Program Organizers: W.W. Milligan, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931; D.L. Davidson, Southwest Research Institute, PO Box 28510, San Antonio, TX 78228-0510; M.F. Henry, General Electric, CRD, K1-MB229, Schenectady, NY 12345; H.W. Sizek, INCO Alloys International, PO Box 1958, Huntington, WV 25720
Tuesday, PM Room: Grand D
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: M.F. Henry, General Electric, CRD, K1-MB229, Schenectady, NY 12345; H.W. Sizek, INCO Alloys International, PO Box 1958, Huntington, WV 25720
GAS PHASE EMBRITTLEMENT OF SUPERALLOYS: D.A. Woodford, Materials Performance Analysis, Inc., 1737 Union Street, Suite 543, Schenectady, NY 12309
This talk describes the phenomenology, mechanisms, and implications of high temperature gas phase embrittlement (GPE), associated with intergranular penetration of oxygen, and other aggressive species such as sulfur and chlorine. Examples of its occurrence in nickel, cobalt, and iron-based superalloys are given, and it is shown to be manifested in a degradation in tensile ductility and impact resistance, creep fracture, and fatigue crack initiation and propagation. The detailed grain boundary processes associated with this embrittlement are explored relative to studies of various grades of nickel. These data lead to radical implications, which should change our general understanding of creep cavitation and high temperature fatigue cracking, and have broad implications relative to the performance of engineering alloys in aggressive environments. A new approach to materials development, design and life assessment, which has evolved from this work will be outlined. Several methods of preventing or alleviating the damage resulting from oxygen penetration are described. In particular, the mechanical integrity of environmentally resistance coatings in preventing GPE becomes a crucial factor.
THE DEVELOPMENT OF HYDROGEN RESISTANT SUPERALLOYS: D.P. DeLuca, Pratt & Whitney, MS 707-20, PO Box 109600, West Palm Beach, FL 33410-9600
Nickel base superalloys are used extensively in the hot sections of Pratt & Whitney's high pressure oxidizer and hydrogen fuel turbopumps now entering service in the NASA Space Shuttle's main engines. Mechanical property tests of cast [[gamma]]' strengthened alloys have shown substantial deleterious effects when conducted in high pressure gaseous hydrogen. Among the most pronounced (and life limiting) effects are orders of magnitude reduction in fatigue life and accelerated fatigue crack growth. Unexpectedly, the most extreme effects are observed at ambient temperature rather than elevated temperature. A summary is presented of Pratt & Whitney's efforts to develop cast [[gamma]]' hot section alloys which are resistant to the unique challenges posed by hydrogen embrittlement at low temperatures. Mechanisms of hydrogen degradation and the effects on fatigue and fracture will be discussed.
A STUDY OF MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN MODEL NI-BASE SUPERALLOYS: S.D. Antolovich, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99163-2920; A de Bussac, SNECMA, BP 81, 91003 Evry, France
This study addresses some aspects of environmental damage in large grained model Ni-base alloys at intermediate temperature (427[[ring]]C) and high frequency (10Hz) for which fatigue damage is expected to predominate. Four different compositions were used to evaluate the effects of APBE, mismatch, and volume fraction of [[gamma]]'. The alloys were heat treated to produce two [[gamma]]' sizes. Testing at both 25[[ring]]C and 427[[ring]]C showed that FCP resistance was slightly increased at elevated temperature, and was higher for compositions and heat treatments that promote reversible, planar slip and increased surface roughness. This suggests the importance of closure effects. Air/vacuum/air tests at elevated temperature allowed for separation of environmental and closure effects through an analysis of FCP data at the air/vacuum transitions. These tests demonstrated a strong negative environmental effect on the FCP resistance of the superalloys tested. However, while oxygen was damaging it was shown to promote crystallographic cracking and thus high roughness.
A MODEL FOR OXIDATION-ASSISTED CREEP CRACK GROWTH IN A PM SUPERALLOY: L. Rémy, P. Bernède, Centre des Matériaux de l'Ecole des Mines, URA CNRS 866, BP 87, 91003 Evry, France
The purpose of this paper is to show the importance of oxidation on creep-fatigue and to outline how oxidation can be taken into account in life prediction, in the case of a PM superalloy, Astroloy. This alloy has been studied in a fine-grained condition (5 mm grain size after extruding and isothermal forging), and tested in creep-fatigue and pure creep in air at 650C. The effects of air, vacuum, and frequency on the crack propagation rates and crack paths will be discussed. A life prediction model is proposed which considers the coupling between diffusion, oxidation, and inelastic strain. The inelastic strain is further partitioned into plastic and viscoplastic components. The model is in very good agreement with experimental results, particularly with respect to the effects of initial Kmax on the transient creep crack growth regime. This work was sponsored by the DRET (French Ministry of Defence) and was conducted in collaboration with SNECMA engineers.
3:40 pm BREAK
GRAIN SIZE DEPENDENCE OF FATIGUE CRACK PROPAGATION IN A P/M Ni-BASE SUPERALLOY: C.P. Blankenship, Jr., M.F. Henry, General Electric, Research and Development, PO Box 8, Schenectady, NY 12301
Processing routes were developed to produce grain sizes of 7, 15 and 90 um, in P/M Ni-base superalloy Rene88DT. Heat treatment schedules were applied to ensure that [[gamma]]' sizes and distributions were identical, regardless of grain size. These microstructures are not indicative of production material, yet they provide a means to isolate the contribution of grain size to fatigue crack propagation resistance. Fatigue crack growth tests were run at temperatures between 1200F and 1400F with various tensile hold times. Significant improvements in time dependent fatigue crack propagation resistance were demonstrated with increasing grain size.
STRESS ACCELERATED GRAIN BOUNDARY OXYGEN EMBRITTLEMENT ON CREEP CRACK GROWTH BEHAVIOR OF IN718 SUPERALLOY: B.S.-J. Kang, G. Zhang, P. Liu, M. Ellathur, Mechanical and Aerospace Engineering Department, West Virginia University, Morgantown, WV 26506
Oxidation environment can significantly reduce the tensile fracture ductility and notch stress rupture lives of several nickel-base superalloys at elevated temperatures. This brittle failure is often referred to as "stress accelerated grain boundary oxygen (SAGBO) embrittlement." In this research, the mechanism of SAGBO embrittlement on IN718 superalloy was investigated. In situ, full field crack tip displacement fields of IN718 SEN specimens under loads at 650[[ring]]C were measured using high temperature moire interferometry and elastic crack growth was observed for specimens tested in air. The experimental stress intensity factors and crack-tip plastic yield zones were found to be in good agreement with the theoretical predictions based on linear elastic fracture mechanics. Post-mortem microstructural analysis showed the existence of Nb-rich oxide films at grain boundaries near the crack tip region. This suggested that SAGBO embrittlement was caused by oxygen diffusion to grain boundaries in high stress regions and reacting with Nb to form thin, brittle Nb2O5 oxide layers on grain boundary surfaces. Chemical composition of the Nb2O5 layers was verified using Auger electron spectroscopy.
INTERACTIONS BETWEEN CREEP, FATIGUE AND ENVIRONMENT IN DAMAGE TOLERANT NIOBIUM ALUMINIDE INTERMETALLICS: J. DiPasquale, F. Ye, W.O. Soboyejo, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1179
The interactions between creep, fatigue and laboratory air/vacuum environments are discussed in this paper for damage tolerant niobium aluminide intermetallics. These include the effects of load/environmental interactions of crack initiation and propagation behavior under monotonic and cyclic loading. The possible creep-fatigue-environment interactions are modeled using fracture mechanics- and chemistry-based models. Coating schemes are also proposed for the control of environmental interactions.
STRESS RUPTURE OF A SiC/SiC COMPOSITE: T.E. Steyer, F.W. Zok, Materials Department, University of California, Santa Barbara, CA 93106. D.P. Walls, United Technologies, Pratt and Whitney, West Palm Beach, FL 33410
The stress rupture response of a SiC/SiC [0/90] plain weave composite has been
studied at a temperature of 900[[ring]]C. At this temperature, failure is
governed by an oxidation embrittlement mechanism. Tests have been performed at
various stresses ranging from the matrix cracking stress (~70 MPa) to values
approaching the composite fast-fracture ultimate tensile stress. The effects of
pre-cracking at a high stress at ambient temperature on the stress rupture
characteristics at 900C have also been examined. The effects of oxidation on
microstructure have been studied using both SEM and scanning Auger microscopy
to examine the evolution of chemical species present in the matrix and
fiber/matrix interphases. The implications of oxidation on durability and
lifetime are explored.
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