Experimental observations on the nucleation and growth of δ′ (Al₃Li) in dilute Al-Li alloys

Abstract

The influence of nucleation temperature and pre-aging thermal history on δ′ (Al₃Li) precipitate distributions was studied in a binary Al-8.2 at. pct Li alloy. The distribution of δ′ in the microstructure is found to be homogeneous and insensitive to variations in pre-aging thermal history when nucleation occurs at large undercoolings (ΔT > 90 °C). When nucleation occurs at low undercoolings (90 °C > gDT > 40 °C), nucleation is homogeneous but the precipitate number density is influenced by preaging thermal history. At very small undercoolings (ΔT < 40 °C) nucleation next to dislocations is favored unless large numbers of excess vacancies are present, in which case homogeneous nucleation can be more prevalent than nucleation next to dislocations. The early stages of δ′ growth are seen to be consistent with bulk diffusion control. Enhanced growth rates are observed when the material contains a higher concentration of excess vacancies. The retention of this enhanced growth for long periods of time suggests that vacancies can be trapped within the structure for long times despite their mobility. Morphological instabilities are found to develop during δ′ particle growth. These instabilities occur at very small particle sizes and cause the normally spherical particles to tend toward a dendritic morphology.