INTEGRALLY DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN NIOBIUM SILICIDE BASED ULTRAHIGH TEMPERATURE ALLOY.

Integrally directional solidification of an advanced Nb silicide based ultrahigh temperature alloy was conducted with the use of special ceramic crucibles. The melt temperature was 2000°C and the withdrawing rate was respectively 5, 10, 15, 30, 50, 80 and 100 µm/s. The directionally solidified microstructure was composed of primary (Nb,X)₅Si₃ rods and (Nb,Ti)ss + (Nb,X)₅Si₃ eutectic colonies (here X represents Ti and Hf elements) that distributed evenly and aligned erectly along the growth direction. The quenched solid/liquid (S/L) interfaces had the morphology of a series of protruded parallel eutectic cells along the growth direction. (Nb,Ti)ss and (Nb,X)₅Si₃ slices grew in a coupled manner in each eutectic cell, forming a typical lamellar eutectic morphology. Both the average diameter of eutectic cells and lamellar spacing in them decreased with the increase of withdrawing rates. The solidification path was that primary (Nb,X)₅Si₃ rods formed firstly and then the retained liquid around solidified into (Nb,Ti)ss + (Nb,X)₅Si₃ eutectic cells. [ABSTRACT FROM AUTHOR]