Materials Week '97: Wednesday AM 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 morning, September 17.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session V

Session Chairpersons: Brian N. Cox, Rockwell Science Center, Thousand Oaks, CA 91360; David Davidson, Southwest Research Institute, San Antonio, TX 78228

DESIGNING PARTICULATE COMPOSITE MICROSTRUCTURES FOR FATIGUE RESISTANCE: David Davidson, Southwest Research Institute, San Antonio, TX 78228

Non-deforming particles channel deformation at the tip of a crack into the matrix regions between particles. Particles also constrain flow in the matrix, lowering the deformation that can occur. This effect has been quantified in several particulate composites; representative results will be shown. Fatigue cracks growing at near threshold values of stress intensity factor are not affected much by the presence of particles because of the small plastic zone of the crack. As stress intensity rises, constraint retards crack tip plasticity which prevents the slope of the da/dN vs. K curve from decreasing to the value found in the unreinforced matrix. Fracture toughness is also lowered because of reduced plasticity. To design tougher composites, constraint must be relaxed by using deformable particles.

9:00 am INVITED

MICROMECHANISMS OF FATIGUE CRACK GROWTH IN MoSi₂-BASED COMPOSITES: F. Ye, Y. Li, R.J. Lederich¹ and W.O. Soboyejo, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH; ¹McDonnell Douglas, P.O. Box 516, St. Louis, MO 63166-0516

The paper presents the results obtained from recent studies of the effects of reinforcement geometry (particles, fibers and layers) on the fatigue crack growth behavior of MoSix/Nb composites. Particulate reinforcement is shown to promote relatively fast growth rates at low stress intensity factor ranges. Intermediate fatigue crack growth rates are demonstrated in Nb layer-reinforced composites. The beneficial effects of synergistic toughening are also demonstrated in hybrid composites reinforced with transformation toughened MoSi₂ and Nb layers. The trends in the fatigue crack growth behavior are rationalized by considering the combined effects of crack-tip plasticity and the crack-tip shielding mechanisms observed in the experiments.

9:20 am

MICROMECHANICS OF FATIGUE & FRACTURE IN LAMELLAR TiAl COMPOSITES: Bimal Kad and Robert J. Asaro, Dept. of Applied Mechanics & Engineering Sciences, University of California San Diego, La Jolla, CA 92093

Finite element based numerical procedures, incorporating physically based crystal plasticity models, are employed to study the evolution of non-uniform deformation, under monotonic, and cyclic loadings, in lamellar TiAl composite microstructures. The impetus for such efforts is to gather fundamental insight into microstructure sensitive deformation mechanisms, and to extract additional information, not obtainable from traditional mechanical property measurements. Computational efforts address constant strain, and stress, amplitude cycling schemes with variable R=0.5, -0.1, and -1.0 loadings. Results, initially presented for a maximum of 100 cycles, indicate that flat S-N curve response can be understood via the 'signature' hydrostatic stress evolution, which decreases with cycling. Such signatory patterns are significantly affected by intrinsic effects such as plastic anisotropy and microtexture. We will present several examples of experimentally observed, and numerically computed results, to identify hot spots for strain localization in monotonic and fully reversed loadings, and prescribe microstructural remedies to alleviate such effects. Numerical procedures extending these analyses to 10,000 cycles, and those incorporating total strain to life criteria will be described.

9:40 am

FATIGUE CRACK GROWTH BEHAVIOR OF NICKEL ALUMINIDE COMPOSITES: P. Ramasundaram, M. Li, F. Ye, Y. Li, N. Katsube, and W.O. Soboyejo, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus OH 43210-1179; Department of Aerospace Engineering, Applied Mechanics and Aviation, 155 W. Woodruff Avenue, Columbus, OH 43210

The effects of composite architecture on the micromechanisms of fatigue crack growth are elucidated for a range of model nickel aluminide based composites. The effects of reinforcement architecture are discussed for Mo-reinforced composites. The effects of layer thickness on the fatigue crack growth behavior are discussed for composites reinforced with ductile phase reinforcements such as Mo, Nb-15Al-40Ti and V. The observed trends in the fatigue crack growth behavior are rationalized by considering the combined effects of crack-tip shielding and crack-tip plasticity.

10:00 am BREAK

10:20 am INVITED

CREEP BEHAVIOR OF MOLYBDENUM DISILICIDE MATRIX COMPOSITES: A.K. Ghosh, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136

High temperature intermetallics such as molybdenum disilicide under steady state creep conditions deform by dislocation creep mechanism exhibiting stress exponent values of 3-4. These intermetallics have weak grain boundaries and are prone to intergranular fracture under tensile creep loading. The addition of SiC particulate reinforcements lead to grain refinement of the matrix while providing load sharing via higher levels of elastic modulus and higher strength levels of the ceramic particulates. Grain refinement brings about an alternative creep mechanism n these intermetallics such as grain boundary sliding accommodated by dislocation glide and climb. The reinforcing particles introduce local stress concentrations at grain boundaries and triple points and can produce an additional component of the matrix creep rate. A creep deformation model has been developed which includes the effect of grain refinement and strain concentration, and particle strengthening effects to predict possible strengthening and weakening effects in the composites as a function of particle volume fraction. It is found that significant volume loading is necessary to produce net strengthening effects in these composites.

10:40 am

CREEP BEHAVIOR OF PARTICULATE NiAl₃ INTERMETALLICS REINFORCED Zn-Al ALLOY: R. Zhang, Z. Wang, Department of Metallurgy & Materials Science, The University of Toronto, Toronto, Ontario

Creep tests were carried out on NiAl₃ intermetallics reinforced commercial Zn-Al Zamak 3 alloy and also the unreinforced reference material at 120°C for a stress range from 20 to 50 MPa. The creep resistance of the composite was found to be significantly higher than that of the unreinforced alloy. For both materials the prediction of the total time to rupture or the time to reach a given creep strain can be made by a single linear relationship between the time to rupture or the time to a specific creep strain and the minimum creep rate. Necking was observed in the unreinforced alloy but not in the composite material, although both materials had similar total creep strain. This observation indicates that the creep deformation in the reinforced material was rather uniformly distributed along the whole gage of the sample. The creep mechanisms and the fracture behavior of the two materials will also be extensively discussed.

11:00 am

COMPRESSION CREEP OF THE NiAl-W INTERMETALLIC FIBER COMPOSITE: T.A. Venkatesh, D.C. Dunand, Massachusetts Institute of Technology, Cambridge, MA 02139

The compressive creep behavior of aligned long fiber reinforced NiAl-W composites, processed by reactive infiltration, is investigated. Three different fiber deformation mechanisms are identified for the case of creeping fibers deforming in a creeping matrix, i.e., longitudinal contraction, lateral deflection and kinking. Simple analytical models are developed to demonstrate the dominant deformation mechanism to be largely determined by two parameters - temperature and initial fiber configuration. Experimental validation of the models is obtained by characterizing the microstructures and creep behavior of NiAl matrix, tungsten reinforcement and NiAl-W composites.

11:20 am INVITED

EFFECTS OF CHANGES IN GRAIN SIZE AND R-RATIO ON FATIGUE OF MONOLITHIC NIOBIUM AND NIOBIUM-BASED "IN SITU" COMPOSITES: William A. Zinsser, Jr., John J. Lewandowski, Dept. of Materials Science & Engineering, Case Western Reserve University, Cleveland OH 44106

Considerable work has focused on the fracture toughness of refractory based systems as well as both intermetallic and ceramic systems toughened with such reinforcements. Less work has focused on the behavior of such systems under cyclic conditions. The present work investigates the effects of changes in grain size and solid solution additions of Zr and Si on the fatigue crack growth behavior of monolithic niobium as well as the effect of cyclic loading on the "in situ" composites based on the Nb-Si system. The effects of changes in the R-ratio and K on the rate of fatigue crack growth and the fracture morphology will be covered and compared to other recent work on such systems.

11:40 am INVITED

ON THE MICROSTRUCTURAL DEPENDENCY OF FRACTURE TOUGHNESS AND CREEP RESISTANCE OF NICKEL/YTRRIA COMPOSITES: Lian-Cho Sun, Leon L. Shaw, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269

The current prime materials for turbine blade airfoils are nickel-base superalloys which have a density of about 8.2 gm/cm³ and are capable of operating at about 1000C for long-time service. However, further improvement in the operating temperature and density of turbine blades relies on either the employment of ceramics, intermetallics and their composites or metal matrix composites (MMCs). In this study, pure nickel and yttrium oxide were selected as a model system of MMCs to investigate the feasibility of processing three-dimensionally-reinforced nickel-based composites through a powder metallurgy approach and to examine the microstructural dependency of mechanical properties of these model composites. Composites with a volume fraction of yttria ranging from 20 to 70% were prepared through hot pressing. The density, microstructure, fracture toughness and creep resistance of these composites were evaluated as a function of the yttria volume fraction and processing conditions. It was found that the fracture toughness and creep resistance of the composites changed dramatically with the volume fraction of yttria, and this could be related to the morphology change of the yttria, reinforcement from discrete particles to a continuous 3-D network in addition to the effect of the yttria volume fraction. The insight to the dependency of the fracture toughness and creep resistance on the microstructure was explored in light of theories of fracture toughness and creep resistance of composites.