Recent progress in TEM observation on age-hardened alloys

Transmission electron microscopy (TEM) has played a significant role in the understanding of the nature of phase transformations in various alloy systems. For example, recent advancement of high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) with a sufficiently small probe size made possible clarification of the structure of GP zones at the atomic level without a priori information on the structure. Thus GP-I zones consisting of two Cu layers, in addition to the commonly-known single layered GP-I, can clearly be imaged, while θ' phase was observed to nucleate at the face of the GP-II zone. In this study/review we also show that high-resolution TEM indicates that the structure of several coherent phases in an aged Al-Li-Cu alloy depends sensitively on the Li content of the alloy.