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SiC composite during monotonic tensile tests using synchrotron X-ray microtomography
ELSEVIER
Materials Science and Engineering A2344236 (1997) 633-635
Damage assessment in an Al/Sic composite during monotonic tensile tests using synchrotron X-ray microtomography J.-Y. Buffiere a$*, E. Maire a, C. Verdu a, P. Cloetens b, M. Pateyron c, G. Peix d, J. Baruchel b a GEMPPM
UMR
’ CREATIS d CNDRI
5510 INSA LYON, 20 b ESRF BP 220, INSA LYON, 20 AI;.A. INSA LYON, 20 AD. A.
Ao. A. Einstein, 69621 Villeurbanne Cedex, France 38043 Grenoble, France Einstein, 69621 Villeurbanne Cedex, France Einstein, 69621 Villeurbanne Cedex, France
Received 5 February 1997; received in revised form 1 April 1997
Abstract High resolution X-ray tomography is used to study the evolution of damage in an Al/Sic composite during monotonic tensile tests at room temperature. Two main damage mechanisms are observed when the plastic regime is reached: (i) The cracking of the matrix on brittle oxides resulting from the processing of the material; (ii) the cracking of Sic particles. The aspect ratio of the broken particles along the tensile direction is found to be high and the damage accumulation rate is different at the surface and in the bulk of the material. Those results are discussed with respect to the resolution of the imaging technique and to the strain level reached during the tests. 0 1997 Elsevier Science S.A. Keywords:
Metal matrix composite; X-ray computed tomography; Damage
1. Introduction The high intrinsic mechanical properties exhibited by metal matrix composites (MMC) make them very attractive materials in transport applications where a gain in weight is highly desirable. However, the poor fracture properties of those materials (e.g. fracture toughness and ductility) has greatly restricted, so far, their use in industrial applications. Therefore, in the last years, much attention has been paid, in the scientific literature, to the fracture process of MMC. In situ mechanical tests have been used by several authors [1,2] and provide very interesting information on the loss of ductility of MMC. However, the relevance of surface examination has to be checked by destructive observations within the material through time consuming techniques like serial sectioning [3] which are not free of artefacts such as cracks or voids blunting. X-ray high resolution computed tomography (HRCT), originally developed for medical applications, is for the moment * Corresponding author. Tel.: + 33 472438854; 472438539; e-mail: [email protected]
fax:
+ 33
0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PII SO921-5093(97)00302-X
the only non destructive method of characterisation which can provide three dimensional (3D) images of damage or of microstructural features within materials in the micrometer range [4-61. In the present paper, the damage mechanisms of a MMC deformed monotonically in tension have been studied by HRCT and scanning electron microscopy (SEM); for the first time, a quantitative comparison of the damage mechanisms observed at the surface and those observed in the bulk, by a non destructive method, is presented.
2. Experimental
methods
The HRCT experiments were conducted at the European Synchrotron Radiation Facility in Grenoble on line ID19. Differential absorption between the constituents (i.e. Al and SIC) is the usual source of contrast used in classical HRCT. In the present paper, we used the so called ‘phase contrast’ technique which greatly improved the detection of SIC particles as well as the detection of cracks in the material. More details about this technique can be found elsewhere [7,8]. The voxel
634
J.-Y.
Buf$ere
et al. /Materials
Science
size in the reconstructed volume, fixed by the experimental setting of the detector, was about 6.5 x 6.5 x 6.5 um3. Given the resolution of the detecting device, a model composite with large SIC particles, has been chosen for the study. It consisted in a 6061 aluminium alloy reinforced with 12 vol.% SIC particles with a mean size of 120 urn; it has been elaborated by a rheocasting route and subsequently submitted to a standard T4 heat treatment. Double shouldered tensile samples similar to those used for SEM in situ tensile tests [l] were used. The cross-section of those samples was 1.58 x 1.47 mm2 and their gage length was 5 mm. Before testing, the samples were mechanically grinded using Sic paper and diamond paste down to 1 urn. The tensile tests were conducted at room temperature using a constant crosshead displacement rate of 150 ym min ~ ‘. A total of four tomographic scans were performed corresponding to the initial undeformed state and to three consecutive steps at increasing values of the plastic deformation. These steps will subsequently be referred as steps 0, 1, 2 and 3. For each scan, the sample was removed from the tensile testing device. All observations were consequently made in the unloaded state. In parallel, a detailed characterisation of the material surface has been performed in a SEM in the initial state and after step 3. The total size of the studied reconstructed volume was 1.58 x 1.47 x 3.32 mm3.
3. Results At the surface of the sample a few SIC particles were observed to have cracked during the test. However, for the experimental conditions investigated, the dominant damage mechanism appeared to be the cracking of the matrix. In the bulk of the sample, the same damage mechanisms were detected. Fig. la, b and c show the reconstructed images of several cracks in the matrix at three increasing values of the depth: 0, 40, and 80 urn, respectively. Some cracks, appearing as bright fringes on the reconstructed images for the specimen/detector distance used, were also clearly detected in the SIC particles. They appeared perpendicularly to the stress direction as illustrated in Fig. 2a and b which shows a reconstructed section in the bulk of the composite at steps 1 and 3, respectively. Careful comparison between SEM observations and tomographic images of the surfaces revealed that, in the SIC particles, cracks with an opening of less than 1 urn could be detected thanks to the phase contrast technique [8]. A systematic quantitative study of the SEM micrographs and of the reconstructed images has been undertaken. 1550 particles were detected and included in the analysis, 384 of them being considered as surface parti-
and Engineering
A234-236
(1997)
633-635
Fig. 1. Tomographic images of the composite showing the shape two matrix cracks (arrows A and B) within the sample.
of
cles. The dimensions of each broken particle has been measured along the x, 4’ and z axis defined in Fig. 2. The corresponding average aspect ratios y/x, V/Z and Z/X have been measured and found equal to 2.28 f 0.98, 1.47 f 0.8 and 1.79 + 0.9, respectively. The evolution with strain of the respective number of broken particles at the surface and in the bulk was also determined. For the two first deformation steps (1 and 2) the number of broken particles at the surface slightly increased and remained higher than in the bulk where almost no cracked particles were detected. After step 3, the number of broken particles increased both at the surface and in the bulk, but this increase was more rapid in the bulk. This difference in the rate of damage accumulation is clearly illustrated by the final slope of the curves in Fig. 3 which represent the evolution of the fraction of newly broken particles (i.e. those broken because of the deformation of the sample) with the applied stress. It can also be seen, from that figure, that the final fraction of newly broken particles is finally higher in the bulk than at the surface.
Fig. 2. Tomographic images of the composite (a) after step 1 and (b) after step 3. The arrow indicates a crack inside a Sic particle detected in the volume of the material and which has been induced by the plastic deformation of the composite.
J.-Y. Buffiere et al. /Materials
50
Science and Engineering A234-236
100
Stress (MPa) Fig. 3. Evolution of the fraction of broken Sic particles (because of the applied stress) in the volume and at the surface of the composite as a function of the stress level reached for each step.
(1997) 633-635
635
Fig. 3 up to a normal value of the rupture stress/strain for this kind of material, suggests that mere surface observations might lead to a rather large under estimation of damage for this type of material. One shall keep in mind that even if very fine cracks can be detected by the phase contrast technique [8] there might be some undetected cracks in the bulk. Hence, the number of broken particles in the volume determined in this study should be considered as a lower bound of the real value. The use of an in situ tensile testing device, currently under development, allowing tomographic images to be recorded under load should help to partly overcome this problem.
4. Discussion 5. Conclusion The early cracking of the matrix observed at the surface of the sample and in the bulk is due to the presence, within the material, of brittle oxides which stem from the rheocasting process conducted in air. Therefore, the matrix cracks should not be regarded as a typical damage mechanism of MMC but rather as an artefact resulting from the way the material was processed.Hence, in what follows, the discussion will focus on the breaking of the Sic particles. Several authors have shown, on the basis of surface observations, that particles elongated in the direction of the stress tend to break more easily [1,3] in relation with the higher stressesdeveloped in these particles. Our results tend to corroborate this trend for particles located in the volume of the sample (aspect ratios y/x and y/z greater than 1). An automatic image analysis procedure for the segmentation in three dimensions of the Sic particles is currently under development. It will provide a statistical description of the distribution of the characteristics of all the particles which will permit a comparison with the average size and aspect ratios of the broken particles. The initial number of broken particles at the surface (step 0) was found different of zero, indicating that a fraction of these particles are pre-damaged during the preparation of the sample. However, it can be seen from the final slopes of the curves shown on Fig. 3 that, once a sufficient stress level has been attained in the sample, the damage accumulation rate is higher in the bulk than at the surface leading eventually to a higher fraction of broken particles in the bulk. This result is in agreement with destructive bulk/surface comparisons made by other authors [9]. From a theoretical point of view, indeed, the cracking of SIC particles is likely to be easier in the bulk than at the surface where part of the elastic energy induced by the loading can be relaxed. In our case, unfortunately, the test ended prematurely because of the presence of large cracks in the matrix. However, an extrapolation of the two curves shown on
HRCT observations show that the damage mechanisms occurring in an aluminium matrix composite during monotonic tensile tests do not differ qualitatively from those observed at the sample surface by SEM. Quantitative analysis reveals that both at the surface and in the bulk, the broken SIC particles are elongated along the stress direction. Besides, for the experimental conditions investigated, the fraction of broken SIC particles appears to be larger in the volume of the sample than at the surface.
Acknowledgements The authors wish to acknowledge Luc Salvo of the GPM2 laboratory in Grenoble for the elaboration of the samplesand B. Barbier of the CREATIS laboratory for reconstructing the volumes.
References [1] E. Maire, C. Verdu, G. Lormand, R. Fougeres, Mater. Sci. Eng. Al96 (1995) 135. [2] L. Manoharan, J.J. Lewandowski, Ser. Metall. 23 (1989) 1801. [3] J. Llorca, A. Martin, J. Ruiz, M. El&s, Metal]. Trans. A 24 (1993) 1575. [4] J.H. Kinney, S.R. Stock, M.C. Nichols, U. Bonse, T.M. Breunig, R.A. Saroyan, R. Nusshardt, Q.C. Johnson, F. Busch, SD. Antolovich, J. Mater. Res. 5 (1990) 1123. [5] T. Hirano, K. Usami, Y. Tanaka, C. Masuda, J. Mater. Res. IO (1995) 381. [6] P.M. Mummery, P. Anderson, G.R. Davis, B. Derby, J.C. Elliott, Ser. Metal]. 29 (1993) 1457. [7] P. Cloetens, R. Barrett, J. Baruchel, J.P. Guigay, M. Schlenker, J. Phys. D: Appl. Phys. 29 (1996) 133. [8] P. Cloetens, M. Pateyron Salome, J.Y. Buffitre, G. Peix, J. Barruchel, F. Peyrin, M. Schlenker, J. Appl. Phys. 81 (9) (1997) 5878. [9] Y. Zong, B. Derby, J. Phys. IV. Colloque C7, supplement J. Phys. III, 3 (1993) 1861.
Materials Science and Engineering A2344236 (1997) 633-635
Damage assessment in an Al/Sic composite during monotonic tensile tests using synchrotron X-ray microtomography J.-Y. Buffiere a$*, E. Maire a, C. Verdu a, P. Cloetens b, M. Pateyron c, G. Peix d, J. Baruchel b a GEMPPM
UMR
’ CREATIS d CNDRI
5510 INSA LYON, 20 b ESRF BP 220, INSA LYON, 20 AI;.A. INSA LYON, 20 AD. A.
Ao. A. Einstein, 69621 Villeurbanne Cedex, France 38043 Grenoble, France Einstein, 69621 Villeurbanne Cedex, France Einstein, 69621 Villeurbanne Cedex, France
Received 5 February 1997; received in revised form 1 April 1997
Abstract High resolution X-ray tomography is used to study the evolution of damage in an Al/Sic composite during monotonic tensile tests at room temperature. Two main damage mechanisms are observed when the plastic regime is reached: (i) The cracking of the matrix on brittle oxides resulting from the processing of the material; (ii) the cracking of Sic particles. The aspect ratio of the broken particles along the tensile direction is found to be high and the damage accumulation rate is different at the surface and in the bulk of the material. Those results are discussed with respect to the resolution of the imaging technique and to the strain level reached during the tests. 0 1997 Elsevier Science S.A. Keywords:
Metal matrix composite; X-ray computed tomography; Damage
1. Introduction The high intrinsic mechanical properties exhibited by metal matrix composites (MMC) make them very attractive materials in transport applications where a gain in weight is highly desirable. However, the poor fracture properties of those materials (e.g. fracture toughness and ductility) has greatly restricted, so far, their use in industrial applications. Therefore, in the last years, much attention has been paid, in the scientific literature, to the fracture process of MMC. In situ mechanical tests have been used by several authors [1,2] and provide very interesting information on the loss of ductility of MMC. However, the relevance of surface examination has to be checked by destructive observations within the material through time consuming techniques like serial sectioning [3] which are not free of artefacts such as cracks or voids blunting. X-ray high resolution computed tomography (HRCT), originally developed for medical applications, is for the moment * Corresponding author. Tel.: + 33 472438854; 472438539; e-mail: [email protected]
fax:
+ 33
0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PII SO921-5093(97)00302-X
the only non destructive method of characterisation which can provide three dimensional (3D) images of damage or of microstructural features within materials in the micrometer range [4-61. In the present paper, the damage mechanisms of a MMC deformed monotonically in tension have been studied by HRCT and scanning electron microscopy (SEM); for the first time, a quantitative comparison of the damage mechanisms observed at the surface and those observed in the bulk, by a non destructive method, is presented.
2. Experimental
methods
The HRCT experiments were conducted at the European Synchrotron Radiation Facility in Grenoble on line ID19. Differential absorption between the constituents (i.e. Al and SIC) is the usual source of contrast used in classical HRCT. In the present paper, we used the so called ‘phase contrast’ technique which greatly improved the detection of SIC particles as well as the detection of cracks in the material. More details about this technique can be found elsewhere [7,8]. The voxel
634
J.-Y.
Buf$ere
et al. /Materials
Science
size in the reconstructed volume, fixed by the experimental setting of the detector, was about 6.5 x 6.5 x 6.5 um3. Given the resolution of the detecting device, a model composite with large SIC particles, has been chosen for the study. It consisted in a 6061 aluminium alloy reinforced with 12 vol.% SIC particles with a mean size of 120 urn; it has been elaborated by a rheocasting route and subsequently submitted to a standard T4 heat treatment. Double shouldered tensile samples similar to those used for SEM in situ tensile tests [l] were used. The cross-section of those samples was 1.58 x 1.47 mm2 and their gage length was 5 mm. Before testing, the samples were mechanically grinded using Sic paper and diamond paste down to 1 urn. The tensile tests were conducted at room temperature using a constant crosshead displacement rate of 150 ym min ~ ‘. A total of four tomographic scans were performed corresponding to the initial undeformed state and to three consecutive steps at increasing values of the plastic deformation. These steps will subsequently be referred as steps 0, 1, 2 and 3. For each scan, the sample was removed from the tensile testing device. All observations were consequently made in the unloaded state. In parallel, a detailed characterisation of the material surface has been performed in a SEM in the initial state and after step 3. The total size of the studied reconstructed volume was 1.58 x 1.47 x 3.32 mm3.
3. Results At the surface of the sample a few SIC particles were observed to have cracked during the test. However, for the experimental conditions investigated, the dominant damage mechanism appeared to be the cracking of the matrix. In the bulk of the sample, the same damage mechanisms were detected. Fig. la, b and c show the reconstructed images of several cracks in the matrix at three increasing values of the depth: 0, 40, and 80 urn, respectively. Some cracks, appearing as bright fringes on the reconstructed images for the specimen/detector distance used, were also clearly detected in the SIC particles. They appeared perpendicularly to the stress direction as illustrated in Fig. 2a and b which shows a reconstructed section in the bulk of the composite at steps 1 and 3, respectively. Careful comparison between SEM observations and tomographic images of the surfaces revealed that, in the SIC particles, cracks with an opening of less than 1 urn could be detected thanks to the phase contrast technique [8]. A systematic quantitative study of the SEM micrographs and of the reconstructed images has been undertaken. 1550 particles were detected and included in the analysis, 384 of them being considered as surface parti-
and Engineering
A234-236
(1997)
633-635
Fig. 1. Tomographic images of the composite showing the shape two matrix cracks (arrows A and B) within the sample.
of
cles. The dimensions of each broken particle has been measured along the x, 4’ and z axis defined in Fig. 2. The corresponding average aspect ratios y/x, V/Z and Z/X have been measured and found equal to 2.28 f 0.98, 1.47 f 0.8 and 1.79 + 0.9, respectively. The evolution with strain of the respective number of broken particles at the surface and in the bulk was also determined. For the two first deformation steps (1 and 2) the number of broken particles at the surface slightly increased and remained higher than in the bulk where almost no cracked particles were detected. After step 3, the number of broken particles increased both at the surface and in the bulk, but this increase was more rapid in the bulk. This difference in the rate of damage accumulation is clearly illustrated by the final slope of the curves in Fig. 3 which represent the evolution of the fraction of newly broken particles (i.e. those broken because of the deformation of the sample) with the applied stress. It can also be seen, from that figure, that the final fraction of newly broken particles is finally higher in the bulk than at the surface.
Fig. 2. Tomographic images of the composite (a) after step 1 and (b) after step 3. The arrow indicates a crack inside a Sic particle detected in the volume of the material and which has been induced by the plastic deformation of the composite.
J.-Y. Buffiere et al. /Materials
50
Science and Engineering A234-236
100
Stress (MPa) Fig. 3. Evolution of the fraction of broken Sic particles (because of the applied stress) in the volume and at the surface of the composite as a function of the stress level reached for each step.
(1997) 633-635
635
Fig. 3 up to a normal value of the rupture stress/strain for this kind of material, suggests that mere surface observations might lead to a rather large under estimation of damage for this type of material. One shall keep in mind that even if very fine cracks can be detected by the phase contrast technique [8] there might be some undetected cracks in the bulk. Hence, the number of broken particles in the volume determined in this study should be considered as a lower bound of the real value. The use of an in situ tensile testing device, currently under development, allowing tomographic images to be recorded under load should help to partly overcome this problem.
4. Discussion 5. Conclusion The early cracking of the matrix observed at the surface of the sample and in the bulk is due to the presence, within the material, of brittle oxides which stem from the rheocasting process conducted in air. Therefore, the matrix cracks should not be regarded as a typical damage mechanism of MMC but rather as an artefact resulting from the way the material was processed.Hence, in what follows, the discussion will focus on the breaking of the Sic particles. Several authors have shown, on the basis of surface observations, that particles elongated in the direction of the stress tend to break more easily [1,3] in relation with the higher stressesdeveloped in these particles. Our results tend to corroborate this trend for particles located in the volume of the sample (aspect ratios y/x and y/z greater than 1). An automatic image analysis procedure for the segmentation in three dimensions of the Sic particles is currently under development. It will provide a statistical description of the distribution of the characteristics of all the particles which will permit a comparison with the average size and aspect ratios of the broken particles. The initial number of broken particles at the surface (step 0) was found different of zero, indicating that a fraction of these particles are pre-damaged during the preparation of the sample. However, it can be seen from the final slopes of the curves shown on Fig. 3 that, once a sufficient stress level has been attained in the sample, the damage accumulation rate is higher in the bulk than at the surface leading eventually to a higher fraction of broken particles in the bulk. This result is in agreement with destructive bulk/surface comparisons made by other authors [9]. From a theoretical point of view, indeed, the cracking of SIC particles is likely to be easier in the bulk than at the surface where part of the elastic energy induced by the loading can be relaxed. In our case, unfortunately, the test ended prematurely because of the presence of large cracks in the matrix. However, an extrapolation of the two curves shown on
HRCT observations show that the damage mechanisms occurring in an aluminium matrix composite during monotonic tensile tests do not differ qualitatively from those observed at the sample surface by SEM. Quantitative analysis reveals that both at the surface and in the bulk, the broken SIC particles are elongated along the stress direction. Besides, for the experimental conditions investigated, the fraction of broken SIC particles appears to be larger in the volume of the sample than at the surface.
Acknowledgements The authors wish to acknowledge Luc Salvo of the GPM2 laboratory in Grenoble for the elaboration of the samplesand B. Barbier of the CREATIS laboratory for reconstructing the volumes.
References [1] E. Maire, C. Verdu, G. Lormand, R. Fougeres, Mater. Sci. Eng. Al96 (1995) 135. [2] L. Manoharan, J.J. Lewandowski, Ser. Metall. 23 (1989) 1801. [3] J. Llorca, A. Martin, J. Ruiz, M. El&s, Metal]. Trans. A 24 (1993) 1575. [4] J.H. Kinney, S.R. Stock, M.C. Nichols, U. Bonse, T.M. Breunig, R.A. Saroyan, R. Nusshardt, Q.C. Johnson, F. Busch, SD. Antolovich, J. Mater. Res. 5 (1990) 1123. [5] T. Hirano, K. Usami, Y. Tanaka, C. Masuda, J. Mater. Res. IO (1995) 381. [6] P.M. Mummery, P. Anderson, G.R. Davis, B. Derby, J.C. Elliott, Ser. Metal]. 29 (1993) 1457. [7] P. Cloetens, R. Barrett, J. Baruchel, J.P. Guigay, M. Schlenker, J. Phys. D: Appl. Phys. 29 (1996) 133. [8] P. Cloetens, M. Pateyron Salome, J.Y. Buffitre, G. Peix, J. Barruchel, F. Peyrin, M. Schlenker, J. Appl. Phys. 81 (9) (1997) 5878. [9] Y. Zong, B. Derby, J. Phys. IV. Colloque C7, supplement J. Phys. III, 3 (1993) 1861.