Effect of alloying on fracture toughness of MoSi₂ at ambient temperature

Six kinds of MoSi₂ ,(Mo,2at%Nb) Si₂ , (Mo,6at% Nb) Si₂, Mo(Si,2at% Al)₂,Mo(Si,4at% Al)₂ and(Mo,2at% Cr) Si₂ alloys were arc-melted. The effects of Nb, Al and Cr alloying on raicrostructure, hardness and fracture toughness of MoSi ₂ at ambient temperature were investigated by means of XRD, SEM-EDS,EPMA,Vickers and indentation toughness method. The results show that all of the three alloys with 2at% Al ,4at% Al and 2at%Cr have only obtained tetragonal C11_b phase; while in the alloy with 6at% Nb,the addition of excessive Nb leads tetragonal C11_b phase to transform partly into hexagonal C40 phase, thus the dual-phase microstructure with grey C11 _b matrix and whiter lamellar C40 phase was observed in this alloy. Also, there are some cracks crossing lamella in the width and almost arranging parallelly in some C40 phase. Nb,Al and Cr alloying are found to decrease hardness and improve fracture toughness of monolithic MoSi₂. The alloys with 2at% Nb and 4at% Al have highest fracture toughness, reaching 5.90 MPa-m^1/2,69% higher than that of monolithic MoSi₂.