Comparation in microstructure evolutions between casted and sintered Co-Cr-Ni alloys

Abstract

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.

Keywords

• co-cr-ni alloy
• powder metallurgy
• vacuum sintering
• sub-rapid solidification
• microstructure evolution