2007 年 71 巻 1 号 p. 2-8
Coupon specimens of 30Nb-40Al-xCr-yMo-zSi (mol%), with x+y+z=30, were oxidised in air at 1573, 1673 and 1773 K for 72 ks. The original specimens and oxidation products were characterised by X-ray diffractometry, scanning electron microscopy and electron probe microanalysis. All the specimens consist of several phases with complex structures. The major phases of the specimens are AlNbCr, Nb5Si3, NbAl3 and Mo3Al, and their minor phases are Cr2Al, Cr5Al8, NbCr2 and Nb2Al. The amount of AlNbCr decreases and that of Nb5Si3 increases as the Si content increases. At 1573 and 1673 K all the specimens, except 30Nb-40Al-15Cr-15Si oxidised at 1673 K, form protective scales with inner α-Al2O3 layers very sound and adherent to the substrates. At 1773 K 30Nb-40Al-7.5Cr-7.5Mo-15S and 30Nb-40Al-10Cr-5Mo-15Si show excellent oxidation resistance with scales consisting mainly of α-Al2O3. The other specimens form poorly protective scales or oxidation zones without scales. In general, the oxidation resistance increases with an increase in the Si content for the Mo-containing alloys. This is attributable to the formation of protective scales containing SiO2 and mullite in the initial oxidation stages. However, an adverse relationship was found for those without Mo. This is explained in terms of the strength of the specimen. This view is supported by the fact that the specimens without Mo were elongated and deformed. These shape changes would have introduced mechanical failure in the scales.