Fatigue crack growth behavior of an oxide dispersion strengthened MA 956 alloy
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I.
INTRODUCTION
THERE is a growing interest in the use of oxide dispersion strengthened (ODS) alloys for high temperature applications because of the development of the mechanical alloying process 1'2 which is being used to produce materials with unique properties that cannot be made by conventional methods. Among the several alloys that have been developed, iron-base alloy MA 956 and nickel-base alloys MA 754 and MA 6000 have been shown to be of particular interest in terms of their strength and other properties. Alloy MA 956 is a body centered cubic alloy with a nominal composition of Fe-20Cr-4.5A1-0.5Ti-0.5Y203. It is stable and has relatively high strength at 1200 ~ and possesses high resistance to oxidation and carburization. In contrast to this ODS alloy,5 a dispersion-free alloy of similar composition was found to undergo significant degradation in hot gas corrosive environments. Tensile and creep properties of this alloy have been studied extensively at selected temperatures up to 1200 ~ 2-5 Both tensile and creep properties were found to vary significantly with stress and strain rate. In particular at low strain rates the tensile and creep ductilities are ,1ery low and are related to localization of deformation, and failure occurs by the nucleation and growth of cracks in the narrow deformation zone. At high strain rates, however, the deformation zone is larger and failure occurs by ductile shear resulting in higher ductility. Table I summarizes the tensile and creep properties of the alloy.
K. S A D A N A N D A and P. SHAHINIAN are Metallurgists, Naval Research Laboratory, Washington, DC 20375. Manuscript submitted August 16, 1983. METALLURGICAL TRANSACTIONS A
While behavior of the alloy under tensile and creep stresses is known, its subcritical crack growth behavior particularly under fatigue and creep has not been reported so far. This is important in terms of its application since its low ductility and high strength could make it very susceptible to crack growth. In fact, it was suggested4 that the low ductility under creep is due to the fact that life is determined by the propagation of a single crack formed at the localized deformation zone. The object of the present study is to examine the fatigue crack growth resistance of MA 956 alloy at 25 and 1000 ~ The crack growth data are analyzed in terms of both linear and nonlinear elastic fracture mechanics along with the detailed fractographic analysis of the fatigue crack surfaces. Finally the behavior of the alloy is examined in terms of existing theoretical models of fatigue crack growth.
II.
EXPERIMENTAL DETAILS
A. Fatigue Crack Growth Measurements Alloy strips of 50 mm width and 3.2 mm thickness were obtained from INCO MAP (a trademark of INCO family of companies) from which 3/4 T compact tenSion specimens with nominal dimensions but with 2.9 mm thickness were machined. Notch orientations were perpendicular (L-T) and parallel (T-L) to the direction of the hot extrusion. The specimens were side grooved to a depth of 5 pct of the thickness on e
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