Effects of alloying elements and heat treatments on the microstructure and mechanical properties of refractory metal silicide-based alloys

The effects of alloying elements and heat treatments on the microstructure and mechanical properties of refractory metal silicide-based alloys have been reviewed in this paper. The silicides in arc-melted Nb-Si based alloys are beta Nb5Si3 after adding W, Mo or Al elements, alpha Nb5Si3 after adding Cr or V, and Nb3Si after adding Ti. The microstructure of the arc-melted Nb-Si-Ti alloys is composed of Nb3Si and Nbss. For the Nb-Si based alloys, adding Ti, Hf and B elements can improve the room temperature fracture toughness, the high temperature strength can be increased by the addition of Mo and W, and adding Cr, Al and Ti can enhance their high temperature oxidation resistance. In the MoSi2 based alloys, the silicides (Mo,W)Si-2, (Mo,Nb)Si-2 or Mo(Si,Ge)(2) are formed after the addition of W, Nb and Ge elements. For the Mo-Si-B alloys, the 72 phase is formed due to the addition of B, and its volume fraction increases with the increase of B content. The volume fraction of alpha-Mo phase plays an important role on the fracture toughness and oxidation resistance of Mo-Si-B alloys. The heat-treatment temperature is always rather higher for the refractory metal silicide based alloys. The microstructure is slightly coarser and the distribution is more uniform after heat treatments and annealing, obviously affecting the mechanical properties of alloys due to the changes of component phases and their volume fractions.