Achieving lower mass density with high strength in Nb stabilised γ/γ′ Co–Al–Mo–Nb base superalloy by the replacement of Mo with V

Abstract

We report new compositions of γ/γ′ Co base super alloys with low mass density where the strengthening γ′ precipitates Co3(Al0.45V0.33Nb0.21) are stabilised with a systematic replacement of Mo with V in Co–10Al–5Mo–2Nb (all in at.%) superalloy. The γ/γʹ microstructure is found to be metastable in nature that decomposes into equilibrium phases after 200 h of aging at 800 °C. Diffraction and atomic-scale compositional analysis reveal that phases are a γ-Co solid solution with composition Co-6.9Al–5V-1.4Nb, a B2 ordered Co(Al,V) phase with composition Co-34.6Al-5.6V and a DO19 ordered Co3Nb phase with composition Co-23.4Nb-1.6Al. Addition of 30 at.% Ni to the Co–10Al–5V–2Nb superalloy increases the γ′-solvus temperature to 1017 °C with improved high temperature strength and stability, concomitantly achieving a yield strength of 701 ± 9 MPa at 770 °C. The γ/γʹ microstructure of the alloy is found to be stable at 800 °C and 900 °C till 1000 h. Vanadium addition, therefore, have marked beneficial effect in cobalt base superalloys indicating good potential for future alloy development.