Directionally Solidified Al₂O₃–Er₃Al₅O₁₂–ZrO₂ Eutectic Ceramics with Interpenetrating or Nanofibrillar Microstructure: Residual Stress Analysis

Abstract

Directionally solidified Al2O3–Er3Al5O12–ZrO2 eutectic ceramic rods were fabricated by the laser‐assisted float zone method at growth rates from 25 to 1200 mm/h. The microstructure of the eutectic rods was studied as a function of the solidification rate and the three eutectic phases present in the material were identified by X‐ray and electron diffraction. The size of the eutectic phases measured in the plane perpendicular to the growth direction decreased when the growth rate increased. The microstructure showed a geometrical pattern morphology at low growth rates, an irregular faceted and interpenetrating morphology at medium rates, and a nanofibrillar morphology in samples solidified at 1200 mm/h. The fibrous pattern consisted of Al2O3 trifold shaped nano‐fibers surrounded by a jacket of Er‐stabilized ZrO2 (t < 50 nm) and embedded in a matrix of Er3Al5O12 of ~150 nm width. The residual stress in the alumina phase due to the thermal contraction mismatch between phases was determined by piezospectroscopy. It decreased with solidification rate from 262 MPa compressive at room temperature in the sample with the coarsest microstructure to 135 MPa in the sample solidified at 1200 mm/h. A linear behavior of the thermoelastic stresses was found in the 110–370 K temperature range. The stress relief temperature at which the stresses become zero markedly decreased with the interphase spacing. The decrease was attributed to thermal activation of diffusional creep in the nanometric ZrO2 phase.

Solidification of Bcc/T1/T2 three-phase microstructure in Mo–Nb–Si–B alloys

“…Figure 3 shows BEIs of microstructures on (a) the longitudinal cross-section and (b) the transverse cross-section of the directionally solidified alloy. The solidification microstructure of the Mo-Nb-Si-B alloy is characterized by elongated T 2 phase regions surrounded by bcc and T 1 phases in an interwoven morphology with a similarity to the interpenetrating multiphase morphology seen in solidification of some oxide systems [34]. The T 2 phase exhibits a plateshaped morphology with a mean thickness of about 3 m (figure 3 (b)) and is elongated along the solidification direction ( figure 3 (a)), whereas the morphology of the bcc and T 1 phases appears to be independent of the solidification direction.…”

Laser Floating Zone Growth: Overview, Singular Materials, Broad Applications, and Future Perspectives

“…Thus, bulk fibers were pro-duced at relatively high speeds (10 to 30 mm/h) in air at atmospheric pressure from raw oxide powder mixtures of Ga 2 O 3 and Eu O 3 with an excess of the former compound. More recently, Ren et al, in 2017 [116], grew polycrystalline Al 2 O 3 /Er 3 Al 5 O 12 /ZrO 2 eutectic ceramics under a nitrogen atmosphere suitable to be used as selective thermal emitter for thermal photovoltaic (TPV) generation [117]. One year before, these researchers also grew Al 2 O 3 -Yb 3 Al 5 O 12 (YbAG) eutectic rods to be selective emitters for thermophotovoltaic (TPV) devices [69].…”

Fabrication, microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics

“…Among the multiphase alumina-based ceramics, directionally solidified Al 2 O 3 -based eutectic oxide composites, such as Al 2 O 3 /YAG, Al 2 O 3 / ZrO 2 , Al 2 O 3 /GdAlO 3 , Al 2 O 3 /YAG/ZrO 2 , etc. have attracted much attention in the past because of their excellent thermo-mechanical behavior [11][12][13][14][15][16][17][18]; in particular, the ternary eutectic Al 2 O 3 /YAG/ZrO 2 system combines the high strength and creep resistance of the alumina and YAG phases with the superior fracture toughness of the zirconia phase. These properties derive directly from the singular lamellar microstructures developed during the solidification process, which improve the microstructural stability of the different phases.…”