Experimental determination and thermodynamic description of the Nb-V binary system

Highlights: • Experiments show that there exists a miscibility gap in the low temperature region of Nb-V binary system. • Two typical phase transition mechanisms of the spinodal decomposition and the nucleation-growth are observed. • The thermodynamic parameters are assessed using CALPHAD approach. The phase equilibria of the Nb-V binary system in the low temperature range below 850 °C have been investigated in order to determine the miscibility gap of its solid solution. By means of the approaches of the isothermal treatment of the alloys and diffusion couples as well as the differential thermal analysis (DTA), it has been proved that there is an immiscible region at low temperature, and the solid solution β (BCC_A2) separated into two phases with the same crystal structure but the different compositions, the Nb-rich β_Nb and the V-rich β_V phases. The microstructures formed after both the spinodal decomposition and the nucleation decomposition have been observed by transmission electron microscopy (TEM). On the basis of the present experimental data as well as the related literature reports, the thermodynamic parameters of the Nb-V binary system were optimized and a 6 set of self consistent thermodynamic data was obtained. By use of the optimized thermodynamic data, the miscibility gap of the Nb-V solid solution and its spinodal decomposition range were calculated and presented.