Hyperfine Interactions 27 (1986) 305-308 305

CRYSTALLISATION STUDIES IN Fe37.sNi37.sCrsMo2SiloB s

GIRISH CHANDRA a, G. RAJARAM a, SHIVA PRASAD b, V. SRINIVAS b,

S.N. SHRINGI b and R. KRISHNAN c

a. Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India. b. Physics Department, Indian Institute of Technology, Powai, Bombay 400076, India. c. Laboratoire de Magnetisme, CNRS, Bellevue, France.

The distribution of hyperfine fields (HF) is obtained at

room temperature for Fe37 5Ni37 5Cr5Mo2SiloB8 annealed at temperatures between 641 K and 74~ K. P(H,T) is also derived for samples annealed at 744 K for over 70 hours. Three steps are noted in the evolutien of the crystalline phases: (I) separation of ~ 35% of Fe atoms into environ- ments with low and high HF's, with increase in T c for the remainder (2) nearly complete separation of the P(H) into

these two components on further annealing at a higher tempe- rature and (3) at yet higher annealing temperatures, conver- sion of the high field phase into one with a lower HF. The observations are tentatively explained in terms of preferential presence of Cr in one of the phases and its subsequent migration

into the other phase.

i. INTRODUCTION

Whether or not distribution of Cr is non-random in Cr contain- ing amorphous alloys has been a matter of controversy [1,2]. Aging and crystallisation studies in a-Fe32Ni36CrI4PI2B 6 suggest tendency towards Cr-rich environment [3,'4]. As part of our studies on the series Fe40 x/2Ni40_x/2CrxMo2Sil0B8, ~ = ? to 5, we have obtained the distribution of hyperfine fields .HF). P(H), from the room temperature Mossbauer spectra of samples with x = 5 annealed at temperatures between 641 K and 744 K in an attempt to follow the evolution of the crystalline phases.

2. EXPERIMENTAL DETAILS AND ANALYSIS

The samples were annealed in a home-made vacuum furnace (vacuum: 10-3 to 10 -4 torr) in which the spectra could be taken in situ. The transmission geometry wasused with a source which gave a line width of 0.25 mm/s for a 25~m thick natural iron absorber. The hyperfine field distributions were obtained using Window's method [5] with 15 terms of a cosine series. In analysing spectra of samples closer to the amorphous phase (temperature of anneal, T A ~ 641 K), a subspectrum line width of %0.42 mm/s was used to account for broadening due to quadrupole interactions. For other spectra, a subspectrum line width of ~ 0.30 mm/s was used.

�9 J.C. Baltzer A.G., Scientific Publishing Company

306 Gir&h Chandra, et al., Crystallisation studies

-6 -4 -2 0 2 4 6 VELOCITY (ram/s)

Crystalllsed Fe40Ni40M~ Sil0B 8

.02

4 , . 0

. 0 2

01:o /\ 4 =.04

375 K

.02

0

.041 3 K o ~ ~ ' - ~ - - .

.08 K

0 I00 2O0 3O0 4O0 HYPERFINE FIELD (KG)

Crystallised Fe37.5Ni37,5Cr 5Mo2Sil0B 8

.04 383 K

0

. 0 4

8 0 -~ . ~ ~ O ~ " "

,O4

8

0 0 . .

~ k 0 ~

1 0

0

-6 -4 -2 0 2 4 6 i00 200 300 400

VELOCITY (ram/s) HYPERFINE FIELD (KG)

Figs.l and 2. P(H,T) for alloys Fe40.X/2Ni40.x/2CrxMo2SiloB 8 , _

X = 0 ~nd 5, annealed at 744 K ~or 70 hours.

The scheme of the annealing runs was as follows: (i) successive annealing runs at 641 ~ 3 K for durations of 3, 1.5, 1.5 and 6 hours each, with cooldown to room temperature for recording the Mossbauer spectrum after each run (ii) one-time anneals between 641 K and 744 K and at temperatures and durations indicated in Fig.4 (iii) successive annealing runs at 744 • 2 K for 6, 6, 6, 6, 6, 12 and 12 hours each. The total annealing time at the highest anneal- ing temperature, T A, during a run is indicated in the figures, along with T A.

3. RESULTS AND DISCUSSION

Prior to the annealing runs, samples with 0 and 5 at.% Cr were

Gir&h Chandra, et al., Cristall&ation studies 307

~nealed at 744 K for ~70 hours. X~ray diffraction shows that these samples are no longer amorphous, but the peaks are rather broad. P(H,T) for the two alloys are shown in Figs.l and 2. Different ordering temperatures for the different peaks suggest that they correspond to Fe in different phases. The difference in the P(H) at the lowest temperature for the two samples is probably the effect of Cr since the X-ray peak positions are quite similar.

Fig.3 shows the P(H) at R.T. for the 5 at.% Cr sample after annealing at 641 K for upto 12 hours. T c shifts from ~378 K for the as-received alloy to ~410 K after an anneal at 641 K. About 35% of the Fe atoms has separated into environments with high (%340 KG) and low (~40 KG) HFs respectively. For T A ~660 K, separation of the rest of the Fe into these two components is nearly complete (Fig.4).

The first anneal at 744 K brings about two changes (i) The high and low HF components (A and B respectively in Fig.5) decrease by roughly equal amounts (2) A component with HF % 280 KG (C) grows. Further annealing at this temperature brings about almost complete conversion of the component A into component C. The position (HF i00 KG) and area of the peak D is virtually unchanged within error.

We offer the following tentative explanation for the observa- tions. (a) the high value of HF of peak A suggests that this com- ponent may be Fe-Ni with possibly some Si but with no Cr. Nearly all Cr is associated with Fe atoms in peak B (It has been shown that f.c.c, crystals depleted in P and Cr form in Fe32Ni36Cr14- PI2B6 [6]). (b) step 1 suggests that equal amounts of Fe from A and B undergo conversion to a new environment initially. (c) step 2 suggests that this process is accompanied by diffusion of Cr into the resulting phase.

ACKNOWLEDGEMENTS

We would like to thank Dr. Boil, Dr. Kunz and Prof. Warlimont of Vacuumschmelze, Hanau for their interest in this work.

0-

4"

0.

4.

O:

4,

0

0

Annealed Fe37.5Ni37.5Cr 5Mo2SiloB 8

~ 4 M ~ ~ ~

0

.02

0

. 0 2

[ o

0

-6-4-20246 ~ITY(~/S)

i00 200 300 400 HYPERPINE FIELD (KG)

Fig.3. Room temperature Mossbauer spectra and P(H) for the x = 5 alloy annealed at 641 K for various anneal times.

308 Girish Ghandra, et al., Crystallisation studies

Annealed Fe37.5Ni37.5Cr5Mo2Sil0B8

0 0 - -

i! ~ 0

-6 -4 -2 0 2 4

VELOCITY (~m/s)

~'A~,

660 K 32 hrs

6 hrs

695 g ~ 6 hrs

731 K

744' K 6 hrs

0 101) 200 300 400

HYPERFINE FIELD (kG)

Fig.4. Mossbauer spectra and P(H) at R.T. for the x = 5 alloy annealed at temperatures

between 641 K and 744 K.

ANNEALED Fe37.5Ni37.5CrLMo2Sil0B8

�9 04 T A = 744 K

.0

6 ' " " .02

0

0 V J . ~ 42 hr

o

6 0 _

-6 -4 -2 0 2 4 6 0 I00 200 300 400 VELOCITY (mm/s) HYPERFINE FIELD (KG}

Fig. 5. Mossbauer spectra and P (H) at R.T. for the x = 5 alloy annealed at 744 K for various

anneal times.

References

[i] G.L. Whittle, S.J. Campbell and A~ Stewart, Phys. 8tat. Sol. 71a (1982) 24 [2] G. Rajaram, Shiva Prasad, Girish Chandra, S.N. Shringi and R. Krishnan,

Phys. Lett., 98A (1983) 57. [3] A.S. Schaafsma, Ph.D. Thesis, University of Groningen (1981).

[4] C.L. Chien, Phys. Rev. B 19 (1979) 81. [5] B. Window, J. Phys. E 4 (1971) 401. [6] M. Yon Heimendahl and H. Oppolzer, Scripta Met. 12 (1978) 1087.