Materials Week '97: Wednesday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday afternoon, September 17.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session VI

Session Chairs: Sheldon Wiederhorn, Materials Science and Engineering Laboratory, National Institute of Standards & Technology, Gaithersburg MD 20899-0001; Leon L. Shaw, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269

CREEP FAILURE MECHANICS FOR CERAMIC MATRIX COMPOSITES: B.N. Cox, C. Argento, Rockwell Science Center, Thousand Oaks, CA 91360

In many current ceramic matrix composites (CMCs), the shielding of matrix cracks by unbroken fibers is compromised by fiber creep. Matrix cracks are therefore observed to grow subcritically. We will review experimental and theoretical studies of matrix cracks bridged by creeping fibers, emphasizing basic concepts. The processes of crack initiation and crack growth will be unified by micromechanical models. The question of whether to expect multiple or dominant single cracks is especially interesting and central to formulating life models. Failure maps will be discussed which show how micromechanical properties of the composite, length scales, notch and ply dimensions, and the steady state matrix cracking stress all influence failure modes.

2:30 pm

HIGH TEMPERATURE FLEXURAL CREEP BEHAVIOR OF REACTION-FORMED SILICON CARBIDE CERAMICS AND FIBER REINFORCED COMPOSITES: M. Singh, NYMA, Inc., NASA Lewis Research Center Group, Cleveland, OH 44135

Flexural creep behavior of reaction-formed silicon carbide (Hi-Nicalon^TM) fiber reinforced composites have been investigated from 1150 to 1350°C in air. These materials were fabricated by the melt infiltration of silicon in to porous carbonaceous preforms. Creep tests were carried out at 75, 150, and 200MPa for 100-200hrs. Microstructural characterization was carried out to identify the creep mechanisms. These results will be compared with literature data on reaction-bonded silicon carbide and CVI SiC/SiC composite materials.

2:50 pm

ROLE OF FIBER CREEP RATE IN THE CRACK GROWTH RATE OF SiC/SiC COMPOSITES: R.H. Jones, C.H. Henager, Jr., C.A. Lewinsohn, Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352

Optimization of the high-temperature structural properties of continuous fiber SiC/SiC composites requires a fundamental understanding of the relationship between fiber, matrix and interface properties and composite properties. Results of a study aimed at relating the fiber creep rate to the subcritical crack growth rate and fracture properties of SiC/SiC composites have demonstrated that the crack growth rate in a bulk composite is controlled by the fiber creep rate. This result was demonstrated for Nicalon-CG and Hi-Nicalon fiber reinforced material where a 100°C shift in the creep strength of the fibers resulted in a similar shift in the crack growth rate of the composite. The fiber creep rate of Hi-Nicalon S and Dow Corning's Sylramic fiber exhibit a 300°C improvement in the creep strength relative to Nicalon-CG. Crack growth rates of composite material made with these fibers are expected to exhibit similar increases. *Research supported by the Office of Basic Energy Sciences of the U.S. Department of Energy with Battelle Memorial Institute under Contract DE-AC06-75RLO 1830.

3:10 pm INVITED

THE MONOTONIC AND FATIGUE BEHAVIOR OF NICALON FIBER REINFORCED ALUMINA COMPOSITES: N. Miriyala, P.K. Liaw, C.J. McHargue, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996; L.L. Snead, Oak Ridge National Laboratory, Oak Ridge, TN 37831

The mechanical behavior of a commercially available continuous fiber reinforced ceramic composite, namely a Nicalon fiber fabrics reinforced alumina composite, was investigated. Flexure specimens were subjected to monotonic and cyclic-fatigue loadings, at ambient temperature and 1000°C, with loading either parallel or normal to the fabric plies. The orientation of the laminate plies to the loading axis, and the exposure of the composite materials for prolonged times at the elevated temperature, resulted in significant differences in the monotonic and fatigue behavior. The damage mechanisms responsible for the observed effects will be the focus of the paper. Research supported by DOE under a subcontract from Lockheed Martin Energy Research Corporation (No. 11X-SV483V), and by the National Science Foundation, under contract No. EEC-9527527 with Mrs. Mary Poats as a contract monitor.

3:30 pm BREAK

3:50 pm INVITED

HIGH TEMPERATURE DESIGN WITH SILICON NITRIDE: Sheldon Wiederhorn, Materials Science and Engineering Laboratory, National Institute of Standards & Technology, Gaithersburg, MD 20899-0001

Abstract not available.

4:10 pm

ELASTIC STRAIN DISTRIBUTION IN THE PHASES OF A CuMo PARTICULATE REINFORCED METAL MATRIX COMPOSITE DURING CREEP DETERMINED BY NEUTRON DIFFRACTION: M.R. Daymond and M.A.M. Bourke, LANSCE, Los Alamos National Labs, NM 87545; D.C. Dunand & C. Lund, MIT, Cambridge, MA 02319

The macroscopic load bearing capability of a composite material is directly related to the strain partitioning occurring between the individual phases. To understand the creep behavior of a composite, and for model validation, we need a direct measure of these strains. Neutron diffraction offers in-situ determination of the elastic strains within a bulk specimen, without the ambiguity implicit in a surface x-ray diffraction measurement. We report the different strain partitioning occurring in a Cu-Mo particulate reinforced MMC for increasing applied tensile loads at room temperature, 300 and 350°C. The results are compared with finite element unit cell predictions, with good agreement.

4:30 pm

ELEVATED TEMPERATURE DEFORMATION BEHAVIOR OF THE INTERFACIAL REGION IN CONTINUOUS FIBER REINFORCED METAL-MATRIX COMPOSITES: I. Dutta, J.E. Funn, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943

This paper will present the results of ongoing studies of creep and stress relaxation behavior of the near-interface region in fiber reinforced metal-matrix composites (MMC). The studies are based on model single fiber composite (SFC) systems representing strongly bonded (W-Pb) and weakly bonded (Quartz-Pb) interfaces. The relative roles of interface diffusion and near-interface matrix creep in both strongly and weakly bonded systems will be discussed, and based on the experimental results, a constitutive law representing the dependence of the steady state strain rate of the interfacial region on temperature and the average interfacial shear stress will be proposed. Such a law will enable the modeling of multi-phase systems while accounting for interfacial strain accommodation by representing the interface as a separate entity with its own constitutive law, and will thus have an impact not only on the field of composites, but wherever there are interfaces between dissimilar materials. Supported by the National Science Foundation under contract # DMR-9423668.

4:50 pm

INDENTATION EXAMINATION OF METAL AND POLYMERIC MICROSTRUCTURE USING PRECISION STRAIN RATE SENSITIVITY: M. Van Prooijen, B.J. Diak, S. Saimoto, Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada, K7L 3N6

Microindentation testing is a very attractive materials testing technique due to its almost non-destructive nature of examination and position localized determination. However, hardness alone is not sufficient to describe the microstructure and we have implemented the measurement of thermodynamic response by precision displacement rate change to correlate strain rate sensitivity to microstructure. In order to distinguish the material response from the testing geometry, various materials, including brass, I.F. steel, aluminum alloys, silica modified polyester paints and ABS blends have been tested. For the case of polymeric material a unique master curve on the apparent activation work versus strain rare plot manifests itself independently of the test temperature and displacement rate used. The unique features of the crystalline dislocated case will be compared to those of the molecular polymeric one.