The　promoting　effect　of　secondary　phase　particles　on　the　refinement　of　matrix　grains　in　a　series　of　Mg-Y1.5-Nd0.6-Zn.x　(x=0,　0.2,　0.4,　0.8,　13)　alloys　deformed　by　friction　stir　processing　(FSP)　was　investigated　in　this　study.　The　volume　fraction　of　secondary　phase　in　the　alloys　increased　with　increment　of　Zn,　and　most　of　the　secondary　phases　were　broken　into　nano-sized　particles　and　dispersed　in　the　matrix　during　thermal　mechanical　processing.　The　resulting　fine　particles　had　a　remarkable　promoting　effect　on　the　grain　refinement　of　the　Mg　matrix　during　mechanical　processing,　thermal　deformation,　and　cooling　with　increasing　volume　fraction　of　the　secondary　phase.　For　example,　the　grains　of　Mg-Y1.5-Nd0.6-Zn1.3　alloy　were　refined　to　an　average　size　of　28　nm,　compared　with　the　more　than　100　nm　grain　size　of　the　Mg-Y1.5-Nd0.6　alloy.　(C)　2015　Elsevier　B.V.　All　rights　reserved.