ROLE OF MATRIX MICROSTRUCTURE ON - ROOM-TEMPERATURE TENSILE PROPERTIES AND FIBER-STRENGTH UTILIZATION OF AN ORTHORHOMBIC TI-ALLOY-BASED COMPOSITE

Microstructure-property understanding obtained for a nominally Ti-25Al -17Nb (at; pet) monolithic sheet alloy was used to heat treat a unidir ectional four-ply SCS-6/Ti-25Al-17Nb metal-matrix composite (MMC) and a fiberless ''neat'' material of the same alloy for enhancing mechanic al properties. The unreinforced alloy and [0](4) composite recorded si gnificant improvements' in ductility and strength, which were related to-the microstructural condition. Modeling of the tensile strength bas ed on fiber fracture statistics helped in understanding how improved m atrix microstructure provided more efficient utilization of fiber stre ngth. In comparison to the [0](4) MMC, improvement of the [90](4) resp onse was negligible, which was related to an alpha(2) stabilized zone around the fiber. A Nb coating on the fiber was used to modify the loc al microstructure, and it produced a modest improvement in strength an d ductility in the transverse direction. Structure-property relations of the matrix under different heat-treatment conditions are described in terms of deformation and failure mechanisms of the constituent phas es; alpha(2) (ordered hexagonal close-packed), B2 (ordered body-center ed cubic), and O (ordered orthorhombic based on Ti2AlNb).