Effect of Mo and Hf on the mechanical properties and microstructure of Nb–Ti–C alloys

The microstructures and mechanical properties of Nb–20Ti–12.5C (at.%) containing different Mo or Hf contents have been investigated. The microstructures were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The alloy without Mo and Hf contains Nb solid solution (Nbss), (Ti,Nb)2C/Nbss eutectic, primary (Ti,Nb)2C and secondary (Ti,Nb)C. The addition of Mo or Hf restrains the precipitation of (Ti,Nb)C. No Mo-rich precipitates were observed. Part of the Hf appeared as (Hf,Ti,Nb)2C-type carbide. Compression testing has been carried out at room temperature and 1473 K. The alloys showed good compression ductility at room temperature and Nb20Ti12.5C20Mo produced a 0.2% offset yield stress of 300 MPa at 1473 K. The room temperature and high temperature strengths increase with increasing Mo, but the room temperature ductility decreases. Hf additions however did not much change the strength because half the Hf dissolved in the carbides and there was not much modification of their volume fraction.