NATURE OF BORON IN a-IRON*

Y. HAYASHIT: and T. SUGENOt

Internal friction measurements were carried out on iron-boron and iron-nickel-boron alloy specimens, in order to define the nature of the boron solid solution in a-iron. An internal friction peak corresponding to a small amount of boron in a-iron was observed. Referring to the lattice parameter measurements, and the diffusion experiment, it can be concluded that boron atoms occupy both substitutional and interstitial positions. The concept of dissociative diffusion explains well the diffusion mechanism of boron i;;;ir;r. The relaxation time of the jump of an interstit,ial boron atom is shown to be 1O-13 exp (13000/

6 . SATURE DG BORE DANS LE FER-ALPHA

Des mesures de frottement interne ont 6tB me&es iL bien sur des Bchantillons d’alliage fer-bore et d’alliage fer-nickel-bore dans le but de dbfinir la nature de la solution solide de bore dans le fer-a. En se &f&rant aus mesures des param&tres du rQseau et) aux expbriences de diffusion, les auteurx concluent que les atomes de bore occupent B la fois des posit,ions interstitielles et de subst.itution. Le concept de la diffusion dissoci& explique bien le mecanisme de diffusion du bore dans le fer-a. Les auteurs montrent que le temps de relaxation correspondant au saut d’un atome interstitiel de bore est de lo-l3 exp (130001 RT) sec.

OBER DIE SATUR DES BORS IS a-EISES

Zur Bestimmung der Satur der festen LGsung van Bor in a-Eisen wurden RIessungen der inneren Reibung an Eisen-Bor- und Eisen-Nickel-Bor-Legierungen durchgefiihrt. Ein Maximum der inneren Reibung, das einer kleinen Menge van Bor in a-Eisen entsprach, wurde beobachtet. Aus Messungen der Gitterperameter und aus Diffusionsexperimenten kann geschlossen werden, dal3 die Boratome sowohl Gitter- als such Zwischengitterpliitze einnehmen. Die Vorstellung der dissoziativen Diffusion erkliirt gut. den Diffusionsmechanismus van Bor in a-Eisen. Zwischengitteratomo ist lo-l3 exp (13000/RT) sec.

Die Relaxationszeit des Sprungs eines Bor-

INTRODUCTION

Any interpretation of the mechanical or physical

properties of alloys is strictly dependent on a funda-

mental knowledge of those alloy systems, such as the

type of solution, solubility, diffusibility and so on.

The equilibrium diagram of the iron-boron system

was studied by various authors(1-3) but the results

obtained show a considerable disagreement in the low

concentration region of boron. It can be said that the

maximum solubility of boron in u-iron is very restrict-

ed.

On arranging alloying elements in iron by their

neut’ral atomic diameter@ it is found that boron lies

midway between the substitutional elements and t,he

interstitial elements. From this fact it can be recog-

nised that boron may take either a subst,itutional

position or an interstitial position in a-iron. The

small amount of solubility makes it difficult to have a

satisfactory understanding of many of the propert)ies

of the iron-boron alloy.

The concept that a boron atom takes a substitutional

posit’ion in a-iron is supported by the lattice param-

eter measurements(5-i) and also by calculations

based on energetic considerations.(l) A similar result’

was concluded by Busby and Wells(s) from the result

of diffusion experiments, i.e. they obtained 62 kcal/mol

as the activation energy of the diffusion of boron in

* Received October 17, 1969. -t Department of Applied Physics, Faculty of Engineering,

University of Tokyo, Tokyo. $ Now at: S.C.K.-C.E.S., MOL, Belgium.

a-iron. But the pre-exponential term of the diffusion

constant is extraordinarily large.

On the contrary, the interstitial nature of boron in

a-iron is supported by several internal friction experi-

ments. Some of them@-ll) reveal a peak super-

imposed on the Snoek peak of carbon in a-iron, which

has been interpreted as an interstitial boron peak. An

internal friction peak of boron superimposed on that of

carbon is. however, deniedbyseveralauthors.(6J2.13) An

other internal friction peak(gJ3*14) supposed to be influ-

enced by boron is observed at a lower temperature t,han

that of the carbon peak. But the relaxat,ion strength

of this peak is small and the stability is not good.

Considering these resuhs. it is not possible to con-

clude that all t’he boron aboms dissolve interstit,ially

in u-iron.

On the other hand. in the case of carbon in iron the

interstitial position is said to be stabilized by the

a,ddition of nickel.

If the interstitial boron atom in iron is stabilized

by the addition of nickel as in the case of carbon in

iron. the internal friction peak caused by boron atoms

can apparent,ly be obtained.

The purpose of this work is to assure the existence

of the internal friction peak attributed to interstitial

boron in u-iron, and also to reconsider the mechanism

of the diffusion of boron in u-iron.

EXPERIMENTAL TECHNIQUES

I pecimens q

The iron (Westinghouse “Puron”) and the iron-

nickel alloy were shaped indo wires of 1 mm dia. and

ACTA METALLURGICA, VOL. 18, JUNE 1970 693

694 ACT.4 METALLURGIC3, VOL. 18, 1970

sheets of 1 mm thickness, and treated with a moist halogen flow to reduce the carbon and the nitrogen. For the preparation of boron alloys the following treatments were carried out; the specimens were surrounded with metallic boron powder and heated in a dry hydrogen atmosphere for 2 hr at 85O’C in order to obtain a Fe,B surface layer. Subsequently speci- mens were annealed in uacuo at, various temperatures for sufficient time to allow boron from the surface layer to enter the core of the specimens. Finally the surface layer was completelg removed by polishing to eliminate any trace of Fe,R. The boron concent of the specimens is shoed bp circles in Fig. 1 for t,he various annealing temperatures. All specimens contain a

certain amount of carbon which was picked up in the treatment of the boron addition. This, however, can be reduced to several ppm of carbon by annealing in a

dry hydrogen flow. This treatment, was employed on some specimens.

Internal friction measurements were carried out both by a torsion pendulun~ method and by a resonant bar technique. The adopted frequencies were l-6 c/s and 200-300 c/s.

1

I 7 9 9

l/TXIO: OK

Fro. 1. Boron content of specimens antlea~cc~ at various temperatures. Data by IlcUride et nt. are also shown.

EXPERIMENTAL RESULTS

Specimens of iron containing boron were quenched in the a region from high temperatures and internal friction measurements were carried out. The results obtained by tlte torsional oscillation (3.1 c/s) are shown in Fig. 2. An internal friction peak appeared at about 50% and it can be attributed to the relaxation of interstitial carbon from the fact that the half height width and the temperature of the peak are exactly those of carbon. It was verified by the shift of the

Tempera lure, “C

90 80 70 6C 5G a0 3c 23 IO C 10 2c /

3

2-

“0

x 7 0

0’ 3.0 35 4c

1000/T, OK

FIG. 2. Interna friction of boron-treated iron quenched from 900°C. Freq. 3.1 c/s. (a), (b), (c) are results of the same specimen and same treatments, showing the lack

of &ability of the peak at - 13%.

peak to higher temperatures by increasing the applied frequency from 1 to 6 C./S. Anot,her peak appeared at - 13”C, though its reproducibility is not good. This peak appears only when the specimen contains boron.

Figure 3 shows the results obtained by the bending oscillation (208 c/s) method. The peak influenced by boron is also observed.

Figures 4 and 5 show the results of internal friction measurements of iron-nickel-boron alloys quenched from 850°C. The applied frequencies are 1.5 and 257 c/s respectively. In Fig. 4 there appears an internal friction peak at about -30°C and the peak considered to be a boron peak can not be distinguished distinctly. This -30% peak is observed on specimens which have experienced a y-x transformation, and the peak temperature does not shift on changing the applied frequencies. The origin of this peak is not yet known. In Fig. 5 the internal friction peak influenced by boron can be seen, although the peak is not a single relaxation peak due to the alloying element. The applied frequencies and the peak temperature are plotted in Fig. 6 and from this plot the relaxation time of the peak can be obtained: + = IO- ‘sexp (13(~~~~~~) sec.

DISCUSSION

The equilibrium solubility of boron in cc-iron can be considered to be about. 50 ppm at 85O’C. X-ray

HATASHI ATD SCGEXO: SATURE OF BOROK-a-IHOS 695

2

PC 0

x 7 0

I

Temperature, “C

2c 40 60 80 I I

Fe B 050°C W.P 200 c/s

_

_

1”

I I 1 I I I , 34 33 32 31 30 29 26 27

1000/T, OK-’

FIG. 3. Internal friction of boron-treated iron quenched from 85O’C. Frequency 208 c/s.

_. -Temperature, “C

-4C -20 0 20 40 60 80 100

I----

I 1 I /

IO t

8 -- t

Fe-Ni-E 850 oc w. Q

9 l-5 c/s

-. , 0 I ,I 5 c/s ~ 4: 40 35 30 2

1000/T. OK-I

FIG. 4. Internal friction of Fe-Xi-B and Fe-Ni quenched from 850°C. Frequency 1.5 c/s.

‘0 X

0

:r.

Temperature, “C

ic 60 80

Fe-NI-8 85oocwa 219 c/s

Fe-N1 85OOC w a 257 c/s

FIG. 5. Internal friction of Fe-Xi-B and Fe-X quenched from 85O’C. Frequency 219 and 257 c/s.

Hasiguti’ Kamoshita

Q z 13 kcol / mol

d\ E1

: / i j ( 1 40 38 36 34 32 30 28 26

I/TX IO3

FIG. 6. Plot of applied frequencies against the inverse temperature at which the peak appears.

measurements show that the dissolution of boron

causes a lattice parameter contraction. If boron

dissolves interstitially it would be expected that an

internal friction peak corresponding to boron solute

would be observed. However, in the present experi-

ments this was not as great as expected. The internal

friction superimposed on that of the interstitial carbon

reported by Thomas, Leak@) and by other authors is

attributed exclusively to carbon as discussed b\ C,D( ::, (‘,<IIs. then equation (3) bwomc~s

Strocchi et ~1.‘~) From the facts mentioned above.

it is impossible to consider that all solute boron atoms

take inberstitial positions in a-iron. The small internal

friction peak observed at a lower temperature than that,

of the carbon peak. which is independent of the carbon

content in the present experiments and is observed

more clearly on the addition of nickel. may be attrib-

uted to t,he inbernal friction peak of interstitial boron

in a-iron. Comparing the solubility limit, of boron and

t,he low temperature-peak height. ib can be said that

most boron solute occupies substitutional positions

and while about 5’,, of boron solute dissoives inter-

stitially.

The vacancy concentration C,, can be assumed to be

about lO-’ at 850°C from the calculation’20) based on

the estimate of vacancy formation energy in x-iron.

which can be deduced b)- comparing the vacancy

formation energy of W: Xb. MO and the melting tem-

perature of W. Nn. 110 and Fe. If the energy of

dissociation is considered to be used to expand the

iron lattice, the equation of Zener.(21J and \Vert

and Zener’22) can be used

14’ = 9!% T,,, ’

p = T,,, ;, 111 p - (ij) The concept that boron occupies both substitutional

and interst,itial positions is suited for the interpreta-

tion of diffusion experiments. Table 1 shows the

results of diffusion experiments on boron, carbon,

nitrogen. nickel and silicon in z-iron.

TABLE 1

D (cm*/sec) D at 850°C Ref.

lo6 esp (-62OOO/RT) 1 x 10-s (8) 0.02 em l--20101RT) 3 x 10-e 11.5) 0.0078 ,x, ( - 18’900/kT) 2 x 10-e (IS; 9.9 exp (-61900/R?‘) 1 x lo-” (17) 0.44 (-4800/m) exp 3 x 10-16 (18)

Fe-B Fe-C Fe-S Fe-Xi Fe-Si

Though the activation energy of diffusion is large,

boron like carbon or nitrogen which are typical inter-

stitial elements. diffuses faster t,han nickel or silicon

which are substitutional elements. This situation can

be rationalized by considering the mechanism of

dissociative diffusion.(lg) Consider the reaction be-

tween sub&itutional and inOerstitia1 boron i.e.

where A’, I are substitut’ional and interstitial boron

atoms respectively, I’ is a vacanc,v. C,. C,., C,q are

concentrations of interstitial boron. vacancy and

substitutional boron respectively. AS and d, are the

dissociation entropy- and energy of substitutional

boron respectively, and f is a numerical fact,or.

When the equilibrium of equation (1) is achieved

an effective diffusion constant can be defined as

follo\Vs

696 ACTA METALLI~RGICA. VOL. 18. 1970

where T,, is melting temperature. ,IA is shear modulus.

At 850°C. putting C,/C,$ = 50-l. we have that

Q = 55 kcal/mol and eSIH = 7 1~ 102. Using Di -

2 x 10-4e-13*000itlT cm2/sec. which was obtained from

the internal friction measurements. we can obt,ain

DefP = 3 x 10s~-ss~ooo’R?’ cm2/sec (6)

Though the numerical values may not be reliable,

the qualitative nature of the diffusion of boron in a-

iron, which is characterized by the high activation

energy and large pre-exponential factor, can be elu-

cidated by somewhat changing the value of t,he as-

sumptions. This result shows that the dissociative

mechanism of diffusion is a good explanation of t,he

diffusion of boron in x-iron.

CONCLUSION

d small fraction of the boron solute atoms occupy

interstitial positions in cr-iron.

The dissociative diffusion mechanism is suited for

the interpretation of the diffusion of boron in a-iron.

ACKNOWLEDGEMENTS

The authors are ver,v grateful to Prof. Ramada of

the University of Tokyo for the analysis of boron.

The authors would also like to acknowledge the vari-

ous supports Dr. Yoshikawa of the National Research

Institute for Metals, and Prof. Sakamoto and other

members of bhe laboratory.

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