Phase Equilibria in the Nb-Fe-Cr-Si System.

Abstract Efforts to improve the CVD process used to elaborate protective coatings against oxidation for niobium alloys have needed a good knowledge of the quaternary Nb–Fe–Cr–Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb 6.6 Fe 1.4 Cr 4 Si 8 , phase has been identified and studied using single X-ray diffraction data. The crystal structure is of a new type, derived from the Cr 11 Ge 8 -type structure (space group Pnma, Z =4, a =13.649(2) A, b =4.953(1) A, c =16.386(3) A, R = R w =5.7%). Higher temperatures stabilize the ternary Nb 2 Cr 3 Si 3 silicide in the Mn 5 Si 3 -type structure (space group P6 3 /mcm, Z =2, a =7.190(2) A, c =4.860(1) A, R =3.2%, R w =2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges.