Modeling of creep and stress relaxation of the nickel-base alloy NiCr20TiAl at isothermal and non-isothermal loading conditions

The design and dimensioning of new as well as the assessment of operating high-temperature components in service require a precise prediction of creep and stress relaxation. The increasing share of renewable energies forces fossil-fired power plants for increasing numbers of start-ups and shut-downs. Consequently, transient loading conditions need to be taken into account. In order to meet this demand, non-isothermal creep equations are necessary, which enables a consistent prediction of creep strain and stress relaxation in a wide range of temperatures and stresses. In this paper, an approach for the visco-plastic modeling of creep and stress relaxation for non-isothermal loading conditions is presented. The strain portions creep, negative creep and initial plasticity, occurring at elevated temperatures are described by temperature-dependent phenomenological equations. Within this paper, the adjustment of the parameters is based on a wide database of hot tensile tests, creep and annealing experiments. The nickel-base alloy NiCr20TiAl has been examined in a temperature range from 450 degrees C to 650 degrees C. The developed material models have been successfully validated with isothermal and non-isothermal relaxation experiments. Further, the recalculation of a staged relaxation test demonstrates the capability of the defined material laws in a wide stress range under isothermal and non-isothermal loading conditions.