Microstructures and mechanical properties of Nb-Mo-Ti-Si-C in-situ composites prepared by arc melting and directional solidification

In order to examine the effect of carbon addition on the mechanical properties of niobium solid solution/niobium silicide eutectic alloys (in-situ composites), (Nb-xMo-22Ti-18Si)-yC(x = 10 and 20 mol%, y = 0, 1.0, 2.4, 4.8, 7.0, 9.1 mol%) alloys were prepared by arc melting and floating zone melting techniques. Each specimen was annealed at the temperature range from 1820 to 1870 K in a vacuum for 100 h. Vickers hardness was measured at room temperature and compressive strength was measured at 1670 K in a vacuum at an initial strain rate of 1×10^-4 s^-1. Specimens containing more than 2.4 mol% carbon are composed of Nb solid solution, (Nb, Mo, Ti)₅Si₃ silicide and (Nb, Ti)C carbide, and the amount of carbide phase increases with increasing carbon content. A small amount of carbon addition lowers both Vickers hardness and 0.2% yield stress at 1670 K. By the addition of more than 4.8 mol% carbon, however, Vickers hardness and yield stress tend to increase. A certain amount of carbon must be soluble in the silicide phase, which possibly changes the crystal structure of the silicide, resulting the softening of this alloy.