Creep behavior of a γ′-strengthened Co-base alloy with zero γ/γ′-lattice misfit at 800 °C, 196 MPa

Abstract

Deformation and structural behavior of an experimental c9-strengthened Co-base alloy during creep at 800°C and 196 MPa have been investigated. The characteristic features of this alloy are zero c/c9-lattice misfit and a fine c/c9-microstructure. In the initial condition, the c9-precipitates in this alloy are small (size of about 100 nm), have polyhedral morphology, and are separated by the very narrow c-channels (width of about 10 nm). The tests performed up to about 1% creep strain (about 500 h creep time) gave creep curves with a slow constant strain rate and without an apparent transient creep, typical for superalloys with nonzero misfit. In this initial stage of creep, entering of the narrow c-channels by dislocations is blocked by a strong Orowan force. The micromechanism of creep was identified as an octahedral glide of h011i superdislocations simultaneously in two phases, c and c9. The c/c9-microstructure with zero misfit shows no rafting but rapidly coarsens isotropically. It is concluded that zero misfit is beneficial at the initial stages of the creep but is unfavourable for longterm creep because of the continuous microstructural coarsening.Recently, Co-base alloys with a c/c9-microstructure similar to that of Ni-base superalloys were discovered. 1 In these Co-base alloys, a face-centred cubic (fcc) solid solution of Co, c, is strengthened by precipitation of a stable ternary Co 3 (Al,W) intermetallic compound, c9, with the ordered structure L1 2 (Cu 3 Au type). It is assumed that these new c9-strengthened Co-base alloys can substitute the Ni-base superalloys in some important applications. The reasons for this assumption are:(i) The discovered Co-base alloys have higher solidus and liquidus temperatures compared with those of Nibase superalloys, which makes them potentially attractive for high-temperature applications.(ii) These Co-base alloys have good castability due to the narrow solidification interval and a low degree of segregation during solidification. This is important for manufacturing of the large single-crystal blades needed for power gas turbines.(iii) These Co-base alloys are c single phase in a wide temperature interval, and the material is then soft and ductile. This could permit high-temperature processing, e.g., forging or rolling, assuming that these alloys could be developed as wrought or sheet materials.Research activities on these new Co-based alloys include: alloy development, 1-7 mechanical testing, 2-8 testing of structural stability 9,10 and oxidation, 2,11 and investigations of deformation micromechanisms. 3,[12][13][14][15] At the current stage of alloy development, several Co-base alloys have been proposed, which have creep strengths at temperatures 800-900°C approaching those of Ni-base superalloys, e.g., Refs. 3-5.