Hyperfine Interactions 94(1994)2219-2222 2219

Thermal stability of nanocrystalline iron-iron carbide composites. A MSssbauer study

Fabio Miani, Paolo Matteazzi, Diego Basset

Dipartimento di Scienze e Tecnologie Chimiche, Universitd di Udine, Via Cotonificio 108, 1-33100 Udine, Italy

and

G6rard Le Ca~r LSGMM, Ecole des Mines, Parc de Saurupt,

F-54042 Nancy Cedex, France

Mechanically synthesized nanocrystalline iron-iron carbide nanocomposites were subjected to isothermal cycles of one hour duration. As-treated and annealed powders were characterized by means of MSssbauer spectroscopy and X-ray diffraction. The grain growth was studied in a temperature range in which E and X iron carbides, detected in the treated amples, have a low thermal stability. Cementite results are substantially stable up to 1100 K and, according to a simplified model of crystal growth, show a low activation energy for crystal growth.

1. Introduction

Mechanical synthesis (mechanosynthesis [ 1 ]) by means of high-energy milling of elemental powders is a promising technique for the production of nanocrystalline materials in significant quantities. In the F e - C system, mechanosynthesis has proven to be effective in producing nanocrystalline iron carbides with an average crystal size in the 10 nm range. In order to achieve nano- or microcrystalline bulks by sintering, it is useful to study the thermal stability of these nanophase materials. Here, we present a study of the thermal stability of nanocrystalline iron-iron carbide composites obtained by mechanical synthesis of elemental powders, which may be valuable for structural mechanical applications [2]. Thorough information on the grain growth kinetics may be extracted from combined X-ray diffraction (XRD) or transmission electron microscopy and differential scanning calorimetry techniques [3,4]. However, here we follow a simpler approach, similar to some recent work [5,6], exploiting results directly from X-ray line broadening.

�9 J.C. Baltzer AG, Science Publishers

2220 F. Miani et al. / Thermal stability of iron-iron carbide

2. Experimental

A mixture of elemental powders of Fe and C (purity >99%) was milled in a high-energy, high-capacity experimental mill [7]. In this system, this experimental mill, typically loaded with 100 g of powder and a 20:1 ball-to-powder ratio (BPR), behaves like the commercial laboratory mill SPEX 8000 in the 3 g powder and a 10:1 BPR configuration [7]. The total mass of the powder loaded into the mill (in an argon atmosphere) was 120 g, i.e. 113.22 g of Fe and 6.78 g of C (wt.% C 5.65); the milling charge was composed of 1989 g of 28.6 mm steel balls, thus providing a BPR of 1: 16.5. At room temperature, in the well-known Fe-Fe3C phase diagram, this content of C corresponds to 85% in weight of cememtite.

The powders were subjected to milling times of 2.5, 5, 7.5 and 10 h. The powders milled for 10 h were annealed by means of thermal cycles of 20 K/min heating rate, and 1 h under isothermal conditions at different temperatures ranging from 673 to 1098 K in an argon atmosphere by means of a Netzsch differential thermal analysis.

3. Results and discussion

The samples were analyzed by means of XRD and M6ssbauer spectroscopy (MS. In the first case, a CoK~ radiation was employed (~. = 0.178897 nm) using a position-sensitive detector and a germanium monochromator. M6ssbauer data (fig. I(A), as-milled powders) were obtained in transmission geometry at room temperature; we used a 57Co source having an intensity of about 10 mCi. A suitable hyperfine field distribution (HFD) including the phases iron, H~igg carbide (FesC2), (approximately Fe2C) carbide, cementite (Fe3C), and a high carbon content F e - C alloy has already been utilized [8] to bring an X-ray consistent phase analysis of the kinetics of phase formation in the as-milled powders; the HFD is depicted in fig. 1 (B), together with the assignment of hyperfine fields. Due to the annealing of defects already at low temperatures [4], in the heat-treated samples an HFD is no longer necessary, and an analysis may be performed with a data fitting of two single components. In fact, already at 400 ~ the identified phases are iron and Fe3C iron carbide (H = 208.9 kG; tSFe = 0.186 mm/s), confirming preliminary annealing experiments [8,9]. Cementite is quite stable in the studied temperature range (fig. 2(A)); however, a partial transformation occurs already below the euctectoid temperature of the Fe-Fe3C phase diagram ( ~- 1000 K), as shown by the increase of iron.

The results obtained by MS were compared to the information provided by XRD. The peaks of the annealed samples were attributed to iron (JCPDS 6-696) and cementite (JCPDS 35-772). As for the line broadening analysis, we could detect the effect of the strain in X-ray only in the spectra of the as-milled powders. In the annealed samples, the Debye-Scherrer formula permits us to follow the crystal size

F. Miani et al. / Thermal stability of iron-iron carbide 2221

-10 -5

~t

o 5 l o velocity (mm/s)

100

8o

6o

A

700 900 1100 Temperature K

E"

Fe5C2 t r Fe2C m l Fe3C

Fe B /--'-'1 Fe-C alloy

1 00 200 3 ~

Hyperfine field H [kG] 400

Fig. 1. (A) MOssbauer spectrum of as-milled powders, 10 h milling time; (B) hyperfine

field distribution.

8O

101 700 9(X) 1100 Temperature K

Fig. 2. (A) RA% of annealed powders (1 h) for iron and Fe3C; (B) crystallite sizes of

iron and cementite.

evolution of the two phases, iron and cementite (fig. 2(B)). We consider a simplified model of crystal growth, with the temperature dependence accounted for only in the Arrhenius term:

d = K exp(-E/RT)t" ,

where d is the crystallite size, E is the activation energy for crystal growth, n is the grain growth exponent, assuming typical values from 1/3 to 1/2, T is the temperature and t is time.

We can then estimate the activation energy by plotting In(d) versus 1/T (fig.3): the slope (independent of n -isochronal annealing-) corresponds to -E /R , where R is the gas constant; this yields E = 20.5 + 3.5 kJ/mol, a low value in comparison to recent reports [3, 5] for metal-metal alloys or even for intermetallic compounds [4]. A limited crystal growth is detected also for the iron matrix, whose crystal size is comparable to that of FeaC. This could be attributed to the well-known effect of dispersed particles on boundary migration [11].

2222 F. Miani et al. / Thermal stability of iron-iron carbide

4 .2

3 .8

3 .6 r .N tO 3.4

, , 3 .2

~ 3

2.8

2.6 0.9 i 111 112 113 114 1.5

1000/T [ K]

Fig. 3. In ( ~ - l I T plot for the estimarion of the activa- tion energy for crystal growth of the Fe3C phase.

4. Conclusions

Among mechanically synthesized iron carbides prepared at initial atomic composition Fe78.2C21.8 cementite is the most stable thermally. MS permits the identification of the presence of a unique carbide already at low temperature (400 ~ confirming the kinetics of the transformation of different carbides (e----> Z---> 0) during the aging of high-carbon martensite [12,13]. The crystal size growth is limited, not exceeding 50 nm in these treatments for the cementite phase.

References

[1] G. Le Ca~r, E. Bauer-Grosse, A. Pianelli, E. Bouzy and P. Matteazzi, J. Mater. Sci. 25(1990)4726. [2] WJ. Kim, J. Wolfenstine, O.A. Ruano, G. Frommeyer and O.D, Sherby, Met, Trans. A23(1992)527. [3] L.C. Chen and F. Spaepen, Nanostructured Mater. 1(1992)59. [4] M.D. Baro, J. Malagelada, S. Surinach, N. Clavaguera and MT. Clavaguera Mora, in: Ordering and

Disordering in Alloys, ed. A.R. Yavari (Elsevier, London, 1992) p. 55. [5] Z. Gao and B. Fultz, Nanstructured Mater., in press. [6] T. Spassov and U. Koster, J. Mater. Sci. 28(1993)2789. [7] D. Basset, P. Matteazzi and F. Miani, Mater. Sci. Eng., submitted. [8] P. Matteazzi, F. Miani and G. Le CaSr, Hyp. Int. 68(1991)173. [9] P. Matteazzi and G. Le CaSr, J. Am. Ceramic Soc. 74(1991)1382. [10] G. Le Ca~r and P. Matteazzi, Hyp. Int. 66(1991)309. [11] R.W. Cahn, in: Physical Metallurgy, eds. R.W. Cahn and P. Haasen (Elsevier, 1983), Ch. 25,

p. 1654. [12] J.M. Genin, Met. Trans. A18(1987)1371. [13] R. Kaplow, M. Ron and N. DeCristofaro, Met. Trans. A14(1983)113.