Scripta METALLURGICA Vol. 25, pp. 2861-2866, 1991 Pergamon Press plc et MATERIALIA Printed in the U.S.A. All rights reserved

Structure o f the P Phase in Vapour Quenched AI-Cr-Fe

B. A. Shollock end B. Cantor Department of Materials, Oxford University, Parks Road, Oxford OXI 3PH, U.K.

Oxford Centre for Advanced Materials and Composites

( R e c e i v e d Augus t 8, 1990) ( R e v i s e d O c t o b e r 9, 1991)

Introduction

The development of advanced aerospace structures relies on the introduction of new materials. Conventional alumininm alloys commonly used by the aircraft industry rapidly lose strength at temperatures above 500K and are therefore not as stable as titanium and nickel alloys. Aluminium alloys, however, have other advantages over titanium such as ease of forming and machining, and over nickel alloys in offering substantial weight reductions. The lack of thermal stability in conventional wrought aluminium alloys has prevented their use in new aerospace applications. In order to meet these needs, a new class of thermally stable aluminium alloys based on transition metal additions, especially chromium and iron, and produced by rapid solidification techniques has been developed. Although most rapid solidification techniques involve quenching from the melt, vapour quenched aluminium-transition metal alloys have been shown to have mechanical properties comparable to liquid quenched materials, with the added advantage of no consolidation to produce bulk material (1 - 3). Other studies have examined both binary and ternary vapour quenched materials (1 - 4), but ternary alloys typically containing 2.6 to 6. lat% Cr and 0.45 to 0.92at% Fe have been the subject of most previous work.

The general microstructure of the vapour quenched ternary alloy in both the as-deposited and aged conditions has been previously reported (1 - 6). For deposits with collector temperatures less than 473K, most of the chromium and iron was in solid solution regardless of the overall alloy composition, as determined by X-ray diffraction. Material deposited at temperatures between 523K and 543K consisted of supersaturated aluminium grains that also contained a uniform dispersion of fine, 3nm to 5nm diameter, unidentified precipitates, which were designated the P phase (1, 2). When substrate temperatures exceeded 593K, the solid solution and fine precipitates were still present, but grain boundary precipitates of CrA17 and FeA16 with diameters of l~an to 2~an were also observed (1 - 5).

Transmission electron microscopy studies of alloys deposited at 553K containing 2.6at% to 6. lat% Cr and 0.45at% to 0.92at% Fe showed that ageing resulted in a uniform dispersion of P phase precipitates. Bickerdike and co-workers (3, 4) reported that after ageing ternary deposits at 533K for 400 hours, the formation of these fine precipitates gave rise to an age-hardening response. At a later stage, nucleation of rods or plates of CrA17" with diameters less than 100nm was observed within the aluminium grains. The precipitation reactions were complete after heating at 673K for a few hours; the fine P phase precipitates developed to a final diameter of 10nm and the CrA17 particles within the aluminium grains had reached 500nm in diameter.

It has been suggested that the P phase provides a thermally stable strengthening mechanism. Although the large precipitates which form in the AI-Cr-Fe vapour quenched alloy are well characterised, details of the early stages of decomposition, and the composition and structure of the P phase precipitates are not known. In view of the significance of the P phase in potentially providing long term high temperature properties, insight into the characteristics of this phase can be used to provide a basis for understanding and improving the mechanical properties of these materials. This note describes an investigation of the crystal structure of the P phase precipitates and their orientation relationship with the aluminium matrix in an aluminium-chromium-iron alloy using a combination of transmission electron microscopy (TEM) and field ion microscopy/atom probe analysis (FIM/AP). Further details of the early stages in the formation of the P phase precipitates and their composition are described elsewhere (6).

Experimental Procedure

The alloy studied in this investigation was an AI - 4at% Cr - 0.5at% Fe alloy deposited at a temperature of 522K by the electron-beam evaporation-vapour condensation technique employed by the Royal Aircraft Establishment, Famborough. Details of the procedure can be found elsewhere (4). The material was heat treated at 623K for times between 1 and 200 hours. Specimens for TEM and FIM/AP were prepared using standard methods. The specimens were examined in a Philips CM12 TEM and a VG FIMI00. The results presented in this paper concentrate on ageing treatments which produce well-developed P phase precipitates.

2861 0036-9748/91 $3.00 + .00

Copyright (c) 1991 Pergamon Press plc

2862 P PHASE STRUCTURE Vol. 25, No. 12

Results

Figure 1 presents a TEM micrograph of the alloy after ageing for 8 hours. A uniform distribution of P phase precipitates with average diameter of approximately 5nm can be seen. Figure 2 presents a corresponding selected area diffraction pattern (SADP). The pattern contains second phase reflections in addition to the intense aluminium spots which arise from an [001] orientation of the matrix. An inner set of eight precipitate reflections are equally spaced around the transmitted beam at 45" intervals, corresponding to a d-spacing of approximately 0.36nm. In addition, an outer set of eight reflections are at a spacing corresponding to a d-spacing of about 0.21nm, and are arranged in four pairs (eg. spots A and B) with 24" between the spots in each pair. The precipitate reflections show no evidence of streaking or splitting.

Figure 3 presents a TEM micrograph of the alloy after ageing for 8 hours oriented with the beam parallel to the [ 111 ] matrix direction. P phase precipitates can again be seen clearly. A corresponding SADP is shown in figure 4. In addition to the intense aluminium matrix reflections, second phase reflections (e.g. spots A and B) with a characteristic arrangement can again be seen. In this pattern, there are six pairs of precipitate reflections at positions corresponding to a d-spacing of 0.21nm with angular separations of about 39" between pairs and 21" within pairs. In addition, twelve equally spaced faint reflections with a d-spacing of approximately 0.23nm are closely associated with each spot in the six pairs (e.g. spot C). Additional faint diffraction spots at positions corresponding to a d-spacing ofapproximately 0.34nm can be seen as six pairs of refiections (e.g. spots D and E) with about 10" between the spots comprising each pair and approximately 50" between pairs.

Discussion

Other studies of rapidly solidified aluminium-iron (7 - I I) and aluminium-platinum alloys (12, 13) have reported similar [001] and [I 1 I] zone axis diffraction patterns to those observed for the P phase. All analyses of the precipitates by previous investigators have relied on TEM diffraction techniques alone, with no information concerning the composition of the precipitates. Fontaine and Guinier (7) proposed that the precipitates in a quenched aluminium-iron alloyhad a face- centered cubic type structure, FeTAl, with a lattice parameter of 0.707nm. In a quenched aluminium-iron alloy, Jacobs et a/. (8) proposed that the precipitate phase had a diamond cubic crystal structure with a lattice parameter of 0.585+0.05nm and the following orientation relationship: (I 1 l)precipitate//(001 )AI; [ 110]precipitate//[010 ]AI. This crystal structure and orientation relationship have been presumed io be correct in subsequefit stfidies. Kamio et al. (9) observed the P phase in an aged A1 - 4wt% Fe alloy produced by melt spinning. The same matrix <001> and < I I I > diffraction patterns as Jacobs et M. were reported, and they quoted the same crystal structure and orientation relationship with no further proof ofits validity. Other studies have quoted the orientation relationship of Jacobs et al., but have suggested an alternative crystal structure to describe the phase. A CaF 2 structure with a lattice parameter of 0.567nm was proposed by Chattopadhyay and co.workers (12, 13) based on their observations of a <I 1 I> selected area diffraction pattern in a rapidly solidified AI - 2at% Pt alloy.

In the present work, in addition to the [001] and [111] zone axis diffraction patterns, a series of zone axis diffraction patterns, [011], [323], [013], and [112], was obtained from the aged material in order to evaluate the crystal structure and orientation relationship of the P phase precipitates. Centered-dark field imaging of all second phase and matrix reflections identified the presence of more than one variant of the precipitate/matrix orientation relationship, and eight variants appeared to account for all observed precipitates. The following orientation relationship was found: { 111 }AI//{ 111 ]p; <321>A1//<110>p. The crystal structure was found to be cubic. Figures 5 and 6 present schematic diagrams of the experimental diffraction patterns shown in figures 2 and 4, respectively.

Previous X-my diffraction studies of material containing the P phase did not reveal any additional reflections other than those arising from the aluminium matrix and FeAI6 and CrAb particles (1, 2). The composition and structure of the P phase were not previously identified. In the present study, FIM/AP analysis was used to determine the composition of the P phase as Fe3AI, and an example of a composition profile obtained by FIM/AP analysis showing the precipitates containing 75at% Fe is presented in Figure 7. The results of FIM/AP investigations are discussed in greater detail elsewhere (6). The Fe3AI phase is a 1)O3 ordered structure with a space group of Fm3m. Partial stereograms for the [001 ] and [ 111 ] zone axes produced using the proposed orientation relationship and incorporating the Fm3m structure factor are shown in figures 8 and 9. Excellent agreement between the observed and predicted angles in the patterns can be seen. All other zone axis patterns showed the same degree of agreement. A commercial software program, "Diffract" (14), was used to generate diffraction patterns based on the eight variants of the orientation relationship with Fe3AI as the precipitate phase, and [001 ] and [ 111 ] simulations showing the major reflections are presented in figures 10 and 11. Again, the calculated patterns agree well with the experimental patterns. The FIM/AP results are in good agreement with the diffraction pattern analysis discussed above. Examination of the structures proposed by previous studies based purely on TEM diffraction show that they belong to the same space group as Fe3AI, thus the same systematic absences would be

Vol. 25, No. 12 P PHASE STRUCTURE 2863

expected in all cases. Convergent beam electron diffraction experiments are being conducted to provide further confirmation.

Conclus ions

The present investigation has identified the structure, composition and orientation relationship of the P phase in vapour quenched aluminium-chromium-iron. The P phase structure is proposed to have a composition of Fe3AI, a phase which has a DO 3 ordered cubic structure and an Fm3m space group, with an orientation relationship of { 11 I}AI//{ 11 l}p; <321>A1//<110>p. The orientation relationship was found to be valid for a large number of zone axis patterns. Compositional analysis provided by FIM/AP studies has provided additional support for the Fe3AI structure. The structure identified in the present study has been shown to be a refinement of those proposed in other investigations.

Acknowledgements

This work has been carried out with the support of the Procurement Executive, Ministry of Defence. The author is grateful to Dis. A. W. Bowen, J. S. Crompton, C. Gilmore and K. Mingard for useful discussions.

References

(1) P.G. Partridge andM. C. McConnell, ActaMet. 35 (1987) 1981. (2) M.C. McConnell and P. G. Partridge, Acta Met. 35 (1987) 1973. (3) R.L. Bickerdike, D. Clarke, J. N. Eastabrook, G. Hughes, W. N. Malt, P. G. Partridge and H. C. Ranson, Proc.

Int. Conf. in Rapidly Solidified Materials (ASM) (1986) 137. (4) R.L. Bickerdike, D. Clarke, J. N. Eastabrook, G. Hughes, W. N. Mair, P. G. Partridge and H. C. Ranson, Int.

J. Rapid Solidification 1 (1986) 1. (5) C.J. Gilmore and A. W. Bowen, EUREM 88, eds. P. J. Goodhew and H. C. Dickinson, Institute of Physics

Conf. Series 93 IOP (1988) 199. (6) B.A. Shollock, D. Phil. Thesis Oxford University (1989). (7) A. Fontaine and A. Guinier, Phil. Mag. 31 (1975) 839. (8) M.H. Jacobs, A. G. Dogger and M. J. Stowell, J. Mat. Sci. 9 (1974) 1631. (9) A. Kamin, H. Tezuka, T. Sato, T. T. Long and T. Takahashi, J. Jpn. Inst. Light Metals 36 (1986) 324. (10) P. Furrer and H. Warlimont, Z. Metall. 64 (1973) 236. (11) E. Blank, Z. MetaU. 63 (1972) 324. (12) K. Chattopadhyay and P. Ramachandramo, Mat. Sci. Eng. 38 ( 1979) 839. (13) K. Chattopadhyay, S. Lele and P. Ramachandrarao, J. Mat. Sci. Letters 13 (1978) 2730. (14) Mierodev Software, Colorado U.S.A.

Figure 1: TEM bright field micrograph of the Al-Cr-Fe alloy after ageing at 623K for 8 hours. Darkly imaging P phase precipitates with diameters of 3 to 5nm can be s e e n .

Figure 2: Corresponding [001] zone axis selected area diffraction pattern for figure 1, showing both matrix and P phase precipitate reflections.

2864 P PHASE STRUCTURE Vol. 25, No. 12

Figure 3: TEM bright field micrograph of the A1-Cr-Fe alloy after ageing at 623K for 8 hours. Darkly imaging P phase precipitates with diameters of 3 to 5rim can be seen.

Figure 4: Corresponding [111] zone axis selected area diffraction pattern for figure 3, showing both matrix and P phase precipitate reflections.

12"

• o 33"

0

o o • 0 0 •

• 200m • 220m • 220p O dohble diffraction

O0

Ooo

e• % ~o

o• o ° o • 0 ~" e ~ o oe

• 220rn • 220p ~ariantA A220p variant B o double diffraction

Figure 5: Schematic diagram of the experimental [001] diffraction pattern, showing matrix and precipitate reflections as well as double diffraction.

Figure 6: Schematic diagram of the experimental [ 111 ] diffraction pattern, showing matrix and precipitate reflections as well as double diffraction.

Vol. 25, No. 12 P PHASE STRUCTURE 2865

1071] 75a t

• . ....t,. t . , , ,' , L " ' ""

I ~ 7:5 O, o 5obo lo6ao 156oo ~o6oo

tluzber c: 10n; ~lumber of 'lor, s

Figure 7: A composition profile obtained from atom probe analysis of the AI-Cr-Fc alloy aged for 8 hours at 623K. The chromium is randomly distributed, however, precipitates containing approximately 75at% iron are present.

OlOm ll0ml 011p

57*

lOOm

i l .0m/ / I "~,,.~liO~

001m

Figure 8: [001] stereogram showing more than one variant of the proposed orientation relationship, providing confirmation of the angular relationships observed in figure 5.

2866 P PHASE STRUCTURE Vol. 25, No. 12

f d/o Z

O]lm

19 °

Figure 9: [ 111 ] stereogram showing two variants of the proposed orientation relationship, providing confirmation of the angular relationships observed in figure 6.

O o o

° o

• • ° • • o o o

• o • • • • ° o o

Figure 10: Computer simulated [001 ] diffraction pattern generated using the eight variants of the orientation relationship.

Figure 11: Computer simulated [111] diffraction pattern generated using the eight variants of the orientation relationship.