On Localized Deformation and Recrystallization as Damage Mechanisms during Thermomechanical Fatigue of Single Crystal Nickel-Based Superalloys

Abstract

Abstract. Thermomechanical fatigue (TMF) in superalloys is growing in importance due to the introduction of advanced cooling systems but also due to the changes in demand and competition within the power generation market; this is requiring many power plants to operate under cyclic conditions. In this paper the TMF behaviour of three different single crystal nickel-based superalloys are compared. It is demonstrated that the deformation and damage mechanisms occurring during TMF are rather different from those traditionally reported for creep or isothermal fatigue. In all cases examined, the deformation is localized within a rather small number of deformation bands. While these bands were found to consist mainly of micro-twins in some alloys, in others they might be better described as slip or shear bands. Furthermore, in some circumstances these bands are prone to recrystallization. In CMSX-4, the intersection points of twins of different orientation act as initiation sites for this process. In the SCA425 alloy -of smaller gamma' content, lower creep resistance and less great oxidation resistance -twinning is observed infrequently; however the deformation is still very localized and in the distorted gamma-gamma' microstructure, along the shear bands, recrystallization is observed. Furthermore the recrystallization is enhanced by oxidation due to the development of a gamma'-depleted zone. In CMSX-4, TCP phases precipitated during long term ageing cause a more dispersed deformation behaviour which prevents recrystallization. Our findings confirm the importance of an inhomogeneous microstructure for good TMF resistance. IntroductionThe aim of the present paper is to summarize our recent efforts to characterize the deformation and damage mechanisms active during thermomechanical fatigue of single crystal Ni-base superalloys. Broadly speaking, thermomechanical fatigue (TMF) failure is promoted when plastic strains cannot be accommodated at low temperatures and creep deformation and/or oxidation occurs at high temperatures. Microstructural stability plays an important role during TMF and it is now established that there are important interactions between the degradation mechanisms occurring at high and low temperatures [1]. Therefore, the deformation and damage mechanisms in single crystal superalloys during TMF are very different from those traditionally reported to occur under creep or isothermal fatigue. Unfortunately, at this stage relatively little is known about the influence of alloy composition on the mechanisms active during thermomechanical fatigue of single crystal superalloys. In this study, the TMF performance of different alloys has been compared in order to reveal new insights into this topic.

A New Single Crystal Superalloy for Power Generation Applications

“…microstructure developed during primary ageing has been studied, since these need to be chosen carefully for optimised mechanical properties. The mechanical properties of several different microstructures have been verified by creep and fatigue testing [12,14]. Based on these observations the following heat treatment conditions are proposed for STAL-15:…”

The Role of Cavitation in Creep-Fatigue Interaction

“…For simulating the role of straining during start-ups and shut-downs, thermal mechanical fatigue (TMF) is often used where both the loading and temperature are varied during the cycle. It is important that the minimum temperature in the cycle is low, since a significant part of the damage is generated at low temperatures [4]. LCF and TMF are commonly performed under strain control.…”

Microstructural aspects of fatigue in Ni-base superalloys

“…-Recrystallization, twinning slip bands and oxidation [86]. In fact, these authors state an essential feature of TMF [86, p. deformation is localized within a rather small number of deformation bands.…”