Comparation in microstructure evolutions between casted and sintered Co-Cr-Ni alloys
Cobalt-chromium-nickel (Co-Cr-Ni) alloys were prepared by two techniques of the sub-rapid solidification (SRS) and vacuum sintering (VS).The effects of the preparation processing and the content of Cr,Ni on the microstructure evolution were investigated in detail.Results show samples under SRS present a typical dual-phase dendrite microstructure of the face centered cubic (γ-fcc) crystal and the hexagonal closest packing (ε-hcp) crystal,which is independent of the raw material composition.The γ-fcc phase forms in dendrite arms,while the ε-hcp phase forms in inter-dendrites.With the increase of the solidification rate,the secondary dendrite spacing decreases.The main solute elements Cr and Ni take segregation between the two phases of γ-fcc and ε-hcp.Cr concentrates in inter-dendrites,and Ni exists in dendrite arms.The microstructure of cobalt chromium nickel alloy prepared by VS is determined by its composition,and the sintered body of low nickel content alloy is single-phase ε-hcp structure,the grains exhibit a sharp coarsening characteristic with the increase of sintering temperature.The sintered body of high nickel content alloy is γ-fcc+ε-hcp dual phase structure,the grains are small and γ-fcc phase exhibits obvious twinning and stacking fault structures.Compared with the casted alloy,the sintered alloy exhibits a marked improvement in the uniform plastic elongation.The dislocation defect is the prime microstructural unit that promotes the improvement in the mechanical property of the sintered alloy with the single-phase structure of ε-hcp,while the defects of the crystal twins and stacking faults are considered as the key factors determining both the strength and toughness for those with the dual-phase structure of ε-hcp and γ-fcc.
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