Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

Highlights

• IP and OP TMF test with optional dwell times were conducted on a DS Ni-base alloy.

• 20 min dwell times reduce both IP and OP TMF lives by more than 80%.

• Dwells in OP tests lead to softening of the material due to extensive microtwinning.

• Dwells in IP tests give rise to significant internal cavity damage.

• Forces exerted by the extensometer may induce transversal creep during the dwells.

Abstract

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T = 100–750 °C and optional dwells of 20 min at 750 °C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {1 1 1}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.

Introduction

During service of gas turbine engines, the turbine blades are subject to various loading conditions. Temperature gradients during repeated start-up and shut down operations may induce thermomechanical fatigue (TMF) damage. Depending on the phase angle between mechanical loading and temperature, the total TMF damage consists of varying portions of fatigue, creep and environmental damage [1], [2], [3]. During steady state service, the exposure to high temperatures under load may degrade the material due to creep, oxidation and microstructural changes [4], [5], [6], [7], [8], [9]. The interaction of temperature changes and applied cyclic strain during TMF and the degradation processes during steady state service often determines the service lifetime of the blades. In order to provide reliable lifetime predictions, a detailed knowledge about the material behavior under service conditions is necessary. In land-based gas turbines, the blades of the first and second stage are commonly made of directionally solidified (DS) Ni-base superalloys that contain a minimized amount of grain boundaries in the blade axis direction. That way, the creep resistance in this direction is improved and intergranular creep cavitation as well as preferential grain boundary oxidation is mostly suppressed. However, transgranular creep damage in a DS Ni-base superalloy was reported even at the relatively low temperature of 760 °C [10]. γ′-strengthened DS and single crystal Ni-base alloys may exhibit anisotropic deformation behavior dependent on their orientation, temperature and strain rate [11], [12], [13]. Reasons for this are the orientation and temperature dependent tension–compression asymmetry of the γ′-phase [14], [15] as well as time and temperature dependent dislocation reactions at γ–γ′-interfaces [13]. More recently, microtwin deformation at intermediate temperatures was observed which may also result in anisotropic deformation behavior [16]. Dwell times representing steady state service typically reduce the lifetime of Ni-base superalloys in TMF or isothermal fatigue tests [17], [18], [19]; however positive effects have also been reported [20], [21]. Because of the high costs of DS Ni-base alloys, laboratory tests are rare and only few literature data about their behavior under near service conditions are available. The objective of this study is to provide a better understanding about how dwell times influence the deformation, damage and lifetime behavior of Alloy 247 LC DS under TMF loading. Tests were conducted in laboratories of Karlsruhe Institute of Technology (KIT) and of Institute of Physics of Materials (IPM) in Brno.