Effect of carbon on microstructure and high-temperature strength of Nb-Mo-Ti-Si in situ composites prepared by arc-melting and directional solidification

Improvement of high-temperature strength of Nb-(10-20)Mo-22Ti-18Si alloys by adding carbon and directional solidification technique has been investigated. Vickers hardness HV and compressive strength at 1470 and 1670 K were examined. The as-cast specimen without C composes of Nb solid solution (Nb_SS) and (Nb, Mo, Ti)₅ Si₃ silicide, while (Nb, Ti)C carbide obviously appears when contain more than 2.4 mol% carbon. The amount of carbide and silicide phases increases with increasing carbon content. The directionally solidified samples have the same phases as the as-cast ones and show coarse microstructure slightly oriented to the direction of growth. A small amount of carbon addition lowers both the HV and strength at 1470 and 1670 K, where corresponds with the appearance of the carbide phase. By adding more than 4.8 mol% carbon, however, the HV, 0.2% yield strength (σ₀.2) and maximum strength (σ_max) tend to increase. The directional solidification samples show higher strength and work hardening rate than the as-cast ones at corresponding C content. The 10 mol% Mo sample is weakened by C addition, but a considerable improvement in strength at elevated temperatures can be predicted when more than 10 mol% C is added to the 20 mol% Mo material. The sliding and debonding of the Nb_SS/silicide interfaces is dominant in compressive damage at elevated temperatures.