Chromium addition and environmental embrittlement in Fe3Al

Scripta METALLURGICA et MATERIALIA

Vol. 24, pp. 2119-2122, 1990 Printed in the U.S.A.

Pergamon

Press plc

CHROMIUM ADDITION AND ENVIRONMENTAL EMBRITTLEMENT IN Fe~d" C. G. McKamey and C. T. Liu Metals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, TN 37831-6115 ( R e c e i v e d August 23,

1990)

Introduction Iron aluminides based on Fe~,l afford excellent oxidation properties at relatively low cost, making them candidates for use as structural material in corrosive environments (1,2). Iron aluminides produced by conventional ingot metallurgy have been investigated for years but generally have exhibited low room temperature ductilities (in the range of 3-5%) (3-5). [A ductility of 15% has been reported in Fe~A1 alloys with wrought structure produced from powder (6).] Recently, efforts have been devoted to understanding and improving their ductility through control of grain structure, alloy additions and material processing (7. 16). Studies at this laboratory have now shown that the ambient temperature ductility can be increased significantly by additions of up to 6% Cr (17-19). This increase in ductility was earlier attributed to increased cleavage strength, easier cross slip due to lower antiphase boundary (APB) energy, and solid softening (18,19). Very recent studies of FeAl and Fe3AI in various tensile testing environments have indicated that both alloy systems are relatively more ductile at room temperature when tested in vacuum or dry oxygen (20,21). Ductilities of 12-18% were attained in both iron aluminide systems in an oxygen pressure of 6.7 x 104 Pa, while only 2-4% ductility was achieved in normal laboratory air. The low ductility in air tests to environmental embrittlement involving generation of atomic hydrogen at the crack tips which is transported into the specimen during stressing producing brittle cleavage failure. The atomic hydrogen is apparently produced by the reaction of aluminum atoms at the crack tips with water molecules in the air. In light of these results, it seems appropriate to reexamine the mechanism by which chromium produces improved ductility at room temperature in laboratory air and to correlate it with the environmental effects on mechanical properties. In the current investigation, we have evaluated room temperature tensile properties of the binary alloy (Fe-28AI, at.%) and ternary alloy containing chromium (Fe-28AI-4Cr) as a function of surface condition and heat treatment. The results indicate that, although chromium may affect cleavage strength and APB energies, its most significant effect on room temperature ductility is to modify the protective surface oxide, resulting in a minimization of environmental embritflement. Experimental Procedures Binary Fe-28AI and Fe-28AI-4Cr (at.%) were prepared by arc melting and drop casting, using commercially pure aluminum and iron. The alloy ingots were clad in stainless steel and hot rolled at 1000850°C, then warm-rolled bare at 650-600°C to 0.75 mm sheet. Tensile specimens with a gage section of 12.7 x 3.2 x 0.75 mm were punched and prepared for tensile testing. Three different surface conditions were tested: (1) the as-rolled surface (AR) consisting of a bronze-colored oxide which formed during the rolling procedure, (2) an electropolished surface (EP) produced by first grinding on 320 grit emery paper to

"Research sponsored by the U.S. Department of Energy, Fossil Energy AR&TD Materials Program and Division of Materials Sciences, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

2119 0036-9?48/90 $3.00 + .00

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remove the as-rolled oxide and then electropolishing in a solution of one part sulfuric acid and thirteen parts methanol for 4 rain at 6-7 V, and (3) an oxidized surface produced by first cleaning the surface as in (2) above, then annealing in air to produce a thin oxide coating (OC). Three different heat treatments were evaluated for each surface condition: (1) lh/900*C+2h/700*C in vacuum to preserve the as-prepared surface and produce the B2 structure, (2) lh/900*C+2h/700*C in vacuum plus 4d/500"C in vacuum to produce the DO3 structure, and (3) 11V850"C plus 4d/500"C in air to produce a thin oxide coating and DO3 order. All these heat treatments resulted in fully reerystallized microstructures with grain sizes on the order of 150-200/~m. Tensile tests were performed on an Instron testing machine in air at room temperature using a strain rate of 3.3 x 10"3/s. At least two specimens were tested in each condition. Fracture surfaces were examined in a ISI Super III-A scanning electron microscope (SEM) operated at 25 kV. Results Table 1 shows the results of tensile tests in air at room temperature for Fe-28A! and Fe-28AI-4Cr as a function of annealing treatment and surface condition. AR (as-received) refers to specimens that r~etain the as-rolled surface oxide; EP refers to specimens whose surfaces were cleaned by first grinding on 320 grit emery paper, followed by electropolishing; OC refers to the specimens oxidized by heat treating in air. For the binary alloy, yield strengths, ultimate tensile strengths, and elongations were relatively insensitive to surface condition within each heat treatment set, indicating no effect of the oxide presence on the tensile properties. The lower yield strengths for specimens heat treated for 4 days at 500"C are due to D03 ordering. However, the environmental embrittlement, as noted in a previous study (21), does not seem to be sensitive to crystal structure. Ductilities for all binary specimens ranged between 3.6 and 5.6%. Ultimate stress values sealed with the ductility. Yield strengths of the chromium-containing alloy ranged between 192 and 256 MPa. The lower values compared to the binary are due to a solid-solution softening effect produced by the chromium which was noted in an earlier study (19). Several observations can be made concerning the ductilities of the chromium-containing specimens: (1) The specimens which were electropollshed to remove oxide and then annealed in vacuum to minimize oxide reformation (EP) showed elongations comparable to the binary alloy whether in the B2 or DO3 structure. (2) Leaving the as-rolled oxide coating on the specimens (AR) TABLE 1 Effect of Surface Condition and Heat Treatment on Tensile Properties of Fe.,~AI and Fer~d+Cr Tested in Air at RT Fe-28AI

Fe-28AI-4Cr

Heat treatment (h/*C)

Surface condition attesting'

Yield (MPa)

11900+2/700 vac "

EP AR

387 398

559 587

4.1 4.3

256 199

364 433

4.0 7.8

"+96/500 vac "

EP AR

267 277b

515 551b

5.4 5.6b

192 196

356 423

5.8 7.2

1/850+96/500 air "

EP+OC AR

270 285

429 466

3.6 3.6

236 248

483 461

8.2 7.0

Ult. Elong. (MPa) (%)

Yield (MPa)

°EP=electropolishod; AR=as-received, no post-roUing surface treatment; OC=oxide coating. ~Data from different ingot of same composition.

Ult. Elong. (MPa) (%)

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resulted in an observable increase in ductility for the chromium-containing alloys compared to both the binary and to chromium-containing specimens which had been electropolished. (3) The chromiumcontaining specimens which were annealed for lh at 850"C and 96 h at 500"C in air had comparable ductilities whether they were electropolished ( E P + O C ) or left with the as-rolled oxide coating. This suggests that the heat treatment in air produced a thin protective oxide coating on previously electropolished specimens. (4) Also, the ductility values for the chromium-containing specimens annealed in air were comparable to those for the AR specimens, but higher than the ductility of the EP specimens, again suggesting the formation of a protective oxide coating. Figure 1 shows a typical both alloys for all conditions 15% intergranular fracture. intergranular failure and the

SEM fractograph. In general, the fracture mode is transgranular cleavage in tested. A few specimens showed a mixed failure mode with approximately 5However there appeared to be no correlation between the presence of the specimen composition, heat treatment, or surface condition. Discussion Our studies (20-21) of iron aluminides have demonstrated that FegSd and FeAI are intrinsically quite ductile at room temperature and that the low ductilities commonly observed in air tests are due to an extrinsic dynamic effect - environmental embrittlement. Many aluminum alloys also exhibit environmental embrittlement at ambient temperatures (see, for example, Ref. 22-27). The aluminum alloys are generally not embrittled by dry hydrogen at ambient pressure, but they are susceptible to embrittlement in environments containing moisture. The embrittlement involves the following reaction at the metal surface (22): 2 A l + 3 H20--~A1203 + 6 H .

Fig. 1. SEM fractograph of Fe-28Al-4Cr in the A R This effect appears to be a dynamic phenomenon, surface condition after a heat treatment of occurring only during stressing when fresh unoxidized lh/900°C plus 2h/700°C. Tensile elongation was material is exposed to the atmosphere. 8.7%. Our earlier studies had indicated several differences in properties between binary Fe3Al and the ternary alloy with chromium. (1) Yield strengths were decreased slightly indicating some solid solution softening. (2) The presence of fine wavy slip lines in alloys with chromium as opposed to coarse straight lines in the binary indicated that cross slip from one {110} plane to another in the {110}(111) system is easier when chromium is present. (3) The spacing between dislocations in the four-fold superdislocations was increased, indicating a decrease in anti-phase boundary (APB) energy. (4) A change in fracture mode was noted with addition of chromium from one of near 100% transgranular cleavage to one of mixed cleavage and intergranular failure, suggesting that chromium enhances cleavage strength and partially suppresses cleavage fracture. The results of the present study, however, indicate that the beneficial effect of chromium comes from surface-oxide modification, rather than modification of bulk properties. The fact that chromium additions result in improved ductility may indicate a change in oxide chemistry and properties or a change in the kinetics of oxide formation, thereby reducing the water-vapor reaction. Whatever the mechanism, the result is an alloy in which this type of environmental embrittlement is minimized. Through further composition modifications, control of grain structure, and fabrication procedures, ductilities of near 20% have been attained at this laboratory (16).

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Conclusion The Current study clearly demonstrates that the addition of chromium to Fe~Al results in a change in the nature of the protective oxide coating. At this point it is not clear whether this change is a change in structure or composition of the oxide, or a change in the kinetics of oxide formation. Like many other intermetallics (27), Fe~AI is susceptible to environmental embrittlement in the presence of water vapor. However, this embrittlement, which involves the reaction of water vapor with aluminum atoms and the release of atomic hydrogen at the crack tip, can be minimized through compositional modifications, specifically with the addition of chromium. References 1. J. H. DeVan, p. 107-15 in Oxidation of Hi~h-Tem~erature Intermetallics, ed. T. Grobstein and J. Doychak, The Minerals, Metals and Materials Society, 1989. 2. J. L Smialek, J. Doychak, and D. J. Gaydosh, pp. 83-95 in Oxidation of High-Temperature Intermetallics, ed. T. Grobstein and J. Doychak, The Minerals, Metals and Materials Society, 1989. 3. J. F. Nachman and W. J. Buehler, Naval Ordnance Lab., NAVORD Rept. 4130 (December 5, 1955). 4. F. X. Kayser, Ford Motor Company, WADC-TR-57-298, 1957. 5. E. R. Duffy and J. F. Nachman, Bureau of Mines Open File Report 112-76 (U.S. Dept. of Commerce, National Technical Information Service PB-259-253) June 1976. 6. R. G. Bordeau, AFWAL-TR-87-4009, Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, Ohio, May 1987. 7. W. R. Kerr, Metall. Trans. 17A, 2298 (1986). 8. C. G. McKamey, J. A. Horton, and C. T. Liu, p. 321-27 in MRS Symp. Proc., Vol. 81, High Temperature Ordered Intermetallic Alloys, ed. N. S. Stoloff, C. C. Koch, C. T. Liu, and O. lzumi, 1987. 9. G. Culbertson and C. S. Kortovich, AFWAL-TR-4155, Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, Ohio, March 1986. 10. M. G. Mendiratta, S. K. Ehlers, D. K. Chatterjee, and H. A. Lipsitt, Metall. Trans. 18A, 283 (1987). 11. J. A. Horton, C. T. Liu, and C. C. Koch, pp. 309-21 in High Temperature Alloys: Theory and Design, ed. J. O. Stiegler, AIME, 1984. 12. R. G. Bordeau, AFWL-TR-87-4009, Development of Iron Aluminides, Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, Ohio, May 1987. 13. M. G. Mendiratta, S. K. Ehlers, D. M. Dimiduk, W. R. Kerr, S. Mazdiyasni, and H. A. Lipsitt, pp. 393. 404 in MRS Syrup. Proe. Voi. 81, High Temperature Ordered Intermetallic Alloys, ed. N. S. Stoloff, C. C. Koch, C. T. Liu, and O. Izumi, 1987. 14. M. G. Mendiratta and H. A. Lipsitt, pp. 155-62 in MRS Syrup. Proc. Vol. 39, High Temperature Ordered Intermetallic Alloys, ed. C. C. Koch, C. T. Liu, and N. S. Stoloff, 1985. 15. R. S. Diehm, M. P. Kemppainen, and D. E. Mikkola, Mater. Man. Proc. 4(1), 61 (1989). 16. V. K. Sikka, C, G. Mcl~amey, C. R. Howell, and R. H. Baldwin, ORNI./TM-11465 (March 1990). 17. C. G. McKamey, C. T. Liu, J. A. Horton, and S. A. David, '~Development of Iron Aluminides," pp. 275284 in Fossil Energy AR&TD Materials Program Semiannual Progress Report for the Period Ending March 31, 1988, ORNI.JFMP-88/1 (July 1988). 18. C. G. McKamey, J. A. Horton, and C. T. Liu, Scrivta Metall. 22, 1679 (1988). 19. C. G. McKamey, J. A. Horton, and C. T. Liu, J. Mater. Res, 4(5), 1156 (1989). 20. C. T. Liu, E. H. Lee, and C. G. McKamey, Scripta Metall. 23, 875 (1989). 21. C. T. Liu, C. G. McKamey, and E. H. Lee, Scripta Metall. 24, 385 (1990). 22. M. P. Speidel, pp. 329-351 in Hydrogen Damage. ed. C. D. Beachem, ASM Publication, 1977. 23. R. J. Gest and A. R. Troiano, Corrosion 30(8), 274-79 (1974). 24. Zhao-Xiong Tong, Shi Lin, and Chi-Mei Hsiao, Metall. Trans. 20A, 925 (1989). 25. R. E. Ricker and D. J. Duquette, Metall. Trans. 19A, 1775 (1988). 26. G. M. Bond, I. M. Robertson, and H. K. Birnbaum, Acta Metall. 36(8), 2193 (1988). 27. C. T. Liu and C. G. McKamey, pp. 133-151 in High Temperature Aluminides and Intermetallics, ed. S. H. Whang, C. T. Liu, D. P. Pope, and J. O. Stiegler, The Minerals, Metals and Materials Society, 1990.