0s
THE
STRUCTURE BOUNDARIES
AXD MIGR\TION BETWEES F.C.C. G.
BXROi
and
H.
OF ISCOHEREXT IXTERPH_%SE AND B.C.C. CRYSTALS* GLEITER:
Observations by transmihm ctectron microscopy on the rnigrarivn of interphase b~~~uvlnrienhe::v++l r-brrrss (f.c.c.-structure) and &bras (b.c.c.-structure) are reported. The observations auggrat r&r interphase boundary migration ma:- occur by the movement vf step< with a height of about 2f)_l~ri _k along the plane of the boundary. The mechanism may be understood in term3 of the different atomic structure of the steps and the segments of the bounclarws bet-seen them. STRTCT~RE
ET
JIIGRATIOS
DE JOISTS CRIST_iCS
ISTERPHsiSES CFC ET CC
ISCOHERESTS
ESTRE
LEs
Les auteura prCeentent des observations de lu migration dea joints interphases entre le laiton x (irructure cfc) et Ic laiton $ (atructurc cc) effectu6es par microscopic Plectronique en transmis&n. Lea observations +ugg&rcnt clue la migration clu joint interphase peut Ptre procluite p;tr le dCplitcr~~~nt 11r marches ayant une httuteur de -lit B I00 A environ suivant plan du jomt. Le m&xnismr peut P:re ~sphc~u6 par la diff&ence de structure atomiqur eutre les marches et les szgmvntti de joint clui ICSli+ez:. Z1-R
STRCKTTR
I-SD
W.iSDERl_-SG ISKOHXRESTER PHdSESGRESZFL:$CHES ZWISCHES K.F.Z. VSD K.R.Z. KRISTALLES
Es w-id iiber elektronenmikroskopinche B:obachtLmgen cler Vanclerung van Pha~en,~re:lzAlif;~n zwiachen r-Messing (k.f.a.-sicruktur) uncl ~-Messing (k.r.z.-Struktur) bcrichtet. Die Beobachcun_-n ergeben, da0 die IVanderung der PhasengrenzALche rlurch eine Bewgung van Stufen dcr H&e 211 bii 100 .% entlang cler Grenzebene erfolgt. Der Mechanismus kann anhancl cler atomiatischen Srruktur -let Stufen. die sich van cler Struktur tier cbr Grenzfliichcnse~mente zwischcn den unterschciclet. ~rz_+u veretanden nerden. INTRODUCTION
In recent years a considerable number of inveatig&ions hare been carried out on the migration of incoherent grain boundaries in single phase(l) and multi phase alloys. w Little is known. however. on the atomic mechanism of the migration of incoherent interfaces between crystals being different in structure and composit iou (incoherent interphase boundaries). It is the purpose of this paper to report observations br_ transmission electron microscopy on the migration of incoherent interphase boundaries between x-brass (f.c.c. structure) and B-brass eryvatals (b.c.c. structure). ESPERIMENTAL
PROCEDURES
The material used was a two phase (%-- and $brass) copper-zinc alloy containing 55 wt. % copper. In order to obtain a high density of interphase boundaries between the r- and ,$-phase, the material was 90 per cent cold rolled and subsequently recrystallized at 450°C for 20 min and then cooled in air to room temperature. Specimens suitable for transmission electron microscopy were prepared by electropolishing the material at -25°C in a D2electrol_vte (Struers Co.) at 10y(3) b>- the standarcl window technique.
* Receive{1 June 2.5, 1953. i Inatitut fiir Werketoffe. Ruhr-Cniversitiit Bochum, I63 Bochum. Wejt Germany. : University Ssarbriicken, Fwchbereich AngewnncltePhysik, Bau 2. GF saarbriicken, n-est Germany. AC’Th
JIETALL’CIRGICA,
VOL.
2”.
FEBRI-AR\-
1974
The investigations were carried out tith a Philips electron microscope EM 300 operated at 100 k\‘. Interphase boundary migration in the thin foils was induced by heating the specimens in the hot stage of the electron.. microscope to temperatures between 160” and 320°C. It follows from the equilibrium phase diagram of Cu-Zn that the volume fraction of x- and $-brass changes as a function of remperature. Since the alloy was cooled in air from about 3~?‘c’ the volume fraction of both phases present in the recrystallized material corresponds approximatel)to room temperature. If the specime% are heated to a temperature between 160” and 3_“O’C t,he alloy equilibrates according to the phase diagram by increasing the volume fraction of x-brass ate the expense of b-brass by the migration of the x,/,3 interphase boundaries. In order to study the interphase b+jundary migration process in the electron microscope,. the following procedure was used. At the beginning an electron micrograph of a certain specimen area containing an interphase boundary was taken. Then the specimen xas held in the hot stage of the microscope for about 30 set at 160X’. Subsequently the specimen was cooled back to room temperature and a second micrograph of the same area was taken. It was not possible to take the picture at high temperatures because of specimen drift. This procedure was repeated several times. In each heating cycle. the temperature u-as increased by about %C. Therefore in the second heating cycle the specimen was held for 30 set at 168’C. then 30 set at 17652. etc. 141
ESPERIMESTAL
RESC;LTS
From the sequence of electron micrographs from
the
following (about
heating results
5 per
experiments emerged.
cent)
of the
obtained
described X
above
considerable
inter-phase
the
portion
boundaries
FIG. 1. Enlarged portion of the boundary shown in Fig. ?(a). It may be noticed that the thickness fringes are displaced at the points where thex intersect the lines.
“(11) Frc. 2. Dark field electron micrographs shorting a pattern of lines in an interphase boundary betlwen z-bras !lower crystal) and /?-brass (upper cr+al). The spacing of the lines varies along the plane of the botmdary depending on the inclination of the boundary. The sequence shows the movement of the lines along the plane of the boundary and boundary migration when the specimen was heated in the electron microscope according to the heating procedure described in the test. (a) Boundan_ prior to heating. (b) The same boundary s above after heating the specimen 30 see at lS4’C. A movement of the lines at t,he lower right Corner of the figure ma) be noticed. (c) The same bonndan- as above after heating the material for 30 set at 2.21’c. All lines have moved resulting in appreciable boundary migration. (cl) The same boundary 8s above after heating the zqecimen fior 30 SPCto 2T2’C. At the righ& side of the boundary new line; have appeared. at the left end others have cl&>appeared. The boundary migratccl approximately 300 A. In the middle of the picture the bounclav has bul.ged out (A). This local change in inclination resulted m 8 local increnae of the line density.
exhibited a pattern of lines the characteristic features of which may be summarized as follow examined,
(Figs. 1 and 2 a-d). (1) The thickness fringes of the boundaw are displaced by approximately %I-100 -1 at the points Z(b)
where the thickness
fringes intersect
the lines (Fig. 1).
BAR0
ASD
GLEITER:
STRUCTURE
_AXD
YIGRATIOS
(2) The lines are not parallel. The spacing of the lines varies along the boundary (Figs. la-d). (3) The spacing of the lines was observed to be correlated with the inclination of the boundary. A change in the boundary inclination resulted in a change of the line density (Fig. 2d). (4) If the specimens were heated to 160°C or higher, interphase boundary migration occurred. At the same time the lines were observed to move across the plane of the boundary (Fig. “a-d). DISCUSSION
The observed displacements of the thickness fringes at the intersection with the lines indicates that the lines represent steps in the interphase boundary. From the displacement of the thiclmess fringes the height of the steps was calculated to be in the order of 20-100 .?,. Hence the interphase boundary has a shape as indicated schematically in Fig. 3. If the mobility of the steps and the straight segments (e.g. segment bc, de Fig. 3) were the same, the rate of migration of the steps (v,) and the straight segments (ua) should be equal, since the driving force* is the same for both parts of the boundary. Apparently this is not so. From Figs. 2(a-d) it is clear that the migration rate of the straight boundary segments is negligible in comparison to the velocit,y of the steps (Lag< vJ. As the mobility of the steps is much higher than the mobility of the straight boundary segments between the steps, we conclude that the atomic structure of the steps may be different
OF
ISTERPH;\SE
BOUSDARIES
113
from the atomic structure of the straight boundary It appears, therefore, that interphase segments. boundary migration occurs essentially b_v the movement of the steps across the plane of the boundary. This result may be understood if it is assumed that the interphase boundary has a structure as is indicated schematically in Fig. 4. Recent observations(l) suggest that an z/P interphase boundary may have a structure similar to that of a grain boundary. By analogy to the step structure of an asymmetrical tilt boundarv(5) it mav therefore be possible that an interphaie boundary which is slightly pulled out of the inclination of good atomic fit (e.g. by surface tension forces) has a step structure consisting of long segments of good atomic fit (e.g. the segment bc, de,
Fm. 4. Schematic structure of an interface boundary containing a step which is several atomic layers high. In the horizontal parts of the boundary the atomic fit is good. An “open” boundary structure results, however, in the step.
etc. in Fig. 3) and small steps where the fit is poor and hence an “open” boundary structure results (Fig. 4). If such a boundary is forced to migrate the atoms can easily diffuse across the boundary in the “open” structure of the steps resulting in a high step mobility. The opposite may be true for the long segments of good atomic fit: in those segments diffusion may be difficult and therefore a low boundary mobility results. These results are in agreement with observations on the migration semi-coherent interfaces.c6)c7) ACKNOWLEDGEMENTS
The authors very much appreciated the valuable discussions with Dr. P. H. Pumphrey. The financial suhport of the Deutsche Forschungsgemeinschaft is greatfully acknowledged. REFERENCES
FIN. 3. Schematic sketch of the shape of an interphase boundary with steps. Interphase boundary migration occurs by the movement of the steps along the plane of the boundary with a velocity V, and migration of the segments bc and de with a migration rate v,. * The driving force for interphase boundary migration is proportional to the difference in free energy between the a-and j%brass crystals.
1. H. GLEITER and B. CHALMERS, Progress in Nateriala Science, Vol. 16, edited by B. CHALMERS, J. ‘A’. CHRISTIAX and T. B. Mlass~~ssx. Pergamon Press 19;‘. 2. J. W. CHRISTLAS, in Physical Jfetallurgy, edited by R. W. CAES. Xorth-Holland (1970). 3. K. M_~~ER, Thesis, University Bochum (1973). 4. G. B.iRo and H. GLEITER, Acta Net. 21, 1405 (1973). 5. H. GLEITER, Phy8. StatzraSoEidi (b) 45, 9 (1971). 6. &I. G. EUL, H. T. AARO~SOS and K. R KIXSXAX, Swface Science 31,257 (1971). 7. G. C. WEATTHERLT, ActaMet. 19, 181 (1911).