Formation of alumina on NbAl alloys

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

“…The diffusion distances of the alloying elements are estimated to be less than 500 nm, so that morphologies and compositions of the former BCC1, BCC2, and Laves phases retain while oxidation of these phases is accompanied by large volume expansion. This behavior is somewhat different from the oxidation behavior of conventional Nb alloys, as well as developmental Nb-Si-Al-Ti alloys, where a preferable metal oxide scale forms resulting in a decrease in the concentration of the alloying elements inside the Nb matrix, an increase in the oxygen solubility and eventually internal oxidation [15,28]. Reduced diffusion rates of the alloying elements in high-entropy alloys were noted in previous publications [6,12,13] and it was attributed to a reduced concentration of free vacancies in these multiprincipal-element alloys.…”

“…Indeed, feeding the surface layer with a sufficient amount of the alloying elements, which would form a stable oxide scale, has been considered to be the main issue during oxidation of conventional refractory alloys [14]. Heavy alloying of refractory metals with other elements, such as Al, Cr, Ti, Hf, etc., has been shown to be quite effective in obtaining remarkably improved high-temperature oxidation resistance for at least two reasons: (i) it reduces oxygen solubility and diffusivity in the matrix material and (ii) provides sufficient amount of the elements to form a stable oxide scale [15,16]. In this study, we report on the oxidation behavior of a high-entropy refractory alloy NbCrMo 0.5 Ta 0.5 TiZr.…”

Cyclic oxidation response of multiphase niobium-based alloys

“…The early efforts, which were reviewed extensively by Stringer [10] and others, [11,12,13] were largely unsuccessful because of the formation of Nb 2 O 5 , a nonprotective oxide, which was too dominant to be altered by alloying addition. Subsequent studies [14][15][16][17][18][19] were concentrated on an aluminidebased system and the use of Al addition to improve oxidation resistance of Nb-based alloys through the formation of a protective alumina layer. While the formation of a protective alumina was feasible, the resulting alloys had low melting points and were too brittle to be used as structural materials.…”

Recent advances in high-temperature niobium alloys

“…The two matrices considered in the work-Nb-25Ti-42AI-3Cr-4V and Nb-25Ti-38AI-3Cr-4V (in at.%)-are of interest since both form continuous alumina protective layers at elevated temperatures. 1 A variety of reinforcements were added to these matrices and vacuum hot-pressed to achieve full density. Results on samples aged at 1,371 °C indicated that yttria and alumina are the most stable reinforcements.…”