Microstructure and creep behaviour of alloy 800HT

The microstructure and creep behaviour of alloy 800HT (X10 NiCrAlTi 32 20 HT) was studied in the temperature range between 700 and 900 C under stresses ranging from 30 to 185 MPa after solution treatment and isothermal overageing. The creep behaviour, damage development and microstructure characteristics, such as particle size and dislocation density, were investigated and discussed with respect to the creep deformation. In order to obtain these informations, creep tests, metallographic investigations and electron microscopy methods (SEM, TEM, EDX) were used. It was found that specimens exhibited a longer creep life after solution treatment compared to those aged before testing. The difference in creep life and in the development of intercrystalline creep damage was found to increase at lower creep stresses at a given temperature. This phenomenon is attributed to the precipitation of fine carbides during the early stages of creep, which strengthen the material and reduce the creep rate. For longer creep or ageing times, the particle coarsening leads to an acceleration of the creep strain rate. The increasing creep damage was enhanced by the formation of grain-boundary carbides. A model was developed to describe the influence of carbide precipitation during creep on the behaviour of Alloy 800HT under different creep temperatures and stresses. Comparison with the experimental results showed that the model predicts the creep behaviour accurately in the range of stresses and temperatures investigated in the present study. (orig.)