Tension/compression asymmetry of [001] single-crystal nickel-based superalloy DD6 during low cycle fatigue

In order to study the tension/compression asymmetry of (001) single-crystal nickel-based superalloy DD6 during low cycle fatigue, fully reversed cycle fatigue tests were conducted at 760 °C and 980 °C, with strain rates of 10−3 and 5×10−3 s−1, respectively. The total strain ranges were varied from ±0.5% to ±1.2%. The DD6 single crystals show that tensile stress (T) is higher than compressive stress (C) at the initial cycles in the above conditions. Under the strain rate 10−3 s−1at strain ranges ±0.7%/760 °C and ±0.55%/980 °C or under the strain rate 5×10−3 s−1at ±0.7%/980 °C, the tension-compression asymmetry of C>T was observed. According to the above experiments, an advanced rate-dependent constitutive model based on the crystal plasticity theory was proposed. The effects of elastic deformation (for T>C at the initial cycles of LCF), dislocation accumulation in the matrix channel (for kinematic hardening) and the shearing of precipitates by Superlattice Intrinsic Stacking Faults (SISF) (for kinematic softening) were taken into account. Numerical simulations were conducted using the proposed constitutive equations, such as monotonic tension, compression and cyclic deformation. The present model is shown to be successful in simulating the inelastic behavior of DD6 single crystals under low cycle fatigue.