Microstructure dependence of stress relaxation behavior of powder-processed Ni‐base superalloys

Abstract

The stress relaxation behavior at 700 °C of two experimental and one commercial powder-possessed Ni-base superalloys possessing different microstructural characteristics were studied following the application of two different strain levels (0.6% and 2%) in order to investigate how phosphorus additions, secondary γ′ microstructure and tertiary γ′ microstructure affect the stress relaxation behavior, respectively. Analysis of the characteristic stress relaxation behavior using the natural logarithm of plastic strain rate ln(ε˙) vs. stress σ curves, revealed that the compositional and microstructural variations impact the stress relaxation behavior differently following deformation to a total strain of 0.6%. No changes in the stress relaxation behavior or γ′ microstructure was observed in nominally identical alloys RRHT5P1 and RRHT5P2 that contained 0.013 wt % P and 0.041 wt % P, respectively. Changing the cooling rate from the solution heat treatment temperature for these alloys from 1 °C/s to 2 °C/s, however, did lead to modest changes in both the γ′ microstructure and the stress relaxation behavior. Significant variations in the stress relaxation behavior were quantified as a function of the tertiary γ′ microstructure in a commercial Ni-base superalloy, RR1000. Reducing the amount of tertiary γ′ precipitates by either a prolonged aging process or multi-step aging process contributed to a greater stress drop during stress relaxation, regardless of the variations in the secondary γ′ microstructure. Following the application of a total strain of 2%, the stress relaxation process for all the samples was observed to be insensitive to compositional and microstructural variations.