Grain boundary precipitation in αβ brass

GRAIN

BOUNDARY

PRECIPITATION

M. 5. WOOD

IN a/p BRASS*

and A. HELLAWELLt

Specimens of an a/b brass (46 wt. o/ozinc) were cast to produce columnar grains which had an approximately common [lo01 sxis. After heat treating to produce grain boundary precipitation of the a-phase, these specimens were examined microscopically and by means of X-ray back-reflection Laue photographs of individual grains. The results showed that side plates of precipitate are produced by grain boundary migration, and have an orientation determined by the growing grain of the matrix. Although the density of potential nuclei in a grain boundary is related to the misorientation between the grains, the growth and shape of the precipitate are controlled by movement of the boundary and the orientation of the boundary plane itself. PRECIPITATION

AUX

JOINTS DES GRAINS

DANS LES LAITONS

a//I

Des Bchantillons de laiton a/@ (46% en poids de zinc) ont et& co&s pour obtenir des grains allong& dont l’axe [loo] a une orientation commune. API& un traitement destine a produire une precipitation de la phaee a aux joints des grains, ces 4chantillons ont Bti examines microscopiquement et aux rayons X par la methode de Laune en retour. Les result&s montrent que des plaquettes de pr6cipiGe apparaissent g&e au deplacement des joints. Leur orientation est en relation avec le grain de la matrice qui coalesce. La croissance et la forme du pr6cipit5 dependent du mouvement du joint et de l’orientation du plan du joint, bien que la densit des germes potentiels dans un joint soit en relation avec la d&orientation entre lea grains. KORNGRENZENAUSSCHEIDUNC

IN a/b MESSINU

Proben aus a/b Messing (46 Cew.% Zink) wurden so vergossen, da2 ein Stengelgeftige entstand, dessen KSrner nahezu eine gemeinsame [loo]-A&se hatten. Nach W&rmebehandlung sur Erseugung einer Korngrenzenausscheidung der a-Phase wurden diese Proben mikroskopisch und mittels LaueRiickstrahlaufnahmen von einxehmn Kornern untersucht. Die Ergebnisse se&an, daS durch Komgrenxenwanderungplattenformige Ausscheidungen entstehen. Deren Orientierungwird be&runt durch das wachsende Korn der Matrix. Obwohl die Dichte der moglichen Keime in einer Komgrense mit dem Orientierungsunterschied zwischen den Kornern ausammenb&ngt, wird doch das Wachstum und die Gestalt der Ausscheidung durch die Bewegung der Urenxe und durch die Orientierung der Crenaebene se1bst bestimmt.

INTRODUCTION

The present work has been concerned with the rate of precipitation in grain boundaries, and with the shapes of the precipitate particles which are formed. Gruhl and Amman(l) have attempted to relate the thickness of grain boundary precipitate in some copper base alloys to the misorient&ion between grains. Specimens were produced by casting to give long columnar grains having an approximately common [loo] growth axis. They obtained only very poor correlation between their results and the graingrain misorient&ion. As pointed out by Cahn(2), part of this lack of correlation was probably due to the fact that their measurement included both nucleation and growth processes, since these might proceed at different rates. Aaronson(3) has also examined grain boundary precipitation of primary ferrite in low carbon steels. He suggests that the shape of the precipitate particles must be dependent on the type of boundary structure l Received June 22, 1960. t Department of Metallurgy, University of Oxford.

ACTA

METALLURGICA,

VOL.

9, MAY

1961

which exists; this is determined largely by the orientation of the boundary plane itself, with respect to the adjacent grains. EXPERIMENTAL The material used was an a/B brass containing 45 wt.% of zinc. Both the /l matrix and the a precipitate have cubic crystal structures, and the habit planes for the precipitation are well known,(‘P) being {Ill}a 11{110)/l and [IlOla 11[lll]& Polycrystalline specimens were prepared by a method similar to that used by Gruhl and Amman(l), and after homogenizing in the one phase /3 region for several hours at 6OO”C,they were transferred directly to a salt bath at 520°C to precipitate the a phase. Annealing times from 3 to 30 min were employed. The actual specimens were sections out from chill cast ingots. These latter were produced by casting within a hot graphite mould, into the base of which there fitted a water cooled copper plug. Long columnar grains were produced, having [lOO] directions within 10” of the specimen axis. t6) Castings were some 5 cm 429

WOOD

AND

HELLAWELL:

GRAIN

in length, and 1 cm in diameter, a typical cross section contained about 30 grains. Apart from a small angular scatter along the axis of the castings, the grain boundaries observed in sections could be described as tilt boundaries of variable symmetry. Specimens were electropolished, using phosphoric acid, and were etched with alcoholic ferric chloride to give satisfactory contrast. The orientations of grains were determined from back-reflection Laue photographs which were taken with a microbeam camera.

RESULTS

and DISCUSSION

Microscopic examination showed that a variety of forms of a precipitate occurred. Among these, the so-called Widmanstiitten side plates, referred to by Aaronson(3), showed some interesting features. Typical examples are illustrated by Figs. 1 and 2. In the Crst place, these side plates only occurred in those boundaries which were concave towards a large expanding grain, that is, in boundaries which were moving. It was established which grains were growing by microscopical examination before and after the homogenizing treatment. One side of the plates nearly always lies parallel to the nearest (110) habit plane of the expanding /3 grain. The other side, that inclined towards the boundary plane, exhibits a step-like formation. On the longer plates these steps are found at almost equal distances from the grain boundary. The most probable explanation of these shapes seems to be that the boundary has migrated from that position where the longer plates of precipitate nucleated, at the narrower ends, and growth has proceeded due to this boundary migration, and not by the plates of precipitate growing outwards from the boundary into the grain. As the boundary moved, some of the shorter plates also nucleated, slowing down the rate of migration, and producing the corresponding steps on the existing plates. The precipitate has therefore grown along that edge attached to the grain boundary, and the boundary provides, in this case, a diffusion short circuit by means of which, zinc atoms are able to diffuse away from the growing edges of the plates. When the boundary is retarded, as by the nucleation of fresh precipitate, the plates thicken by growing outwards on that side nearest the boundary. Ultimately the boundary becomes filled with precipitate and is completely held up. The thickening of the precipitate plates rarely takes place on that surface which is inclined away from the boundary, and this generally lies parallel to the habit plane of the matrix. The

BOUNDARY

PRECIPITATION

429

inclination of the precipitate plates to the grain boundary is controlled by the angle between the boundary surface itself, and the nearest habit plane of the a sheets, which in these alloys was predominantly the (IlO}@ plane. Whenever the boundary plane coincides with the habit plane of an expanding grain, a continuous sheet of precipitate is formed (Fig. 3). The density of precipitate particles in a boundary is related to the intergranular misorient&ion, at least for the low angle boundaries <5’. If, however, the boundary is a low angle (110) tilt boundary, the nuclei grow so rapidly in the boundary plane that a relatively dense sheet of precipitate is formed. This means that the apparent density of precipitation cannot be regarded as a simple function of the intergranular angle and the number of nuclei initially present. It follows from the above argument, that where boundary migration is slow, the precipitate will be able to grow more rapidly along the boundary plane, and be unlikely to show the well defined crystallographic symmetry of Figs. 1 and 2. Precipitate of this type is shown in Fig. 4. It is also notable, that where the a precipitate has grown together, a large number of growth twins occur. Some specimens were annealed for short periods to produce selective nucleation of boundaries. No correlation could be found between those boundaries which first showed signs of precipitate and the intergranular angle existing across them. Many large angle boundaries did not produce any visible precipitate at all. In so far as any correlation could be found between rates of nucleation and the types of boundary, it seemed that migrating boundaries produced precipitate most rapidly, a typical example being illustrated by Figs. 5 and 6. It would seem, that although the density of potential nuclei in a boundary must be related to the intergranular angle, these nuclei are unlikely to grow unless the boundary is actually moving. If our interpretation of Figs. 1-5 is correct, then some of the previous work can be better understood. Firstly, the poor correlation between precipitate thickness and boundary misorientation, in the work of Gruhl and Amman(l), is only to be expected, since their results did not take into account the orientation of the boundary surfaces, nor the relative mobility of boundaries. Secondly, the various categories of precipitate described by Aaronsonc3) are explicable in terms of the three variables which describe our own observations. The detailed shape of the side plates requires some

430

WOOD

AND

HELLAWELL:

GRAIN

BOUNDARY

PRECIPITATION

431

FIQ. 3. Sheet precipitate of CCwhich forms in the /I boundaries when the boundary plane coincides with the (110) /? habit plane. x 750

FIG. 4. Irregular

a precipitate

explana especially the contrast between the straightt and stepped sides of the plates. There: are two considerations : (1) If one compares two posisible step like projections A and B, as shown in Fig. 7, 01ne can see that the ratio of volume-tosurface area in the element A will exceed that for

showing twins.

x 1000

element B, as long as the angle 8 is less thar1 9o". From energetic considerations alolle, it is therefcIre to be expected that the a plate will thicl ten by g :owth on that side inclined towards the j3 boun dary. (2) The contact angles at the triI)le /?--a--B junc:tions will approach the equilibrium val ues wwhenever there

432

ACTA

METALLURGICA,

VOL.

9, 1961

FIG. 5. Selective precipitation at one grain boundary. The intergranular misorient&ions in this case were 20” for the active boundary and 23” and 20” for the other two inactive boundaries. x 500

FIG. 6. Showing the grain boundary movement ocourring during the formation of the precipitate shown in Fig. 5.

FIG. 8. Showing the adjustments to contact angles to be expected at a grain boundary. The broken lines indicate the initial non-equilibrium shape of the precipitate.

is adequate

time for adjustment.

rate of boundary

Thus, whenever the

is reduced,

there will be

adjustments at the B-a--/l junctions. The equilibrium values of the contact angles as determined by Smith”) indicate a ratio of boundary surface energies a/l//?/l =

a

FIG. 7. To show the relative an&x/volume two possible elements A and B.

migration

ratios for

1, this value is an average one, and does not for variations in the &3 intergranular angles. of u precipitate can approach equilibrium junctions by altering its shape as shown in

account A plate at the Fig. 8.

WOOD

AND

HELLAWELL:

GRAIN

BOUNDARY

PRECIPITATION

433

Contact 810143the low energy hebit plane is mainteined on the “open” side of the plate, and adjustment of the contact angles tekes place with curveture of the &l boundery or by projection of the a plate into the next

part in determining the rate of precipitation end the subsequent growth forms. The shepes of individual precipitate plates c8n be

gmin.

and in terms of boundary tensions.

hum

On the “closed” side of the plebe, the equilibcan be attained either by butwerd

growth

from the a ph1t.8,producing a step, or by curv8ture of t.he B boundary towards

the a plate at the point of

contact. Some exemples of the adjustments in shape can be seen on the side platea in Figs. 1 and 2. Greater magnification of the interfaces and junctions will require the use of replica techniques and electron microscopy.

CONCLUSIONS The rate of precipitstion in grain boundaries, and the shapes of the precipitcrte perticles 8re only partially explicable in terme of the intergranular misorient&ions and the orient&ions of the boundery planes. Metallogrsphic

examination

indicates

thet

the

movement of the grain boundary plays 8n important

quelitatively understood from energetic considerations

ACKNOWLEDGMENTS The authors wish to thank professor W. HumeRothery for laboratory 8ccommodetion and facilities, Mr. R. Eborell of the British Non-ferrous Research Association for kindly preparing brass rods of the desired composition, and Dr. J. W. Christian for helpful discussion of the results. REFERENCES 1. W. GRUEL and D. AMMAN, AC& Met. 8, 347 (1955). 2. J. W. CAliN, AC& Met. 4, 217 (1956). 3. H. I. AAIWMON, Phaaa Tmneformdti in Met&. Institute of Metals, London (1955). 4. J. WEERTB, 2. Phys. 78. 1 (1932). 5. 0. T. MARZKE, Tmtae. Aw. Inat. Min. (M&U.) Engru. 104, 64 (1933). 6. D. WAL~N and B. CHALMERS. Tmn.9. 4-r. Inst. Min. (M&U.) Enqru. 216, 447 (196Qj. 7. C. S. SMITH. Trans. Aw. Znat. Min. (Me&all.)Engm. eOr, 2387 (1948).