A FIELD-ION MICROSCOPY AND ATOM PROBE STUDY OF AGEING BEHAVIOUR OF A Co-BEARING MARAGING STEEL

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A FIELD-ION MICROSCOPY AND ATOM PROBE STUDY OF AGEING BEHAVIOUR OF A

Co-BEARING MARAGING STEEL

W. Sha, A. Cerezo, G. Smith

To cite this version:

W. Sha, A. Cerezo, G. Smith. A FIELD-ION MICROSCOPY AND ATOM PROBE STUDY OF

AGEING BEHAVIOUR OF A Co-BEARING MARAGING STEEL. Journal de Physique Colloques,

1989, 50 (C8), pp.C8-407-C8-412. �10.1051/jphyscol:1989869�. �jpa-00229967�

COLLOQUE D E PHYSIQUE

Colloque C8, Suppl6ment au n o l l , Tome 50, novembre 1989

A FIELD-ION MICROSCOPY AND ATOM PROBE STUDY OF AGEING BEHAVIOUR OF A CO-BEARING MARAGING STEEL

W. SHA, A. CEREZO and G.D.W. SMITH

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3PH, Great-Britain

Abstract

-

This paper reports the first atom probe field-ion microscopy (APFIM) study of the ageing reactions in a Co-bearing (C-300) maraging steel (Fe-l8.5%Ni-9.0%Co-4.8%Mo-0.6%Ti-O. l%Al-O.I%Si-0.03%C (wt%)), using both the conventional APFIM and the position-sensitive atom probe (POSAP). Two families of intermetallic phases (Fe7M06 and Ni3(Mo,Ti) types) have been found to contribute to age-hardening. The com~osition and morphology of these precipitates were studied for different ageing times (0.5- 128 hours at 510 C) to investigate the ageing sequence. The observation of the (Fe,Ni,Co)7Mog p-phase is of interest since this is contrary to most published electron diffraction work, but is supported by thermodynamic calculations. Austenite reversion has been found to start after ageing for 4 hours and the reverted austenite approaches the predicted equilibrium composition after ageing for 8 hours.

1

-

INTRODUCTION

Maraging steels are a class of ultrahigh-strength martensitic steels which are age-hardened by the precipitation of intermetallic compounds19 2.3. Since the 1960s, a large amount of research work has been carried out on the physical metallurgy of these alloys and this interest has been renewed in the last few years, prompted by the sharp drop in cobalt availability in the late 1970s. Although much work has focused on what happens in hardening reactions during ageing, the exact nature of the precipitation process is still, to a large extent, unknown. This is due to the hi h density of extremely tine precipitates embedded in the martensite matrix (of the order of lOnm in size and I023m-$in density), which is beyond the resolution for chemical analysis of most electron microscopes. Genemlly, it is believed that A3B type phases (Ni3Ti, Ni3Mo, and Ni3(Mo,Ti)) form at the initial ageing stage4. 5 while the more stable A2B phase (Fe2Mo) replaces them after longer ageing times6. But, different results have been reported even on the same material and under the same heat treatment.

APFIM7.8 has been proved to be capable of revealing the distribution of small precipitates and accurately analysing their compositions. In the more recently developed position-sensitive atom probe (POSAP), time-of-flight mass spectrometry is combined with position-sensing to produce a system which can determine both the chemical identity and initial positions of single atoms field evaporated from the specimen surface'). The unique, powerful facility has enabled us to investigate the precipitation process on an atomic scale.

2

-

EXPERIMENTAL

The alloy studied was Vasco Max (2-300 steel (composition shown in table 1). This alloy was chosen partly because it has been the subject of a recent TEM / field emission STEM X-ray investigation by Vanderwalke& lo. Small plates of steel in the as-received condition were cut into bars of about 0.7 x 0.7 x 10mm3, sealed in silica tubes with vacuum <

10-5 mbar, annealed at 8 16OC for one hour, and water quenched. They were subsequently aged at 5 10°C for different times of 0.5, 1, 2,4, 8 and 128 hours and air cooled. The Oxford VG FIMIOO atom probe, with a position-sensitive detector mounted on it, was used in these experiments. Atom probe analyses were conducted at 65K or lower, with a pulse fraction of 20% or occasionally 15%, and a dc voltage ranging from 5 to 18kV. The accuracy of analysis was tested using as-quenched material and the composition obtained was in agreement within statistical error with the nominal composition. Figure 1 shows the spectra obtained from the conventional energy compensated atom probe and the POSAP.

TABLE 1 THE NOMINAL COMPOSITION OF C-300 MARAGING STEEL

Weight percent Atomic percent

C 0.03 max 0.14 max

Fe bal.

bal.

Ni 18.5 18.2

Co 9.0 8.8

Mo 4.8 2.9

Ti 0.6 0.7

A1 0.1 0.2

Si 0.1 0.2

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989869

Ions Ions

120

-

⁵⁰⁰⁰

-

2+ 2+

Fe

100

-

4000

-

80

-

3000

-

60

-

2000

-

26 27 28 29 30 31 32 33 34 10 15 20 25 30 35 40 45 50 55

Mass-to-charge Ratio Mass-to-charge Ratio

a) b)

Fig. 1 Mass spectra ofaged C-300 maraging steel obtained by

(a) conventional energy compensated and (b) position-sensitive atom probes 3

-

^RESULTS

Figure 2 shows field-ion micrographs of the alloy after ageing. The preci itates appears to be coherent, or semi- coherent with the martensite matrix, in agreement with previous authors'> l f There is some variation from place to place of density of the precipitates. Conventional atom probe analyses showed that there were two families of precipitates. The first kind has an average composition of Fe3gi3Nil4+2Co3*lMo43k2Si3i2, or (Fe,Ni,Co)53Mo43Si3, and sometimes contains some residual Ti. The composition of these precipitates is found to be quite stable (figure 3a), and they continue growing when ageing proceeds (figure 4). The composition suggests that this is a Fe7Mog p-phase with some Fe substituted by Ni and a small amount by Co. The presence of Si is reasonable if one considers the affinity between Mo and Si in the steel. Analysis shows that there is an atomically sharp interface between the precipitates and the martensite matrix (figure 5). It should be noted that apart from the fine spherical precipitates, there are some relatively large plaie-like ones as shown in figures 2a and 2b, although much less frequent, which have also been identified as Fe7Mog type. We believe that these are precipitates at martensite lath boundaries, though the possibility of being at prior austenite grain boundaries could not be completely ruled out. This has been suggested by the early work of Spitzig et all*. Magnee et al3has mentioned that Mo preferred to segregate at lath martensite boundaries and we did find instances of single atomic layer decoration of Mo, which might be the very initial stage of boundary precipitation.

Fig. 2 FIM images o f the precipitated area i n aged (2-300 maraging steel. The local magnification difference makes the precipitates appear larger than reality.

(a) Ageing time 1 hour, 10.4kV (below BN), 5 IK, 3 x 1 0 - h b a r Ne. The isolated particle just above the centre of the image was identified by AP analysis as NidMo,Ti). The other bands of bright particles are - .

of F e 7 ~ G t y p e .

(b) Aaeinn time 2 hours. 13.4kV. 64K. 1 x 10-5mbar

~ e . Rot; the relatively large plateilike Fe7Mog precipitate.

(c) Ageing time 8 hours, 14.3kV, 64K, 2 x 10-5mbar Ne. Note the large, plate-like, dark-imaged, reverted austenite region to the left above the probe hole. The bright-imaged particle just below the probe hole aperture was identified by AP analysis as Fe7Mog.

0 0

.1 1 10 100 1000 1 1 10 100 1000

Ageing time

(hours)

Ageing time

(hours)

a) b)

Fig. 3 Compositions of the two kinds of precipitates versus ageing time. (a) Fe7Mog type, (b) Ni3(Mo,Ti) type.

The error bars shown are the estimates of standard deviation calculated for each dbndition.

Field evaporation sequences revealed that the other kind of precipitate, which images less brightly than Fe7Mog, was rod-shaped rather than disc-like. A rough estimation of the rod length and diameter is shown in figure 4. The low growing speed also explains the fact that maraging steels usually display no particular sensitivity to overageing, double maraging or underageing treatments. The compositions of it vary more widely (figure 3b). The variation of the composition data from different precipitates in the samples aged for 2 hours was too great to allow a meaningful average composition to be quoted. It can be concluded, nevertheless, that this is a Ni3(Mo,Ti) type precipitate with some Ni substituted by Fe and a small amount by Co. The presence of Al in this precipitate is due to the chemical affinity between Ti and Al in the material. The fact that this is an intersolution of Ni3Mo and NijTi is another cause of the lack of consistency of the measured compositions. It was found that the Mo- and Ni-rich regions d o not quite overlap with the Ti-rich part of the precipitates in samples aged for shorter time, the former being broader than the latter. An explanation would be that Ti is relatively active and has a fast diffusion rate, while Mo incorporates into the precipitates later and Ni also diffuses slower. The precipitate Ti content tends to decrease with increasing ageing time up to 4 hours because of the insufficient supply of Ti from the matrix during the growing of the precipitates, as well as the incorporation of Mo. As ageing proceeds to longer time, Mo tends to incorporate into the more stable p-phase, which

results in a drop of Mo content in this precipitate after ageing for 8 hours and the precipitate becomes nearly pure Ni3Ti.

Some interfacial impurity (Si, C, and Cr) segregation between Ni3(Mo,Ti) and the martensite matrix, although not very significant, has been found.

Size parameter (nm)

A Spheroidal diameter of Fe7M% phase

Rod length of Ni3 (Mo,Ti) phase Rod diameter of Ni3 (Mo,Ti) phase

0

t . . .

. . . I

. . .

. . . I

. .

. . . I

. . . . . . -

^{1 I}

.1 1 10 100 1 000

Ageing time (hours)

Fig. 4 Apparent size of the two kinds of precipitates (spheroidal diameter of Fe7Mog and rod length and diameter of Ni3(Mo,Ti)) obtained from field-ion image observations.

Mo lons I101

I

Matrix

90

- -

¹

70

-

50

-

-

^{I n m}

20 40 60 80 100 120 140 160 Fe+Ni+Co lons

Fig. 5 Ladder diagram showing atomically sharp interface between Fe7Mog type precipitate a n d martensite matrix in C-300 maraging steel.

We also observed the coexistence of these two kinds of precipitates adjacent to each other (figures 6-7), the next step towards the decomposition of Ni3(Mo,Ti). We believe that Ni3(Mo,Ti) forms first because of its smaller lattice misfit with the martensite matrix and the more stable p-phase forms later. The adequate supply of Mo from the Ni3(Mo,Ti) and relatively high energy state near these interfaces would make the adjacent area to be the ideal place for p-phase to nucleate. Such a process would happen most strongly after 2 hours ageing, which makes the chemical analyses of Ni3(Mo,Ti) type precipitates in specimens aged for this time different from one to other.

As expected, the matrix near precipitates was found to have a lower content of precipitated alloying elements, while the matrix far away from them approaches the bulk composition of the material. There is some non-randomness in the atom probe analysis result for the element distribution in the matrix far away from precipitates, but further work is needed to investigate this in more detail.

Austenite reversion after prolonged ageing (24 hours) has been detected and analysed (table 2). The high Ni and Ti contents in the sample aged for 4 hours suggest that austenite forms more easily in Ni-enriched regions after decomposition of Ni3(Mo,Ti). It is interesting to relate this with the fact that some Ni3(Mo,Ti) decomposes after 2 hours ageing.

a) b)

Fig. 6 POSAP grey scale images showing two kinds of precipitates near to each other in C-300 maraging steel aged for 2 hours.

(a) Element distribution map of Mo, (b) Element distribution map of Ti.

1500 2500 3500 Number of lons

4500 500 1500 250b 3500 4500 Number of lons

Fig. 7 Composition rofiles of C-300 maraging steel aged for 4 hours across a'p

-

Nij(Mo,Ti)

-

^{y region.}

It is interesting to note tRe depletion of Co at the interfaces between two kinds of precipitates.

4

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DISCUSSION ON THE ESTABLISHMENT OF U-PHASE

One difference between the results from this experiment and from many of the previous works is that we found the composition of the stable strengthening phase to correspond to FqMo6 y-phase while many other researchers, most using electron microscope selected area diffraction (SAD) and some X-ray diffraction of extracted particles, have identified it as Fe2Mo Laves-phase. Apart from the composition analysis, there are other facts which can support our result.

a) Using the Thermo-Calc Alloy Data Basel3, the equilibrium state of C-300 steel in 510°C has been calculated.

Equilibrium phases relevant to C-300 in this database include ferrite, austenite, HCP phase, Fe2Mo Laves-phase, Ni3Mo y-phase, N i m o 0-phase, NiMo &phase, p-phase, R-phase, and a-phase. The calculated equilibrium is ferrite and austenite, together with p-phase. It is only when the p-phase has been suspended that the Fe2Mo Laves-phase will appear, and R and a-phase are the next two candidates.

b) The low degree of symmetry of the likely precipitate phases, namely Fe2Mo, FqMog, FeMo, Ni3Mo, and Ni3Ti, makes their diffraction patterns difficult to distinguish. The lattice distortion makes the situation even worse, as there is no chance to see all the diffraction lines or spots, with diffraction lines being usually broadened and spots diffuse. In the case of SAD, the situation is further complicated because of double diffraction and because the diffraction spots from precipitates are often overshadowed by the stronger ones from matrix.

TABLE 2 COMPOSITIONS OF REVERTED AUSTENITE

c) Although many other workers reported the existence of Fe Mo precipitates, there are some previous authors who described the formation o f high M o p- and a-phases22 12. 14, ^f59 16, though it is unlikely for the high temperature o- phase17 to exist at such a low temperature. Of course it may be that in steels having different compositions, and/ or under different heat treatments, Fe2Mo would be the precipitate formed'.

Ageing time (hours) 4

8 128

Equilibrium prediction*

5

-

CONCLUSIONS

Two kinds of precipitates exist in aged C-300 maraging steel, namely Ni3(Mo,Ti) with some Ni substituted by Fe and a small amount by C o and with some AI, mostly rod-like; and Fe7M06 p-phase with an average composition of (Fe,Ni,Co)53Mo43Si3, mostly spherical but some with less regular shapes, and having a sharp interface with matrix.

Both kinds of them form after ageing for 30 minutes, but Mo tends to delocalize from Ni3(Mo,Ti) and forms F q M % after longer ageing. Some Fe7M06 intermetallics can nucleate along martensite lath boundaries, forming relatively large plate-like precipitates. Thermodynamic calculations give a result that FqMog rather than Fe2Mo is the equilibrium phase. In addition, there appears to be some composition variation in the martensite matrix. Reverted austenite has been detected in sample aged for 4 hours and after ageing for 8 hours it approaches the predicted equilibrium austenite composition.

*

Calculated from Thermo Calc Data Base.

Fe 3823 54.0k0.8

54k2 56.5

Acknowledaements

The authors would like to thank Dr M G Hetherington for discussions on thermodynamic calculations and statistics, and Professor Sir Peter Hirsch, FRS, for the provision of laboratory facilities. Mr T J Godfrey and other members of Oxford FIM Group are acknowledged for invaluable technical assistance. The materials were supplied by D M Vanderwalker.

Ni 49+3 37.3k0.8

39k2 38.4

References

[ l ] S Floreen, Metallurgical Reviews, Review 126, 13, (1968), 115.

[2] R F Decker and S Floreen, in Maraging Steels: Recent Developments and Applications, R K Wilson, ed, TMS, 1988, 1.

[3] A Magnee, J M Drapier, J Dumont, D Coutsourdis, and L Habraken, Cobalt-Containing High Strength Steels, Centre D'Infonnation Du Cobalt, Brussels, 1974.

[4] J Zhu, H Li, L Zhang, Acta Metall Sin, 22, (1986), A304.

[S] D M Vanderwalker, Mct Tmns, 18A, (1987), 1191.

161 M Fukamachi, Y Kawabe, K Nadazawa, S Muneki, J Jpn Inst Met, 47, (1983), 237.

[7] G D W Smith, in Metals Handbook, 9th Edition, Vol 10, ASM, 1986, 583.

[8] M K Miller, International Materials Reviews, 32(5), (1987), 221.

[9] A Cerezo, T J Godfrey, and G D W Smith, Rev Sci Instrum, 5 9 , (1988), 862.

1101 D M Vanderwaker, in Maraging Steels: Recent Developments and Applications, R K Wilson, ed, TMS, 1988, 255.

[I I] J B Lecomte, C Servant, and G Cizeron, J Mater Sci, 20, (1985), 3339.

I121 W A Spitzig, J M Chilton, and C J Barton, Tmns ASM, 6 1, (1968), 635.

[13] Bo Sundman, Bo Jansson, and Jan-Olof Andersson, The Thermo-Calc Databank System, Calphad 9 , (1985), 150.

1141 B R Bane j e e and J J Hauser, in Transformation and Hardenability in Steels, Climax Molybdenum Co, Ann Arbor, 1968, 133.

1151 J R Mihalisin and C G Bieber, J Metals, 18, (1966), 1033.

1161 W R Bandi, J L Lutz, and L M Melnick, J Iron Steel Inst, 207, (1969), 348.

[17] T B Massalski, ed, Binary Phase Diagrams, ASM, 1986.

A1 1k 1 0.3k0.1

0 0.4 Co

6+2 5.1k0.4

4+ 1 3.6

Mo 3+ 1 2.9k0.3

3-+ 1 1.1

Ti 3 2 1 0.4k0.1

0 0.0