Miniature specimen assessment of creep of the single-crystal superalloy LEK 94 in the 1000°C temperature range

The creep behavior of the Ni-base superalloy LEK 94 in the 1000 degrees C range has been investigated. Miniature tensile specimens were employed, which can be taken out of a thin walled blade or used when only limited amounts of material are available. We show that this miniature test technique yields reasonable creep data, including stress exponents n and apparent activation energies Q(app). The n values are close to 5 and the Q(app) values are close to 500 kJ/mole, independent of the crystallographic direction. Creep rates are fastest for samples oriented in the [110] direction, followed by [001] samples. The lowest creep rates are observed for the [111] direction. This difference is related to a difference in resolved shear stresses and in the number of activated crystallographic slip systems. As a striking new result, it was observed that the onset of tertiary creep occurs earlier and rupture strains are significantly smaller in the case of [110] specimens as compared to [100] specimens, while the [111] specimens show an intermediate behavior. These effects are related to the alignment of cast porosity along the axis of primary dendrites. When cast micropores align perpendicular to the axis of the applied tensile stress, the onset of fast tertiary creep occurs early and small rupture strains are observed.