joan mattuck george feick, john and francis galliganithis report was prepared as an account of...

Joan Berkowitz - Mattuck

and Francis Galligani George Feick, John

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National ~ ~ r o ~ ~ ~ ~ aftct %e ~d~imistratio Technical ~n~ormation Facility

NASA CR-72847 ADL 71965

FINAL REPORT

APPLICATION OF YTTRIUM COATINGS ON CHROMIUM-BASE ALLOYS BY METALLIDING

Joan Berkowitz-Mattuck,

George Feick, John O'Brien

and Francis Galligani

ARTHUR D. LITTLE, INC. Cambridge, Massachusetts 02140

Prepared f o r

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

March 31, 1971

CONTRACT NAS 3-13477

Technical Management NASA-Lewis Research Center Cleveland, Ohio 44135

Joseph R. Stephens, Project Manager

Arthur D Little, Inc.

ABS TRACT

An attempt was made to form a dense, adherent yttrium oxide-chromium oxide coating on a high strength

chromium alloy (Cr-7Mo-2Ta-0.09C-O.l(Y+La)), in order

to protect the alloy from nitrogen embrittlement due to

air exposure.

electrolytically from a molten fluoride bath.

Yttrium was deposited on the alloy surface

Excess

salt was removed by vacuum evaporation, and the mixed

Y203-Cr203 was formed by preoxidation at 1500'F (1089'K) in %/%O gas mixtures.

the necessary oxidation/nitridation resistance at 2100'F (1422'K), probably because of non-uniform coverage and

lack of coherency.

The coating failed to provide

iii

Arthur D Little, Inc

TABLE O F CONTENTS

Page

L i s t of Figures

L i s t of Tables

I. SUMMARY

11. INTRODUCTION

111. EXPERIMENTAL WORK

A. STARTING MATERIAL

B. METALLIDING PROCEDURES

C. CLEANING AND PRE-OXIDATION

D. CUTILE-BRITTLE TRANSITION TEMPERATURE T E S T S

E . OXIDATION T E S T I N G

I V . RESULTS

A. METALLIDING

B. DUCTILITY

C. OXIDATION

V. DISCUSSION

V I e CONCLUSIONS

V I I . RECOMMENDATIONS

v i i

i x

1

3

5

5

5

9

10

1 6

19

19

26

31

37

39

41

REFERENCES 43

V


LIST OF FIGURES

Figure No.

1 Optical Micrograph of Cross-Section of Chromium Alloy Cr-7Mo-2Ta-0.09C-Oe1Y Showing Inclusion

Content of Alloy

2 Metalliding Apparatus

3 Schematic Diagram of Metalliding Cell

4 Apparatus Used for Performance of DBTT Test

5 Load-Deflection Curve of a Ductile Chromium Alloy

Specimen

6 Load-Deflection Curve of a Brittle Chromium Alloy

Specimen

7 Photograph of Ductile and Brittle Chromium Alloy

Specimens

8 Schematic Diagram of Oxidation Test Apparatus

9 Optical Micrograph of Cross-Section of Chromium

Alloy Cr-7Mo-2Ta-Oa09C-0.1Y Showing Chromium

Boride Layer

Page

6

7

8

11

13

14

15

ii

20

vii


LIST OF TABLES

Table No.

I.

I1 a

I11 e

IV.

v.

VI.

VI1 . VI11 e

IX.

X.

Yttriding of Pretreated Alloy Specimens

Yttriding of As-Received Alloy Specimens

Effect of Time, Temperature, and Current Density on Yttriding of As-Received Chromium Alloy

S p ec imens

Effect of Cleanup Procedures on the Quality of

Yttrided Deposits

DBTT of the As-Received Chromium Alloy

DBTT of the Borided Chromium Alloy

Cyclic Oxidation Tests of


Cyclic Oxidation Tests of 12230K for 6 Minutes at

(1280 A/m2)


1223’K for 6 Minutes at

Sample N o . 40143

Sample No. 52862

Samples Yttrided at

0.825 A/in. 2

Page

22

23

25

27

29

30

32

34

35

Samples Yttrided at 36

0.825 A/in.’ (1280 A/m2)

ix

Arthur D Little,Inc.

I. S U m Y

Chromium alloys are highly susceptible to pickup of interstitial

impurities, especially nitrogen, at elevated temperatures. The success-

ful application of chromium alloys in airbreathing engines with long

life requirements is dependent on the development of coatings that will

both prevent nitrogen embrittlement and improve high 'iemperature oxidation resistance. Since nitrogen is virtually insoluble in oxides, a program was initiated to develop and evaluate a dense, adherent coating

of yttrium oxide-chromium oxide on a high strength chromium alloy

(Cr-7Mo-2Ta-0.09C-0.1 (Y+La)) as a means for protecting the alloy.

A three step coating procedure was used. Yttrium was deposited on the alloy surface electrolytically from a molten fluoride bath containing

77.8 wt % LiF and 22.2 wt % YF3.

using an yttrium anode, were identified as 1740°F (1223°K) for 360 seconds

Excess molten salt

Optimum electrodeposition conditions,

at a current density of 0.825 A/ina2 (1.28 x 10 3 A/m 2 ). was removed by vacuum evaporation at 1920°F (1323°K) for 600 seconds. mixed Y 0 -Cr203 was formed by preoxidation at 1500°F (1089°K) in H /H 0

The

2 3 2 2 mixtures.

Coated samples were screened for oxidation/nitridation resistance

in cyclic furnace tests at 2100°F (1422'K) in slow flowing dry air.

Spalling was observed after 100-160 hours total time at temperature, and

net weight gains exceeded 30 mg/in.:! (0.0465 kg/m ) prior to the onset

of sp all ing e

2

As received chromium alloy samples had a DBTT of approximately 625°F

(602°K).

DBTT by at least 100°F (55"K), although several yttrium coated samples

retained ductility at 650°F (616°K). All samples were brittle at

800°F (700°K) after the cyclic oxidation test.

Fused salt electrodeposition of yttrium usually increased the

The failure of the coating system developed to provide the requisite

oxidation/nitridation resistance is ascribed to a lack of uniformity and

coherency in the Y 0 -Cr 0 formed. 2 3 2 3

1


11. INTRODUCTION

Utilization of chromium alloys for jet engine components in long-

time service applications (600-3000 hours) at temperatures between 2100°F (1422°K) and 2400°F (1589°K) is critically dependent on the development

of oxidationlnitridation resistant coatings.

have been formulated with promising ductility in the as fabricated condi-

tion. However, these alloys are drastically embrittled by absorption of nitrogen during high temperature exposure. Furthermore, the oxidation

resistance of the high strength alloys is inadequate above about 2000°F (1367 O K ) .

High strength chromium alloys

The choice of coating materials that might inhibit nitrogen diffusion

to a chromium alloy substrate is limited by the potential embrittling ef- fects of the coating components. While the data in the literature is

often conflicting, it has been reported that additions of titanium, nickel,

aluminum, silicon, tantalum, manganese,beryllium, iron, tungsten (ref.1)

and boron (ref.2) all raise the ductile-brittle transition temperature

(DBTT) of chromium. Since coating components commonly diffuse into the

substrate during high temperature service, elements that raise the DBTT

of the substrate are to be avoided in environmental coatings for chromium

alloys. It has been reported that additions of zirconium, yttrium, cesium

and several other rare earths lower the DBTT of chromium, while gold and

silver do not affect the DBTT (ref.2). The compatibility of copper with

chromium alloys is uncertain (refs. 1 and 21, and recent data on silicon (ref.3) suggests that it may not have a severe embrittling effect.

The purpose of the present program was the development of a coating

system for a high strength Cr alloy, Cr-7Mo-2Ta-O.09C-Oe1(Y+La), which

would provide oxidation/nitridation resistance for a minimum of 200 hours

at 2100°F (1422°K) with consistent retention of ductility, and total

oxidation-nitridation weight change of less than 3 mg/ine2 (4.65 x 10

kg/m ) .

to provide the necessary protection. Oxides are well-recognized as excel-

lent diffusion barriers for nitrogen. It is almost impossible to sinter oxides in the presence of nitrogen, for example, since there is no mechanism

by which nitrogen can diffuse out of closed pores (ref04)*

-3

2 The basic approach was to use an yttrium-chromium oxide coating

3


While t h e l a c k of d u c t i l i t y of ox ides i s a d e t e r r e n t t o t h e i r use

g e n e r a l l y , a t h i n dense l a y e r of o x i d e , p o r e f r e e , should provide t h e neces-

s a r y b a r r i e r t o n i t r o g e n wi thout s e r i o u s l y degrading t h e p r o p e r t i e s of t h e

a l l o y such as t h e notch s e n s i t i v i t y , o r t h e DBTT of t h e e n t i r e system.

The complex y t t r i u m chromium oxide w a s s e l e c t e d as t h e b e s t ox ide

f o r t h e c u r r e n t a p p l i c a t i o n , on t h e b a s i s of t h e f i n d i n g s of Seybol t ( r e f . 5 )

and o t h e r s ( r e f s . 6-8) w i t h r e s p e c t t o t h e o x i d a t i o n r e s i s t a n c e of

Y-containing chromium a l l o y s . Seybol t p r o v i d e s good evidence t h a t Y 0

d i s s o l v e s i n C r 0 and t h a t Y C r O formed a t t h e a l l o y oxide i n t e r f a c e ,

e f f e c t i v e l y b locks d i f f u s i o n of chromium. Even more important f o r o u r

purposes , s i n c e w e are p r i m a r i l y i n t e r e s t e d i n Y C r O as a b a r r i e r f o r

n i t r o g e n , i s t h a t t h e adherence of t h e complex Y 0 - C r 0 oxide scale i s

v a s t l y b e t t e r t h a n t h a t of p u r e C r 2 0 3 , probably as a r e s u l t of a keying

e f f e c t of Y 0 p a r t i c l e s . 2 3

2 3

2 3’ 3’

3

2 3 2 3

The method explored f o r forming t h e d e s i r e d Y203-Cr 0 c o a t i n g w a s 2 3 based on a two s t e p p r o c e s s involv ing f i r s t , e l e c t r o l y t i c d i f f u s i o n of

y t t r i u m o n t o t h e a l l o y i n a f u s e d s a l t b a t h , and t h e n c o n t r o l l e d low pres-

s u r e o x i d a t i o n i n a n i t r o g e n - f r e e atmosphere t o form t h e mixed Y 0 - C r 2 O 3

p r o t e c t i v e b a r r i e r . 2 3

4


111. EXPERIMENTAL WORK

A. S t a r t i n g Material

W e r ece ived 90 p i e c e s of Cr-7Mo-2Ta-Oe09C-0.1(Y+La) a l l o y from NASA, which w a s d iv ided i n t o t h r e e l o t s l a b e l e d A, B and C. Microscopic examin-

a t i o n of t h i s material showed t h a t it conta ined a l a r g e number of i nc lu -

s i o n s i n agreement wi th t h e r e s u l t s of S l augh te r e t a1 ( r e f . 9 ) , F igu re 1

shows a photomicrograph of t h e a l l o y s t r u c t u r e ( l o t C ) .

The DBTT of t he a l l o y w a s found t o be 600 t o 625°F (589 t o 602°K).

Details of t h e DBTT measurements are g iven i n s e c t i o n D below.

The a l l o y w a s c u t i n t o s t r i p s 2" (5 .1 cm) l ang by 1/2" (1.3 cm) wide

by about 1/16" (0.16 cm) t h i c k by means of a f i n e s i l i c o n ca rb ide wheel.

A h o l e w a s d r i l l e d i n t o one end of each s t r i p by which i t could be hung

i n t h e m e t a l l i d i n g c e l l , and la ter i n t h e o x i d a t i o n furnace . P r i o r t o

u s e , t h e s t r i p s w e r e c a r e f u l l y c leaned by scrubbing wi th de t e rgen t and

d r i e d wi th methanol e

B. M e t a l l i d i n g Procedures

A g e n e r a l view of t h e m e t a l l i d i n g appa ra tus i s shown i n F igure 2 and

a schematic diagram of t h e c e l l i s g iven i n F igu re 3. The p r i n c i p l e of

t h e o p e r a t i o n i s similar t o e l e c t r o p l a t i n g except t h a t it is c a r r i e d ou t

a t high tempera ture i n a molten s a l t e l e c t r o l y t e , under cond i t ions such

t h a t i o n s reduced a t t h e cathode d i f f u s e i n t o i t s s u r f a c e . A l l r uns were

c a r r i e d o u t i n an argon atmosphere except as noted .

Pre-bor id ing , abandoned e a r l y i n t h e program, was c a r r i e d o u t i n a

e u t e c t i c mix tu re of KF, LiF and NaF c o n t a i n i n g about 0.2 mole % boron added

by bubbl ing gaseous BF i n t o t h e molten sal t . The anode c o n s i s t e d of 99.8% 3 pu re c r y s t a l l i n e boron lumps, .2-21 i n . (5-50 mm) i n d iameter , he ld i n a

copper mesh Easke t , 5" (12.7 cm) long and 1" (2.54 cm) i n diameter., The mesh

s i z e i s 1 2 w i r e s / i n . (471 w i r e s / m ) , The chromium a l l o y , suspended from a

1/8" (0 ,32 cm) tungs t en rod , se rved as t h e cathode, Two t o s i x a l l o y

samples are bor ided s imul taneous ly . For t h e pre-boriding s t e p , t h e c e l l w a s

opera ted a t 1650'F (1173°K) wi th a c u r r e n t d e n s i t y of 30 A / i n a 2 (46.5 A/m )

f o r t h e i n i t i a l 1-1/2 hours , and 20 A/ ine2 (31 ,0 A/m ) f o r t h e nex t 1.7 hours .

2

2

5

FIGURE 1: OPTICAL MICROGRAPH OF CROSS SECTION OF CHROMIUM ALLOY Cr-7Mo-2Ta-0 e 09C-0.7 Y SHOWING INCLUSION CONTENT OF ALLOY

Etched electrolytically in a Solution of Sodium Hydroxide. Magnification 62.5X

6


FIGURE 2: METALLIDIMG APPARATUS

7


GLASS E

I NSULAT

:LE CTRODE

'ED DOORS

' GAS

COOLED

GA I ,S KET

FIGURE 3 SCHEMATIC DIAGRA OF M E T A L L I D I N G CELL

8

Arthur I 1 I -it t IC, I nc

dur ing overn ight o p e r a t i o n . Except f o r a few pre l iminary runs which were

a c c i d e n t a l l y made under n i t r o g e n , an argon atmosphere w a s used. This

procedure i s e s s e n t i a l l y t h a t of t h e Cook p a t e n t ( r e f . 1 0 ) ass igned t o t h e

General E lec t r ic Company.

The b a s i c d e s i g n of t h e y t t r i d i n g appara tus w a s t h e same as t h a t f o r

b o r i d i n g , except f o r a few minor m o d i f i c a t i o n s . A n i c k e l screen w a s b u i l t

t o f i t i n t o t h e top p a r t of t h e c e l l t o hold y t t r i u m c h i p s , which can be

used i n t h i s system f o r g e t t e r i n g t h e argon t o a s s u r e u l t r a - c l e a n condi-

t i o n s . The e l e c t r o l y t e i s 77.8 w t % LiF and 22.2 w t % YF The anode 3" c o n s i s t s of two p u r e 1" (2.54 cm) y t t r i u m cubes, s t r u n g onto an 1/8"

(0.32 cm) t u n g s t e n rod.

as-received o r borided. Before t r y i n g t o y t t r i d e any of t h e chromium

samples, a number of n i c k e l cathodes w e r e y t t r i d e d f o r c e l l clean-up pur-

poses. I n t h i s o p e r a t i o n i m p u r i t i e s as w e l l as y t t r i u m are depos i ted on

t h e n i c k e l . Clean, s t a b l e o p e r a t i o n is i n d i c a t e d when t h e measured g a i n

i n weight of t h e n i c k e l cathode corresponds t o t h e e q u i v a l e n t c a l c u l a t e d

from t h e charge f low i n ampere-hours.

The cathode i s a g a i n t h e chromium a l l o y e i t h e r

C . Cleaning and Pre-Oxidation

One of t h e problems w i t h t h e p r e s e n t y t t r i d i n g p r o c e s s i s t h e removal

of t h e t i g h t l y adher ing l a y e r of s o l i d i f i e d e l e c t r o l y t e which covers t h e

sample s u r f a c e a f t e r t rea tment . One way of removing t h i s is t o t a k e ad-

vantage of t h e s m a l l b u t f i n i t e s o l u b i l i t y of LiF i n water. The coated

samples w e r e h e l d i n a l a r g e excess of h o t water f o r a number of hours o r

sometimes overn ight . This t rea tment s o f t e n e d t h e s a l t l a y e r enough so

t h a t i t could b e scrubbed o f f w i t h a steel brush. I n many cases, however,

no a p p r e c i a b l e c o a t i n g remained a f t e r t h i s t rea tment .

Cleaning methods based on water soaking and scrubbing are open t o

t h e s u s p i c i o n t h a t any l a y e r of depos i ted y t t r i u m w i l l be des t royed by

t h e c l e a n i n g process .

A c l e a n i n g method which is f r e e from t h i s o b j e c t i o n involves subl iming

o f f t h e f l u o r i d e sal ts a t h igh temperature i n a good vacuum. The vapor pres -

s u r e of LiF is known t o b e 1 t o r r ( 1 3 3 N/mZ) a t 2100°F(14220K), bu t w e were

9


unable t o f i n d d a t a on YF3* Analogy w i t h s i m i l a r f l u o r i d e s , however, sug-

g e s t s t h a t i t s vapor p r e s s u r e w i l l b e cons iderably lower t h a n t h a n of LiF.

Experiments w e r e made by h e a t i n g sa l t -covered , y t t r i d e d chromium a l l o y -4 2 samples i n a vacuum of 3 x 10

1920°F (1323'K).

s i s t o r and t h e f u r n a c e w a s evacuated w i t h an o i l - d i f f u s i o n pump equipped

wi th a water-cooled t r a p . The chamber w a s purged twice w i t h argon p r i o r t o

r a i s i n g t h e temperature .

t o r r ( 4 x lom2 N/m ) f o r ten minutes a t

The samples w e r e h e a t e d by r a d i a t i o n from a g r a p h i t e re-

Pre-oxida t ion t r e a t m e n t s w e r e done f o r one hour a t 1500°F (1089°K) i n

H /H 0 mixtures w i t h hydrogen a t a i r temperature f lowing through w a t e r a t

200°F(366"K),

o x i d a t i o n of Cr-0.22Y g r e a t l y improved t h e n i t r i d a t i o n r e s i s t a n c e of t h e

a l l o y .

2 2 Work a t Westinghouse ( re f .11) had p r e v i o u s l y shown t h a t pre-

D. D u c t i l e - B r i t t l e T r a n s i t i o n Temperature Tests

The a p p a r a t u s used f o r performance of t h e DBTT test i s shown, as i t

i s mounted on t h e I n s t r o n , i n F igure 4. It can be s e e n t h a t t h e bottom

s u p p o r t f o r t h e specimen i s a t t a c h e d t o t h e l o a d c e l l . The bottom span is

1 i n c h (2.54 cm), w i t h s h o u l d e r s a t t h e specimen s u r f a c e of 0.050 inch

(0.127 cm). Radius of c u r v a t u r e of t h e punch i s 0.250 i n c h (0.635 cm)

( z 4 t ) . Support rods t o t h e crosshead and load c e l l are broken w i t h Micarta

i n s e r t s t o reduce h e a t f l o w along t h e suppor ts . I n a d d i t i o n , t h e top and

bottom s u p p o r t s are water cooled w i t h soldered-on copper c o i l s . The f u r -

nace i s c o n t r o l l e d by a c o n t r o l l e r which r e a d s an i ron-cons tan tan thermo-

couple a t t a c h e d t o t h e bottom specimen suppor t about 1 cm from t h e specimen.

An impor tan t f e a t u r e of t h e DBTT tes t i s t h e "clam s h e l l " opening of

t h e f u r n a c e which a l lows specimens t o b e i n s e r t e d i n t h e lower jig and

removed wi th ease. Approximately two specimens can be run p e r hour i n t h e

f u r n a c e ; t h u s , many specimens can be t e s t e d each day. The h i g h e s t o p e r a t i n g

temperature of t h e furnace i s 1200°F (922°K).

i n test e v a l u a t i o n of t h e chromium a l l o y is 1000°F (811°K).

b e no reason t o t es t a material w i t h a DBTT above t h i s temperature .

The h i g h e s t temperature used

There should

1 0

Arthur 1) little,Inc

FIGURE 4 : APPARATUS USED FOR PERFORMANCE OF DBTT TEST

11


When a sample i s p u t i n t h e fu rnace i t i s no t t e s t e d u n t i l it has been

a t t h e d e s i r e d tempera ture f o r a t least f i v e t o t e n minutes. It i s assumed

t h a t t h i s t i m e i s s u f f i c i e n t f o r uniform tempera ture t o be ob ta ined i n t h e

sample r e g i o n of t h e furnace .

During t h e DBTT tes t t h e c rosshead speed i s 2 c m p e r minute. This i s

as c l o s e t o t h e sugges ted 1 inch p e r minute (0 .42 m m / s e c ) g iven i n t h e MAB

s p e c i f i c a t i o n s as can be achieved wi th t h e metric I n s t r o n a t ADL. For con-

venience t h e c h a r t speed used i s 20 cm p e r minute, which means t h a t 1 c m

of c h a r t t ravel corresponds t o 1 mm of crosshead travel. It has been found

t h a t a s e t t i n g of 100 kg f u l l scale of t h e load ce l l i s s u f f i c i e n t f o r t h e

DBTT tests.

I n t h e DBTT tes t , t h e sample i s cons idered t o be d u c t i l e i f t h e c ross -

head t r a v e l s 10 mm and t h e specimen i s i n t a c t . With t h i s travel t h e spec i -

men bends t o a 90' angle . A s r e p o r t e d h e r e , t h e DBTT is t h e lower test

temperature a t which t h i s c o n d i t i o n i s s a t i s f i e d .

. I n i n i t i a l tes ts on t h r e e samples, an approximate va lue of t h e DBTT

w a s ob ta ined by pushing t h e samples 2 .mm, t hen dropping t h e tempera ture i n

50°F (28'K) i n t e r v a l s and pushing t h e same amount u n t i l t h e samples broke.

It w a s subsequent ly found t h a t t h e DBTT obta ined by pushing a number of

samples a t unique tempera tures i s 25'-50°F (14-28'K) lower than t h a t ob ta ined

by s e q u e n t i a l l y dropping t h e tempera ture of a sample a f t e r l i m i t e d deforma-

t i o n . For t h i s reason t h e p re l imina ry technique w a s dropped i n f avor of

o b t a i n i n g t h e DBTT by deforming samples a t unique tempera tures .

r e p o r t e d w e r e ob ta ined by t h e l a t t e r technique .

All DBTT's

Typica l l oad -de f l ec t ion curves from t h e I n s t r o n f o r t h e a l l o y i n t h e

d u c t i l e and b r i t t l e cond i t ions , i r r e s p e c t i v e of i t s i n i t i a l c o n d i t i o n , are

shown i n F igu res 5 and 6. A s seen i n F igu re 4 , when a sample b reaks , even

i n i t s most b r i t t 1 6 cond i t ion , y i e l d i n g precedes f r a c t u r e , and about 1 mm

of p l a s t i c d e f l e c t i o n can occur . No samples broke i n t h e s e tests i n t h e

absence of p l a s t i c deformation. When a sample does f r a c t u r e , i t i s ca ta -

s t r o p h i c , and t h e f r a c t u r e cannot be a r r e s t e d by s topping t h e I n s t r o n .

Represen ta t ive tes t samples a f t e r d u c t i l e and b r i t t l e behavior are

shown i n F igu re 7. It is seen t h a t t h e r e i s no ambiguity between d u c t i l e

1 2

Arthur 1) Little, Inc

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14


FIGURE 7 : PHOTOGRAPH OF DUCTILE AND BRITTLE CHROMIUM ALLOY SPECIMENS (SPECIMENS 9 AND 10)

15


and b r i t t l e samples. l i n e wi th t h e a x i s of t h e punch. F r a c t u r e w a s n o t s een around t h e h o l e

used t o suppor t t h e samples i n t h e m e t a l l i d i n g cel ls , s i n c e t h e h o l e is

too f a r from t h e s t r e s s e d r eg ion t o i n f l u e n c e f r a c t u r e .

m e n a sample f r a c t u r e d , t h e f r a c t u r e w a s always i n

E. Oxida t ion T e s t i n g

Oxida t ion sc reen ing tests were c a r r i e d ou t on a s e l e c t e d se t of op-

t i m a l l y coa ted samples. The specimens w e r e suspended on a s t a i n l e s s steel

j i g and i n s e r t e d i n t o a pre-heated M u l l i t e t ube , surrounded by a Globar

( s i l i c o n ca rb ide ) furnace . A i r from a compressed a i r tank w a s d r i e d over

potassium p e r c h l o r a t e and Drierite, and w a s flowed s lowly p a s t t h e samples

a t one atmosphere p r e s s u r e . A schematic diagram of t h e test appa ra tus is

shown i n F igu re 8. For t h e f i r s t twenty hours , samples w e r e r epea ted ly

hea ted a t 2100'F (1422'K) f o r two hours , a i r quenched t o room temperature

and weighed, t hen rehea ted . For t h e nex t 180 hours , o r u n t i l f a i l u r e ,

twenty hour h e a t i n g c y c l e s were used.

16

Arthur D Little,Inc

IV. RESULTS

A. Metalliding

It was recognized very early in the program that the low solubility of yttrium in chromium and the absence of intermetallics in the Y-Cr

binary system precluded the possibility of forming an yttrided diffusion

layer on the chromium alloy surface by the metalliding process.

seem possible, in principle, to enrich the alloy surface in yttrium by

pre-treating with boron or nickel. Yttrium forms intermetallics with both these elements. The experiments summarized in Table I were done to assess the feasibility of the pre-treatment approach. Figure 9 shows the micro- structure of the borided alloy.

It did

Yttriding of the borided chromium alloy samples (Runs 11310 and 11413)

resulted in a weight gain which was only 15% of that which would have been observed if all the yttrium arriving at the surface diffused into the samples.

CrB to be present. The boriding pre-treatment results in the formation 2 of CrB Although there is every reason to expect that yttrium can be metallided on boron, with the formation of one or more yttrium boride phases, yttrium apparently cannot

readily be electrochemically diffused into CrB2.

mens showed an unduly high DBTT (cf. IV., B.) the pre-boriding approach was not pursued further.

X-ray diffraction of a borided sample after yttriding shows only

by diffusion of boron into the alloy surface, 2

Since all borided speci-

Three alloy specimens from lot "A" were electroplated with nickel to a depth of, 0.5 mil(,Ol mm) (runs 31339, 31340 and 31641). The plated samples

were yttrided at 1740°F(1223'K) and a current density of 0.4375 A/in.

(679 A/m ) for times of 1,3, and 6 minutes, to achieve various depths of yttrium penetration. X-ray analysis clearly identified the presence of several nickel-yttrium phases. The DBTT was of the order of 750-775'F(672-686"K).

Preliminary experiments with these samples indicated that the coating tended

to blister in water despite attempts to diffuse the nickel into the base

alloy by annealing in the metalliding bath for about an hour before yttriding.

2

2

All the nickel plated samples were brittle at 700'F (644'K)

19


CHROMIUM BORIDE LAYER

FIGURE 9: OPTICAL MICROGRAPH OF CROSS SECTION OF CHROMIUM ALLOY Cr-7Mo-2Ta-0.09C-O.lY SHOWING CHROMIUM BORIDE LAYER

Etched Electrolytically in a Solution of Sodium Hydroxide. Magnification 250X

20


and one a t 800'F (700'K) e Nickel p re -p la t e s of 0.2 and 1 m i l (0.5 x

and 2.5 x 1 0 mm) o f f e r e d no advantage over t h e 0.5 m i l (1.3 x 10 mm)

p l a t e .

-2 -2

S ince t h e p r e - t r e a t e d samples of Table I a l l e x h i b i t e d an i n c r e a s e i n

DBTT as compared t o t h e as-received a l l o y , it w a s of i n t e r e s t t o determine

t h e e f f e c t of y t t r i d i n g p e r se on d u c t i l i t y . Run 11312 w a s made wi th an as-

r ece ived sample under t h e y t t r i d i n g c o n d i t i o n s of run 11310, where t h e

sample has been pre-borided. While no s i g n i f i c a n t amount of y t t r i u m w a s

depos i t ed i n e i t h e r case, t h e as - rece ived sample remained d u c t i l e .

The remainder of t h e program w a s d i r e c t e d towards op t imiza t ion of t h e

y t t r i d i n g parameters f o r as-received a l l o y specimens. I f y t t r i u m could

n o t be d i f f u s e d i n t o t h e a l l o y , t h e r e w a s s t i l l a p o s s i b i l i t y t h a t y t t r i u m

might be p l a t e d o u t on to t h e s u r f a c e , a l though fused s a l t e l e c t r o p l a t i n g

i n g e n e r a l r e s u l t s i n d e n d r i t i c o r powdery d e p o s i t s .

A s i n d i c a t e d i n Table 11, t h e as - rece ived chromium a l l o y samples were

i n i t i a l l y y t t r i d e d a t 1700'F(1200'K) f o r 5-1/2 minutes a t a c u r r e n t d e n s i t y 2 2 of 1 3 1 A/in. (2030 A/m ) ( r e f .12 ) . X-ray d i f f r a c t i o n revea led an unknown

cub ic phase on t h e s u r f a c e which w a s subsequent ly shown by X-ray f luo rescence

t o c o n t a i n y t t r i u m .

a me ta l log raph ic s e c t i o n i n d i c a t e d t h a t t h e r e w a s no d i f f u s e d l a y e r p r e s e n t .

This y t t r i u m may have been due t o r e s i d u a l salt , s i n c e

An as- rece ived chromium a l l o y sample from Lot C , 1/2" x 2" (1.27 cm x

5.08 c m ) , w a s y t t r i d e d a t 1775'F(1238'K) f o r 180 minutes a t a c u r r e n t d e n s i t y

of 0.163 A/in. (252 A/m ) (Run No.13023). A f t e r y t t r i d i n g , t h e sample w a s

scrubbed i n h o t water t o romove t h e f l u o r i d e salts on i t s s u r f a c e and from

i t s appearance w a s c l ean . This sample w a s t hen examined by X-ray d i f f r a c t i o n

and X-ray f luo rescence .

on ly elements noted w e r e t h o s e of t h e s u b s t r a t e , X-ray d i f f r a c t i o n showed

t h e presence of t h e compound C r C on t h e s u r f a c e . Apparent ly , t h e a l l o y

has picked up carbon i n t h e ce l l .

YC w a s confirmed by X-ray d i f f r a c t i o n , and some evidence w a s found f o r s u l -

f u r and n i t r o g e n i m p u r i t i e s as w e l l . S ince t h e m e t a l l i d i n g process demands

a ve ry c l e a n system, anhydrous HF w a s bubbled through t h e s a l t b a t h f o r 1-1/2

hours a t 1775'F(1238'K) i n an a t tempt t o remove t h e i m p u r i t i e s as v o l a t i l e

2 2

Yt t r ium w a s no t d e t e c t e d by f luo rescence and t h e

23 6 The presence of carbon i n t h e s a l t b a t h as

2

C2H2' H2S, and NH2. A f t e r t h e clean-up procedure wi th HF, a chromium a l l o y

21


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specimen from l o t "A" w a s y t t r i d e d a t 1740'F (1223'K) f o r 3 hours a t a cur-

r e n t d e n s i t y o f 0.163 A/ inB2(252 A/m ) (Run No.30634).

a n a l y s i s s t i l l showed t h e presence of a chromium c a r b i d e phase C r

t h e s u r f a c e . A ser ies of f i v e y t t r i d i n g runs w e r e c a r r i e d o u t under s i m i -

l a r c o n d i t i o n s wi th one m i l chromium p l a t e d s t ee l cathodes, 2" x 6"

(5 .1 x 15.2 c m ) , t o remove t h e r e s i d u a l carbon. F i n a l l y , f o u r chromium

a l l o y specimens from l o t "A" were y t t r i d e d i n l i k e manner, and were found

t o b e f r e e from carb ide . Unfor tuna te ly , no y t t r i u m appeared t o have d i f -

fused i n t o t h e a l l o y on t h e b a s i s of X-ray evidence.

2 X-ray d i f f r a c t i o n

23'6 at

Continued e f f o r t d i d r e s u l t i n some y t t r i u m d e p o s i t i o n a t t h e a l l o y

s u r f a c e , a l though as i n d i c a t e d i n Table I1 r e s u l t s were h i g h l y non-

reproducib le . Runs 40143, 40645, and 40746, f o r example, w e r e made under

nominally t h e same c o n d i t i o n s , b u t t h e appearance of t h e c o a t i n g s and t h e

e x t e n t of embr i t t l ement of t h e s u b s t r a t e were v e r y d i f f e r e n t .

The runs summarized i n Table I11 w e r e made t o determine t h e e f f e c t s

of t i m e , t empera ture , and c u r r e n t d e n s i t y on c o a t i n g q u a l i t y . Runs 41553,

41451, and 4135013 w e r e made a t t h r e e d i f f e r e n t temperatures under condi-

t i o n s of c o n s t a n t c u r r e n t d e n s i t y and t i m e . A l l c o a t i n g s w e r e s p o r t y and

non-uniform, and t h e DBTT w a s of t h e o r d e r of 100°F ( 5 5 ° K ) h i g h e r than t h a t

of t h e as-received a l l o y f o r a l l coated specimens. Thus, no important e f -

f e c t s of temperature w e r e observed. Runs 41452T and 41452B were designed

t o e v a l u a t e t h e e f f e c t s of h igh c u r r e n t dens i ty-shor t t i m e c o n d i t i o n s and

low c u r r e n t densi ty- long t i m e c o n d i t i o n s . Both c o n d i t i o n s l e d t o more

severe embr i t t l ement than t h e prev ious i n t e r m e d i a t e t ime-current d e n s i t y

c o n d i t i o n s . Run 41350B, made w i t h n i c k e l p l a t e d specimens a l s o r e s u l t e d

i n s e r i o u s embr i t t l ement .

From t h e experiments summarized i n Table 111, it w a s impossible t o

i d e n t i f y optimum y t t r i d i n g condi t ions . None of t h e c o a t i n g s were uniform

i n appearance; a l l seemed t o e m b r i t t l e t h e s u b s t r a t e . Furthermore, f o u r

samples from Run No. 41451 f a i l e d c a t a s t r o p h i c a l l y w i t h i n f o u r hours i n

t h e c y c l i c o x i d a t i o n tes t as d e s c r i b e d i n III., E. R e s u l t s w e r e so much

poorer t h a n t h o s e obta ined previous ly from Run No. 40143, made under normal-

l y s i m i l a r c o n d i t i o n s , t h a t w e f e l t t h e s a l t b a t h must have become contam-

i n a t e d e

24

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Numerous a t t empt s w e r e made, as summarized i n Table I V , t o c l e a n up

t h e c e l l and t o o b t a i n an opt imized y t t r i u m d e p o s i t . Yt t r ium n i t r i d e w a s

found f a i r l y c o n s i s t e n t l y on t h e s u r f a c e of y t t r i d e d samples, a l though t h e

c e l l w a s c o n t i n u a l l y purged wi th argon dur ing ope ra t ion , and a t t empt s were

made t o remove n i t r o g e n from t h e b a t h by adding HF and Y 0 The la t te r ,

i t w a s thought , would release as NH (g) any n i t r o g e n he ld as NH F, and

indeed a l k a l i n e fumes were d e t e c t e d a f t e r t h e oxide a d d i t i o n s . Incorpor-

a t i o n of a g a t e v a l v e , t o prevent back-d i f fus ion of n i t r o g e n , w a s beyond

t h e scope of t h e program. The presence of YN on t h e s u r f a c e appa ren t ly

d id n o t e m b r i t t l e t h e s u b s t r a t e a l l o y . A sample from Run No. 52862 proved

t o be d u c t i l e a t 650°F (616°K) i n s p i t e of t h e s u r f a c e n i t r i d e . This r e s u l t

i s n o t too s u r p r i s i n g , s i n c e t h e g e t t e r i n g of n i t r o g e n by y t t r i u m a t t h e

s u r f a c e of t h e a l l o y could a c t u a l l y p reven t t h e d i f f u s i o n of n i t r o g e n i n t o

t h e base metal . A t h i n b r i t t l e s u r f a c e l a y e r , fur thermore , need no t

n e c e s s a r i l y e m b r i t t l e t he system.

2 3"

3 4

It can b e seen from Table I V t h a t t h e vacuum evapora t ion s t e p used t o

remove r e s i d u a l sa l ts r e s u l t e d i n s u r f a c e ox ida t ion . This i s of cour se a -4 2 r e s u l t of t h e r e l a t i v e l y poor vacuum used (about 10

I n a number of runs , y t t r i u m b o r a t e s were found a f t e r vacuum evapora t ion .

I n t e n s i v e c l ean ing removed t h e sou rce of d i f f i c u l t y .

t o r r ) (1 .3~10-~N/m ) .

Optimum d e p o s i t i o n c o n d i t i o n s were i d e n t i f i e d as 1740°F(1223"K) f o r 6 2

minutes a t a c u r r e n t d e n s i t y of 0.825 A/in.2(1280 A/m ). These c o n d i t i o n s

gave t h e most c o n s i s t e n t r e s u l t s and t h e b e s t adherent d e p o s i t s . Although

y t t r i u m w a s s u c c e s s f u l l y p l a t e d on t h e chromium a l l o y s u r f a c e s , t h e d e p o s i t s

w e r e f o r t h e most p a r t non-uniform, t h i n , grey, and probably no t coherent .

I n no case w a s any i d e n t i f i a b l e d i f f u s i o n coa t ing formed on t h e chromium

a l l o y .

B. D u c t i l i t y

1. As-Received Chromium Alloy

The t h r e e p re l imina ry samples from l o t "C" r e f e r r e d t o above gave

a DBTT of approximately 625°F (602°K).

t u r e s , t h e DBTT obta ined w a s 600°F (589°K). Data f o r t h e s e tests are g iven

On samples pushed a t s i n g l e tempera-

26

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27

Arthur 11 Little, lnc.

i n Table V. I n t h i s t a b l e t h e amount of crosshead d e f l e c t i o n a f t e r y i e l d -

i n g and t h e presence o r absence of f r a c t u r e i s noted.

To determine i f the tempera ture and t i m e used f o r b o r i d i n g might i n f l u -

ence t h e DBTT, several samples w e r e vacuum annealed a t 1650°F(1173'K) f o r

n i n e t e e n hours under t h e temperature-time c o n d i t i o n s used f o r bor id ing .

The samples from l o t "C" w i t h as-received s u r f a c e s w e r e s e a l e d i n evacu-

a t e d q u a r t z tubes and t h e n annealed. It w a s noted t h a t t h e s e samples were

s l i g h t l y darkened a f t e r t h e annea l , i n d i c a t i n g t h a t they picked up oxygen.

The test r e s u l t s are given as Samples 15 and 16 i n Table V.

(644"K), Sample 15 w a s n o t f a r from be ing d u c t i l e .

temperature of 725'F (658°K) t h i s sample would have been d u c t i l e . Although

t h e i n d i c a t e d DBTT i s about 100°F (55°K) h i g h e r than t h a t of t h e as-received

a l l o y , i t i s thought t h a t t h e o x i d a t i o n of t h e sample dur ing annea l ing h a s

r a i s e d t h e DBTT, r a t h e r t h a n t h e annea l ing t rea tment . It w i l l be noted

l a t e r t h a t t h e DBTT of t h e bor ided samples i s about 30C"F (422'K) higher

than t h a t of t h e as-received samples w i t h t h e same c o n d i t i o n of t i m e and

temperature . Thus, it would appear t h a t annea l ing , p e r se, dur ing t h e

b o r i d i n g treatment h a s l i t t l e o r no i n f l u e n c e on t h e DBTT.

A t 700°F

P o s s i b l y a t a test

2. DBTT of t h e Borided Chromium Alloy

T e s t r e s u l t s on DBTT of t h e borided l o t "C" chromium a l l o y are

shown i n Table V I .

w a s 950°F (783'K). This means t h a t t h e DBTT of t h e borided samples i s

900-950°F (775-783"K), about 300'F (422'K) h i g h e r than t h a t of t h e as-

r e c e i v e d samples. The exact DBTT has n o t been obta ined , b u t t h e r e i s no

doubt about t h e s e r i o u s increase i n DBTT caused by bor id ing .

The lowest temperature noted t o o b t a i n a d u c t i l e sample

It w a s found t h a t t h e bor ided samples oxid ized dur ing t h e DBTT tes t .

A d a r k c o a t i n g formed on t h e samples i n d i c a t e d poor o x i d a t i o n r e s i s t a n c e .

I n c o n t r a s t , t h e as-received a l l o y samples showed no evidence of d i s -

c o l o r a t i o n a t tes t temperatures as h igh as 800°F (700°K). P o s s i b l y t h e

poorer o x i d a t i o n r e s i s t a n c e of t h e bor ided samples c o n t r i b u t e s t o t h e i r

cons iderably h i g h e r DBTT.

To determine i f a b e t t e r s u r f a c e b e f o r e b o r i d i n g would improve t h e

DBTT, one sample w a s po l i shed on 600 g r i t b e f o r e bor id ing . This sample,

28


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Arthur D Little, In

No. 1 3 as shown i n Table V I , w a s b r i t t l e a t 800°F (700°K).

t h e high DBTT of t h e bor ided samples cannot be improved apprec iab ly by

s u r f a c e t r ea tmen t .

Apparent ly ,

One sample from l o t "A" w a s a l s o borided (see Table V I , No. 17)

and w a s found t o be b r i t t l e a t 750°F (672°K).

of t h e chromium sample a l s o does not l e a d t o s i g n i f i c a n t improvement of

t h e DBTT a f t e r bo r id ing .

Thus, t h e i n i t i a l q u a l i t y

3. DBTT of Y t t r i d e d Samples

The d u c t i l i t y d a t a on a l l t h e y t t r i d e d samples are summarized

i n Tables I - I V . The bor ided samples were b r i t t l e a t 800°F (700°K) and

below.

b r i t t l e a t 700°F (644°K). One sample w a s b r i t t l e a t 800°F (700°K). The

load -de f l ec t ion curves i n d i c a t e t h a t most samples would have a DBTT of

t h e o r d e r of 750-775'F (672-686°K).

The samples which were n icke l -p l a t ed be fo re y t t r i d i n g were

The samples which were y t t r i d e d as-received w e r e , f o r t h e most

p a r t b r i t t l e a t 700'F (644"K), an i n c r e a s e of t h e DBTT of a t least

100°F (55"K), Some were b r i t t l e a t 700, 800, and 900°F (672, 700,

and 755°K).

Two samples (40143 and 52862) were found t o be d u c t i l e a t 650°F

(616°K). The reason f o r t h e s e f avorab le r e s u l t s i s no t known. It i s

p o s s i b l e t h a t t h e embr i t t l ement is caused by pickup of n i t r o g e n i n t h e

c e l l dur ing treatment. I f t h i s i s t h e case, t h e r e i s no obvious reason

why t h e s e two samples f a i l e d t o p i ck up n i t rogen .

C. Oxidat ion

The y t t r i d e d sample, No. 40143, which had a dense, adherent , ve lve ty

b l ack coa t ing , and which had r e t a i n e d d u c t i l i t y up t o and inc lud ing t h e

vacuum evapora t ion s t e p w a s c y c l i c a l l y exposed a t 2100°F (1422°K) i n

slow f lowing dry a i r wi th t h e r e s u l t s given i n Table V I I . This sample

f a i l e d t o m e e t t h e goa l of no more than 3 mg/ine2 (4.65 x 10

weight g a i n i n 200 hours .

-3 kg/m2)

31


TABLE VII. - CYCLIC OXIDATION TESTS OF SAMPLE NO. 40143

(Yt t r ided a t 1650°F(11730K) f o r 30 minutes

a t 0.163 A/in.2 (252 A/m ) ) 2

T i m e a t Temperature, H r s .

Weight Gain

(kp/m2) 2 m P / i n ,

2 ( --- 1 --

1.4 (2.17) 2

2 0.6 (0.93)

2

2

0.7 (1.09)

0.2 (0.31)

2 1.1 (1.70)

0.7 (1.09) 2

2 0.4 (0.62)

0.2 (0.31) 2

2 0.4 (0.62)

2.9 (4.5) 20

20 1.9 (2.92)

2.9 (4.5) 20

1.5 (2 a 32) 20

20 - 1.7 (2 63) - 16.6 mg/in. 2 (25.6 kg/m2) T o t a l 120 hours

20 45.9 spa l l i ng (71 .0 )

32


One of t h e f o u r samples comprising run 52862 w a s t e s t e d f o r d u c t i l i t y

immediately a f t e r removal from t h e s a l t b a t h , and proved t o b e d u c t i l e a t

650°F (616°K) l i k e t h e u n t r e a t e d a l l o y . The t h r e e remaining samples w e r e

soaked i n water f o r several days and vacuum hea t t r e a t e d t o remove re-

s i d u a l sa l ts . One of t h e t r e a t e d samples w a s p reoxid ized f o r one hour

i n f lowing 5% 0 -95% A r a t 1650°F (1172°K). 2 which s p a l l e d badly. Subsequent p reox ida t ion t r ea tmen t s were done a t 1500°F

(1089°K) i n H / H 0 mixtures w i t h hydrogen a t one atmosphere f lowing through

w a t e r a t 120°F(323"K). A second t r e a t e d sample w a s c y c l i c a l l y oxid ized i n

s l o w moving a i r a t 2100°F(1442"K), t o g e t h e r w i t h an u n t r e a t e d c o n t r o l spec i -

men. Weight ga ins w e r e lower f o r - t h e y t t r i d e d sample, bu t were still t o o h i g h

f o r p r a c t i c a l a p p l i c a t i o n s . Ne i the r sample w a s d u c t i l e a t t h e end of t h e

test.

A heavy oxide c o a t i n g r e s u l t e d

2 2

The r e s u l t s are g iven i n Table VIII.

One s a m p l e from each of t h e l o t s 281011 and 281113, and two samples

from 281013 w e r e c y c l i c a l l y oxid ized i n s low f lowing d ry a i r a t 2100°F

(1422"K), w i t h no p reox ida t ion .

a f t e r two 2-hour exposures and proved t o be b r i t t l e . The o t h e r samples

were c a r r i e d through t h e 200 hour o x i d a t i o n / n i t r i d a t i o n test, and showed

h ighe r weight g a i n s than t h e d e s i r e d 3 mg/ inB2 (4.65 x kg/m ). T e s t

r e s u l t s are summarized i n Table I X .

The 281113 sample w a s t e s t e d f o r d u c t i l i t y

2

Four samples from l o t 282121T were p reox id ized i n hydrogen-water

vapor as desc r ibed above, and sub jec t ed t o c y c l i c o x i d a t i o n t e s t i n g . Three

samples from l o t 282123 were t e s t e d s i m i l a r l y . Both sets of r e s u l t s are

g iven i n Table X.

While a l l of t h e weight changes are unacceptably h igh , i t i s clear

from Tables I X and X t h a t t h e magnitude of t h e weight change i n c r e a s e s

from p o s i t i o n (1) t o p o s i t i o n (4) i n t h e furnace . The tempera tures a t

t h e s e f o u r p o s i t i o n s were o r i g i n a l l y measured i n a s t a t i c system and were

found t c r b e f a i r l y uniform, 2090°F (1416°K) a t (l), 2100°F (1422°K) a t (2)

and ( 3 ) , and 2086'F (1414°K) a t ( 4 ) . The gas f low a l t e r e d t h e temperature

d i s t r i b u t i o n cons ide rab ly t o 2060'F (1400°K) a t (l), 2092°F (1417'K) a t

( 2 ) , 2112°F (1429°K) a t ( 3 ) , and 2119°F (1432°K) a t (4 ) . Hence t h e in-

c r eas ing weight change wi th p o s i t i o n corresponds t o i n c r e a s i n g tempera tures .

33


TABLE VIII. - CYCLIC OXIDATION TESTS OF SAMPLE NO. 52862 2 (Yttrided at 1560°F(1123"K) f o r 120 min. at 0.163 A/in. )

2 (252 A/m ) and an Untreated Control)

Weight Gain Weight Gain of of

Yttrided Sample Untreated Control Time at Temperature mg/in.2 (ka/m2) mg /in. 2 (ka/m2>

(hrs)

2 4.6 (7.14) 2.9 (4.5)

2 2.6 (4.04) 2.6 (4.04)

2 0.9 (1 a 39) 1.7 (2 e 64

2 0.3 (0.465) 2.2 (3.42)

2 0.4 (0.62) 1.9 (2 95)

2 0.6 (0.93) 2.1 (3.26)

2 1.4 (2 e 18) 2.0 (3 e 10)

- - -

Total 14 hrs 10.8 mg/ina2 (16.8 kg/m2) 15.4 mg/ine2 (23-9 kg/rn2)

34

Arthur I 1 Little, Inc

TABLE IX. - CYCLIC OXIDATION TESTS OF SAMPLES

YTTRIDED AT 1740°F(12230K) FOR 6 MINUTES AT 0.825 A/in.2(1280 A/m2) (NO PREOXIDATION)

Weight Gains, mg/in. 2 (kg/m 2 Sample No.:

Time at Temperature

Furnace Position: 2 hrs. 2 2 2 2 2 2' 2 2 2 20 20 20 20 20 20 20

* Spalling

281011

(1) 1.6 (2.5) 0.2 (0.3) 3.2 (5.0) 0.6 (0.9) 0.8 (1.2) 0.7 (1.1) 0.9 (1.4) 0.0 (0.0) 0.3 (0.5) 0.2 (0.3) 5.0 (7.8) 4.4 (6.8) 2.6 (4.0) 2.1 (3.3) 2,2 (3.4) 6.6 (10.2) 2.8* (4.3)

(2) 3.5 (5.4) 0.9 (1.4) 3.9 (6.0) 1.0 (1.5) 1.1 (1.7) 1.3 (2.0) 1.6 (2.5) 0.1 (0.15) 0.4 (0.6)

0.5 (0.8) 10.2 (15.8) 8.8 (13.6) 5.3 (8 .2)

6.2* (9.6) 11.3*(17 5) -16.1 (-25) -20.1 (-31)

(3) (4 1 3.7 (5.7) 6.1 (9.5) 1.3 (2.0) 2.7 (4.2) 3.6 (5.6) DBTT 1.6 (2.5)

1.7 (2.6) 1.7 (2.6) 2.3 (3.6) 0.4 (0.6) 0.5 (0.8)

1.0 (1.5) 14.3 (22.2) 11.3 (17.5) 9.7 (15.0)

-33 2 (-51 a 5) -62.3 (-96.5) -27.6 (-42 e 8) -19.2 (-29.8)

35


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36


V. DISCUSSION

The b a s i c concept of u s ing a dense adherent Y 0 - C r 0 c o a t i n g as a 2 3 2 3 n i t r o g e n d i f f u s i o n b a r r i e r t o p reven t embr i t t l ement of chromium a l l o y s

remains, w e t h i n k , a v a l i d one, a l though w e w e r e unable t o form such a

c o a t i n g under t h e p r e s e n t program. W e d i d e l e c t r o d e p o s i t Y o r YN on t h e

s u r f a c e of a chromium a l l o y , and w e d i d form Y 0 - C r 0

However, n e i t h e r t h e y t t r i u m con ta in ing s u r f a c e l a y e r , no r t h e mixed

oxide w a s depos i t ed as a uniform, po re - f r ee , coherent coa t ing which might

be impenet rab le t o n i t r o g e n .

i n many cases. 2 3 2 3

Due t o t h e i n s o l u b i l i t y of y t t r i u m i n chromium, o r i n any of t h e com-

ponents of t h e a l l o y , and t h e absence of Y-Cr i n t e r m e t a l l i c s , t h e m e t a l -

l i d i n g c e l l could n o t be opera ted i n t h e u s u a l d i f f u s i o n mode. I n s t e a d ,

cond i t ions were sought f o r e l e c t r o p l a t i n g of y t t r i u m . The h i s t o r y of

e l e c t r o p l a t i n g from fused s a l t media, d a t i n g from t h e t i m e of Michael

Faraday, i s almost e x c l u s i v e l y a d ismal d e s c r i p t i o n of t h e format ion of

powdery o r d e n d r i t i c d e p o s i t s . Only t h e r e f r a c t o r y metals, aluminum-

manganese a l l o y s , and i r i d i u m have been e l e c t r o p l a t e d as coherent de-

p o s i t s from molten s a l t b a t h s , and even f o r t h e s e materials, success w a s

n o t achieved e a s i l y . One of t h e necessa ry , bu t n o t s u f f i c i e n t , r equ i r e -

ments f o r e l e c t r o p l a t i n g i n s a l t b a t h s i s met icu lous c l e a n l i n e s s . Thus,

a g a t e v a l v e system t o permit i n t r o d u c t i o n of chromium a l l o y s i n t o t h e

s a l t b a t h i n t h e t o t a l absence of a i r would c e r t a i n l y be mandatory i f

t h e work w e r e t o be pursued.

our experiments a lmost c e r t a i n l y c o n t r i b u t e d t o t h e poor q u a l i t y of t h e

d e p o s i t s . The s u c c e s s f u l fused sa l t p l a t i n g of t h e r e f r a c t o r y metals

depended i n p a r t on p r e c i s e c o n t r o l of t h e average va lence of m e t a l i o n s

i n s o l u t i o n ( r e f . 1 3 ) . However, t h e r e f r a c t o r y metals are commonly

mul t i -va l en t ; y t t r i u m is no t .

The p e r s i s t e n t n i t r o g e n contaminat ion i n

The non-uniformity of t h e y t t r i u m d e p o s i t ob ta ined i n t h e m e t a l l i d -

i n g c e l l w a s r e f l e c t e d i n non-uniform oxide coverage i n t h e p reox ida t ion

and subsequent c y c l i c o x i d a t i o n experiments . The oxides formed were

porous, f l a k y , and v a r i a b l e i n c o l o r over t h e s u r f a c e of a s i n g l e specimen.

37

Arthur 13 Little, Inc

V I 1 e RECOMMENDATIONS

There are methods o t h e r t h a n m e t a l l i d i n g and p r e o x i d a t i o n by which

an y t t r ia -chromia d i f f u s i o n b a r r i e r w i t h t h e d e s i r e d p h y s i c a l charac te r -

i s t ics might b e achieved. I o n p l a t i n g of y t t r i u m onto chromium, f o r

example, could provide a wel l -adherent t r u e d i f f u s i o n l a y e r which might

subsequent ly b e oxid ized t o a p r o t e c t i v e Y 0 - C r 0 f i l m .

a n argon g a s d i s c h a r g e would b e s t r u c k between a chromium a l l o y cathode

and a hea ted y t t r i u m anode.

would be i o n i z e d i n t h e plasma, could s t r i k e t h e chromium s u r f a c e wi th

s u f f i c i e n t l y high energy t o p e n e t r a t e t h e s u r f a c e w i t h t h e formation of

a s t a b l e d i f f u s i o n zone. One of t h e g r e a t advantages of i o n p l a t i n g ,

p a r t i c u l a r l y f o r active metal s u b s t r a t e s , i s t h a t s p u t t e r c l e a n i n g pre-

cedes and accompanies t h e a c t u a l p l a t i n g process , so t h a t s u r f a c e im-

p u r i t i e s are e f f e c t i v e l y e l imina ted .

I n i o n p l a t i n g 2 3 2 3

Vaporizing y t t r i u m atoms, some of which

Another p o s s i b i l i t y might b e t o d e p o s i t Y 0 - C r 0 d i r e c t l y onto 2 3 2 3 t h e a l l o y s u r f a c e by chemical vapor d e p o s i t i o n .

of Y C 1 (g) and C r O C1 (g) might produce t h e d e s i r e d product , a l though n o t

n e c e s s a r i l y as a n adherent coa t ing . A l t e r n a t i v e l y , y t t r i u m might be

chemical ly vapor d e p o s i t e d by hydrogen r e d u c t i o n of Y C 1 (g) a t t h e chrom-

ium a l l o y s u r f a c e , and subsequent ly oxid ized . Adhesion a g a i n would be

t h e major problem, s i n c e chemical vapor d e p o s i t i o n , u n l i k e i o n p l a t i n g ,

w i l l not g i v e a d i f f u s i o n type i n t e r f a c e i f t h e c o a t i n g i s i n s o l u b l e i n

t h e s u b s t r a t e and forms no compounds w i t h i t .

Hydrolysis of a mixture

3 2 2

3

A l l t h i n g s cons idered , t h e i o n p l a t i n g approach appears t o b e most

promising. Adherent, f u l l y dense, pore- f ree c o a t i n g s are obta ined even

a t t h i c k n e s s e s of t h e o r d e r of -08 m i l (2000 1). formation of t h e mixed oxide , Y 0 - C r 2 0 3 , some chromium might be i o n

p l a t e d s imul taneous ly w i t h t h e y t t r i u m , from a s e p a r a t e l y powered source .

I n o r d e r t o promote

2 3

39

Arthur I> little, Inc

V I . CONCLUSIONS

1. W e have been unable t o produce any y t t r ium-conta in ing d i f f u s i o n

c o a t i n g on t h e a l l o y Cr-7Mo-2Ta-Oe09C-01Y by t h e m e t a l l i d i n g

p rocess , d e s p i t e t h e f a c t t h a t f r e e y t t r i u m has been found by

X-ray d i f f r a c t i o n on t h e a l l o y s u r f a c e a f t e r t r ea tmen t .

2. The presence of n i t r o g e n i n t h e cel l e l e c t r o l y t e appears unavoid-

a b l e wi th t h e p r e s e n t exper imenta l equipment. Some n i t r o g e n from

t h e a i r appea r s t o e n t e r t h e c e l l dur ing o p e r a t i o n , d e s p i t e t h e

use of y t t r i u m c h i p s over t h e e l e c t r o l y t e as a g e t t e r f o r 0 and

N2. t h e cause of t h e f a i l u r e of d i f f u s i o n . However, y t t r i u m e l e c t r o -

depos i t ed on t h e a l l o y s u r f a c e o f t e n appeared as YN.

of t h e s u r f a c e n i t r i d e d i d no t e m b r i t t l e t h e s u b s t r a t e .

2 W e do no t cons ide r t h e presence of n i t r o g e n i n t h e c e l l t o be

The presence

3 . The f a i l u r e of t h e d i f f u s i o n p rocess appears t o be due t o t h e in -

h e r e n t p r o p e r t i e s of t h e system. The a l l o y i s a l r eady s a t u r a t e d

wi th y t t r i u m i n s o l u t i o n and chromium forms no i n t e r m e t a l l i c com-

pounds wi th y t t r i u m .

4 . W e have succeeded i n producing s u r f a c e l a y e r s con ta in ing Y 0 and

YOF on t h e a l l o y s u r f a c e by o x i d i z i n g s u r f a c e l a y e r s of plated-on

material. These c o a t i n g s , however, do n o t enhance t h e o x i d a t i o n /

n i t r i d a t i o n r e s i s t a n c e of t h e base a l l o y , and do no t m e e t t h e goa l

of 3 rng/in.2 (4 .65 x

2 3

2 kg/m ) weight g a i n i n 200 hours .

5. P re t r ea tmen t of t h e a l l o y s u r f a c e by boron d i f f u s i o n formed C r B 2

which d id no t pe rmi t t h e subsequent d i f f u s i o n of y t t r i u m i n t o t h e

s u r f a c e . S ince boron raises t h e d u c t i l e - b r i t t l e t r a n s i t i o n temperature

(DBTT) of t h e a l l o y t o an unacceptab le degree, t h i s approach w a s no t

pursued

6 . P l a t i n g t h e a l l o y s u r f a c e wi th n i c k e l pe rmi t t ed t h e d i f f u s i o n of

y t t r i u m wi th t h e format ion of n i cke l -y t t r ium i n t e r m e t a l l i c compounds.

P re l imina ry experiments , however, showed t h a t t h i s l a y e r tended t o

f l a k e o f f t h e base a l l o y and t h e y t t r i d e d samples w e r e b r i t t l e . A more

e f f e c t i v e d i f f u s i o n s t e p p r i o r t o y t t r i d i n g may overcome t h i s problem.

41

Arthur I> little, Inc.

REFERENCES

1.

2.

3.

4.

5.

6.

7 .

8.

9.

10.

11.

12.

13.

A. H , S u l l y , E. A. Brandes, and K. W. M i t c h e l l , J . I n s t . of Metals

- 81, 585 (1952-53).

W. H. Smith and A. U. Seybol t , J. Electrochem. SOC. 103, 347 (1956).

J.R.Stephens and W.D.Klopp, T ransac t ions A I M E , Z , l 9 7 5 (1969).

R. L. Coble, J. Am. C e r a m . S O C . , 45, 123 (1962).

A. U. Seybo l t , Corrosion Sc ience , 5, 263 (1966).

E. J. F e l t e n , J. Electrochem. SOC. 108, 490 (1961).

W. C. Hagel, Trans. Am. SOC. Metals, 56, 583 (1963).

J. M. F ranc i s and W. H, Whitlow, Corrosion Sc ience 5, 701 (1965).

E. R. S l augh te r , J. R. Hughes, and W. F. Moore, NASA CR-72545,

December, 1968.

N. C. Cook, U . S. P a t e n t No. 3,024,176.

R. C . Svedberg, WANL-L-551, A p r i l 8, 1970.

N. C. Cook, U. S. P a t e n t Appl ica t ion , Ser ia l No. RDCD-834.

S. Senderoff and G. W. Mel lors , Science 153, 1475 (1966).

43


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