Observations on etch pits and sub-boundaries in columbium

LETTERS

twin.

It is apparent

from the polarized

TO

THE

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339

light micro-

graph of Fig. lc that the same family of intersecting twins can result in intersections

of different

types.

Furthermore, in this particularinstanceitwouldappear that the thickness of the crossed twin (lighter twin) is an important factor. The parallel markings in the background of this micrograph again represent slip on a set of {lOiO} planes. Another example of this third type of twin intersection is strikingly

illustrated

in the polarized

micro-

graph of Fig. le. Here the (lOT2) crossing twin is very thin and penetrates the thick (lOT2) twin only a short distance. It is significant that the nature of this penetration is the same when the twin starts anew on the other side of the thick twin.

This suggests that,

in

spite of the absence of a secondary crossing twin, the strain of the impinging thin twinistransmitt,edthrough the lattice of the thick twin.

Moreover, this transmis-

sion of strain probably occurs along a’ rational crystallographic plane, since the twin displacement in intersections appears

of

this

kind

(as pointed

to be proportional

crossed twin.

out

to the thickness

In this connection

above) of the

it is further signifi-

cant that one edge of the V-type notch appearing in the thick twin of Fig. lc is parallel to the line joining the displaced twin and corresponds

to a (lOi2)

It is also appa.rent from the micrograph

plane.

that the other

edge of the notch is parallel to the active {lOiO} slip planes, whose traces are evident

in the background.

Variations of this third type of twin intersection

are

shown in Figs. If and lg. The intersections in Fig. If between pairs of (lOi2} twins are interesting in that both twins are displaced in the region of impingement. It may be noted that this kind of intersection where one of the twins (presumably is noticeably

thinner.

occurs

the crossed twin)

The notable feature in the other

FIG. 2(b)

modification,

shown in Fig. lg. is the stepped configu-

ration at the impinged

interface.

Close examination

reveals that the fine steps occur alternately on planes parallel to the twinning planesof the intersectingtwins. A fourth type of twin intersections micrograph

is shown in the

of Fig. lh, where the intersecting

are again of the {lOi2}

type.

twins

In this kind of intersec-

tion, one twin appears to pass through another, as a thread through a needle. It is particularly noteworthy that neither twin has suffered a displacement

and that

the pierced twin has apparently

Further-

been split.

more, the split surface appears to be parallel to one of the two secondary surface

twins radiating from the impinged

of the adjacent

twin intersection

between the same pair of (lOi2)

which

For purposes of generalization,

the micrographs

Fig. 2 show examples of (112) twin intersections, types

2 and 3, in alpha-iron

It should also be mentioned of twin

intersections

is

twins.

deformed

in

above

at -196%.

that similar observations

in alpha-uranium

have

been

reported by CAHN,~) who was primarily interested in confirming geometry

the twinning of intersecting

elements in uranium by the pairs of twins. F. D. RON

RCA Laboratories Princeton, New Jersey, U.S.A. Reference 1. R. W. CAHN Plastic Acta Met. 1, 49 (1953). * Received

November

deformation

in

alpha

uranium.

28, 1956.

Observations on Etch Pits and Sub-boundaries in Columbium* The correlation FIG. 2(a)

of one

dislocation

with each pit

at low-angle boundaries on germanium single crystals grown from the melt was demonstrated by

340

ACTA

METALLURGICS,

Fm. 1. Substruoture in a large grain of arc-cast columbium. x 100

Vogel et uZ.(lJ In some cases, however, the formation of etch pits on metals appears to be related to the interaction of impurities and dislocations. Etch pits at small-angle boundaries on single crystals of zinc grown from the melt were obtained only after adding cadmium as an impurity,(2) and on aluminum sheet given various heat treatments the origin of etch pits could not be explained solely on the basis of dislocations emerging from the grains, and it was assumed that the pits were due to the presence of impurities around dislocations.(3)

FIG. 2. Region of grain shown in Fig.1. Low linear etch-pit density at a sub-boundary. x 1000

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5,

1957

FIG. 3. Region of grain shown in Fig. 1. High linear etch-pit density at a sub-boundary. x 1500

The authors have observed well-defined etch-pit arrays at sub-boundaries on large grains of arc-cast columbium which analysed 0.16 wt.. ‘I?;>Ta, 0.15 Xi, 0.02 Ti, 0.005 Fe, 0.86 0,, 0.05 C, and 0.01 N,. After aging the as-cast metal at elevated temperatures, the etch-pit arrays showed marked changes which presumably resulted from an association of inlpurities and dislocations. Success in developing etch pits was achieved by electrolytic polishing followed by etching. The polishing solution consisted of 90 ml cont. HeSO, and 10 ml of 48% HF. This solution is also used for

I?IG. 4. Etch-pit arrays in subgrains, near a grain boundary and a sub-boundary after aging at 1750°C. Same grain a~ shown in Fig. 1, x 1500

LETTERS

TO

the electrolytic polishing of tantalum.(4) Polishing was done at room temperature, using mild agitation and a current density of 0.1 amp. per sq. cm. The etching reagent consisted of 10 ml cont. H&lo,, 10 ml of 48%. HP, 10 ml H,O, and a few drops of H,O,. For etching, the specimen was immersed in the etchant and agitated. The cellular substructure found on a typical etched large grain of the as-cast columbium is shown in Fig. 1. At a higher magnification the faint sub-boundary patterns in Fig. 1 were resolved into arrays of closely spaced etch pits whose linear density varied throughout the grain, as shown in photomicrographs of two different regions, Figs. 2 and 3. Additional pits were developed only after prolonged etching. A Laue back-reflection pattern obtained from this grain showed the closely spaced spots associated with low-angle boundaries. After aging at 1750°C for 2 hours in a vacuum of less than 0.5 ,u and furnacecooling, the reactivity increased. The arrays developed after short etching times and the etch pit density at the sub-boundaries and in the subgrains increased, whereas the size of the pits decreased. In addition, a zone impoverished in etch pits was observed near the grain boundaries, and to a lesser extent at the subbouudaries . The etch-pit arrays found after aging are shown in Fig. 4. There was no microscopic evidence of a second phase accompanying the etch pits in either the as-cast or annealed metal. These observations on the formation of etch pits on as-cast columbium are similar to those observed by Wyon and Lacombe(3) for aluminum sheet, and can also be interpreted in terms of the interaction of impurity elements and dislocations. Because of the high rates of soliditication after arc-melting, there would be little tendency for the impurity elements to condense and form Cottrell’s atmospheres in the as-cast metal except at the more active dislocation sites such as the sub-boundaries. Aging the as-cast metal at an elevated temperature followed by slow cooling would permit the migration and condensation of the impurity elements at other dislocation sites within the subgraius and at the boundaries. The presence of these impurities would tend to promote the etching of dislocations while at the same time the reduction of strain-energy of the dislocations due to impurities would decrease the size of the etch pits. The depleted zones near the grain boundaries found after aging can be attributed to a larger migration of impurities to the boundaries because of the greater structural defects in these regions. The elements having the greatest tendency to condense at dislocations would be those with an 4

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atomic radius different than that of columbium and a low solubility in the metal. This assumed association between impurities and dislocations also implies that there is no guarantee that all the dislocations that are present are represented by etch pits. Similar etch-pit arrays at sub-boundaries have been observed in arc-cast tantalum electrolytically polished and etched with the same mixtures. A. B. MICHAEL Fansteel Metallurgical Corporation F. J. HVEGEI,~ North Chicago, Illinois References 1. F. L. VOUEL, W. G. PFANN. H. E. COREY, and E. E. THOMAS Phys. Rev. 90, 489 (1953). 2. J. J. GILMAN J. Metals 8, No. 8, 998 (1956). 3. G. WYON and P. LACOMBE Report of the Conference on Defects in Crystalline Solids, Bristol, 1954. p. 187. Physical Society (1955). 4. G. W. WENSCH, K. B. BRUCKART,and M. CONOLLY Metal Progress 61, No. 3, 81 (1952). * Received December 5, 1956. t Now with the Research Division, Minneapolis Honeywell Regulator Company, Hopkins, Minnesotct.

Luder’s

Bands*

Previously, Luder’s band propagation in 1010 steel wires was described and discussed.(r) The band propagation was said to depend on stress and temperature. Our observations of Luder’s line propagation in 1020 annealed steel flat bars pulled in tension has shown that the speed of propagation is strongly dependent on the loading rate and that surface imperfections also play an important part in their speed. The Luder’s line appears at an angle of approximately 25-40” to the normal of the direction of loading and has geometry similar to slip in a single crystal. The lines commonly start at the grips and move across the specimen, under constant load, until they encounter an imperfection or another Luder’s line. Such imperfections may bend the line (Fig. l), slow it, or even temporarily stop the Luder’s line (Fig. 2). Luder’s line collision with an obstacle, such as a notch, is sometimes accompanied by localized yielding. Severe notches may serve as “collectors” of roving Luder’s line deformation. Thus, notches in some cases do not initiate yielding, but plastic deformation at the notch occurs only after the Luder’s line collides with a notch. The above observations were made at a loading rate of approximately 0.0250.1 in./min and possibly these effects would not be as apparent at higher speeds. Since it is difficult to believe that shear can occur across grains of different orientations at exactly the annealed