Sponsored by: Jt. SMD/MSD Nuclear Materials and MSD Flow and Fracture Committees and FEMS (Federation of European Materials Societies)
Program Organizers: R.J. Arsenault, Deptartment of Materials Science and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; David Cole, CRREL, 72 Lyme Rd., Hanover, NH 03755; Todd Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; Gernot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; Sivan Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; Howard Sizek, Inco Alloys International Inc., Huntington, WV 25705-1771
Monday, AM Room: Orange County 3
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: H. Sizek, Mechanical Testing, Inco Alloys International Inc., Huntington, WV 25720-1958; Julia Weertman, Department of Materials Science and Engineering, Northwestern University
OPENING REMARKS: J.B. Cohen, Northwestern University
8:35 am Invited
HIGH TEMPERATURE DEFORMATION IN ULTRAHIGH CARBON STEELS: O.D. Sherby, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305; O.A. Ruano, M. Carsi, CENIM, Avda. Gregorio del Amo, 8, Madrid, Spain; T. Oyama, WESGO Co., 477 Harbor Blvd., Belmont, CA 94002
Ultrahigh carbon steels (UHCSs) contain from 1 to 2.1% carbon and have been shown to be superplastic at elevated temperature and strong, tough and tensile ductility at low temperature. The basis for achieving such properties is the ability to prepare these steel with fine ferrite grain sizes (1 to 5 um) and fine spheroldized carbides. The elevated temperature mechanical properties of these steel will be described with special emphasis on their behavior at large strains utilizing torsion and tension tests. The influence of carbon content, grain size and subgrain size on the deformation characteristics are considered. The results are discussed on the basis of deformation mechanisms involving grain boundary sliding and diffusion-controlled dislocation creep, with consideration for the important influence of grain growth during plastic deformation.
9:00 am Invited
CREEP BEHAVIOR OF INTERMETALLICS WITH SPECIAL REFERENCE TO Ni3Al: T.S. Rong, I.P. Jones, R.E. Smallman, School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
Intermetallic compounds are currently showing promise as high temperature structural materials, particularly for airspace applications. Two areas of considerable concern, however, for intermetallics in general, are low temperature brittleness and high temperature creep resistance. In this paper, it is this latter aspect which is reviewed. Conventional strain-rate deformation of intermetallics has been extensively studied. By comparison, investigation of creep deformation has been limited to only a few intermetallics, and particularly Ni3Al, which is considered to be a basic compound for understanding intermetallics in general. The aim of the present paper is to outline the current understanding of creep of Ni3Al. Its creep properties will be summarized with respect to stress and temperature dependence, and the microstructures corresponding to the different stages and temperatures of creep outlined. The mechanisms which are thought to control creep deformation, especially at intermediate temperatures below the peak in the flow stress, will be discussed. General comments of creep behavior in intermetallic will then be made.
STRAIN AND STRAIN RATE BEHAVIOR OF A LOW CARBON 18Cr-12 Ni (304L) STAINLESS STEEL UNDER CONDITIONS OF CREEP-LOW CYCLE FATIGUE INTERACTION: J. Cadek, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 616 62 Brno, Czech Republic
Results of an investigation of the application of a low frequency "rectangular" cyclic stress, during both the initial and advance stages of primary creep, on the time dependent strain and strain rate behavior of a low carbon 18Cr-12Ni(304L) stainless steel are presented. The strain rates immediately before, as well as immediately after, any stress decrement and/or increment in any loading cycle are measured accurately. It is shown that at any given relative applied stress amplitude [[Delta]][[sigma]]/[[sigma]] and superimposed applied stress period [[Delta]]tc, these strain rates depend linearly on time. The relations between these strain rates and the relative amplitude of stress cycling, [[Delta]][[sigma]]/[[sigma]] are also linear at any constant cyclic loading period [[Delta]]tc. This finding makes it possible in principle, to estimate the contribution of the net strain due to superimposed cyclic loading. Further, it is shown that the creep strain rate as measured after the cyclic loading superposition lasting [[Sigma]][[Delta]]tc = 36 ks, is only relatively slightly affected by this superimposed loading. An interpretation of strain and strain rate behavior under conditions of cyclic loading superposition is based on such processes as creep strain strengthening and creep recovery, though the backward straining after any stress reduction must be taken into account as well. All the above mentioned processes depend not only on temperature and the "mean" applied stress, but especially on cyclic loading superposition history.
RECENT ADVANCES IN IMPRESSION CREEP TESTING:James C.M Li, Materials Science Program, Dept of Mechanical Engineering, University of Rochester, Rochester, NY 14627
Impression creep is a new localized mechanical test using a flat end cylindrical indenter. Unlike the usual indentation test in which a conical or a pyramidal indenter is used for which there is no steady state even under a constant load, with a cylindrical indenter both transient and steady state stages are clearly visible. So far the technique has been applied to metals and alloys, polymers, ionic crystals, and molecular solids. Some recent results on Sn-Pb solder, Pb single crystal, ABS polymer and selenium will be reviewed. A single mechanism is proposed for the eutectic Sn-Pb alloy. In addition, impression recovery for PMMA and impression fatigue for glasses will be reported. The possibility of obtaining local mechanical information will be shown.
THE EFFECTS OF MICROSTRUCTURAL CONTROL ON THE MECHANICAL BEHAVIOR OF Cr2Nb-BASED INTERMETALLIC ALLOYS: J.A. Cook, P.K. Liaw, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200; C.T. Liu, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6155
Microstructural evaluations and mechanical testing of Laves-phases alloys based on Cr2Nb were examined in order to optimize microstructural and mechanical properties through secondary processing techniques and thermo-single-phase Cr2Nb alloys are very hard and brittle due to the complicated crystal structure(C-15). The following results were revealed through examination of the Cr-Cr2Nb two-phase region:(a) with increasing amounts of the soft chromium-rich phase, the compression strength and hardness decrease; (b) the annealing treatments studied provided the best break-up of the coarse/brittle eutectic structure in the 94 at.% Cr-6 at.% Nb(CN-7) alloy; (c) 2 days (d) at 1200deg.C, lead to a substantial improvement in the room-temperature compressive ductility over the as-cast condition and previous annealing treatments (3d at 1100deg.C). In addition, Hot Isostatic Pressing (HIPping) + annealing led to a substantial refinement of the brittle Cr2Nb phase in the eutectic structure in the CN-7 (Cr-6 at.% Nb) composition. HIPping led to only a marginal refinement of the brittle, interconnected Cr2Nb phase in the eutectic structure in the CN-4(Cr-12 at.%) composition. A combination of hot forging and annealing is promising in refining the brittle Cr2Nb phase in the eutectic structure of the CN-7 composition.
10:30 am BREAK
PHENOMENA AND MECHANISM OF HIGH TEMPERATURE CREEP OF SiC/2124 Al METAL MATRIX COMPOSITES: S.H. Hong, Kyung H. Chung, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technical, 373-1 Kusung-dong, Yusung-gu, Taejon, 305-701, Korea
The high temperature creep behavior of SiC/2124Al metal matrix composites was investigated at 300deg.C. The steady state creep rates of SiC/2124Al composites were sensitively dependent on the aspect ratio and alignment of SiC reinforcements. Based on the concept of load transfer between reinforcement and matrix, a new parameter of effective aspect ratio was proposed considering the combined effects of aspect ratio and alignment on load transfer efficiency. The steady state creep rates and sub-grain sizes were measured similar when the effective stress for creep deformation of 2124Al matrix is identical. It is suggested that the SiC reinforcements enhance, the creep resistance of SiC/2124Al composites by reducing the effective stress for creep deformation of 2124Al matrix. A modified power law creep equation was proposed to describe the high temperature creep behavior of SiC/2124Al composites.
TENSILE AND COMPRESSIVE CREEP PROPERTIES OF ALUMINUM CONTAINING 25 VOL% OF ALUMINA DISPERSOIDS: A.M. Redsten, D.C. Dunand, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 8-328, Cambridge, MA 02139
The creep properties in both tension and compression of dispersion-strengthened cast aluminum (DSC-Al) containing 25 vol.% of 0.3 um alumina particles are presented for two very different grain sizes (micron-and millimeter-range). Continuum models predict that diffusional creep is the dominant deformation mechanism for fine-grained DSC-Al. Experimental data for this grain size, however, indicate that power-law creep controls the deformation and that diffusional creep is negligible. This result is discussed in terms of reduced mobility of grain-boundary dislocations due to the presence of particles. The stress exponent and activation energy of both fine-and coarse-grained DSC-Al are significantly higher than those for pure aluminum, indicating a threshold stress inn the power-law creep region. The measured power-law threshold stresses are compared to predictions from models based on dislocation-particle interactions. Finally, deformation is significantly faster in tension than in compression, a result discussed in terms of cavitation.
INTERPRETATION OF CREEP BEHAVIOR IN TERMS OF A MODIFIED POWER LAW: F.A. Mohamed, Department of Chemical Engineering and Materials Science, University of California, Irvine, California 92717
Recent experimental data have suggested that the low-stress creep behavior of some materials may be best described by a modified creep power law which incorporates a threshold stress. The presence of a threshold stress signifies that an effective stress, rather than the applied stress, is responsible for the observed creep rate. Micrograin superplastic alloys and powder metallurgy Al alloys used as matrices in the development of discontinuous SiC-Al composites provide typical examples for materials whose low-stress creep characteristics are interpreted in terms of a modified creep power law.
CONSTANT STRUCTURE CREEP STUDIES OF THE MECHANISMS OF DEFORMATION IN DISPERSION STRENGTHENED COPPER: S.E. Broyles, J.C. Gibeling, Division of Materials Science and Engineering, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616
A new series of constant structure, stress reduction creep experiments on
oxide dispersion strengthened copper alloys is described. The results of these
experiments reveal that the constant structure creep rate is found to be
significantly higher than the steady state rate corresponding to the reduced
stress, reflecting the important role of dislocation density in the creep
process. By considering the effects of a high dislocation density, the steady
state and constant structure creep behavior can be described by a new model
based on the thermodynamics and kinetics of dislocation detachment from hard
obstacles. Unlike previous models, the current method does not require the
introduction of threshold stresses or comparisons with arbitrary values of the
activation energy for creep. A relaxation parameter, k, is shown to provide a
measure of the degree to which a dislocation's line energy is reduced at the
particle matrix interface. However, the behavior of dispersion strengthened
materials can be adequately described by a combination of an athermal flow
stress, which represents the strength of the microstructure, and a Helmholtz free
energy of activation, delta-F. The magnitudes of delta-F and
are determined from the constant structure creep data and are shown to be in
the range expected for deformation controlled by relatively strong obstacles.
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