| Aluminum-lithium (Al-Li) alloys have recently generated a prominent amount of interest among the automotive, aerospace, and construction industries. Principally through introduction of solute lithium, the lightest metallic species, considerable reductions in density have been achieved simultaneously with significant increases in elastic modulus. In these alloys, precipitation hardening gives rise to {dollar}deltaspprime{dollar} (Al{dollar}sb3{dollar}Li) or T1 (Al{dollar}sb2{dollar}CuLi) strengthening phases.; Nondestructive techniques (NDE) were employed to analyze effects of isothermal aging on 2090 and 8090 Al-Li alloys at 165{dollar}spcirc{dollar}C, 175{dollar}spcirc{dollar}C, and 185{dollar}spcirc{dollar}C. Ultrasonic time-of-flight and attenuation measurements were conducted at regular intervals during 24 hour aging processes for room temperature and elevated temperature (e.g. real time) conditions. Rockwell hardness confirmed increases in mechanical strength during room temperature analysis. Results for the 2090 alloy yielded an average 0.8% increase in longitudinal velocity, accompanied by a 35.5% increase in Rockwell hardness and a maximum R{dollar}sb{lcub}rm B{rcub}{dollar} = 81. From velocity data, the shear (G), bulk (k), Young's (E) moduli, and Poisson's ratio ({dollar}nu{dollar}), were calculated. After aging, the 2090 alloy demonstrated an average Young's modulus (E) of 75.1 GPa, which was about 5.2% higher than E values for conventional Al alloys and was consistent with other studies. The 8090 alloy yielded similar results.; Ultrasonic data confirmed the presence of minima-like anomalies that occurred at faster rates with increased isothermal aging temperature. These minima are thought to be associated with formation of the T1 (Al{dollar}sb2{dollar}CuLi) phase and {dollar}not deltaspprime{dollar} (Al{dollar}sb3{dollar}Li), more commonly associated with Al-Li strength. Kinetic calculations provided an activation energy of approximately 3.83 kcal/mole associated with these minima. The Avrami expression yielded a transformation mode parameter (n) of 0.55-0.58 at 165{dollar}spcirc{dollar}C, consistent with coarsening of thin platelets, e.g. T1 precipitates. The n-values decreased with increasing isothermal aging temperature, indicating morphological changes in precipitate growth.; In addition, Rayleigh surface wave techniques indicated a 500 {dollar}mu{dollar}m deep lithium-depleted region on an 8090 alloy surface. The current investigation successfully demonstrated the effectiveness of ultrasound in the study of the aging process in Al-Li alloys. |
