Influence of the microstructure on the formation of alumina scales on near stoichiometric RuAl produced by arc melting

Oxidation of ruthenium aluminide-based alloys: The role of microstructure and platinum additions

“…8) are composed of alternating layers of Al 2 O 3 and Ru-rich phases next to the outer Al 2 O 3 layer after cyclic oxidation at 1100 C. The outer Al 2 O 3 layer appears to be fragile and porous and thus unprotective. The formation of the alternating multilayer structure beneath the Al 2 O 3 scale was observed in near-stoichiometric RuAl isothermally oxidized at 1350 C [4], in a eutectic RuAl alloy [33], and in near-stoichiometric RuAl alloys [13,15] oxidized between 1000 C and 1300 C. A similar alternating multilayered structure has been observed in the IrAl and Ir x Ru 1ÿx Al systems with alternating layers of Al 2 O 3 and Ir-rich or Ir/Ru-rich phases during isothermal oxidation at temperatures up to 1400 C [32,34]. The formation of the layered structure has been attributed to the slower nucleation and growth kinetics of the Al 2 O 3 at the reaction front compared to the inward diffusion rate of oxygen [34].…”

“…The formation of the layered structure has been attributed to the slower nucleation and growth kinetics of the Al 2 O 3 at the reaction front compared to the inward diffusion rate of oxygen [34]. Soldera reported that the formation of the alternating layered structure in stoichiometric RuAl was due to the higher inward flux of oxygen through the Ru-rich layer than the outward flux of aluminum [15]. Conversely, Bellina proposed that this multilayer structure formed when cracks developed in both the Al 2 O 3 and the Ru-rich layer, thus permitting oxygen to reach the RuAl matrix.…”

“…For either bulk or coating applications, detailed knowledge of the oxidation properties and their dependence on composition and microstructure is of great significance. Limited isothermal oxidation studies on RuAl alloys have shown relatively good oxidation resistance up to 1000 C [2,4,5,14]. Soldera and co-workers reported that single phase RuAl produced by reactive sintering showed excellent oxidation resistance at 1000 C due to the formation of a dense, protective Al 2 O 3 scale [14].…”

“…Limited isothermal oxidation studies on RuAl alloys have shown relatively good oxidation resistance up to 1000 C [2,4,5,14]. Soldera and co-workers reported that single phase RuAl produced by reactive sintering showed excellent oxidation resistance at 1000 C due to the formation of a dense, protective Al 2 O 3 scale [14]. Conversely, a porous and discontinuous a-Al 2 O 3 scale was formed during isothermal oxidation of an arc-melted near-stoichiometric RuAl alloy, due to the vaporization of the Ru oxides at 1000 C [15].…”

“…Soldera and co-workers reported that single phase RuAl produced by reactive sintering showed excellent oxidation resistance at 1000 C due to the formation of a dense, protective Al 2 O 3 scale [14]. Conversely, a porous and discontinuous a-Al 2 O 3 scale was formed during isothermal oxidation of an arc-melted near-stoichiometric RuAl alloy, due to the vaporization of the Ru oxides at 1000 C [15]. Further investigations conducted by Bellina and co-workers on nominally single phase RuAl isothermally oxidized at 1100 C concluded that the formation of a d-Ru layer beneath the oxide scale is detrimental to the formation of a compact protective Al 2 O 3 scale [13].…”