ThesecondaryhardeningreactionisaccompaniedwithprecipitationoffinecarbidesinhighCoNiultrahighstrengthsteel.Thecrystalstructureoftheprecipitatingcarbidesisunambiguouslydeterminedbymicrobeamdiffractionintransmissionelectronmicroscopy.Itisidentifiedthattheneedle-shapedcarbidesareM2Cwithahexagonalstructure.TheconcentrationofsubstitutionalalloyingelementsinthecarbidesquantifiedbyenergydispersiveX-rayspectroscopy(EDS)alsosupportstheresultabove.ThespatialstructureofM2CisidenticalwithL'3type.Metalatomsareinaclosepackedhexagonalstructure,thecarbonatomspartlydistributewithrandomintheoctahedralintersticesandthefillingprobabilityislessthan1/2.Particularattentionwaspaidtotherelationshipofneedle-shapedcarbides/ferritematrixatsecondaryhardeningpeaktemperedat482℃for5h.Observationbyhighresolutiontransmissionelectronmicroscopy(HRTEM)confirmsthatcarbideswithblack-whitecontrastarefullycoherentwithferriteandhaveinpidualcrystalstructure,andthecoarsenedcarbideswithmoirefringearepartiallycoherentwithmatrix.TheorientationrelationshipbetweenM2CandferritematrixisdirectlyobservedandidentifiedwiththewellestablishedP-Srelationship[001]α//[111]c.
