The　aging　strengthening　behavior　and　coarsening　process　of　Al3Er　precipitates　were　investigated　in　AI-0.045Er　(in　at.%)　alloys　aged　isothermally　at　300-400　degrees　C,　using　Vickers　micro-hardness　measurement　and　transmission　electron　microscopy　(TEM)　observation.　The　isothermal　hardness　curves　showed　that　the　peak　hardness　decreased　slightly　with　the　increase　of　temperature.　From　the　TEM　observations,　the　radius　of　Al3Er　precipitates　at　peak　hardness　was　measured　to　be　2.4　+/-　0.4　nm.　The　coherency　of　Al3Er　precipitates　started　to　get　lost　at　the　radius　of　8.0-9.1　nm,　which　were　much　larger　than　the　predicted　value.　The　difference　was　speculated　to　be　due　the　reduction　of　misfit　between　Al3Er　precipitates　and　matrix　when　particles　were　very　small.　From　the　analysis　of　the　coarsening　of　coherent　Al3Er　particles　using　LSW　theory,　the　diffusion　activation　energy　and　the　diffusivity　pre-exponential　constant　of　Er　in　Al　were　deduced　to　be　77.2　+/-　5.3　kJ/mol　and　(4.3　+/-　2.2)　x　10(-12)　m(2)　S-1,　respectively.　Finally,　the　strength　increment　caused　by　Al3Er　particles　was　evaluated　using　precipitation　strengthening　theory.　It　was　found　that　when　the　particle　radius　was　larger　than　2.4　nm,　the　Orowan　bypass　mechanism　dominated.　In　radius　of　0-1.4　nm,　a　less　coherency　strengthening　contribution　indicated　a　reduction　of　lattice　misfit　between　particles　and　matrix.　From　the　strength　plateaus　in　radius　of　1.4-2.4　nm,　the　anti-phase　boundary　(APB)　energy　of　Al3Er　precipitates　was　deduced　to　be　0.60　+/-　0.03　J/m(2).　(C)　2014　Elsevier　B.V.　All　rights　reserved.