Tension/compression asymmetry of a single-crystal superalloy in virgin and degraded condition

The mechanical behaviour at room temperature of a single-crystal superalloy exposed to long-term ageing at elevated temperature has been investigated, a topic important for the material's resistance to thermal mechanical fatigue. Specimens with several different crystallographic orientations were plastically deformed in either tension or compression before and after the long-term furnace exposure. While the thermally activated degradation of the microstructure causes a reduction in yield limit of up to 25% for specimens initially deformed in the [0 0 1] and [0 1 1] directions, none or only moderate reduction was seen for specimens initially deformed along the [(1) over bar 1 1] direction. This can be explained by the strong correlation between yield limit reduction and the amount of gamma' coarsening. By introducing an isotropic degradation function in a newly developed crystal plasticity model, the constitutive behaviour of both virgin and degraded material has been described with good agreement with the experimental results. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.