The research on scandium additions in aluminium alloys has gained some interests over the last decades, due to the precipitation of Al₃Sc phase particles, which have the beneficial effect on mechanical properties, weldability and corrosion resistance. But Sc is a very expensive strategic
metal. Therefore, a breakthrough of the selection of RE is expected. However, only a limited number of possibilities exist in Al–based alloys for the formation of coherent strengthening particles that are thermodynamically stable and have the ordered, L1₂ structure. It involves the addition of some suitable alloying elements such as La, Ce, Y, Sc and especially to Sc, while the effects of other single elements are scarcely referred. Recent studies have considered Er as a promising alloying element in Al–alloy, and it has been shown that a small Er addition can improve the material properties in several Al–alloy systems. Most of the beneficial effects are attributed to the formation of the Al₃Er phase in these alloys.
However, few research have been done on the effect of homogenizing treatment on the precipitation of Er modified 5083 alloy. Although the Al₃Sc and Al₃(Sc,Zr) particles are wildly studied, but few studies about coherency and coarsening behaviors of Al₃Er have been reported. The present investigation provides some information about orientation relationships of the precipitates with matrix and the coherency behavior of Al₃Er phase.
Al₃Er phase precipitates durinhomogenizing of the Er modified 5083 alloy produced by direct chill casting were analyzed by transmission electron microscopy (TEM).The results show that many particles with different shapes and sizes precipitate during homogenizing. Spheric precipitates with the size between 10—50 nm is Al₃Er phase which are good coherent/semi–coherent with matrix; the orientation relationships between matrix and the spheric precipitates are derived:[111]_m//[111]_sp, (110)_m//(110)_sp, (101)_m//(101)_sp; [011]_m//[011]_s, (100)_m//(200)_sp, where the indexs m and sp indicate matrix and spheric precipitate, respectivety.

The radius for coherent/semi-conherent transition of Al₃Er particle is determinined as 12---20 nm by TEM analysis; The calculated value of critical transition radius (r_t=9.44 nm) of the Al₃Er precipitates is correspondent with the experiments.