L1₂-Strengthened Cobalt-Base Superalloys

Abstract

The discovery of the γ -Co 3 (Al,W) phase with an L1 2 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 -L1 2 phase and γ + γ superalloys. Relatively low stacking fault energies and phase stability of the γ phase compared with those in Nibase 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 -Co 3 (Al,W), thus improving the high-temperature strength of Co-base γ phase and γ + γ two-phase superalloys.8.1

Effect of doping Mg on the microstructure and mechanical behaviours of a novel Co-9Al-4.5W-4.5Mo-2Ta-0.02B alloy at room- and high-temperatures

“…This mechanism is the same as that in the g 0 -Ni 3 Al compound [20,21]. TEM analysis confirmed that slip of ordinary a/2〈1 0 1〉{1 1 1} dislocations through the g-Co SS matrix, rafting of the g 0 precipitates perpendicular to the external compressive stress axis [22] and glide of the superlattice intrinsic and extrinsic stacking faults in the g 0 precipitates [23,24] are the main creep mechanisms of the Co-Al-W base alloys.…”

Predicting the morphologies of γʹ precipitates in cobalt-based superalloys

“…Coarsening, creep, and rafting behaviors are different in cobalt-based γ/γ superalloys than in nickel-based superalloys: both creep and coarsening rates and precipitate shape and alignment under applied stress are different. These differences arise from a complex interplay of the variations in interfacial energies, misfit strains, and elastic stiffnesses from system to system [6][7][8]. Because these processes occur over long time scales, an understanding of the thermodynamic driving force affecting microstructural evolution should accelerate alloy development.…”

Effects of Nb and W Additions on the Microstructures and Mechanical Properties of Novel γ/γ’ Co-V-Ti-Based Superalloys

“…The ordered γ' (L1 2 )-precipitates phase was reported in Cobalt-based alloys for the first time in 1971 [9]. Subsequently, Sato et al reported new γ' phase Co 3 (Al, W) in 2006 [7], which has accelerated the researchs of Cobalt-based superalloys and renewed the worldwide scientific interest [10][11][12][13]. Currently, several γ' (L1 2 )-precipitates in Cobalt-based alloys, such as tungsten containing Co 3 (Ge, W) [14] and Co 3 (Ga, W) [15], have been discovered and effects of alloying elements B, V, Ti, Cr, Mn, Fe, Ni, Nb, Mo, Ta, Re and Hf in novel Cobalt-based superalloys have also been investigated [7,[16][17][18][19][20][21][22][23].…”