Fatigue/Creep Interactions in Ni 3 Al-Base Alloys
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FATIGUE/CREEP INTERACTIONS IN Ni 3 Al-BASE ALLOYS G.M. Camus, D.J. Duquette and N.S. Materials Engineering Department Rensselaer Polytechnic Institute Troy, New York 12180-3590
Stoloff
ABSTRACT Stress-controlled fatigue tests and fatigue crack growth rate tests respectively have been carried out on two Ni 3 A1 Cr/Zr alloys, IC 218 at 0 600%C and 800*C, and IC 221 at 800 C, in vacuum, at various test frequencies. Decreasing the test frequency and/or increasing the temperature leads to a decrease in the number of cycles to failure, and a gradual disappearance of a fatigue fracture zone. In fatigue crack propagation tests, the crack growth rate only decreases at the lowest frequency and remains constant in the major part of the frequency range investigated. The fatigue propagation mode in all cases is intergranular. These trends are shown in both cases to be related to a true creep component but, under fatigue crack growth test conditions, crack blunting intervenes gradually as the frequency is decreased, leading therefore to a less severe frequency effect.
INTRODUCTION Large improvements in high temperature ductility have been achieved in boron-doped Ni 3 AI through additions of chromium [1,2]. Creep/stressrupture properties have been improved through additions of zirconium [2,3]. Simultaneously it has recently been shown [4] that these Ni 3 AI alloys also have improved fatigue properties in terms of crack initiations as well as crack propagation. However, little work has been done on the creepfatigue behavior of Ni 3 Al alloys. This phenomenon, in which during cyclic stresses at elevated temperature, both fatigue and creep damage interact to reduce a specimen's fatigue life is of major interest for eventual high temperature applications. In their recent study of a directionally solidified Ni 3 AI (B,Hf) alloy, Bellows et al. [5,6] have concluded that there is a very good intrinsic creep/fatigue resistance at 0 0 450 C and 760 C under stress-controlled fatigue in vacuum. They were not so sure of the eventual behavior of an equiaxed polycrystal Ni 3 Al since some intergranular failures were noted. Also, a study of the fatigue crack propagation behavior of a Ni 3 Al+B alloy containing some substantial additions of cobalt and hafnium, at 400%C, has shown no frequency effect on the crack growth rate in vacuum, but a strong dependence upon the frequency when the tests are performed in air [7]. The purpose of the present work was to investigate frequency effects on the high temperature stress-controlled fatigue life and fatige crack propagation properties of IC 218 and IC 221, respectively, two Ni 3 AI B/Cr/Zr alloys, in vacuum. In the case of the IC 218 alloy, temperature and stress-range were additional experimental variables.
EXPERINENTAL PROCEDURE The IC 218 and IC 221 alloys were provided by Oak Ridge National Laboratory. Processing, chemical compositions (at %), heat treatments performed on these alloys and the resulting grain size are listed in Table I.
Mat. Res. Soc. Symp. Proc. Vol. 133. 01989 Materials Resea
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