A new ordered structure in the niobium-nitrogen system

Scripta M E T A L L U R G I C A

Vol. 9, pp. 949-953, 1975 Printed in the United States

A NEW ORDERED STRUCTURE

Pergamon

IN THE N I O B I U M - N I T R O G E N

Press,

Inc.

SYSTEM

P. Jung and T. Schober Institut

fur F e s t k 6 r p e r f o r s c h u n g ,

Kernforschungsanlage

J~lich,

(Received June

517 JUlich,

18,

Germany

1975)

INTRODUCTION With the e x c e p t i o n of the Nb-N system, and N was obtained Ref.

(4), V-O

In the V-N system four d i f f e r e n t

(3,5,6), Nb-O

superlattices

structure w i t h a unit cell c o n t a i n i n g

s y s t e m only one s u p e r l a t t l c e above superlattice failed to produce

was observed

in V. E x t e n s i v e evidence

for s u p e r l a t t i c e

Such evidence will be p r e s e n t e d

We may g e n e r a l l y

expect the o b s e r v a t i o n

were found

(8,9).

(I). One of them has

27 metal cells.

In the Ta-N as the

in the Nb-N system

formation - see,

for example

(10-14).

Some of the present results have

(15).

I at.% of N is quenched

of ordering

of N in Nb when

I) more than

in w h i c h may be achieved by using thin speci-

and 2) ageing is performed

growth of N b 2 N - p r e c i p i t a t e s

(7) and Ta-O

investigations

in this work.

elsewhere

mens,

ordering of O

(4) with the same structure

previous

briefly appeared

roughly

for interstitial

for the group Vb metals V, Nb and Ta: for V-N as quoted in

(I-3), Ta-N see Ref.

a b.c.c,

evidence

at s u f f i c i e n t l y

low temperatures

to delay the

(10). EXPERIMENTAL

High purity burizing

(Marz-grade)

treatment

were s u b s e q u e n t l y

Nb foils of 25 um thickness were first given a decar-

in a UHV system degassed

(19OO°C,

5 min)

h e a t i n g of the foils at 18OO°C at

of 4 x 10 -5 and I x 10 -4 Torr,

respectively.

The samples

were q u e n c h e d by switching off the heating current.

The N - c o n c e n t r a t i o n s

gravimetrically

Transmission

copy

determined

to be 1.4 and 2.9 at %.

(TEM) and r e s i s t i v i t y m e a s u r e m e n t s

(16). The specimens

were a n n e a l e d

high purity Nb specimens less than

and

at 23OO°C in a v a c u u m of 10 -10 Torr for 30 minutes.

N i t r o g e n d o p i n g was performed by resistance n i t r o g e n pressures

10 -5 Torr of oxygen,

were performed

at 330°C at pressures

the gas a b s o r p t i o n

10 ppm/hour.

949

electron micros-

using standard below

during annealing

were

techniques

10 -7 Torr. Using was found to be

950

NEW ORDERED

STRUCTURE

IN Nb-N

V01.

9, No. 9

RESULTS AND D I S C U S S I O N After q u e n c h i n g

neither ordering

TEM in the 1.4 at % samples. increase

in r e s i s t i v i t y

effects nor precipitates

From the electrical

In the 2.9 at % specimens precipitates

(11) on d e f o r m e d precipitation

by

of these samples an

was derived of about 4.6 u~cm per at % of N. This value

is in good a g r e e m e n t with the p r e v i o u s l y

platelike

were observable

resistivity

p u b l i s h e d value of 4.1 u~cm

the d i s l o c a t i o n s

of Nb2N.

samples,

during q u e n c h i n g

were found to be decorated with small

In agreement with the work of Dollins

{1OO} planes were

(10).

found as habit planes

reduced the r e s i s t i v i t y

and Wert

(Fig.

I). This

of these specimens by

about 30 %.

FIG.

I

H e t e r o g e n e o u s p r e c i p i t a t i o n of Nb2N platelets along dislocations in a 2.9 at % Nb-N sample as observed directly after quench. Habit plane of p l a t e l e t s is {100}. K i n e m a t i c a l bright field TEM image.

I After

1jim

annealing

,

at 330°C for 30 minutes

observed by TEM in both samples. served in V-N

(I) and Ta-N

very faint superlattice

Their geometry

(3) and is s c h e m a t i c a l l y

shown in Fig.

FIG.

002

222

000

110

220

reflections were

is identical with the one ob2

2

Schematic view of the reciprocal lattice of the ordered Nb-N phase. (OO1) and (110) sections are shown. Open circles: superlattice points. Full circles: Nb-matrix.

Vol.

9, No.

9

NEW ORDERED

Resistivity measurements stages of s u p e r l a t t i c e

STRUCTURE

were used to investigate

formation

Fig.

3 shows the relative

both c o n c e n t r a t i o n s work

versus the annealing

for N b 2 N - p r e c i p i t a t i o n .

by a p r o n o u n c e d time.

The c o n t r i b u t i o n

unchanged

(11) had demon-

resistivity

of

It is also known from other

of Nb2N n u c l e a t i o n

1OO0 hrs at 330°C

to the initial de-

The superlattice

after that time while Nb2N n u c l e a t i o n of about 60 % in Fig.

in resisti-

of specimens

is expected to be small since the resistivity

in r e s i s t i v i t y

superlattice

the initial

decrease

period of a p p r o x i m a t e l y

only very slowly after the first two hours. decrease

quantitatively

change in residual

(10) that there is an incubation

crease in r e s i s t i v i t y

951

since recent studies on Ta-N

strated that ordering of N is a c c o m p a n i e d vity.

IN Nb-N

continues.

decreases

reflections

Thus,

remain

the initial

3 is almost e x c l u s i v e l y

due to

formation.

1.0 t-

~

i

i

~

i

i

i

p(t~l p(o)

1!

o 06

o2

~.

0{ 0

---~--

330°Cl 10

I

,s0% 1

I 210

I

30

FIG.

I ,0

1 40

I time [hours1 50

3

R e s i d u a l r e s i s t i v i t y versus time during the anneals. The values are n o r m a l i z e d with respect to the q u e n c h e d - i n resistivity. Two d i f f e r e n t c o n c e n t r a t i o n s were u s e d : 1.4 at % (+) and 2.9 at % (o). The a n n e a l i n g temperatures are given at the b o t t o m of the Figure. In o r d e r to accelerate raised after

precipitation

16 hours to 450°C. A n n e a l i n g

increase of the r e s i s t i v i t y monstrated

of Nb2N the annealing for 30 m i n u t e s

in both samples.

of the s u p e r l a t t i c e

Optical m i c r o s c o p y

and TEM revealed

was

at 450°C caused an

This increase has already been de-

in the Ta-N system and may be i n t e r p r e t e d

ordering parameter

temperature

in terms of a reduced

at higher t e m p e r a t u r e s

that coarsening

(17).

and growth of Nb2N precipi-

tates was completed after about 20 hours at 450°C. After that period no superlattice r e f l e c t i o n s stable ordered

also be c o n c l u d e d in Ta

could be seen anymore by TEM signifying

structure

that the meta-

has t r a n s f o r m e d to the e q u i l i b r i u m Nb2N-phase.

from this and other work

(17)) only at c o n c e n t r a t i o n s

(10) that N orders

above roughly

1 at %.

in Nb

It may

(as well as

952

NEW O R D E R E D STRUCTURE

IN Nb-N

Vol.

9, No. 9

Some samples were also p r o d u c e d w i t h N c o n c e n t r a t i o n s a r o u n d 20 at %. Here, areas w e r e o b s e r v e d after q u e n c h i n g w h e r e p r e c i p i t a t i o n had fully o c c u r r e d in the form of large N b 2 N plates on {1OO} - see Fig.

4. The areas c o n t a i n i n g pre-

c i p i t a t e s w e r e s e p a r a t e d by w i d e bands d i s p l a y i n g the typical p r e v i o u s l y o b s e r v e d in V-N of Fig.

(2) and Ta-N

4. D i f f r a c t i o n p a t t e r n s

strong s u p e r l a t t i c e reflections. Fig.

(SAD's)

"tweed structure"

(4) - see also left and right hand side of such "tweed" areas always showed

A typical example of a SAD is p r e s e n t e d in

5. D a r k field o b s e r v a t i o n s w i t h s u p e r I a t t i c e beams in the "tweed area"

r e v e a l e d the sizes of the m i c r o d o m a i n s w h i c h w e r e of the o r d e r of 10 tab - see Fig.

6.

FIG.

4

C e n t e r of the picture: Nb?N p r e c i p i t a t e s in a h i g h l y aoped (20 at %) N b - N sample as observed d i r e c t l y after quench. Note d e n u d e d areas around precipitates. Left and right hand side: the typical "twaed structure" is o b s e r v e d w h e r e the ordered alloy is present. Kine~Batical TEM b r i g h t field image.

10o11 I

• 1,um

e.-.-,.-IlOO]

I

FIG.

5

(110)-selected area d i f f r a c t i o n p a t t e r n of a "tweed" area in Fig. 4: s u p e r s t r u c t u r e reflections are clearly v i s i b l e at g e n e r a l p o s i t i o n s I/3 g. All the d i f f e r e n t domains c o n t r i b u ted to the f o r m a t i o n of this d i f f r a c t i o n pattern.

Vol.

9, No.

9

NEW ORDERED STRUCTURE

IN Nb-N

953

FIG.

6

Dark field image using a s u p e r l a t t i c e reflection of the "tweed" areas in Fig. 4: m i c r o - d o m a i n s appear bright.

Q,1pm

ACKNOWLEDGMENT The authors a c k n o w l e d g e Dr. H. Wenzl.

helpful d i s c u s s i o n s

with Prof.

Mr. H.J. B i e r f e l d p r o v i d e d valuable

Thanks are due to Dr. D.M. Kroeger for carefully

W. Schilling and

technical

assistance.

reading the manuscript.

REFERENCES 1. H. Epstein, 2. G. H6rz,

B. Goldstein,

J. L e s s - C o m m o n M e t a l s

3. M. Cambini,

Mat.

4. K. Trenzinger,

Res. Bull.

6. D. Gunwaldsen, 7. J. Niebuhr, 9. M.P.

A.G.

13. T. Schober,

11,

17, 711

G. Antesberger, A. Maisseu,

10, 246 Cryst.

Scripta Met.

stat.

(1966) 13, 910

(1969)

18, 119

(1966);

(1972)

7, 731

J. Vicens,

phys.

16. T. Schober, V. Soraji~,

Metallography

sol.

6, 183

17. P. Jung,

Acta Met.

123

22,

(1974)

(1969)

15. P. Jung and T. Schober, K. Trenzinger,

34, 97

Metals

(1966)

Sc° J. 6, 96

G. Nouet,

(1974)

Krug, J. L e s s - C o m m o n

191

Soy. Phys.

B.L. Eyre, Metals

14. A. Deschanvres, 237 (1974)

M.P.

22, 937

G. H6rz, J. Nucl. Mater.

C. Wert, Acta Met.

12. N. Dahlstrom,

Acta Met.

J. L e s s - C o m m o n M e t a l s

Khachaturyan,

(1973)

(1974)

J. L e s s - C o m m o n Metals

10. E. Gebhardt, W. DHrrschnabel, 18, 134 (1966)

7, 717

(1974)

R.A. McCune,

D. Potter,

J. Rennet,

11. C. Dollins,

35, 207

9, 1469

J. L e s s - C o m m o n Metals

Usikov,

Scripta Met.

P. Jung, W. Schilling,

5. J.L. Henry, S.A. O'Hare, 21, 115 (1970)

8. S. Steeb,

D. Potter,

(1973) J. L e s s - C o m m o n M e t a l s

(a), to be p u b l i s h e d (1973)

(1974)

(1975)

34,