简介：Surfacenanocrystallization(SNC)hasprovedtobeaneffectiveapproachtoimprovetheoverallpropertiesofbulkmetallicmaterials.Recently,anewsurfacenanocrystallizationtechnique,i.e.,surfacemechanicalgrindingtreatment(SMGT),wasdeveloped.Inthiswork,agradientnano-microstructurewasachievedinthesurfacelayeroftheAISI52100steelbyusingSMGT.Weobtainedaminimumgrainsizeofabout7nminthetopsurfacelayer.Thetotalthicknessofthedeformedlayerisover200micrometer.Meanwhilethesurfaceroughnessisratherlow.Ferritegrainsweredeformedtodifferentextentsvaryingwithdepthfromthetopsurface.Gradientgrainsizeswereformedfromtopsurfacetodeepmatrixwhichofferedagreatopportunitytostudytherefinementprocessoftheferritegrains.Itisfoundthatdislocationactivitiesplayadominantroleintheprocess.Attheinitiatestage,dislocationsaccumulatedandinteractedtoformdensedislocationwallsandcells.Increasingstrainandstrainrateinducedmoredislocationwallsincells,formingfinercells.Thisprocedurecontinueduntilnanograinsformedatthetopmostsurface.Theexistenceofcementiteparticlesinferritematrixgreatlyfacilitatestheferriterefinementprocess.Boundariesbetweenferritesandcementitesofferedmanydislocationsourceswhichacceleratethepropagationofdislocations.Dislocationwallswereblockedbycementiteswhichcertainlyleadtofinerdislocationcells.Theexistenceofcementitesmakesiteasiertogeneratefreshdislocationwallsinsub-microngrains.Astraingradientwasformedfromacementiteparticletosurroundingferritegrains.Thisstraingradientgivesrisetomoregeometricnecessarydislocations.Asferritegrainsizedecreasedlessthanthatofcementiteparticles,fragmentationoccurredincementites.Hardsecondphasewasusuallyconsideredasbrittle.Inthiswork,evidencesofdeformation(tracesofdislocationactivities)incementitesweredistinct.Sincethestressconcentrationinthephaseboundary(espec