Developing　ultra-high　strength　in　rare-earth-free　Mg　alloys　using　conventional　extrusion　process　is　a　great　challenge.　What　is　even　more　difficult　is　to　achieve　such　a　goal　at　a　lower　processing　cost.　In　this　work,　we　report　a　novel　low-alloyed　Mg-2Sn-2Ca　alloy　(in　wt.　%)　that　exhibits　tunable　ultra-high　tensile　yield　strength　(360-440　MPa)　depending　on　extrusion　parameters.　More　importantly,　there　is　little　drop　in　mechanical　properties　of　this　alloy　even　when　it　is　extruded　at　a　speed　several　times　higher　than　those　used　in　the　reported　high　strength　Mg　alloys.　Examination　of　as-extruded　microstructures　of　this　Ca-containing　Mg　alloy　reveals　that　the　ultra-high　strength　is　mainly　associated　with　the　presence　of　surprisingly　submicron　matrix　grains　(down　to　similar　to　0.32　mu　m).　The　results　suggest　that　the　Ca　addition　promotes　accumulations　of　the　pyramidal　dislocations,　which　eventually　transform　into　the　low　angular　grain　boundaries　(LAGBs).　The　high　number　density　of　LAGBs　separate　the　alpha-Mg　matrix　via　either　discontinuous　dynamic　recrystallization　(DDRX)　mechanism　in　the　early　stage　or　the　continuous　dynamic　recrystallization　(CDRX)　mechanism　in　the　later　stage　of　extrusion,　which　effectively　enhances　the　nucleation　rates　of　the　DRXed　grains.　More　importantly,　large　amount　of　Ca　segregation　along　LAGBs,　accompanied　with　dynamically　precipitated　Mg2Ca　nano-phases,　are　detected　in　the　present　non-severely　deformed　samples.　It　is　the　combination　of　solute　segregations　and　numerous　Mg2Ca　nano-precipitates　that　contributes　to　the　formation　of　the　ultra-fine　grains　via　pinning　mechanism.　The　ultrafine　grains　size,　Ca　enrichment　in　most　LAGBs,　and　residual　Mg2Ca　nano-precipitates　would　in　turn　contribute　significantly　to　the　enhancement　of　the　yield　strength　of　the　as-extruded　Mg-2Sn-2Ca　(wt.%)　alloy.　The　low　content　of　alloying　elements　and　the　fast　one-step　extrusion　process　render　the　present　alloys　low-cost　and　thus　have　great　potential　in　large-scale　industry　applications.　(C)　2018　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.