Effect of microstructure on the high-temperature deformation behavior of Nb-Si alloys

Deformation behavior of Nb-Si-Zr alloys is investigated at various temperatures ranging from R.T. to 1670 K. The master alloy ingots composed of Nb-18.1 at% Si-1.5 at% Zr doped with Mg are Ar-arc-melted. The ingots contain Nb rods (radius: 1 μm) in Nb₃Si matrix formed by eutectic reaction. Alloys are subjected to heat treatments at 1923 K for 4-100 h to obtain a large Nb network structure with small silicide (α-Nb₅Si₃) particles by decomposing Nb₃Si matrix into Nb and Nb₅Si₃ through a eutectoid reaction. Compression tests are conducted at room temperature in air and at elevated temperatures in Ar atmosphere. At 1471 K the maximum strength is 500 MPa and compressive ductility is higher than 10% with a strain rate of 1.0 × 10^-4 s^-1, while at room temperature the maximum strength is over 1500 MPa and compressive ductility is about 1.5%. The high-temperature deformation obeys a power-law type equation. The stress exponent n is evaluated to be 4.8 and the apparent activation energy is 350 kJ/mol. The Vickers indentation at room temperature revealed that the crack propagation at room temperature is suppressed effectively by ductile Nb. This suggests that the Nb aggregate in the network structure acts as a large Nb grain containing fine Nb₅Si₃ particles, which might be beneficial for ductility at low temperatures.