a constant holding time of 20 min, were tested. The objective was to understand the influence of the brazing temperature on the final microstructure and properties of the joints. The mechanical properties of the metal/ceramic (M/C) joints were assessed from bond strength tests carried out using a shear solicitation loading scheme. The fracture surfaces were studied both morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The degradation behaviour of the M/C joints was assessed by means of electrochemical techniques.

It was found that using a Ag–26.5Cu–3Ti brazing alloy and a brazing temperature of 850 °C, produces the best results in terms of bond strength, 234 ± 18 MPa. The mechanical properties obtained could be explained on the basis of the different compounds identified on the fracture surfaces by XRD. On the other hand, the use of the Ag–34.5Cu–1.5Ti brazing alloy and a brazing temperature of 850 °C produces the best results in terms of corrosion rates (lower corrosion current density), 0.76 ± 0.21 μA cm⁻². Nevertheless, the joints produced at 850 °C using a Ag–26.5Cu–3Ti brazing alloy present the best compromise between mechanical properties and degradation behaviour, 234 ± 18 MPa and 1.26 ± 0.58 μA cm⁻², respectively. The role of Ti diffusion is fundamental in terms of the final value achieved for the M/C bond strength. On the contrary, the Ag and Cu distribution along the brazed interface seem to play the most relevant role in the metal/ceramic joints electrochemical performance.