IMPROVING MECHANICAL-PROPERTIES OF NEXTEL 610 REINFORCED AL-224 ALLOY THROUGH THETA-PHASE PRECIPITATION AT THE FIBER-MATRIX INTERFACE

A novel approach toward axial and transverse composite strengthening through controlled delamination of interfaces in alumina fiber reinforced Al-(224.2) is presented. During annealing at 350-degrees-C, secondary CUAl2 (0) precipitates coarsen preferentially at the fiber-matrix interface. Observations of fiber pull-out indicate that the interface delamination and crack bridging mechanism for axial toughening was activated by coarsening of theta precipitates to a critical size and distribution. The peak sigma11 of 972 MPa (91% of rule of mixtures (ROM)) after 10 h at 350-degrees-C correlates with a minimum in the mean free distance between theta precipitates and a minimum in transverse strength sigma22. However, thermal exposures for longer than 100 h at 350-degrees-C resulted in an increase in sigma22 to 345 MPa and an acceptable decrease in sigma11 to 70% of ROM (724 MPa). This value of sigma22 is comparable with wrought 2xxx-T7 alloys, and is extremely high for most continuously reinforced metal matrix composite applications. The increase in sigma2 correlates with an increase in the metal-fiber bond area fraction at the interface, which results from theta precipitate coarsening.