The contrasting roles of creep and stress relaxation in the time-dependent deformation during in-situ cooling of a nickel-base single crystal superalloy

Chinnapat Panwisawas,David M Collins,Neil D’Souza,Ayan Bhowmik

doi:10.1038/s41598-017-10091-w

Abstract

Time dependent plastic deformation in a single crystal nickel-base superalloy during cooling from casting relevant temperatures has been studied using a combination of in-situ neutron diffraction, transmission electron microscopy and modelling. Visco-plastic deformation during cooling was found to be dependent on the stress and constraints imposed to component contraction during cooling, which mechanistically comprises creep and stress relaxation. Creep results in progressive work hardening with dislocations shearing the γ′ precipitates, a high dislocation density in the γ channels and near the γ/γ′ interface and precipitate shearing. When macroscopic contraction is restricted, relaxation dominates. This leads to work softening from a decreased dislocation density and the presence of long segment stacking faults in γ phase. Changes in lattice strains occur to a similar magnitude in both the γ and γ′ phases during stress relaxation, while in creep there is no clear monotonic trend in lattice strain in the γ phase, but only a marginal increase in the γ′ precipitates. Using a visco-plastic law derived from in-situ experiments, the experimentally measured and calculated stresses during cooling show a good agreement when creep predominates. However, when stress relaxation dominates accounting for the decrease in dislocation density during cooling is essential.

Highlights

The micromechanical deformation of single crystal nickel-base superalloys during cooling after solidification is not well understood
Transmission electron microscopy (TEM) is one such method, observations are typically post-deformation and do not reveal the temporal evolution of deformation or kinetic mechanisms, where the latter constitutes the critical basis for any analysis
If TEM is used in conjunction with a technique that permits time resolved measurements, such as neutron diffraction, this complimentary approach could offer sufficient information to elucidate the governing micro-mechanisms that may lead to recrystallisation

Summary

Introduction

The micromechanical deformation of single crystal nickel-base superalloys during cooling after solidification is not well understood. If TEM is used in conjunction with a technique that permits time resolved measurements, such as neutron diffraction, this complimentary approach could offer sufficient information to elucidate the governing micro-mechanisms that may lead to recrystallisation This approach provides accurate data for numerical model validation used for the quantification of stresses and strains during casting. A further aim of this study is to calculate the macroscopic stresses and strains that develop in an incremental temperature interval during cooling via a numerical modelling approach To this end an expression for the inelastic strain is required, which is either isotropic hardening (i.e. without invoking any specific slip system) or a visco-plastic strain rate, when rate-dependent deformation occurs. It should be emphasised that this aspect is critical for establishing a reliable materials database in terms of plastic strain and its correspondence to dislocation density[20]

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Conclusion