|Return To Program Contents Page|
Session Chairperson: J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801
DAMAGE TOLERANCE IN BRITTLE MATERIALS: R.O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720
A critical factor in the design of advanced materials for structural application is the development of sufficient damage tolerance, i.e., resistance to fracture and subcritical crack growth in the presence of pre-existing flaws. In practical terms, this implies designing microstructures with adequate fracture toughness based on toughening mechanisms that are resilient to cyclic loading, and where appropriate elevated temperatures and adverse environments. In this presentation, such considerations will be reviewed for a number of "low ductility" advanced materials, including ceramics (Si3N4, SiC) and intermetallics (g-TiAl, MoSi2, Nb3Al) in the form of monolithic, composite and layered materials.
PROCESSING, MECHANICAL BEHAVIOR, AND MICROSTRUCTURAL CHARACTERIZATION OF LIQUID PHASE SINTERED INTERMETALLIC-BONDED CERAMIC COMPOSITES: C.B. Thomas, P.K. Liaw, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200; T.N. Tiegs, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115
Intermetallic-bonded ceramic (IBC) composites with various amounts of a WC or TiC carbide phase mixed with a Ni3Al or FeAl binder phase were successfully fabricated by liquid phase sintering and hot isostatic compaction methods. The influence of carbide type, binder type, and binder content on the flexural strength, hardness, fracture toughness, and corrosion resistance of IBC composites was investigated. Experimental techniques used to characterize the mechanical behavior included Vickers hardness, four-point flexure at 25 and 800°C, indentation fracture, indentation strength, and mass loss corrosion testing. In addition, the microstructure was characterized using optical and scanning electron microscopy to examine the polished and etched microstructures, the cracks located at the corners of the hardness indents, and the fracture surfaces produced by flexure testing. Correlations were made between the mechanical properties of flexural strength and hardness and the microstructural parameter of contiguity. Experimental results indicate that the strength of IBC composites with Ni3Al at 800°C compares favorably to the room-temperature strength. At 25°C, plastic deformation of the Ni3Al and FeAl binders occurs, but the strength of the composite with FeAl is higher due to strong bonding along the carbide-binder interfaces. WC-based composites yielded higher flexural strength and fracture toughness values, while TiC-based composites produced higher hardness values. Increased binder amounts produced higher values of flexural strength and fracture toughness due to a reduction in the number of carbide-carbide interfaces and an increase in plastic deformation in the crack-tip region, respectively.
INTERFACIAL SLIDING ALONG SOLDER/INTERMETALLIC INTERFACES: C. Zhang, D. Yao, J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801
Intermetallic cells formed during reactive wetting of metal substrates often lead to a microcellular interface morphology. Evidence is given of interfacial sliding as the crack grows, along these interface, subcritically at very low strain energy release rates under cyclic loading. Physical model was constructed to capture the salient aspects of the interfacial sliding mechanism. The model predicted that the crack shielding should arise from interfacial sliding and the magnitude of the crack shielding depended on loading-mode, roughness of the interface, and sliding resistance of the interface. Experiments designed to test the validity of the model will be described and comparisons will be made between model predictions and experimental results.
3:30 pm BREAK
4:00 pm INVITED
MICROMECHANISMS OF CRACK-TIP DEFORMATION AND TOUGHENING TITANIUM ALUMINIDES IN INTERMETALLICS: W.O. Soboyejo, C. Mercer, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1179; P.B. Aswath, Materials Science Program, Department of Mechanical Engineering, The University of Texas at Arlington, P.O. Box 19301, Arlington, TX 76019
The micromechanics of crack-tip deformation and toughening are elucidated for a range of 2- and -based titanium aluminides. Crack-tip deformation is shown (via crack-tip transmission electron microscopy) to occur by a combination of microcracking and slip under cyclic loading. Toughening mechanisms in 2-based alloys (crack-tip blunting, bridging, deflection and microcrack shielding/anti-shielding) are also modeled using micromechanics. Similarly, the crack tip deformation and toughening mechanisms in gamma-based titanium aluminides (deformation-induced twinning and slip) are elucidated for a range of experimental and near commercial alloys. Micromechanical models are also presented for the prediction of toughening in gamma alloys. The paper shows clearly that crack-tip shielding mechanisms do not necessarily result in slower fatigue crack growth rates.
SHEAR LIGAMENT TOUGHENING AND MICROMECHANICAL MODELLING IN IRON ALUMINIDE INTERMETALLICS: Scott X. Mao, Department of Mechanical Engineering, University of Calgary, Calgary, Canada T2N 1N4
The fracture behavior of an iron aluminide intermetallica in air environment was studied. At room temperature, round tensile specimens were tested at different strain rates. By carefully examining the lateral surface of the tensile specimens, ligament-like structures that connected between microcracks were found. SEM pictures show that these structures, which can be called shear ligments, undergo ductile fracture by shearing. This type of fracture dissipates more energy and was believed to enhance the fracture toughness of the material. By use of a micromechanical model of shear ligment toughening, fracture toughness of the material, K, was estimated at different strain rate. The values of critical parameters, which are ligment length, area fraction and work to fracture by shear were obtained from SEM observations on different tensile specimens. K was found to be decreased towards lower strain rates. This strain rate effect was similar to that obtained experimentally. This outcome verified the importance of shear ligments in determining the materials fracture toughness.
MICROMECHANISMS OF CRACK-TIP DEFORMATION IN ZIRCONIA TOUGHENED INTERMETALLICS: F. Ye, G-Y. Lu, P. Ramasundaram and W.O. Soboyejo, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, Ohio 43210-1179
The micromechanisms of crack-tip deformation will be elucidated for zirconia toughened nickel aluminide and molybdenum disilicide intermetallic composites reinforced with partially stabilized zirconia particles with different particle sizes. The transformation toughening components associated with different stabilizers (ceria, magnisia and yttria) are also quantified using micromechanics models and experimental results obtained from laser Raman spectroscopy and optical interference analyses. Shielding contributions from other applicable toughening mechanisms are also discussed. Stable fatigue crack growth in zirconia toughened intermetallics is attributed to the effects of kinematic irreversibility due to stress-induced martensitic transformations under cyclic loading. The relative contributions from microcracking and slip phenomena (to kinematic irreversibility) are also discussed.
|Search||Technical Program Contents||1997 Annual Meeting Page||TMS Meetings Page||TMS OnLine|