Hardening mechanisms in a dynamic strain aging alloy, HASTELLOY X, during isothermal and thermomechanical cyclic deformation

Abstract

Isothermal cyclic deformation tests were conducted on HASTELLOY X with a total strain range of ±0.3 pct at several temperatures and strain rates. Cyclic hardening exhibited a broad peak between about 200 °C and 700 °C, with a maximum near 500 °C of about 80 pct increase in stress amplitude, Δσ/2, at failure. The present work examines the mechanisms contributing to this marked cyclic hardening. Cr₂₃C₆ precipitation on dislocations contributed to hardening, but only with sufficient time above about 500 °C. The substantial hardening rate at lower temperatures or shorter times was attributed to solute drag. The contribution of solute drag was evidenced in tests at both 400 °C and 600 °C by a continually decreasing strain rate sensitivity of the AcrJ2. Solute drag alone produced very considerable cyclic hardening. The increase in AcrJ2 after 1000 cycles at 427 °C was 75 pct of the maximum observed at higher temperatures where carbides did precipitate. Additionally, thermomechanical tests were conducted between various temperature limits, but with the same ±0.3 pct mechanical strain range. Hardening was bounded by isothermal behavior at the temperature limits of the thermomechanical cycles, except for tests between 400 °C and 600 °C which exhibited extreme hardening. However, microstructural examination did not suggest a cause. Specimens subjected to thermomechanical cycles appeared similar to those isothermally cycled at the maximum temperature of the thermomechanical cycle, including those from the 400 °C to 600 °C tests.