Abstract:With the melting temperature of 2323 K and withdrawing rate of 100 μm/s, Nb-Ti-Si based ultrahigh temperature alloy has been directionally solidified with the use of crucibles. The directionally solidified (DS) specimens were subsequently heat-treated in two ways: either at 1723 K/50 h (HT1) or at 1623 K/50 h 1723 K/50 h 1373 K/50 h (HT2) respectively. XRD, SEM and EDS have been employed to investigate the influence of heat-treatments on the microstructures and fractographies of the directionally solidified alloys. The results show that after heat-treatment, the volume fraction of the large-size primary silicides decreases. Both kinds of heat-treatments could effectively alleviate or even eliminate the segregation in the alloys. The original boundaries between the (Nb,X)5Si3 Nbss eutectic cells in the DS specimens have thoroughly disappeared after heat-treatments. Specifically, silicides have been more evenly distributed after HT2 than HT1. Compared with DS specimens, the average room temperature fracture toughness of the specimens has increased by 12.3% to 19.2 MPa?m1/2 while the average tensile strength has increased by 26.6% and the maximum value reaches 933.2 MPa after HT2. These improvements could be mainly attributed to the more effective dispersion strengthening of (Nb,X)5Si3 silicide particles and the shape change as well as size increase of the ductile Nbss phases after HT2.