Effect of carbides on embrittlement of Fe3Al based intermetallic alloys

Scripta Materialia,Vol. 36, No. 6, pp. 667-671.1997 Ekevis Science Ltd Copyright0 1997 Acta Metehgice Inc. Printedin the USA. All rights reserved 1359-6462/97$17.00 + .OO

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EFFECT OF CARBIDES ON EMBRITTLEMENT Fe,Al BASED INTERMETALLIC ALLOYS

OF

R.G. Baligidad, U. P&ash, A. Radhakrishna and V. Ramakrishna Rao Defence Metallurgical Research Laboratory Hyderabad-500 058, India

P.K. Rao and N.B. Ballal Indian Institute of Technology Powai, Bombay-400 076, India (Received August 14,1996) (Accepted October 18, 1996) Introductiog Iron aluminides based on Fe,Al are being considered for high temperature structural applications (1,2). These alloys are known to be susceptible to environmental embrittlement in the presence of atmospheric moisture (3). This embrittlement has been proposed to be due to diffision of nascent hydrogen liberated by the chemical reaction between ahuninium present in the alloy and the atmospheric moisture. The same reaction has also been attributed to induce surface cracking during cutting and machining when a waterbased coolant is used. Most of the available literature is on alloys prepared from high purity raw materials containing very low levels of carbon (CO.01 wt.%). Additions of carbon in the range of 0.14 to 0.50 wt.% result in formation of hard (and brittle) Fe,AlC precipitates (4). Here we report on the effect of these carbides on the environmental embrittlement of Fe,Al. ExDerimentaI Procedure 30 kg ingots of iron aluminides containing about 16 wt.% (28 at.%) ahuninium and 0.14, 0.27 and 0.50 wt.% carbon were produced by a combination of air induction melting and electroslag remelting (ESR). The melting practice has been described in detail elsewhere (4-7). The ESR ingots were held at 1000°C for 1 hour and hot forged in a one tonne press with die platens at room temperature. The cast and forged ESR ingots were cut in longitudinal direction by a high speed abrasive silicon carbide cut-off wheel. The cut-off sections were machined, ground to 180 grit finish and then etched with Aqua Regia (50% HCl + 50% HN03 by volume) and photographed at low (1 x) magnification. For optical metallography the

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sections were mechanically polished to 0.5 pm grade diamond powder finish and etched with an etchant consisting of 33% HNO, + 33% CH,COOH + 33% H,O + 1% HF by volume. The tensile and impact samples were cut and machined from longitudinal sections of cast and forged ESR ingots using a water-based coolant. The details of sample preparation and testing procedures have been given elsewhere (4-7). To screen the samples from environmental moisture during testing, tensile tests were also carried out after coating the samples with linseed oil. Results

The ESR ingots exhibited columnar grains having an average grain diameter of 1750 pm. The solidification structure was retained in the forged ESR ingot containing 0.14 wt.% carbon which has undergone 40% reduction during forging. The ESR ingots containing 0.27 and 0.50 wt.% carbon were forged to reduction ratios of 60% and 70% respectively and exhibited recrystallized grains. The microstmcture of forged ESR ingots is shown in Figures la to c. The tensile and impact properties of cast and forged ESR ingots are listed in Table 1. The Charpy impact energy decreases with increasing carbon content. Coating of samples by linseed oil failed to show any significant improvement in tensile ductility. The SEM examination of tensile and impact fracture surfaces confirmed that all the specimens failed by cleavage. Discussion

The ductility of Fe,Al based alloys may be slightly improved by leaving a thin film of oil such as that achieved during an oil quench, on the specimen (8). The film acts as a barrier between the atmospheric water vapour and ahuninium in the sample, thus reducing the tendency for environmental embrittlement. It has also been reported (9, 10) that the ductility of wrought binary Fe,Al as well as multicomponent Fe,Al-based alloys is higher when the material is in unrecrystallized and partially recrystallized condition. It was suggested that the presence of highly elongated grains with very few transverse boundaries that intersect the specimen surface reduces hydrogen atom diffusion and the resulting environmental embrittlement thereby increasing the ductility. In the present work, neither the oil coating nor the grain structure, columnar in cast condition and equiaxed after forging, had any significant effect on ductility. We have also reported earlier that the longitudinal specimens of ESR cast Fe-15.27AL0.074C ingots having columnar grains oriented along the axis and transverse specimens cutting across the several columnar grains did not show significant difference in ductility (5). It should be reiterated that the material in the present work differs from that used in other studies in two aspects, the columnar grain sizes in the present work are much coarser and the material has significant amount of carbon. The columnar grains obtained after ESR are of = 1750 pm in width as compared to E 30 pm width of elongated grains in warm worked material reported in the literature (10). However, recrystallized grains in the forged ingots are of similar sizes as studied elsewhere. This suggests that the effect of grain size on environmental embrittlement seems to be absent for the alloys studied in the present work. It is well documented that carbide particles can act as hydrogen traps and lower hydrogen difisivity in steels (11,12). Iron aluminides also have a bee-derivative structure and are iron based. It may be, therefore, argued that hydrogen trapment by Fe,AlC precipitates lowers hydrogen difisivity in these alloys and reduces the susceptibility to environmental embrittlement. There is evidence that an Fe-28at.% Al-Sat.%Cr-0.2 at.% (~0.05 wt.%) C alloy also failed to exhibit susceptibility to environmental embrittlement (13). The reported ductility values with and without oil film were similar. This again can be attributed to hydrogen trapment by chromium carbide particles present in the alloy.

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Figure 1. Optical micrographs of (a) Fe-15.6Al-0.14C alloy after 40% forging reduction (b) Fe-16.3AI-0.27C alloy after 60% forging reduction and (c) F3-15.6Al-0.5C ahoy after 70% forging reduction. (b) and (c) show recrystallized grains with Fe,AIC precipitates aligned along forging direction. Fe-15.6Al-O.14Calloy exhibited mostly cast columnar structure (Fig. 2b) though there was some recrystallization as shown in (a) above.

There are reports of the embrittling effect of carbon in Fe,Al where it has been reported to lower tensile ductility, though no reasons were given for the loss in ductility (14). In contrast, we have found that high

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TABLE 1 Effect of Test Environmenton Room TemperatureTensile Properties of ESR as Cast and as Forged Fe- 16AlAlloys __________________~~~------~~~~~~~~~~~~~~-----~~~~~~~~~~~~~~~~~~~~~~~~____________ UTS @Spa) YS WPa) El (%) Condition

composition (wt.%)

_------------

-____________

Air oc Air ____________________~-------~~~~~~~~~~~~~--~~-~~~~~~~~~~~~~~~~~~~~~~~~_~__________

Cast

Fe-15.60Al-0.14C

Fe-16.30Al-0.27C

Fe-15.60Al-0.50C

447

380

460

____________

CVN

oc

Air

oc

360

3.5

4.6

5.5 5.0

Air

Forged

430

452

360

380

5.0

6.4

Cast

526

546

415

416

4.4

5.0

3.5

Forged

553

545

407

425

4.4

4.6

3.0

Cast

545

583

425

434

4.2

5.0

3.0

Forged

560

600

458

464

4.7

6.0

3.6

_____________________~~~~~~~~~~~~~~~~_____~~~~~~~~__~~_______________________________ UTS

:

Ultimate oc

;

tensile

streagth;

YS

:

Yield

strength1

El

:

Elongation;

Oil coating

strength, some ductility (3.5 to 5.0%) and enhanced high temperature properties could be achieved by addition of carbon in the range of 0.14 to 0.50 wt.% (Table 1). Fe,AlC phase is a hard and brittle phase and its presence may be normally expected to embrittle the Fe,Al based alloys. Their impact resistance does, in fact, deteriorate with increase in volume fraction of Fe,AlC (Table 1) probably because during impact testing diffusion controlled processes such as hydrogen embrittlement play only a limited role. The improved tensile ductility may be attributed at least partly to trapment effects already discussed. Further, the high carbon alloys in the present work exhibited excellent machinability even when a water based coolant was used. This contrasts with the very poor machinability due to surface cracking during cutting and machining reported in the literature for low carbon alloys (3). These low carbon alloys do not contain Fe,AIC precipitates and are probably more susceptible to cracking. We also reported on the existence of microcracks in the alloys containing 0.013 to 0.06 wt.% carbon prepared by a combination of air induction melting and electroslag remelting, whereas the alloys with higher carbon contents were found to be free from these cracks (7). These cracks were attributed to higher hydrogen contents that may be expected in low carbon alloys. However, hydrogen trapment effects may also play a role in reducing tendency for cracking. These low carbon alloys contain correspondingly lower volume fractions of Fe,AlC precipitates and thus have fewer trapment sites for hydrogen rendering them more susceptible to cracking. Conclusions Environmental embrittlement is a major cause for low ductility in Fe,Al-based intermetallic alloys. Hydrogen can be picked up by the alloy during (a) processing, (b) machining/cutting and (c) testing. The presence of carbides may reduce the tendency for embrittlement during all the three stages. No systematic studies are available in the literature on hydrogen trapment by second phase particles in intermetallic systems. It is suggested that these effects can play an important role in limiting susceptibility for embrittlement in intermetallics.

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The authors (RGB, Up, AR and VRR) are greatful to the Director, DMRL for the interest and permission to publish this work. The authors wish to thank Dr. D. Banerjee for his keen interest and encouragement. References 1. U. P&ash, R.A. Buckley, H. Jones and C.M. Sellars, ISIJ lnt., 31, 113 (1991).

2. C.G. McKamey, J.H. Devan, P.F. Tortorelli and V.K. Sikka, J. Mater. Res., 6,1779 (1991). 3. N.S. Stoloff and CT. Liu, lntermetahics, 2, 75 (1994). R.G. Baiigidad, U. P&ash, V. Ramakrishna Rao, P.K. Rao andN.B. Ballal, ISIJ lnt., 36 (1996). R.G. Baligidad, U. Prakash, V. Ramakrishna Rao, P.K. Rao andN.B. Ballal, Iron Making and Steel Making, 21,324 (1994). R.G. Bahgildad, U. P&ash, V. Ramakrishna Rao, P.K. Rao and N.B. Ballal, ISIJ lnt., 35,443 (1995). R.G. Baliglldad, U. Prakash, V. Ramakrishna Rao, P.K. Rao and N.B. Ballal, Communicated to ISIJ lnt. S. Vyas, S. Viswanathan and V.K. Sikka, Ser. Metall., 27, 185 (1992). C.G. McKamey and D.H. Pierce, Ser. Metall., 28, 1173 (1993). P.G. Sanders, V.K. Sikka, C.R. Howell and R.N. Baldwin, Ser. Metall., 252365 (1991). J.P. Hirth, Metall. Trans., 1IA, 861 (1980). F. Gehrmann, H.J. Grake, E. Riecke, Hydrogen Transport and Cracking in Metals, Edited by A. Turnbull, Institute of Materials, London (1995), 216. 13. V.K. Sikka, S. Viswanathan and S. Vyas, High Temperature Ordered Intermetallic Alloy V, MRS Symp. Proc. Vol., 288, MRS, Pittsburgh (1993), 971. 14. W.R. Kerr, Metall. Trans., 17A, 2298 (1986). 4. 5. 6. 7. 8. 9. 10. 11. 12.