Creep transients in the deformation of anisotropic nickel-base alloys

The transient creep behaviour of two anisotropic nickel-base alloys (viz directionally solidified IN738LC and the in situ composite γ-γ-Cr₃C₂) has been examined following a wide range of stress changes. The response following an unloading loading sequence depends critically on the magnitude of the stress change and on the time spent on the reduced load. When a proportion of the stress is removed and immediately replaced delay periods of zero creep. Δt′, before creep recommences are obtained in both alloys although the magnitudes of Δt′ are 3–100 times greater for the composite than for the superalloy. When the reduced stress is maintained for long periods (⪆ 10 h) conventional primary creep, rather than delay periods are observed. A model is proposed that accounts for the transient strains in terms of the stress redistributions between fibre and matrix in the composite and across the dendritic structure in the superalloy. The implications of the results to the cyclic creep behaviour of these materials are discussed.