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

Earlier studies of Nextel 610 (seeded sol-gel-derived Al2O3 multifilam ent tow developed by 3M) reinforced Al-224.2 composite panels manufact ured by pressure infiltration casting revealed interface delamination as a result of precipitation of the equilibrium theta precipitate and preferential growth at fiber/matrix interface sites. As a result, stre ngths as high as 91% of rule of mixtures (ROM) were observed. 75% ROM strengths were observed after extended heating at 350-degrees-C and he nce in situ coarsening of the interface theta precipitates. Transverse specimens displaying this still acceptable tensile strength exhibited values of 490 MPa (approaching that of wrought 2xxx alloys). This com bination of high axial and transverse strengths from material in the s ame condition represents a significant improvement in this class of ma terials. This process is termed ''discontinuously coated interfaces'' (DCI(TM)). In the present study, the kinetics of the coarsening proces s were measured at 350 and 400-degrees-C using a new algorithm for the quantitative metallography determinations of the particle distributio n at the interface. The interfacial theta coarsening kinetics follows a classical t1/4 rate law. However, the particle size distribution is contrary to the theoretical predictions. Axial flexure tests on materi als subjected to heat treatments at 350 and 400-degrees-C producing cl ose particle sizes result in similar flexure strengths. Thus, it has b een demonstrated that the precipitate size and hence distribution at t he interface is the most important factor controlling the mechanical p roperties of the composite.