Analysis of the Effective and Internal Cyclic Stress Components in the Inconel Superalloy Fatigued at Elevated Temperature

Miroslav Šmíd,Martin Petrenec,Jaroslav Polák,Karel Obrtlík,Alice Chlupová

doi:10.4028/www.scientific.net/amr.278.393

Abstract

Cyclic multiple step test in strain control have been performed on cylindrical specimens of cast polycrystalline Inconel 738LC and 792-5A superalloys at 800 °C in laboratory atmosphere. Hysteresis loops were analyzed according to the statistical theory of hysteresis loop. The effective and internal stress components were evaluated. The effective stress of γ´ precipitate has significant influence on the stress-strain response both materials. The stress amplitude in IN 792-5A is higher than in IN 738LC at approximately same total strain amplitude due to significantly higher effective stress of γ´ phase. Cyclic hardening/softening curves and cyclic stress-strain curves using short-cut procedure were obtained. Cyclic hardening/softening behavior depends both on temperature and strain amplitude. Low amplitude straining is characterized by the saturation of the stress amplitude. In high amplitude straining slight softening was found. The cyclic stress-strain curves for both materials can be fitted by power law. Cyclic stress-strain response in terms of internal and effective stress components is discussed in relation to microstructural parameters of the materials. The observation of surface relief revealed the presence of persistent slip markings.

Highlights

Precipitation strengthened nickel base superalloys exhibits excellent high temperature strength and hot corrosion resistance and they are used for production of blades and discs of gas turbine engines [1]
Cyclic stress-strain response: Fig. 2a shows the dependence of the stress amplitude σa on number of cycles N during the test at increasing total strain amplitudes
The results of total strain controlled fatigue tests in symmetrical strain cycling and the observations of surface relief produced by cyclic straining in cast Inconel 738LC and 792-5A polycrystals at 800 °C can be summarized as follows: 1. Cyclic straining results in stabilized behaviour in Inconel 792-5A, while cyclic softening is observed in Inconel 792-5A

Summary

Introduction

Precipitation strengthened nickel base superalloys exhibits excellent high temperature strength and hot corrosion resistance and they are used for production of blades and discs of gas turbine engines [1]. It is important to study low cycle fatigue properties of these materials. Structure of nickel base superalloys generally consist of two components, matrix γ and precipitate γ (L12 type structure) with different mechanical properties. Analysis of hysteresis loops shape can provide important information about the sources of the high cyclic stress in such complex materials. The aim of this paper was to conduct cyclic test at 800°C of two commonly used cast polycrystalline superalloys (IN 738LC and IN 792-5A) and study cyclic stress-strain response of these materials using generalized statistical theory of the hysteresis loop [2]

Objectives

Results

Conclusion