Microstructure and creep strength of different γ/γ′-strengthened Co-base superalloy variants

The discovery of the γ′-Co3(Al,W) phase with an L12 structure provided Co-base alloys with a new strengthening mechanism, enabling a new class of high-temperature material: Co-base superalloys. This review discusses the current understanding of the phase stability, deformation, and oxidation behaviors of γ′ single-phase and γ + γ′ two-phase alloys in comparison with Ni-base γ′-L12 phase and γ + γ′ superalloys. Relatively low stacking fault energies and phase stability of the γ′ phase compared with those in Ni-base alloys are responsible for the unique deformation behaviors observed in Co-base γ′ and γ + γ′ alloys. Controlling energies of planar defects, such as stacking faults and antiphase boundaries, by alloying is critical for alloy development. Experimental and density functional theory studies indicate that additions of Ta, Ti, Nb, Hf, and Ni are effective in simultaneously increasing the phase stability and stacking fault energy of γ′-Co3(Al,W), thus improving the high-temperature strength of Co-bas...

L12-Strengthened Cobalt-Base Superalloys

Elastic moduli and thermal expansion coefficients of medium-entropy subsystems of the CrMnFeCoNi high-entropy alloy

Effects of quaternary alloying elements on the γ′ solvus temperature of Co–Al–W based alloys with fcc/L12 two-phase microstructures

Partition behavior of alloying elements and phase transformation temperatures in Co–Al–W-base quaternary systems

Phase equilibria between the y and y' phases at 900°C in the Co-(10-70)Ni-Al-W system were determined by electron probe microanalysis (EPMA) and X-ray diffractometry (XRD). It was found that the "/ phase with L1 2 structure continuously exists from the Co side to the Ni side in Co-Ni-Al-W system and that it widens to the low W region with increasing Ni content. The partition of Al into the y' phase increased with Ni content, while the W changed from a y' former to a y former by increase of Ni content. Differential scanning calorimetry (DSC) measurements also revealed that the y' solvus temperature increases with Ni content, while the solidus temperature is hardly affected by such content. The lattice parameter of the y and y' phases and the mismatch decreased with increasing Ni content, which caused the morphologic change of the γ' precipitates from cubes to spheres.

Phase Equilibria and Microstructure on γ' Phase in Co-Ni-Al-W System

Ductility enhancement by boron addition in Co–Al–W high-temperature alloys

Phase equilibria and thermodynamic calculation of the Co–Ta binary system

The phase equilibria between the γ (A1) and liquid phases and those between the γ, e (A3), σ (D8b) and α (A2) phases at temperatures between 750 and 1300 °C in the Co-Cr-Ni system were determined by differential scanning calorimetry and electron probe microanalysis. It was found that the γ solidus and liquidus temperatures decrease with increasing Cr or Ni content and that the difference between them increases with increasing Cr content. The phase equilibria at 800, 900, 1000, 1100, 1200 and 1300 °C were determined using two- or three-phase alloys, and the phase boundaries due to the magnetically induced phase separation in the γ and e phases were also observed at 850, 800 and 750 °C by the diffusion couple method.

Kazuya Shinagawa

Papers

Phase Equilibria and Microstructure on γ' Phase in Co-Ni-Al-W System

Ductility enhancement by boron addition in Co–Al–W high-temperature alloys

Phase equilibria and thermodynamic calculation of the Co–Ta binary system

Experimental Determination of Phase Equilibria in the Co-Cr-Ni System

Phase equilibria and mechanical properties of the Ir–W–Al system