Yonghua Liu
Published: 2001
Total Pages: 122
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Two ternary phases in Nb-Si-B system, T2(Cr5B3-type tetragonal structure) and D8(Mn5Si3-type hexagonal structure), were prepared. The oxidation resistances of both T2 and D88 were found better than binary Nb5Si3 at 1000 0C, but still inadequate. The oxide scale is the mixture of Nb2O5 and borosilicate glass. The severe cracks were developed in the matrix and scale due to the large volume expansion associated with the formation of Nb2O5, SiO2 and B203. In Nb-Mo-Si-B system, all observed phases contain both Mo and Nb because Nb/Mo substitution occurs. The suitable Nb/Mo substitution can avoid the formation of Mo3Si and offer the possibility of ductile phase toughening. The lattice parameters for T1, T2 and D88 increase linearly with the increase of the Nb/Mo ratio. Oxidation tests indicated that the substitution of Nb for Mo decreased the oxidation resistance of boron doped Mo5Si3 dramatically. All specimen exhibited catastrophic oxidation behavior at 800 0C and converted to oxides completely after 20-50 hours exposure to flowing air. Although a parabolic behavior was achieved for most compositions at 1200°C, the high rate constants indicated that the scales formed are not protective. The existence of four layers in scale implies that the oxidation mechanism is complex and need further study in detail.