Elevated temperature diffusion in the systems Nb-Pt, Nb-Se, Nb-Zn, Nb-Co, Ni-Ta, and Fe-Mo

JOURNAL

OF NUCLEAR

MATERIALS

ELEVATED

3, No. :( (l!%l)

?ci%%%,

TEMPERATURE

NORTH-HOLCAP\‘I)

PITBLISHtW(:

oc.).. .\MS’I’E:lklM

DIFFUSION IN THE SYSTEMS

Nb-Pt, Nb-Se, Nb-Zn, Nb-Co, Ni-Ta, AND Fe-MO

Received

The

electron

determine occurring A

in

total

binary

probe

the

microanalyzer

composition

bimetal

diffusion

of eighteen systems

and

phases

: Nb-Pt,

Nb-Se,

1960

has been used to

phases produites dans le systeme Nb-Co & savoir Nb&O

extent,

et NbCoa n’ont

couples

were

19 September

of at

phases 1100” C.

determined

Nb-Zn,

Nb-Co.

pas 6th antdrieurement

la litterature.

in six

region

Xi-Ta,

de niobium

comportait

lui aussi une

contenant

b peu pres

5 y& en poids

dans la phase cobalt,

limite

de solubilite

and expressible

phase

existants.

in small whole

systems

formations

in that Nb-Se

Nb-Se2 while Nb-Zn and NbZns. to preclude

in the

reported This

taining

showed phases

5 weight Ni-Ta

strong with

pure

one phase:

would seem

previously

an extended niobium

limit

diffusion

TaNi2,

TaNia phase on cooling

TaNis,

thermocouples

are

percent

cracking

Taxi,,

t,he zones, only

system

with the solubility

phase diagrams.

NbZnz,

and platinum

and NbPta

and are Nb&os

also exhibited

about

agreement

fissuration

NbzSea, and

NbzZno,

des

montree La

zones et

TaNi,,

la

du tantale

par

refroidissement

de

pur pr&ipitant

apparaissant

une seule phase

grain and

tantalum from

dix-huit Nb-Pt,

Der

con-

in cobalt

in

formation.

von

Phesen

gesamt

zones which

zu ermitt,eln,

Ni-Ta,

in the

wurden

ermittelt:

stoohiometrische,

1100” C.

a et6 employ6

et l’etendue

Nb-He,

(lurch

kleine

und Nb-Zn

NbzRes und NbSez,

des couples

On

arrive

Nb-Co,

de diffusion

B un total

binaires

Ni-Ta

:

et Fe-MO. Dans avec des

que

tandis NbZns.

dans la formation Nb-Se

presente

que Nb-Zn La

qui former

NbaPt,

exclure

I’emploi

platine

au contact

NbSe,

produit

diffusion

NbZn,

entre

NbPt,

de leurs phases NbzZna,

le niobium

NbPt2

prolong6

Nb&ea,

de

du niobium

et

en ce NbSez,

NbZns

et)

et le platine

et NbPta

semblerait

thermocouples h 1100”. Les

de seules

auf.

bri Nb-Zn

Ins-

6 bindren Nb-Co,

Zahlen Die

Platin

unter

Rildung

diirfte

einen langeren

1100” C ausschlietlen.

bri

von

Kontakt,

Die beiden

Nb-Co hergestellt

be-

Systeme

als bei Nb-Se NbZn,

NbsZns,

Die Diffusion zwischen

und NbPts

im System

1100” C

Nb-Zn,

ahneln sieh insofern,

Pt-Thrrmoelementen

de

differents

und

bei

waren.

ganze

Zusammensetzung

NbSe,

NbPtg

in einer

in den folgenden

Xb-Pt,

ver-

Dabei weisen die Phasen stet,s einr

Nb-Se

Niob

des phases

wurde

vorhanden

18 Phasen

und Fe-Mo.

schroibbare

formules a coefficient,s numtiriques entiers et faibles. Les systemes Nb-Se et Nb-Zn sont quelque pen sens

Fe-MO

und die Ausdehnung

die

Diffusionsprobe

Systemen

The

comprised

tous les cas les phases sont stoechiometriques

semblables

1100” C.

Rdntgenfluoreszenz-Mikroanalysator

gegluhten

in existing

TasNie

k sonde Bloctronique

Nb-Zn,

phases

les zones

MozFes.

wendet, urn die Zusammensetzung

precipitating

phases pour six systemes Nb-Se,

Les

un-

1100” C. Fo-Mo produced

la composition b

pro-

importante

grains.

de

region

shown

produced

and TaNi,

B 1100” dans

bimetalliques

Ni-Ta

une

la dc

dans la phase TaNi

2~ partir

and ;“u’bCoJ.

MozFes.

Le microanalyseur

avec

et TaaNis comprenaient

NbZnz und NbZns vorkommen. pour determiner

du couple

montraient

formation

TaNi

avec

produisait

diffusion qui

en accord

dans les diagrammes

at 1100” C. The only phases

Nb-Co

in the literature

system

Nb-Se,

use of platinum

with niobium

produced

Xb-Se

duisait

niobium

NbPtz,

The

alike in their phase

Nb-Zn,

between

NbPt,

prolonged

in contact

exhibits

produces

Diffusion

to form NbsPt,

numbers.

are somewhat

2

dans

Ca systeme

&endue

and Fe-MO. In all cases the phases are stoichiometric and Nb-Zn

signalees

NbsPt,

NbPt,

Gebrauch mit

einzigen

Niob

von bei

Phasen, die

werden konnten,

namlich

NbsCo2 und NbCo.l sind bisher in der Literatur

nicht

erwahnt. In diesem System wurde such in Ubereinst,immung mit den in der Literat’ur angefuhrten Zustandsdiagrammen

ein ausgedehnter

Mischkristall-

bereich mit rund 5 Gew. - oi, Niob in Kobalt Bei der Ni-Ta-Diffusion Zonen. TaNi

Im

einzelnen

entstanden

sind die Phasen

und TaaXiz vorhanden,

Phase beim Abktihlen scheidet. MozFes

Im System auf.

wobei

beobachtet .

rissige und kiirnige TaNi:%, TaKiz,

sich in der TaNiz-

von 1100” C reines Tantal Fe-Mo

aus-

baute sich nur die Phase

ELEVATED

1.

TEMPERATURE

261

DIFFUSION

tained within a vacuum

Introduction This report is the second in a series on the

about

5 x IO-5 mm I-g.

effects and results of binary diffusions at elevated

was maintained

temperatures

Northrup

and covers

Nb-Se, Nb-Zn,

Nb-Co,

the systems

Ni-Ta,

first report 1) was concerned

Nb-Pt,

and Fe-MO. The with the diffusion

chamber

at

controller

thermocouple

operating

Temperature

& 5” C by using

control

a Leeds

a R/X%-l3

affixed directly

at

y.

and Rh

to the specimen.

At the end of the desired diffusion

time the

of niobium with stainless steels and their major

specimen was cooled quickly to room tempera-

constituents

ture by quenching in helium, removed

and served to shed new light on

the phase formations

in the Nb-Cr and Nb-Ni

chamber, and immediately

from the

mounted in Bakelite.

the Nb-Fe

A cross section of the diffusion zone, well away

phase diagram. Five of the systems reported herein yield new phase formation information, while one, Fe-MO, corroborates the present

from the outer edge was given a goad metallographic polish. The extent and condition of the zone(s) and its boundaries was usually apparent under t,he optical microscope without etching.

systems

as well as to corroborate

phase diagram. As in the first report

all analyses

of the

diffusion couples were made with the electron probe microanalyzer. This instrument has allowed diffusion times to be cut to as little as 24 hours and has successfully elim~ated the need for costly and time-consuming wet chemical or radioactive tracer analyses. Even the 2-micronwide diffusion zones in Nb-Pt and Ni-Ta were measured with reasonable precision and accuracy electrons.

2.

using

the

finely

focused

beam

of

In

any

case,

no

etching

was

done

on

any

specimen prior to its analysis in the electron probe microanalyzer.

3.

of

Eieetron

Probe Meas~emen~

All analytical measurements entailed the use 25-keV electrons focused to a beam one

micron in diameter. Standard, curved LiFcrystal X-ray optics were used with argon or krypton Geiger counter detectors, and the X-ray inOensities were recorded on Brown strip-chart recorders. The analytical results shown in figs. 2, 4, 8, 10 and 12 were obtained from

Specimen Preparation

The niobium used in these investigations was Fansteel 99.7 y0 grade, the cobalt was Kulite

continuous scans which produced simultaneous qLlanti~,ative analyses for the diffusing elements.

reactor grade, the platinum,

Phase thicknesses and diffusion zone lengths are those produced for the respective systems under

and tantalum mercially

zinc, molybdenum,

were the purest materials

obtainable,

and

the

selenium

comwas

the stat’ed time-temperature

conditions.

A more

vacuum distilled metal. Quarter-inch squares of each material, except selenium and zinc, were cut, metallographically polished on one face,

complete description of the electron probe mi~roanalyzer is contained in the previous report and will not be repeated here.

and lightly spot-welded under helium to form the appropriate couples. Because of selenium’s low melting point and high vapor pressure, it was first distilled into quartz ampules ; then clean, polished niobium plates were introduced

In the previous report, percent composition was obtained from X-ray intensity using ealibration curves extrapolated from measurement of chemically analyzed standards. Since that report, a new calculation technique has been developed 2) which obviates the need for calibration standards and, in some 50 compounds and alloys tested, has yielded accuracy of a few percent of the amount present. Calculation has also shown that the calibration curves of the first report were quite correct’.

and the ampules were evacuated and sealed for the diffusion treatment. For the Nb-Zn system, niobium wire was dip coated with molten zinc prior to a~ealing in air. All ~ffusions were carried out singly in a small tungsten-wound furnace which was con-

4.

Results

of niobiram wit,b ~~lat,~~~~irn for LXX

Diffusion hours

at, 1100~ (‘1 ~~rodll~~e(~zones

somewhat

variable

conDinuons voiding.

but’ completely of cracking

zone.

in widt~h along t-he length

These

were

identified.

~1~

inrest2gated

the

NbsPb

phase,

of

proceeding

from t*he Nb to the 13. as Nb&%. NbPt. and NbPta. Phase diagraI~~s of this present

01

The zone co~~taj~le(~four phases, each

of which varied the

in width

wit.h no evidence

t.hat were

NbPka. system

which

was

by Greer1l-ield and Neck in 1!bX

Distoocrr f Microns 1

Photogra~~ls of the u~et~hed and etched specimen are shown in figs. la and lb, while the

\\

Al3

C iI3

corn~ositjoll across the zone is graphed in fig. 2. & the widest point the zone WRM about, 40 microns (0.0016) across. An interesting observation of this diffusion is t,hat t,he NbRs and NbPts phases do not, occur as a continuous feature of the diffusion zone. The*y appear. always together. as irregular layers strongly indenting the NbPt phale which constitutes

the major

portion

of the ~.~ffl~sioj~

zone. Comparison of the e-khed and unetehed specimens of tig. 1 indicatXes that aqua regisl has little effect, on the NbPts or NbR, phases but atStacks the Nb&% and NbPt rapidly. The formation of this 30-micron diffusion zone bet,ween Nb and Pt after 18% hours at 11c)O”C indicates t-hat. Pt, or P1. alloy thermocouples cannot be used in direct contact wit-it Nb for extended periods of time at elevnt,eti ~e~~~erature if precise tamperat3rre measurements are required. Nb-Se

Fig. 3 shows an unetched diffusion zone fn a ~liob~um-seleI~j~~rn couple formed af-ter 24 hours

ELEVATED

TEMPERATURE

263

DIFFUSION

is not

between Nb$3es and NbSes. There is no repre-

in hard-

sentation of 0 o/o Nb (100 o/o Se) in fig. 4 because

ness made it impossible to achieve a good polish

with the vapor deposition method, the formation

at

C. The

1100”

appearance

visually

pleasing because the extreme variation

on the pure elements and t,he three intermediat,e

of stoichiometric

phases simLl~t,~neously. However,

permit

bhe diffusion

zones are seen to be more uniform in width than

compounds

buildup

was too rapid to

of a 100 y/,_ Se layer

on the

surface.

in the Nb-Pt system. Going from the Nb toward the Se, the int’ermediate

phases were identified

Nb-Zn

as NbSe, NbsSes, and NbSes and having respective widths of about 14, 18, and 170 microns as showl~ in fig. 4. Some void formation appears between the NbSe and NbsSea phases and also

by

Niobium-zinc specimens had been prepared the Metallurgy Division of NRL for air

corrosion tests and these corrosion specimens were used rather than preparing separate

Nb Fig.

3.

Diffusion

zone

formed

1 IOW’ C by vapor deposition

after

of selenium

24 hours

I Nb2Zn3

at

+ NbZn

onto niobium Fig.

5.

48 hours

Nb-Zn

dip-coated

in air

at

NbZn

so-

- so

80-

-80

$ 5 60-

-60 <

for of

phase

E, %

-40

z .?30-

-

E

-30”; 0 ;i

s 20-

4.

diffused precipitate

E

zL g40-

Fig.

in t!he NbsZns

the

- 50 5

;: 50-

0'

specimen

980’ C. Note

-70

?O-

iiz

INbZn21 NbZn3

-20

I

IO

I

20 30 40 Distance (Microns)

Composition

190

200

Fig. 6.

across the Nb-Se diffusion

shown

in fig.

NbZn

I

3

couple

Nb-Zn

Zn diffused

specimen

in vacuum then

averaging for

35 weight

195 hours

in air at 980” C

percent

at 870” C and

diffusiou couples;

consequent Iy. the thickijesses

of t$he various layers in figs. 5 ailtl 6 may ha,vc

mu

a

0-m

tlistaiue of some 130 mictroiis. ‘I’lrr

nib&o2 pha,se shows voids near t*he outer edges

little sigllifica~l~e from the ~~i~llsio~~s~all~l~oi~~~. of the di~l~~il~il coul)le. There is no i~~ll~e(~ia~,~ However, the positive j(fellt,ificatioIl of the e~l)Ia~la,tjo~~ fi)r this ~~~lelionlel~(~~~ utilesr; it phases helps elucidate the kinetics of the represent,8 the laoduct,ion of a three-coml,onel,tprotective

action of Zn on Nb at tcmperatnres

of 1000* C and should prove invaluable formulation

of a phase diagram for this system.

Fig. Ei shows a Nb specimen coated

in t)he

in molten

which was dip

Zn and then heated

for 4X

system from a sligbt~ oxide or nitride contamination or is a physical t’o bimetal

action

straining of the zone tlire

during cooling.

The post~ulatetl Xb( !OZphase 4) was not, found in this investigat-ioil.

hours

in air at 980"c! to l~oduce the three dist,inct layers. Elect,ron probe measurements

Xi-T%

showed that the layer next to the Nb is a NbaZns matrix co~it.aining a ~re~ipit,ate of NbZn ; m3xts

Fig. 9 and 1.0 show the four distinct phases identified in the Ilickel-t~ant.alum system after

to this is a pure phase of NbZna and finally a

diff&ion for 36 hours at 1100” C. The wellknown TaNia comprised about 50 y0 of the zone area wit’h the other stoichiometric compounds TaNia, TaNi, and TaaNiz sharing the remainder.

NbZns phase. The specimen in fig. 6 was prepared by diffusing 35 wt y0 Zn into a Nb cube in vacuum at 870” C for 195 hours prior to heatSing in air at 980’ C. Only two metallic phases, NbZn and NbzZns, were present, t,ogether with an overlaying layer of niobium oxide. A most interesting feature here is that the NbZn appeared as a separate precipitate

and discret,e phase rather than as a in t,he Xb2Zna.

Nb-Co

Interdiffusion of niobium wit,h cobalt for DO hours at, 1100” C produced a three-phased zone having a total width of about, X60 microns as shown in figs. 7 and 8. The narrow 6-micronwide phase was identified as Nb&os, while the main zone graded from 27 at o/oNb down t,o

20 at o/oover a distance of 100 microns. At the

90

higher Nb percentage, this phase corresponds approximately to the previously assumed 3) Nb&os phase; however, t,he lower n’b percentage end of the zone agrees with the stoichiometric NbCo4. It is difficult to say whether t’hix phase represents a transition from Nb&os t’o NbCod or a NbCo4 matrix with up t’o 16.5 wt “/b Nb in solution at the rich end. It does trot, seem logical to consider t,his as NbaCob with Co in solid solution to the extent of 37 wt. 76. On the Co side of t,he d~ffus~ol~ zone, there is a HO-I~ti~ron-aide phase of constant 6 wt “/b Nb. After t,hat t.he Nb composition decreases to

80 - 70 E - 605 - 502 - 40*

10

20

30

40

100

110

Distance (Microns)

120

130

140

150

z e

ELEVATED

Severe cracking

and voiding is apparent, of TaNi

between the “grains” Ni

and

the

TaNi

100 o/o tantalum

TEMPERATURE

zone.

fig. 9,

and between the

Small

were found

265

DIFFUSION

“fingers”

80 70

of

60

within the TaNi

50

zone. These probably precipitated on cooling and represent an unknown solubility of Ta in TaNiz at 1100” C. The identifications

of TaaNiz and of TaNi

confirm the assumptions Speidel5)

of Kubaschewski

and Therkelsen 6) respectively

and con-

cerning the ~omposit,ions of t,he peritectically formed compounds. In addition the establishment of the presence

of a TaNi phase is in agreement wit.h t,he eutectic point established by Therkelsen.

:I/ , , , , I j IO Fig. 10.

20 30 40 Distance (Microns)

50

60

Composition across a Ni-Ta diffusion couple after 96 hours at 1100” C

Fe-MO _4

30-micron-wide

zone graded

from

39 to

at. o/0 (52-50 wt %) molybdenum was formed as a result of iron-molybdenum diffusion 37.5

at 1100” C for 66 hours. This corresponds to the i phase which has already been established for the Fe-MO phase diagram and designat’ed a,s MosFes wit,h a slight solubilit,y for iron. At the Fe-MoeFes interface there was continuous, erratic cracking, fig. 11, similar to that observed in several of the Nb couples reported in t.he first report. and always occurring near the Nb-poor int,erface.

%ig.

9.

~‘h~)t~~mi~rographs of

two different

Xi-Ta

diffusion zones. Note grain growth in the TaNis phase and severe cracking between the grains and the Ni. The finger-like precipitate seen in the TaNi A-TasNiz,

B-TaNi,

is pure Ta.

C-TaNin _t Ta.

Fig. 11. Fe-MO diffusion couple formed at 1100” C. Continuous erratic cracking occurs at the Fe-MozFea interface

go-

wit81z

the electron probe microanalyzer.

presents

80-

E g 70::

results

obtained

shown

that each ~1ltern~ediat.e l)hase

stoichiometric

-:mI

‘I’al)lc i

a list#ing of Lhese phases. All of the from

diffusion

composit,ion

cottplcs

expressible

IGLYC

is of a in small

whole numbers.

=: 50-

.I

TABLE

&I p 40-

Phases

= $303

20

furmed

during

binary

cliffusion

E

* ;i

20-

IO

0'

IO

20

30

Distance Fig.

12.

Composition couple

The concentration

40

Ni-Ta

the

in fig.

Fe-MO

diffusion

of MO across the diflksion

pure Fe. Conclusion identifications

of phases

of niobium

NbSrz

NbzZns,

NbsCoz,

XbZna,

NbZna

NbCof TaNiz,

TaNis,

KbPt:,

TaNi,

TaaNiz

MozFea

References ‘)

L. S. Birks,

resulting

with platinum,

and R.

E. Seebold,

with

Cr, Fe, Ni, Mo, and Stainless

Mat.

3 (1961)

Diffusion

of Nb

Steel. .J. 511~1.

249-459

2) L. S. Birks, J. Appl. Phys. 31 (1960) 1297 3, W. Koster and W. Mulfinger, Z. Metallkundc (1938)

from the diffusion

Nbl’tz,

11

MozFea were found and there was no appreciable diffusion of Fe into pure MO or of MO into

Positive

Fe-MO

NbPt,,

r\‘bSe, K b&a, NbZn,

, ! / j /

Nb-Co

60

zone is shown in fig. 12. No phases other than

5.

Nb-Zn

i

(Microns)

across shown

50

Nb,Pt,,

Nb-Pt Nb-Se

IO-

30

348-350

“1 H. S. ~Vallbaum, Z. K&t. 103 (1941) 391-402 5, 0. Kubaschewski and N. Speidel, J’. Inst. Metals 75 (1949)

417--430

6) E. Therkelsen,

Metals and Alloys

4 (1933) 105-108