Influence of precipitate morphology on tensile creep of a single crystal nickel-base superalloy

The influence of the γ′ precipitate morphology on the creep properties of [0 0 1]-orientated specimens of a nickel-base single crystal superalloy has been investigated. Specimens with two different γ′ microstructures, namely as-heat treated crystals with cuboidal γ′ particles and crystals given a compression pre-treatment to form a raft γ′ microstructure orientated parallel to the stress axis, were crept at 800 and 1000 °C. The analysis of the experiments show that, at 800 °C and 600 MPa, the alloy with cuboidal γ′ precipitates exhibited both lower creep rates and longer rupture lives compared to those with rafted γ′. TEM examination showed that, in addition to {1 1 1} 〈1 1 0〉-slip system operated in γ matrix, the γ′ precipitates are cut by superlattice dislocations and stacking fault. It indicated that dislocations could shear the semi-coherent γ′ rafts relatively easily compared to the coherent cuboidal γ′. At 1000 °C and 200 MPa, the alloy with raft γ′ microstructure exhibited an extremely low minimum creep rate and longer rupture life. The dislocations movement by the combination of climbing and gliding processes and the glide–climb along γ/γ′ vertical interfaces are impeded most strongly. This indicates that the rafted structure is beneficial to improving creep resistance of the alloy under the test condition.