Effect of heat-treatment on microstructure and mechanical behavior of directionally solidified Nb-Ti-Si based alloy

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). XRD, SEM and EDS have been employed to investigate the influence of heat-treatments on the microstructures and fractography 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% (19.2 MPa·m^½) 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.