Download Application of grain boundary engineering concepts

Materials Science and Engineering, A185 (1994) 39-43
39
Application of grain boundary engineering concepts to alleviate
intergranular cracking in Alloys 600 and 690
C. Cheung and U. Erb
Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Ont. K 7L 3N6 (Canada)
G. Palumbo
Ontario Hydro Research Division, 800 Kipling Avenue, Toronto, Ont. M8Z 5S4 (Canada)
(Received Augusl 13. 1993)
Abstract
Nickel-based alloys used in nuclear steam generator tubing have been found to be susceptible to intergranular stress
corrosion cracking while in service. Following a recently developed model, grain boundary engineering concepts may be
used to alleviate these concerns. This paper presents a report on the grain refinement approach in the proposed model
which results in a higher probability of arresting stress corrosion cracks before reaching a critical length at which failure
occurs. For this purpose, an electroplating system was developed to produce ternary Ni-Cr-Fe alloy coatings. Electroplating conditions are given for the production of Alloys 600 and 690 having an average grain size in the range
100-250 nm.
1. Introduction
Alloy 600 (UNS N06600) has been used in the
nuclear industry for steam generator tubing applications; however, this alloy has been shown to be prone
to both primary and secondary side intergranular stress
corrosion cracking (IGSCC)(see for example refs, 1
and 2). More recently, Alloy 690 has been considered
as a replacement option. However, its long-term resistance to cracking has yet to be fully addressed. In this
context, Palumbo et al. [3] have shown that a grain
boundary engineering approach may alleviate these
concerns. A model was presented which indicates that
the probability P of arresting an intergranular crack is
given by
p =fsp2 + 2 [f~p( l --~p )]
(l )
where f~ is the fraction of interfaces in the material
which are susceptible to cracking or unfavourably
oriented to the stress axis (note that f0 is strongly
dependent on the grain aspect ratio) and fsp is the fraction of non-susceptible grain boundaries. The probability g of arresting a crack within a length L less than
Lcr~t at which material failure occurs was shown to be
given by the following equation:
1 -g=(1
- e ) 2L/~t
(2)
where d is the average grain size. From the argument
put forth, the probability of crack arrest can be
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increased by three fundamental approaches: increasing
~p, decreasing d or altering the grain shape (i.e. changing f0). In other words, a material's resistance to IGSCC
can be increased by increasing the number of nonsusceptible grain boundaries in the material, decreasing
the overall grain size or having shape-modified grains
oriented favourably to the stress axis.
The first approach takes advantage of the properties
of special grain boundaries. In a recent paper, Aust et
al. [4] pointed out the importance of special Z boundaries; they are less susceptible to solute segregation,
more stable in terms of grain boundary sliding, fracture
and cavitation, have greater resistance to localized
corrosion etc. Palumbo et al. [5] confirmed that low Y
boundaries are more resistant to IGSCC in Alloy 600.
Thus one can alter the bulk properties of the material
by incorporating more of these boundaries into the
material. Palumbo et al. [6] have also shown that such
an increase in special boundaries may be achieved
through annealing twin formation. According to this
analysis, a "twin-limited microstructure" consisting
entirely of low Z boundaries can be achieved for a
polycrystal with twin boundary densities approaching
two thirds. More recently, a thermomechanical process
has been developed by Palumbo [7] through which the
population of special boundaries by twinning is
increased to values in excess of 65% from nominally
about 10% in conventionally processed polycrystalline
Alloy 600.
© 1994 - Elsevier Sequoia. All rights reserved
40
C. Cheung et al.
/
Alleviation of intergranular cracking in Alloys 600 and 690
The second and third approaches involve grain size
and shape control. It has been shown in previous
studies that a stress corrosion crack dictates at one
triple junction and then propagates along a grain boundary directly to another junction where it is arrested
and has to be re-initiated for further propagation [5].
Therefore decreasing the overall grain size indicates
the increase in the number of triple junctions in the
material and accordingly the probability of arresting a
crack before it reaches a critical length Lcrit at which
failure occurs. For example, a component of 1 mm
thickness having a conventional grain size of 50 /zm
possesses approximately 20 triple junctions as possible
sites for crack arrest across its entire thickness (Fig.
1 (a)). On the contrary, if the grain size is reduced to 0.1
/~m (100 nm), a coating of only 50/~m thickness, which
is the diameter of 1 grain in the conventional material,
has approximately 500 triple junctions across the coating (see Fig. l(b)). As a result, the probability of crack
arrest is increased considerably, leading to a higher
resistance to cracking.
Several techniques are now available for producing
nanocrystalline materials such as sputtering, gas condensation and electroplating [8]. Sputtering is hard to
control while gas condensation is capital intensive and
the production rate is rather low. Moreover, the
deposition of such a nanocrystalline coating onto a
finished product by sputtering and gas condensation is
technologically difficult. Electroplating, on the contrary, is much more suitable for large-scale industrial
applications in terms of control, flexibility, cost and the
relatively high rate of production. Electroplating is also
highly useful for producing materials with different
grain shapes (see Fig. 1 (c)), which has been previously
shown to affect the parameter f0.
The overall objective of this study is to develop an
electroplating technique for producing IGSCCresistant Ni-Cr-Fe alloys through grain size reduction
and grain shape control. Such a process if developed as
a coating technology could be applied as an in-situ
remedial measure for the rehabilitation of nuclear
steam generators [9]. In this paper, results pertaining to
the grain size reduction approach are presented.
2. Experimental details
Past research in electrodeposition of Ni-Cr-Fe
ternary alloys has dealt with compositions which
approximate that of 18-8 Cr-Ni stainless steel and a
large number of different electrolytes have been
utilized [10-18]. However, electrodeposition of alloys
which have a higher Ni content has received only very
little attention. Although sulphate baths are most
frequently used, chloride baths have proved their
Lcrit
(a)
v
Lcrit
(b)
m
Lcrit ~(
(c)
Fig. 1. Schematic diagrams showing intergranular stress corrosion crack arrest; (a) conventional crystalline material; (b) with a
nanocrystalline coating; (c) with a grain-shape modified coating.
viability in increasing the electrical conductivity of the
electrolyte and increasing the limiting current density
for Ni and Fe deposition. In addition, they are sometimes preferred because of their high efficiency of
anodic dissolution [19]. Therefore a chloride bath is
considered most suitable for plating alloys with a high
Ni content, such as Alloys 600 and 690. Furthermore,
electroplating of thin deposits (35/zm or less) has been
the major focus of previous work. An exception is the
work of el-Sharif et al. [20] who have plated in excess of
800 /zm. Moreover, none of the papers gives a
systematic analysis of grain size. One paper reported
that "the individual grains are too small to detect" [20].
The nominal ratio of the base constituents is 75
wt.% Ni, 15 wt.% Cr and 10 wt.% Fe for Alloy 600 and
60 wt.% Ni, 30 wt.% Cr and 10 wt.% Fe for Alloy 690.
It should be noted that commercial Alloys 600 and
690 contain various alloying elements and impurities
such as C, Mn, Cu, Si and S. However, it is almost
C. Cheung et aL
/
Alleviation of intergranular cracking in Alloys 600 and 690
impossible to develop an electrodeposition process
taking every component into consideration. Therefore
in the present work, attention is only given to the major
constituents, namely Ni, Cr and Fe. Based on the available literature, one of the most promising baths is
described by el-Sharif et al. [20], yielding electrodeposits of high Ni content up to 54 wt.%. The bath
contains chloride salts with boric acid in an electrolyte
made up of a 50:50 N, N-dimethylformamide (DMF)water mixture by weight (Table 1). The bath also contains additions of ammonium chloride and sodium
chloride.
Since both Alloy 600 and Alloy 690 are Ni rich,
modifications of the bath were necessary. These
included increasing the Ni concentration and decreasing the Cr concentration in the solution. In addition,
since DMF promotes Cr deposition as shown by
Watson et al. [21], DMF was not used in the present
study. With these considerations, a bath with composition listed in Table 2 was prepared. The cathode
material was chosen to be copper because it has previously been shown that electrodeposits obtained on Cu
substrates are more satisfactory [22] and alloys with the
highest Ni content were obtained on Cu [20].
Electrolysis was carried out in a standard 2 1 vessel
with an anode of high density graphite. The solution
was mechanically stirred at about 200 rev min-~. No
diaphragms were used during plating. The pH of the
electrolyte was not adjusted but monitored during the
experiments.
Electrodeposits were obtained in a range of temperatures (23-52 °C) and current densities (0.05-0.15
TABLE 1. Bath composition after el-Sharif et al. [ 19]
Component
Concentration
[Cr(H20)4C12]CI'2H20
NiCI2"6H20
FeCI2"6H20
NH4CI
NaC1
B(OH).~
DMF
Deionized water
0.8 mol
0.2 mol
0.03 mol
0.5 mol
0.5 mol
0.15 mol
50O g
500 g
TABLE 2. Bath composition used in present study
Component
Concentration
[Cr(H20)4CI2]CI'2H20
NiCI 2"6H20
FeCI2"6H20
NH4CI
NaC1
B(OH)3
0.28 mol
0.42 mol
0.025 mol
0.47 mol
0.51 mol
0.16 mol
41
A cm 2). Analysis of deposit composition was carried
out by energy-dispersive X-ray spectroscopy in a conventional scanning electron microscope.
3. Results and discussion
The effects of temperature and current density on
the composition of the alloy are summarized in Fig. 2.
At 52 °C (Fig. 2(a)), the Fe content is too low throughout the entire range of current densities investigated.
At higher current densities, the Ni-to-Cr ratio in the
alloy is about 2 to 1. This ratio decreases with current
density to about 1 to 1 at the lowest value of 0.05 A
c m -2. For the experiments carried out at 32°C (Fig.
2(b)), the general trend is that both the Ni and the Cr
content decrease with increasing current density. The
Fe content, on the contrary, increases from about 1
wt.% at 0.05 A c m -2 to about 55 wt.% at the highest
current density of 0.15 A cm-2. At room temperature
(23°C) (Fig. 2(c)), the Ni content decreases with
increasing current density while, at intermediate
current densities, Cr and Fe content go through a minimum and maximum respectively.
From the results shown in Fig. 2, an operating
window of current density and temperature can be
identified for which ternary Ni-Cr-Fe alloys with composition close to Alloy 600 can be produced. This
window is at a temperature of 32°C and a current
density of about 0.1 A c m -2, as indicated in Fig. 2(b)).
Similarly, electrodeposits with composition close to
that of Alloy 690 can be obtained at a temperature of
23 °C and a current density of 0.05 A c m -2, as indicated by the window shown in Fig. 2(c).
The typical microstructure of the deposits is shown
in Fig. 3. They are generally quite rough with nodular
structure containing a high degree of porosity. The
nodules generally vary from 5 to about 50/~m in size
and consist of arrangements of small grains. The grain
size is of the order of 100-250 nm as shown in the high
magnification scanning electron micrograph (Fig. 3(b)).
The deposits are brittle and exhibit networks of
microcracks. Therefore they may not yet meet the
requirements for the particular application discussed in
the previous sections.
The current efficiencies obtained in the present
study are fairly low, between 5 and 20%, which is to be
expected for deposition of Cr and/or Cr alloys. This is
due to the low deposition potential of Cr, the formation
of chromium hydroxide at low pH and the associated
overpotential for hydrogen evolution [21]. As a result,
the overall deposition rate is very low.
From this preliminary study, it can be concluded that
the deposition of ternary Ni-Cr-Fe alloys with composition close to those of Alloys 600 and 690 is
/ Alleviation of intergranular cracking in Alloys 600 and 690
C Cheung et al.
42
70
6O
,..,--'"""
[ ! Ni C o n t e n t
Cr Content[
Fe C o n t e n t
~.:: ...
t=
50
0
.,a
40
"'"'-,.
T=52Oc
"'"'-,
..
"-~................. . .................
~c~ 3o
~ 2o
I0
I~ ..................................
• ................
i
0.04
0.08
(a)
~
................
•
[
i
i
i
i
0.08
0.10
0.12
0.14
0.16
Current Density (A/ern 2)
Ni Content
Cr Content
Fe Conten!
7O
6O
"~
T=32°C
50
i
',,
,/
,,'
/*
"~
=
0
40
0
30
0
(.9
20
i
10
"
"'-
...,a - y
,
0.04
,--"]"
,
,
0.06
!
0.08
,
0.10
,
,
0.12
0/14
[~ Ni C o n t e n t
Cr C o n t e n t
Fe C o n t e n t
7O
"',
60
T=23°C
o 40
:~
/y,.,,.,
-.~...
0
0.18
Current Density (A/era 2)
(b)
20
,.'
""
lO
"', .,,~
.."
"""
"-..
.
Fig. 3. Scanning electron micrographss of e l e c t r o d e p o s i t e d
nanocrystalline ternary Ni-Cr-Fe alloys; (a) low magnification;
(b) high magnification.
. ..---
,"
0
¢
0.04
0.06
(c)
i
i
i
0.08
0.10
0.12
~
i
i
0.14
0.16
Current Density (A/cm 2 )
Fig. 2. Effect of current density on t h e a l l o y c o m p o s i t i o n
(a) at 52 °C; (b) at 32 °C; (c) at 23 °C.
at
different temperatures:
possible. However, remedies need to be found to
improve the overall quality of the deposits. At present,
studies are being carried out using complexing agents
such as sodium citrate and ethylenediaminetetra-
acetate. In previous studies, it has been shown that such
additions have beneficial effects on the codeposition of
ternary Ni-Cr-Fe alloys [ 12, 13, 18]. Such complexing
agents put the deposition potentials of the individual
metals closer together, hence making codeposition
easier [18]. Also, levelling agents might be needed to
reduce the roughness of the deposits. In addition,
further reduction in grain size may be achieved by
pulse plating as previously demonstrated with pure Ni
[23] and Zn-Ni alloys [24].
C. Cheung et al.
/
Alleviation of intergranular cracking in Alloys 600 and 6(8)
4. Conclusion
T h e application of a "grain b o u n d a r y engineering"
concept to increase the resistance to I G S C C of Alloy
600 and Alloy 690 has b e e n discussed. O n e a p p r o a c h
to achieve this is to decrease the grain size of the
material, which in practice could be achieved by the
electrodeposition of a nanocrystalline coating onto the
base alloy of conventional grain size. T h e results f r o m
this preliminary study have shown that deposition of
nanocrystalline ternary N i - C r - F e alloys with compositions close to those of Alloys 600 and 690 can be
accomplished. A plating bath containing the metal
chloride salts, sodium chloride, a m m o n i u m chloride
and boric acid was found to be successful.
Acknowledgments
Fruitful discussions with Dr. K. T. Aust and Dr. E S.
Gonzalez are gratefully acknowledged. T h e authors
would like to thank the National Sciences and
Engineering Research Council of C a n a d a for financial
support. O n e of the authors (C.C.) was the recipient of
a Queen's G r a d u a t e Award.
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