Constitutive modeling of the creep behavior of single crystal superalloys under non-isothermal conditions inducing phase transformations

The prediction of the viscoplastic behavior of Ni-based single crystal superalloy is still a challenging issue due to the non-isothermal loadings which can be encountered by aeronautical engines components such as high pressure turbine blades. Under particular in-service conditions, these materials may experience temperature cycles which promote the dissolution of the strengthening γ′ phase of the material on (over)heating, and subsequent precipitation on cooling, leading to a transient viscoplastic behavior.New internal variables representing the microstructural changes under those specific thermal loadings have been introduced in the framework of crystal plasticity using a macroscopic approach (no representation of the γ/γ′ microstructure of the alloy) to account for the transient creep behavior induced by microstructure changes. This modeling approach captures first order effects on the creep behavior due to (a) γ′ precipitates volume fraction evolution of each kind of particles of a bimodal distribution of precipitates (which evolves according to thermal history), (b) recovery of the dislocation density and, (c) material orientation.In addition, a damage law keeping in memory all the thermal history and recovery processes has been introduced to account for the unconventional post-overheating creep life.This model is calibrated on non-isothermal creep experiments on [0 0 1] oriented single crystals made of MC2 alloy. It is able to predict creep strain (primary, secondary, tertiary), whatever the temperature history of the material. In addition, it can be used to quantify the effect of slight variation of the as-received γ′ volume fraction on the creep behavior.