Room Temperature Strength and Fracture Toughness of Two-phase Nb_ss/Nb₅Si₃ Alloyed with Mo

Abstract

Room temperature compressive strength and fracture toughness of two-phase Nb_ss/Nb₅Si₃ intermetallics alloyed with Mo are investigated in terms of chemical compositions, and microstructures that were modified by various processing techniques; arc melting, isothermal forging, non-crucible directional solidification(DS). Three types of microstructures are characterized by processing; maze-like structure for arc melting, equiaxed structure for isothermal forging and lamellar structure aligned to growth direction for DS. The Nb₅Si₃, coexisting with Nb_ss in Nb-xSi-15Mo alloys, is found to change the crystal structure from α to β accompanied by the increasing Mo content and heat treatment temperature. The yield strength of the two-phase alloy consisting of α-Nb₅Si₃ and Nb_ss is higher than that obtained in the alloy consisting of β-Nb₅Si₃ and Nb_ss, irrespective of the same chemical composition and volume fraction for both the alloys. The fracture toughness of the arc-melted alloys with a maze-like structure is found to be higher than that of the DS alloys with a fine aligned microstructure. On the basis of experimental results, it is suggested that the fracture toughness of the two-phase Nb_ss/Nb₅Si₃ alloys depends primarily on morphology, thickness and solid solution hardenability of the incorporated Nb_ss but does weakly on volume fraction of constituent phases and phase transformation.