DAMAGE EVOLUTION IN Ti-SiC UNIDIRECTIONAL FIBER COMPOSITES

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1 Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN 97- DAMAGE EVOLUTION IN Ti-SiC UNIDIRECTIONAL FIBER COMPOSITES Jay C. Hanan, Geoffrey A. Swift, Ersan Üstündag, Irene J. Beyerlein, Bjørn Clausen, Jonathan D. Aler, Ulrich Lienert and Dean R. Haeffner () Departent of Materials Science, California Institute of Technology, Pasadena, CA 95 () Materials Science and Technology Div., Los Alaos National Lab., Los Alaos, NM () Advanced Photon Source, Argonne National Laboratory, Argonne, IL 649 ABSTRACT Fiber fractures in etal-atrix coposites often initiate daage zones that grow until the coposite fails. To better understand the evolution of such daage fro a icroechanics point of view, a odel Ti-atrix/SiC-fiber coposite was studied for the first tie. Using high energy X-rays and a sall sapling volue, the daage zone around a broken fiber was investigated. The growth of this zone was onitored in situ under applied tensile stress by easuring the responses of the fibers and the atrix. The diffraction data was copared to a odified shear lag odel, which considers the elastic response of the atrix and the fibers. A coparison of the odel and data shows a correlation on the trend of the data, while the nature of the daage region incurs discrepancies. Results indicate a need for further refineent of the odel, revealing the necessity of incorporating such factors as residual stress in the syste and plasticity in the atrix. INTRODUCTION The transfer of load fro a broken fiber to the rest of a fiber-reinforced coposite is one of the fundaental icroechanical processes deterining strength. In order to predict strength, one needs to understand the details of this load transfer. In-situ easureents of strain can help validate and refine predictive odeling of strength in fiber coposites. This study used synchrotron X-rays to investigate, for the first tie, the evolution of in-situ strains in a daaged Ti-atrix/SiC-fiber coposite. The diffraction results were copared to predictions fro a icroechanics odel []. This odel accounts for the linear elastic co-deforation of fiber and atrix in unidirectional fiber coposites containing any configuration of ultiple fractures. EXPERIMENTAL PROCEDURE The studied coposite syste corresponds to the geoetry presented in the echanics odel described in ref. []. It consisted of a single row of unidirectional SiC fibers (SCS-6, 4 µ diaeter) in a Ti-6Al-4V atrix, prepared by a proprietary technique at M Corp. (St. Paul, MN 5544). Fig. shows the scheatic of a typical specien. The fibers are uniforly spaced with an average center-to-center distance of about 4 µ. The area fraction of the fibers was %. On the specien exained, a sall region of the atrix was reoved via acid etching (5% HF aqueous solution) to expose the SiC fibers. One fiber was subsequently broken in the exposed region, denoted fiber. The atrix was left intact around and behind the exposed region. A strain gage was attached easuring the applied acroscopic longitudinal strain (parallel to fibers) as well as that in the transverse direction (Fig. ).

2 ISSN 97- This docuent was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This docuent is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific counity. All copyrights for the docuent are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN 97- The saples were exained using 5 kev X-rays (wavelength, λ =.496 Å) at the -ID-C bea line, Advanced Photon Source. At this energy, the transitted bea intensity is about 58% of the incident bea intensity. For an exponential decay in the transitted bea intensity fro the saple surface inward, the center of gravity of the sapling volue was calculated to be 9 µ fro the surface facing the incoing bea. Therefore, these easureents are representative of the entire thickness of the saple. A four-circle gonioeter was used. The diffraction vector was along the fiber axis yielding longitudinal strain in the plane of the coposite. The diffraction data was collected with a scintillator detector equipped with a Si () analyzer crystal. The X-ray bea size was defined by slits. A Si diode continuously onitored the transitted bea intensity. First, the location of the intact buried fibers iediately around the daage region was deterined using absorption contrast and a x µ bea size. Fig. shows the daage region and the location of the fibers around it. A reference spot on the saple was used to correct for shifts during loading. The greatest source of shifts cae fro the saple slipping at the grip region. A second calibration of position for each data point was the local absorption contrast. By translating the saple in the y direction, the position of the bea with respect to the center of the fiber was readily observed. Prior to each strain ap, in order to assure that the fiber axis was aligned with the x-axis, this calibration procedure was perfored at both extree x positions of the region of interest. Grip Matrix Fibers Strain Gage Bea Exposed Fiber Region W =.5 σ,ε σ,ε Thickness, t =. L = 6. Figure. Scheatic showing the saple geoetry. Fibers are represented by black lines between a gray atrix (illustration only not to scale). To deterine the effect of the broken fibers on the neighboring fibers and atrix regions, the daaged saple was scanned with a 9 x 9 µ bea (Fig. ). Fits to individual reflections were perfored using the ethod of least squares assuing a Lorentzian peak profile for each phase. The three nearest fibers adjacent to the broken fibers (no., -, and -), the broken fibers (no. and, the latter broke during the experient), and the intervening atrix regions were scanned along fiber axes for a distance of fiber diaeters in each direction away fro the break in 8 µ steps. Additionally, at.89 fro the break, one Figure. Absorption contrast iage of the daage region. The darker a region the higher the absorption. The daage region is evidenced by the bright region near the center of the iage. The periodic change in intensity along y corresponds to the position of SiC fibers in the atrix. Soe iportant fibers are labeled.

4 Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN 97- fiber and its adjacent atrix region were scanned at each load to obtain a easure of the in-situ applied far-field strain in the saple. Relative changes in the elastic lattice strains in the atrix and fibers were obtained by onitoring one reflection fro the ajority phases in each: ( ) fro α-ti and () fro β-sic. To reduce experiental errors, especially those due to specien displaceent, Si powder (NIST, Standard Reference Material 64a) was attached to the specien as an internal standard. Displaceent errors for each applied stress, which averaged about µε, were subtracted fro the easured strains. In addition, noinally stress-free references of Ti and SiC were scanned. The XRD data fro the references allowed the deterination of the absolute values of strains in each coponent. The references were obtained by etching away one surface of the coposite dissolving the atrix with a 5% HF acid aqueous solution to expose the fibers, which then easily separated because of theral residual stresses. The Si standard powder was also placed on the surface of these reference saples. The coposite was stressed in tension using a custo-built load frae. A load cell on the frae was connected to a coputer so that the applied load could be recorded siultaneously with the strain gage strains. The loading data was synchronized with the diffraction data. RESULTS AND DISCUSSION The transitted bea intensity shows alignent of the fibers with the x-axis and clearly reveals the region of the atrix etched by HF (Fig. ). Along with collecting the transitted bea intensity, the intensity of α-ti ( ) reflection was onitored using the θ detector set at.5. The resulting plot gives the position of grains in the saple suitably oriented for diffraction (Fig. ). As shown, the α-ti ( ) ap is highly discontinuous due to the large grain size found in the atrix (~9 µ). Siilarly, an intensity ap was collected fro the atrix using the β SiC () reflection (Fig. 4, this data was collected siultaneously with the data for Fig. ). The nearly continuous nature of the SiC reflection reveals the drastic difference in grain size for the two phases (less than µ for the fibers and about 9 µ for the atrix) [,]. The residual theral strains were easured around the daage region under no applied load. The reoval of the atrix around this area significantly relaxed the longitudinal residual strains in the fibers. The residual strains given by the far-field fiber ( 5 µε) approach the values at each extree position easured along the fibers. Within the error of the easureent, these strains also agree with the bulk residual Fibers Ti Daage Zone Figure. Map of α-ti ( ) reflection indicating the location of diffracting Ti grains. With a grain size of ~9 µ, few grains are oriented for diffraction at a given θ angle.

5 f f f f Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN 97- strains and those already quoted in literature [,4]. Since the odel [] does not consider the residual strains, the residual strain data was subtracted fro the total easured strains when stress was applied. Therefore, only the relative strains were used in the coparison of experiental data with odel predictions. Fig. 5 exhibits the odel coparison with experiental data fro the fibers. The fiber strains were noralized with respect to the far-field value. The diensionless length [] is given by = (.4 µ - ) x, where x is position along the fiber. It becae Figure 4. Map of β-sic () reflection indicating the location of the buried fibers. The oval outlines the daage region. obvious fro the strain data [] that two fibers were broken (no. and ) and this assuption was included in the odel. This odel also assues the fibers are behaving as if the atrix is intact adjacent to the broken fibers. Overall, a reasonably good agreeent is observed between the data. a).6.4 b) Fiber Fiber Fiber c) Fiber d) Fiber - Figure 5. Coparison of strains fro odel predictions (designated by lines) and XRD data fro fibers (sybols). The applied tensile stress varied between 4 and 4 MPa. Strains were noralized with respect to the applied far field value, ε f = 7 to 6 µε. A typical error bar is shown beneath the data in graph (d).

6 Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN a).5 b) c).5 d) Figure 6. Coparison of strains fro odel predictions and XRD data for the atrix in various regions: a) the region between the two broken fibers (no. and ), b) the atrix region between an intact and broken fiber (no. and ), c) the atrix region between two intact fibers (no. and ), and d) also a region between an intact and broken fiber (no. and ). The applied tensile stress varied between 45 and 4 MPa. Strains were noralized with respect to the applied far field value, ε = 4 to 4 µε. The error bars denote the 95% confidence liits for the center position of the peak. and odel predictions. However, a higher data density would have iproved the coparison between the two around the daage zone. Note that the larger deviation in the strain profile along fiber no. is likely due to the width of initial daage in the fiber. Fig. 6 copares the odel predictions with easured atrix strains. While experiental errors of less than µε were found in the fibers, strain uncertainties of µε to greater than 7 µε were observed in the atrix. In addition, not every location had diffracting atrix grains leading to a lower data density copared to fibers. This results fro the relatively large grain size of the atrix copared to the sapling volue (the graininess proble). That eans, at a given location, only a grain or two is likely contributing to the intensity of the ( ) reflection. As a result, the ( ) intensity varies treendously accopanied with error in peak position. Furtherore, the strain values fluctuate due to intergranular effects. All of these observations confir the difficulty of perforing strain easureents on a scale coparable to grain size. The experiental difficulties notwithstanding, the strain data fro the atrix qualitatively agrees with odel predictions. Its ability to collect strain data fro both the fibers and the atrix provides XRD with an advantage copared to optical ethods such as Raan and piezospectroscopy, ethods that can usually investigate only the fibers (see e.g., [5]). With XRD it is possible to obtain a ore coplete description of the deforation in a fiber coposite at length scales approaching those possible with optical ethods.

7 Copyright (c)jcpds-international Centre for Diffraction Data, Advances in X-ray Analysis, Volue ISSN 97- Several shortcoings of the icroechanics odel were noted in this study. First, as entioned above, it does not account for residual stresses/strains. Although the initial residual strains around the daage zone were easured and subtracted fro the total strains under applied stress, it was iplicitly assued that these residual strains did not vary during loading. This assuption could be invalidated by any plastic deforation in the atrix. The acroscopic echanical behavior of both the atrix and fibers was independently deterined with XRD using a large sapling volue [] and it was seen that global plasticity in the atrix did not coence below 5 MPa applied coposite stress. This property would lower the probability of plastic deforation around the daage zone, but plastic deforation ight still occur since soe stress concentration was expected near the daage region. Another proble with the odel was that it could not account for the half-reoved atrix in the daage region. To eliinate these uncertainties, experients were perfored recently using an area detector [6]. For the new experients, the step size was reduced to increase the data density and a well-defined daage zone was used in the for of a hole. This data is currently being analyzed while the icroechanics odel is being iproved. The results will be reported in a future publication. SUMMARY As an initial investigation of the icroechanical behavior of a odel Ti-SiC coposite using synchrotron radiation, notable results were obtained. The behavior of the coposite under tensile loading was studied in both the fibers and the atrix, and the strains were copared to a odified shear-lag odel. Considering the coplicated residual strain profile, the odel predictions are well within reason. While the odel and data overall agree, there are a few instances where the assuptions break down and the predictions are inconsistent with the data. In the narrow region near the break where uch activity is predicted, higher spatial resolution is necessary to better characterize the in-situ behavior. Nevertheless, this work has shown that the general behavior predicted by the odel is evidenced in both the fiber and atrix of the coposite. ACKNOWLEDGMENTS The authors are grateful to Dr. H. Deve at M Co. for providing the speciens and helpful discussions about the properties of the coposites. This study was supported by the National Science Foundation (CAREER grant no. DMR ) at Caltech and a Laboratory-Directed Research and Developent Project (no. 4) at Los Alaos. The work at the Advanced Photon Source was supported by the U.S. Departent of Energy, Office of Basic Energy Sciences under contract no. W--9-ENG-8. REFERENCES. I.J. Beyerlein and C.M. Landis, Mechanics of Materials,, -5 (999).. J.C. Hanan, G.A. Swift, E. Üstündag, I.J. Beyerlein, B. Clausen, J.D. Aler, U. Lienert and D.R. Haeffner, subitted to Metall. Mater. Trans. A ().. J.C. Hanan, E. Üstündag, I.C. Noyan, D.R. Haeffner and P.L. Lee, Adv. X-Ray Anal., 44 (). 4. P.J. Withers and A.P. Clarke, Acta Mater., 46 (8), (998). 5. J. He, I.J. Beyerlein and D.R. Clarke, J. Mech. Phys. Solids, 47, (999). 6. J.C. Hanan, G.A. Swift, E. Üstündag, I.J. Beyerlein, U. Lienert and D.R. Haeffner, unpublished results ().