Deformation and fracture behavior of an Al2O3/YAG composite from room temperature to 2023 K

The deformation and fracture behavior of a newly developed Al2O3/YAG composite, fabricated by the unidirectional solidification of a eutectic composition, was investigated by means of bending tests between room temperature and 2023 K, on specimens whose longitudinal directions were parallel (L specimen) and perpendicular (T specimen) to the direction of solidification. At low temperatures, the composite fractured in a brittle manner at all displacement speeds. At high temperatures, it fractured in a brittle manner at high displacement speed, but in a ductile manner with accompanying plastic deformation at low speed for both L and T specimens. The brittle-ductile transition temperature became higher at higher displacement speeds in both L and T specimens, while it was slightly higher in the latter specimen. The crack propagated dominantly through YAG with lower ductility than Al2O3, as a consequence of which the fraction of YAG in the fracture surface was higher than the structural value estimated from a polished surface. The strength at room temperature of both L and T specimens was maintained up to 2023 K at high displacement speeds. At low displacement speeds, the strength decreased beyond 1900 K as a result of the enhanced plastic deformation. The stress exponent for plastic deformation at high temperatures (1823-2023 K) was 5-6, suggesting that the plastic deformation is controlled by a dislocation mechanism during bending as well as in compressive and tensile loading. The fracture toughness tended to increase with increasing temperature and with decreasing displacement speed, especially in L specimens. (C) 2001 Elsevier Science Ltd. All rights reserved.