Materials Week '97: Tuesday 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 Tuesday afternoon, September 16.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session IV

Session Chairs: Linda S. Schadler, Dept. of Materials Science and Engineering, Rennselaer Polytechnic Institute, Troy NY 12180; John J. Lewandowski, Dept. of Materials Science & Engineering, Case Western Reserve University, Cleveland, OH 44106

DESIGNING METAL MATRIX COMPOSITES WITH DISCONTINUOUS REINFORCEMENT FOR HIGH CREEP STRENGTH: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616

Metal matrix composites (MMCs) with discontinuous reinforcement are attractive for high temperature applications because of their creep strength. An additional benefit of discontinuously reinforced MMCs is the ease of fabrication through a number of processing routes. Although the increase in creep strength because of the second phase reinforcement is well documented, the scientific issues related to creep deformation are not so well understood. Also, the magnitude of creep strengthening depends on the nature of reinforcement and the matrix material. By using a 'dislocation creep mechanism map' it is possible to predict the level of creep strengthening in aluminum matrix composites. The role of interparticle spacing on the creep strength of various aluminum and titanium matrix composites is discussed through a detailed analysis.

2:30 pm INVITED

EFFECT OF MATRIX MICROSTRUCTURE ON FATIGUE BEHAVIOR OF AN Al-2080/SiC COMPOSITE: Christoph Andres*, J. Wayne Jones*, and John E. Allison+, *Department of Materials Science, The University of Michigan, MI; +Ford Research Laboratory, Ford Motor Company, Dearborn, MI

The main focus of this study is the influence of the matrix precipitate structure on the fatigue behavior of powdermetallurgy 2080-Al and 2080-Al reinforced with SiC particles. The materials were subjected to either T6 or T8 heat treatments. The thermo-mechanical T8 heat treatment was utilized to achieve uniformly dispersed S' precipitates. A certain number of T8 and T6 samples were further aged to induce an overaged structure. This overaging led to a lower fatigue strength compared to the peakaged counterparts. The fatigue behavior of a 2080-Al with four different SiC particle volume fractions (0, 10, 20, 30 vol.%) and two different particle sizes of @ 4 mm (FEPA grade F - 1000), @ 10 mm (FEPA grade F-600) was studied. At a given SiC particle volume fraction peakaged and overaged T6 conditions showed higher fatigue strengths than the T8 conditions despite coarser and more inhomogeneously distributed precipitates. TEM studies have been conducted in order to understand the effect of precipitate size and precipitate spacing on the fatigue behavior.

2:50 pm INVITED

EFFECT OF REINFORCEMENT ON CRACK INITIATION AND EARLY CRACK GROWTH OF A P/M PROCESSED Al-2080/SiC COMPOSITE: Christoph Andres*, J. Wayne Jones*, and John E. Allison+, *Department of Materials Science and Engineering, the University of Michigan, Ann Arbor, MI; +Ford Research Laboratory, Ford Motor Company, Dearborn, MI

The low cycle and high cycle fatigue crack initiation and small crack growth process of a particle reinforced aluminum matrix composite Al-2080/SiC has been evaluated. A controlled matrix microstructure was achieved by using a thermo-mechanical heat treatment (T8), which led to a homogeneous distribution of S'-precipitates. Four different SiC particle volume fractions (0, 10, 20, 30 vol.%) and three different particle sizes of @ 4 mm (FEPA grade F-1000), @ 10 mm (FEPA grade F-600) and @ 36 mm (FEPA grade F-280) have been investigated. The influence of these composite variables on fatigue crack initiation will be examined in detail. The experimental results show for a given loading condition, larger particle sizes, and higher volume fractions tend to favor particle-cracking. In many cases large iron-or silicon-rich intermetallic inclusions were responsible for crack nucleation. To understand the crack initiation and propagation behavior surface replication and SEM studies have been conducted and will be reviewed.

3:10 pm INVITED

HIGH TEMPERATURE CREEP OF AN Al-7005 COMPOSITE REINFORCED WITH Al₂O₃ PARTICULATES: Yong Li and Terence G. Langdon, Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453

A creep investigation was conducted over temperature from 573 to 773 K on an Al-7005 metal matrix composite reinforced with 20 volume per cent of Al₂O₃ particulates. The creep data, which extend over six orders of magnitude of strain rate, show curvatures in the logarithmic plots of strain rate versus stress, thereby indicating the existence of a threshold stress. By incorporation of a threshold stress into the analysis, it is shown that the true stress exponent of the composite is close to 4.4 and the true activation energy is close to ~ 120 kJ mol^-1: these values are in reasonable agreement with earlier reports for an Al-5 wt.% Zn alloy. Therefore, the results indicate that the creep mechanism in the Al7005 composite is essentially identical to that reported in dilute Al-Zn alloys.

3:30 pm BREAK

3:50 pm INVITED

INFLUENCE OF MICROSTRUCTURE ON THE HIGH CYCLE FATIGUE BEHAVIOR OF Al-BASED METAL MATRIX COMPOSITES: Don Lesuer, Chol Syn, T.G. Nieh, L-342, Lawrence Livermore National Laboratory, Livermore, CA 94551

It is generally recognized that the high cycle fatigue behavior of metal matrix composites (MMCs) can be superior to that of the matrix alloy. For composites in which defect-related failures can be avoided, these improved fatigue properties are related to the increases in strength and stiffness that are produced in the MMC. In this paper we explore the influence of microstructure on the high cycle fatigue response of particle-reinforced aluminum alloys. Experimental work has been performed to establish the cyclic stress - strain behavior and resulting stress - life response for a 6090/SiC/25p-T6 composite. The material exhibited fatigue properties that are not dominated by defects and thus approach the inherent fatigue response of the composite. The results have been compared to the fatigue behavior of other particle-reinforced aluminum alloys. This work has resulted in a quantitative understanding of the influence of microstructure on the high cycle fatigue response of particle reinforced metals.

4:10 pm

HIGH-TEMPERATURE RUPTURE OF PARTICULATE REINFORCED AND UNREINFORCED 2124 UNDER MULTIAXIAL STRESS STATES: Ahmadali Yousefiani, Farghalli A. Mohamed, James C. Earthman, Materials Science and Engineering, Dept. of Chemical and Biochemical Engineering, University of California, Irving, CA 92697

Creep rupture experiments were performed on 2124 Al under three different stress states (uniaxial tension, biaxial shear and triaxial tension) in both an unreinforced and a 10 vol.% SiC_p reinforced condition. Rupture times are compared for the three states with respect to the maximum principal stress, von Mises effective stress, and the principal facet stress. The results of this comparison along with microstructural observations regarding the cavitation behavior of the alloy are discussed in reference to the effect of the presence of the reinforcing particles on observed high temperature damage.

4:30 pm

CREEP BEHAVIOR OF A METAL MATRIX COMPOSITE: I.M. Daniel, Northwestern University, Evanston IL; H.J. Chun, Yonsei University, Seoul Korea

The creep characteristics of a silicon carbide/ aluminum (SiC/Al) unidirectional composite were measured under transverse tensile loading over a temperature range from 204°C (400°F) to 288°C (550°F). It was found that the minimum creep strain rate of the composite can be described by an Arrhenius type power law relation similar to the one used for the unreinforced matrix. This creep rate for the composite is less sensitive to stress amplitude and temperature than that of the matrix material. The increased creep resistance of the composite is attributed to redistribution of the stresses in the matrix and to matrix stress relaxation around the relatively rigid fibers. Creep tests were also conducted during thermal deformation cycling. The latter was found to increase creep significantly above that under isothermal conditions, even at temperatures equal to or higher than the peak cyclic temperature. The creep behavior was also analyzed by means of a micromechanical model based on the average field theory. The measured creep strains at various stress amplitudes and at various temperatures were in favorable agreement with predictions.

4:50 pm INVITED

FATIGUE CRACK GROWTH ALONG GRAPHITE/EPOXY INTERFACE: James Ryan, J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801

Effect of graphite surface-modification on fatigue crack growth resistance of graphite-epoxy interface was examined using flexural peel specimens. Edge surfaces of pyrolytic graphite were treated in an oxygen plasma for various times and subsequently bonded to a toughened epoxy to form flexural peel specimens. Fatigue crack growth rates were measured in the plasma-treated and untreated specimens as a function of strain energy release rate. Fatigue crack resistance of plasma-treated specimens was notably higher, with the fatigue threshold doubled at an optimal treatment time. X-ray photoelectron spectroscopy, electron microscopy and surface profilometry studies indicated that both chemistry and morphology of the graphite surface were changed by the plasma treatment. High temperature annealing was used to restore the original surface chemistry and fatigue experiments were then performed on annealed specimens to separate chemical and morphological effects. Chemical modification turned out to be secondary, contributing less than 30% to the overall improvement from the plasma treatment.