Periodic structures in ternary diffusion couples
Citation for published version (APA):Osinski, K., Gehring, A. P., Bastin, G. F., & Loo, van, F. J. J. (1983). Periodic structures in ternary diffusion couples. In F. J. Kedves, & D. L. Beke (Eds.), Diffusion in metals and alloys : international conference, 1982, Tihany, Hungaria: proceedings (pp. 469-472). (Diffusion and Defect Monograph Series; Vol. 7). Trans Tech Publications.
Document status and date: Published: 01/01/1983
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PERIODIC STlUC1'URFS IN TERNARY DIFEUSICN COOPLFS
K. Osinski, A.P. Gehring, G.F. Bastin and F.J.J. van Loo
Iaboratory of Physical O1emi.stry,
Eindhoven University of Tedmology, 'llie Netherlands
Introduction
During the reaction in the temary diffusion ccuples Fe
3Si-Zn and
Co
2Si-Zn reactionlayers with periodic structures develop(l). Fig. 1
and 2 give exanples of the d>served reactionlayers. 'llie thin bands consist of resp. FeSi and CoSi precipitates and
are
in fact two-ph.asebands canposed of netal-silicides and netal-zinc intennetallics (500
fig. 3). '!'he different "cells" in the reactionlayer of the Co
2Si-Zn
couple correspond with different grains in the Co
2Si substrate which
points to a relation he~ the observed periodicity and
grain-orientation. Co
2Si has a hexagonal structure. In the Fe3Si-Zn ccuple such a relationship is net te he expected hecause Fe
3Si is cubic. This paper deals only with the results of the Fe
3Si-Zn couplës. First
we will discuss the diffusion rredlanism of the fOIlllation of a two-phase
band. Finally
we
will discuss two IOOdels which might e.xplain thisunusual phencm:mon.
Diffusion mechanism of fomation of the two-tttase band
Zinc is the only diffusing eatpOnent in the FeZn
10 (=0) and FeZn13 (=0 intennetallic carp::ml<ÏS(2) • 'lli.is neans that the two-phase band is for-: ned at the Fe
3Si substrate. With EJ:MA we were able te neasure the total
concentration of a two-};i1aSe band viz. Fe
a
Si7Zn10• When we suppose
the two-phase band te he canposed of FeSi and FeZn
lO the m:>lar ratio of these two carp::ml<ÏS in a band will he 7:1. (N.B. the' stoechiatetric ratio is 1:2).
we
can ncM draw a diffusionpath.(3) of a Fe3Si-Zn caJple on the 39SoC Fe-Zn-Si isothei:m. This has been done sd1e!natically in fig. 4. Every two-phase band in the cS or 7; layer should correspond
with a loop of the diffusionpath into the FeSi-oand Fesi-l; two-phase
regions. For clarity this has oot been drawn in fig. 4.
Fran fig. 4
we
can see that the diffusion path crosses the (l-FeSi-othree-phase triangle te the :reasured band eatp)Sition marked by 1
a1m:Jst harizontally. Sc there is on~ a weak Si concentration gradient
across the substrate;two-phaseband interface and consequently hardly any Si diffusion will occur across this interface. The consequence of this imld:>ility of Si is (see also fig. 5) that when Zn reacts with the
substrate at position 1 the anount of Fesi in the twc::.>-Fhase band will
be dictated by the anount of Si in the Fe
3Si substrate. '!he quantity of 0 fo.med between the Fesi in the bK>-phase band is nt:M toe little
te satisfy the mass-balance. Sc Fe is forced to diffuse throogh the
bK>-phase bard and reacts with Zn at position II
te
fonn the 0 phasebebind the bK>-phase band. (Note: the 0 phase in the two-phase band and
the 0 phase bebind this band are foD'l'l:rl in a different way). Thus both
half-rea.ctions occuring resp. at I and II are:
7Fe3S~ + lOZn + Fe
a
Si7Zn10 + 13Fe 13Fe
+
130Zn + 13FeZn10
'Die diffusion velocity of Fe through this two-phase band is prd>ably
very ICM, the consequenCes· of which
we
will discuss in the next chapter.SUnmarizing
we
can say that the main characteristic diffusional features of the reaction in a Fe3Si-Zn couple
are:
- there is a very fast diffusing cCJ11?Onent viz. Zn - there is a ccnp:>nent whiCh is alm:>st imnobile Viz. Si
- there is a ccnp:>nent whiCh is forced to diffuse but its diffusion
velocity is very lew viz. Fe.
Discussion
In this section
we
will give sare general remarks on this phenanenonand discuss two m::x3els whiCh might eJq?lain the c:bse:rved' pheni:mana.
'!he question is why the reaction does oot proceed by a continuous grCMth of the different reaction layers but rather proceeds in this
disconti-In oor view there are two IOOdels which can account for the abserved phenarena., in both
cases
i t will appear that events at the substrate/two-phase baOO. interface play an inp:>rtant role.
'!he first roodsl is that when the two-Iilase band, which is fonred at
the substrate read1es a critical thickness i t is lifted of the
sub-strate due to nechanical stresses accarpanying the grcwth of the
two-phase band. After this lift-off we have a "fresh11 surface on which
the reaction can start afresh. Ex:peri..nents in which we used diffusion couples prepa;red fran thin (20011) Fe
3Si substrates and a
V'aPOUr-deposited Zn layer as Zn sarrce confinood this roodsl because a remark-able change in band thickness was abserved (see fig. 7). However, the
lack of cracks at the interface may refute this roodel.
'!he second model is that after a fixed t.iIre a sudden fOIJ1lation of the
ö-phase occurs at the interface due to an enrichment of Fe (see fig. 8). After this sudden ö fonnation the whole process of band fOlIllation
could start fran the beg~g. This enrichment may arise because the
excess Fe which is fonred during the develq:rnent of the two-phase band carmot diffuse away anymore CMing to its lON diffusion velocity. We believe that an explanation of the abseJ:Ved phenarena lies in one of these two roodsls or a canbination of bath.
At the m:::ment
ether
techniques (TEM)are
being applied in order to give a more clear image of the processes occuring at the interface.References
1) K.
Osinski,
A.W. Vriend, G.F. Bastin and F.J.J. van 100,Z. Metallk.de. 73 '(1982) 258.
2) M. Onishi, Y. wakamatsu and H. Miura,
Trans. Jap. Inst. Met. 15 (1974)
", h
F~g. 1. Fe3Si-Zn, 24 , 3950C (BEI) ====:100p
Si
Fig. 2. Co2Si-Zn, 44h, Fig. 3. Magnification
3950C (BEI) : lOOp of a band in a
Fe3Si-Zn couple
Fig. 4. Fe-Zn-Si 3950C isotherm with diffusim path of Fe3Si-Zn couple (schematic drawing)
Fig. 5., Diffusion rcechani.sm at Fe3Si substrate (see also text)
Fig. 7. "'Ihi.n substi'ate Fe3Si-Zn, 20h , 3950C
Zn
reaction layer
Fig. 6. Expected (left) and abserved (right) rrcrphology
in Fe3Si-Zn oouple
Fig. 8. Fe3Si-Zn, 2h , 3950C SUpposed sudden ê fonnation
