*Sponsored by:* **MSD Computer Simulation Committee**

*Program Organizers:* S.P. Chen, Los Alamos National Lab., Los Alamos, NM
87545; M.P. Anderson, Exxon Research Center, Clinton Township, Route 22 East,
Annandale, NJ 08801

Wednesday, AM Room: Marquis 1&2

February 7, 1996 Location: Anaheim Marriott Hotel

*Session Chairman:* S. P. Chen, Los Alamos National Laboratory, Los
Alamos, NM 87545; M. P. Anderson, Exxon Research Center, Clinton Township,
Route 22 East, Annandale, NJ 08801

**9:00 am**

**THE YIELD STRESS OF THE COHERENT NANOLAYER COMPOSITE Cu-Ni:**
S. I. Rao, P. M. Hazzledine, UES Inc., 4401 Dayton-Xenia Road, Dayton, OH
45432

There are good experimental and theoretical reasons for believing that epitaxial multilayers have a yield stress which is low both when the layers are very thin and when they are very thick. At intermediate thickness there is a large peak in the yield stress which, in the case of Cu-Ni, occurs at a double-layer thickness of about 20 nm. At thickness below 10nm., the material is expected to be fully coherent and therefore to contain large alternating biaxial stresses caused by the strains required to match the lattice parameters of Cu and Ni. In order for the multilayer to yield, dislocations must cross interfaces from regions of biaxial tension to regions of biaxial compression and vice versa. The interfaces between these regions present two forms of obstacle to gliding dislocations, the Koehler (elastic modulus mismatch) obstacle and a 'chemical' obstacle caused by the fact that neither the stacking fault energies nor the dislocation core structures are the same in Cu and Ni. This paper reports the results of atomistic simulations of dislocations crossing Cu-Ni interfaces in the presence of coherency stresses. The calculations employ the embedded atom method with suitable Cu, Ni and Cu-Ni potentials. The results are used to discuss the variation of the strengths of Cu-Ni multilayers with thickness when the layers are very thin and fully coherent.

**9:30 am**

**FINITE ELEMENT MODELING OF PARTICLE CRACKING IN SiC-REINFORCED ALUMINUM
ALLOYS:** P. Scarber, Jr., G. M. Janowski, Dept. of Materials Science and
Engineering, The University of Alabama at Birmingham, Birmingham, AL
35294-4461

Simulations of an aluminum matrix composite reinforced with 9 vol% SiC particles possessing varying surface defect geometries were performed using a nonlinear axisymmetric finite element model. Preliminary results have shown that the tendency for a reinforced particle to crack is not only dependent upon the existence of surface flaws, but also on the types of flaws. Model results have shown that long slender surface defects do not promote particle cracking during tensile loading. The residual stresses created during cooling from solutionizing temperatures have been shown to be a major factor in the tendency for a reinforced particle to crack. These results will be analyzed using composite strengthening theory with consideration being given to the effects of matrix strength and particle volume fraction. This research was supported by Alabama-NSF EPSCoR and the Alabama Space Grant Consortium under contract number NGT-40010.

**10:00 am**

**MODELING OF STATISTICAL TENSILE STRENGTH OF SHORT-FIBER**

**COMPOSITES:** Y. T. Zhu, W. R. Blumenthal, M. G. Stout, T. C. Lowe,
Materials Science and Technology Division, Los Alamos National Laboratory, Los
Alamos, NM 87545

This paper develops a statistical strength theory for three-dimentionally (3-D) oriented short-fiber reinforced composites. Short-fiber composites are usually reinforced with glass and ceramic short fibers and whiskers. These reinforcements are brittle and display a range of strength values, which can be statistically characterized by a Weibull distribution. This statistical nature of fiber strength needs to be taken into account in the prediction of composite strength. In this paper, the statistical nature of fiber strength is incorporated into the calculation of direct fiber strengthening, and a maximum-load composite criterion is adopted to calculate the composite strength. Other strengthening mechanisms such as residual thermal stress, matrix work hardening, and short-fiber dispersion hardening are also briefly discussed.

**10:30 am**

**THE ROLE OF GRAIN BOUNDARY SLIDING AND REINFORCEMENT MORPHOLOGY ON THE CREEP
DEFORMATION BEHAVIOR OF DISCONTINOUSLY REINFORCED COMPOSITES: **S. B. Biner,
Ames Laboratory, Iowa State University, Ames, IA 50011

The role of grain boundary sliding behavior on the creep deformation characteristics of discontinously reinforced composites is investigated numerically together with the other influencing parameters: reinforced aspect ratio, grain size and interfacial behavior between the reinforcement and the matrix. The results indicate that the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behavior. Experimentally observed higher creep exponent values or stress dependent creep exponent values for the composites could not be explained solely by the mechanism of grain boundary sliding. This work was supported by USDOE, Office of Basic Energy Sciences, Div. of Materials Science under contract no. W-7405-ENG-82.

**11:00 am**

**CONSTRAINED METAL DEFORMATION IN AN INTERMETALLIC/METALLIC MICROLAMINATE
COMPOSITE:** J. Heathcote, G. R. Odette, G. E. Lucas, Materials Department
University of California, Santa Barbara, CA 93106

The stress-displacement function sigma(u), is one of the fundamental properties that determines mechanical properties in ductile phase reinforced composites. This function is determined for the constrained metal layers of an intermetallic/metallic microlaminate composite by a fracture reconstruction/modeling technique. First, quantitative topographical measurements are made on conjugate fracture surfaces by confocal microscopy. Then, height profiles from the two surfaces are overlaid and sequentially separated to recreate the deformation fracture process. The recreation sequence provides a direct assessment of the large scale geometry changes during constrained deformation. This information is used to guide the development of a finite element method (FEM) simulation of constrained deformation to model the sigma(u) function of the metal layers. The model predictions are compared to other independent evaluations of sigma(u). Knowledge of the sigma(u) function allows for the prediction of resistance curves and fracture strengths for various testing geometries.

**11:30 am**

**MODELING OF BRITTLE/BRITTLE AND BRITTLE/DUCTILE LAMINATES:** **THE
EFFECTS OF THE INTERFACIAL COHESION:** S. P. Chen, Los Alamos National
Laboratory, Los Alamos, NM 87545

A spring-network model was used to simulate the deformation and fracture
behaviors of brittle/brittle and brittle/ductile laminates. The effects of the
interfacial cohesion, moduli, grain boundary cohesion, yield stress will be
presented and comparted with available experiments.

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