PVAD-AlAS 431 ILLINOIS5 UNI v AT URBANA DEPT OF METALLURGY AND MINING--ETC F/A 7/4 9I CHARACTERIZATrON OF AS-GROWN DISLOCATION STRUCTURE IN NIORIUM B--ETCIU)I SEP A1 S R STOCK ,H CHEN, H K BRNBAUM NOi 5C11

UNCLASSIFIED 0047-C1 2N

Emoorh.a.

CHARACTERIZATION OF AS-GROWN DISLOCATION STRUCTUR~E

IN JIOBIUM BY X-RAY DIFFRACTION TOPOGRAPHY,

?'R. Stock, Haydn /Chnand H rn ba urn

ONR Contract USNJ0014-75-C-1O12,

University of Illinois at Urbana-Champaign

Department of Metallurgy and Mining Engineering D7Urbana, Illinois 61801 -.. E

This document is unclassified. Distribution anid

reproduction for any purpose of the U.S. Governm'etn

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8110 5 060

CHARACTERIZATION OF AS-GROWN DISLOCATION STRUCTURE IN

NIOBIUM BY X-RAY DIFFRACTION

TOPOGRAPHY

by

S. R. Stock, Haydn Chen and H. K. Bi rnba um

Depdrtment Of >',etd I Iurcy and I'i ni ngE Lng inee ri ng

and th. aeral s Resea rch Labo!rdtory

Uni vr.,!s ity o I I! inois att Urb aii a C a pin

Urba n i, I L 6 1801

The behavior of dislocations in b.c.c. metals has been

extensively examined usinq relatively macroscopic methods, such

as deformation studies and etch pitting, as well as with

microscopic TEM methods. While these studies have led to

significant increases in the understanding of dislocation

behavior, the need remains for a method for studying the

microscopic behavior of dislocations in relatively thick

specimens. One such method, x-ray diffraction topography, has

not been extensively applied due to the relatively high x-ray

absorption of many of the metals of interest, such as niobium,

and the consequent need to prepare highly perfect thin

crystals. In the present note we report the preparation of such

niobiuvi crystals and the use of Lang topography to characterize

their dislocation structures.

Single crystal niob)ium specimens of thickness suitable for

transmiission topographic studies ;ere grown by recrystallization

of a heavily deform~ied polycrystalline ribbon of niobium,;. Ine

procedure consisted of resistance heatinqg at 2200K and 2.6 x 10-8

Pa for times on the order of an hour. It should be noted

howeever, that due to outgassing of the n ioh;ul l, protracLed

annealing .'s required to obt in this pressure. DuIinI thi

annea e. ,Ited .a qr'owth ( ec o:i,i.ry riocr'jstall 1 iz~iti o )

occurred which was apparently dri ven by the s:;rfa e energy of ihe

niobium-vapor inLerface. Grains were prou .iced having surfaLe

area of several cm 2 and having <110> approximat;ly normal to the

surface. Rapid cooling, achieved by cessation of the heating

current, was necessary in order to mini.iizf, severe oxidation at

1/

. . . .. . , , ,..2

intermediate teiperatures. In the particular crystal described

here, the recrystallization anneal was folloived by equilibration

with 2.9 x 10 - 3 Pa of pure N2 clas to introduce nitrogen solutes

into the niobiunt. The nitrogen solutes served to reduce the

sensitivity of tile crystal to strains. The thickness of the

crystal reported in the present note is 76 pin, and the crystal '1

has a surface normal about five degrees fro:m [110]. Single

crystals grown by this method have total dislocation densities of

10 cin/c 3 or less.

A cenventional Lang camera using MoK radiation from a

inicrofocus generator operating at 50 kV and 3.8 wiA was used for

the topography. Ilford 1.4 rInclear emulsions with a thickness of

50 pmn recorded the topographs. Because pt - 1, where p is the

linear absorption coefficient and t is the crystal thickness,

kinematical contrast is expected to predo, iinate. The Burgers

vectors of soi~i of the dislocations in a ntvjork were deterviinn

using the follooing criteria. Neylecting elastic anisotropy, a

dislocation will be coripletely invisible if q • b = 0

dcd q • b x v :; 0, ihcre g is the diffraction vector, b is the

B'.,rqers vector ari , is the dislocition line vector. Residual

cor!trdst nay occur if i . b-- 0, h::t y b x ut- 0; if both

condi ions hold h,, ('t;riel is . isLiC lly anisotropicc as

niob i u; ; or i f t he rc o sPreq a io,i of i pur i ty ato, IS to t i.

dislocations. An a IditiOudl requiremen t for networks is that the

su-m of Burgers vectors of dislocations inretir.y at a node 'must he

zero.

Projue tion tu',ojrp;)h:; w,er taken with the following

(A2

diffraction vectors: 200, 020, 002, 110, 011, 121 and 211.

Figure I shows tWo of these, the 110 and 011 topographs, wii ch

exhibit a variety of features including several different

subgrains and a network of dislocations labeled "N." The thick

dark line extending through the dislocation array is a scratch

placed on the surface. Topographs taken before and after the

scratch was introduced show that the network was only slightly

disturbed and that no dislocations propagated from the immediate

vicinity of the scratch. Feature "0" is a "dent" which was

present prior to crystal growth as a result of local plastic

deformation. The dislocations w'hich formed this "dent" annealed

out, and the remaininq deformation is elastically accomrIodated.

The gradient of elastic strains about the "dent" leads to unusual

contrast of the dislocation network in the 011 topograph: rdther

than the more usual enhanced diffraction at defects, the

radiation is scattered away froi the defect leading to a decrease

in diffracted intensity relative to the backgrOund. This

contrast is similar to that observed in elastically bent silic n

crysta Is (e eran and BI ech , 192) and can be expected fro:m

dynamical scattering thLuo ry (Iart, 1931).

The netwo rk of di sloc a tions is labeled in Fig. 2 wit tihe

Burgers vectors deteriin ed using the above contrast criteria.

Residual contrast, however, proved to he a ',ajor problemi in

identifying a consistent set of Burgers vectors. Optical

densitometry was requi red to conclusively determine whether the

images were in contrast or in residual contrast. The difference

in transriitted intensity between the dislocation image and the

background, normalized relative to the intensity transmitted

through the unexposed portiorns of the emulsion, was used as a

parar,,eter to deterni ne whother the image was in contrast or was

exhibiting residual contrast. As can be seen in Fig. 2, the

network's dislocation Burgers vectors consist of <111>, <100>,

and <110> types. While it is energetically favorable for two

a/2<111> slip dislocations to combine to form a single a<100>

dislocation, the reaction of two a/2<111> dislocations to form a

single a<ll> is energetically unfavorable. It appears, however,

that during the high te;perature anneal both a<100> and

a<110> dislocation segmients form in the network. The fractions

of Burgers vectors of each type are 60% a/2<111>, 25%" a<100> and

150, a< 110>. Networks conmpri sed of th? sarie types of dislocations

were observed by Dingley and Hale (1966) using TEM in Fe, Fe -.

0.7 j.I;n and 2 1/4%Cr - IIo steel. They rep)orted si m ilar

distributions of 6urgers vector types with 60 a/2<1II>,

20 a I10 > and 20%, a<10)> ty)es.

Nearly per fect n iob i un crys tIs have been reported before

but apparently the perfocLion of strain-dnnealed and

recrystall izel ribbon speci:7ens hcis not been previously

Oxa:il ied. Re e , ' ::an '!~ I IJR 1 i .11 (1961) have prepared

rl io*,.n1,, crys al; frui, th,. , .'hlc contained vi despreid

net;urks of disloc.atIo:is an :1 dislocation densities approaching

l[)?C./ci 3 . Using 1- crijci'ole-less pulling iiethod, Naramoto (l / 3)

has also grown excellent niobi,:i crystals, continimy long

dipoles of a/2<1l1> eddie dislocations, long segmients of a/2<1lI>

screw type, :hart a<iO > sc;:1ents and small 1/2<111> pris:ttic

Sloops and helices. The presence of the prisviatic loops and

helices was ascribed to vacancy precipitation during growth. The

absence of loops and hel ices in the present study could be due to

a variety of factors. The small specimen thickness and tihe

availability of the surface as a defect sink would tend to limit

vacancy loop formation. The relatively good vacuum used would

also minimize oxide formation during cooling and would limit the

subsequent vacancy injection at moderate temperatures. Studies

in which loops and helices were observed in molybdenum (Becker

and Pegel, 1969) and in niobium ( Naramoto, 1978 and Zedler, 1967)

were characterized by slow:er :ooling rates dnd poorer

vacuu,%I (10 - 5 - 1 0 - 6 torr).

AC K 0!L E 0 G E ,N I-S

The authors would ike tn thdnr" Dr. P. E. Zapp for pruvi-/ y

the particular crystal described here and flr. I). L. Zi erath of

the Illi nois State Geologi cal Survey for his help with the

optical densitoiietry. We gratefully ackre..il ,dkie the us of t.h.e

x-ray facilities a tha ,o t qria is .R oe ;c!r Lah.)ruLory aI th0

s, upport of the ff 0 f f ic ff ,if R esar 1h Lhro ,1! COntract ,",'"ri 1'

I

REFER FNUCE S

1. Becker, C., and Pegel, B., 1969, Phys. Stat. Sol., 32, 443.

2. Dingley, D. J., and Hale, K. F., 1966, Proc. Roy. Soc., A295,

55.

3. Hart, Ml., 1981, Chrceiaino Crystal Growith Defects by

X-ray__Methods, p. 216.

4. Mperan, E. S. , and BIlech, I. A., 1972, J. Appi. Phys ., 43,

265.

5. Narai~ioto, H. , 197211, Crystal Growth, -44, 475.

6. Reed, R. E. , Gwber;,an, H. D. , and Baldwhin, T. 0. , 1967, J.

Phys. Chem. Sol ids Suppi . ,1, 829.

7. Zedler, E. , 1967, J. Appi. Pkys. , 38, 2046.

FIGURE CAPTIONS

Fig. 1. Lang topographs of a niobiu:n crystal with surface normal

approximately [110]. a) Subgrains "A" and "B,"

dislocation network "N," scratch "S" and dent "D" are

seen with g = [110]. b & c) Contrast of the

dislocations in the network differs for g = [110] and

g = [011], respectively, due to the influence of bending

from the nearby dent "D.

Fig. 2. Dislocation Burgers vectors for a network "N" in a

niobiur crystal with surface normal approximately

[110]. Different line segments, single, dcuble, dashed

and dotted, represent dislocation lines with a/2<111>,

a<110>, a<100> and undetermined Burgers vectors,

respectively. The direction of Burgers vectors which

lie in the plane of the foil are indicated by arrows.

- 0

0.

1-6-

CC7

oil a

* -/

O-2A4A

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3 REPORT TITLE

Characterization of As-Grown Dislocation Structure in Niobium by X-rayDiffraction Topography

4 DESCRIPTIVE NOTES (Ty.pe of report and inclusive date*)

Technical Report September 1981S AUTHOR(S) (Last name, first name. initial)

Stock, Stuart S., Chen, Haydn and Birnbaum, Howard K.

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13 P BSTRACT

The use of X-ray topography to study dislocation structures inniobium is described.

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NiobiumX-ray topography

Dislocation structures

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