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Deformation and fracture of L12 (Al,Fe)3Ti
S c r i p t a METALLURGICA e t MATERIALIA
Vol.
24, pp. 2 1 8 7 - 2 1 9 0 , 1 9 9 0 ; Printed in the U.S.A.
Pergamon P r e s s p l c All rights reserved
DEFORMATION AND FRACTURE OF LI 2 (AI,Fe)3Ti Z.L. Wu, D.P. Pope and V.Vitek Department of Materials Science and Engineering University of Pennsylvania, Philadelphia, 19104 (Received August
20,
1990)
lntn~lctiqn Intermetallic AI3Ti is an attractive material for aerospace applications as it combines low density with a high melting temperature. In stoichiometric form A13Ti has a tetragonal DO n structure and is very brittle, but it can be transformed into the LI 2 form by adding a few atomic percent of elements such as Cu, Cr, Fe, Mn, Ni and Zn (1-7). Since the L12 structure has a higher symmetry than the DO22 structure, it has been expected that the transformation will improve the ductility. The A164.tFe9.TTi25.7alloy used in this study has the L12 structure. In this work, compression tests were performed to determine the temperature dependence of the flow stress, the strain rate sensitivity of the flow stress, the strain to failure and the fracture mode. Three-point bending tests were also performed. Exoedmental The alloy used in the study was an as-cast ingot, 5.5 cm in diameter and 16 cm in length, of nominal composition AI67FegTi24,provided by Martin Marietta laboratories. The ingot was homogenized at 1100°C for 60 hours in an argon atmosphere. The phase distribution and chemical composition of the ingot were examined using the x-ray powder diffraction technique and quantitative energy dispersive x-ray analysis in an SEM. Compression specimens of dimension 3x3x5 mm and average grain size about 0.5-0.7 mm were cut using EDM from the outer, porosity-free regions of the ingot. The specimens were mechanically polished on grade 600 SiC paper before testing. The flow stress was measured in compression on an Instron testing machine at four different strain rates at temperatures between 77K and 1300K. Three-point bending tests were performed at room temperature on 2.4x6x20 mm samples at two different strain rates. The bending axis was parallel to the 6 mm dimension. Fracture tests were carded out in compression at five different temperatures, and the fracture surfaces were examined using scanning electron microscopy. Results and gi~c~tssi0ns X-ray powder diffraction and full quantitative energy dispersive x-ray analysis confu-med that the alloy has an L12 matrix with a few volume percent of second phase panicles. The chemical composition of the matrix is A164.tFeg.TTi25.7. The compositions of the second phase vary but most of them contain more that 70 at% titanium, and are believed to be due to incomplete melting of some of the Ti powder during the casting process. The flow behavior as a function of temperature and strain rate is shown in Fig. 1. The flow stress shows a small increase between 300K and 850K, and changes rapidly at low and high temperatures, similar to that of the platinum-based L12 alloys with A1, Ga and In (8-10). The flow stress is also strain rate sensitive. For example, the flow stress increases by about 60 MPa at 780K due to a ten-fold increase in strain rate from 1.75xi0-4 s"t to 1.75x10 -3 s"1. The overall temperature dependence of flow behavior for different strain rates is essentially the same. However it should be noted that the shape of the flow stress vs. temperature curve differs somewhat from that of the AI2aFe3Ti 8 alloy reported by Kumar and Pickens (I 1), who found a more gradual increase at low
2187 0036-9748/90 $5.00 + .00 Copyright (c) 1990 Pergamon Press plc
2188
DEFORMATION
OF AI-Fe-Ti
Vol.
24, No. ii
temperatures and a higher overall strength. The difference may be the result of the differences in composition and heat treatrnen. Compression tests were performed to fracture at a strain rate of 1.75x10 -4 s"t at 77, 297, 829, 974 and 1273K (Fig. 2). Substantial ductility was observed at all temperatures. At 1273K, no fracture occured even above 70% plastic strain (Table 1). As shown in Fig.3 transgranular cleavage is the dominant fracture mode. Many secondary cracks are also visible in the fractured samples as seen in Fig.4 which shows a cross-sectional area perpendicular to the compressive axis of a specimen tested to failure at 829K. Note the proliferation of cracks. The samples deformed at 297 and 829K show about the same compressive ductility, but the former sample shattered at failure, making it impossible to observed the secondary cracks. Although the alloy showed substantial ductility in compression, it is very brittle in bending. Fig.5 shows load-deflection curves measured in three-point bending tests at two different strain rates at room temperature. Note that the sample tested at the higher strain rate failed at a smaller load and deflection. The fracture mechanism is clearly transgranular cleavage (Fig.6). A fractured bend sample was then sectioned perpendicularly to the fracture surface and no secondary cracks were observed. Unlike in the compression tests, once cracks are initiated in bending, they propagate quickly, leaving no chance for multiple cracking to occur. We believe that much of the apparent ductility observed in the low temperature compression tests, Fig.2, is in fact the result of the proliferation of cracks seen in these samples. No such cracks form in bending since the first crack to form propagates catastrophically. Conclusions AI64.6Fe9.TTi25.7has a flow behavior similar to Pt3Al-type alloys. The flow stress is strain rate sensitive and exhibits a weak positive temperature dependence at elevated temperatures. The alloy shows substantial ductility in compression, while it is very brittle in bending. The fracture mode is predominantly transgranular cleavage. The ductility seen in compression tests at low temperatures is probably the result of a proliferation of microcracks in these samples. Acknowled=ements This work was supported by AFOSR under Grant AFOSR-89-0062. Research facilities were provided by the LRSM supported by the NSF program under Grant DMR88-19885. The authors are grateful to K.S. Kumar for providing the AI-Ti-Fe alloy ingot and to W.J. Romanow and R. Hsiao for experimental assistance. TABLE I. Fracture Stresses and Corresponding Plastic Strains at Different Temperatures Temperature (K)
Strain (%)
Stress(MPa)
77
7.5
748
297
14.5
957
829
18.1
823
974
19.0
662
1273
>70
991"
* no fracture occurs. .References 1. 2. 3. 4.
A.Raman, K.Schubert, Z. Metallk. 56, 40 (1965). A.Raman, K.Schubert, Z. Metallk. 56, 99 (1965). P.Virdis, U.Zwicker, Z. Metallk. 62, 46 (1971). S.Mazdiyasni, D.B.Miracle, D.M.Dimiduk, M.G.Mendiratta and
Vol.
2~, No.
Ii
DEFOR~L~TION OF AI-Fe-Ti
2189
P.R.Subramanian, Scripta metall. 23,327 (1989). 5. H.Mabuchi, K.Hirukawa, Y.Nakaya.ma, Scripta metall. 23,1761 (1989). 6. $.Zhang, ,r.p.Nic, D.E.Mikkola, Scripta metall. 24, 57 (1990). 7. H.Mavuchi, K.Hirukawa, H.Tsuda and Y.Makauama, Scripta metal!. 24,505 (1990). 8. D.M.Wee, O.Noguchi, Y.Oya, T.Suzuki, Trans. JIM 21,237 (1980). 9. D.M,Wee, D.P.Pope and V.Vitek, Acta metall, 32, 829 (1984). 10. F.E.Heredia, G.Tichy,D.P.Pope and V.Vitek, Acta metall. 37, 2755 (I989). 11. K.S.Kumar, LR.Pickens, Scripta metall. 22, 1015 (1988). 500
lo~o /
2g7K
~v
8C0 •
400. A
al o. 6c0 • ~-
300-
3: O
400. 200' • 0
¢,1 o
.
200 •
4 . 3 5 x 1 0 "4 s - I 8 . 7 ~ x 1 0 ~1 1 " l
• 1 O0
~
127
1 . 7 5 x l 0 "3 s" l .
.
.
i
500
.
.
.
.
,
"
I (]00
TEMPERATURE
0
"
1500
(K)
Fig. 1 Temperature dependence of the flow stress at different strain rates.
,
0
20
L
4o STRAIN
,
;
60
$0
100
(%)
Fig. 2 Stress-strain curves at the temperatures indicated.
Fig. 3 Fracture morphologies after fracture in compression at (a) 77K, (b) 297K. (c) 829K and (d) 974K.
2190
DEFO~L~TION
OF AI-Fe-Ti
Vol.
24, No.
50
4O
3O
0 ,,,J
20'
10 '
2.54x 104 minis 1.27xi0 "1 mints
Deflection
Fig. 4 A cross-section perpendicular to the compressive axis of a sample tested to failure in compression at 829K. Note the multiple cracking.
(10 -2 ram)
Fig. 5 Load-deflection curves for the samples testext in three-point bending at room temperature at cross head speeds 2.54x10 -4 minis and 1.27×10 -3 minis.
Fig. 6 Fracture surface after deformation in three-point bending.
ii
Vol.
24, pp. 2 1 8 7 - 2 1 9 0 , 1 9 9 0 ; Printed in the U.S.A.
Pergamon P r e s s p l c All rights reserved
DEFORMATION AND FRACTURE OF LI 2 (AI,Fe)3Ti Z.L. Wu, D.P. Pope and V.Vitek Department of Materials Science and Engineering University of Pennsylvania, Philadelphia, 19104 (Received August
20,
1990)
lntn~lctiqn Intermetallic AI3Ti is an attractive material for aerospace applications as it combines low density with a high melting temperature. In stoichiometric form A13Ti has a tetragonal DO n structure and is very brittle, but it can be transformed into the LI 2 form by adding a few atomic percent of elements such as Cu, Cr, Fe, Mn, Ni and Zn (1-7). Since the L12 structure has a higher symmetry than the DO22 structure, it has been expected that the transformation will improve the ductility. The A164.tFe9.TTi25.7alloy used in this study has the L12 structure. In this work, compression tests were performed to determine the temperature dependence of the flow stress, the strain rate sensitivity of the flow stress, the strain to failure and the fracture mode. Three-point bending tests were also performed. Exoedmental The alloy used in the study was an as-cast ingot, 5.5 cm in diameter and 16 cm in length, of nominal composition AI67FegTi24,provided by Martin Marietta laboratories. The ingot was homogenized at 1100°C for 60 hours in an argon atmosphere. The phase distribution and chemical composition of the ingot were examined using the x-ray powder diffraction technique and quantitative energy dispersive x-ray analysis in an SEM. Compression specimens of dimension 3x3x5 mm and average grain size about 0.5-0.7 mm were cut using EDM from the outer, porosity-free regions of the ingot. The specimens were mechanically polished on grade 600 SiC paper before testing. The flow stress was measured in compression on an Instron testing machine at four different strain rates at temperatures between 77K and 1300K. Three-point bending tests were performed at room temperature on 2.4x6x20 mm samples at two different strain rates. The bending axis was parallel to the 6 mm dimension. Fracture tests were carded out in compression at five different temperatures, and the fracture surfaces were examined using scanning electron microscopy. Results and gi~c~tssi0ns X-ray powder diffraction and full quantitative energy dispersive x-ray analysis confu-med that the alloy has an L12 matrix with a few volume percent of second phase panicles. The chemical composition of the matrix is A164.tFeg.TTi25.7. The compositions of the second phase vary but most of them contain more that 70 at% titanium, and are believed to be due to incomplete melting of some of the Ti powder during the casting process. The flow behavior as a function of temperature and strain rate is shown in Fig. 1. The flow stress shows a small increase between 300K and 850K, and changes rapidly at low and high temperatures, similar to that of the platinum-based L12 alloys with A1, Ga and In (8-10). The flow stress is also strain rate sensitive. For example, the flow stress increases by about 60 MPa at 780K due to a ten-fold increase in strain rate from 1.75xi0-4 s"t to 1.75x10 -3 s"1. The overall temperature dependence of flow behavior for different strain rates is essentially the same. However it should be noted that the shape of the flow stress vs. temperature curve differs somewhat from that of the AI2aFe3Ti 8 alloy reported by Kumar and Pickens (I 1), who found a more gradual increase at low
2187 0036-9748/90 $5.00 + .00 Copyright (c) 1990 Pergamon Press plc
2188
DEFORMATION
OF AI-Fe-Ti
Vol.
24, No. ii
temperatures and a higher overall strength. The difference may be the result of the differences in composition and heat treatrnen. Compression tests were performed to fracture at a strain rate of 1.75x10 -4 s"t at 77, 297, 829, 974 and 1273K (Fig. 2). Substantial ductility was observed at all temperatures. At 1273K, no fracture occured even above 70% plastic strain (Table 1). As shown in Fig.3 transgranular cleavage is the dominant fracture mode. Many secondary cracks are also visible in the fractured samples as seen in Fig.4 which shows a cross-sectional area perpendicular to the compressive axis of a specimen tested to failure at 829K. Note the proliferation of cracks. The samples deformed at 297 and 829K show about the same compressive ductility, but the former sample shattered at failure, making it impossible to observed the secondary cracks. Although the alloy showed substantial ductility in compression, it is very brittle in bending. Fig.5 shows load-deflection curves measured in three-point bending tests at two different strain rates at room temperature. Note that the sample tested at the higher strain rate failed at a smaller load and deflection. The fracture mechanism is clearly transgranular cleavage (Fig.6). A fractured bend sample was then sectioned perpendicularly to the fracture surface and no secondary cracks were observed. Unlike in the compression tests, once cracks are initiated in bending, they propagate quickly, leaving no chance for multiple cracking to occur. We believe that much of the apparent ductility observed in the low temperature compression tests, Fig.2, is in fact the result of the proliferation of cracks seen in these samples. No such cracks form in bending since the first crack to form propagates catastrophically. Conclusions AI64.6Fe9.TTi25.7has a flow behavior similar to Pt3Al-type alloys. The flow stress is strain rate sensitive and exhibits a weak positive temperature dependence at elevated temperatures. The alloy shows substantial ductility in compression, while it is very brittle in bending. The fracture mode is predominantly transgranular cleavage. The ductility seen in compression tests at low temperatures is probably the result of a proliferation of microcracks in these samples. Acknowled=ements This work was supported by AFOSR under Grant AFOSR-89-0062. Research facilities were provided by the LRSM supported by the NSF program under Grant DMR88-19885. The authors are grateful to K.S. Kumar for providing the AI-Ti-Fe alloy ingot and to W.J. Romanow and R. Hsiao for experimental assistance. TABLE I. Fracture Stresses and Corresponding Plastic Strains at Different Temperatures Temperature (K)
Strain (%)
Stress(MPa)
77
7.5
748
297
14.5
957
829
18.1
823
974
19.0
662
1273
>70
991"
* no fracture occurs. .References 1. 2. 3. 4.
A.Raman, K.Schubert, Z. Metallk. 56, 40 (1965). A.Raman, K.Schubert, Z. Metallk. 56, 99 (1965). P.Virdis, U.Zwicker, Z. Metallk. 62, 46 (1971). S.Mazdiyasni, D.B.Miracle, D.M.Dimiduk, M.G.Mendiratta and
Vol.
2~, No.
Ii
DEFOR~L~TION OF AI-Fe-Ti
2189
P.R.Subramanian, Scripta metall. 23,327 (1989). 5. H.Mabuchi, K.Hirukawa, Y.Nakaya.ma, Scripta metall. 23,1761 (1989). 6. $.Zhang, ,r.p.Nic, D.E.Mikkola, Scripta metall. 24, 57 (1990). 7. H.Mavuchi, K.Hirukawa, H.Tsuda and Y.Makauama, Scripta metal!. 24,505 (1990). 8. D.M.Wee, O.Noguchi, Y.Oya, T.Suzuki, Trans. JIM 21,237 (1980). 9. D.M,Wee, D.P.Pope and V.Vitek, Acta metall, 32, 829 (1984). 10. F.E.Heredia, G.Tichy,D.P.Pope and V.Vitek, Acta metall. 37, 2755 (I989). 11. K.S.Kumar, LR.Pickens, Scripta metall. 22, 1015 (1988). 500
lo~o /
2g7K
~v
8C0 •
400. A
al o. 6c0 • ~-
300-
3: O
400. 200' • 0
¢,1 o
.
200 •
4 . 3 5 x 1 0 "4 s - I 8 . 7 ~ x 1 0 ~1 1 " l
• 1 O0
~
127
1 . 7 5 x l 0 "3 s" l .
.
.
i
500
.
.
.
.
,
"
I (]00
TEMPERATURE
0
"
1500
(K)
Fig. 1 Temperature dependence of the flow stress at different strain rates.
,
0
20
L
4o STRAIN
,
;
60
$0
100
(%)
Fig. 2 Stress-strain curves at the temperatures indicated.
Fig. 3 Fracture morphologies after fracture in compression at (a) 77K, (b) 297K. (c) 829K and (d) 974K.
2190
DEFO~L~TION
OF AI-Fe-Ti
Vol.
24, No.
50
4O
3O
0 ,,,J
20'
10 '
2.54x 104 minis 1.27xi0 "1 mints
Deflection
Fig. 4 A cross-section perpendicular to the compressive axis of a sample tested to failure in compression at 829K. Note the multiple cracking.
(10 -2 ram)
Fig. 5 Load-deflection curves for the samples testext in three-point bending at room temperature at cross head speeds 2.54x10 -4 minis and 1.27×10 -3 minis.
Fig. 6 Fracture surface after deformation in three-point bending.
ii