- Email: [email protected]
High temperature tensile and creep properties of a cast aim and ESR intermetallic alloy based on Fe3Al
Materials Science and Engineering A231 (1997) 206 – 210
High temperature tensile and creep properties of a cast aim and ESR intermetallic alloy based on Fe3Al R.G. Baligidad *, U. Prakash, A. Radhakrishna Defence Metallurgical Research Laboratory, Hyderabad 500 058, India Received 3 February 1997
Abstract A high carbon intermetallic Fe-16 wt.% A1-1.1 wt.% C alloy based on Fe3Al was melted under a flux cover by air induction melting (AIM). The AIM ingots exhibited excellent elevated temperature tensile properties in the temperature range (600–800°C) studied, in contrast to poor properties expected in ingots melted without a flux cover. Subsequent processing of the AIM ingots through electroslag remelting (ESR) resulted in improvement in ductility. However, the AIM ingots exhibited better creep properties because of their coarser gain structure. The presence of large (1.1 wt.%) amount of carbon in the alloy resulted in significant improvement in elevated temperature tensile as well as creep properties over those reported for Fe3Al based intermetallic alloys with lower carbon contents. These improvements in mechanical properties are attributed to the extensive precipitation of Fe3AlC phase and to the formation of a duplex Fe3Al-Fe3AlC structure at such high levels of carbon. It is suggested that carbon may be an important alloying addition to Fe3Al-based intermetallic alloys. © 1997 Elsevier Science S.A. Keywords: Intermetallics; Fe3Al; Carbon addition; Duplex Fe3Al-Fe3AlC structure; Elevated temperature properties
1. Introduction Intermetallic alloys based on the iron aluminide Fe3Al are being considered for high temperature structural applications [1,2]. They also provide potential replacement for the more expensive high temperature structural alloys based on strategic elements such as nickel, cobalt and chromium. So far Fe3Al based alloys have been generally produced only in small quantities with high purity raw materials, employing techniques such as arc melting and drop casting, vacuum induction melting and rapid solidification followed by powder compaction. Though these techniques give high quality material they are not cost effective or amenable for bulk production. Attempts to make bulk quantities by
* Corresponding author. Fax: +91 040 239683. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 9 7 ) 0 0 0 7 8 - 6
air induction melting (AIM) resulted in extensive gas porosity [3]. Recently, we have reported that use of a suitable slag cover during air induction melting of a high carbon Fe3Al based intermetallic alloy results in ingots free from gas porosity [4]. The cast AIM ingots exhibited excellent room temperature tensile properties. here we report elevated temperature tensile and creep properties of the cast AIM alloy. The effect of subsequent processing through electroslag remelting (ESR) on these properties is also reported.
2. Experimental procedure First, 30 kg melts of Fe-16 wt.% Al (28 at %A1)-1.1 wt.% C alloy were taken in a medium frequency air induction melting furnace. The AIM ingots were subsequently processed through electroslag remelting. The
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210 Table 1 Chemical analysis of AIM and ESR ingots (wt.%) Element
Aim ingot
ESR ingot
Al Mn Si P C S O N
15.38 0.310 0.210 0.016 1.100 B0.001 B0.001 0.0025
15.24 0.310 0.180 0.015 1.100 B0.001 B0.001 B0.001
207
Machine at a strain rate of 0.05 min-1. The details of sample preparation and testing procedures are given elsewhere [5]. Constant load creep and stress rupture tests were carried out at 600°C and 140 MPa. Specimens of 5 mm gauge diameter and 25 mm gauge length were machined and polished using 600 grit abrasive. The details of sample preparation and creep testing procedures are given elsewhere [6]. Tensile and creep fracture surfaces were observed in a JEOL JSM-840 scanning electron microscope (SEM).
3. Results melting practice has been described in detail elsewhere [4]. Longitudinal sections of AIM and ESR ingots were cut using a high speed abrasive silicon carbide cut-off wheel. The cut-off sections were machined and ground to 180 grit finish and then etched with Aquaregia (50% HCl+ 50% HNO3 by volume) for macrostructural examination. The longitudinal ingot sections were mechanically polished to 0.5 mm grade diamond powder finish for electron probe microanalysis (EPMA) studies. The sections were subsequently etched using an etchant consisting of 33% HNO3 +33% CH3COOH+ 33%H20+1% HF by volume for examination by optical microscopy. Longitudinal ASTM-E8M tensile specimens of 4 mm gauge diameter and 20 mm gauge length were machined and polished using 600 grit abrasive. Tensile tests were carried out at 600, 700 and 800°C in a 100 kN Instron 1185 Universal Testing
The chemical analysis of AIM and ESR ingots is given in Table 1. The AIM ingot exhibited coarse equiaxed grains whereas relatively finer columnar grains aligned almost parallel to the ingot axis were observed in the ESR ingot (Fig. 1a and b). EMPA and optical microscopy showed that the AIM and ESR ingots exhibited significant amount of coarse Fe3AlC precipitates (Fig. 2a and b). The elevated temperature mechanical properties of AIM and ESR ingots are shown in Fig. 3a and b. ESR ingot exhibits better strength and ductility compared to those exhibited by AIM ingot at all temperatures. The elevated temperature strength decreases while the ductility increases with increasing test temperature. At 600 and 700°C all the samples exhibited cleavage failure (Fig. 4a). On increasing the test temperature the failure mode changed to mixed (dimples+ cleavage) failure at 800°C (Fig. 4b).
Fig. 1. Macrostructure of longitudinal sections of (a) AIM and (b) ESR ingot.
208
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
The AIM ingot exhibited a higher creep life (550 h) compared to that (410 h) exhibited by the ESR ingot. The ESR ingot exhibited a minimum creep rate of 0.02% h-1 and elongation of 26%. The corresponding values could not be measured for the AIM ingots. The creep fracture surfaces exhibited a predominantly transgranular failure with some evidence of cavitation around the Fe3AlC precipitates (Fig. 5).
4. Discussion The improvement in elevated temperature tensile properties after ESR (Fig. 3a and b) may be attributed to the refined axially oriented columnar grain structure free from internal detects [7]. The better creep properties of the AIM ingot, on the other hand, may be attributed to the coarser grain size exhibited by the AIM ingots. The yield strength decreases sharply on testing at temperatures above 600°C (Fig. 3a and b). The reasons for high yield strength at 600°C have been discussed in detail elsewhere [5]. The decrease in yield strength at higher temperatures may be attributed to the increased dis-
Fig. 3. Showing variation of tensile properties of (a) AIM and (b) ESR ingots with temperature. The room temperature data is taken from Ref. [4].
Fig. 2. Optical micrographs of longitudinal sections of (a) AIM and (b) ESR ingots showing extensive preceipitation of Fe3AlC phase.
location mobility with increase in temperature which also leads to increased elevated temperature ductility. The AIM and ESR ingots exhibited cleavage tensile failure when tested at room temperature [4]. The transition from cleavage to mixed (dimple +cleavage) failure mode, observed in the present work at 800°C, occurs at relatively lower (700°C) temperature for low (0.14 to 0.50 wt.%) carbon ESR alloys [8]. These alloys exhibited ductile dimple failure at 800°C which is accompanied by a sharp increase in tensile ductile (Fig. 6). This sharp increase in tensile ductility is not observed in the present work. The low carbon alloys also show comparatively lower elevated temperature strength and higher ductility at the corresponding test temperatures (Fig. 6). There is no significant increase in strength on increasing the carbon content from 0.14 to 0.50 wt.%. However, 50–100% increase in elevated temperature strength is observed on increasing carbon content from 0.50 to 1.1 This may be attributed to the sharp increase in volume
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
209
fraction of the Fe3AlC precipitates on addition of 1.1 wt.% carbon. As is evident from Fig. 6 (and Fig. 4b and Fig. 5), these precipitates also form a continuous network resulting in a duplex 2Fe3Al-Fe3Al-C structure which was absent in the alloys with lower carbon content. Although is reported that the creep and stress rupture properties of Fe3Al-based alloys are significantly enhanced on addition of carbon no appreciable improvement was observed on further increasing carbon content from 0.14 to 0.5 wt.% [6]. However, improvements of up to an order of magnitude are observed in minimum creep rate as well as rupture life on increasing the carbon content from 0.5 to 1.1 wt.% (Fig. 7). This is consistent with the increase in elevated temperature strength. It would appear that carbon may be an important alloying addition to Fe3Al-based intermetallic alloys.
5. Conclusions
Fig. 4. SEM fractographs showing the change in tensile fracture mode from cleavage (a) at 600°C (and 700°C) to mixed (dimple+ cleavage) failure at 800°C (b) in AIM ingots. Similar failure modes were observed in tensile samples of ESR ingots.
(1) Air Induction Melting using slag cover yields Fe3Al-based intermetallic alloys with superior elevated temperature mechanical properties. Subsequent processing through electroslag remelting leads to improvement in tensile ductility. However, the AIM ingots exhibited better creep properties because of their coarser grain structure. (2) Addition of 1.1 wt.% carbon results in significant improvement in elevated temperature tensile as well as creep properties of Fe3Al-based intermetallic alloys. These improvements in mechanical properties are attributed to the significant amount of precipitation of Fe3AlC phase and to the formation of a duplex Fe3Al-Fe3AlC structure at such high levels of carbon.
Acknowledgements
Fig. 5. SEM fractograph sowing predominantly transgranular failure with some evidence of cavitation around the FE3AlC precipitate in creep tested samples of AIM ingots. Similar failure mode was observed in creep samples of ESR ingot.
The authors are grateful to the Director, DMRL for permission to publish this work. The authors wish to thank Dr D. Banerjee for his keen interest and encouragement, Mr D.G. Deshpande for tensile, Dr M.C. Pandey and Mr D.V.V. Satyanarayana for creep testing and Mr A. Ramalingeswara Rao for typing this manuscript.
210
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
Fig. 6. Showing variation of tensile properties of cast ESR alloys with temperature. Figs (a) – (c) are taken from Ref. [8]. The carbon content increases from (a) 0.14; (b) 0.27 and (c) 0.5 to (d) 1.1 wt.%.
References [1] C.G. McKamey, J.H. Devan, P.F. Tortorelli, V.K. Sikka, J. Mater. Res. 6 (1991) 1779. [2] U. Prakash, R.A. Buckley, H. Jones, C.M. Sellars, ISIJ Int. 81 (1991) 1113. [3] V.K. Sikka, High Temperature Ordered Intermetallic Alloys IV, MRS Sump Proc., Vol. 133, MRS, Pittsburgh, 1991, pp. 907. [4] R.G. Baligidad, U. Prakash, A. Radhakrishna, Indian Patent filed, December 1996. [5] R.G. Balligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao and N.B. Ballal, Scripta Mater., in press. [6] R.G. Baligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, ISIJ Int. 36 (1996) 1215. [7] R.G. Baligidad, U. Prakash, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, Ironmaking Steelmaking 21 (1994) 324. [8] R.G. Baligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, Nickel and Iron alumines: Processing, and applications, ASM, 1997, to be published.
Fig. 7. Showing variation of creep and stress rupture properties of case ESR alloys with carbon content. The data for alloys with 0.14, 0.27 and 0.5 wt.% carbon is taken from Ref. [6].
.
High temperature tensile and creep properties of a cast aim and ESR intermetallic alloy based on Fe3Al R.G. Baligidad *, U. Prakash, A. Radhakrishna Defence Metallurgical Research Laboratory, Hyderabad 500 058, India Received 3 February 1997
Abstract A high carbon intermetallic Fe-16 wt.% A1-1.1 wt.% C alloy based on Fe3Al was melted under a flux cover by air induction melting (AIM). The AIM ingots exhibited excellent elevated temperature tensile properties in the temperature range (600–800°C) studied, in contrast to poor properties expected in ingots melted without a flux cover. Subsequent processing of the AIM ingots through electroslag remelting (ESR) resulted in improvement in ductility. However, the AIM ingots exhibited better creep properties because of their coarser gain structure. The presence of large (1.1 wt.%) amount of carbon in the alloy resulted in significant improvement in elevated temperature tensile as well as creep properties over those reported for Fe3Al based intermetallic alloys with lower carbon contents. These improvements in mechanical properties are attributed to the extensive precipitation of Fe3AlC phase and to the formation of a duplex Fe3Al-Fe3AlC structure at such high levels of carbon. It is suggested that carbon may be an important alloying addition to Fe3Al-based intermetallic alloys. © 1997 Elsevier Science S.A. Keywords: Intermetallics; Fe3Al; Carbon addition; Duplex Fe3Al-Fe3AlC structure; Elevated temperature properties
1. Introduction Intermetallic alloys based on the iron aluminide Fe3Al are being considered for high temperature structural applications [1,2]. They also provide potential replacement for the more expensive high temperature structural alloys based on strategic elements such as nickel, cobalt and chromium. So far Fe3Al based alloys have been generally produced only in small quantities with high purity raw materials, employing techniques such as arc melting and drop casting, vacuum induction melting and rapid solidification followed by powder compaction. Though these techniques give high quality material they are not cost effective or amenable for bulk production. Attempts to make bulk quantities by
* Corresponding author. Fax: +91 040 239683. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 9 7 ) 0 0 0 7 8 - 6
air induction melting (AIM) resulted in extensive gas porosity [3]. Recently, we have reported that use of a suitable slag cover during air induction melting of a high carbon Fe3Al based intermetallic alloy results in ingots free from gas porosity [4]. The cast AIM ingots exhibited excellent room temperature tensile properties. here we report elevated temperature tensile and creep properties of the cast AIM alloy. The effect of subsequent processing through electroslag remelting (ESR) on these properties is also reported.
2. Experimental procedure First, 30 kg melts of Fe-16 wt.% Al (28 at %A1)-1.1 wt.% C alloy were taken in a medium frequency air induction melting furnace. The AIM ingots were subsequently processed through electroslag remelting. The
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210 Table 1 Chemical analysis of AIM and ESR ingots (wt.%) Element
Aim ingot
ESR ingot
Al Mn Si P C S O N
15.38 0.310 0.210 0.016 1.100 B0.001 B0.001 0.0025
15.24 0.310 0.180 0.015 1.100 B0.001 B0.001 B0.001
207
Machine at a strain rate of 0.05 min-1. The details of sample preparation and testing procedures are given elsewhere [5]. Constant load creep and stress rupture tests were carried out at 600°C and 140 MPa. Specimens of 5 mm gauge diameter and 25 mm gauge length were machined and polished using 600 grit abrasive. The details of sample preparation and creep testing procedures are given elsewhere [6]. Tensile and creep fracture surfaces were observed in a JEOL JSM-840 scanning electron microscope (SEM).
3. Results melting practice has been described in detail elsewhere [4]. Longitudinal sections of AIM and ESR ingots were cut using a high speed abrasive silicon carbide cut-off wheel. The cut-off sections were machined and ground to 180 grit finish and then etched with Aquaregia (50% HCl+ 50% HNO3 by volume) for macrostructural examination. The longitudinal ingot sections were mechanically polished to 0.5 mm grade diamond powder finish for electron probe microanalysis (EPMA) studies. The sections were subsequently etched using an etchant consisting of 33% HNO3 +33% CH3COOH+ 33%H20+1% HF by volume for examination by optical microscopy. Longitudinal ASTM-E8M tensile specimens of 4 mm gauge diameter and 20 mm gauge length were machined and polished using 600 grit abrasive. Tensile tests were carried out at 600, 700 and 800°C in a 100 kN Instron 1185 Universal Testing
The chemical analysis of AIM and ESR ingots is given in Table 1. The AIM ingot exhibited coarse equiaxed grains whereas relatively finer columnar grains aligned almost parallel to the ingot axis were observed in the ESR ingot (Fig. 1a and b). EMPA and optical microscopy showed that the AIM and ESR ingots exhibited significant amount of coarse Fe3AlC precipitates (Fig. 2a and b). The elevated temperature mechanical properties of AIM and ESR ingots are shown in Fig. 3a and b. ESR ingot exhibits better strength and ductility compared to those exhibited by AIM ingot at all temperatures. The elevated temperature strength decreases while the ductility increases with increasing test temperature. At 600 and 700°C all the samples exhibited cleavage failure (Fig. 4a). On increasing the test temperature the failure mode changed to mixed (dimples+ cleavage) failure at 800°C (Fig. 4b).
Fig. 1. Macrostructure of longitudinal sections of (a) AIM and (b) ESR ingot.
208
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
The AIM ingot exhibited a higher creep life (550 h) compared to that (410 h) exhibited by the ESR ingot. The ESR ingot exhibited a minimum creep rate of 0.02% h-1 and elongation of 26%. The corresponding values could not be measured for the AIM ingots. The creep fracture surfaces exhibited a predominantly transgranular failure with some evidence of cavitation around the Fe3AlC precipitates (Fig. 5).
4. Discussion The improvement in elevated temperature tensile properties after ESR (Fig. 3a and b) may be attributed to the refined axially oriented columnar grain structure free from internal detects [7]. The better creep properties of the AIM ingot, on the other hand, may be attributed to the coarser grain size exhibited by the AIM ingots. The yield strength decreases sharply on testing at temperatures above 600°C (Fig. 3a and b). The reasons for high yield strength at 600°C have been discussed in detail elsewhere [5]. The decrease in yield strength at higher temperatures may be attributed to the increased dis-
Fig. 3. Showing variation of tensile properties of (a) AIM and (b) ESR ingots with temperature. The room temperature data is taken from Ref. [4].
Fig. 2. Optical micrographs of longitudinal sections of (a) AIM and (b) ESR ingots showing extensive preceipitation of Fe3AlC phase.
location mobility with increase in temperature which also leads to increased elevated temperature ductility. The AIM and ESR ingots exhibited cleavage tensile failure when tested at room temperature [4]. The transition from cleavage to mixed (dimple +cleavage) failure mode, observed in the present work at 800°C, occurs at relatively lower (700°C) temperature for low (0.14 to 0.50 wt.%) carbon ESR alloys [8]. These alloys exhibited ductile dimple failure at 800°C which is accompanied by a sharp increase in tensile ductile (Fig. 6). This sharp increase in tensile ductility is not observed in the present work. The low carbon alloys also show comparatively lower elevated temperature strength and higher ductility at the corresponding test temperatures (Fig. 6). There is no significant increase in strength on increasing the carbon content from 0.14 to 0.50 wt.%. However, 50–100% increase in elevated temperature strength is observed on increasing carbon content from 0.50 to 1.1 This may be attributed to the sharp increase in volume
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
209
fraction of the Fe3AlC precipitates on addition of 1.1 wt.% carbon. As is evident from Fig. 6 (and Fig. 4b and Fig. 5), these precipitates also form a continuous network resulting in a duplex 2Fe3Al-Fe3Al-C structure which was absent in the alloys with lower carbon content. Although is reported that the creep and stress rupture properties of Fe3Al-based alloys are significantly enhanced on addition of carbon no appreciable improvement was observed on further increasing carbon content from 0.14 to 0.5 wt.% [6]. However, improvements of up to an order of magnitude are observed in minimum creep rate as well as rupture life on increasing the carbon content from 0.5 to 1.1 wt.% (Fig. 7). This is consistent with the increase in elevated temperature strength. It would appear that carbon may be an important alloying addition to Fe3Al-based intermetallic alloys.
5. Conclusions
Fig. 4. SEM fractographs showing the change in tensile fracture mode from cleavage (a) at 600°C (and 700°C) to mixed (dimple+ cleavage) failure at 800°C (b) in AIM ingots. Similar failure modes were observed in tensile samples of ESR ingots.
(1) Air Induction Melting using slag cover yields Fe3Al-based intermetallic alloys with superior elevated temperature mechanical properties. Subsequent processing through electroslag remelting leads to improvement in tensile ductility. However, the AIM ingots exhibited better creep properties because of their coarser grain structure. (2) Addition of 1.1 wt.% carbon results in significant improvement in elevated temperature tensile as well as creep properties of Fe3Al-based intermetallic alloys. These improvements in mechanical properties are attributed to the significant amount of precipitation of Fe3AlC phase and to the formation of a duplex Fe3Al-Fe3AlC structure at such high levels of carbon.
Acknowledgements
Fig. 5. SEM fractograph sowing predominantly transgranular failure with some evidence of cavitation around the FE3AlC precipitate in creep tested samples of AIM ingots. Similar failure mode was observed in creep samples of ESR ingot.
The authors are grateful to the Director, DMRL for permission to publish this work. The authors wish to thank Dr D. Banerjee for his keen interest and encouragement, Mr D.G. Deshpande for tensile, Dr M.C. Pandey and Mr D.V.V. Satyanarayana for creep testing and Mr A. Ramalingeswara Rao for typing this manuscript.
210
R.G. Baligidad et al. / Materials Science and Engineering A231 (1997) 206–210
Fig. 6. Showing variation of tensile properties of cast ESR alloys with temperature. Figs (a) – (c) are taken from Ref. [8]. The carbon content increases from (a) 0.14; (b) 0.27 and (c) 0.5 to (d) 1.1 wt.%.
References [1] C.G. McKamey, J.H. Devan, P.F. Tortorelli, V.K. Sikka, J. Mater. Res. 6 (1991) 1779. [2] U. Prakash, R.A. Buckley, H. Jones, C.M. Sellars, ISIJ Int. 81 (1991) 1113. [3] V.K. Sikka, High Temperature Ordered Intermetallic Alloys IV, MRS Sump Proc., Vol. 133, MRS, Pittsburgh, 1991, pp. 907. [4] R.G. Baligidad, U. Prakash, A. Radhakrishna, Indian Patent filed, December 1996. [5] R.G. Balligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao and N.B. Ballal, Scripta Mater., in press. [6] R.G. Baligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, ISIJ Int. 36 (1996) 1215. [7] R.G. Baligidad, U. Prakash, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, Ironmaking Steelmaking 21 (1994) 324. [8] R.G. Baligidad, U. Prakash, A. Radhakrishna, V. Ramakrishna Rao, P.K. Rao, N.B. Ballal, Nickel and Iron alumines: Processing, and applications, ASM, 1997, to be published.
Fig. 7. Showing variation of creep and stress rupture properties of case ESR alloys with carbon content. The data for alloys with 0.14, 0.27 and 0.5 wt.% carbon is taken from Ref. [6].
.