Synergy of Cr concentration and withdraw rate on microstructure evolution of directionally solidified Nb-14Si-24Ti alloys

The synergy of Cr concentration and withdraw rate on the microstructure of directionally solidified and heat treated Nb-14Si-24Ti-xCr (x=0, 2, 8, 10, 14 at-%) alloys was investigated. The phases present in the directionally solidified alloys were sensitive to Cr concentration and withdraw rate. The addition of Cr changed the solidification path of ternary alloys. High Cr concentration stabilised the Nb₅Si₃ and facilitated the formation of Cr₂Nb during the directional solidification process. However, the effect of Cr was suppressed by the increase in withdraw rate. The competition between the effect of Cr addition and withdraw rate determined the microstructure of alloys and resulted in a four-phase microstructure. During heat treatment, the Nb₃Si phases in Nb-14Si-24Ti-0Cr alloys were stable, while in the Cr contained alloys, the Nb₃Si decomposed into NbSS/Nb₅Si₃ eutectoid through the eutectoid reaction (Nb₃Si←Nb_SS+Nb₅Si₃).