PHASE-EQUILIBRIA IN THE NB-FE-CR-SI SYSTEM

Efforts to improve the CVD process used to elaborate protective coatin gs against oxidation for niobium alloys have needed a good knowledge o f the quaternary Nb-Fe-Cr-Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb6.6Fe1.4Cr4Si8, ph ase has been identified and studied using single X-ray diffraction dat a. The crystal structure is of a new type, derived from the Cr11Ge8-ty pe structure (space group Pnma, Z=4, a=13.649(2) Angstrom, b=4.953(1) Angstrom, c=16.386(3) Angstrom, R=R-w=5.7%). Higher temperatures stabi lize the ternary Nb2Cr3Si3 silicide in the Mn5Si3-type structure (spac e group P6(3)/mcm, Z=2, a=7.190(2) Angstrom, c=4.860(1) Angstrom, R=3. 2%, R-w=2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges. (C) 1998 Elsevier Science S.A.