Characterization of the microstructures produced by different solutionizing quench rates in aluminum-lithium alloys
The microstructure in lightweight aluminum-lithium alloys produced by two different quench rates after solution heat treatment were investigated using conventional transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The formation mechanism, size and distribution of the phase δ’ (Al3Li) were compared, since the precipitate affects many of the mechanical properties in the age- hardening Al-Li system. The metastable δ’ phase has the L12 ordered structure based on the fcc lattice with a lattice parameter difference of -0.08% with the aluminum matrix. The equilibrium phase δ (AlLi) nucleates independently with the B32 structure. TEM yields direct evidence of δ’ formation, size and distribution which is unattainable by other methods due to the size of the precipitates and the presence of isostructural β’ (Al3Zr) dispersoids. Small blocks of Al-2.4Li-0.lZr and Al-2.6Li-lCu-0.5Mg-0.5Zr (wt.%) were solutionized at 550°C for 2 hrs, then either quenched in 0°C water or in still, ambient air. Electropolished foils were viewed in a Philips 301 (l00kV) or JEM 200CX (200 kV).