Effect of niobium on microstructure and mechanical properties of high carbon Fe–10.5 wt.% Al alloys

Materials Science and Engineering A368 (2004) 131–138

Effect of niobium on microstructure and mechanical properties of high carbon Fe–10.5 wt.% Al alloys R.G. Baligidad∗ Defense Metallurgical Research Laboratory, Ministry of Defense, P.O. Kanchanbagh, Hyderabad 500 058, India Received 12 July 2003; received in revised form 7 October 2003; accepted 7 October 2003

Abstract The effect of niobium on the microstructure and mechanical properties of high carbon Fe–10.5 wt.% Al alloys has been investigated. The alloys were prepared by a combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot-forged and hot-rolled at 1373 K. The hot-rolled alloys were characterized. The ternary Fe–10.5 wt.% Al–(0.7 and 0.9 wt.%) C alloys exhibited two-phase microstructure of large volume fraction of Fe3 AlC0.5 precipitates in Fe–Al (␣) matrix. Addition of niobium to Fe–10.5 wt.% Al–(0.7 and 0.9 wt.%) C alloys resulted in the precipitation of small volume fraction of niobium carbide precipitates in Fe–Al (␣) matrix in addition to large volume fraction of Fe3 AlC0.5 precipitates. The addition of up to 2 wt.% Nb to high carbon Fe–10.5 wt.% Al alloys has no effect on the yield strength at both room temperature and 873 K as well as creep properties at 140 MPa and 873 K, but it has reduced the room temperature tensile elongation at higher (2 wt.%) concentration. In the present work, it has also been observed that alloys containing high (0.9 wt.%) carbon, exhibited higher yield strength at room temperature as compared to alloys containing low (0.7 wt.%) carbon. The increase in strength with small increase in carbon may be attributed to the significant increase in volume fraction of Fe3 AlC 0.5 precipitates. © 2003 Elsevier B.V. All rights reserved. Keywords: Iron aluminides; Niobium addition; Microstructure; Mechanical properties; Electroslag remelting

1. Introduction Iron aluminides based on Fe3 Al are attractive for high temperature applications owing to their low density, low material cost, good wear resistance, and excellent corrosion resistance in oxidizing and sulphidizing atmospheres [1–3]. They are potential low cost replacement for austenitic and ferritic steels in many applications including heating elements, furnace fixtures, heat exchanger piping, automobile, and other industrial valve components. Although, binary Fe3 Al alloys can exhibit brittle behavior at room temperature, poor toughness, poor machinability, and low strength as well as poor creep resistance at temperatures above 873 K, improvements in these respects may be achieved by alloying additions and process control [3,4]. Recent developments have indicated that the room temperature ductility of Fe3 Al alloy can be improved by addition of Cr, Ce, and combined addition of Zr and C [5–8]. The addition of Nb, Mo, and W has improved the high temperature strength and creep ∗

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resistance [9–11]. Recently, it has been shown that the carbon may be an important alloying addition to Fe–Al alloys containing 8.5–16 wt.% aluminum. The addition of carbon leads to a improved strength, creep resistance, machinability, and resistance to environmental embitterment [12–15]. The purpose of the present paper is to report the effect of a combined addition of carbon and niobium on microstructure and mechanical properties of hot-rolled Fe–10.5 wt.% Al alloy at ambient temperature and 873 K.

2. Experimental procedures Forty-kilogram melts of five alloys were prepared by a combination of air induction-melting with flux cover (AIMFC) and electroslag remelting (ESR). The nominal compositions of alloys are shown in Table 1 (all compositions are in wt.% unless otherwise specified). The melting practice has been described in details elsewhere [16,17]. The ESR ingots of 80-mm diameter were held in the hearth furnace at 1373 K for 1 h and hot-forged in a 1000 kg hammer forge to a thickness of 24-mm. The forged

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Table 1 Nominal alloy compositions of ESR hot-rolled alloys Alloy

Alloy composition (wt.%)

1 2 3 4 5

Fe–10.5Al–0.7C Fe–10.5 Al–0.9C Fe–10.5Al–0.7C–0.5Nb Fe–10.5Al–0.9C–0.8Nb Fe–10.5Al–0.9C–2Nb

imens of 5-mm gauge diameter and 25-mm gauge length were machined and polished using 600-grit abrasive. All the creep tests were carried out at 873 K and 140 MPa till the specimen failed.

3. Results and discussion Scanning electron micrograph of hot-rolled ESR ingots of Alloys 1–5 are shown in Figs. 1 and 2. The composition of the alloy phases determined by EDAX is summarized in Table 2. The matrix in Alloys 1–5 contains about 90% iron and 10% aluminum (Table 2). X-ray diffraction analysis confirmed the matrix to have Fe–Al (␣) phase (Fig. 3). The composition of the second phase particles (Precipitate-1, Table 2) which appear relatively dark in back scattered electron micrographs of SEM of Alloys 1–5 (Fig. 2) was found to be within the range of composition of perovskite phase Fe3 AlC (Table 2). This was confirmed to be Fe3 AlC0.5 phase by X-ray diffraction analysis (Fig. 3). The EPMA quantitative analysis of fine, very small fraction of bright precipitates (Precipitate-2, Table 2) observed in Alloy 3, 4, and 5 (Fig. 2c-e) and bright cuboidal shaped precipitates observed in Alloy 5 (Figs. 2e and 4) indicates that the precipitates are enriched with Nb and may probably be identified as niobium–carbide (Nb2 C). The EPMA quantitative analysis of dark phase (Fe3 AlC0.5 ) and matrix of Alloys-3, 4, and 5 revealed no obvious presence of niobium (Table 2), suggesting that there is no solubility of Nb in both Fe–Al and Fe3 AlC0.5 phases and all the Nb was used in the formation of niobium–carbide precipitates. The volume fraction of Fe–Al matrix, Fe3 AlC0.5 , and Nb2 C precipitates were calculated assuming all the niobium in the alloy to be Nb2 C phase and

ESR billets were subsequently hot-rolled at 1373 K to thickness 12 mm. Longitudinal sections of hot-rolled ESR ingots were mechanically polished to 0.5 ␮m grade diamond powder finish and etched with an etchant composed of 33% HNO3 + 33% CH3 COOH+33% H2 O+1% HF by volume for microstructural examination. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) studies were carried out to determine the matrix and precipitate composition and to identify the phases present in the alloys. For X-ray diffraction (XRD) studies in a Philips 3710 diffractometer, the hot-rolled ingots were machined and powder samples were prepared from the turning, ground and sieved to −200 mesh (75 ␮m) size. The bulk hardness measurements were made on hot-rolled microscopy samples using a Vickers hardness machine with 30-kg load. Longitudinal ASTM-E8M tensile specimens of 4.0-mm gauge diameter and 20-mm gauge length were machined from hot-rolled ESR alloys. Tensile tests were carried out at room temperature and at 873 K in 100 kN Instron 1185 Universal Testing Machine at a strain rate of 0.05 m–1 . The selected tensile fracture sample surfaces were examined in SEM. Constant load creep and stress rupture tests were carried out on hot-rolled ESR alloys. For these tests specTable 2 EPMA quantitative analyses of ESR hot-rolled alloys Alloy

Alloy composition (wt.%)

Elements

Matrix (wt.%)

Precipitate 1 (dark in color, wt.%)

Precipitate 2 (bright in color, wt.%)

1

Fe–10.5Al–0.7C

Fe Al C

90.5 8.9

83.2 13.5 3.3

Nil

2

Fe–10.5Al–0.9C

Fe Al C

90.0 9.8

83.7 13.2 3.0

Nil

3

Fe–10.5Al–0.7C–0.5Nb

Fe Al C Nb

90.0 9.9 – –

83.6 13.2 3.2 –

3.7 – 6.2 90.1

4

Fe–10.5Al–0.9C–0.8Nb

Fe Al C Nb

89.8 10.1 – –

83.4 13.2 3.4 –

3.6 – 6.4 90.0

5

Fe–10.5Al–0.9C–2Nb

Fe Al C Nb

90.3 9.8 – –

83.8 13.2 3.0 –

2.7 – 6.5 90.8

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Fig. 1. SEM secondary electron micrographs showing large volume fraction of precipitates in hot-rolled ESR ingots of (a) Fe–10.5Al–0.7C [22], (b) Fe–10.5Al–0.9C, (c) Fe–10.5Al–0.7C–0.5Nb, (d) Fe–10.5Al–0.9C–0.8Nb [22] and (e) Fe–10.5Al–0.9C–2Nb alloys.

the excess carbon over and above required for the formation of Nb2 C in the alloy to be present in the Fe3 AlC0.5 phase (Table 3). The addition of 0.5% Nb to Alloy 1 and 0.8% Nb to Alloy 2 has resulted in the formation of 0.5 and 0.7% volume fraction of Nb2 C precipitates, respectively, without any significant change in the volume fraction of Fe3 AlC0.5 precipitates (Table 3). Whereas the increase in the Nb addition from 0.8 to 2.0% in Alloy 2 has resulted in the increase in the volume fraction of Nb2 C precipitates from 0.7 to 1.8% with significant reduction in the volume fraction of Fe3 AlC0.5 precipitates (31–27%). This is because, more amount of carbon is required to form a given volume fraction of Nb2 C as compared to Fe3 AlC0.5 . Niobium addition to the binary Fe3 Al alloy has been found to increase the yield strength at temperature up to 923 K. This has been attributed to a solid solution strengthening as well as the formation of fine Fe2 Nb precipitates in binary Fe3 Al and niobium carbide precipitates in Fe3 Al alloys containing small amount of carbon as an impurity, which strengthen both the matrix and grain boundaries [18,19].

In the present work, the addition of niobium to ternary Fe–10.5Al–(0.7 and 0.9) C alloys did not improve the yield strength both at room temperature and 873 K (Table 4). This is mainly because there is no solid solubility of niobium in high carbon Fe–10.5 wt.% Al alloys. Though, all

Table 3 Volume fraction of phases present in ESR hot-rolled alloys Alloy

Alloy composition (wt.%)

Volume fraction of phases (%) Fe3 AlC0.5

Nb2 C

Fe–Al

1 2 3 4 5

Fe-10.5Al–0.7C Fe–10.5 Al–0.9C Fe–10.5Al–0.7C–0.5Nb Fe–10.5Al–0.9C–0.8Nb Fe–10.5Al–0.9C–2Nb

25 32 24 31 27

– – 0.5 0.7 1.8

75 68 75.5 68.3 71.2

Density of Fe3 AlC0.5 : 6.124 g/cm3 [23], density of Fe–Al: 6.700 g/cm3 , density of Nb2 C: 7.820 g/cm3 [24], calculation for volume fractions of carbide phases were made considering all the carbon in the alloys to be in the form of Nb2 C and Fe3 AlC0.5 precipitates.

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Fig. 2. SEM back scattered micrographs showing large volume fraction of dark precipitates in hot-rolled ESR ingots of (a) Fe–10.5Al–0.7C [22] and (b) Fe–10.5Al–0.9C alloys and large volume fraction of dark precipitates and small volume fraction of bright precipitates in hot-rolled ESR ingots of (c) Fe–10.5Al–0.7C–0.5Nb, (d) Fe–10.5Al–0.9C–0.8Nb [22] and (e) Fe–10.5Al–0.9C–2Nb alloys.

Fig. 3. XRD traces using Cu K␣ radiation showing bcc and Fe3 AlC0.5 peaks in Fe–10.5Al–0.9C–2Nb alloys and similar peaks were observed in Fe–10.5Al–0.7C, Fe–10.5Al–0.9C, Fe–10.5Al–0.7C–0.5Nb, and Fe–10.5Al–0.9C–0.8Nb alloys.

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Fig. 4. BSE images and line scan of EPMA observed in hot-rolled ESR ingots Fe–10.5Al–0.9C–2Nb alloy.

the niobium added to the ternary alloys resulted in the formation of hard (2050 HV) [20] niobium carbide precipitates, the volume fraction is very small (0.5–0.7%) especially in the case of Alloys 3 and 4 (Table 3). In the case of Alloy 5, the volume fraction of Nb2 C is more (1.8%) but it has resulted in significant reduction (32–27%) in the

volume fraction of Fe3 AlC0.5 precipitates which has about 600 HV hardness [21] and is an important strengthening phase in iron aluminides. It can also be observed from the Table 4 that the Alloy 2 and 4 exhibited much higher room temperature yield strength (803–820 MPa) as compared to (720–740 MPa) Alloys 1 and 3. This may be attributed to the

Table 4 Mechanical properties ESR hot-rolled alloys Alloy

1a 2 3 4 5a

Alloy composition (wt.%)

Fe–10.5Al–0.7C Fe–10.5 Al–0.9C Fe–10.5Al–0.7C–0.5Nb Fe–10.5Al–0.9C–0.8Nb Fe–10.5Al–0.9C–2Nb

Hardness (HV)

315 348 339 345 340

600 ◦ C tensile properties

RT tensile properties UTS (MPa)

YS (MPa)

El (%)

UTS (MPa)

YS (MPa)

El (%)

883 980 845 941 868

720 820 740 803 781

1.8 3.0 2.0 2.4 1.0

318 365 334 360 345

300 330 320 345 310

60 53 75 75 64

UTS: ultimate tensile strength, YS: yield strength, El: elongation. a After [22].

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Fig. 5. SEM photographs showing the change in tensile fractures made from transgranular cleavage (a) at room temperature to ductile dimple (b) at 873 K in hot-rolled ESR ingots of Fe–10.5Al–0.9C–2Nb alloys. Similar failure modes were observed in other four alloys.

presence of higher volume fraction (31–32%) of Fe3 AlC0.5 precipitates in Alloys 2 and 4 as compared to (24–25%) Alloys 1 and 3. It has been reported [22] earlier that Alloy 4 (Fe–10.5Al–0.9C–0.8Nb) exhibit higher strength as compared to Alloy 1 (Fe–10.5Al–0.7C). The reasons for this improvement was felt due to the presence of Nb-rich precipitates. However, it is clear from the present study that the improvement in strength is mainly due to the increase in the volume fraction of Fe3 AlC0.5 precipitates with the increase in carbon content from 0.7 to 0.9 wt.%. It has been reported [10] earlier that the addition of Nb to Fe3 Al alloys is not beneficial to room temperature tensile ductility. The poor room temperature ductility (Table 4)

observed in the alloys containing 2% Nb (Alloy 5) may be attributed to the formation of coarse segregated cuboidal shaped niobium carbide precipitates (Fig. 2e). The tensile elongation of all the five alloys at 873 K is significantly higher (60–75%) as compared to room temperature (1–3%). The transgranular cleavage failure (Fig. 5a) was observed in the tensile samples of all the five alloys tested at room temperature. At 873 K the failure mode changed to ductile dimple (Fig. 5b). The increase in ductility at 873 K for all the alloys may be attributed to the increased dislocation mobility at high temperature. The creep resistance of the binary Fe3 Al alloy is very poor, the creep rupture life of the Fe–16Al alloy is reported

R.G. Baligidad / Materials Science and Engineering A368 (2004) 131–138 Table 5 Creep properties of ESR hot-rolled alloys Alloy Alloy composition (wt.%)

Creep properties (600 ◦ C, 140 MPa) Rupture Life (h)

MCR (%/h)

El (%)

1 2 3 4 5

2.4 2.6 3.1 2.6 3.5

11 10 2 8 7

58 63 89 54 70

Fe–10.5Al–0.7C Fe–10.5Al–0.9C Fe–10.5Al–0.7C–0.5Nb Fe–10.5Al–0.9C–0.8Nb Fe–10.5Al–0.9C–2Nb

MCR: minimum creep rate, El: elongation.

to be only 5.8 h at 873 K and 134 MPa [18]. By adding 2 wt.% Nb the creep-rupture life of the binary alloy is significantly increased to greater than 1975 h [19]. The creep resistance of the high carbon Fe–10.5Al alloys (Alloys 1 and 2) is also very poor (Table 5). Addition of niobium up to 2 wt.% to high carbon Fe-10.5 alloys did not exhibit any significant improvement in either creep life or minimum creep rate (Table 5). This is consistent with behavior of strength at 873 K with niobium addition. The reason for poor creep properties in the alloys containing niobium may be attributed to the formation of small volume fraction of coarse segregated niobium carbide precipitates which may not be very effective barriers to the dislocations. It would appear from the present work that the benefits of niobium addition achieved in iron aluminides based on Fe3 Al, could not be achieved in the case of Fe–Al alloys containing high carbon.

4. Conclusions The effect of a combined addition of carbon and niobium as well as variation in concentration of carbon and niobium on structure and properties of ESR hot-rolled Fe–10.5Al alloys has been studied. The results are summarized as follows. 1. The hot-rolled ESR ingots of Fe–10.5Al–0.7C and Fe–10.5Al–0.9C alloys exhibits a two-phase microstructure of large volume fraction of Fe3 AlC0.5 precipitates in Fe–Al (␣) matrix. Addition of 0.5 and 0.8% Nb to Alloys 1 and 2, respectively, resulted in the precipitation of very small volume fraction of niobium carbide precipitates in Fe–Al (␣) matrix in addition to large volume fraction of Fe3 AlC0.5 precipitates. Increase in niobium content from 0.8 to 2% has resulted in the formation of cuboidal shape niobium carbide precipitates in addition to small volume fraction fine niobium carbide precipitates and large volume fraction of Fe3 AlC0.5 precipitates. 2. No improvement in room temperature and 873 K yield strength as well as creep properties were observed by the addition of niobium to high carbon Fe–10.5Al alloys, but has reduced the room temperature tensile elongation at higher (2%) concentration.

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3. Alloys containing high (0.9%) carbon exhibited higher yield strength at room temperature as compared to alloys containing low (0.7%) carbon. The increase in strength with small increase in carbon may be attributed to the significant increase in volume fraction of Fe3 AlC0.5 precipitates.

Acknowledgements The authors are grateful to the Defence Research and Development Organization, Ministry of Defence, New Delhi for the financial support in carrying out this research work. The authors wish to thank Dr. D. Banerjee, Director DMRL for his interest and encouragement. The authors are would like to thank fellow officers and staff of ERG (melting and casting), ACG (chemical analysis), GMS (sample making), CDG (radiography, forging and rolling), MBG (tensile and creep), SFAG (Metallography, SEM and EPMA).

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