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, AM Room: Grand D
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: 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
WELCOME AND INTRODUCTORY REMARKS: W.W. Milligan, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931
A PHENOMENOLOGICAL EXAMINATION OF TIME DEPENDENT CRACK PROPAGATION IN THE PARIS AND NEAR THRESHOLD REGIMES IN NICKEL-BASE SUPERALLOYS: M.F. Henry, General Electric, CRD, K1-MB229, Schenectady, NY 12345
Time dependent fatigue crack propagation has been observed in nickel-base superalloys in ambient environment at temperatures of 400[[ring]]C and above. This paper discusses generic techniques for the phenomenological treatment of these data. The behavior can be treated as a thermally activated process. Fully time dependent data is extrapolated to estimate cyclic periods required to achieve this full time dependence. In addition, the behavior of these materials in the near threshold regime is explored. Compared to the behavior in the Paris Law regime, crack propagation rates in the near threshold regime are seen to have an inverse relationship with both temperature and cyclic period. The similitude of the cyclic stress intensity factor, crack closure, creep interaction and environmental interaction are examined as potential cause of this "cross-over" behavior.
ENVIRONMENTALLY ASSISTED CRACK GROWTH IN NICKEL-BASE SUPERALLOYS AT ELEVATED TEMPERATURES: R.P. Wei, S.F. Chen, M. Gao, Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015
Environmental enhancement of crack growth in nickel-base alloys is re-examined in the light of recent understanding of the role of niobium. In this presentation, results from a coordinated study of crack growth kinetics (in oxygen and water vapor), surface chemistry, and microstructure in a commercial IN718 are reviewed. Crack growth is suggested to result from the formation and rupture of niobium oxides at the grain boundary surfaces, and to be controlled by the rate of oxidation and decomposition of niobium carbides at these boundaries. Additional data on a niobium-free, Ni18Cr18Fe ternary alloy and on data from the literature are presented to corroborate this interpretation. The technological significance of these findings is considered. Research supported by the National Science Foundation under Grant DMR-9102093.
FATIGUE/TIME-DEPENDENT CRACK GROWTH IN ALLOY 718: H. Ghonem, Mechanics of Materials Laboratory, Department of Mechanical Engineering, University of Rhode Island, Kingston, RI 02881
This paper summarizes studies focusing on the crack growth behavior of Alloy 718 due to a combined influence of environment and loading frequency at 650[[ring]]C. The parameters that have been examined include the crack growth rate, fracture surface morphology and slip line density at and below the fracture surface. Results of these studies suggest that the governing mechanism of the crack tip response is the degree of homogeneity of plastic deformation and associated slip density. For loading conditions promoting homogeneous plastic deformation with a high degree of slip density, the creep-environmental damage contribution is shown to be limited thus permitting the dominance of the cyclic damage effects which is characterized by a transgranular crack growth mode and lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and subsequent fracture. On the basis that the variation in crack growth damage mechanism in Alloy 718 from cyclic-dependent to environment-dependent is governed by the slip character in the crack tip region, experimental observations under different fatigue-creep-environment conditions are examined. Approaches to modify the alloy resistance to environmental effects through microstructure control as well as sensitizing by mechanical conditioning are discussed.
THE EFFECT OF OXYGEN PARTIAL PRESSURE ON TIME DEPENDENT CRACK PROPAGATION IN NI-BASE SUPERALLOYS: P.F. Browning, M.F. Henry, General Electric, Research and Development, PO Box 8, Schenectady, NY 12301; K. Rajan, Rensselaer Polytechnic Institute, Department of Materials Science and Engineering, Troy, NY 12180
Time dependent crack propagation in elevated temperature, oxygen bearing environments is thought to occur as a result of oxygen penetration along grain boundaries in front of the advancing crack tip. However, direct evidence to support this argument is lacking, and there is little understanding of either the mechanism(s) by which oxygen acts to reduce intergranular cohesion or the rate limiting step in oxygen transport. In this study, H2/H20 environments are used to control oxygen partial pressure and investigate its effect on crack tip oxide phase stability and crack propagation rate in two Ni-base superalloys used for structural applications in the aircraft engine industry. Experimental results are compared with those obtained by other workers in high vacuum, controlled oxygen pressure studies.
10:15 am BREAK
ENVIRONMENTAL EMBRITTLEMENT OF IRON ALUMINIDES UNDER MONOTONIC AND CYCLIC LOADING: D.A. Alven, N.S. Stoloff, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590
The tensile properties and fatigue crack growth behavior at room temperature in air, in hydrogen, and in oxygen for two Fe3Al alloys: Fe-Al-Cr and Fe-Al-Cr-Zr, are reported. In the case of the ternary alloy hydrogen and air were embrittling while in the Fe-Al-Cr-Zr alloy hydrogen was the only embrittling environment. Crack growth behavior was adversely affected by moisture, while crack growth rates increased as frequency was decreased for both alloys. Oxygen provided the lowest crack growth rates in each case. Fractographic features have been determined as a function of environment. Fatigue crack growth results have been compared with those of other iron aluminides. Zirconium appears to improve the crack growth characteristics of Fe3Al-type alloys.
EFFECT OF ENVIRONMENTAL DAMAGE ON THE CREEP BEHAVIOR OF A Ti-24Al-11Nb ALLOY: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616
The influence of environmental damage on the creep behavior of a Ti-24 Al-11 Nb alloy has been studied by pre-oxidizing the specimens at 923 K for 30-495 h. All the specimens show extensive surface cracking. The quantitative results of surface cracking agrees well with Riedel model. The technological implication of surface cracking is presented. The pre-oxidized specimens show change in creep strength and life. The dependence of creep life on stress of pre-oxidized specimens is different at lower and higher stresses. The origin of this difference is explained in terms of concurrent embrittlement kinetics.
CREEP-FATIGUE INTERACTIONS DURING HIGH-TEMPERATURE FATIGUE-CRACK GROWTH IN INTERMETALLIC ALLOYS: K.T. Venkateswara Rao, R.O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
High temperature fatigue-crack propagation behavior in aluminide and silicide intermetallic alloys is reviewed with emphasis on the role of creep deformation, creep-fatigue interactions and environment. Specifically, crack-growth data in various two-phase ([[gamma]]+[[alpha]]2) TiAl alloys and TiNb/[[gamma]]-TiAl composites at 800[[ring]]C, and MoSi2, MoSi2 composites and Nb3Al alloys at 1200[[ring]]C are presented. It is found that creep is the more dominant damage mechanism in MoSi2-based materials at 1200[[ring]]C than mechanical fatigue. Crack growth at high temperatures is governed by mutual competition between creep cavitation damage at the grain boundaries and crack bridging by the viscous SiO2 glassy phase. However, the crack-growth results are well characterized by the linear-elastic fracture mechanics parameter, DK. The influence of creep is less prominent in TiAl alloys for temperatures up to 800[[ring]]C; fatigue crack growth rates are fastest in the 600-650[[ring]]C range just below the ductile-to-brittle transition temperature for [[gamma]]-TiAl. Potential implications of such crack growth response on the damage-tolerant approach to component design and their potential structural use are also discussed. Work supported by the U.S. Air Force Office of Scientific Research.
ELEVATED-TEMPERATURE FATIGUE-CRACK GROWTH IN GAMMA-TITANIUM ALUMINIDES: A. McKelvey, K.T. Venkateswara Rao, R.O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
Intermetallic alloys based on the [[gamma]]-TiAl system are promising
materials for many elevated-temperature applications, in particular to replace
titanium alloys and nickel-based superalloys in gas-turbine engines. In light
of these applications, ambient and high-temperature fatigue-crack growth
behavior has been measured in several commercial TiAl-base alloys, including
the Ti-48Al-2Cr-2Nb ("GE") and Ti-46.5Al-2Cr-3Nb-0.2W ("K5") alloys, in both
the equiaxed duplex and lamellar ([[alpha]]2 + [[gamma]]) microstructures, at
ambient, 600 and 800C (above and below the ductile-brittle transition
temperature for TiAl). To delineate contributions from intrinsic
mechanisms (which promote fatigue damage) and extrinsic mechanisms (which
impede crack growth), "long-crack" tests have been performed under both
constant load ratio (Kmin/Kmax = 0.1 to 0.7) and constant-Kmax/increasing Kmin
loading conditions. Crack-profile analysis and Auger spectroscopy, in
combination with extensive fractography, have been used to evaluate the roles
of microstructural damage and crack-tip shielding in order to delineate the
salient microstructure features controlling crack growth. Work supported by the
U.S. Air Force Office of Scientific Research.
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