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This report was prepared as an account of work sponsored by the United
States Government. Neither the United States nor the United States Atamic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their emplayees, makes any wairanty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights,

 

 
ORNL~TM~4380

Contract No. W~7405-eng~26

METALS AND CERAMICS DIVISTION

INFLUENCE OF AGING ON THE IMPACT PROPERTIES OF HASTELLOY N,
HAYNES ALLOY NO. 25, AND HAYNES ALLOY NO. 188

H, E. McCoy and D. T. Bourgette

DECEMBER 1973

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION

for the
U.S. ATOMIC ENERGY COMMISSION

IDGE NATIONAL LABOSATORY LIBRARIES

EERRERALRR

3 Y45, 0550204 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Abstract . « .+ . . .+ . . .
Introduction . . . . . . .
Experimental Details . . .
Impact Data . . . « . . .
Tensile Properties . . . .

Metallogranhic Examination

Scanning Electron Microscope

Discussion of Results . .
Summary . . 0 v 0 4 0 . .
Acknowledgments . . . . .

References » + &+ v o o o &

CONTENTS
Observations
9 . - ] . » -

Page

10
21
21
25
26
26
P

 

INFLUENCE OF AGING ON THE IMPACT PROPERTIES OF HASTELLOY N,
HAYNES ALLOY NO. 25, AND HAYNES ALLOY NO, 188

 

H. E. McCoy and D. T. Bourgette

ABSTRACT

Samples of Hastelloy N and Haynes alloy WNos. 25 and
188 were solution annealed, aged at temperatures over the
range 650 to 900°C, and impact tested at 25 and 300°C.
The impact energy decreased for most aging conditions,
with the property changes being least for Hastelloy N and
greatest for Haynes alloy No. 25. Small aged tensile
samples showed that aging reduced the fracture strain at
25°C. The fractures of the tensile samples were examined,
and some of the notched impact specimens were viewed
optically. The reduction in toughness of Hastelloy N
correlated well with the amount of grain boundary precipi-
tate. Haynes alloys Nos. 25 and 188 formed carbides and
Laves phase., These alloys had a strong tendency to frac-
ture intergranularly even in the solutiom-annealed condition.
The tendency increased with aging, and the increased
amounts of grain boundary precipitate likely account for
the reduction in toughness.

 

INTRODUCTION

Hastelloy N is a solution-strengthened nickel-base alloy that was
developed for good strength and corrosion resistance at about 650°C. No
intermetallic compounds have been identified in this alloy, but carbides
precipitate and cause modest changes in the properties.!** Haynes
alloys Nes. 25 and 188 are cobalt~base alloys that are solid solution
strengthened and suitable for use at temperatures up to 1000°C. These
cobalt-base alloys experience some property changes due to carbide precipi~
tation, but the formation of brittle Laves phases of the AsB type is the
main cause of embrittlement.3”’

Generally, neithexr the carbides nor the Laves phases cause drastic
embrittlement at elevated temperatures, but their effect increases as
the temperature is decreased. The degree of embrittlement is also
increased by increasing strain rate and by notches and other disconti-
nuities that magnify the average stress. One of the applications that
would subject the structural material to impact loading at low tempera-
tures is in isotope power supplies intended for use in space. The
material would be aged for several thousand hours at an elevated service
temperature, cooled gradually while still in orbit or during reentry,
and possibly exposed to impact loading during reentry. Thus, the iImpact
properties after aging are of major concern in this application.
Although the experimental program described in this report was
undertaken to provide necessary information for the design of isotope
power supplies, numerous other applications require a knowledge of the
impact properties of these alloys after prolonged aging. Our specific
program involved aging samples of Hastelloy N, Haynes alloy No, 25, and
Haynes alloy No. 188 up to 4000 hr at remperatures from 650 to 900°C
and measuring their impact properties at 25 and 300°C. Budgetary
limitations prohibited detailed phase identification, and only limited
metallographic examination was possible.

EXPERIMENTAT DETAILS

Experimental material of the three allovs was obtained in the form
of 1/2-~in.-thick plate. The vendor's and ORNL's chemical analyses are
given in Table 1. The Hastelloy N was double vacuum melted and the
Haynes alloys Nos. 25 and 188 were air melted and vacuum arc remelted.

Standard Charpy V-notch impact specimens were wachined in conformance
with the specifications in ASTM Standard E 23-66. The samples were
solution annealed 1 hr at 1150°C in argon and cooled rapidly to ambient
temperature. They were wrapped in nickel foil and sealed in quartz in
argon at 0.33 atm and aged at various temperatures. Following aging
they were broken from the capsule and impact tested at 25 and 300°C
according to ASTM Standard E 23-66.

IMPACT DATA

The results of impact tests on Hastelloy N at 25°C are shown in
Fig. 1. The material was quite tough in the solution-annealed condition,
with an impact energy of 164 ft-1b. Aging at 650 and 700°C caused a
gradual decrease in the impact energy to values of 75 and 95 ft-1b,
respectively, and then the impact energy increased with further aging.
Aging at 900°C resulted in a steady decrease to about 30 ft-1b after
aging 4000 hr. Aging at 800°C had little effect for 500 hr, but longer
aging resulted in a rapid decrease in the impact energy to a value of
40 ft-1b after aging 4000 hr.

The impact results shown in Fig. 2 for Hastelloy N tested at 300°C
are qualitatively similar to those shown in Fig. 1 for 25°C., The impact
energies were higher at 300°C, with a value of 235 ft-1b for the solution-
annealed material and a minimum value of 70 ft~1lb for samples aged at
900°C.

The fractures of the Hastelloy N iwmpact specimens are shewn in
Fig, 3. The lower smooth part of each sample is the machined portion of
the notch. The appearance of the fracture is a qualitative measure of
the toughmess, with terms of “granular" (brittle) and "fibrous™ (ductile)
used commonly in the literature. All the samples had fibrous fractures,
and three of the samples tested at 300°C had sufficient touginess not
to part. The widih of the sample at the base of the notch is acother
measure of plasticitv. as the sample deforms longitudinaily, it wmust
contract laterally. Thus, the greater the reduction in width ar the
base cof the notch, cthe greater the tougimess. All the samples contraciea
Table 1. Chemical Analyses of Alloys

 

Contents, wt %

 

 

 

 

 

 

Element Hastelloy N Alli?yfiibeS Allagaiz?slSSC
Vendor ORNL Vendor ORNL Vendor ORNL
Cr 7.1 7.0 19.8 18.1 22.7 20.3
Fe 0.47 2.4 6 1.6 1.6
Ni Bal 75.6 10.0 9.5 21.1 21.4
Co 0.08 Bal 52.1 Bal 39,5
Mo 16.7 15.7 0,57 0.39
W 0.03 14.5 13.3 14.1 15.3
C 0.052 0.051 0.12 0.12 0.09 0.11
51 0.02 0.05 0.08 0.23 0.14 0.33
Mn 0.02 0.02 1.3 1.2 0.81 0.69
Ti 0.06 0.05 <0, 01 0.01
Al 0.13 0.2 0.1 1
Cu 0.02 0.02 0.03 0.03
P 0.004 0.0020 0.020 0.120 0.009 0.011
S 0.006 0.0020 0,009 0.003 0.001 0.001
B ¢.001 0.0020 0.0050 0.0005
La 0.07 0.07

 

Procured as arc-melted plate 48 % 24 x 1/2 in. from Allvac Metals,
heat 5960.

bprocured as hot-rolled, pickled, and annealed plate 36 x 76 X 1/2 in.
from Stellite Division, Union Carbide Corporation, heat 1860-6-1813.

“Procured as plate 30 X 12 % 1/2 in. from Stellite Division, Union
Carbide Corporation, heat 1880-8-0132.

appreciably at the base of the notch, but the samples aged at 900°C
{(rows 2 and 4) showed progressively less reduction in width with
increasing aging time.

The variation of impact energy at 25°C with aging time 1s shown in
Fig. 4 for Haynes alloy No. 25 aged at various temperatures. The impact
energy in the solution-annealed condition was 70 ft-1b, compared with
164 fr~1b (Fig. 1) for Hastelloy N. Aging at 650, 700, 800, and 900°C
caused a progressive decrease in impact energy with aging time, and the
IMPACT ENERGY (ft—1b)

Fig. 1.
for Hastelloy N.

120

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Variation of Notch~Impact Energy at 25°C With Aging Time
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AGING TIME (hr)

Variation of Notch-Impact Energy at 300°C With Aging Time
Samples annealed 1 hr at 1150°C before aging.
 

Fig. 3. Fracture Surfaces of Hastelloy N Impact Samples. The aging
time increases in each row from zero on the left to 4000 hr on the right.
The samples in the top row were aged at 650°C and tested at 25°C, those
in the second row were aged at 900°C and tested at 25°C, those in the
third row were aged at 650°C and tested at 300°C, and those in the bottom
row were aged at 900°C and tested at 300°C.

QRNL~DWG 71-1C685R

 

 

IMPACT ENERGY (ft —Ib)

 

 

 

 

 

 

 

 

 

 

 

 

AGING TIME (hr}

Fig. 4. Variation of Notch-Impact Energy at 25°C With Aging Time
for Haynes Alloy No. 25. Samples annealed 1 hr at 1150°C before aging.
rate of decrease increased with increasing aging temperature. The lowest
impact energles resulting from aging were from 3 to 8 fi-1b.

The effects of agiag on the impact energy of Haynes alloy No. 25
at 300°C are shown in Fig. 5 and are qualitatively similar to the effects
noted at 25°C (Fig. 4). The lowest value noted at a test temperature
of 300°C was 5 ft-1b for a sample aged at 650°C.

The fracture surfaces in Fig. 6 also reflect the large effect of
aging on the impact properties. The top row was azed at 650°C and tested
at 25°C. The fracture appearance changed from fibrous to graanular, and
the reduction in width at the base of the notch decreased with increasing
aging time. The second row was aged ai 850°C and tested at 25°C; the
fracture appearance and the reduction in width indicate a high rate of
ambrititlement during aging at 850°C. The samples in the third and fourth

rows were aged at 650 and 850°C, respeciively, and tested at 300°C. They
rafiect progressive embrittlement wilh increasing aging time.
The variation of impact energy at 25°C of faynes alloy No. 188 with
ig aled

aging tiwe is shown in Fig, 7. The impact energy in the solution-anne
condition was 58 ft-lb, compared with 70 ft-1b for Haynes alioy No. .5
and 164 fi-1b for Hastelloy N. Aging at 650°C gradually reduced the
impact energy excepi ftor a possible incvease after aging for 50 hr. Aging
at 700°C reduced the impact energy for the first 500 hr, and further aging
caused a slight improvement. Aging at 800 and 900°C veduced the impact

6

entergy to values of 9 and fi~1b, respectively. The crossover

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since the same effect was noted at a test temperature of 300°C (Fig. 8).

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No. 188 are shown in Fig. 8., These data show the improvement in impact
properties after aging 50 hr at 650°C and the crossover in properties
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AGING TIME (hr)

Fig. 8. Variation of Notch~Impact Energy at 300°C With Aging Time
for Haynes Alloy No. 188. Samples annealed 1 hr at 1150°C before aging.

The fracture surfaces of the Haynes alloy No. 188 samples shown in
Fig. 9 show the decrease in ductility with aging. The fracture appearance
changed from fibrous fo granular, and the reduction in width at the base
of the notch decreased as the aging time increased.
 

 

   

Fig. 9. Fracture Surfaces of Haynes Alloy No. 188 Impact Specimens.
The aging time increases in each row from zero on the left to 4000 hr on
the right. The samples in the top row were aged at 650°C and tested at
25°C, those in the second row were aged at 900°C and tested at 25°C, those
in the third row were aged at 650°C and tested at 300°C, and those in the
bottom row were aged at 900°C and tested at 300°C. ‘

TENSILE PROPERTIES

Small tensile specimens having a gage section 1/2 in. long and 1/8 in.
in diameter were machined from halves of several tested impact specimens.
These specimens were tensile tested at 25°C at a strain rate of 0.1/min,
and the results are summarized in Table 2. Although aging Hastelloy N at
900°C for 100 hr reduced the impact energy from 164 to 82 ft-1b (Fig. 1,
P. 4) the uniaxial tensile properties changed very little (Table 2, speci-
mens 11440 and 11441). Aging for 4000 hr at 900°C reduced the impact
energy to 35 ft~1b, and the tensile properties indicate a general decrease
in yield and tensile strength, very small changes in axial strain, and a
sizeable decrease in the reduction in avrea (Table 2, specimens 11440
and 11448). : '

The tensile properties of Haynes alloy No. 25 were changed markedly
as a result of aging. Aging for 50 hr at 850°C increased the yield stress
10

Table 2. Results of Tensile Tests at 25°C

 

Stress, psi

 

 

 

 

Aging Strain, 7% Reduction
Specimen T Ultimat T in Area, %
(hr) °o Yield mate Fracture Uniformn Total e
Tensile
Hastelloy N
11440 0 60, 300 114,400 105,700 51,2 55.6 £1.5
11441 100 900 59,000 118,100 112,300 50.5 54.4 41.5
11448 4000 90¢ 55,500 166,800 100,200 41.5 56.4 27.7
Haynes Alloy No. 25
11446 0 71,700 138,400 137,500 43.2 43.6 27.9
11447 50 850 79,900 113,300 113,300 14.4 14.6 9.1
11445 4843 850 87,500 133,000 133,000 12.7 12.9 5.8
Haynes Alloy No. 188
11444 0 69,800 143,700 141,200 51.2 53.5 40.0
11443 100 900 69,200 139,300 138,500 37.0 37.9 27.3
11442 2000 900 71,100 137,300 137,300 21.1 21.6 7.7

 

211 specimens solution annealed 1 hr at 1150°C before aging.

and decreased all the ductility parameters (Table 2, specimens 11446 and
11447). These same aging conditions caused the impact energy to decrease
from 70 to 6 ft-1b (Fig. 4, p. 5). Aging for 4843 hr at 850°C caused the
yield stress to increase and the ductility parameters to decrease further.
These same aging conditions caused the impact energy to decrease to about
3 ft~1b,

Aging at 900°C caused very little change in the strength parameters
of Haynes alloy No. 188 (Table 2, specimens 11444, 11443, and 11442).
However, the ductility parameters decreased with aging. The impact
properties after 0, 100, and 2000 hr aging at 900°C were 58, 27, and
6 ft-1b, respectively (Fig. 7, p. 7).

METALLOGRAPHIC EXAMINATION

No phase identification work was done on these samples, so we will
only indicate the phases that are likely present on the basis of other
studies. Carbides of the M;C and MgC types are formed' in Hastelloy N,
but the M,C type should predominate in a heat such as 6960, which
contained only 0.05% Si. The "M" in this carbide consists of about 80% Mo
and 20% Cr. The microstructure of Hastelloy N after solution heat
treatment is shown in Fig. 10. The grain boundaries were quite jagged
and contained some fine carbide. Some coarser primary carbide was also
present within the grains. Aging at 650°C caused a decrease in the
impact energy for the first 500 hr and a slight increase with further
aging (Fig. 1, p. 4). The microstructural change observed at 650°C was
carbide precipitation along the grain boundaries, and no obvious charac-
teristic of this precipitation could account for the slight minimum in
the impact energy. Figure 11 shows the microstructure after 4000 hr
aging at 650°C. Carbide has precipitated om and adjacent to the grain
11

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Fig. 11. Typical Photomicrograph of Hastelloy N Annealed 1 hr at
1150°C and Aged 4000 hr at 650°C. Impact energy at 25°C was 108 fr-1b.
Etchant: glyceria regia.
12

boundaries. Aging at 700°C caused carbide precipitation similar to that
noted at 650°C. Aging at 800°C initially caused grain boundary carbide
precipitation and some general precipitation (Fig. 12). After prolonged
aging at 800°C, the grain boundary carbide agpglomerated, and the amount

of precipitate within the grain increased (Fig. 13). However, the impact
energy continued to decrease with further aging at 800°C (Fig. 1, p. 4) even
though the grain boundary carbide appeared to become discontinuous. At
900°C the carbide formed preferentially along the grain boundaries

(Fig. 14). It was initially quite fine and became coarser and agglomerated
with further aging time. The impact energy decreased with increasing
carbide formation and seemed to reach a plateau of about 30 ft-1b after

the carbide became quite coarse (Fig. 1, p. 4).

Wlodek® has studied the embrittlement of Haynes alloy No. 25 and the
phases that are formed. He found that the solution-annealed alloy contained
M¢C and Zr(N,C). Aging over the range 650 to 1100°C formed Cr,3Cg and a
Laves phase, A,B. After prolonged aging the major constituents were MgC
and the Laves phase. These various microconstituents assumed different
morphologies at different aging temperatures, so it is unot possible to
identify microconstituents solely by optical metallography, and our phase
identifications are largely conjecture.

The microstructure of solution-annealed Haynes alloy No. 25 is shown
in Fig. 15; it contains a primary carbide, likely MgC. Even though aging
at 650°C caused a steady decrease in the impact energy from 60 to 2 ft-1b
(Fig. 4, p. 5), the microstructure changed only slightly. After aging for
4843 hr at 650°C (Fig. 16) the grain boundaries etched very quickly
(likely because of the presence of carbide or Laves precipitate), and
some precipitate was visible in the grains. Aging at 700°C produced a
steady decrease in the impact energy (Fig. 4, p. 5), and large amounts of
precipitate (likely Laves phase) were visible in the microstructure
(Fig. 17). Aging at 800°C resulted in coarser precipitate than noted at
700°C, and the grain boundaries etched very readily (Fig. 18). Aging
at 850°C for 50 hr caused the impact energy to decrease from 50 to 6 ft-1b
(Fig. 4, p. 5), but the microstructure showed little change from the
solution~annealed condition (compare Figs. 15 and 19). The main difference
is the more rapid etching characteristics of the grain boundaries after
aging. Aging 4843 hr at 850°C caused the impact energy to decrease only
slightly from that noted after aging 50 hr; however, the amount of
precipitate increased dramatically (Fig. 20). These observations lead to
the speculation that the impact property changes in Haynes alloy No. 25
result primarily from the grain boundary precipitate structure and to a
lesser extent on the precipitate within the grains.

Herchenroeder® studied the aging characteristics of four heats of
Havnes alloy No. 188. He found that the solution—annealed alloy centained
primary MgC and that aging formed secondary MsC, M23Ce, and a Laves phase
of the A,B type. Over the range 700 o 900°C the MgC teaded strongly to
transform to M,3Ce on aging. The carbide transformation would make more
of the group VIB materials available to form the A;BE phase,

Aging at 650°C caused the impact energy of Haynes alloy No, 188 to
decrease (Fig. 7, p. 7). The microstructure changed very little during
the 2000 hr aging (Fig. 21). The primary change was the formation of
precipitates along the grain boundaries so that these regions etched
13

    

 

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glyceria regia.

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/
[
=5
-
s,
o
¢

o,
= = e .
o
T, R
@ & i
a 2 ¥ -
% - > P
. - e
Kad ({ g&
& =
- Ny 4 e
=7 %, - ’
, .
o ~
T N o £ s D

Fig. 1l4. TPhotomicrographs of Hastelloy N Annealed 1 hr ai 1150°C
and Aged at 900°C. 500x. (Reduced 39.5%). (a) 50 hr, (b) 100 hr, and
(c) 4000 hr. Impact energies at 25°C were 114, 83, and 36 ft-1b,
respectively. FEtchant: glyeeria regia.
15

 

 

  

 

 

" ‘ ; y{_* s %27109160 7 l _
v : » ‘ 5 C e
&
* -
L Q‘
[
: é" ®
» R
}
i
i
i
+
i
i
§
#
P F
'
* L .
. :: f"‘
Ve Lo ! P
"‘"*r.h‘"-}? - . *
Py i
v,vi'g;"'fl»- . i B . , r
aw.im*{} \M@ o D ; ) L e T e
)::_:Gi- 5 : o o \ f"_,.n" o S : E‘%
waj~fc_ W - - . . L ' s
M_ a 'vm-m,m ff . 1 - g . >

. 15. Typical Photomicrograph of Haynes Alloy No. 25 Annealed 1 hr
C. TImpact energy at 25°C was 70 ft~1b. Etchant: glyceria regia.

20

104

 

G005 .

 

 

 

   

P=

8 [

inr S ol
e ol w0

f o :

aph of Haynes Alloy No. 25 Annealed
650°C.  Impact energyv st 2370 was

-
16

  
  

¥-109208 |

 
 

 

T

 

005

500X

0005 in.

5

 

 

10007 in.

Fig. 17. Typical Photomicrograph of Haynes Alloy No. 25 Annealed
L hr at 1150°C and Aged 4843 hr at 700°C. Twmpact energy at 25°C was
5 ft-1b. Etchant: glyceria regia.

oo

T

 

19.00% in.

500X

I

0.007 INCHES

{0005 .

o Y vy A gt . ) o . 4
s :«?:-j Q ’y b2 . ’ J J
w2 . . EEET 3 ; '

&

 

 

 

i0.007 in.

Fig. 18. Typical Photomicrograph of Haynes Alloy No. 25 Annealed
1 hr at 1150°C and Aged 4843 hr at 800°C. Tmpact energy at 25°C was
7 ft-1b. Etchant: glyceria regia.
17

 

. Gar . | : y ; Y-109212
g > oo R 2 { g ol * : L .
2 C T A ~
- ) % 9 T
" 45 o o e
f: i hfi,
. & -
. » # & £
L%
& s
#
b f
r
o
&
= -
& P
-.»a
- - e,
. &
- &
o & x - Y.

Fig. 19. Typical Photomicrograph of Haynes Alloy No. 25 Annealed
1 hr at 1150°C and Aged 50 hr at 8530°C.
Etchant: glyceria regia.

  
   

R ‘;‘Q{_;. 2 i’i -
- -

& -
& f‘

Fig. 20. Typical Photomicrograph of Haynes Alloy No. 25 Annealed
1 hr at 1150°C and Aged 4843 hr at 850°C.
Etchant: glyceria regia.

S
)

D

 

i0.00% in.

{

 

10,003

Q0%

O,007 INCH

16508

 

 

Impact energy at 25°C was 6 ft-1b.

10,001 in.

1

 

0.00% i,

HO0X

T

15605 in

 

 

Impact energy at 25°C was 3 ft~1b.
18

    
       
 

Cem &\\ : Ym10%216
. / . 3 -%.-»;? - * . I
N e
1

> ‘3 k- %t

R
®

   

¥
E ] . 4

L
. LEEX.

. -
(b) -3 S

Fig. 21. Typical Photomicrographs of Haynes Alloy No. 188.
(a) Annealed 1 hr at 1150°C. TImpact energy at 25°C was 59 ft-1b.
(b) Annealed 1 hr at 1150°C and aged 2000 hr at 650°C. Impact energy
at 25°C was 22 fi~-1b. Etchant: glyceria regia.

0007 INCHFES

[0.000 .

i0,00b in.

'0.003 .

500X

 

 

 

0.007 INCHES

to.005 m

10.007 n.

10.00: in.

'0.002 in. T

500X

T

 

 

10.007 in.
19

nore readily. The results of Herchenroeder® indicate that a sample aged at

650°C would contain M;3Ce and MgC, but likely not Laves phase. The
microstructure resulting from 2000 hr aging at 700°C contained copious
grain boundary precipitate, likely carbides (Fig. 22). Aging at 800°C
for 2000 hr resulted in the formation of more precipitates, priwmarily
grain boundary (Fig. 23). Herchenrceder's work would indicate that this
sample contained Mj3Ce and Laves phase. Aging at 900°C caused the grain

boundary precipitate to agglomerate and a large amount of precipitate to
form within the grains (Fig. 24)-
M»2Cg, MgC, and Laves phase,

The samples in Fig. 24 likely contain

Fig, 22. Typical Photo-~
micrograph of Haynes Alloy
No. 188 Annealed 1 hr at
1150°C and Aged 2000 hr at
700°C. Impact energy at
700°C was 24 ft=1b. Etchant:
gilyceria regia, 500x%.

Fig. 23. Typical Photo~
micrograph of Haynes Alloy
No, 188 Annealed 1 hr at
1150°C and Aged 2100 hr at
g00°C.  Impact ewnergy at
25°C was 9 ft-1b. Etchant:
glyceria regla. 500%,

 
20

 

 

 

 

. ~t ) > ag . ¢ L e 3 . 2 e o4 .%;,w"j”‘ © Y-109228 T
- on T ma W .
.‘ » 3‘ . %?‘ : ‘4; . , .- ,, L @,-r* %.r‘ -, .
e ! .‘ - > F ] : ‘“- ¢3' > 'n‘ *c: » t’f F ® £
2 @é - & f }v, e e ot 1 ¢ o ‘*“ &y * ’ ‘ - 3 g
i é . - T, s W e %o = o
. ' e . . ‘,—m%*%' ¥4 "fi@*&” S
6 "4 ~ & - 75 ""6. "e \. - ‘?‘w* s * *® *w e SF T
. * . _o" e . ° . o.‘ * o %)
- L . - »
- . \‘:} ‘o a f. "
- § 3 o e & a - ,“‘h_.";. 1 * " o - g
- . ¥ LA o, ¢ -~ “r
* t e . a 0 ® e
. .. el ‘,‘ % >4 P - - - - L B » f -
‘Q 7 “ - .'# 7 ‘.w.‘. ’ - - “: ’
.
& e -
’ . s - F V. R #"’%fl. . < o [ . k . . - * =
- «» = *» . A - b » ¥ =
MM . n"’t:* " - \ o s 9 “ - o " i p
., . o ® - O ° L RN\ = v e T, 2
T e e’ . : - o “y A T e e &
. w el T o' B L * 6 » .‘::"N 2 A W ot e L 0
- o 4 . P . &, >
: ot - s sy Lo oW .1 - . o ' \”""v!} et LT 2o
- o % - : ‘\O
“ o
O
O
<
Wy
o
<
1<
£
i~
O
Q
1
rfi
c
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e
L
T
LI
Zlo
o
~
810
S
£
0
o
<
1<
£
M~
o
<
|2

 

Fig. 24. Typical Photomicrographs of Haynes Alloy No. 188 Annealed
1 hr at 1150°C and Aged at 900°C. (a) Aged 100 hr, impact energy at
25°C was 28 ft-1b. (b) Aged 2000 hr, impact energy at 25°C was 6 ft-1b.
Etchant: glyceria regia.

 

 
21

SCANNING ELECTRON MICROSCOPE OBSERVATIONS

The fractures of the nine tensile samples described in Table 2 (p. 10)
were examined in the scanning electron microscope. Typical views of the
Hastelloy N fractures are shown in Fig. 25. Some portions of the fracture
were intergranular, but the entire surface shows deformatlon markings
characteristic of ductile fracture. The 1ntergranular cracks perpendicular
to the plane of view would be expected because of the complex stress state
that developed before fracture. As the specimen necked, three-dimensional
stresses developed that would tend to open cracks perpendicular to the
applied stress. The features of the fracture surface changed very little
as a result of aging.

The photomicrographs shown in Fig. 26 of the Haynes alloy No. 25
tensile specimens show that the fractures are primarily intergranular.

The main change that occurred with aging was a reduction in the amount of
deformation (evidenced by dimpling) that occurred before fracture.

The fracture surfaces of the Haynes alloy No. 188 tensile specimens
shown in Fig. 27 are quite similar to those of Haynes alloy No. 25, The
predominant difference is the more ductile appearance of the Haynes alloy
No. 188. Although the fractures were almost entirely intergranular,
the fracture surfaces are dimpled from plastic deformation. We saw no
features in Fig. 27 that obviously changed with aging, although the
reduction in area changed from 40.0 to 7.7%. :

DISCUSSION OF RESULTS

All three alloys underwent substantial losses in toughness as a
result of aging over the temperature range 650 to 900°C. Hastelloy N
forms only carbides during aging, but the alloy lost about 80% of its
toughness during aging at 800 and 900°C (Fig. 1, p. 4). Aging at
650 and 700°C caused losses of about 50%. These losses in toughness
seem to correlate with the amount of grain boundary precipitate.

Haynes alloys Nos. 25 and 188 are more complex than Hastelloy N in
that they form Laves phases. The aging characteristics of these two
alloys are compared in Fig., 28 in terms of the reduction in the impact
energy. One of the objectives in the development of alloy No. 188 was
to reduce the embrittlement due to Laves phase that occurred in Haynes
alloy No. 25. The comparison in Fig. 28 shows that this was partially
successful, since the time required to cause a certain loss in impact
energy was much greater for alloy No. 188. This is particularly true for
the larger losses in impact energy. For example, at 800°C alloy No. 25
lost 75% of its impact energy in 30 hr, but alloy No. 188 required 600 hr.

Haynes alloys Nos. 25 and 188 both formed Laves phase, but alloy No.188
formed less. Both alloys formed carbides. The toughness of a material
can be reduced by the matrix or the grain boundaries becoming brittle so
that a low~energy fracture path is defined. The carbide and Laves phases .
could produce either of these effects. It is quite likely, although
not necessarily the case, that the formation of either phase in quantities
sufficient to embrittle the matrix would harden it also. It is also
possible for the matrix to be strengthened sufficiently to cause the
grain boundaries and adjacent regions to deform;preferentially to the
22

     

  

Fig. 25. Scanning flectron
Micrographs of the Fractures of
Hastelloy N Tensile Specimens Frac-
tured at 25°C. (a) Solution annealed.
(b) Aged 100 hr at 900°C. (c) Aged
4000 hr at 900°C. Left 100X,

Right 300x,
23

1R62238

 

Fig. 26. Scanning Eleciron Micrographs of the Fractures of Haynes
Alloy No. 25 Tensile Specimens Frazctured at 25°C. 100%. {a) Solution
annealed.  (b) Aged 50 he at 850°C. (c) Aged 4843 hr at §50°C.
24

g _ s (R~62229

 

 

4
()
Fig. 27. Scanning Electron Micrographs of the Fractures of Haynes

Alloy No. 188 Tensile Specimens Tested at 25°C. 100x. (a) Solution
annealed. (b) Aged 100 hr at 900°C. (c) Aged 2000 hr at 900°C.

 
25
ORNL* DWG 73-3196
]}:}’

H;’«‘\T’NLS AILOY 25 W)
T

DB e s e e e Ty

 

 

 

 

300 | 52

  

-*-'-HA YNES ALLOY 188

  

 

 

 

 

 
 

 

 

 

 

U :
ol )

AGING TIME (hr)

Fig. 28. Reduction in Tmpact Energy at 25°C Due to Aging for Haynes
Alloys Nos. 25 and 188. All samples annealed 1 hr at 1150°C before
aging.

matrix. The yield stress of Haynes alloy No. 25 (Table 1, p. 3) increased
about 20% with aging, and the ductility parameters decreased. This

alloy showed a strong tendency to fail intergranularly in the solution-
annealed condition, and the tendency toward intergranular fracture
increased. There was less evidence of grain boundary plasticity with
aging, indicating that the grain boundaries themselves became less

ductile with aging. The stronger matrix may have contributed to the
embrittlement, but the grain boundary embrittlement seemed to be the
dominant factor.

The data for Haynes alloy No. 188 are even less definitive of an
embrittlement mechanism. The solution-annealed material fractured
predominantly intergranularly. The yield and tensile stresses changed
very little during aging, but the ductility decreased (Table 2, p. 10).
Although the fracture was intergranular, considerable deformation occurred

. before the fracture (Fig. 27), and this did not change appreciably with

aging. Thus, there is no evidence to support the premises that the matrix
was strengthened or that the grain boundaries were embrittled by aging.

SUMMARY

Impact samples of Hastelloy N, Haynes alloy No. 25, and Haynes
alloy No. 188 were solution annealed and aged up to 4000 hr at tempera-
tures over the range 650 to 900°C. Impact tests were run at 25 and
300°C, and all alloys underwent reduction in toughness due to aging. The
reduction in toughness was least for Hastelloy N, intermediate for alloy
No. 188, and greatest for alloy No. 25. The changes in Hastelloy N were
due to carbide precipitation, and those in Haynes alloys Nos. 25 and 188
26

were likely due to the combined effeclts of carbide and Laves phase
precipitation.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the contributions of J. M. Newsome,

T. N. Jones, and W. J. Stelzman for the aging and lmpact testing;

H. R. Tinch for the metallography; R. G. Berggren, R. G. Donnelly, and

W. R. Martin for review of the manuscript; the ORNL Graphic Arts Depariment
for preparation of the drawings, and the Metals and Ceramics Division
Reports Office for preparatioon of the manuscript.

REFERENCES

H. E. McCoy and R. E. Gehlbach, "Influence of Irradiation Temperature
on the Creep~Rupture Properties of Hastelloy N," Nucl. Technol.
11(1): 4560 (1971).

H. E. McCoy, Jr., 4n Evaluation of the Molten-Salt Reactor Fxperiment
Hastelloy N Surveillance Specimens — Fourth Group, ORNL-TM~3063
(March 1971).

S. T. Wlodek, Embrittlement of a Co-Cr-W (L-605) Alloy,
R~-61-FPD-538 (December 1961).

R. B. Herchearoeder, Aging Characteristics "Haynes" Developmental
Alloy No. 188, Report No. 7513, Technology Departmeni, Stellite
Division, Union Carbide, Xokomo, Indiana (August 1, 1968).

D. T. Bourgette, Effect of Aging Time and Temperature on the Impact
and Tensile Behavior of L[-605 — A Cobali~Base Alloy, ORNL~TM-3734
(April 1973).
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