Influence of Additional Elements on the Oxidation Behaviour of Nb-Al-Cr Base Alloys
Coupon specimens of 30Nb-40Al-(30-x)Cr-xMo (mol %), with x being 0, 7.5, 15, 22.5 or 30, and of 30Nb-40Al-(22.5-y)Cr-7.5Mo-ySi, with y being 5 or 10, were oxidised in air in a temperature range 1473 to 1773 K for 72 ks. The influence of additions of 0.05 and 0.1% Hf to 30Nb-40Al-22.5Cr-7.5Mo was also examined. Conventional metallographic examinations were performed for the specimens before and after the oxidation. The major phase of the specimens is AlNbCr, and their minor phases are Cr2Al and NbAl3, except for high Mo-content specimens which form AlNbMo, Nb2Al, Mo3Al and MoAl3. The additions of Si lead to additional formation of Nb5Si3. Virtually Al2O3 scales (a dense and continuous Al2O3 layer with a thin layer consisting of AlNbO4, CrNbO4 and/or SiO2 on it) are formed at temperatures up to 1673 K, except for the high Mo-content specimens which form poorly protective scales. The additions of Hf increase the mass gain to some degree. 5Si forms protective scales at temperatures up to 1723 K, and 30Cr and 10Si up to 1773 K.
Shigeji Taniguchi, Toshio Maruyama, Masayuki Yoshiba, Nobuo Otsuka and Yuuzou Kawahara
S. Taniguchi et al., "Influence of Additional Elements on the Oxidation Behaviour of Nb-Al-Cr Base Alloys", Materials Science Forum, Vols. 522-523, pp. 625-632, 2006