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Influence of strain rate on the fracture toughness of Al-8Fe-1V-2Si at 315°C
Scripta METALLURGICA et M A T E R I A L I A
Vol.
24, pp. 1 0 0 9 - 1 0 1 3 , 1990 P r i n t e d in the U . S . A .
Pergamon Press plc All rights reserved
INFLUENCE OF STRAIN RATE ON THE FRACTURE TOUGHNESS OF AI-8Fe-IV-2Si AT 315°C
P. Krawczyk* and K. V. Jata** *Wright Research and Development Center, Materials Laboratory, U. S. Air Force, Dayton, Ohio 45433 **University of Dayton Research Institute, Dayton, Ohio 45469 (Received December 27, 1 9 8 9 ) ( R e v i s e d M a r c h 21, 1 9 9 0 ) Introduction Aluminum alloys produced via rapid solidification processing (RSP) exhibit comparable mechanical properties to conventional aluminum alloys at ambient temperature but, at temperatures as high at 315°C, they are far superior (i). The improved performance of these alloys at elevated temperature has made them strong candidates for a variety of future aerospace applications, such as the aft fuselage of the Advanced Tactical Fighter (ATF), missile fins and winglets, rocket motor cases, and various gas-turbine engine components (2). This improved performance is the direct result of RSP. RSP allows for the presence of large volume fraction of fine second phase particles in these alloys which, when exposed to elevated temperatures for an extended period of time, do not coarsen due to their low diffusion coefficients (3). This lack of coarsening, despite the fact that alloys produced via RSP inherently have fine grain size, gives these alloys their elevated temperature strength and excellent creep resistance
(3). Since there is great interest in using these alloys in future aerospace systems at elevated temperatures as high as 315"C, research is being performed to better understand the mechanisms that govern fracture behavior under quasi-static and fatigue loading (3,&,5,6). Elevated temperature aluminum alloys have been shown to exhibit an increased toughness at elevated temperature due to thin sheet toughening (5). This toughening mechanism involves the creation of free surfaces or delaminations perpendicular to the crack front. As a result, the effective thickness of the material being tested is reduced, giving rise to a plane stress condition which increases the stress intensity necessary to drive the crack front, leading to an increased toughness (7,8). The present work is aimed at characterizing the fracture behavior of an elevated temperature AI alloy, AI-8Fe-IV-2Si, at 20 and 315°C under various strain rates via R-Curve determination. This is of interest since many aerospace components are designed using RCurves, and the fracture behavior under various strain rate conditions is desirable if a thorough understanding of this alloy is to be achieved. Experimental Procedures The material used in this investigation was an AI-SFe-IV-2Si extrusion produced by Allied Signal, Inc. Nominal dimensions of the supplied extrusion were 114.3 mm wide and 25.4 mm thick. Compact tension specimens, nominally 6.35 mm thick with a width of 38.1 mm, were removed from the half thickness of the extrusion in the L-T orientation. Specimens were tested to develop fracture toughness data through R-Curve determination as outlined in ASTM Standard E561. Fracture behavior was studied at 20°C and 315°C, at strain rates ranging from 3.3xi0 "5 s "I to 3.3xi0 "3 s "I. The 315°C temperature was achieved using a resistance furnace controllable to ±2°C. Crack extension, load, and crack opening displacement were continually recorded using a data acquisition system developed for a HP9816 computer system. Increment in effective crack extension, and elastic stress intensity were obtained at each temperature using
0036-9748/90 Copyright (c) 1 9 9 0
1009 $ 3 . 0 0 + .00 Pergamon Press
plc
I010
STRAIN
RATE
AND
TOUGHNESS
those procedures outlined in the ASTM E561 standard.
Vol.
24,
No.
6
K C values were calculated by fitting R-
Curves with a power-law equation and analytically determining the instability point associated with the tangency point between the R-Curve and the applied K Curve for the specimen geometry. A transmission electron microscope (TEM) and SEM were used to determine microstructure and fracture mode. Results and Discussion The fine submicrometer sized grains and the nanometer-sized All2(Fe,V)3Si dispersoids that are achieved using RSP in this alloy are shown in the bright field TEM micrographs, Figure I. K C, Kr.25 and Kr.ma x values at 20 and 315°C are shown in Table i. At room temperature the
TABLE 1 Fracture Toughness of AI-SFe-IV-2Si at 20 and 315°C for Different Strain Rates
AI-SFe-IV-2Si
Strain Rate
20°C
Kr.25
315°C
Kr.ma x
Kc
Kr.25
Kr.ma x
Kc
(MPa-m I/2)
(MPa-m I/2)
(s "I)
(MPa.m I/2)
(MPa-m I/2)
(MPa-m I/2)
(MPa-m I/2)
3.33 x 10 .3
27.8
29.5
29.7
29.7
66
50.6
3.33 x 10 .4
27.7
27.7
27.7
29.0
40
38.4
3.33 x 10 .5
27.4
.
14.4
36.7
31.4
.
.
.
influence of strain rate on K values is minimal whereas at 315°C a monotonic increase in the K r values and K C is observed, r Specimens tested at room temperature did not exhibit any delaminations, whereas at 315°C the lowest and the intermediate strain rate tested specimens exhibited delaminations. At the highest strain rate, delaminations were absent (Figure 2). The increase in fracture toughness at 315"C as compared to 25°C has been explained by the crack divider toughening (or thin-sheet toughening) mechanism (5), which has been explained briefly in the introduction of the present paper. Our results are consistent with Chan's observations (5) at low and intermediate strain rates. However, (at 315°C) for the highest strain rate test, delaminations are not observed, while an improvement in K C is still noted. This result was also verified on another high temperature AI alloy, AI-IOFe-2V-2Si (an experimental alloy supplied by Allied Signal Inc.), suggesting this may be a general observation for high temperature AI alloys based on AI-Fe system. This improvement in fracture toughness at 315"C with increasing strain rates is an
Vol.
~,
No.
6
SLR~IN
~&TE
AND
TOUGHNESS
10ll
observation not reported previously. The increase in the fracture toughness cannot be attributed to a thin sheet toughening mechanism since crack extension is not accompanied by delaminations in the loading plane at the fastest strain rate, Figure 2. High temperature ai~minu~n alloys are extremely strain-rate sensitive and it has been demonstrated that they exhibit enhanced strength, modulus, and fracture strain with increasing strain rate at elevated temperatures (9). Since fracture toughness is dependent on these parameters, it appears that they contribute in a major way to this obse~#ed increase in toughness at the highest strain rate employed for the present study. Mechanical property data at ]15"C for different strain rates were obtained, as shown in Table 2, to explain the o b s e ~ e d e~nancement in fracture tou~nness. TABLE 2 Mechanical
Temperature
Properties
of AI-SFe-IV-2Si
Strain Rate -I
at 20 and 315°C for Different
Strain Rates
Yield Strength
UTS
Elong.
R.A.
E
MPa
MPa
%
%
GPa
°C
s
20
4 x I0 "4
413.4
434.1
315
i.i x i0 "4
186
!98
315
4 x 10 .4
192.9
315
6.3 x 10 .2
269.&
Such trends have also been observed in our laboratory alloys (AI-12Fe-IV-2Si and AI-IOFe-2V-2Si). Fracture using the following expression:
!!
--
75.79
9
22
54.43
199.8
13
24
57 !B
284.5
18
62
61.32
for other elevated temperature alumin,~ toughness calculations were performed
Klc,c ~ (n2E ay s ef) 0"5 where ef - ef/2 for plane stress and el/3 for plane strain.
Since delaminations
are present at
the slowest strain rate, the state of stress in this sample is effectively reduced to a state of plane stress, whereas at the fastest strain rate it is plane strain, as discussed above. Calculations using the above equation show that fracture toughness increases with increasing strain rate by a factor of two, suggesting that fracture toughness in high temperature AI alloys should be controlled largely by the failure strain and strength of the alloy at higher strain rates. SEM micrographs of the fracture surfaces at the slowest and fastest strain rates (Figure 3) show different failure modes. At the slowest strain rate, failure occurs by a mixture of brittle fracture of prior particle boundaries and ductile microvoid coalescence, whereas at the fastest strain rate, fracture mechanism is totally by classical ductile microvoid coalescence. This change in failure mode may also contribute to the increased fracture toughness with increasing strain rate since more energy is required to advance the crack with a void nucleation growth and linkage mechanism.
ST~IN
I01~
RATE
AND
TOUGHNESS
VOI
.
24 , NO.
6
Conclus~oB l"ne fracture toughness of the alloy AI-SFe-IV-2Si is strain-rate sensitive at 315°C.
At
this temperature delaminations are observed for 3.3xi0 "5 s "I and 3.3xi0 "4 s "I but not aC -3 -I 3.3xi0 s The improved toughness at the highest strain rate should be attributed to enhanced strength and fracture strain of the alloy. The improvement is also accompanied by a change in failure mode where brittle failure of prior particle boundary occurs at slower strain rates and ductile microvoid coalescence occurs at higher strain rates. .
Acknowledgements This work was performed under U. S. Air Force Contract No. F33615-88-C-5437. was the contract monitor.
Mr. N. Ontko
References i.
2.
3. 4. 5. 6. 7. 8. 9.
A. M. Brown, D. J. Skinner, D. Raybould, S. K. Das, R. L. Bye and C. H. Adam, Aluminum Alloys Their Physical and Mechanical Properties, Editors, E. A. Starke, Jr., and T. H. Sanders, Jr., Vol. II, p. 1029, ~MAS Ltd., Westmidlands, U.K., (1986). P. S. Gilman, M. S. Zedalis, J. M. Peltier and S. K. Das, Proceedings of AIAA-.~HS-ASEE, Conference on Aircraft Design Systems and Operations, Atlanta, CA, September 7-9, 1988, AIAA-88-4444, p. i, AIAA, Washington, D.C. (1989). Dispersion Strengthened AI Alloys, Edited by Y. W. Kim and W. M, Griffith, AIME, Warrendale, PA, U.S.A. (1988). D. J. Skinner, M. S. Zedalls and P. S. Gilman, Mater. Sci. Eng., Vol. A 119, p. 81 (1989). K. S. Chan, Metall. Trans., Vol. 20A, p. 155, (1989). H. H. Smith, D. J. Michel and J, R. Reed, Metall. Trans., Vol. 20A, p. 2425, (1989). J. D, Embury, N. J. Perch, A. E. Wraith and E. S. Wright, Trans. AIME, 239, p. 114, (1967). K. T. Venkateswara Rao, W. Yu and R. O. Ritchie, Metall. Trans., A, P. &85, (1989). D. J. Skinner, H. S. Zedalis and J. Peltier, "Light-Weight Alloys for Aerospace Applications," eds. E. W. Lee, E. H. Chia and N. J. Kim, TMS, Warrendale, PA, p. 71, (1989).
FIG. I.
BF T~M pictures showing (a) fine grains and (b) nanometer sized dlspersoids in AI-8FeIV-2Si extrusion.
Vot.
FIG.
15,
2.
FIG. 3.
No.
6
STRAIN
RATE A N D
TOUGHNESS
1013
Macrophotographs showing the presence and absence of delaminations at the (a) slowest and (b) fastest strain rates, respectively, at 3!5°C in AI-SFe-IV-2Si.
Fracture surfaces at the (a) slowest and (b) fastest strain rates, (a) mixture of brittle and ductile failure and (b) total ductile microvoid coalescence.
Vol.
24, pp. 1 0 0 9 - 1 0 1 3 , 1990 P r i n t e d in the U . S . A .
Pergamon Press plc All rights reserved
INFLUENCE OF STRAIN RATE ON THE FRACTURE TOUGHNESS OF AI-8Fe-IV-2Si AT 315°C
P. Krawczyk* and K. V. Jata** *Wright Research and Development Center, Materials Laboratory, U. S. Air Force, Dayton, Ohio 45433 **University of Dayton Research Institute, Dayton, Ohio 45469 (Received December 27, 1 9 8 9 ) ( R e v i s e d M a r c h 21, 1 9 9 0 ) Introduction Aluminum alloys produced via rapid solidification processing (RSP) exhibit comparable mechanical properties to conventional aluminum alloys at ambient temperature but, at temperatures as high at 315°C, they are far superior (i). The improved performance of these alloys at elevated temperature has made them strong candidates for a variety of future aerospace applications, such as the aft fuselage of the Advanced Tactical Fighter (ATF), missile fins and winglets, rocket motor cases, and various gas-turbine engine components (2). This improved performance is the direct result of RSP. RSP allows for the presence of large volume fraction of fine second phase particles in these alloys which, when exposed to elevated temperatures for an extended period of time, do not coarsen due to their low diffusion coefficients (3). This lack of coarsening, despite the fact that alloys produced via RSP inherently have fine grain size, gives these alloys their elevated temperature strength and excellent creep resistance
(3). Since there is great interest in using these alloys in future aerospace systems at elevated temperatures as high as 315"C, research is being performed to better understand the mechanisms that govern fracture behavior under quasi-static and fatigue loading (3,&,5,6). Elevated temperature aluminum alloys have been shown to exhibit an increased toughness at elevated temperature due to thin sheet toughening (5). This toughening mechanism involves the creation of free surfaces or delaminations perpendicular to the crack front. As a result, the effective thickness of the material being tested is reduced, giving rise to a plane stress condition which increases the stress intensity necessary to drive the crack front, leading to an increased toughness (7,8). The present work is aimed at characterizing the fracture behavior of an elevated temperature AI alloy, AI-8Fe-IV-2Si, at 20 and 315°C under various strain rates via R-Curve determination. This is of interest since many aerospace components are designed using RCurves, and the fracture behavior under various strain rate conditions is desirable if a thorough understanding of this alloy is to be achieved. Experimental Procedures The material used in this investigation was an AI-SFe-IV-2Si extrusion produced by Allied Signal, Inc. Nominal dimensions of the supplied extrusion were 114.3 mm wide and 25.4 mm thick. Compact tension specimens, nominally 6.35 mm thick with a width of 38.1 mm, were removed from the half thickness of the extrusion in the L-T orientation. Specimens were tested to develop fracture toughness data through R-Curve determination as outlined in ASTM Standard E561. Fracture behavior was studied at 20°C and 315°C, at strain rates ranging from 3.3xi0 "5 s "I to 3.3xi0 "3 s "I. The 315°C temperature was achieved using a resistance furnace controllable to ±2°C. Crack extension, load, and crack opening displacement were continually recorded using a data acquisition system developed for a HP9816 computer system. Increment in effective crack extension, and elastic stress intensity were obtained at each temperature using
0036-9748/90 Copyright (c) 1 9 9 0
1009 $ 3 . 0 0 + .00 Pergamon Press
plc
I010
STRAIN
RATE
AND
TOUGHNESS
those procedures outlined in the ASTM E561 standard.
Vol.
24,
No.
6
K C values were calculated by fitting R-
Curves with a power-law equation and analytically determining the instability point associated with the tangency point between the R-Curve and the applied K Curve for the specimen geometry. A transmission electron microscope (TEM) and SEM were used to determine microstructure and fracture mode. Results and Discussion The fine submicrometer sized grains and the nanometer-sized All2(Fe,V)3Si dispersoids that are achieved using RSP in this alloy are shown in the bright field TEM micrographs, Figure I. K C, Kr.25 and Kr.ma x values at 20 and 315°C are shown in Table i. At room temperature the
TABLE 1 Fracture Toughness of AI-SFe-IV-2Si at 20 and 315°C for Different Strain Rates
AI-SFe-IV-2Si
Strain Rate
20°C
Kr.25
315°C
Kr.ma x
Kc
Kr.25
Kr.ma x
Kc
(MPa-m I/2)
(MPa-m I/2)
(s "I)
(MPa.m I/2)
(MPa-m I/2)
(MPa-m I/2)
(MPa-m I/2)
3.33 x 10 .3
27.8
29.5
29.7
29.7
66
50.6
3.33 x 10 .4
27.7
27.7
27.7
29.0
40
38.4
3.33 x 10 .5
27.4
.
14.4
36.7
31.4
.
.
.
influence of strain rate on K values is minimal whereas at 315°C a monotonic increase in the K r values and K C is observed, r Specimens tested at room temperature did not exhibit any delaminations, whereas at 315°C the lowest and the intermediate strain rate tested specimens exhibited delaminations. At the highest strain rate, delaminations were absent (Figure 2). The increase in fracture toughness at 315"C as compared to 25°C has been explained by the crack divider toughening (or thin-sheet toughening) mechanism (5), which has been explained briefly in the introduction of the present paper. Our results are consistent with Chan's observations (5) at low and intermediate strain rates. However, (at 315°C) for the highest strain rate test, delaminations are not observed, while an improvement in K C is still noted. This result was also verified on another high temperature AI alloy, AI-IOFe-2V-2Si (an experimental alloy supplied by Allied Signal Inc.), suggesting this may be a general observation for high temperature AI alloys based on AI-Fe system. This improvement in fracture toughness at 315"C with increasing strain rates is an
Vol.
~,
No.
6
SLR~IN
~&TE
AND
TOUGHNESS
10ll
observation not reported previously. The increase in the fracture toughness cannot be attributed to a thin sheet toughening mechanism since crack extension is not accompanied by delaminations in the loading plane at the fastest strain rate, Figure 2. High temperature ai~minu~n alloys are extremely strain-rate sensitive and it has been demonstrated that they exhibit enhanced strength, modulus, and fracture strain with increasing strain rate at elevated temperatures (9). Since fracture toughness is dependent on these parameters, it appears that they contribute in a major way to this obse~#ed increase in toughness at the highest strain rate employed for the present study. Mechanical property data at ]15"C for different strain rates were obtained, as shown in Table 2, to explain the o b s e ~ e d e~nancement in fracture tou~nness. TABLE 2 Mechanical
Temperature
Properties
of AI-SFe-IV-2Si
Strain Rate -I
at 20 and 315°C for Different
Strain Rates
Yield Strength
UTS
Elong.
R.A.
E
MPa
MPa
%
%
GPa
°C
s
20
4 x I0 "4
413.4
434.1
315
i.i x i0 "4
186
!98
315
4 x 10 .4
192.9
315
6.3 x 10 .2
269.&
Such trends have also been observed in our laboratory alloys (AI-12Fe-IV-2Si and AI-IOFe-2V-2Si). Fracture using the following expression:
!!
--
75.79
9
22
54.43
199.8
13
24
57 !B
284.5
18
62
61.32
for other elevated temperature alumin,~ toughness calculations were performed
Klc,c ~ (n2E ay s ef) 0"5 where ef - ef/2 for plane stress and el/3 for plane strain.
Since delaminations
are present at
the slowest strain rate, the state of stress in this sample is effectively reduced to a state of plane stress, whereas at the fastest strain rate it is plane strain, as discussed above. Calculations using the above equation show that fracture toughness increases with increasing strain rate by a factor of two, suggesting that fracture toughness in high temperature AI alloys should be controlled largely by the failure strain and strength of the alloy at higher strain rates. SEM micrographs of the fracture surfaces at the slowest and fastest strain rates (Figure 3) show different failure modes. At the slowest strain rate, failure occurs by a mixture of brittle fracture of prior particle boundaries and ductile microvoid coalescence, whereas at the fastest strain rate, fracture mechanism is totally by classical ductile microvoid coalescence. This change in failure mode may also contribute to the increased fracture toughness with increasing strain rate since more energy is required to advance the crack with a void nucleation growth and linkage mechanism.
ST~IN
I01~
RATE
AND
TOUGHNESS
VOI
.
24 , NO.
6
Conclus~oB l"ne fracture toughness of the alloy AI-SFe-IV-2Si is strain-rate sensitive at 315°C.
At
this temperature delaminations are observed for 3.3xi0 "5 s "I and 3.3xi0 "4 s "I but not aC -3 -I 3.3xi0 s The improved toughness at the highest strain rate should be attributed to enhanced strength and fracture strain of the alloy. The improvement is also accompanied by a change in failure mode where brittle failure of prior particle boundary occurs at slower strain rates and ductile microvoid coalescence occurs at higher strain rates. .
Acknowledgements This work was performed under U. S. Air Force Contract No. F33615-88-C-5437. was the contract monitor.
Mr. N. Ontko
References i.
2.
3. 4. 5. 6. 7. 8. 9.
A. M. Brown, D. J. Skinner, D. Raybould, S. K. Das, R. L. Bye and C. H. Adam, Aluminum Alloys Their Physical and Mechanical Properties, Editors, E. A. Starke, Jr., and T. H. Sanders, Jr., Vol. II, p. 1029, ~MAS Ltd., Westmidlands, U.K., (1986). P. S. Gilman, M. S. Zedalis, J. M. Peltier and S. K. Das, Proceedings of AIAA-.~HS-ASEE, Conference on Aircraft Design Systems and Operations, Atlanta, CA, September 7-9, 1988, AIAA-88-4444, p. i, AIAA, Washington, D.C. (1989). Dispersion Strengthened AI Alloys, Edited by Y. W. Kim and W. M, Griffith, AIME, Warrendale, PA, U.S.A. (1988). D. J. Skinner, M. S. Zedalls and P. S. Gilman, Mater. Sci. Eng., Vol. A 119, p. 81 (1989). K. S. Chan, Metall. Trans., Vol. 20A, p. 155, (1989). H. H. Smith, D. J. Michel and J, R. Reed, Metall. Trans., Vol. 20A, p. 2425, (1989). J. D, Embury, N. J. Perch, A. E. Wraith and E. S. Wright, Trans. AIME, 239, p. 114, (1967). K. T. Venkateswara Rao, W. Yu and R. O. Ritchie, Metall. Trans., A, P. &85, (1989). D. J. Skinner, H. S. Zedalis and J. Peltier, "Light-Weight Alloys for Aerospace Applications," eds. E. W. Lee, E. H. Chia and N. J. Kim, TMS, Warrendale, PA, p. 71, (1989).
FIG. I.
BF T~M pictures showing (a) fine grains and (b) nanometer sized dlspersoids in AI-8FeIV-2Si extrusion.
Vot.
FIG.
15,
2.
FIG. 3.
No.
6
STRAIN
RATE A N D
TOUGHNESS
1013
Macrophotographs showing the presence and absence of delaminations at the (a) slowest and (b) fastest strain rates, respectively, at 3!5°C in AI-SFe-IV-2Si.
Fracture surfaces at the (a) slowest and (b) fastest strain rates, (a) mixture of brittle and ductile failure and (b) total ductile microvoid coalescence.