Characterization of Rafting and its Influence on TMF

In thermomechanical fatigue (TMF) testing and life prediction of Ni-base superalloys used in gas turbine engines, the evolution of the two phase γ-γ’ microstructure is often neglected.  During service, the turbine components form gradients of aged microstructures that are dependent on the localized temperature and stress state.  In the case of Ni-base superalloys, the aged microstructures are of the form of coarsened γ’ precipitates with morphological changes called rafting.  These changes in microstructure have been shown to influence both the stress-strain and TMF behavior.

Prior work has attempted to address whether γ-γ’ rafting is damaging or in some special cases advantageous, and also to see if this behavior can be avoided through further optimization of the initial microstructure constituents.  While most of these attempts have been unsuccessful, it is now commonly understood that rafting can lead to creep acceleration and a reduction in creep strength, although these are not mutually exclusive, and is therefore important to include in a holistic life prediction methodology.

This study is aimed at developing structure-property relationships based on rafting experiments.  One of the end goals is to develop a microstructure-sensitive constitutive model that captures how changes in microstructure from coarsening and rafting influence the mechanical behavior, including the temperature dependence on the yield and hardening response as well as TMF, so that these influences can be readily predicted in the design and life analysis of hot section components.  This paper focuses on the experimental characterization and observations in a particular directionally-solidified (DS) Ni-base superalloy.

Special axial fatigue specimens were designed to facilitate pre-rafting through placement in a creep frame.  Compared to virgin samples, specimens whose γ-γ’ microstructure had been rafted normal to the stress axis under relative short-term creep conditions exhibited an increased yield strength by a factor of 1.2 at temperatures below 700°C and a reduced strength above.  The opposite effect was observed for rafts lying parallel to the stress axis.  When the tensile rafting conditions are more severe, for example a 300hr creep exposure, the yield strength is reduced.