Hisatoshi Hirai

Influences of Al content and secondary phase of Mo5(Si,Al)3 on the oxidation resistance of Al-rich Mo(Si,Al)2-base composites

Abstract The microstructures and oxidation behavior of Mo(Si,Al)2-base composites were investigated. A high Al content up to 38 mol% and the effect of the secondary Mo5(Si,Al)3 phase were specifically examined. Microstructure observations indicated that the Al solubility limit at 1680 K extends to 43 and 10 mol% for Mo(Si,Al)2 and Mo5(Si,Al)3, respectively. The composites containing about 10 vol.% Mo5(Si,Al)3 exhibits excellent oxidation resistance in the temperature range between 780 and 1780 K, so that the Mo5(Si,Al)3 up to about 10 vol.% is acceptable and has no detrimental influence on the oxidation resistance. At temperatures from 1480 to 1780 K, a continuous alumina layer develops on the Mo5(Si,Al)3 as well as on the Mo(Si,Al)2. The alumina growth rate is nondistinctive in the Al range investigated, and it is controlled by inward oxygen diffusion through the alumina at 1580 K and higher.

Tensile and Creep Properties of the Refractory Nb Base In-Situ Composite

Niobium-base in-situ composite Nb-18Si-5Mo-5Hf-2C (in mol%) was prepared and heat-treated at 2070 K for 20 hour. The uni-axile tensile tests at high temperature ranges and the constant load tensile creep tests at 1570 K were performed. The specimen tensile-tested at 1470 K exhibited the excellent UTS of 450 MPa, and the brittle to ductile transition temperature is between 1470 and 1670 K. The specimens creep tested showed good creep strength; the stress exponent is about 5. The tensile fracture surface of the in-situ composite is complex and attributed to cleavage of the Nb5Si3, Nbss/Nb5Si3 interface separation, ductile rupture of the Nbss and correlations of these. On the otherhand, the fracture surface of creep tested consists of intergranular above 150 MPa and transgranular below 120 MPa with severely deformed Nbss.