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TiAl-based alloys have been regarded as potential substitute materials for heavy Ni super-alloys. To understand the relationship of the processing-microstructure-hot deformation behavior of TiAl-based alloys fabricated via the spark plasma sintering technique, alloys were sintered at a temperature range of 1150-1350°C. Subsequently, isothermal compression tests were carried out at 1125°C with a strain rate of 0.1 s−1. The pre-alloyed powders, which showed dendritic structures, were mainly composed of α2 phase and a few of γ phase. Alloys composed of near gamma, duplex, and fully lamellar microstructures were obtained after sintering at 1150-1250°C, 1300°C, and 1350°C, respectively. The compressive peak stress increased with increasing sintering temperature, and the alloy sintered at 1250°C exhibited a better hot deformability. Moreover, dynamic recrystallization and phase transformation of α2 → γ occurred during isothermal compression. [ABSTRACT FROM AUTHOR]

Grain growth behavior of a powder metallurgy (P/M) Ti-22Al-25Nb alloy was investigated by applying a series of isothermal treatment tests over a wide range of temperatures and holding times. An isothermal treatment scheme was conducted in the B2 phase region (1070-1110 °C) and α2 + B2 phase region (1010-1050 °C) at holding times of 10, 30 min, 1, 2, and 3 h, respectively. The effects of temperature and holding time on the microstructure evolution and microhardness of the P/M Ti-22Al-25Nb alloy at elevated temperatures were evaluated using optical microscope, scanning electron microscope, x-ray diffraction, and Vickers hardness test techniques. The results revealed that the alloy’s treated microstructure was closely linked to temperature and holding time, respectively. The change law of B2 grain growth with holding time and temperature can be well interpreted by the Beck equation and Hillert equation, respectively. The B2 grain growth exponent n and activation energy Q were acquired based on experimental data in the α2 + B2 and B2 phase regions. In addition, the grain growth contour map for the P/M Ti-22Al-25Nb alloy was constructed to depict variations in B2 grain size based on holding time and temperature. [ABSTRACT FROM AUTHOR]

Hot deformation is an effective way to tackle major problems in powder metallurgy, i.e., inferior mechanical properties and low relative density. To characterize the hot deformation behavior of NiAl-based alloy manufactured by hot pressing sintering, the isothermal compression tests were performed in the deformation temperature range of 1100–1300 °C with strain rate of 0.001–1 s−1. The result indicates that calculated hot activation energy Q is 326.31 kJ/mol. The processing efficiency maps and instability maps of NiAl-based alloy were established to optimize deformation parameters on the basis of dynamic material model. They were validated through microstructure evolution. The microstructure observation revealed that fine grains, dislocation pile-up, cracks appear in high efficiency, low efficiency, and instability domains, respectively. According to effective processing window revealed by processing maps, hot forging of sintered billets was performed. The elevated temperature elongation increases from 17.86% to 74.87% after forging. The stripping feature is found on fracture surface after forging. [ABSTRACT FROM AUTHOR]

A study has been undertaken to verify the feasibility of using a powder metallurgy (P/M) approach to fabricate Ti–22Al–25Nb alloys. Pre-alloyed powders with a nominal composition of Ti–22Al–25Nb (at%) obtained by argon atomization were sieved to the spherical size less than 180 μm and used for the fabrication of P/M Ti–22Al–25Nb alloys via hot pressing in vacuum. Vacuum hot pressing sintering was carried out in a temperature range of 950–1200 °C with a pressure of 35 MPa for 1 h followed by furnace cooling. Microstructure and phase composition examinations of the as-atomized powders and hot pressed (HP׳ed) samples were conducted by applying optical microscopy, back-scatter electron imaging and X-ray diffraction analysis. Tensile tests were studied at room temperature and 650 °C, respectively. The results showed that all HP׳ed samples were composed of coarse equiaxed B2 grains, fine lamellar O phase inside the B2 grains, and some α 2 along B2 grain boundaries. The elongations of HP׳ed samples were less than 3.95%, indicating the bad ductility at room temperature. However, the elongations were improved as the tensile temperature increased to 650 °C. The sample sintered at 1050 °C exhibited a better ductility with the elongation of 7.97% at 650 °C than that of other samples. [ABSTRACT FROM AUTHOR]

Ceramic substrate with three kinds of small-hole arrays (the minimum diameter is 400 μm) was manufactured by micro powder injection molding. The homogeneity, thermal, and rheological properties of the feedstock was characterized by means of SEM, Archimedes method, TGA, DSC, and capillary rheometer, respectively. The feedstock has good uniformity and the viscosity of feedstock accords with the pseudo-plastic behavior which is suitable for micro powder injection molding. The test results also show that the linear shrinkage of small holes is lower than the substrate which is important to mold design and size contraction of the sample. Moreover, the porosity of the sintered substrate is lower than that of the thin wall between two neighborhood small holes. Good surface roughness of the sintered samples is obtained by using sub-micron ZrO powder which is even lower than molded surface. The relative density and hardness of the ceramic substrate with small-hole arrays sintered under 1,500°C for 2 h is 98.3% and 13.68 GPa, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Vacuum hot-pressing sintering (VHPS) was used for fabricating Nb-15W and Nb-15W-10Cr (at.%) alloys under 30 MPa for 60 min at 1800 °C in pure argon atmosphere. Microstructure and oxidation resistance of both materials were investigated. Microstructure analysis showed that the as-sintered Nb-15W alloy only consisted of α-Nb (W) solid solution and the as-sintered Nb-15W-10Cr alloy composed of α-Nb (W, Cr) solid solution and small amount cubic type NbCr2 Laves phase due to addition of 10 at.% Cr. The oxidation at 1300 °C for 4 h exhibited that the addition of 10 at.% Cr increased the oxidation rate of Nb-15W alloy. The effects of Cr on the oxidation behaviors are discussed in detail. [Copyright &y& Elsevier]

Abstract: In order to increase the production efficiency of powder injection moulding for SiC parts, thermal debinding was performed in air furnace without gas shield. Bending tests were performed to evaluate the strength of samples debound under different temperatures. The effects of oxidation on debinding process were also analyzed. Analysis indicates that air could accelerate the debinding rate of green parts without defects occurring. The bending strength of debound samples increases from 6.55MPa to 11.58MPa as the pre-sintering temperature increases from 550°C to 850°C. On the other hand, the bending strength of the samples pre-sintered at 1200°C in argon atmosphere is only 11.52MPa. It was found that, blanks have enough strength for transport after being pre-sintered in air atmosphere at 850°C. The technology could reduce the requirement for heating equipment and enhance the efficiency of debinding for SiC parts. [Copyright &y& Elsevier]

Abstract: The effect of heating rate in Resistance Sintering on resulting microstructures and properties was not widely explored. For the purpose of obtaining γ-TiAl alloys with fine grains, synthesis of coarse pre-alloyed TiAl powders was carried out with relatively high heating rate on self-built similar sintering device named pulse current auxiliary sintering apparatus (PCAS). The effect of heating rate on microstructures and high temperature ductility was investigated. And high temperature tensile behavior and fractography were compared between the two obtained original microstructures. The results show that relatively high heating rate is beneficial for obtaining fine-grained microstructures. And the resultant intermetallic alloy with equiaxed near gamma microstructures exhibits superplastic properties at relatively low temperature. [Copyright &y& Elsevier]

Abstract: A fine-grained γ-TiAl based alloy was synthesized by Pulse Current Auxiliary Sintering. Tensile properties of the sintered alloy were investigated at temperatures ranging from 800°C to 1000°C. Results showed that the alloy under study displayed superplasticity at temperatures of over 900°C, and the activation energy Q for its superplastic deformation was determined as 212.6–252.2kJmol−1, corresponding to the deformation mechanism of grain boundary sliding accommodated by grain boundary diffusion. Additionally, it was also found that in superplastic deformation, microstructure and cavity evolution took place, such as axial elongation or growing of γ, breaking up of α2, nucleating and growing of cavities in the vicinity of big α2 grains. All the observed phenomena are tried to be explained in the paper. [Copyright &y& Elsevier]

Abstract: Up to now, most TiAl alloys sintered from coarse particles via resistance sintering have been with coarse fully lamellar microstructures and not promising tensile properties. For the purpose of obtaining γ-TiAl based alloys with ideal mechanical properties, synthesis of coarse pre-alloyed powders was carried out on a self-built similar apparatus named pulse current auxiliary sintering. A dense micron-grained γ-TiAl based alloy was successfully synthesized with superplastic characteristics at over 900 °C in air. The maximum elongation obtained is 437%. The maximum m value reaches 0.8 and the mechanism for superplastic deformation in the alloy under study is determined as the grain boundary sliding accommodated by grain boundary diffusion. [Copyright &y& Elsevier]

Abstract: Micro-grained γ-TiAl based alloy obtained via pulse current auxiliary sintering exhibits good room temperature ductility with the common influence of fine grain size and inner twinning microstructure. Superplastic behavior at relatively low temperatures is also observed. It is also noted that the tensile strength of the studied alloy manifests anomalous hardening from room temperature to approximately 600°C as a result of the controlling of dislocations slip, and softening above 600°C due to thermal activation. Based on calculation, the superplastic deformation mechanism in the present work is determined as the grain boundary sliding accommodated by grain boundary diffusion. [Copyright &y& Elsevier]

Abstract: 3Y-TZP/Al2O3 composites were prepared by hot-pressed sintering at 1450°C for 1h under 30MPa in vacuum, and then the fine-grained composite sintered bodies were deformed by superplastic compression at 1500°C. Quantitative texture analysis to the hot-pressed and deformed composites was accomplished via XRD and X-ray pole figure measurements. Highly oriented and textured microstructure was generated by superplastic deformation. The maximum degree of texture was up to 7.3 times random. The microstructure observation based on SEM and TEM was adopted to explain the development and mechanism of texture formation. The results indicated that Al2O3 matrix with fine platelike grains was formed in deformed materials, while the shape of ZrO2 grains did not change obviously. The predominant mechanism for deformation and texture development was grain-boundary sliding and grain rotation, accompanying with anisotropic grain growth. Meanwhile, the intragranular dislocation motion played an important role in the formation of texture. [Copyright &y& Elsevier]

Abstract: Tape casting and hot-press sintering are used to fabricate an Al2O3/3Y-TZP laminated composite. The as-prepared material is deep drawn at high temperature to research its superplastic formability. It is found that the microstructure of the material sintered at 1550°C is fine and no significant residual porosity was detected from SEM observations at the interfaces between the two types of layers. The superplastic forming experiment shows that, when the strain rate is constant, temperature has a great influence on the superplasticity of the Al2O3/3Y-TZP laminated composite. A hat-like part with the greatest deform height is obtained at 1500°C. The processing will be less effective at higher or lower temperature. [Copyright &y& Elsevier]

Abstract: In order to investigate the superplasticity of ceramic/ceramic laminated composite under tensile stress, the superplastic deep-drawing process was simulated by FEM. The results shown that, the strain and stress conditions of laminated composite, made by ceramic with different superplastic formability, are better than that of monolithic ceramic. Consequently, the material may exhibit high superplasticity. To testify this, some experiments were carried out. Tape casting and hot-press sintering are used to fabricate Al2O3/3Y-TZP laminated composite. As-received material is deep-drawing at high temperature to research its superplasticity. The results shown that, when suitable strain rate and forming temperature were used, the as-received material has excellent superplasticity. [Copyright &y& Elsevier]

The present study investigated high-aspect ratio micro-hole array parts which were made by ZrO2 micro-powder with different particle sizes and micro-powder injection molding technology. It analysed the influence of particle sizes on feedstock, debinding and sintering of ceramic nozzles with multi-micro-holes. The forming quality of ceramic nozzles with multi-micro-holes was discussed in this paper. The results show that the two mixed ZrO2 feedstocks have fine uniformity. The average deviation of the feedstock made with 200 nm powder was −2%, and the average deviation of the feedstock made with 100 nm powder was −7.1%. The sample showed certain sintering characteristics which provided better strength (11.10 MPa) to parts after debinding. The linear shrinkage and the density of the two powder samples at different sintering temperatures increased as the sintering temperature increased. If the temperature continued to increase, the linear shrinkage and the density decreased. The highest hardness and flexural strength values of the ZrO2 sample with 200 nm powder used were: 1265.5 HV and 453.4 MPa, and the crystalline particle size was 0.36 μm. The highest hardness and flexural strength values of the ZrO2 sample with 100 nm powder used were: 1425.8 HV and 503.6 MPa, and the crystalline particle size was 0.18 μm. The ceramic nozzles with multi-micro holes shrunk to nearly the same axial, radial and circumferential directions during sintering. After sintering, the roundness of ceramic micro-hole met the user requirements, and the circular hole had a high parallelism in the axial direction. The micropore diameter was 450 ± 5 μm, and it was possible to control the dimensional accuracy within 1.5% after sintering. The study presented a superior application prospect for high-aspect ratio micro hole array parts in aerospace, electronics and biomedicine. [ABSTRACT FROM AUTHOR]

In this paper, one kind of new composite material formed with Nb and SiC was prepared by hot pressing sintering. The influence of the addition of SiC particles on the mechanical properties at room and high temperature was analyzed. The composite material consists of three phases: Nb2C, Nb3Si, and Nb solid solution (Nbss). The fraction of SiC particles added in the Nb matrix was 3%, 5%, and 7%, respectively. Flexural strength, Vickers hardness, and compressive strength at room temperature were improved with the increasing of SiC content. Among them, compressive strength and fracture toughness were higher than those of Nb/Nb5Si3 composites. The compressive strength at high temperature of the new composites was higher than that of Nb-Si alloys, which improved with the increasing of SiC content. [ABSTRACT FROM AUTHOR]

Abstract: Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which induces the residual carbon and increases forming cycle and production cost. In this paper, injection moulded SiC with Al2O3 and Y2O3 as sintering assistant was thermal debound in air and Ar, respectively. The paper investigates the effects of pre-oxidation during debinding stage on the sintering and mechanical property of SiC material. During sintering, the oxide SiO2 is in favour of the shrinkage of debound samples at lower temperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger than the sample without pre-oxidation. Test results by TEM and XRD indicate that SiO2 disappear from the inside of the sintered samples. The loss of SiO2 decreases the content of Al2O3, which affects the formation of YAG (Y3Al5O12). Sintered Sic samples contain α-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of α-SiC phase and intergranular phase. The bending and compression strength values of sintered samples with pre-oxidation reach to 537MPa and 2.89GPa, respectively. These values approach the strength of sintered samples without pre-oxidation (594MPa and 3.0GPa). [Copyright &y& Elsevier]

Abstract: In this paper, Al2O3–Si3N4/ZrO2–Al2O3 laminated composites were fabricated by tape casting and hot press sintering, and the relationships between the process, microstructure, and mechanical properties of Al2O3–Si3N4/ZrO2–Al2O3 laminated composites were determined. The SiAlON phase was found in the Al2O3–Si3N4 layer, and liquid-phase sintering was proposed. Nano-scratch tests were carried out to investigate the interface bonding strength of the laminates. The distribution of residual stresses, generated due to the different coefficients of thermal expansion between the different layers, was estimated according to lamination theory and confirmed using Vickers indentation. When the sintering temperature was 1550°C, the sintered laminated ceramics had good mechanical properties, with a maximum strength and toughness of 413MPa and 6.2MPam1/2, respectively. The main toughness mechanics of laminated composites was residual stress. [Copyright &y& Elsevier]