Effect of Salt Water and High-Temperature Exposure on Boron-Aluminum Composites

Boron-aluminum composites are being considered for aerospace applications, including jet engine fan blades and structural airframe components. The high specific modulus and strength of this material make it attractive in the design of these components while incorporating weight savings and increased structural efficiency. But two significant hostile environments are present that can jeopardize its outstanding properties; namely, enhanced corrosion in saltwater environments and high-temperature (air) exposures that are present in supersonic aircraft applications. This work was planned to identify critical properties that relate these environments to possible aerospace applications. In particular, the application of boron aluminum in jet engine fan blades was instrumental in the choice of experiment methods and analysis. Some of the findings in this investigation are summarized in the following.

Exposure of 2024 and 6061 matrix BORSIC + aluminum composites to synthetic sea salt spray at 95°F and 100 ksi bending stress results in matrix corrosion that is most severe with 2024 matrix material in the as-fabricated condition. Both 6061-F and 6061-T6 matrix composites exhibit general corrosion without preference for regions of high stress, whereas 2024-F and 2024-T6 matrix composites display depths of attack that are approximately twice as great on the tensile surfaces as the compressive surfaces. The flexural strengths of 6061-F and 6061-T6 matrix composites are unaffected by exposure for periods up to one month. The strength of BORSIC + 2024-F and BORSIC + 2024-T6 matrix composites can be reduced by as much as 100 percent and 14 percent, respectively, after one month as a result of matrix attack. The longitudinal strength of BORSIC filament is not degraded as a result of exposure to the humid salt environment.

The longitudinal strength of 61 v/o 5.7-mil BORSIC + 6061-F was degraded approximately 10 percent after 1000 h at 700°F. Transverse properties were unaffected. Uncoated 4.0-mil boron + 6061-F material exhibited a 12 percent decrease in longitudinal strength (175 × 10³ psi to 154 × 10³ psi) with the same exposure conditions. The transverse strength of this material was unaffected.

Although degradation of mechanical properties was achieved under these severe conditions, it is apparent that only the 2024 matrix alloy in saltwater exposure was found to be unsuitable for turbine engine fan blades.