Composite Particles Formation and Behaviour in Al-Li Alloys Alloyed by Sc (Zr) under Heat Treatment and Severe Plastic Deformation

摘要

The mechanism and kinetics of Al3Li/Al3Sc composite particles formation during aging of triple Al-Li-Sc alloys were studied. The particles of such type consist of two isomorphic phases, ordered by L1(2) type. Composite particles in Al-Li-Sc are formed by epitaxial nucleation of delta'(Al3Li) phase on pre-existing Al3Sc precipitates at low temperature aging in the area of existence of Al3Li phase, and form uniform shell around them. The motive force of such heterostructure formation is the minimization of total surface, elastic and chemical energy of the alloy. Dark-field transmission electron microscopy images of such composite particles taken from superlattice reflex 100 are composed of a dark core surrounded by a bright shell. The mutual diffusion of Li and Sc during the aging of Al-Li-Sc alloys is observed. The dark-field transmission electron microscopy image of the composite particle obtained from superlattice reflection became transparent after continuous natural aging of Al- Li-Sc alloys. The presence of the particle in Al matrix can be only seen due to decoration by secondary precipitations of delta'(Al3Li) phase. Composition analysis of such transparent particles was carried out using two-beam kinematical theory of electron diffraction. Such composite particles with transparent cores remain in Al matrix even after severe plastic deformation of Al-Li-Sc alloy.