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

The influence of the gamma' precipitate morphology on the creep properties of [001]-orientated specimens of a nickel-base single crystal superalloy has been investigated. Specimens with two different gamma'microstructures, namely as-heat treated crystals with cuboidal gamma' particles and crystals given a compression pre-treatment to form a raft gamma' microstructure orientated parallel to the stress axis, were crept at 800 and 1000 degrees C. The analysis of the experiments show that, at 800 degrees C and 600 MPa, the alloy with cuboidal gamma' precipitates exhibited both lower creep rates and longer rupture lives compared to those with rafted gamma'. TEM examination showed that, in addition to {111} (110) -slip system operated in gamma matrix, the gamma' precipitates are cut by superlattice dislocations and stacking fault. It indicated that dislocations could shear the semi-coherent gamma' rafts relatively easily compared to the coherent cuboidal gamma'. At 1000 degrees C and 200MPa, the alloy with raft gamma' 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 gamma/gamma' 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. (c) 2006 Elsevier B.V. All rights reserved.