THE EFFECTS OF ARTIFICIAL AGING ON THE MICROSTRUCTURE AND FRACTURE-TOUGHNESS OF AL-CU-LI ALLOY-2195

Aluminum-lithium alloys have shown promise for aerospace applications, and National Aeronautics and Space Administration (NASA) has selected the aluminum-lithium Alloy 2195 for the main structural alloy of the super light weight tank (SLWT) for the space shuttle, This alloy has s ignificantly higher strength than conventional 2xxx alloys (such as 22 19) at both ambient and cryogenic temperatures. If properly processed and heat treated, this alloy can display higher fracture toughness at cryogenic temperature than at ambient temperature. However, the proper ties of production materials have shown greater variation than those o f other established alloys, as is the case with any new alloy that is being transitioned to a demanding application. Recently, some commerci al 2195 plates for the SLWT program were rejected, mostly due to low C FT or FTR at ambient and cryogenic temperatures, Investigation of the microstructure property relationships of Al-Cu-Li based alloys indicat es that the poor fracture toughness properties can be attributed to ex cessive T-1 precipitation at subgrain boundaries. Lowering the aging t emperature is one way to avoid excessive T-1 precipitation at subgrain boundaries. However, this approach results In a significant drop in y ield strength, In addition, low-temperature aging is associated with s luggish aging kinetics, which are not desirable for industrial mass pr oduction. Therefore, the present study was undertaken to develop an ag ing process that can improve fracture toughness without sacrificing yi eld and tensile strength. A multistep heating-rate controlled (MSRC) a ging treatment has been developed that can improve the cryogenic fract ure toughness of aluminum-lithium Alloy 2195, At the same levels of yi eld strength (YS), this treatment results in considerably higher fract ure toughness than that found in Alloy 2195, which has received conven tional (isothermal) aging. Transmission electron microscopy revealed t hat the new treatment greatly reduces the size and density of subgrain -boundary T-1 precipitates. In addition, it promotes T-1 and theta '' nucleation, resulting in a fine and dense distribution of precipitate particles in the matrix. The MSRC aging treatment consists of(a) aging at 127 degrees C (260 degrees F) for 5 h, (b) heating continuously fr om 127 degrees C (260 degrees F) to 135 degrees C (275 degrees F) at a rate of 0.556 degrees C/h (1 degrees F/h), (c) holding at 135 degrees C (275 degrees F) for 5 h, (d) heating continuously from 135 to 143 d egrees C (275 to 290 degrees F) at a rate of 0.556 degrees C/h (1 degr ees F/h), and (e) holding at 143 degrees C (290 degrees F) for 25 h to obtain a near peak-aged condition.