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The effect of nickel on vacancy hardening in iron-rich FeAl
Intermetallics 4 (1996) 5-l I Elsevier Science Limited Printed in Great Britain 0966-9795/96/%09.50 ELSEVIER
0966-9795(95)00011-9
The effect of nickel on vacancy hardening in iron-rich FeAl P. R. Munroe School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 20.52, Australia (Received
20 February
1995; accepted
14 March
1995)
The hardness and defect morphology of alloys with the composition (Fez5_, Ni,)A&s, where 0.1 5 x 5 10 at.% following high-temperature heat treatment and subsequent low temperature annealing was studied. It was found that nickel additions significantly reduced the softening which occurs on low temperature annealing of FeAl and also significantly increased the hardness of this compound. Following low-temperature annealing < 001 > edge dislocations were observed to form. In addition cuboidal voids were formed in those alloys with less than 10 at.% nickel. It is believed that the hardening which occurs in these alloys is associated with the interaction of the nickel atoms with the thermal vacancies which form on high temperature heat treatment. Possible mechanisms for the hardening which occurs are discussed. Key words: iron aluminides,
vacancies,
hardening
1 INTRODUCTION
by vacancies.
hardening behaviour of FeA1.‘4T15Additions of 10 at.% or less nickel, which replace iron, greatly decrease the softening which occurs on low-temperature annealing and also significantly increase the equilibrium hardness following this heat treatment. For example, following heat treatment for two hours at 950°C and air-cooling, Fe-49Al (unless otherwise stated all compositions given will be in atomic percent) exhibited a Vickers microhardness of N 570, but following a subsequent heat treatment at 400°C the hardness of this material was reduced to - 350. In contrast, an alloy with a nominal composition of Fe4sNi5A1so exhibited a hardness of - 610 following heat treatment at 950°C and - 550 following the subsequent 400°C anneal. Furthermore, transmission electron microscope (TEM) studies showed that the defect microstructures which evolved during low-temperature annealing were strongly dependent upon composition.13 It was shown that in FeAl, homogeneous arrays of c-001 > edge dislocations formed, but as the iron content decreased, the dislocation structure became increasingly heterogeneous so that very well-defined subgrains formed in nickel-rich alloys. Cuboidal voids were also formed in those ternary alloys with less than 10 at.% nickel, although the origin of these voids was not made clear.
The ordered intermetallic compound FeAl, which exhibits an ordered bee or B2 structure, has received considerable attention due to its high strength, good oxidation resistance and low density, which make it a candidate material for high temperature applications. It is now well established that a significant concentration of thermal vacancies can be retained in FeAl after heat treatment at even after furnace cooling, high temperature, which can lead to significant increases in hardness and decreases in ductility.iP5 These excess vacancies can be subsequently eliminated by low temperature annealing during which the material softens and large numbers of < 001 > edge dislocations evolve in the microstructure.1,6,7 In contrast, the isostructural compound NiAl also may retain a large concentration of vacancies following high temperature heat treatment.* However, its hardness is relatively independent of heat treatment,3 and, following low temperature heat treatment, voids rather than dislocations form in the microstructure.*-l3 More recently it has been shown that small additions of nickel to nominally stoichiometric FeAl can significantly affect the thermal vacancy 5
6
P.R.Munroe
Data consistent with the earlier work of Kong and Munroe has been reported by Schneibel and co-workers.16,17 The yield strength of Fes0Ni5A1d5, with trace additions of zirconium and boron, following prolonged annealing at low temperature was measured to be - 570 MPa, which was significantly higher than the yield strength of - 320 MPa for Fe5sALi5 following the same heat treatment. Kong and Munroe’* and Schneibel et ~1.‘~ have also investigated the effects of other first-row transition metals on the vacancy hardening of FeAl. Kong and Munroe showed that additions of these elements replacing iron decreased the softening which occurs on low-temperature annealing and increased the overall hardness following lowtemperature annealing. However, the increases in hardness were smaller than those which arose through nickel additions. Consistent with this, Schneibel et aLI9 showed that the first row of transition metals increased the yield strength of FeAl and were able to correlate this increase in strength with atomic size misfit. However, the increase in yield strength for the nickel-containing alloy was significantly higher than that predicted by atomic size misfit calculations. It is clear that nickel additions to FeAl interact with this compound in a more complex manner than other transition metals, and the effect of such additions on the mechanical properties and microstructure of FeAl warrants further investigation. The aim of this paper is to examine the hardness and microstructural development of alloys with a base composition of (Fe55_xNix)A145, where 0.1 5 x 5 lo%, following high-temperature heat treatment and subsequent low-temperature annealing.
2 EXPERIMENTAL Alloys with the nominal composition, (FeSs-x Ni,)&, where x = 0.1, 0.3, 1.0, 3.0 and 10.0 were cast. All of the samples were annealed in air at 950°C for 2 h and cooled to room temperature by either water-quenching or air-cooling. Subsequent low temperature annealing was performed on the air-cooled alloys at 400°C for 120 h. Vickers microhardness testing was performed on each sample using a 300 g load. Each data point obtained was the average of at least 10 measurements. Standard deviations were less than 5%, but error bars were omitted from Fig. 1 to improve clarity. The microstructures of all the materials were examined by both optical and transmission electron microscopy (TEM). Thin foils for TEM
40il
q
Water-Quenched
350
q
AiPco&d
300
q
4G1XX20hows
HardnlZSS
250
FiAl
013
0:l
i
Ni Content (at.%)
Fig. 1. The effect of heat treatment on the Vickers microhardness of alloys with the composition (Fe55-xNix)A145r where 0.1 5 x < 10 at.%. Data for Fe55A145 (after(3)) are also included for comparison.
study were prepared using methods described elsewhere,*O and examined in a Philips 420 transmission electron microscope, furnished with an energy dispersive X-ray spectrometer (EDS), operating at 120 kV.
3 RESULTS
AND
DISCUSSION
The effect of nickel additions to the hardness of iron-rich FeAl as a function of heat treatment is shown in Fig. 1. Data for Fes5Ald5 from the work of Nagpal and Baker3 are included for comparative purposes. However, it should be noted that their samples were initially heat treated at the slightly higher temperature of 1000°C. It is presumably for this reason that a slightly higher hardness was observed for this alloy following the initial heat treatment, since a higher concentration of thermal vacancies would be expected to be quenched in at this slightly higher temperature. It is clear that following heat treatment at 950°C hardness is little affected by composition, although a slight increase in hardness was observed as the nickel content increased. It is also clear that hardness is little affected by the cooling rate from this temperature; the air-cooled samples were slightly softer than those which were water-quenched, but the differences were small. Following subsequent annealing at 400°C of the samples which were air-cooled from 950°C significant changes in hardness as a function of nickel content were observed. As the nickel content increased the hardness increased, such that for Fes4NilA&, for example, the hardness was only reduced by 7% by prolonged lowtemperature annealing, compared with a reduction of about 44% for the binary alloy. It is interesting
Nickel in iron-rich FeAl
to note that even a nickel addition of only 0.1% can increase the hardness of Fes5A145, following prolonged exposure at 4OO”C, from about 250 to about 300. The hardness data obtained here are consistent with the earlier work of Kong and Munroe on alloys with a nominal composition of Feso_, NixAlSO.l4 However, in these prior studies the hardness values which were obtained following both heat treatments were significantly higher than those acquired in this study on aluminium-lean compositions. The microstructure of these materials was examined by optical metallography. In all cases the alloys were, as expected, single-phase with a grain size of about 100 pm. Transmission electron microscopy was performed on all the alloys following heat treatment at 950°C and air-cooling. For all five alloys the microstructure contained a small number of < 001 > dislocations which were edge in
Fig. 3. Bright field transmission electron micrographs subsequent annealing at 400°C for 120 h. Diffraction
I
Fig. 2. Bright field transmission electron micrograph of Fe45Ni10A145 following annealing at 950°C for 2 h and aircooling. Diffraction vector as shown, beam direction [l 1 I].
of Fe,,NiloA&s following annealing at 950°C for 2 h and air-cooling and vectors as shown, beam directions (a) and (b) [OOl], (c) [Ol l] and (d) [012].
8
P. R. Munroe
character (Fig. 2). The density of the dislocations was estimated to be about 1 x lo-‘* m-*, consistent with levels previously observed in binary FeAl after similar heat treatments.13 The atom site location of the nickel in these alloys was determined on these alloys following heat treatment at 400°C by atom location by channelling-enhanced microanalysis methods (ALCHEMI), using methods described elsewhere.*’ It was deduced that the nickel atoms exerted a strong preference for the iron sublattice, consistent with previous studies on near-stoichiometric compositions14315 and recent theoretical predictions.22 Following annealing at 400°C for 120 h an increase in dislocation density was observed in Fe4sNi10A14s (Fig. 3). The dislocation density was estimated at 2 x 10-‘3m-2. The dislocations were distributed in a relatively homogenous manner and all were found to exhibit a < 001 > Burgers vector and be edge in character. For example, those dislocations marked ‘a’ exhibited a line direction, u, of [OlO] and exhibited invisibility when g = 020, and weak contrast when g = Oli This contrast is consistent with these dislocations being edge in character with a Burgers vector, b, of [loo]. Similarly, the contrast from those dislocations marked ‘b’ was consistent with that expected from edge dislocations with a [OlO] Burgers vector and a [loo] line direction. Similar observations were made for the alloy with the nominal composition Fe40Nii0A150 studied earlier. 13 A similar defect structure was observed in Fe52Ni3A145 following heat treatment at 400°C (Fig. 4). That is, an array of < 001 > edge dislocations was noted. The density of dislocations in this alloy was generally similar to that observed in Fe45Ni10A145.However, a small number of voids, about 100 nm in diameter were also observed. These voids were usually cuboidal in shape with faces parallel to (001) planes. The void density was estimated to be about 1 x 1018m-3. In many cases these voids appeared to have interacted with the edge dislocations or had formed at the intersection of two such dislocations, see for example the void marked V in Fig. 4. Similar voids were observed in Fes0_,Ni,A150 alloys with nickel concentrations of less than 7.5% following the same heat treatment.13 Those alloys with less than 3% nickel exhibited similar microstructures following heat treatment at 400°C as Fe52Ni3A145;that is, a mixture of < 001 > edge dislocations and cuboidal voids. However, it was noted that as the nickel concentration
Fig. 4. Bright field transmission electron micrograph of FeS2Ni3A14s following annealing at 950°C for 2 h and aircooling and subsequent annealing at 400°C for 120 h. Diffraction vector as shown, beam direction [OOl].
decreased the dislocation density decreased and the void density increased; in Fe54.9Ni0.1A145very few dislocations and a large number of voids were observed, (Fig. 5). In this sample the dislocation density was estimated to be about 2 x 10-‘2m-2 and the void density was estimated to be about 5 x 1018m-3. Theoretical and experimental studies of the vacancy concentration of FeAl as a function of composition and heat treatment have shown the concentration of vacancies increases as temperature increases and also as the composition approaches stoichiometry. 4,5Conversely, the number of anti-site
Fig. 5. Bright field transmission electron micrograph of Fe54.9Ni0.1A145 following annealing at 950°C for 2 h and aircooling and subsequent annealing at 400°C for 120 h. Diffraction vector as shown, beam direction [OOl].
Nickel in iron-rich FeAl
defects, that is iron atoms on the aluminium sublattice, increases as the alloy becomes iron-rich. It has been shown that much higher hardnesses are obtained nearer stoichiometry.3,4 This indicates that it is vacancies, rather than anti-site defects, which are most influential in determining the mechanical properties of FeAl. Consistently with this, Chang et aL4 correlated theoretical hardness concentrations with hardness as a function of both composition and heat treatment and showed that hardness correlated well with the square root of vacancy concentration. Although it is apparent that vacancies exert significant influence over the mechanical properties of this compound, the mechanism through which vacancies harden FeAl remains unclear. Recently Crimp and Vedula studied the mechanical behaviour of both Fe-40Al and Fe-5OAl as a function of both cooling rate and grain size.23 Hall-Petch analysis of their data suggested that vacancies retained in the lattice following high-temperature heat treatment increased lattice resistance, but had little effect on grainboundary strengthening. However, the alloys studied were subjected to a range of processing conditions and heat treatments, so that more detailed relationships between vacancy concentration and lattice resistance could not be deduced. It has been well established that during heat treatments to remove excess thermal vacancies in FeAl large numbers of dislocations are evolved.‘,6,7 Some differences in the crystallography of these dislocations have been observed with composition and heat treatment, but in general these dislocations have < 001 > Burgers vectors and tend to be homogeneously distributed. Although the dislocation structures observed here were, in general, similar to those previously observed in the (Fe, Ni)Al alloys which were nearer in composition to stoichiometry, the densities of dislocations noted were much lower in these aluminium-lean alloys.i3 Since these dislocations are associated with the removal of thermal vacancies, the lower dislocation density evolved is consistent with a smaller number of thermal vacancies being retained in these alloys following high temperature heat treatment. Also as hardness is strongly affected by vacancy concentration, the lower hardness values of these alloys compared with the near-stoichiometric alloys, is also indicative of a lower number of thermal vacancies being retained following the 950°C anneal. These observations are consistent with the previous studies of vacancy concentration as a function of composition for FeA1.4,5
9
The origin of the voids which were observed in some of the alloys here is presently unclear. Similar voids were also observed in the more stoichiometric alloys, but in that study the smallest nickel concentration was 1%. Here, nickel additions as low as 0.1%, replacing iron, have led to a change in the defect morphology formed, from only < 001 > edge dislocations in FeAl to almost all voids in Fe54.9A145Ni0. 1. FeAl exhibits a high divacancy binding energy for vacancies on the same sublattice. This means that it is relatively easier for two vacancies on the same sublattice to form a divacancy. It is also known that the nickel atoms are preferentially located on the iron sublattice. It is possible that because of the slightly larger atomic radius of nickel that vacancies are attracted to regions of the lattice adjacent to the nickel atoms. It is therefore possible for divacancies to form preferentially in lattice regions around the nickel atoms and that the mobility of these divacancies may then be restricted. These divacancies may then, in turn, attract and trap other, more mobile, divacancies not associated with nickel atoms. If these other divacancies are oriented in other non-parallel < 001 > directions, the interaction of these divacancies may lead to the formation of cuboidal arrays of vacancies and so the nucleation of cuboidal voids. At higher nickel concentrations lower densities of voids were observed. It is possible that as the concentration of nickel atoms increases the number of mobile divacancies may be reduced, so that there may be insufficient numbers of mobile divacancies to nucleate voids. No voids were observed in Fe45Ni10A145,consistent with earlier studies of nominally stoichiometric alloys. l3 Furthermore, it is known that in NiAl the divacancy binding energy is repulsive and such defects are unlikely to form.24 It is possible that in Fe45Ni10A145the nickel additions are enough to decrease the divacancy binding energy sufficiently so such defects do not form, thus inhibiting void formation completely. It is clear that nickel additions replacing iron have two effects on the dependence of hardness of FeAl on heat treatment. First, nickel additions, even additions as low as O.l%, limit the softening which occurs on low-temperature annealing, and second, these ternary additions significantly increase the equilibrium hardness of FeAl following the 400°C anneal. It is possible that nickel additions to FeAl may affect its behaviour in several ways. First, such additions might reduce the concentration of thermal vacancies which form during high-temperature heat treatment.
10
P. R. Munroe
Second, nickel additions may limit the number of thermal vacancies which are eliminated from the lattice during subsequent low-temperature annealing. Third, it is clear that the defect microstructure evolved on low temperature annealing is strongly modified by the addition of nickel. This modified structure may limit either the formation or the mobility of slip-related dislocations during subsequent deformation and so harden the lattice. Alternatively, dislocation mobility may be limited by nickel atoms or thermal vacancies forming solute atmospheres around slip dislocations. The vacancy-hardening behaviour of the (Fe, Ni)Al alloys is consistent with the observations made for binary FeAl. Significantly higher hardnesses were obtained for the near-stoichiometric alloys compared with the (Fe55-xNix)A145alloys. It might be expected that the near-stoichiometric alloys would contain higher concentrations of vacancies than the (Fe55_xNix)A145alloys which in turn would be expected to contain a high fraction of anti-site defects. However, the relative drop in hardness for both alloys during the 400°C heat treatment was, in each case, about the same. Nevertheless, the trends which occur in each set of alloys are the same. It would seem that in these alloys the mechanical properties are strongly influenced by the presence of vacancies and it would seem unlikely that nickel additions greatly reduce the concentration of vacancies which form during high-temperature heat treatment relative to the binary alloy. However, further studies which allow measurement of vacancy concentration for the ternary alloys as a function of composition and heat treatment would allow this possibility to be investigated. It is also possible that the nickel atoms inhibit thermal vacancies being removed from the lattice during annealing at 400°C. This would imply that the nickel atoms interact with the vacancies in some way to form nickel-vacancy complexes. It might also be expected that these defects would induce significant lattice hardening, consistent with the observed hardening in these alloys. However, in the ternary alloys, significant concentrations of defects (whether dislocations or voids) are evolved on low-temperature annealing, suggesting that vacancy removal is not completely inhibited by nickel additions. Again, to determine the extent to which nickel atoms trap vacancies in the lattice would require study of the vacancy concentration in these alloys as a function of composition and heat treatment.
The third possibility is that the defects which form on low temperature heat treatment affect the nucleation and motion of slip dislocations during subsequent plastic deformation. At room temperature, FeAl deforms by the movement of < 111 > dislocations, irrespective of composition25 and NiAl deforms by < 001 > slip.25 It is clear that across the FeAl-NiAl pseudobinary there is a transition in slip vector. For the Fe60_,Ni,A140 pseudobinary strained at room temperature, this transition occurs at about the composition Fe40Ni20A140.26The composition at which this transition occurs at higher aluminium concentrations is not known, although it would seem probable that, in those alloys with 10% nickel or less, that room temperature deformation would occur through < 111> slip. During straining of the (Fe, Ni)Al alloys studied here following low-temperature annealing < 111 > slip dislocations must be nucleated to facilitate plastic deformation. It is possible that the voids or the more heterogenous arrays of < 001 > dislocations which form in the ternary alloys may inhibit dislocation nucleation or motion and harden these materials. Another possibility is that the observed hardness peak which occurs at Fe40NiieA15014,‘5is related to a transformation in slip vector. It is also possible that the nickel atoms, vacancies or even nickelvacancy complexes form atmospheres around the slip dislocations and inhibit their motion. Further studies of the interaction between the defect structure evolved on low-temperature annealing and slip dislocations will allow this possibility to be investigated.
4 CONCLUSIONS The hardness and defect morphology of (Fes5_X Nix)A145, where 0.1 5 x < 10 at.% following high temperature heat treatment and subsequent low-temperature annealing was studied. Nickel additions decreased softening which occurs on low temperature annealing of FeAl but also significantly increased the hardness of this compound. Following annealing at 4OO”C, < 001 > edge dislocations formed in the lattice, but with decreasing nickel concentration an increased tendency to the formation of cuboidal voids was noted. This behaviour was attributed to nickel atoms interacting with the thermal vacancies retained following the high-temperature heat treatment.
Nickel in iron-rich FeAl
ACKNOWLEDGEMENTS The author would like to thank Dr J. Schneibel at Oak Ridge National Laboratory for provision of the samples and Dr J. R. Sellar for useful discussions.
REFERENCES 1. Rieu, J. & Goux, C., Mem. Sci. Rev. Met., 65 (1969) 869. Weber, D., Meurtin, M., Paris, D., Fourdeux, A. & Lesbats,
2.
P., J. de Phys. Collogue, C7 (1977) 332. 3. Nagpal, P. & Baker, I., Metall. Trans., 21A (1990) 2281. 4. Chang, Y. A., Pike, L. M., Liu, C. T., Bilbrey, A. R. & Stone, D. S., Intermetallics, 1 (1993) 107. 5. Xiao, H. & Baker, I., Acta Metall. et Muter., 43 (1995) 391. 6. Morton, A. J. & West, G. W., Proc. 8th Inter. Cong. on Electron Microscopy VI, eds J. V. Sanders & D. J. Goodchild, the Australian Academy of Science, Canberra, 1976 p. 460. 7. Fourdeux, A. & Lesbats, P., Phil. Mug., A45 (1982) 81. 8. Eibner, J. E., Engell, H. J., Schultz, H., Jacobi, H. & Schlatte, G., Phil. Mug., 31 (1975) 739. 9. Yang, W. J. & Dodd, R. A., Scripta Metall., 8 (1974) 237. 10. Epperson, J. E., Gerstenberg, K. W., Berner, D., Kostorz, G. & Ortiz, C., Phil. Mug., A38 (1978) 529. 11. Yang, W., Dodd, R. A. & Strutt, P. R., Metall. Trans., 3 (1972) 2049. 12. Weaver, M. L. & Kaufman, M. J., Scripta Metall. et Muter., 29 (1993) 1113.
11
13. Kong, C. H. & Munroe, P. R., Intermetallics, 2 (1994) 333. 14. Kong, C. H. & Munroe, P. R., Scripta Metall. et Mater., 28 (1993) 1241. 15. Kong, C. H. & Munroe, P. R., Scripta Metall. et Mater., 30 (1994) 1079. 16. Schneibel, J. H. & Specht, E. D., Scripta Metall. et Muter., 31 (1994) 1737. 17. Schneibel, J. H. & Maziasz, P. J., Processing, Properties and Application of Iron Aluminides, eds M. A. Crimp & J. H. Schneibel, TMS, Warrendale, USA, 1994, p. 183. 18. Kong, C. H. & Munroe, P. R., Processing, Properties and Application of Iron Aluminides, eds M. A. Crimp & J. H. Schneibel. TMS, Warrendale, USA, 1994, p. 231. 19. Schneibel, J. H., George, E. P., Specht, E. D. & Horton, J. A., High Temperature Ordered IntermetaIlic Alloys 6, MRS Symp, 1995, in press. 20. Baker, I. & Gaydosh, D., Phys. Stat. Solidi, 96 (1986) 185. 21. Munroe, P. R. & Baker, I., Microbeam Analysis - 1990, eds J. R. Michael & P. Ingram, San Francisco Press, San Francisco, USA, 1990, p. 297. 22. Kao, C. R., Pike, L. M., Chen, S.-L. & Chang, Y. A., Intermetallics, 2 (1994) 235. 23. Crimp, M. A. & Vedula, K. M., Mats. Sci. and Eng., Al65 (1993) 29. 24. Fu, C. L., Ye, Y. Y. & Yoo, M. H., High Temperature Ordered Intermetallic Alloys 5, MRS Symp. Proc. 288, 1993, p. 21 25. Baker, I. & Munroe, P. R., High Temperature Aluminides and Intermetallics, eds S. H. Whang, et al., TMS, Warrendale, PA, 1990, p. 425. 26. Patrick, D. K., Chang, K.-M., Miracle, D. B. & Lipsitt, H. A., High Temperature Ordered Intermetallic Alloys 4, MRS Symp. Proc. 213, 1991, p. 267.
0966-9795(95)00011-9
The effect of nickel on vacancy hardening in iron-rich FeAl P. R. Munroe School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 20.52, Australia (Received
20 February
1995; accepted
14 March
1995)
The hardness and defect morphology of alloys with the composition (Fez5_, Ni,)A&s, where 0.1 5 x 5 10 at.% following high-temperature heat treatment and subsequent low temperature annealing was studied. It was found that nickel additions significantly reduced the softening which occurs on low temperature annealing of FeAl and also significantly increased the hardness of this compound. Following low-temperature annealing < 001 > edge dislocations were observed to form. In addition cuboidal voids were formed in those alloys with less than 10 at.% nickel. It is believed that the hardening which occurs in these alloys is associated with the interaction of the nickel atoms with the thermal vacancies which form on high temperature heat treatment. Possible mechanisms for the hardening which occurs are discussed. Key words: iron aluminides,
vacancies,
hardening
1 INTRODUCTION
by vacancies.
hardening behaviour of FeA1.‘4T15Additions of 10 at.% or less nickel, which replace iron, greatly decrease the softening which occurs on low-temperature annealing and also significantly increase the equilibrium hardness following this heat treatment. For example, following heat treatment for two hours at 950°C and air-cooling, Fe-49Al (unless otherwise stated all compositions given will be in atomic percent) exhibited a Vickers microhardness of N 570, but following a subsequent heat treatment at 400°C the hardness of this material was reduced to - 350. In contrast, an alloy with a nominal composition of Fe4sNi5A1so exhibited a hardness of - 610 following heat treatment at 950°C and - 550 following the subsequent 400°C anneal. Furthermore, transmission electron microscope (TEM) studies showed that the defect microstructures which evolved during low-temperature annealing were strongly dependent upon composition.13 It was shown that in FeAl, homogeneous arrays of c-001 > edge dislocations formed, but as the iron content decreased, the dislocation structure became increasingly heterogeneous so that very well-defined subgrains formed in nickel-rich alloys. Cuboidal voids were also formed in those ternary alloys with less than 10 at.% nickel, although the origin of these voids was not made clear.
The ordered intermetallic compound FeAl, which exhibits an ordered bee or B2 structure, has received considerable attention due to its high strength, good oxidation resistance and low density, which make it a candidate material for high temperature applications. It is now well established that a significant concentration of thermal vacancies can be retained in FeAl after heat treatment at even after furnace cooling, high temperature, which can lead to significant increases in hardness and decreases in ductility.iP5 These excess vacancies can be subsequently eliminated by low temperature annealing during which the material softens and large numbers of < 001 > edge dislocations evolve in the microstructure.1,6,7 In contrast, the isostructural compound NiAl also may retain a large concentration of vacancies following high temperature heat treatment.* However, its hardness is relatively independent of heat treatment,3 and, following low temperature heat treatment, voids rather than dislocations form in the microstructure.*-l3 More recently it has been shown that small additions of nickel to nominally stoichiometric FeAl can significantly affect the thermal vacancy 5
6
P.R.Munroe
Data consistent with the earlier work of Kong and Munroe has been reported by Schneibel and co-workers.16,17 The yield strength of Fes0Ni5A1d5, with trace additions of zirconium and boron, following prolonged annealing at low temperature was measured to be - 570 MPa, which was significantly higher than the yield strength of - 320 MPa for Fe5sALi5 following the same heat treatment. Kong and Munroe’* and Schneibel et ~1.‘~ have also investigated the effects of other first-row transition metals on the vacancy hardening of FeAl. Kong and Munroe showed that additions of these elements replacing iron decreased the softening which occurs on low-temperature annealing and increased the overall hardness following lowtemperature annealing. However, the increases in hardness were smaller than those which arose through nickel additions. Consistent with this, Schneibel et aLI9 showed that the first row of transition metals increased the yield strength of FeAl and were able to correlate this increase in strength with atomic size misfit. However, the increase in yield strength for the nickel-containing alloy was significantly higher than that predicted by atomic size misfit calculations. It is clear that nickel additions to FeAl interact with this compound in a more complex manner than other transition metals, and the effect of such additions on the mechanical properties and microstructure of FeAl warrants further investigation. The aim of this paper is to examine the hardness and microstructural development of alloys with a base composition of (Fe55_xNix)A145, where 0.1 5 x 5 lo%, following high-temperature heat treatment and subsequent low-temperature annealing.
2 EXPERIMENTAL Alloys with the nominal composition, (FeSs-x Ni,)&, where x = 0.1, 0.3, 1.0, 3.0 and 10.0 were cast. All of the samples were annealed in air at 950°C for 2 h and cooled to room temperature by either water-quenching or air-cooling. Subsequent low temperature annealing was performed on the air-cooled alloys at 400°C for 120 h. Vickers microhardness testing was performed on each sample using a 300 g load. Each data point obtained was the average of at least 10 measurements. Standard deviations were less than 5%, but error bars were omitted from Fig. 1 to improve clarity. The microstructures of all the materials were examined by both optical and transmission electron microscopy (TEM). Thin foils for TEM
40il
q
Water-Quenched
350
q
AiPco&d
300
q
4G1XX20hows
HardnlZSS
250
FiAl
013
0:l
i
Ni Content (at.%)
Fig. 1. The effect of heat treatment on the Vickers microhardness of alloys with the composition (Fe55-xNix)A145r where 0.1 5 x < 10 at.%. Data for Fe55A145 (after(3)) are also included for comparison.
study were prepared using methods described elsewhere,*O and examined in a Philips 420 transmission electron microscope, furnished with an energy dispersive X-ray spectrometer (EDS), operating at 120 kV.
3 RESULTS
AND
DISCUSSION
The effect of nickel additions to the hardness of iron-rich FeAl as a function of heat treatment is shown in Fig. 1. Data for Fes5Ald5 from the work of Nagpal and Baker3 are included for comparative purposes. However, it should be noted that their samples were initially heat treated at the slightly higher temperature of 1000°C. It is presumably for this reason that a slightly higher hardness was observed for this alloy following the initial heat treatment, since a higher concentration of thermal vacancies would be expected to be quenched in at this slightly higher temperature. It is clear that following heat treatment at 950°C hardness is little affected by composition, although a slight increase in hardness was observed as the nickel content increased. It is also clear that hardness is little affected by the cooling rate from this temperature; the air-cooled samples were slightly softer than those which were water-quenched, but the differences were small. Following subsequent annealing at 400°C of the samples which were air-cooled from 950°C significant changes in hardness as a function of nickel content were observed. As the nickel content increased the hardness increased, such that for Fes4NilA&, for example, the hardness was only reduced by 7% by prolonged lowtemperature annealing, compared with a reduction of about 44% for the binary alloy. It is interesting
Nickel in iron-rich FeAl
to note that even a nickel addition of only 0.1% can increase the hardness of Fes5A145, following prolonged exposure at 4OO”C, from about 250 to about 300. The hardness data obtained here are consistent with the earlier work of Kong and Munroe on alloys with a nominal composition of Feso_, NixAlSO.l4 However, in these prior studies the hardness values which were obtained following both heat treatments were significantly higher than those acquired in this study on aluminium-lean compositions. The microstructure of these materials was examined by optical metallography. In all cases the alloys were, as expected, single-phase with a grain size of about 100 pm. Transmission electron microscopy was performed on all the alloys following heat treatment at 950°C and air-cooling. For all five alloys the microstructure contained a small number of < 001 > dislocations which were edge in
Fig. 3. Bright field transmission electron micrographs subsequent annealing at 400°C for 120 h. Diffraction
I
Fig. 2. Bright field transmission electron micrograph of Fe45Ni10A145 following annealing at 950°C for 2 h and aircooling. Diffraction vector as shown, beam direction [l 1 I].
of Fe,,NiloA&s following annealing at 950°C for 2 h and air-cooling and vectors as shown, beam directions (a) and (b) [OOl], (c) [Ol l] and (d) [012].
8
P. R. Munroe
character (Fig. 2). The density of the dislocations was estimated to be about 1 x lo-‘* m-*, consistent with levels previously observed in binary FeAl after similar heat treatments.13 The atom site location of the nickel in these alloys was determined on these alloys following heat treatment at 400°C by atom location by channelling-enhanced microanalysis methods (ALCHEMI), using methods described elsewhere.*’ It was deduced that the nickel atoms exerted a strong preference for the iron sublattice, consistent with previous studies on near-stoichiometric compositions14315 and recent theoretical predictions.22 Following annealing at 400°C for 120 h an increase in dislocation density was observed in Fe4sNi10A14s (Fig. 3). The dislocation density was estimated at 2 x 10-‘3m-2. The dislocations were distributed in a relatively homogenous manner and all were found to exhibit a < 001 > Burgers vector and be edge in character. For example, those dislocations marked ‘a’ exhibited a line direction, u, of [OlO] and exhibited invisibility when g = 020, and weak contrast when g = Oli This contrast is consistent with these dislocations being edge in character with a Burgers vector, b, of [loo]. Similarly, the contrast from those dislocations marked ‘b’ was consistent with that expected from edge dislocations with a [OlO] Burgers vector and a [loo] line direction. Similar observations were made for the alloy with the nominal composition Fe40Nii0A150 studied earlier. 13 A similar defect structure was observed in Fe52Ni3A145 following heat treatment at 400°C (Fig. 4). That is, an array of < 001 > edge dislocations was noted. The density of dislocations in this alloy was generally similar to that observed in Fe45Ni10A145.However, a small number of voids, about 100 nm in diameter were also observed. These voids were usually cuboidal in shape with faces parallel to (001) planes. The void density was estimated to be about 1 x 1018m-3. In many cases these voids appeared to have interacted with the edge dislocations or had formed at the intersection of two such dislocations, see for example the void marked V in Fig. 4. Similar voids were observed in Fes0_,Ni,A150 alloys with nickel concentrations of less than 7.5% following the same heat treatment.13 Those alloys with less than 3% nickel exhibited similar microstructures following heat treatment at 400°C as Fe52Ni3A145;that is, a mixture of < 001 > edge dislocations and cuboidal voids. However, it was noted that as the nickel concentration
Fig. 4. Bright field transmission electron micrograph of FeS2Ni3A14s following annealing at 950°C for 2 h and aircooling and subsequent annealing at 400°C for 120 h. Diffraction vector as shown, beam direction [OOl].
decreased the dislocation density decreased and the void density increased; in Fe54.9Ni0.1A145very few dislocations and a large number of voids were observed, (Fig. 5). In this sample the dislocation density was estimated to be about 2 x 10-‘2m-2 and the void density was estimated to be about 5 x 1018m-3. Theoretical and experimental studies of the vacancy concentration of FeAl as a function of composition and heat treatment have shown the concentration of vacancies increases as temperature increases and also as the composition approaches stoichiometry. 4,5Conversely, the number of anti-site
Fig. 5. Bright field transmission electron micrograph of Fe54.9Ni0.1A145 following annealing at 950°C for 2 h and aircooling and subsequent annealing at 400°C for 120 h. Diffraction vector as shown, beam direction [OOl].
Nickel in iron-rich FeAl
defects, that is iron atoms on the aluminium sublattice, increases as the alloy becomes iron-rich. It has been shown that much higher hardnesses are obtained nearer stoichiometry.3,4 This indicates that it is vacancies, rather than anti-site defects, which are most influential in determining the mechanical properties of FeAl. Consistently with this, Chang et aL4 correlated theoretical hardness concentrations with hardness as a function of both composition and heat treatment and showed that hardness correlated well with the square root of vacancy concentration. Although it is apparent that vacancies exert significant influence over the mechanical properties of this compound, the mechanism through which vacancies harden FeAl remains unclear. Recently Crimp and Vedula studied the mechanical behaviour of both Fe-40Al and Fe-5OAl as a function of both cooling rate and grain size.23 Hall-Petch analysis of their data suggested that vacancies retained in the lattice following high-temperature heat treatment increased lattice resistance, but had little effect on grainboundary strengthening. However, the alloys studied were subjected to a range of processing conditions and heat treatments, so that more detailed relationships between vacancy concentration and lattice resistance could not be deduced. It has been well established that during heat treatments to remove excess thermal vacancies in FeAl large numbers of dislocations are evolved.‘,6,7 Some differences in the crystallography of these dislocations have been observed with composition and heat treatment, but in general these dislocations have < 001 > Burgers vectors and tend to be homogeneously distributed. Although the dislocation structures observed here were, in general, similar to those previously observed in the (Fe, Ni)Al alloys which were nearer in composition to stoichiometry, the densities of dislocations noted were much lower in these aluminium-lean alloys.i3 Since these dislocations are associated with the removal of thermal vacancies, the lower dislocation density evolved is consistent with a smaller number of thermal vacancies being retained in these alloys following high temperature heat treatment. Also as hardness is strongly affected by vacancy concentration, the lower hardness values of these alloys compared with the near-stoichiometric alloys, is also indicative of a lower number of thermal vacancies being retained following the 950°C anneal. These observations are consistent with the previous studies of vacancy concentration as a function of composition for FeA1.4,5
9
The origin of the voids which were observed in some of the alloys here is presently unclear. Similar voids were also observed in the more stoichiometric alloys, but in that study the smallest nickel concentration was 1%. Here, nickel additions as low as 0.1%, replacing iron, have led to a change in the defect morphology formed, from only < 001 > edge dislocations in FeAl to almost all voids in Fe54.9A145Ni0. 1. FeAl exhibits a high divacancy binding energy for vacancies on the same sublattice. This means that it is relatively easier for two vacancies on the same sublattice to form a divacancy. It is also known that the nickel atoms are preferentially located on the iron sublattice. It is possible that because of the slightly larger atomic radius of nickel that vacancies are attracted to regions of the lattice adjacent to the nickel atoms. It is therefore possible for divacancies to form preferentially in lattice regions around the nickel atoms and that the mobility of these divacancies may then be restricted. These divacancies may then, in turn, attract and trap other, more mobile, divacancies not associated with nickel atoms. If these other divacancies are oriented in other non-parallel < 001 > directions, the interaction of these divacancies may lead to the formation of cuboidal arrays of vacancies and so the nucleation of cuboidal voids. At higher nickel concentrations lower densities of voids were observed. It is possible that as the concentration of nickel atoms increases the number of mobile divacancies may be reduced, so that there may be insufficient numbers of mobile divacancies to nucleate voids. No voids were observed in Fe45Ni10A145,consistent with earlier studies of nominally stoichiometric alloys. l3 Furthermore, it is known that in NiAl the divacancy binding energy is repulsive and such defects are unlikely to form.24 It is possible that in Fe45Ni10A145the nickel additions are enough to decrease the divacancy binding energy sufficiently so such defects do not form, thus inhibiting void formation completely. It is clear that nickel additions replacing iron have two effects on the dependence of hardness of FeAl on heat treatment. First, nickel additions, even additions as low as O.l%, limit the softening which occurs on low-temperature annealing, and second, these ternary additions significantly increase the equilibrium hardness of FeAl following the 400°C anneal. It is possible that nickel additions to FeAl may affect its behaviour in several ways. First, such additions might reduce the concentration of thermal vacancies which form during high-temperature heat treatment.
10
P. R. Munroe
Second, nickel additions may limit the number of thermal vacancies which are eliminated from the lattice during subsequent low-temperature annealing. Third, it is clear that the defect microstructure evolved on low temperature annealing is strongly modified by the addition of nickel. This modified structure may limit either the formation or the mobility of slip-related dislocations during subsequent deformation and so harden the lattice. Alternatively, dislocation mobility may be limited by nickel atoms or thermal vacancies forming solute atmospheres around slip dislocations. The vacancy-hardening behaviour of the (Fe, Ni)Al alloys is consistent with the observations made for binary FeAl. Significantly higher hardnesses were obtained for the near-stoichiometric alloys compared with the (Fe55-xNix)A145alloys. It might be expected that the near-stoichiometric alloys would contain higher concentrations of vacancies than the (Fe55_xNix)A145alloys which in turn would be expected to contain a high fraction of anti-site defects. However, the relative drop in hardness for both alloys during the 400°C heat treatment was, in each case, about the same. Nevertheless, the trends which occur in each set of alloys are the same. It would seem that in these alloys the mechanical properties are strongly influenced by the presence of vacancies and it would seem unlikely that nickel additions greatly reduce the concentration of vacancies which form during high-temperature heat treatment relative to the binary alloy. However, further studies which allow measurement of vacancy concentration for the ternary alloys as a function of composition and heat treatment would allow this possibility to be investigated. It is also possible that the nickel atoms inhibit thermal vacancies being removed from the lattice during annealing at 400°C. This would imply that the nickel atoms interact with the vacancies in some way to form nickel-vacancy complexes. It might also be expected that these defects would induce significant lattice hardening, consistent with the observed hardening in these alloys. However, in the ternary alloys, significant concentrations of defects (whether dislocations or voids) are evolved on low-temperature annealing, suggesting that vacancy removal is not completely inhibited by nickel additions. Again, to determine the extent to which nickel atoms trap vacancies in the lattice would require study of the vacancy concentration in these alloys as a function of composition and heat treatment.
The third possibility is that the defects which form on low temperature heat treatment affect the nucleation and motion of slip dislocations during subsequent plastic deformation. At room temperature, FeAl deforms by the movement of < 111 > dislocations, irrespective of composition25 and NiAl deforms by < 001 > slip.25 It is clear that across the FeAl-NiAl pseudobinary there is a transition in slip vector. For the Fe60_,Ni,A140 pseudobinary strained at room temperature, this transition occurs at about the composition Fe40Ni20A140.26The composition at which this transition occurs at higher aluminium concentrations is not known, although it would seem probable that, in those alloys with 10% nickel or less, that room temperature deformation would occur through < 111> slip. During straining of the (Fe, Ni)Al alloys studied here following low-temperature annealing < 111 > slip dislocations must be nucleated to facilitate plastic deformation. It is possible that the voids or the more heterogenous arrays of < 001 > dislocations which form in the ternary alloys may inhibit dislocation nucleation or motion and harden these materials. Another possibility is that the observed hardness peak which occurs at Fe40NiieA15014,‘5is related to a transformation in slip vector. It is also possible that the nickel atoms, vacancies or even nickelvacancy complexes form atmospheres around the slip dislocations and inhibit their motion. Further studies of the interaction between the defect structure evolved on low-temperature annealing and slip dislocations will allow this possibility to be investigated.
4 CONCLUSIONS The hardness and defect morphology of (Fes5_X Nix)A145, where 0.1 5 x < 10 at.% following high temperature heat treatment and subsequent low-temperature annealing was studied. Nickel additions decreased softening which occurs on low temperature annealing of FeAl but also significantly increased the hardness of this compound. Following annealing at 4OO”C, < 001 > edge dislocations formed in the lattice, but with decreasing nickel concentration an increased tendency to the formation of cuboidal voids was noted. This behaviour was attributed to nickel atoms interacting with the thermal vacancies retained following the high-temperature heat treatment.
Nickel in iron-rich FeAl
ACKNOWLEDGEMENTS The author would like to thank Dr J. Schneibel at Oak Ridge National Laboratory for provision of the samples and Dr J. R. Sellar for useful discussions.
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13. Kong, C. H. & Munroe, P. R., Intermetallics, 2 (1994) 333. 14. Kong, C. H. & Munroe, P. R., Scripta Metall. et Mater., 28 (1993) 1241. 15. Kong, C. H. & Munroe, P. R., Scripta Metall. et Mater., 30 (1994) 1079. 16. Schneibel, J. H. & Specht, E. D., Scripta Metall. et Muter., 31 (1994) 1737. 17. Schneibel, J. H. & Maziasz, P. J., Processing, Properties and Application of Iron Aluminides, eds M. A. Crimp & J. H. Schneibel, TMS, Warrendale, USA, 1994, p. 183. 18. Kong, C. H. & Munroe, P. R., Processing, Properties and Application of Iron Aluminides, eds M. A. Crimp & J. H. Schneibel. TMS, Warrendale, USA, 1994, p. 231. 19. Schneibel, J. H., George, E. P., Specht, E. D. & Horton, J. A., High Temperature Ordered IntermetaIlic Alloys 6, MRS Symp, 1995, in press. 20. Baker, I. & Gaydosh, D., Phys. Stat. Solidi, 96 (1986) 185. 21. Munroe, P. R. & Baker, I., Microbeam Analysis - 1990, eds J. R. Michael & P. Ingram, San Francisco Press, San Francisco, USA, 1990, p. 297. 22. Kao, C. R., Pike, L. M., Chen, S.-L. & Chang, Y. A., Intermetallics, 2 (1994) 235. 23. Crimp, M. A. & Vedula, K. M., Mats. Sci. and Eng., Al65 (1993) 29. 24. Fu, C. L., Ye, Y. Y. & Yoo, M. H., High Temperature Ordered Intermetallic Alloys 5, MRS Symp. Proc. 288, 1993, p. 21 25. Baker, I. & Munroe, P. R., High Temperature Aluminides and Intermetallics, eds S. H. Whang, et al., TMS, Warrendale, PA, 1990, p. 425. 26. Patrick, D. K., Chang, K.-M., Miracle, D. B. & Lipsitt, H. A., High Temperature Ordered Intermetallic Alloys 4, MRS Symp. Proc. 213, 1991, p. 267.