Lifetime estimation of Cr2AlC MAX phase foam based on long-term oxidation and fracture mechanisms

Abstract

Oxidation kinetics and mechanical behaviours of Cr2AlC foam with a porosity of 53 vol.% were investigated. Microstructures of Cr2AlC foams oxidised in the temperature range 1173 to 1473 K for times between 0 and 100 h were examined. Uniaxial compression tests were performed at different temperatures in the range 298–1398 K to assess mechanical properties. The oxidation formed cohesive Al2O3 layers on the Cr2AlC matrix, beneath which porous Cr7C3 was formed. The oxidation kinetics can be expressed by a parabolic law. An excessive oxidation took place first in thin struts, where a breakage Al2O3 layer occurred, followed by an oxygen inflow and decomposition of inner material. At 298 K, non-oxidised Cr2AlC foam fracured intergranularly. Slight oxidation improved compressive strength, as the Al2O3 layer (2.5 µm or thinner) can prevent cracks to propagate from inside outward. However, an excessive oxidation deteriorated any improvement due to the breakage of Al2O3 layer in thin struts followed by the material decomposition. At 1273 K and 1398 K, non-oxidised porous Cr2AlC fractured intergranularly, accompanied by a plastic deformation around small Al2O3 particles segregated at grain boundaries. Oxidised Cr2AlC foams with the Al2O3 thickness of 2.5 µm had a slightly higher brittle-to-plastic transition temperature (~1273 K) than dense Cr2AlC. A thicker Al2O3 layer (~5 µm) was required to reinforce the material due to inferior mechanical properties of Cr2AlC at high temperatures. On the basis of the elucidated oxidation and fracture mechanisms, a safety criterion for high-temperature applications was suggested.