Sponsored by: Joint 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
Thursday, AM Room: Orange County 3
February 8, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: R.J. Arsenault, Metallurgical Materials Laboratory, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616
ON THE DEFORMATION MECHANISM IN HARPER-DORN CREEP: J.N. Wang, Chemistry and Materials Sciences, L-370, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551-9900
Harper-Dorn creep, a Newtonian dislocation creep, has been observed in many metallic, ceramic and silicate materials. Based on an early Weertman's model for power law creep, a microphysical model for Harper-Dorn creep is developed. Two assumptions are made. One is that a steady dislocation density is established not only by the applied stress but by the Peierls stress as well. The other is that the flow process is dislocation glide plus climb being rate-controlling. It is shown that the predicted dependencies of dislocation density and strain rate on the Peierls stress are in very good agreements with experimental data on a wide range of materials.
AN INVESTIGATION OF THE TRANSITION CONDITIONS BETWEEN DIFFUSIONAL CREEP, HARPER-DORN CREEP AND POWER LAW CREEP: J.N. Wang, Chemistry and Materials Sciences, L-370, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551-9900
The information about the transition condition between different creep regimes is essential both for developing new materials for engineering applications and for understanding and modelling geological deformation processes in the Earth. It is recently found that the Peierls stress of a crystal is an important factor determining the transitions between lattice diffusional creep, Harper-Dorn creep and power law creep. Both theoretical predictions and experimental data consistently show that a high Peierls stress leads to a high stress marking the transition from power law creep to Harper-Dorn and diffusional creep and to a small grain size representing the transition from Harper-Dorn creep to diffusional creep. Experimentally confirmed theoretical or semi-empirical rate equations are used to construct generalized deformation mechanism maps. Such maps clearly illustrate that Harper-Dorn creep may be an important mechanism controlling the high-temperature deformation of materials with high Peierls stresses in wide ranges of grain size and stress.
THE ROLE OF GRAIN BOUNDARY CHARACTER DISTRIBUTION ON CREEP DEFORMATION OF Ni-16Cr-9Fe AT 0.38 Tm: V. Thaveeprungstiporn, Department of Nuclear Engineering, University of Michigan, Ann Arbor, MI 48109; G.S. Wae, Department of Nuclear Engineering and Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109
During high temperature creep deformation, lattice dislocations interact with grain boundaries creating extrinsic grain boundary dislocations (EGBDs) which may then be dissociated, absorbed or trapped in the boundaries depending on the type and structure of the grain boundaries. Evidence indicates that dislocation absorption in random high angle boundaries (HABs) is much easier than that in coincidence-site-lattice boundaries (CSLBs). Thus the rate of formation and absorption of EGBDs during creep deformation is of practical importance since it may be related to the rate at which back-stresses on following dislocations are relaxed and this relaxation may control the rate of deformation. This research reveals that by increasing CSLB populations in Ni-16Cr-9Fe alloy the flow stress was increased by 15%-30% and its steady-state creep rate was lowered by a factor of 40. A modified recovery creep theory that includes the effect of grain boundary character distribution will also be discussed for a better understanding of high temperature creep deformation.
PRIMARY CREEP IN DISPERSION STRENGTHENED MATERIALS: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616
Primary creep is perhaps the most important stage for a majority of creep applications, yet it has received very little attention. The understanding of the influence of second phase particles on primary stage inn quite inadequate. The primary creep data of a number of dispersion strengthened aluminum alloys is analyzed. The magnitude of primary creep scales with the interparticle spacing and it appears to be linked with the steady state creep mechanism. Based on the present results, the primary creep response of a dispersion strengthened material can be divided in three regimes; (a) low primary strain when the interparticle spacing is smaller than grain size. (b) medium primary strain when the interparticle spacing is similar to the grain size, and (c) large primary strain when the interparticle spacing is larger than grain size. The primary decay constant for composites is significantly different from mechanically alloyed materials. A model is suggested to explain the interparticle dependence of the primary creep.
CREEP MECHANISMS AND ACCELERATED COARSENING IN A LAMELLAR EUTECTIC: T. Plookphol, D.S. Stone, Department of Materials Science and Engineering, University of Wisconsin, MS&E Building, Madison, WI 53706; S-M. Lee, Samsung Electronics Co. Ltd, Suwon, South Korea
The microstructure of a eutectic solder joint coarsens during thermal fatigue. To gain insight into this process, we investigate a related one: acceleration, due to prior deformation, of coarsening during an isothermal anneal. Eutectic lead-tin specimens are deformed to 10% at room temperature and strain rates 10-6/s< <10-1/s, then annealed at 125deg.C. Beyond 50% volume fraction coarsened material (V = 0.5), the rate of coarsening can be expressed as V=f ( )xVdeg. where Vdeg. is Vdeg. in absence of prior deformation. The multiplicative factor f( ) peaks at ~10-4/s. Based on evidence from creep, load relaxation, and tensile tests, we propose f( ) reflects the ability of the microstructure to store dislocations during deformation. The peak in f( ) is likely related to colony boundary sliding.
10:10 am Break
HIGH TEMPERATURE DEFORMATION MECHANISM OF Al2O3/NiAl COMPOSITES: K. Xu, R.J. Arsenault, Metallurgical Materials Laboratory, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115
Long screw dislocations with jogs or super jogs were observed in NiAl matrix composites after high temperature deformation of NiAl matrix composites. Thermally activated motion of those jogged screw dislocations are believed to be the major mechanism to explain the mechanical behavior or NiAl matrix composites at high temperature. However, the jogged screw dislocation mechanism requires the existence of only vacancy-producing jog in the matrix during the deformation. Barret and Nix also proposed a creep model which indicated that the alternative arrangement of vacancy-producing and vacancy-absorbing jogs would make the mechanism break down. In this paper, a multiple cross slip of screw dislocation model due to the local interaction between screw dislocation and spherical particles based on deformation of NiAl matrix composites is proposed. As a result, groups of single sign jobs are produced in screw dislocation with large group interspacing. This multiple cross slip mechanism may give the survival condition for jogged screw dislocation mechanism for high temperature deformation.
CREEP-FATIGUE CRACK PROPAGATION BEHAVIOR OF HAYNES HR160 ALLOY: Weiju Ren, Peter K. Liaw, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 7996-2200; R.W. Swindeman, B.G. Gieseke, Metal and Ceramic Division, Oak Ridge National Lab, Oak Ridge, TN 37831-6155; G.Y. Lai, Haynes International, Inc., 1020 West Park Ave., Kokomo, ID 46902-9013
Creep-fatigue crack growth behavior was studied on a Haynes HR160 alloy, a newly commercialized high-temperature superalloy. The test temperature investigated ranged from 850deg.C to 1000deg.C. The results indicated that creep crack propagation rates can be uniquely described by the time-dependent fracture mechanics parameter, C*(t), in the form of da/dt = A[C*(t)]q. This correlation was found to be independent of test temperature, load level and specimen size. Using the time-dependent fracture mechanics parameter, the creep-fatigue crack propagation behavior has been successfully correlated with the creep crack growth rates. Microstructural characterization showed that the creep crack grew mainly in an intergranular fracture, while load cycling induced a mixed intergranular and transgranular crack growth mode.
PLASTIC DEFORMATION KINETICS OF FCC METALS AT LOW HOMOLOGOUS TEMPERATURES: W.D. Cao, H. Conrad, Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695-7517
The plastic deformation kinetics of polycrystalline Ag, Cu and Al representing a range in stacing fault energy were investigated at 78 to 300 K. The parameters in the thermally activated plastic strain rate equation
were determined from the effects of temperature and strain rate cycling on the flow stress at a constant structure. Both o and G increased with stacking fault energy. The rate controlling dislocation mechanism was deduced to be the cutting of forest dislocations. The derived dislocation jog and stacking fault energies were inn accord with those obtained by other methods of measurement.
DISLOCATION LINE TENSION AND SPACE CURVES: C.S. Hartley, Department of Mechanical Engineering, Florida Atlantic University, Boca Raton, FL 33431-0991
A component of the restoring force on dislocations having the form of space is
identified by differentiation of the vector line tension. This component acts
in the direction of the binormal to the space curve and is proportional to the
torsion, while the familiar "effective line tension" force is proportional to
the curvature and acts in the direction of the principal normal to the curve.
The component in the direction of the binormal vanishes for plane curves. These
concepts are applied to analyze the formation and stability of dislocation
helices from straight dislocations in the presence of non-equilibrium vacancy
concentrations and general stress fields. It is shown that, in the isotropic
approximation, the circular helix is stable with respect to a straight
dislocation originally lying along the axis of the helix only if the
dislocation is a screw. The equilibrium form of a general space curve immersed
in an arbitrary stress field and non-equilibrium vacancy concentration is given
implicitly by relations which equate the corresponding forces to the self
force produced by the dislocation.
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