Hydrogen effects on low-cycle fatigue of the single-crystal nickel-base superalloy CMSX-2
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I.
INTRODUCTION
N I C K E L - b a s e d single-crystal superalloys are commonly used for turbine engine applications. Specialized applications, such as space shuttle main engine pumps, necessitate improved susceptibility to hydrogen embrittlement, and this has led us to investigate the factors controlling hydrogen performance in two nickel-based single-crystal F/F' superalloys, CMSX-2 and PWA 1480J ~'21 These studies have conclusively demonstrated that hydrogen can significantly affect deformation structure, flow, and fracture of these alloys, often resulting in a serious degradation of tensile properties. There have been surprisingly few studies on the hydrogen-affected cyclic deformation of superalloys, and these have been mostly in government and company reports, t31 This encouraged us to undertake a systematic study on the role of hydrogen in cyclically deformed CMSX-2. The variables in this research included charging procedure, strain amplitude, and specimen geometry. Low-cycle fatigue behavior was investigated since this type of fatigue is of most general interest for intended applications. II. M A T E R I A L A N D EXPERIMENTAL PROCEDURE
A single melt of CMSX-2 superaltoy was supplied from the vendor in the form of single-crystal bars in the assolution heat-treated state of 3 hours at 1315 ~ with orientations within 10 deg of [001]. Its chemical composition is in weight percent: 5.6A1, 1.1Ti, 8.0Cr, 4.6Co, 6.0Ta, 8.0W, 0.6Mo, and balance Ni. Fatigue specimens were machined according to ASTM specification E-606 (diameter = 4.2 mm). A few hollow specimens M. D O L L A R , Associate Professor, and I.M. BERNSTEIN, Chancellor, are with the Metallurgical and Materials Engineering Department, Illinois Institute of Technology, Chicago, IL 60616. A. D O M N A N O V I T C H , H. PINCZOLITS, Graduate Students, and W. KROMP, Associate Professor, are with the Solid State Physics Department, University of Vienna, A-1090 Vienna, Austria. Manuscript submitted December 6, 1990. METALLURGICAL TRANSACTIONS A
(diameter = 10 mm, gage length = 13 m m , wall thickness = 0.45 mm) were also machined to study the effects of specimen geometry and relative hydrogen enrichment on subsequent embrittlement. All specimens were encapsulated in argon and subjected to a heat treatment developed primarily to improve creep resistance, 141 i.e., 1050 ~ hours/air cool + 850 ~ hours/air cool. The detailed characteristics of the macrostructure and microstructure of CMSX-2 have been presented elsewhere. ~5~The most relevant microstructural features are presented in Figure 1, showing a high volume fraction of about 65 pct cuboidal F' particles, with the {100} faces parallel to the [001] growth direction of the single crystal. They have an edge length of 400 to 600 nm and are surrounded by narrow ribbons of F solid solution. Sets of at least two specimens were machined from individual single-crystal bars to eliminate differences resulting from varying crystallographic orientation of the supplied bars. At least one specimen of a set was tested in t
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