Compatibility of Zr2AlCZr_{2}AlC MAX phase-based ceramics with oxygen-poor, static liquid lead-bismuth eutectic

This work investigates the compatibility of $Zr_{2}AlC$ MAX phase-based ceramics with liquid LBE, and proposes a mechanism to explain the observed local $Zr_{2}AlC$/LBE interaction. The ceramics were exposed to oxygen-poor ($C_{O}\le2.2 \cdot10^{-10}$ mass%), static liquid LBE at 500{\deg}C for 1000 h. A new $Zr_{2}(Al,Bi,Pb)C$ MAX phase solid solution formed in-situ in the LBE-affected $Zr_{2}AlC$ grains. Out-of-plane ordering was favorable in the new solid solution, whereby $\textit{A}$-layers with high and low-Bi/Pb contents alternated in the crystal structure, in agreement with first-principles calculations. Bulk $Zr_{2}(Al,Bi,Pb)C$ was synthesized by reactive hot pressing to study the crystal structure of the solid solution by neutron diffraction

Study of the devitrification behaviour of a barium magnesium aluminosilicate glass-ceramic

This work presents the most important aspects of the devitrification behaviour of a barium magnesium aluminosilicate (BMAS) glass-ceramic with the Ba-osumilite stoichiometry. The melt-derived BMAS glass-ceramic was first pulverised and subsequently hot pressed to produce sintered powder compacts, some of which were subjected to post-sintering heat treatments in air. The BMAS glass devitrification behaviour was studied by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction (XRD). This study revealed that at temperatures = 1000 degrees C caused the transformation of all metastable phases to Ba-osumilite. Understanding the crystallisation behaviour of the BMAS glass-ceramic is essential in order to use this material as matrix in fibre-reinforced composites suitable for high-temperature structural applications. (c) 2006 Elsevier Ltd. All rights reserved.status: publishe

Formation of a metastable celsian-cordierite eutectic structure during the crystallization of a Ba-osumilite glass-ceramic

This work addresses the formation conditions and stability of a celsian-cordierite eutectic structure produced during the crystallization of a barium magnesium aluminosilicate (BM AS) glass-ceramic with the Ba-osumilite stoichiometry. The melt-derived BMAS glass-ceramic was pulverized and subsequently hot pressed to produce sintered powder compacts, some of which were subjected to post-sintering heat treatments in air. The study of the devitrifled BMAS glass revealed that a metastable celsian-cordierite eutectic structure resulted from the prolonged annealing of the BMAS glass-ceramic between 860° and 1000°C. The celsian-cordierite eutectic structure transformed to the thermodynamically stable Ba-osumilite phase when annealed at temperatures > 1000°C. © 2006 The American Ceramic Society.status: publishe

Topotactic transformations in Hf-Al-C MAX phase compounds:Synthesis and characterization of nanolaminated Hf₂AlC, Hf₃AlC₂ and Hf₅Al₂C₃

The synthesis of high-purity Mn+1AXn (MAX) phase ceramics in the Hf-Al-C ternary system from near-stoichiometric feedstock powder mixtures has been exceptionally challenging due to the rapid concurrent formation of persistent, ultrafine HfC impurities. This work synthesized ceramics containing the nanolaminated Hf5Al2C3 ‘superstructure’, showing that it comprises alternating Hf2AlC and Hf3AlC2 atomic stackings. For the first time, the Hf5Al2C3 complex structure was associated with the topotactic transformation of Hf2AlC into Hf3AlC2, which is observed upon heating the powder compact to temperatures higher than 1500 °C; moreover, an inverse decomposition reaction of Hf3AlC2 into Hf2AlC was observed as result of further heating the powder compact to temperatures exceeding 1600 °C. The crystal structure and lattice parameters of the Hf5Al2C3 ‘superstructure’ were determined. MAX phase ceramics containing up to 40–45 wt% Hf3AlC2/Hf2AlC were produced with HfC as the main competing phase. The hardness and damage tolerance of these MAX phase ceramics were also evaluated

Reactive spark plasma sintering of Ti3SnC2, Zr3SnC2 and Hf3SnC2 using Fe, Co or Ni additives

This work studied the effect of adding 10 at% Fe, Co or Ni to M-Sn-C mixtures with M = Ti, Zr or Hf on MAX phases synthesis by reactive spark plasma sintering. Adding Fe, Co or Ni assisted the formation of 312 MAX phases, i.e., Ti3SnC2, Zr3SnC2 and Hf3SnC2, while their 211 counterparts Ti2SnC, Zr2SnC and Hf2SnC formed in the undoped M-Sn-C mixtures. The lattice parameters of the newly synthesized Zr3SnC2 and Hf3SnC2 MAX phases were determined by X-ray diffraction. Binary MC carbides were present in all ceramics, whereas the formation of intermetallics was largely determined by the selected additive. The effect of adding Fe, Co or Ni on the MAX phase crystal structure and the microstructure of the produced ceramics was investigated in greater detail for the case of M = Zr. A mechanism is herein proposed for the formation of M3SnC2 MAX phases.status: publishe

Resonant-based identification of the elastic properties of layered materials: Application to air-plasma sprayed thermal barrier coatings

This work introduces a resonant-based, mixed numerical-experimental method for the determination of the in-plane elastic properties of the constituent materials of laminates. This non-destructive method identifies elastic properties from the resonant frequencies of beam-shaped layered specimens, using a set of finite element models. The method is demonstrated on a thermal barrier coating system made of NiCoCrAlY bondcoat and yttria-stabilised zirconia topcoat deposited by air-plasma spraying on stainless steel. The stainless steel was found to be elastically anisotropic, while both bondcoat and topcoat exhibited in-plane isotropy. Moreover, the topcoat Poisson's ratio approached zero, and the bondcoat properties varied with the coating thickness. Scanning electron microscopy was used to correlate the identified elastic properties with the coating microstructure. (c) 2007 Elsevier Ltd. All rights reserved.status: publishe