WETTING AND PREMELTING PHASE TRANSITIONS IN 43° <100> TILT GRAIN BOUNDARIES IN Fe-5at.% Si

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WETTING AND PREMELTING PHASE

TRANSITIONS IN 43° TILT GRAIN BOUNDARIES IN Fe-5at.% Si

E. Rabkin, L. Shvindlerman, B. Straumal

To cite this version:

E. Rabkin, L. Shvindlerman, B. Straumal. WETTING AND PREMELTING PHASE TRANSITIONS

IN 43° TILT GRAIN BOUNDARIES IN Fe-5at.% Si. Journal de Physique Colloques, 1990, 51 (C1),

pp.C1-599-C1-603. �10.1051/jphyscol:1990194�. �jpa-00230363�

WETTING AND PREMELTING PHASE TRANSITIONS IN 43' <loo> TILT GRAIN BOUNDARIES IN Fe-Sat.% S5

E.I. RABKIN, L.S. SHVINDLERMAN and B.B. STRAUMAL

Institute of Solid State Physics; Academy of Sciences of the URSS, Chernogolovka, Moscow d'istrict 142432, U.R.S.S.

A b s t r a c t

-

P e n e t r a t i o n o f Sn and Zn along a 43" <loo> t i l t g r a i n boundary (TiGB) i n a body-centred c u b i c (bcc) Fe - 5 at.% S i a l l o y i s s t u d i e d i n t h e temperature range o f 652 - 975°C. Wetting t r a n s i t i o n o f GB by the S n - r i c h m e l t i s observed a t T, = 810

+

5'C. For Zn p e n e t r a t i o n along t h e GB, t h e w e t t i n g f i l m i s observed a t a l l temperatures studied. The r e g i o n behind t h e w e t t i n g f i l m i s observed t o have an unusually h i g h GB d i f f u s i v i t y o f Zn (by two o r d e r o f magnitude), f o l l o w e d by a r e g i o n o f "usual" GB d i f f u s i v i t y . The t r a n s i - t i o n from usual t o anomalous GB d i f f u s i v i t y D16, where D' i s t h e GB d i f f u s i o n c o e f f i c i e n t and 6 i s t h e GB width, takes p l a c e a t t h e c o n c e n t r a t i o n C,,

.

The value C, i s independent o f t h e annealing time, b u t depends s t r o n g l y on t h e temperature. Such a phenomenon may be explained i n terms o f t h e s o - c a l l e d p r e m e l t i n g phase t r a n s i t i o n "GB -+thin w e t t i n g f i l m on t h e boundary".

1. I n t r o d u c t i o n

-

Phase t r a n s i t i o n s i n two-dimensional systems, p a r t i c u l a r l y t h e phase t r a n s i t i o n s accompanying w e t t i n g on i n t e r f a c e s i n s o l i d s [ l ] , have a t t r a c t e d considerable a t t e n t i o n i n r e c e n t years. Wetting o f GBs by t h e m e l t i n two-component systems was e v i - denced i n Zn-Sn [2], AI-Sn [3], AI-Pb [3], e t c . The t h e o r e t i c a l l y p r e d i c t e d [4] phase t r a n s i t i o n s accompanied by f o r m a t i o n o f t h i n l i q u i d l a y e r s on boundaries (premelting, pre- w e t t i n g ) were never observed e x p e r i m e n t a l l y i n s o l i d s . The present study attempts t o i n - v e s t i g a t e t h e w e t t i n g and premel t i n g t r a n s i t i o n s i n two model systems having h i g h enthalpy o f mixing, namely Fe-Sn and Fe-Zn

[5].

2. Experimental Methods

-

A l l experiments were c a r r i e d o u t w i t h 43' <loo> TiGBs o f an Fe - 5 at.% S i a l l o y . The b i c r y s t a l s were grown by electron-beam zone m e l t i n g technique i n vacuum o f t h e o r d e r o f 1 0 - ~ T o r r w i t h a growth r a t e o f 1 mm/min. A l a y e r o f Sn o r Zn was a p p l i e d t o t h e specimens by immersing them i n t o t h e melt. Before immersion t h e specimens were ground mechanically f o l l o w e d by chemical p01 i s h i n g i n a s o l u t i o n c o n t a i n i n g 80%

H 0,

+

^{14% H,O}

+

6% HF. The samples were annealed i n t h e temperature range o f 652 -975°C.

~ k e Sn o r Zn c o n c e n t r a t i o n p r o f i l e s were determined by means o f e l e c t r o n probe micro- a n a l y s i s ( F i g . l a ) .

3. R e s u l t s

-

Figures l b and l c r e v e a l t y p i c a l c o n c e n t r a t i o n p r o f i l e s o f

Sn

on t h e specimen c r o s s - s e c t i o n s a f t e r two d i f f e r e n t d i f f u s i o n annealing treatments. An a l l o y e d l a y e r formed by d i s s o l u t i o n o f Fe i n l i q u i d Sn d u r i n g t h e i n i t i a l p e r i o d o f annealing may be observed on t h e sample surface. The b u l k d i f f u s i o n l a y e r may be found f u r t h e r deep i n s i d e t h e b i - c r y s t a l . I s o c o n c e n t r a t i o n l i n e s p a r a l l e l t o t h e sample surface appears t o suggest t h a t no p r e f e r e n t i a l p e n e t r a t i o n o f Sn t o o k p l a c e along GBs ( F i g . l b ) . This i s t r u e f o r a l l b i - c r y s t a l samples annealed a t temperatures below 810 - 815'C. A t temperatures above 810

-

815"C, t h e s i t u a t i a n changes d r a s t i c a l l y . A t h i n ( r o u g h l y 1 pm) l a y e r enriched i n Sn along GBs ( F i g . 1 ~ ) i n d i c a t e s t h a t a phase t r a n s i t i o n o f GB w e t t i n g by t h e m e l t (FeSi)-Sn has occured a t T, = 810

-

815°C. I t i s important t o mention t h a t w e t t i n g t r a n s i t i o n above T,, perhaps, does n o t change t h e GB p r o p e r t i e s ahead o f t h e Sn-enriched w e t t i n g l a y e r . How- ever, such changes i n boundary p r o p e r t i e s ahead o f t h e w e t t i n g l a y e r have been observed i n course o f Zn p e n e t r a t i o n along t h e GBs. F i g u r e I d i l l u s t r a t e s t h e Zn c o n c e n t r a t i o n d i s t r i - b u t i o n a f t e r annealing a t 908°C f o r 3 h. The s i m i l a r i t y between t h e c o n c e n t r a t i o n d i s t r i - b u t i o n s between F i g s . 1 ~ and I d a r e worth n o t i n g . However, t h e r e i s a d i s t i n c t d i f f e r e n c e as f a r as Sn and Zn d i f f u s i o n along t h e GB i s concerned. A r e g i o n w i t h h i g h d i f f u s i v i t y can be c l e a r l y seen below t h e GB l a y e r o n l y i n F i g . l d ( t h e beginning and end being marked by arrows). The GB d i f f u s i v i t y (D'6) i n t h i s r e g i o n i s v e r y l a r g e , approximately 2 orders o f magnitude h i g h e r than c h a r a c t e r i s t i c values o f

D'&

f o r GB s e l f - and h e t e r o d i f f u s i o n i n bcc Fe [6-91. F o l l o w i n g t h i s region, t h e GB appears t o have a D ' 6 value n e a r - t o t h e usual one [6-91. For e s t i m a t i n g D'6, a sequence of c o n c e n t r a t i o n p r o f i l e s perpendicular t o t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990194

Cl-600 COLLOQUE DE PHYSIQUE

GB

and parallel to the sample surface has been measured (Fig.la), and the C, values have been calculated from the maxima of these profiles corresponding to the

GB

position. The

D'6

values can be determined from the slope of the variation of logC against the depth y.

These logC,,-y plots for different temperatures are displayed in ~ig.9. These plots consist of an initial part having a low gradient followed by a steep slope. The concentration C,, (the concentration at which

D'S

changes) has been obtained from the point of intersection of these two parts having different slopes. From experimental evidences in Fig.2 it may be suggested that a phase transition accompanies by the formation of the thin wetting layer at C, near the GBs. However, similar transition has not been observed in the experiments with in. At the points of intersection in Fig.2, the

GB

thickness

6

may increase abruptly.

Assuming

D'

constant above and below C,, , the

GB

thickness

6

can be estimated by ratio of

D ' 6

above and below C,, . The value of

6,

so obtained is approximately 10'. The temperature dependence of C, is displayed in Fig.3 in which the variation of C appears rather com- plicated. Above the Curie point, the greater the bulk solubility !:mit CO the lower is the corresponding C,, Near the Curie point, this correlation may not be valid since the line has a protuberante towards the lower Zn concentrations.

Fig.1

(a)

The scheme of concentration measurement in a bicrystal after diffusional an- nealing. The isoconcentration lines and the plot of electron probe shift are drawn.

The measurements were carried out by local X-ray spectral analysis.

_(b)

Five iso-

concentration profiles of Sn after annealing at 792°C for

50

h. CO is the Sn solu-

bility limit. (c,d) The Sn and Zn concentrations in the same bicrystal after an-

nealing at 908°C for

3

h. The region of rapid

GB

diffusion below the wetting layer

is marked by the arrows.

the change of

D ' 6

slope changes with temperature (cf. Fig.3). ?l)

^{975'C, 1.75}

^h;

(2) 857"C, 7.5

h;

(3) 80g°C,

21 h;

(4) 790°C,

63 h;

(5) 745'C, 102

h.

Fig.3 The temperature dependences of

C

and

Co.

The line of solubility limit of Zn in

pure Fe is also included (botktsolidus and solvus)

[IO].

r is an intermetallic

compound and

L

is the liquid.

Cl-602 COLLOQUE DE _PHYSIQUE

Fig.4 The dependences o f excess f r e e energy o f w e t t i n g l a y e r R on t h e l a y e r thickness 1.

ac, i s t h e s u r f a c e t e n s i o n o f t h e interphase " c r y s t a l - w e t t i n g phase", a i s t h e GB s u r f a c e t e n s i o n (marked by dashed h o r i z o n t a l l i n e s ) . The hyperbola and t h e s t r a i g h t l i n e l -ag (a-c) may y i e l d R as p e r Eq. ( l ) . (a) Wetting l ayer on a boundary i s n o t f e a s i b l e . (b) a,, i s tangent t o R, and a t h i n l a y e r o f t h i c k n e s s l, appears on t h e boundary ( t h e p r e m e l t i n g t r a n s i t i o n ) . (c) I n course o f f u r t h e r decrease i n ag, t h e l a y e r t h i c k n e s s grows. (d) A t ag = 0, t h e l a y e r t h i c k n e s s 1 approaches i n - f l n i t y (complete w e t t i n g ) .

4. Discussion

-

The GB w e t t i n g phenomena, along w i t h t h e common f e a t u ~ e s o f t h e surface wetting, deserves s p e c i a l a t t e n t i o n f o r many reasons. I n general, t h e w e t t i n g l a y e r f r e e energy Rs may be expressed as f o l l o w s [l]:

Rs = 2UCF

+

l ~ g

+

^v(1)

,

(1)

where a, i s t h e s u r f a c e t e n s i o n o f t h e " c r y s t a l - w e t t i n g phase" interphase, 1 i s t h e w e t t i n g r a y e r t h i c k n e s s and ag i s t h e excess f r e e energy o f t h e w e t t i n g phase. The l a s t term describes t h e i n t e r a c t i o n o f t h e " c r y s t a l - w e t t i n g phase" interphases. The s i t u a t i o n i n which t h e w e t t i n g l a y e r t h i c k n e s s approaches i n f i n i t y (1 ^-+a) and t h e concerned phases tend t o c o e x i s t (ag 4 0 ) i s c a l l e d t h e complete wetting. I n o r d e r t o analyse t h e R be- haviour, V(1) may be assumed t o have a simple h y p e r b o l i c form,

V(1) =

w / l n .

⁽²⁾

The f u n c t i o n R s ( l ) d e f i n e d by Eqs.(l) and (2) has a minimum a t 1 = l,. I f ag value i s l a r g e ( f a r from phase coexistence l i n e ) , then R s ( l o )

>

a

,,

and t h e w e t t i n g l a y e r on the boundary i s n o t n o t i c e a b l e ( i . e . t h e boundary i s "c1ean1l'j (Fig.4a). When ag i s such t h a t R ! l o ) = U, t h e boundary transforms i n t o a t h i n l a y e r o f a w e t t i n g phase o f t h i c k n e s s 1 (Flg.4b). t h i s i s t h e s o - c a l l e d p r e m e l t i n g t r a n s i t i o n o f GB i f t h e w e t t i n g phase i s l i q u i d . When ag decreases, t h e 1 value increases and approaches i n f i n i t y a t ag = 0 (Figs.4c,d). The experimental d a t a s u b s t a n t i a t e s t h e scheme d i s p l a y e d i n Figs.4a-d by showing a w e t t i n g l a y e r on GB above t h e r e g i o n o f accelerated GB d i f f u s i o n . Consequently, complete w e t t i n g occurs i n t h e systems as explained i n Fig.4d. A t C = C,, ag = 0 and f o r small d e v i a t i o n s i n C,, ag may be expanded i n t o a s e r i e s i n terms

OF

C,

-

C,. Truncating t h e f i r s t term o f t h i s expansion, i t f o l l o w s

g = b(C,

-

C,)

.

^{( 3 )}

The R ( l ) f u n c t i o n determined by use o f Eqs.(l) t o (3) has a minimum a t 1, = [b(Co

-

C , ) / ~ W ] - ~ / ( " ' ~ :

When t h e c o n d i t i o n R = a,, i s s a t i s f i e d , then t h e p r e m e l t i n g t r a n s i t i o n occurs, and t h e GB i s replaced by a ?ayer o f a Zn-enriched phase. For t h i s c o n d i t i o n one may o b t a i n :

The Eq.(4) correlates the bulk solubility limit with the concentration at which the pre- melting transition occurs at the boundary. But the surface tensions

a,

and

a,,

also depend on the concentration. Thus comparison of Eq.(4) with the experimental data may not be feasible. The solid solubility limit line (solvus) in Fe-Zn system is near to the metastable curve of solid solution decomposition [10]. This means that the

a,,

value de- creases sharply as the peritectic transformation point is approached (T

=

782°C).

According to Eq. (4), this leads to a decrease in C,, , which is a1 so observefle?n ,the con- cerned experiment (Fig. 3).

References [l] P.C.De Gennes, Rev. Mod. Phys. 57 (1985) 827.

[2] A. Passerone,

N.

Eustathopoulos and P. Desr6, J. Less-Common Metals 52 (1977) 37.

[3] K.K. Ikeuye and C.S. Smith, Trans. AIME 185 (1949) 762.

[4] C. Rottman, "Theory of Phase Transitions at Internal Interfaces", to be published.

[5] R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser and K.K. Kelley, "Selected Values of the Thermodynamic Properties of Binary Alloys", American Society for Metals, Metals Park, Ohio (1973).

[6]

D.W.

James and G.W. Leak, Phil. Mag. 12 (1965) 491.

171 C. Leymonie and R. Lacombe, Mem. Sci. Rev. Metall. 57 (1960) 285.

[8]

H. Hansel, L. Stratmann, H. Keller and H.J. Grabke, Acta Met. 33 (1985) 659.

[g] J. Geise and C. Herzig, Z. Metallk. 76 (1985) 622.

[l01

0.

Kubaschewski , IRON-Binary Phase Diagrams, Springer-Verlag, Berl in; Verl ag

Stahleisen m.b.H., Dusseldorf (1982).