Relations Between Oxidation Induced Microstructure and Mechanical Durability of Oxide Scales

Abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID : 18118 Abstract Most industrial heat-resistant stainless steels contain silicon as a minor constituent. At high temperature, the internal formation of amorphous silica reduces oxidation rates but decreases the metal/oxide interface toughness. Tensile testing experiments performed on AISI 304L previously oxidized in synthetic air for 50 h at 900 or 1000°C showed a relation between the silica morphology and location and the crack patterns. A micromechanical modeling using cohesive zone models to describe interfaces fracture behavior is proposed to investigate relevant parameters controlling the silica/alloy interface debonding. Calculations carried out using the finite elements method have shown that location of silica inclusions and silica/metal interface toughness are key parameters determining the cracks pattern morphology and the critical strain at failure.

Relation between microstructure induced by oxidation and room-temperature mechanical properties of the thermally grown oxide scales on austenitic stainless steels

“…The low diffusion coefficient of oxygen in fcc-austenite lattice, compared to the grain boundary diffusion coefficient, results in silica internal oxidation along the steel grain boundaries (i.e. intergranular formation of silica, perpendicularly to the metal/oxide interface) as already observed in literature [16,22,28,[34][35][36]. This internal silica seems to play a key role in the damage behaviour of the system.…”

“…Crack formation lowers the average stress in the oxide while a high stress concentration zone develops in the metal beneath the crack tip. Experimental crack patterns are well reproduced considering a silica/metal interface weaker than an oxide/metal interface [28]. In this case, after transverse cracks form in the oxide scale, decohesion of metal/silica interface occurs beneath oxide crack root, preventing oxide scale spallation.…”

“…Indeed, on one hand, the oxide toughness appears to be strong since no cracks are observed on the oxide surface. On the other hand, silica interlayer may locally weaken the metal/oxide interface [28,30]. Thus, compressive stress will initiate buckling of the oxide scale over area weakened by silica interlayer, then through-thickness cracks form in the oxide due to tensile stress generated by this buckling.…”

“…At this point, the mechanism for oxide cracks initiation is not clear but seems related to the presence of silica inclusions along grain boundaries in the metal underneath. Moreover, results from tensile testing numerical simulations have shown that crack formation in the oxide scale is a necessary condition to break metal/inclusion interfaces [28]. Crack formation lowers the average stress in the oxide while a high stress concentration zone develops in the metal beneath the crack tip.…”

“…Most often, authors report transverse cracks in the coating with an increasing density of cracks until reaching a saturation value when spallation takes place [19]. Sometimes saturation is reached without any spallation: in this case saturation is due to localized deformation in the ductile substrate preventing stress transfer to the coating [21,28].…”