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Al2O3 particulate composites by instrumented impact
Composites Science and Technology 46 (1993) 287-292
!
FRACTURE TOUGHNESS E V A L U A T I O N OF 2040-AI/A1203 PARTICULATE COMPOSITES BY INSTRUMENTED IMPACT M. K. Surappa & P. Sivakumar Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India (Received 17 July 1991; revised version received 23 January 1992; accepted 10 March 1992)
strength. ~ Toughness values of these composites depend on the technique of measurement 2 and also on material variables including fibre volume fraction,a-5 size 3'4'6 and shape 7 of reinforcement, matrix alloy composition 3'8 and the method employed for the fabrication of composites. 2 The appropriate method for the evaluation of toughness of metal matrix composites (MMCs) is still being debated. 9'1° One of the methods used for the measurement of toughness is instrumented impact testing. By means of this technique it is possible to measure both fracture energy, E, and dynamic fracture toughness, Kxd, values. A literature survey indicates that little work has been done on the instrumented impact testing of discontinuous fibre reinforced aluminium matrix composites, except in a few cases. Furthermore, no work has been done on the impact toughness of aluminium alloy composites reinforced with alumina particles. Hence, in the present investigation the authors report results of limited studies on the evaluation of the toughness of 2024-A1/10 wt%-A1203 composites and its variation with temperature in the range -196 to 100°C.
Abstract
Composites consisting of an aluminium alloy (2024A1) reinforced with 4 l*m sized A1203 particles were prepared by the casting route. Fracture energy, E, and dynamic fracture toughness, Kid , values for composites and unreinforced alloys were evaluated by means of an instrumented impact test at temperatures varying from -196 to 100°C. The magnitude of the observed reduction is greater in the case of the fracture energy values than for the fracture toughness values as a result of the alumina additions. Furthermore, the dynamic fracture toughness of both composites and unreinforced alloys shows a negative dependence on temperature. By contrast, fracture energy values for the composites decrease with increase in temperature, whereas the fracture energy for the unreinforced alloy increases with increase in temperature. SEM examination of fracture surfaces of composites reveals multiple cracking of alumina particles. Keywords: aluminium matrix composites, 2024AIA1203 composites, instrumented impact-test, dynamic fracture toughness, temperature dependance of fracture energy
2 EXPERIMENTAL PROCEDURE 1 INTRODUCTION
Composites of a 2024 aluminium alloy, of the composition shown in Table 1, reinforced with 10 wt% of AI20 3 particles of 4/,m average size, were prepared by the ingot metallurgy route. Cylindrical billets of the composites were extruded into 15 mm 2 sections, the extrusion ratio being 8.72. Unreinforced alloy was also extruded under similar conditions for comparison purposes. From these extruded sections, 10 mm 2 Charpy V-notch specimens (with 45 ° included angle) were machined. Impact tests were carded out according to the ASTM E23 method in a standard Dynatup instrumented impact unit. Tests were carried out at -196, 24 and 100°C. In the case of the low temperature tests, specimens were kept in liquid nitrogen for 30 min prior to the tests. The time lapse between removal of specimen from liquid nitrogen
Evaluation of static and dynamic properties of aluminium matrix composites reinforced with discontinuous reinforcements such as particles, whiskers or short fibres of SiC or A1203 is being pursued with great interest worldwide. In general, both ambientand elevated-temperature tensile properties of aluminium alloy composites reinforced with ceramic particles (A1203 or SiC) are superior to those of unreinforced alloys. However, these composites are generally reported to suffer from poor fracture toughness and other dynamic properties including fatigue crack growth rate (FCGR) and fatigue
Composites Science and Technology 0266-3538/93/$06.00 © 1993 Elsevier Science Publishers Ltd. 287
288
M. K. Surappa, P. Sivakumar Table 1. Composition of 2024 Ai alloy Elements (wt%) Cu
Mg
Mn
Si
Cr
4.42
0.4
0 . 7 4 0.17 <0.1
AI Remainder
and the test was less than 2 s and a similar procedure was adopted for the tests at boiling water temperature (100°C). At each test temperature, impact tests were carried out on duplicate specimens in the case of unreinforced alloy and on at least three specimens in the case of composites. Fracture surfaces were examined by means of scanning electron microscopy (SEM). 3 RESULTS AND DISCUSSION 3.1 Microstmcture The microstructure of an as-cast 2024-Al/10wt%A1203 composite is shown in Fig. la. It is evident that alumina particles are segregated to interdendritic regions. Similar distributions have been reported in cast 2024/SIC and 2024/A1203 particulate composites. H The distribution of alumina in the extruded composite, as shown in Fig. lb, is considerably better. 3.2 Load/energy versus time plots Figure 2a shows load and energy versus time plots for an extruded 2024 aluminium alloy specimen obtained from an instrumented impact test at room temperature. The figure reveals the ductile mode of failure. Load and energy versus time plots (Fig. 2(b)) for the extruded 2024/10wt% Al203 composites tested at room temperature reveal semi-brittle fracture behaviour. It is evident that the composites tested at -196°C (Fig. 2(c)) show a somewhat ductile mode of failure compared to that at 24°C (Fig. 2(b)) and 100°C (Fig. 2(d)). The maximum load sustained by
Fig. la. Microstructure of as-cast 2024-Al/10wt%-A1203 particulate composite.
Fig. lb. Microstructure of extruded 2024-A1/10 wt%-At20~ particulate composite. composite Charpy specimens decreased from 5 kN at -196°C to 4 kN at 24°C and <4 kN at 100°C. 3.3 Fracture energy Table 2 shows the fracture energy values for the extruded 2024 aluminium and its composites at different temperatures. The data indicate a large degree of scatter both for unreinforced alloy and composites, especially at liquid nitrogen temperature. It is evident that at all temperatures the fracture energy values for the composites are lower than those of the unreinforced alloy in the extruded condition. The reduced fracture energy values for the composites could be attributed to a change in the initiation and propagation energy values. Table 2 shows that the presence of A1203 particles results in a decrease in both the crack initiation and the crack propagation energy. This is also reflected in the load versus time plot for the unreinforced alloy and the composites (Fig. 2). SEM photographs of fractured surfaces show dimple fracture in the case of the unreinforced alloy and a shear type of fracture in the composites. Occasionally, dispersed alumina particles seem to have undergone multiple fracture. Figure 3a shows the variation in the fracture energy of composites and unreinforced alloy with temperature. It is clear that in the case of the unreinforced alloy, the fracture energy increases from 216 kJ/m 2 at -196°C to 312 kJ/m 2 at 24°C and remains the same at 100°C. The increase in the fracture energy for the unreinforced alloy with temperature is expected, but the composites, by contrast, show a decrease in fracture energy with increase in temperature. This is very clearly discernible in the case of the initiation energy, but the trend is unclear in the case of the propagation energy. This kind of anomalous behaviour, i.e. a decrease in fracture energy with increase in temperature, has been observed in Al/Li alloys. ~2 In the present investigation the observed decrease in fracture energy for the composites with an increase in temperature could be possibly attributed to
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Table 2. Dynamic fracture energy values Material
2024A1 (extruded)
2024-A1/10 wt%-AizO3 (extruded)
Temperature (°C)
Fracture initiation energy (kJ/m 2)
Fracture propagation energ), (kJ/m ~)
Total dynamic fracture energ), (kJ/m ~)
-196
36 40
85 272
121 312
216
24
24 24
288 288
312 312
312
100
24 24
287 287
311 311
311
-196
10 10 25 23
52 31 41 62
62 41 66 85
63.5
4 10 13
32 63 48
36 73 61
56.5
9 4 5 6
40 43 27 36
49 47 33 42
42.5
24
100
Mean value of total fracture energy (kJ/m z)
290
M. K. Surappa, P. Sivakumar aging effects. In fact, hardness measurements on samples after holding for 30 min at 100°C showed a marginal increase in hardness (Table 3). However, this needs further study.
320
E
240 3.4 Fracture toughness Dynamic fracture toughness values for the unreinforced alloy and composites as a function of temperature (Fig. 3b) show that the addition of A1203 particles leads to a decrease in dynamic fracture toughness (Kta). Moreover, unlike the fracture energy values, the dynamic fracture toughness decreases with increase in temperature for both the unreinforced alloy and the composites, as shown in Table 4. The
2 0 2 4 AI ahoy (extruded} ~2024 AI * 10% AI203 L composite (extruded)
•
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u_ 80
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Table 4. Dynamic fracture toughness values
_
100
Material
Fig. 3a. Dynamic fracture energy versus temperature. 2024A1 (extruded)
[~ 20
2024-Ai/10wt%-AlaO3 (extruded)
"-" 10 O
O_
Kxd (MPa V~)
-196
29-4 31-53
30.5
24
23.9 25.7
24.8
100
23.87 23.87
23-9
-196
20-06 22-31 24-24 23-00
22.3
18-89 20.74 15-(~
18.2
17.52 12.93 14-36 16-5
15-3
24 •
-200
Temperature (°C)
2024 AI a l t o y
(exlruded)
[2024 AI + I0 % AI203 LCO~posite (extruded)
~
J
-loo
o
100
lo6
Temperature, *C Fig. 3b. Dynamic fracture toughness versus temperature.
Table 3. Hardness values Material
Test temperature
Hardness (VHN--kg/mm 2)
24
80-7 84-6 77.9
81
24 (After holding for 30 min at 100°C)
86.7 83.1 85.1 88.9
86
24
84-5 83-3 83.8
84
93.7 85-6 88-9
89.4
(oc)
2024 AI (extruded)
2024-A1/10 wt%-A1203 composite (extruded)
24 (After holding for 30 min at 100°C)
Mean value (VHN--kg/mm 2)
Mean value (MPa Vm)
Fracture toughness evaluation of 2024-Al/A1203 particles
291
(a) 300
9 200 %
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i -100
I 0
<1
100
(b) .
.
.
.
.
.
.
.
.
.
"
Temperoture, °C
Fig. 4. Variation of AE (Em-Ecomp) and AK (K~dm-K~dcomp)with temperature. difference in magnitude of toughness values between composites and the unreinforced alloy, indicated as (Kid)m--(KId)comp, and of energy values, ( E ) m (E)~omp, are plotted as a function of temperature in Fig. 4. It is clear that the deleterious effect of the A1203 additions on fracture energy increases with increase in temperature. By contrast, its influence on dynamic fracture toughness values initially decreases with increase in temperature.
3.5 Fractography The SEM micrograph of the fractured surface of a Charpy impact specimen of the extruded 2024 aluminium alloy (Fig. 5) shows a dimple fracture, while fractographs of Charpy impact specimens of extruded 2024-Al/10wt%-A1203 composites (Figs 6-8) reveal that there is a smaller number of dimples compared to the extruded unreinforced alloy, indicating more limited ductility. A fractograph of the composite at -196°C (Fig. 6) shows a large number of dimples compared to those at 24°C (Fig. 7) and 100°C (Fig. 8), thus exhibiting greater toughness at -196°C.
Fig. 6. SEM fractographs of extruded 2024-Al/10wt%A1203 composite Charpy specimen tested at -196°C. (a)
(b)
Fig. 5. SEM fractograph of extruded 2024 aluminium alloy Charpy specimen tested at 24°C.
Fig. 7. SEM fractographs of extruded 2024-A1/10wt%A1203 composite Charpy specimen tested at 24°C. (a) Magnification x200.
292
M. K. Surappa, P. S i v a k u m a r
in a decrease in the values with increase in temperature. (4) Dynamic fracture toughness (K~d) values for both the unreinforced alloy and the composites decrease with increase in temperature. (5) SEM fractographic observations confirm the observed dynamic fracture energy and dynamic fracture toughness values of the composites.
ACKNOWLEDGEMENTS
Fig. 8. SEM fractograph of extruded 2024-Al/10wt%A1203 composite Charpy specimen tested at 100°C.
Figure 6(a) shows more shear deformation around the particles in specimens tested at -196°C. Figure 6(b) shows a dimple rupture. Particle fracture is not seen in SEM fractographs of composites tested at -196°C. Figure 7(a) shows secondary cracks on the fracture surface of a composite Charpy specimen tested at 24°C, and Fig. 7(b) shows a particle fracture at 24°C. However, less particle fracture is seen in the fractographs of composites tested at 24°C. SEM fractographs of the composite at 100°C (Fig. 8) show fewer dimples compared to those at -196 and 24°C, in agreement with the low fracture energy value at 100°C. Figure 8 shows particle cracking and voids around alumina particles.
4 CONCLUSIONS (1) Load and energy versus time plots generated during instrumented impact testing reveal a semibrittle mode of failure in the case of 2024-A1/10 wt%A1203 particulate composites. (2) The dynamic fracture energy and dynamic fracture toughness of the 2024 aluminium alloy decrease as a result of the addition of 10wt% of A1203 particles of 4/~m size in the present study. (3) Dynamic fracture energy values for the unreinforced alloy increase with increase in temperature, whereas the addition of alumina particles results
Thanks are due to Prof. Kishore for his comments on fractographs. Thanks are also due to ARDB and DST for their financial assistance during the course of this investigation, and Mr S. Srinivas Murthy for conducting the impact tests.
REFERENCES 1. Logsdon, W. A. & Liaw, P. K., Eng. Frac. Mech., 24 (1986) 737-51. 2. Friend, C. M., Mat. Sci. Tech., 5 (1988) 1-7. 3. Kamat, S. V., Hirth, J. P. & Mehrabian, R., Acta Metall., 37 (1989) 2395-402. 4. Flom, Y. & Arsenault, R. J., Acta Metall., 37 (1989) 2413-23. 5. Duralcan MMCs, Data Report Package. Dural Aluminium Composites Corporation, San Diego, CA 92121, May-June 1989. 6. Flom, Y. & Arsenault, R. J., In VI 1CCM and II ECCM (Conf. Proc.), Vol. 2, ed. F. L. Matthews, M. Bunsell, J. Hudgkinson & J. Mohton. Elsevier Applied Science, London, 1987, pp. 189-98. 7. Hasson, D. F. & Crowe, C. R., In Strength of Metals and Alloys, ed. P. O. Kettunen & T. K. Lepisto. Pergamon Press, Oxford, 1988, pp. 1515-20. 8. McDanels, D. C. & Signorelli, P. A., Evaluation of low-cost aluminium composites for aircraft engine structural applications. NASA Technical Memorandum 83357, 1983. 9. Roebuck, B., Gorley, T. A. E. & McCartney, L. N., Mat. Sci. Tech., 5 (1989) 105-16. 10. Roebuck, B. & Lord, J. D., Mat. Sci. Tech., 6 (1990) 1199-209. 11. McCoy, J. W. & Wawner, E. F. In Int. Symp. on Advances in Cast Reinforced Metal Composites, ed. S. G. Fishman & A. K. Dhingra. ASM International, Metals Park, Ohio, 1988, pp. 237-42. 12. Webester, D., In Proc. I11 Int. A I - L i Conf., ed. C. Baker, A. D. J. Gregson, S. J. Morris & C. J. Reel. Institute of Metals, London, 1985, pp. 602-9.
!
FRACTURE TOUGHNESS E V A L U A T I O N OF 2040-AI/A1203 PARTICULATE COMPOSITES BY INSTRUMENTED IMPACT M. K. Surappa & P. Sivakumar Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India (Received 17 July 1991; revised version received 23 January 1992; accepted 10 March 1992)
strength. ~ Toughness values of these composites depend on the technique of measurement 2 and also on material variables including fibre volume fraction,a-5 size 3'4'6 and shape 7 of reinforcement, matrix alloy composition 3'8 and the method employed for the fabrication of composites. 2 The appropriate method for the evaluation of toughness of metal matrix composites (MMCs) is still being debated. 9'1° One of the methods used for the measurement of toughness is instrumented impact testing. By means of this technique it is possible to measure both fracture energy, E, and dynamic fracture toughness, Kxd, values. A literature survey indicates that little work has been done on the instrumented impact testing of discontinuous fibre reinforced aluminium matrix composites, except in a few cases. Furthermore, no work has been done on the impact toughness of aluminium alloy composites reinforced with alumina particles. Hence, in the present investigation the authors report results of limited studies on the evaluation of the toughness of 2024-A1/10 wt%-A1203 composites and its variation with temperature in the range -196 to 100°C.
Abstract
Composites consisting of an aluminium alloy (2024A1) reinforced with 4 l*m sized A1203 particles were prepared by the casting route. Fracture energy, E, and dynamic fracture toughness, Kid , values for composites and unreinforced alloys were evaluated by means of an instrumented impact test at temperatures varying from -196 to 100°C. The magnitude of the observed reduction is greater in the case of the fracture energy values than for the fracture toughness values as a result of the alumina additions. Furthermore, the dynamic fracture toughness of both composites and unreinforced alloys shows a negative dependence on temperature. By contrast, fracture energy values for the composites decrease with increase in temperature, whereas the fracture energy for the unreinforced alloy increases with increase in temperature. SEM examination of fracture surfaces of composites reveals multiple cracking of alumina particles. Keywords: aluminium matrix composites, 2024AIA1203 composites, instrumented impact-test, dynamic fracture toughness, temperature dependance of fracture energy
2 EXPERIMENTAL PROCEDURE 1 INTRODUCTION
Composites of a 2024 aluminium alloy, of the composition shown in Table 1, reinforced with 10 wt% of AI20 3 particles of 4/,m average size, were prepared by the ingot metallurgy route. Cylindrical billets of the composites were extruded into 15 mm 2 sections, the extrusion ratio being 8.72. Unreinforced alloy was also extruded under similar conditions for comparison purposes. From these extruded sections, 10 mm 2 Charpy V-notch specimens (with 45 ° included angle) were machined. Impact tests were carded out according to the ASTM E23 method in a standard Dynatup instrumented impact unit. Tests were carried out at -196, 24 and 100°C. In the case of the low temperature tests, specimens were kept in liquid nitrogen for 30 min prior to the tests. The time lapse between removal of specimen from liquid nitrogen
Evaluation of static and dynamic properties of aluminium matrix composites reinforced with discontinuous reinforcements such as particles, whiskers or short fibres of SiC or A1203 is being pursued with great interest worldwide. In general, both ambientand elevated-temperature tensile properties of aluminium alloy composites reinforced with ceramic particles (A1203 or SiC) are superior to those of unreinforced alloys. However, these composites are generally reported to suffer from poor fracture toughness and other dynamic properties including fatigue crack growth rate (FCGR) and fatigue
Composites Science and Technology 0266-3538/93/$06.00 © 1993 Elsevier Science Publishers Ltd. 287
288
M. K. Surappa, P. Sivakumar Table 1. Composition of 2024 Ai alloy Elements (wt%) Cu
Mg
Mn
Si
Cr
4.42
0.4
0 . 7 4 0.17 <0.1
AI Remainder
and the test was less than 2 s and a similar procedure was adopted for the tests at boiling water temperature (100°C). At each test temperature, impact tests were carried out on duplicate specimens in the case of unreinforced alloy and on at least three specimens in the case of composites. Fracture surfaces were examined by means of scanning electron microscopy (SEM). 3 RESULTS AND DISCUSSION 3.1 Microstmcture The microstructure of an as-cast 2024-Al/10wt%A1203 composite is shown in Fig. la. It is evident that alumina particles are segregated to interdendritic regions. Similar distributions have been reported in cast 2024/SIC and 2024/A1203 particulate composites. H The distribution of alumina in the extruded composite, as shown in Fig. lb, is considerably better. 3.2 Load/energy versus time plots Figure 2a shows load and energy versus time plots for an extruded 2024 aluminium alloy specimen obtained from an instrumented impact test at room temperature. The figure reveals the ductile mode of failure. Load and energy versus time plots (Fig. 2(b)) for the extruded 2024/10wt% Al203 composites tested at room temperature reveal semi-brittle fracture behaviour. It is evident that the composites tested at -196°C (Fig. 2(c)) show a somewhat ductile mode of failure compared to that at 24°C (Fig. 2(b)) and 100°C (Fig. 2(d)). The maximum load sustained by
Fig. la. Microstructure of as-cast 2024-Al/10wt%-A1203 particulate composite.
Fig. lb. Microstructure of extruded 2024-A1/10 wt%-At20~ particulate composite. composite Charpy specimens decreased from 5 kN at -196°C to 4 kN at 24°C and <4 kN at 100°C. 3.3 Fracture energy Table 2 shows the fracture energy values for the extruded 2024 aluminium and its composites at different temperatures. The data indicate a large degree of scatter both for unreinforced alloy and composites, especially at liquid nitrogen temperature. It is evident that at all temperatures the fracture energy values for the composites are lower than those of the unreinforced alloy in the extruded condition. The reduced fracture energy values for the composites could be attributed to a change in the initiation and propagation energy values. Table 2 shows that the presence of A1203 particles results in a decrease in both the crack initiation and the crack propagation energy. This is also reflected in the load versus time plot for the unreinforced alloy and the composites (Fig. 2). SEM photographs of fractured surfaces show dimple fracture in the case of the unreinforced alloy and a shear type of fracture in the composites. Occasionally, dispersed alumina particles seem to have undergone multiple fracture. Figure 3a shows the variation in the fracture energy of composites and unreinforced alloy with temperature. It is clear that in the case of the unreinforced alloy, the fracture energy increases from 216 kJ/m 2 at -196°C to 312 kJ/m 2 at 24°C and remains the same at 100°C. The increase in the fracture energy for the unreinforced alloy with temperature is expected, but the composites, by contrast, show a decrease in fracture energy with increase in temperature. This is very clearly discernible in the case of the initiation energy, but the trend is unclear in the case of the propagation energy. This kind of anomalous behaviour, i.e. a decrease in fracture energy with increase in temperature, has been observed in Al/Li alloys. ~2 In the present investigation the observed decrease in fracture energy for the composites with an increase in temperature could be possibly attributed to
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l~g. 2. Load and energy versus time plots for: (a) extruded 2024 aluminium alloy at 24°C; (b) extruded 2024-A1/10 wt%-Al2Os composite at 24°C; (c) extruded 2024-A1/10 wt%-AI2Os composites at -196°C; (d) extruded 2024-AI/10 wt%-A12Os composite at 100°C.
Table 2. Dynamic fracture energy values Material
2024A1 (extruded)
2024-A1/10 wt%-AizO3 (extruded)
Temperature (°C)
Fracture initiation energy (kJ/m 2)
Fracture propagation energ), (kJ/m ~)
Total dynamic fracture energ), (kJ/m ~)
-196
36 40
85 272
121 312
216
24
24 24
288 288
312 312
312
100
24 24
287 287
311 311
311
-196
10 10 25 23
52 31 41 62
62 41 66 85
63.5
4 10 13
32 63 48
36 73 61
56.5
9 4 5 6
40 43 27 36
49 47 33 42
42.5
24
100
Mean value of total fracture energy (kJ/m z)
290
M. K. Surappa, P. Sivakumar aging effects. In fact, hardness measurements on samples after holding for 30 min at 100°C showed a marginal increase in hardness (Table 3). However, this needs further study.
320
E
240 3.4 Fracture toughness Dynamic fracture toughness values for the unreinforced alloy and composites as a function of temperature (Fig. 3b) show that the addition of A1203 particles leads to a decrease in dynamic fracture toughness (Kta). Moreover, unlike the fracture energy values, the dynamic fracture toughness decreases with increase in temperature for both the unreinforced alloy and the composites, as shown in Table 4. The
2 0 2 4 AI ahoy (extruded} ~2024 AI * 10% AI203 L composite (extruded)
•
~, 160
/
u_ 80
0
--
;
-200
-1C)0 0 Temperature~ °C
I
Table 4. Dynamic fracture toughness values
_
100
Material
Fig. 3a. Dynamic fracture energy versus temperature. 2024A1 (extruded)
[~ 20
2024-Ai/10wt%-AlaO3 (extruded)
"-" 10 O
O_
Kxd (MPa V~)
-196
29-4 31-53
30.5
24
23.9 25.7
24.8
100
23.87 23.87
23-9
-196
20-06 22-31 24-24 23-00
22.3
18-89 20.74 15-(~
18.2
17.52 12.93 14-36 16-5
15-3
24 •
-200
Temperature (°C)
2024 AI a l t o y
(exlruded)
[2024 AI + I0 % AI203 LCO~posite (extruded)
~
J
-loo
o
100
lo6
Temperature, *C Fig. 3b. Dynamic fracture toughness versus temperature.
Table 3. Hardness values Material
Test temperature
Hardness (VHN--kg/mm 2)
24
80-7 84-6 77.9
81
24 (After holding for 30 min at 100°C)
86.7 83.1 85.1 88.9
86
24
84-5 83-3 83.8
84
93.7 85-6 88-9
89.4
(oc)
2024 AI (extruded)
2024-A1/10 wt%-A1203 composite (extruded)
24 (After holding for 30 min at 100°C)
Mean value (VHN--kg/mm 2)
Mean value (MPa Vm)
Fracture toughness evaluation of 2024-Al/A1203 particles
291
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9 200 %
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i -100
I 0
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100
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.
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.
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.
"
Temperoture, °C
Fig. 4. Variation of AE (Em-Ecomp) and AK (K~dm-K~dcomp)with temperature. difference in magnitude of toughness values between composites and the unreinforced alloy, indicated as (Kid)m--(KId)comp, and of energy values, ( E ) m (E)~omp, are plotted as a function of temperature in Fig. 4. It is clear that the deleterious effect of the A1203 additions on fracture energy increases with increase in temperature. By contrast, its influence on dynamic fracture toughness values initially decreases with increase in temperature.
3.5 Fractography The SEM micrograph of the fractured surface of a Charpy impact specimen of the extruded 2024 aluminium alloy (Fig. 5) shows a dimple fracture, while fractographs of Charpy impact specimens of extruded 2024-Al/10wt%-A1203 composites (Figs 6-8) reveal that there is a smaller number of dimples compared to the extruded unreinforced alloy, indicating more limited ductility. A fractograph of the composite at -196°C (Fig. 6) shows a large number of dimples compared to those at 24°C (Fig. 7) and 100°C (Fig. 8), thus exhibiting greater toughness at -196°C.
Fig. 6. SEM fractographs of extruded 2024-Al/10wt%A1203 composite Charpy specimen tested at -196°C. (a)
(b)
Fig. 5. SEM fractograph of extruded 2024 aluminium alloy Charpy specimen tested at 24°C.
Fig. 7. SEM fractographs of extruded 2024-A1/10wt%A1203 composite Charpy specimen tested at 24°C. (a) Magnification x200.
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M. K. Surappa, P. S i v a k u m a r
in a decrease in the values with increase in temperature. (4) Dynamic fracture toughness (K~d) values for both the unreinforced alloy and the composites decrease with increase in temperature. (5) SEM fractographic observations confirm the observed dynamic fracture energy and dynamic fracture toughness values of the composites.
ACKNOWLEDGEMENTS
Fig. 8. SEM fractograph of extruded 2024-Al/10wt%A1203 composite Charpy specimen tested at 100°C.
Figure 6(a) shows more shear deformation around the particles in specimens tested at -196°C. Figure 6(b) shows a dimple rupture. Particle fracture is not seen in SEM fractographs of composites tested at -196°C. Figure 7(a) shows secondary cracks on the fracture surface of a composite Charpy specimen tested at 24°C, and Fig. 7(b) shows a particle fracture at 24°C. However, less particle fracture is seen in the fractographs of composites tested at 24°C. SEM fractographs of the composite at 100°C (Fig. 8) show fewer dimples compared to those at -196 and 24°C, in agreement with the low fracture energy value at 100°C. Figure 8 shows particle cracking and voids around alumina particles.
4 CONCLUSIONS (1) Load and energy versus time plots generated during instrumented impact testing reveal a semibrittle mode of failure in the case of 2024-A1/10 wt%A1203 particulate composites. (2) The dynamic fracture energy and dynamic fracture toughness of the 2024 aluminium alloy decrease as a result of the addition of 10wt% of A1203 particles of 4/~m size in the present study. (3) Dynamic fracture energy values for the unreinforced alloy increase with increase in temperature, whereas the addition of alumina particles results
Thanks are due to Prof. Kishore for his comments on fractographs. Thanks are also due to ARDB and DST for their financial assistance during the course of this investigation, and Mr S. Srinivas Murthy for conducting the impact tests.
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