Constitutive Model for Anisotropic Creep Behaviors of Single-Crystal Ni-Base Superalloys in the Low-Temperature, High-Stress Regime

Abstract

A crystallographic constitutive model is developed to capture orientation-sensitive primary and secondary creep behaviors within approximately 20 deg from the [0 0 1] orientation in single-crystal superalloys for the low-temperature and high-stress regime. The crystal plasticity-based constitutive formulations phenomenologically incorporate experimentally observed dislocation micromechanisms. Specifically, the model numerically delineates the nucleation, propagation, and hardening of a$\langle 1 { 1 2} \rangle$ dislocations that shear multiple $\gamma^{\prime }$ precipitates by creating extended stacking faults. Detailed numerical descriptions involve slip-system kinematics from a/2$\langle 1 { 1 }0 \rangle$ dislocations shearing the $\gamma$-phase matrix, a$\langle 1 { 1 2} \rangle$ stacking fault dislocation ribbons shearing the $\gamma^{\prime }$-phase precipitate, interactions between a/2$\langle 1 { 1 }0 \rangle$ dislocations to nucleate a$\langle 1 1 2\rangle$ dislocations, and interactions between the two types of dislocations. The new constitutive model was implemented in the finite-element method (FEM) framework and used to predict primary and secondary creep of a single-crystal superalloy CMSX-4 in three selected orientations near the [0 0 1] at 1023 K (750 °C) and 750 MPa. Simulation results showed a reasonable, qualitative agreement with the experimental data. The simulation results also indicated that a/2$\langle 1 { 1 }0 \rangle$ matrix dislocations are important to limit the propagation of a$\langle 1 { 1 2} \rangle$ dislocations, which leads to the transition to secondary creep.