STUDIES ON TUNGSTEN (WHFC) FILAMENT-REINFORCED NBAL3-BASE ALLOY

Hafnium carbide dispersion strengthened tungsten (WHfC) filaments were used as a ductile phase reinforcement to improve the low temperature toughness of an oxidation resistant NbAl3-base alloy with Cr, Y and W additions. The WHfC/NbA1CrYW composite was prepared by the powder cloth technique. The composite specimens were thermally cycled between 500 and 1300 K to examine the influence of CTE mismatch on matrix cracking. The thermal stability was assessed by annealing at 1500 K in argon for up to 100 h. The mechanical properties of the monolithic NbAlCrYW and the WHfC/NbAlCrYW composite were measured from bend tests between 300 and 1300 K in as-fabricated, thermally cycled and annealed conditions. The composite was significantly tougher and stronger than the monolithic NbAlCrYW alloy at all temperatures and in all conditions. The ultimate bend strength and work-of-fracture of the composite decreased with increasing temperature. Thermal cycling up to 1000 times resulted in severe cracking of the matrix due to the CTE mismatch. Interfacial reactions at 1500 K between the NbAlCrYW matrix and the WHfC filament formed a complex phase rich in Cr, Al and W. The thickness of the reaction zone exhibited parabolic growth. The formation of this phase resulted in significant degradation in toughness and strength after annealing at 1500 K for 100 h.