Abstract
Niobium silicide-based in situ composites are Nb base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. In microstructure-property studies of these alloys, the differentiation between the alphaNb5Si3 and beta5Si3 is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) A1 (5 or 4 at.%) and Ta (6 at.%) on the microstructures of as-cast and heat-treated Nb-24Ti-18Si in situ composites have been studied. The main phases observed in the as-cast and heat-treated (at 1400 °C /1500 °C for 100 h / 200 h) alloys were the niobium solid solution, (Nb,Ti)ss, the silicides, (Nb,Ti)3Si, alphaNb5Si3 and beta5Si3, and a Cr-rich C14 silicide Laves phase. In Nb-24Ti-18Si (KZ3) and Nb-24Ti-18Si-5Cr (KZ4), (Nb,Ti)ss was the primary phase, while in the Al-containing alloys, Nb-24Ti-18Si-5Al (KZ7), Nb-24Ti-18Si-5Cr-5Al (KZ5), Nb-24Ti-6Ta-18Si-5Cr-5Al (KZ6), Nb-24Ti-18Si-8Cr-4Al (KZ2) and Nb-24Ti-6Ta-18Si-8Cr-4Al (KZ8), beta5Si3 was the primary phase The as-cast microstructure of KZ3 consisted of primary (Nb,Ti)ss dendrites together with faceted (Nb,Ti)3Si and a rod-like eutectic of (Nb,Ti)ss and (Nb,Ti)3Si. The A1 addition changed the nature of the eutectic; an irregular eutectic of (Nb,Ti)ss and beta5Si3 was formed in the as-cast KZ7. The Cr stabilised the eutectic of (Nb,Ti)ss and beta5Si3 to lower temperatures and promoted the formation of Ti5Si3 in KZ8. In KZ2 and KZ8, the high Cr-content caused the formation of the Cr-rich C14 silicide Laves phase without destabilizing the beta5Si3. This Laves phase existed at the depressions of (Nb,Ti)ss dendrites and probably was formed congruently from the remaining liquid. This Laves phase with Si > 2.5 at.% can be classified as the ternary p-phase proposed by Goldschmidt and Brant, after considering that Si and A1 atoms substitute for Cr atoms and that Ti and Ta atoms substitute for Nb atoms. During heat-treatments, the beta5Si3 phase transformed to alphaNb5Si3 according to the precipitation reaction beta5Si3 → alphaNb5Si3 + (Nb,Ti)ss, which retarded by Ta and Cr. After heat-treatment, the Ta-containing alloys were homogenised to a lesser extent compared to non-Ta-containing alloys. The Ti5Si3 that formed during solidification in KZ8 was not eliminated after homogenization at 1400 °C for 100 h. Liquation was observed after 100 h at 1500 °C in KZ2 and KZ8, because of their high Cr-content. Thermo-gravimetric analysis was performed at 800 °C for the 100 h heat-treated alloys. "Pest oxidation" was observed in KZ3. The A1 and Cr additions suppressed this oxidation mechanism. Higher oxidation rates and lower adherence of the oxide scale were observed with increase of the Cr concentration and/or the Ta addition. The Cr-rich silicide Laves phase and Nb5Si3 silicides with very low oxygen contents existed in the oxide scales, indicating that the oxygen solubility in these phases is quite low and that they are not oxidised easily. The presence of the oxidation resistant Cr-rich C14 silicide Laves phase in the 1400 °C for 100 h heat-treated KZ2 and KZ8 did not affect the oxidation rate at 800 °C, but decreased the oxidation rate at 1200 °C.