Microstructural stability of binary Al-Li alloys at low temperatures

Binary Al-Li alloys containing 6.5, 9.5 and 11.5 at.%Li have been aged for 24 h at 423 K and then subjected to long term exposure at 343, 373 and 403 K. Proof stress and fracture energy have been measured as a function of exposure time, showing that little or no embrittlement occurs at 343 K, but the alloys undergo increasing degrees of embrittlement as the exposure temperature is raised to 403 K. Transmission electron microscopy and electrical resistivity measurements show that an exposure temperature of 343 K results in very slow precipitation of fine delta', but at high exposure temperatures the dominant process is growth of pre-existing delta' particles that have been produced by the 423 K pre-age. These results are discussed in terms of the kinetics of precipitation and the importance of quenched-in vacancies. (C) 1997 Acta Metallurgica Inc.