The　microscopic-deformation　mechanisms　of　an　extruded　magnesium　alloy　with　and　without　precipitates　[Guinier-Preston　(GP)　zones]　subjected　to　cyclic　deformation　were　investigated　by　in-situ　neutron-diffraction　(ND)　measurements　and　crystal-plasticity　modeling.　The　relationship　between　the　macroscopic-cyclic-deformation　behavior　and　the　microscopic　responses　(particularly　twinning　and　detwinning)　at　the　grain　level　was　established.　The　general　deformation-mechanism　evolution　in　the　solution-state　(ST)　sample　was　similar　to　that　in　the　peak-aged-state　(PA)　sample　over　fatigue　cycles.　Both　samples　plastically　deformed　by　extension　twinning　during　compression,　and　by　a　sequential　process　of　detwinning　and　dislocation　motion　under　reverse　tension.　The　main　difference　is　that　in　the　PA　sample,　the　presence　of　precipitating　particles　constrains　the　twinning/detwinning　behaviors,　which　leads　to　an　increase　in　the　participation　of　dislocation　slip　in　the　plastic　deformation　and　then　induces　a　strengthening　effect　during　cyclic　loading.　Based　on　the　combination　of　the　previous　in-situ　ND　results　and　crystal-plasticity　model,　our　work　provides　a　comprehensive　analysis　of　the　interaction　between　the　precipitation　strengthening　and　twinning/detwinning　mechanism　under　the　whole　multi-cycle　cyclic　loading　and　their　effect　on　the　macro-　and　micro-mechanical　behavior　of　the　precipitate-strengthened　magnesium　alloys.　©　2024