Effect of heat treatment on the structure and properties of niobium alloys

1. Microstructure of the niobium alloys studied after annealing at 1300–1400°C is two-phase. Solid-solution decomposition proceeds over the whole area of a cast grain and over grain boundaries. 2. With annealing temperature increased to 1600°C there is carbide solution and formation of a single-phase structure, and with a further increase to 2000°C there is grain growth and development of polygonization. 3. Maximum ductility of cast alloys 1 and 2 is attained after annealing at 1400 and 1600°C, respectively. These annealing temperatures correspond to a minimum value of lattice parameter and x-ray line width, indicating that an equilibrium solid solution is obtained. 4. The optimum annealing schedule for alloy 1 ingots is 1400°C for 10 h and for alloy 2 it is 1600°C for 10 h. These schedules provide maximum ductility for cast alloys with structural and property stability.