Effect of Ti/Nb/Ta addition on the γ/γ' coherent microstructure in low-density and high-strength Co-Al-W-Mo-based superalloys

Coherent precipitation of cuboidal gamma'-Co3(Al,W) nanoparticles in face-centered-cubic (FCC)-gamma matrix is of great significance for improving high-temperature mechanical properties of Co-based superalloys. The present work developed a series of low-density Co-based superalloys in light of the cluster composition formula of [Al1-(Co,Ni)12]((Al0.5(Ti/Nb/Ta)0.5W0.5)(Mo0.5Cr0.5Co0.5)), where the addition of Ti, Nb, and Ta is mixed with an equimolar ratio. It is found that these designed alloys with different combinations of Ti/Nb/Ta, Ti/Nb, and Ti/Ta possess the coherent microstructure of cuboidal gamma' nanoprecipitates in the FCC-gamma matrix. The microstructural evolution of coherent gamma/gamma' during aging at 1173 K shows that these superalloys exhibit higher thermal stability at high temperatures. Even after aging for 10 0 0 h, there do not exist any other precipitated phases on grain boundaries, except the coarse gamma' precipitates. Also, the coarsening rate constants of cuboidal gamma' nanoprecipitates in these alloys are very low ( K = 5.76-6.03 nm3/s), which is mainly ascribed to a moderate lattice misfit ( e = 0.28 %-0.45 %) between gamma and gamma'. The stable gamma/gamma' microstructure renders the alloys with prominent mechanical properties, as evidenced by the high yield strength of crYS = 438-445 MPa at 1143 K. A large amount of stacking faults appear after compressive deformation and Lomer-Contrell dislocation locks are also formed due to the reaction of partial dislocations of stacking faults. Moreover, the microhardness (285-320 HV) in each alloy keeps almost constant with the aging time. Besides, these superalloys have a relatively lower density (8.67- 8.89 g/cm3), among which the alloy containing Ti0.25Ta0.25 also exhibits a much higher gamma' solvus temperature (1361 +/- 2 K) than those of the existing Co-Al-W-based superalloys. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.