Supersaturated solid solutions and metastable phases formation through different stages of mechanical alloying of FeTi

NmStruchued Mataials. Vol. 10. No. 3. pp. 365-374.1998 Ekevia Science Ltd 0 1998 Acta Mdallurgica Inc. Printedin the USA. All rights reserved 0965-9773/98 $19.00 + .CO

Pergamon

PII s0965-9773(98)00077-4

SUPERSATURATED SOLID SOLUTIONS AND METASTABLE PHASES FORMATION THROUGH DIFFERENT STAGES OF MECHANICAL ALLOYING OF FeTi A.A. Novakova*, O.V. Agladze*, S.V. Sveshnikov*, and B.P. Tarasov** *Moscow State University, Department of Physics, 117234, Moscow, Russia **Institute for New Chemical Problems RAS, 142439, Chemogolovka, Russia (Accepted February 5.1998) Abstract- Elemental equiatomic Fe-72 powder mixture was mechanically alloyed in high energy ball mill, XRD, DTA and Mossbauer spectroscopy (at liquid nitrogen temperature) were utilized to monitor the kinetics as well as the accompanied structural andphase transformations through different stages of milling. Our experiments showed that formation of nanocrystalline FeTi compound proceeds via the formation of the supersaturated solid solutions pTi(Fe) and cll-Fe(Ti) at the intelface. After 36 hours of milling, the main part of powder mixture transformed not only to FeTi but also to FezTi inter-metallic compound. The transition of last part of supersaturated solidsolutions pTi(Fe) to those intermetallicphases was observedafter annealing of this sample at 600°C. 01998 Acta Metallurgica Inc.

INTRODUCTION The intermetallic alloy FeTi is one of the best known and especially suitable hydrogen storage material. However,crystallineFeTi suffers severecracking on hydrogen absorption, which has proved to be a serious impediment to its widespread use. For these reasons some interest has been devoted in nanocrystalline alloys. During last decade mechanical alloying of FeTi system has been studied by several groups (l-3), but no single phase amorphous or nanocrystalline material was produced. Furthermore,the main attention in those works was paid to the bee p-Ti(Fe) solid solution formation. The authors suggested that FeTi-phase was subsequently produced via the formation of fl-Ti(Fe) at the interface. The existence of another intermetallic hexagonal FezTi phase and high-temperature a-Fe(Ti) solid solution were ignored. The equilibrium phase diagram (4) is shown in Figure 1. In fact, for Fe-Ti system the maximum equilibrium solid solutions are:2 1%Fe in p-Ti at 1085°C and 10%Ti in a-Fe at 1290°C. Both of them decrease at room temperature. Intermetallicbee FeTi and Fe2Ti with Cl4 hexagonal structuream stable phases at room temperature.In our opinion, the above mentioned factors could cause a non-single phase material formation during the ball milling of Fe-Ti system. It must be also noted that Fe2Ti alloy is not a hydrogen storage material. 365

AA NOVAKOVA,OV AGUDZE, SV SVESHNIKOV AND BP TARASOV

I

---I-----

.--

.-_---__ 1 i 1

00

1UU

Figure 1. The equilibrium phase diagram for Fe-Ti system (4).

The aim of this work is to study the structural transformations through different stages of equiatomic powder mixture Fe andTi milling and the thermal stability of the mechanically alloyed (MA) samples, especially intermetallic hydrogen storage Fe’Ii phase.

EXPERIMENTAL Equiatomic powder mixtures of Ti(99.99%) and Fe(99.99%) were ball milled to give nominal atomic composition FeTi in a planetary ball mill. High energy ball milling was performed with a stainless steel vial and balls in Ar atmosphere with 2O:l ball-to-powder weight ratio. The powder samplings were carried out in a glove box under an argon atmosphere at the intervals of 2,22 and 36 hours, without intermediate openings to extract samples. X-ray diffraction (XRD), differential thermoanalysis (DTA) and Mossbauer spectroscopy (MS) were used for studying structural and phases transformations during different stages of milling and through thermal treatment of samples after ball milling. XRD was performed at diffractometer ADP-1 with Fe& radiation, equipped with single crystal Si (111) monochromator. The phase identification was carried out by using ASTM data. DTA was made by using derivatograph with a heating rate of 20”C/min up to 750°C in argon atmosphere. MS was performed at liquid nitrogen temperatures (80K) using constant acceleration spectrometer with Cos7@h) source. The spectra were fitted by means of the UNIVEM program. All the isomer shift values are given relative to metallic Fe.

MECHANCALALLOYINGOF FeTi

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RESULTS AND DISCUSSION

Mossbauer spectroscopy is sensitive to the chemical and structural environment of iron atomson a nearest-neighbor length scale and therefore allows a-Fe(Ti) and p-Ti(Fe) to be detected at very low concentrations. Such parameter as hyperfine magnetic field value(H), is very sensitive to replacement Fe by Ti. The H is equal to 337 kOe for pure a-Fe at the liquid nitrogen temperature and decreases by 19 kOe per unit of replacement 1 atoms Fe by Ti together with decrease of isomer shift to -0.01 mm/s (5). Furthermore, the iron-containing phases in the Fe-Ti system such as FeTi and FezTi, are readily distinguished in the Mossbauer spectra because they also have their own parameters, such as quadrupole splitting and isomer shift (6- 10). The Fe2Ti phase has hexagonal structure Cl4 (Laves phase MgZn;! type). The Ti atoms occupy a single crystallographic site, whereas there are two non-equivalent site types for Fe atoms in a cell: 2 atoms on sites (O,O,O). (0,0,1/2)- (Fel) and 6 atoms occupied sites +(x.x,1/4), +(2x,x,1/4), +(x,2x,1/4), x=1/6 (Fez). The magnetic moments of (Fez) atoms aligned ferromagnetically in layers with 2=1/4, while the interactions between layers were antiferromagnetic. (Fet) atoms placed in the center of symmetry between two antiferromagnetic planes and the resulted magnetic fieldon it is zero (11). Therefore, the Fe2Ti phase has a complicated MS at the temperatures below 275 K (Neel temperature for this phase): a six-line subspectrum with H value 92 kOe from (Fez) atoms and a doublet from (Fet) atoms. Mossbauer spectroscopy also gives the opportunity to estimate the relative quantity of phases presented in the sample by measurements of its subspectra intensities, and the degree of structure disordering by measurement of line width. Figure 2 shows Mossbauer spectra (MS) obtained at liquid nitrogen temperature from a mixture of equiatomic elemental powders Fe and Ti after various milling times. Two main components are clearly visible. The first component, due to a-Fe, is a magnetically ordered six-line pattern (H=337 kOe), the one present at t=O.After 2 hours of milling (MS-2h) the several six-line patterns with different hyperfine magnetic field values, corresponding to solid solutions a-Fe(Ti) with various concentrations of dissolved Ti ate visible. The second nonmagnetic pattern appears gradually and consists of several components, corresponding to disordered p-Ti(Fe), FeTi and Fe;?Ti phases. The Mossbauer hyperfine parameters are different from those of the amorphous and crystalline FeTi and FezTi. This controversy is attributed to the disordered interfacial phases formed at the gram boundaries. The spectrum from the sample after 22 hours of ball milling (MS-22h) is typical for random nonequilibrium mixture state. The sextets with various values of hyperfme magnetic field appear in spectrum after 22 hours of ball milling. The line widths of all sextets increased from 0.35 up to 0.6 mm/s by comparison with ones in (MS-2h). The relative intensity of pure a-Fe subspectrum decreased 25%, together with extension of summary relative intensity of solid solutions subspectra. The intensity of FeTi phase subspectrum increased, and its line width extended from 0 6 (MS-2h) up to 1.2 mm/s (MS-22h). After 36 hours of milling the central component dominates the spectrum.Their lines widths decreased from 1.2 (MS-22h) down to0.6mm/s(MS-36h).The widthsofFeTiandFe2Tisubspectradecreaseddown too.55 mm/s .The results of MS fittings are presented at the Table 1. XRD for all samples was performed in the 20” - 130” range of 28 scale with step of measurements 0.1”. The lines with visible intensity were obtained only in 45” - 65” range. The XRD patterns for samples after 2, 22 and 36 hours of ball milling are shown in Figure 3. The intensive and wide reflections were observed at the XRD pattern only after 2 hours of milling. Through prolonged time of milling, the intensities of XRD linesdecreased, together with extension

AA NOVAKOVA,OV AGLADZE.SV SVESHNIKOV AND BP TARASOV

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Figure 2. MS of the samples in initial state (a), after 2 (b), 22 (c) and 36 (d) hours of BM.

of lines widths in the XRD pattern from the sample after 22 hours of milling. The profile of line was modified together with intensity increase in the (XRD-36h). The XRD patterns analysis was performed using the superposition reflections of @cc) structures - a-Fe, &Ti and FeTi, and seven reflections of hexagonal structure (FezTi), which have their own relative intensities at the 45”-65” range of 28 scale. Each reflection was described by a Gaussian function. Since we have observed solid solutions a-Fe(X) formation in the (MS-2h), we included such reflections of solid solutions a-Fe(Ti) in our model to describe XRD pattern. On the other hand, we included in our model the reflections for j3-Ti(Fe) solid solutions with various concentration using the results (3). All reflections of a-Fe(Ti) and &Ti(Fe) solid solutions were incorporated into the model as displaced matrix ~1 lO> reflections. Their displacements have been determined by dependence of reflection position and concentration of dissolved elements (3). A

MECHNKAL ALLOYING OF

46

48

50

52

54

56

Fen

58

369

60

62

64

[deg.] Figure 3. XFD patterns for the samples after 2 (a), 22(b) and 36 (c) hours of BM. (0) - FeTi, ( v ) - FQTi, (V) - FezTiOs, (0) - p-Ti, (0) - a-Fe, (ti) the solid solutions regions. 20

small amount of FezTiOs oxide presented in XRD pattern probably is due to oxidation during the sampling for X-ray analysis. The change of reflections width with increase of scattering angle was taken into account by including of (l/Cos Cl)multiplier. The models for other two samples, after 22 and 36 hours of ball milling were carried out in the same way. The initial positions and relative intensities for Gaussian lines which were included in the XRD models for the samples after 2,22 and 36 hours of ball milling are shown in Table 2.

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AA NOVAKOVA,OV AGLADZE,SV SVESHNIKOV AND BP TARASOV

TABLE 1 Fitting Results of the Mossbauer Spectra of Equiatomic Powder Mixture Fe-Ti after Different Milling Time and after Thermal Treatments

The fitting of our experimental data lets us reconstruct the process of structural transitions between phases which take place during different stages of mechanical alloying. It is known that a structure consisting of alternating layers is usually formed at the surface of milling powder after a short attrition time (12). The main transformations occurred already after 2 hours of MA: as the Mossbauer data show, 73% of bee a-Fe reacted with Ti as a result of grain refinement, deformation and diffusion processes. Furthermore, the XRD pattern analysis showed that practically all initial a-Ti transformed to p-Ti @cc). Thus, the favorable conditions for formation of supersaturated bee a-Fe(Ti) and B-Ti(Fe) solid solutions and FeTi were created. Practically the mixture is random (quasi-amorphous) after 22 hours of MAand consists of disordered FeTi, FezTi and supersaturated solid solutions a-Fe(Ti) and l3-Ti(Fe) with very high concentrations of dissolved elements. The quantities of Fetid and Fe2Ti in the mixture increase with duration of MAat the expense of decrease of concentration of solid solutions. 83% of the mixture consists of Fe’li and Fe2Ti phases, with the proportion of their subspectra intensities 5:3. It is possible to assign the broad line in MS with inhomogeneities. The small quantity of u-Fe (-6%) presented in the sample after 36 hours of ball milling can be explained by contamination originating from the milling vial and balls. Hence, the mixture after 36 hours of ball milling consists of Fell and Fe2Ti phases in disordered state and small quantity (about 11%) of a-Fe(Ti) and 8-Ti(Fe).

MECHANICAL ALLOYINGOFFetid

550

600

700

650

371

750

800

Tempcralure [“Cl Figure 4. The DTA curve for Fe-Ti mixture after 36 hours of BM. I, I

-10

-5

II m

I

OV [mm/s]

I

5

Figure 5. MS of the sample after 36 hours of BM and heat treatment at 600°C.

10

372

AA NOVAKOVA,OV

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47

48

1

I

49

50

AGLADZE, SV

I

I

I

51

SVESHNIKOVAND BP

52

53 54 20 [deg.]

TAFIASOV

I

I

I

I

55

56

57

58

Figure 6. XRD pattern from the sample after 36 hours of BM and heat treatment at 600°C. (0) - FeTi, ( V) - FezTi, (0) - p-Ti, (* ) - Tie.

1, # -10

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11

11

11

11

5

V [mm/s] Figure 7. MS of the sample after 36 hours of BM and heat treatment at 1000°C.

’

10

MECHANICAL ALLOYINGOF Fe1

373

TABLE 2 The Index, Relative Intensities, Interplanes Dimensions and Reflection Positions at the 28 Scale for Phases, Formatted in Fe-Ti System Phase



I/I1

d (A)

28 (0)

a-Fe



100

2.85

57.40

a-Ti

1 I I

P-Ti

I I

<002>

I 1

100

1

2.34

1

48.98

40 100

1

2.24

1

51.30

1

2.33

1

49.20

FeTi



100

2.11

54.80

FezTi



50

2.39

47.70

<013> <020>

75 10

2.20 2.07

52.20 55.89

<112>

100

2.04

56.04

<021>

100

2.00

58.02

I

1

TIO2 -

The thermal stability of this sample has been investigated by DTA. Two exotbermic peaks--the small one in the 580 - 630°C temperature range and the large one in the 680 - 740°C range are presented in the DTA curve shown in Figure 4. In order to verify the phase transformation characteristic of the first peak, a portion of the sample was heat treated at the 600°C in the calorimeter for 30 minutes. The Miissbauer spectrum and the XRD pattern from this sample are shown in the Figure 5 (MS-36h+6OO”C) and Figure 6. (XRD-36h+6OO”C). The analysis of XRD pattern showed intensive FeTi reflection, and the sets of reflections for Fe;?Ti phase and Tio2 phase. The positions and intensities of Gaussian lines, which were used for approximation of XRD picture are shown in Table 2. The half-width of FeTi phase reflection is equal 0.26” (of 28 scale), while ~110~ reflection for Fe in crystalline state is 0.2”.

374

AA NOVAKOVA,OV AGUDZE, SV SVESHNIKOV ANDBP TAFUSOV

The (MS-36h+6OO”C) consist of a doublet which we described as a superposition of FeTi and FezTi subspectra with proportion of relative areas 54 . The results of MS fitting are presented in Table 1. The line widths of subspectra for FeTi and Fe2Ti are smaller than the same in the (MS-36h). Hence, the broad transformation observed between 580°C and 610°C by calorimetry showed that no new crystalline phase is formed. We observed the growth of more ordered FeTi and Fe2Ti at the expense of u-Fe and B_Ti(Fe). It is reasonable to assume that the first broad peak in the DTA curve is due to the reaction of the unalloyed solid solutions p-Ti(Fe) and contaminated a-Fe with FeTi and FezTi phases. For investigation of the second phase transformation, a portion of the sample after 36 hours of ball milling was annealed at 1000°C during 30 minutes. Only subspectra from Fe2Ti phase is presented in the (MS-36h+lOOO”C) spectrum which is shown in Figure 7. Fitting results are summarized in Table 1. They are in accordance with results of work (6) where has been shown that FeTi phase in polycrystalline samples transformed to FezTi and a-Ti at 700°C. Therefore, based on these data, we can attribute the sharp exothermic peak in 670-740°C range to the transformation FeTi + FezTi+cz-Ti . CONCLUSIONS The study of structural transformations through Fe-Ti equiatomic mixture ball milling and through thermal treatment after milling can be summarized in the following: (a) The simultaneous formation of disordered Fe-Ti, Fe2Ti and bee solid solutions a-Fe(Ti) and /$Ti(Fe) with various concentrations of dissolved elements on grain boundaries was observed at the initial stages of MA process in the system. (b) After 36 hours of ball milling 83% of mixture consists of two highly disordered phases: FeTi and Fe2Ti. The 11% of mixture consists of solid solutions p-Ti(Fe) and a-Fe(Ti) with high concentrations of dissolved elements. There is also 6% of contaminated a-Fe. (c) The annealing of this sample at 600°C leads to the formation of more ordered Feli and Fe2Ti phases and the solid solutions disappear. REFERENCES 1.

Chu, B.L., Lee, SM., Perng, TX, International Journal of Hydrogen Energy 1991,16,413.

2. 3.

Bckert, J., Schultz, L., Urban, K., Journal ofNon-Crystalline Solids, 1990, 16,413. Zaluski, L., Tassier, P., Ryan, D.N., Doner, C.B., Zaluska,A., Strom-Olsen,J.O., Trudeau,M.L., Schultz, R., Journal of Materials Research, 1993,8,3059. Kabachezski, O., Iron-Binary Diagrams, Springer-Verlag,Berlin, 1982. Vie, J., Campbell, J.A., Journal of Physics F: Metallurgical Physics, 1973, 3,645. Liou, S.H., Chien, C.L., Journal ofApplied Physics, 1984,55(6), 1820. StuPe.1,M.M., Ron, M., Weiss, B.Z., Journal of Applied Physics, 1976,47,6. Harada,H.,Ishibe,S.,Konishi,R.,Sasakura,H.,JapaneseJournalofAppliedPhysics, 1985,24(9), 1141. Wertheim, G.K., Wemick, J.H., et al., Journal of Applied Physics, 1970,41,1325. Gonser, U., Galvb da Silva, E., Preston, R.S., Journal ofApplied Physics, 1985,57(4). Brown, P.J., Deportes,J., Ouladdiaf,B., Journal of Phy sits of Condensed Matter, 1992,4,10015. Koch, C.C., Annual Review of Materials Science, 1989.19.121.

4. 5. 6. 7. 8. 9. 10. 11. 12.