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The mechanical properties of dense, hot-pressed ZrB2-30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot-pressed or after hot-pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot-pressed ZrB2-SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot-pressed ZrB2-SiC. Neither ZrB2 grain size nor SiC cluster size changed for these heat-treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB2-based ceramics. [ABSTRACT FROM AUTHOR]

The flash phenomenon occurs when oxide ceramics are heated above a threshold temperature under an applied electric field. It is defined as an abrupt increase in the conductivity of the specimen. The specimen then can be held in this state of high conductivity by switching the power supply from voltage to current control. Here, we report on the emergence of new X-ray diffraction peaks in 3 mol% yttria-stabilized zirconia (3 YSZ) when the specimen is held in this current controlled state. These peaks are indexed as a pseudocubic phase of zirconia. The peaks extinguish and reappear when the field is turned off and on. The specimen temperature in the flash state is measured from the thermal expansion of platinum, which is placed as a thin film on a small portion of the specimen surface. Experiments without the electric field, at even higher temperatures than those measured with the platinum standard, do not show any change of phase, thus ruling out Joule heating as the cause of this phenomenon. The time dependency of the growth and dissolution of the pseudo cubic phase is reported. These in situ experiments were carried out at the Advanced Photon Source Synchrotron at the Argonne National Laboratory. [ABSTRACT FROM AUTHOR]

Tantalum carbides are commonly processed by hot pressing, canned hot-isostatic-pressing, or spark plasma sintering because of their high melting temperatures and low diffusivities. This paper reports processing of dense ζ- Ta4 C3− x by reaction sintering of a Ta and TaC powder mixture ( C/ Ta atomic ratio = 0.66). ζ- Ta4 C3− x is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face-centered-cubic and hexagonal-close-packed Ta stacking sequences interrupted by stacking faults and missing carbon layers. This structure leads to easy cleaving on the basal planes and high fracture toughness. A key step in processing is the hydrogenation of the Ta powder to produce β- TaH x, a hard and brittle phase that enables efficient comminution during milling and production of small, equiaxed Ta particles that can be packed to high green density with the TaC powder. Studies of phase evolution by quantitative X-ray diffraction during sintering revealed several intermediate reactions: (1) decomposition of β- TaH x to Ta; (2) diffusion of C from γ- TaC to Ta leading to the formation of α- Ta2 C y' with the kinetics described by the Avrami equation with an exponent, n = 0.5, and an activation energy of 219 kJ/mole; (3) equilibration of α- Ta2 C y' and γ- TaC0.78 phases; and (4) formation of ζ- Ta4 C2.56 from the equilibrated α- Ta2 C and γ- TaC0.78 phases with the kinetics characterized by a higher Avrami exponent ( n ≈ 3) and higher activation energy (1007 kJ/mole). The sintered material contained ~0.86 weight fraction ζ- Ta4 C2.56 and ~0.14 weight fraction γ- TaC0.78 phases. The microstructure showed evidence of nucleation and growth of the ζ- Ta4 C2.56 phase in both the α- Ta2 C and γ- TaC0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane. [ABSTRACT FROM AUTHOR]

Easy sinterable Ti3 SiC2 powder was synthesized from a powder mixture with a molar ratio of 1.0 Ti, 0.3 Al, 1.2 Si, and 2.0 TiC by heating at 1200°C in the flowing Ar. Here, the Al powder acts as a deoxidation agent and provides a liquid phase for the reaction. The powder compacts subjected to pressureless sintering at 1300°C in Ar had a relative density up to 99%. The results of chemical analysis and the measured lattice constant suggest that the Al-Si liquid phase was formed at approximately 1200°C and that liquid-phase sintering was promoted by the 0.1 molar ratio of Al and the 0.2 molar ratio of Si remaining in excess. The three-point bending strength, fracture toughness, and electrical resistivity of the sintered samples were 380 MPa, 4.1 MPa m1/2, and 0.34μΩm, respectively. [ABSTRACT FROM AUTHOR]

Reactive sintering of 3 Ti: Sn:2 C and 3Ti:Sn:2C:0.6Fe powder mixtures is studied in the temperature range 510°C-1200°C under argon. It is demonstrated that the recently discovered Ti3 SnC2 phase is formed, provided that Fe is added to a 3 Ti: Sn:2 C reactant mixture within the synthesis conditions used. Using dilatometric and X-Ray diffraction analyses, the formation mechanism of Ti3 SnC2 is discussed. Results show that at low temperature (about 510°C), tin is consumed to form Fe x Sn y intermetallics. At high temperature (about 1060°C), tin is newly available to form Ti3 SnC2 due to the melting of Fe x Sn y. Then, the intermediate phases, TiC and Ti2 SnC, and/or Ti5 Sn3, TiC, C, and Ti are dissolved in the ( Fe + Sn) liquid phase and Ti3 SnC2 very likely precipitate from the melt. The second part of the study deals with the optimization of the Fe content in the initial 3Ti:Sn:2C reactant powder mixture to synthesize samples with larger Ti3 SnC2 content by hot isostatic pressing. [ABSTRACT FROM AUTHOR]

MAX phase Ti2 Al(1− x) Sn x C solid solution with x = 0, 0.32, 0.57, 0.82, and 1 was synthesized by pressureless sintering of uniaxially pressed Ti, Al, Sn, and TiC powder mixtures. Annealing in air atmosphere at 200°C-1000°C triggered a sequence of oxidation reactions which reveal a distinct influence of solid solution composition on the oxidation process. With decreasing Al/ Sn ratio, the characteristic temperature of accelerated oxidation reaction of A-element was reduced from 900°C ( x = 0) to 460°C ( x = 1). SnO2 was formed at temperatures significantly lower than TiO2 (rutile) and Al2 O3. Substitution of A-element in MAX phase solid solution by low-melting elements such as Sn may offer potential for reducing oxidation-induced crack healing temperatures. [ABSTRACT FROM AUTHOR]

Using spark plasma sintering, Ti3 AlC2/ W composites were prepared at 1300°C. They contained 'core-shell' microstructures in which a Ti xW1− x 'shell' surrounded a W 'core', in a Ti3 AlC2 matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the Ti x W1− x interfacial layer providing strong bonding between Ti3 AlC2 and W, and by the presence of hard W. Microstructural development during high-temperature oxidation of Ti3 AlC2/ W composites involves α- Al2 O3 and rutile ( TiO2) formation ≥1000°C and Al2 TiO5 formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO2-doped alumina and the effect of W addition, fine (<1 μm) Al2 O3 grains formed dense, anisomorphic laths on Ti3 AlC2/5 wt% W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO2-doped Al2 O3 clusters on Ti3 AlC2/10 wt% W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti3 AlC2/ W composites beneficially by causing formation of Ti x W1− x thus altering the defect structure of Ti3 AlC2, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al2 O3 laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti3 AlC2/5 wt% W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect. [ABSTRACT FROM AUTHOR]

In this article, the reactivity of Ti2AlC powders, with 3 and 10 μm alumina, Al2O3, fibers during pressure-assisted sintering is explored. Samples were fabricated by hot-isostatic-pressing ( HIPed) or hot-pressing ( HPed), and characterized by X-ray diffraction, differential thermal analysis, and electron microscopy-both scanning and transmission-equipped with energy dispersive X-ray spectroscopes. Samples prepared at 1300°C were fully dense, with no apparent reaction between fiber and matrix. In samples HPed to 1500°C, even pure Ti2AlC powders dissociated to Ti3AlC2 according to: 2 Ti2AlC = Ti3AlC2 + TiAl x ( l) + (1- x) Al ( l/ v), with x < 1. More severe Al loss results in the formation of TiC y. The presence of the Al2O3 fibers delayed densification enough to allow most of the Al and some of the Ti to escape into the vacuum of the hot press or react with the encapsulating glass during HIPing a resulting in a more intensive dissociation of the Ti2AlC. Although, in principle Ti2AlC can be reinforced with Al2O3 fibers, the processing/use temperature will have to be kept below 1500°C, as, at that temperature the fibers, used here, sinter together. [ABSTRACT FROM AUTHOR]

In this study, a well-dispersed γ-Y2Si2O7 ethanol-based suspension with 30 vol% solid loading was prepared by adding 1 dwb% polyethylene imine dispersant, which allows feeble magnetic γ-Y2Si2O7 particles with anisotropic magnetic susceptibility to rotate in a 12 T strong magnetic field during slip casting, resulting in the development of a strong texture in green bodies. Pressureless sintering gives rise to more pronounced grain growth in the textured sample than in the untextured sample prepared without the magnetic field due to the rapid migration of the grain boundaries of the well-oriented grains, which was revealed by constant-heating-rate sintering kinetics. It was found that the use of two-step sintering is very efficient not only for inhibiting the grain growth but also for enhancing the texture. This implies that controlled grain growth is crucial for enhancing texture development in γ-Y2Si2O7. [ABSTRACT FROM AUTHOR]