Effect of alloying additions on the phase equil ibria and Nb2 Nb 5Si 3 eutectic morphologies of Nb2Mo2Si alloys

The alloying effect s of 3 at %M ( M = Cr , Al , Ti) on the phase equlibria and eutectic morphology of
Nb220Si210Mo cast alloy were studied by SEM , EDS and XRD. All the studied alloys were prepared by arc
melting. The Nb220Si2 10Mo cast alloy contains niobium solid solution (Nb SS) andβNb 5Si 3two phases. A large volume f raction of lamellar eutectic (Nb SS 2βNb 5Si 3) appears in the microst ructure. The Cr additions change the phase relationship in Nb220Si210Mo cast alloy. A Laves phase of Cr 2Nb precipitates together with Nb SS,βNb 5Si 3in Nb220Si210Mo23Cr cast alloy , while no third phase was observed in 3 at %Al or 3 at %Ti added alloys. The Nb SS 2Nb 5Si 3 eutectic st ructure loses it s st raight lamellar characteristic in Cr doped alloy , whereas the lamellar characteristic of Nb SS 2Nb 5Si 3eutectic was enhanced by Al additions. The lamellar eutectic morphology changes to featheriness as a result of Ti additions. The lat tice parameters of Nb andβNb 5Si 3change with the alloying additions : The lat tice parameters of Nb decrease in all the alloys ; the c/ a value ofβNb 5Si 3 decreases in Nb220Si210Mo23Cralloy , while the c/ a value ofβNb 5Si 3 increases in the other three alloys.