10 2011 t1 BinczykSledzionaGradon, Odlewnictwo
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Kompozyty 11: 1 (2011) 49-54
Franciszek Binczyk
*
, Józef Śleziona, Paweł Gradoń
Silesian University of Technology, Chair of Materials Technology, ul. Krasińskiego 8, 40019 Katowice, Poland
*
Corresponding author: Email: franciszek.binczyk@polsl.pl
Otrzymano (Received) 03.02.2011
MODIFICATION OF MACROSTRUCTURE OF NICKEL SUPERALLOYS
WITH COBALT NANOPARTICLES
The paper presents the results of studies on the effect of combined surface and bulk modifications on the macrostructure
of castings made from remelted, postproduction IN713C, IN100 and MAR247 waste alloys. Surface modification was car
ried out by applying onto the wax pattern surface, a coating containing zirconium silicate and cobalt aluminate. Bulk modifi
cation was carried out when the cast liquid alloy was passed through a special ceramic filter containing, among others, cobalt
aluminate. The nanoparticles of cobalt, acting as crystallisation nuclei, are the product of a modifier reaction (CoAl
2
O
4
inocu
lant) with the chemically active constituents of the nickel alloys and with the additional filter components in the form of
Al and Ti powders. The filters were placed in the mould pouring basin. The beneficial effect of combined modification on the
macrostructure (equiaxial crystals) and mechanical properties was stated. The effect of the active constituents present in the
nickel alloys and in the filter material was confirmed. A particularly strong refining effect on the macrostructure of equiaxial
crystals was obtained in the MAR247 alloy, which contained the strongest chemically active additives of Hf, Ta and Nb.
A hypothetical model of the surface and bulk modification was developed. A strong influence of the alloy pouring temperature
on the modification effect was reported. Modification was most effective when the pouring temperature did not exceed
1440
O
C.
Keywords: nickel superalloys, macrostructure, modification, pouring temperature, nanoparticles, CoAl
2
O
4
inoculant
MODYFIKOWANIE MAKROSTRUKTURY NADSTOPÓW NIKLU
NANOCZĄSTKAMI KOBALTU
Zamieszczono wyniki badań wpływu łącznego modyfikowania powierzchniowego i objętościowego na makrostrukturę od
lewów wykonanych po przetopie odpadów poprodukcyjnych ze stopów IN713C, IN100 i MAR247. Modyfikację powierzch
niową zrealizowano poprzez naniesienie na model woskowy powłoki zawierającej krzemian cyrkonu i glinian kobaltu. Mo
dyfikację objętościową zrealizowano podczas odlewania ciekłego stopu przez specjalne filtry ceramiczne, zawierające między
innymi glinian kobaltu. Nanocząstki kobaltu, pełniące rolę zarodków krystalizacji, są produktem reakcji modyfikatora
(inoculant CoAl
2
O
4
) z aktywnymi chemicznie składnikami stopów niklu oraz dodatkowymi składnikami filtrów w postaci
proszków Al i Ti. Filtry umieszczono w zbiorniku wlewowym formy. Stwierdzono korzystny wpływ łączonej modyfikacji na
makrostrukturę (kryształy równoosiowe) i właściwości mechaniczne. Potwierdzono oddziaływanie aktywnych składników
obecnych w stopach niklu i materiale filtrów. Szczególnie silny efekt rozdrobnienia makrostruktury kryształów równoosio
wych uzyskano dla stopu MAR247, który zawiera najbardziej aktywne chemicznie dodatki Hf, Ta i Nb. Opracowano hipo
tetyczny model modyfikacji powierzchniowej i objętościowej. Stwierdzono istotny wpływ temperatury odlewania stopu na
efekt modyfikowania. Efekt modyfikowania jest szczególnie widoczny przy temperaturze odlewania poniżej 1440
O
C.
Słowa kluczowe: nadstopy niklu, makrostruktura, modyfikacja, temperatura, nanocząstki, inokulant CoAl
2
O
4
INTRODUCTION
Currently, the near-net-shape castings of aircraft
engine parts are made from modern grades of nickel
and cobalt alloys
such as RENE-77, IN-100 IN-713C,
and MAR-247 [1, 2]. On solidification, these alloys
develop a specific type of macrostructure, composed of
frozen and columnar grains. Structures of this type are
prone to crack formation and propagation, resulting in
fatal failure of aircraft engines [3, 4]. Therefore, every
attempt should be made to obtain a structure of equiax-
ial grains within the whole casting volume.
The fundamental problem in the casting technology
of these alloys is how to control the type and size of the
grains for the specific operating conditions of different
parts of an aircraft engine. Along with an increase of
grain size, high-temperature creep resistance improves,
and therefore a structure of this type is most desirable in
the case of rotating parts operating in a combustion
chamber. Castings of a fine-grained structure offer
higher mechanical properties at low temperatures and
better resistance to thermal fatigue, and as such are
50
F. Binczyk, J. Śleziona, P. Gradoń
successfully used for fixed elements operating at
low temperatures. A schematic diagram is shown in
Figure 1.
a stepped test piece with steps of 6 mm (C1), 11 mm
(C2), 17 mm (C3), 23 mm (C4) thickness, shown in
Figure 2. The surface modifying coating (applied on the
wax pattern) consists of a mixture containing 10%
CoAl
2
O
4
and 90% zirconia powder and a binder in the
form of colloidal silica. As demonstrated in Figure 2,
the macrostructures in the specimen cross-sections
show the effect of modification - superficial or penetra-
ting to a very small depth only. Probably, after solidifi-
cation of the first solid layer, the liquid alloy is cut off
from the modifier-feeding source (the "source" of Co
particles). Pointing towards the central areas, a zone of
very fine columnar crystals is formed. As literature data
suggest, the mechanism of the modifying effect of
cobalt aluminate CoAl
2
O
4
has not been fully explained
yet.
Fig. 1. Schematic representation of low and hightemperature property
effects on microstructure
Rys. 1. Schemat wpływu mikrostruktury na nisko i wysokotempera
turowe właściwości
Literature data mention numerous studies looking
for some means to improve the macro- and microstruc-
ture of nickel superalloys by using the techniques of
refining [5, 6] and modification. A simple solution is
surface modification with nanoparticle inoculants,
which are included in the composition of a coating
directly touching the casting surface [7-11]. In [12], the
results of the modification of the Inconel 718 alloy with
microadditions of cobalt oxide CoO were presented.
A minor degree of structure refinement with slight im-
provement of mechanical properties was obtained. The
authors of this study investigated the solidification pro-
cess [13,14] and the effect of bulk modification taking
place in an IN-713C nickel superalloy on the stereolo-
gical parameters of its macrostructure [15-17]. In [18],
the results of the studies on the effect of repeated re-
melting of IN-713C, IN-100 and MAR-247 alloys on
the chemical composition and mechanical properties
were presented. The results disclosed in these studies
can serve as a good example of the beneficial effect that
inoculating with a mixture of zirconium silicate,
CoAl
2
O
4
inoculant, Al and Ti powders, and colloidal
silica is expected to have on the crystallisation and
refinement of equiaxial grains.
C1 6 mm C2 11 mm C3 17 mm C4 23 mm
Fig. 2. Macrostructure on surface and in crosssections of cast stepped
test piece after surface modification
Rys. 2. Makrostruktura powierzchni oraz przekrojów poprzecznych
odlewu próby schodkowej modyfikowanego powierzchniowo
In [19] it has been observed that after pouring liquid
metal into a mould coated with an inoculant, the cobalt
particles present in the CoAl
2
O
4
aluminate may undergo
reduction due to the effect of the chemically active
constituents of the nickel alloys, among others – alumi-
nium:
2
Al = Co +
4
Al
2
O
3
CoAl
2
O
4
+
3
3
Owing to a strong agreement that exists between the
crystal lattice of the nickel superalloy matrix (? phase)
and high-temperature Co particles, these particles can
play the role of the crystallisation nuclei, provided they
are not dissolved in the liquid alloy first. Therefore, the
lower the temperature and the shorter the time of con-
tact between the liquid alloy and mould surface, the
stronger the modifying effect is, which is particularly
well visible in the thin elements of the stepped test
piece. Therefore, it has been decided to undertake re-
search on bulk modification as a source of crystallisa-
tion nuclei formation before the liquid alloy enters the
mould cavity (for example, in a gating system when
the liquid alloy is flowing through the filtrating
element).
RESEARCH PROBLEM
The surface modification applied so far (with
a modifier placed in the internal layer of the mould),
though enables making near-net-shape castings satis-
fying the predetermined requirements, does not guaran-
tee obtaining a homogeneous structure of equiaxial
crystals. The modifying effect occurs only in a thin
surface layer of the casting and decreases strongly with
a decreasing cooling rate, as seen in the elements of
Kompozyty 11: 1 (2011) All rights reserved
51
Modification of macrostructure of nickel superalloys with cobalt nanoparticles
THERMODYNAMIC CALCULATIONS
In [20], a model of the thermo-chemical phenomena
taking place at the ceramic mould - liquid alloy inter-
face (IN-713C, IN-100 and MAR-247 alloys) was de-
scribed. The content of the main elements in the exam-
ined alloys is compared below:
-
constituents of nickel alloys. The most intense reactions
will occur between the cobalt aluminate and hafnium
(g), tantalum (f) and niobium (e). Less intense reactions
may take place with aluminium (d), titanium (c) and
chromium (b). Hence it can be concluded that the po-
tential nuclei for "in situ" crystallisation can form ac-
cording to the models shown in Figure 4.
IN-713C (0,19% C, 13,25% Cr, 6,13% Al, 0,83% Ti,
4,21% Mo, 2,19% Nb, remainder Ni),
-
MAR-247 (0,15% C, 8,56% Cr, 5,59% Al, 10,0%
Co, 1,08% Ti, 0,65% Mo, 1,40% Hf, 9,98% W,
3,17% Ta, remainder Ni),
-
IN-100 (0,16% C, 8,56% Cr, 5,66% Al, 13,41% Co,
4,66% Ti, 3,01% Mo, 0,8% V, remainder Ni).
Studies were carried out on a mould with a modify-
ing coating based on zirconium silicate and cobalt alu-
minate. The results of an X-ray microanalysis of the
precipitates on the mould and casting surface revealed
the strongest chemical activity of
Al, Ti, Cr and Nb
in
the IN-713C alloy,
Al, Ti, Cr and V
in the IN-100
alloy, and
Al, Ti, Cr, Hf and Ta
in the MAR-247 al-
loy.
Hence it follows that chemical reactions take place
when the liquid nickel alloy is flowing through the filter
and contacts the surface where the particles of cobalt
aluminate CoAl
2
O
4
are present. To check this point of
view, the free enthalpy of possible chemical reactions
was calculated. The results of the computations made
by HSC software [21] are shown in Figure 3.
a. CoOAl
2
O
3
= Al
2
O
3
+CoO
b. 3(CoOAl
2
O
3
) +2Cr = 3Co + Cr
2
O
3
+ 3(Al
2
O
3
)
c. CoOAl
2
O
3
+ 1/2Ti = Al
2
O
3
+ Co + 1/2(TiO
2
)
d. CoOAl
2
O
3
+ 2/3Al = 4/3 (Al
2
O
3
) + Co
e. 2(CoOAl
2
O
3
) + Nb = 2Co + NbO
2
+ 2(Al
2
O
3
)
f. 5(CoOAl
2
O
3
) + 2Ta = 5Co + Ta
2
O
5
+ 5(Al
2
O
3
)
g. 2(CoOAl
2
O
3
) + Hf = 2Co + HfO
2
+ 2(Al
2
O
3
)
Fig. 4. Models of operation of components of modifying mixture with
active alloying constituents (a) and active components of modi
fying mixture (b)
Rys. 4. Modele sposobu oddziaływania składników mieszanki mody
fikującej z aktywnymi składnikami stopu (a) i aktywnymi
składnikami mieszanki modyfikującej (b)
MATERIALS AND METHODS OF INVESTIGATION
The melting charge was composed of post-
production waste IN-713C, MAR-247 and IN-100 al-
loys in the form of gating systems and defective
castings. The ceramic moulds with modifying coating
were made by the investment process. The modifying
filter was placed in the pouring basin. To avoid rapid
heat losses, the moulds were "wrapped" with wool insu-
lation, as shown in Figure 5. The modifier-containing
ceramic filters (made according to the author’s genuine
idea) are shown in Figure 6.
100
a b c d e f g
0
800 900 1000 1100 1200 1300 1400 1500
-100
-200
-300
Temperature,
O
C
Fig. 3. Free enthalpy G in regards to temperature for CoAl
2
O
4
reaction
with selected constituents of nickel alloys
Rys. 3. Wyniki obliczeń entalpii właściwej reakcji CoAl
2
O
4
z wybranymi
składnikami stopów niklu
Fig. 5. Ceramic mould with thermal insulation and modifying filter
placed in pouring basin
Rys. 5. Forma ceramiczna z izolacją cieplną i filtrem modyfikującym
w zbiorniku wlewowym
The positive value of the specific enthalpy makes
the decomposition of cobalt aluminate (a) within the
examined range of temperatures impossible. Cobalt
aluminate can enter into reaction with some specific
Melting was carried out in a vacuum induction fur-
nace, type IS 5/III, made by Leybold - Heraeus, in
a crucible rammed from MPi refractory material based
Kompozyty 11: 1 (2011) All rights reserved
52
F. Binczyk, J. Śleziona, P. Gradoń
on MgO. The charge weight was 8,5 kg. Melting was
conducted in a vacuum of 10÷2 hPa. Before pouring,
argon at a pressure of 900 hPa was introduced to the
furnace chamber. The temperature of the melt in the
crucible was measured with a Pt-PtRh10 immersion
thermocouple, and additionally with a laser pyrometer.
The temperature of pouring was 1465°C. An example
of a casting made from the IN-713C alloy is shown in
Figure 6.
Fig. 8. Macrostructure of castings (13x25x100 mm) for investigated Ni
alloys after surface and bulk modification
Rys. 8. Makrostruktura odlewów z badanych stopów po modyfikowaniu
powierzchniowym i objętościowym
HYPOTHETICAL MODEL OF MODIFICATION
WITH COBALT NANOPARTICLES
The nuclei-forming power of cobalt nanoparticles
has been confirmed by the results of X-ray microanaly-
sis of the distribution of this constituent in the outer
layer of the IN-713C alloy casting. The microanalysis
was made with an optical glow discharge GDS GD
Profiler HR emission spectrometer. The results are
shown in Figure 9.
Fig. 6. Casting made from IN713C alloy
Rys. 6. Odlew ze stopu IN713C
RESULTS AND DISCUSSION
The effectiveness of the modification was evaluated
from the results of the casting macro- and microstruc-
tural examinations. Figure 7 shows the macrostructure
of the starting charge materials, while Figure 8 shows
the same macrostructure after modification.
The macrostructure of the charge materials (unmodi-
fied castings) is in a prevailing part composed of co-
lumnar crystals. As might be expected, the combined
treatment of surface and bulk modification resulted in
the formation of equiaxial crystals within the whole
casting volume. As follows from Figure 7, the highest
degree of grain refinement was obtained in the casting
made from the MAR-247 alloy. This is probably due to
the very active influence of the additions of hafnium
and tantalum present in this alloy. Thus, earlier findings
following from the thermodynamic calculations have
been confirmed. In the remaining castings made from
the IN-713C and IN-100 alloys, the effect of modifica-
tion is less intense due to the weaker reducing effect of
aluminium and titanium.
Fig. 9. Cobalt concentration in the surface layer of IN713C alloy casting
Rys. 9. Stężenie kobaltu w warstwie powierzchniowej odlewu ze stopu
IN713C
A hypothetical model of the modifying effect of Co
particles on the nuclei-forming process in regards to the
cooling rate (the value of undercooling) is shown in
Figure 10.
The modifying effect of cobalt particles can be dis-
cussed with a breakdown into the several successive
stages of the process:
Stage 1
(in the range of T
cast
to T
rCo
- the tempera-
ture of the dissolution of cobalt particles in the alloy
melt): The formation of Co particles as a result of the
Fig. 7. Macrostructure of charge materials (casting diameter 30 mm)
Rys. 7. Makrostruktura wsadu wyjściowego
Kompozyty 11: 1 (2011) All rights reserved
53
Modification of macrostructure of nickel superalloys with cobalt nanoparticles
reduction process induced by the chemically active
alloying elements, such as Al, Ti, Cr, Hf, present in the
alloy and in the modifying mixture. Immediate dissolu-
tion of these particles in the melt at a temperature above
T
rCo
. When the alloy temperature at the time of pouring
considerably exceeds the level of T
rCo
, the modification
does not occur, as confirmed by the results of experi-
ments carried out for a casting pouring temperature
above 1470
O
C. Only a weak surface modifying effect is
possible, due to a sudden drop in the molten alloy tem-
perature at the mould surface.
Stage
2
(in the range of T
rCo
to T
sol
):
The Co particles are stable and can unite into clusters of
a size critical for the given undercooling, which de-
pends on the cooling rate in, e.g. the subsequent ele-
ments of a stepped test piece.
Stage
3
(in the range of T
liq
to T
sol
):
On the thus formed nuclei, the grains grow and their
size depends on the number of crystallisation nuclei
formed by the high-temperature Co particles combina-
tion. The number and size of the nuclei depend on the
degree of alloy undercooling in selected parts of the
casting.
Stage
4
(after casting solidification):
Regular solidification of the casting with a well-defined
macrostructure. Possible changes in microstructure are
the result of structure ordering (? to ?' transformation)
with the formation of microporosities, stresses and oth-
er precipitation processes.
quence of modification is an obvious increase of the
mechanical properties, yield strength and tensile
strength in particular. Compared to an unmodified melt,
after the combined modification, these properties grow
by about 10 to 15%. Unfortunately, all this is at the cost
of the decreasing high-temperature creep resistance
[22].
To sum up, it can be concluded that:
1.
The basic parameter determining the effect of modi-
fication (both surface and bulk) is the pouring tem-
perature of nickel alloys.
2.
The effect of modification is particularly well visible
at a pouring temperature below 1440
O
C. Hence it
can be concluded that the temperature T
rCo
should be
lower than 1440
O
C.
3.
Surface modification (modifier in the first layer of
the ceramic mould) occurs only on the surface, be-
cause the "source" of Co particles is cut off from the
liquid alloy as soon as the first solid layer is formed
on the mould surface. On the thus formed nuclei,
only columnar crystals can grow.
4.
The higher the cooling rate, the finer the columnar
grains formed in surface modification and equiaxial
grains formed in bulk modification are.
Acknowledgements
The financial support of Structural Funds in the
Operational Programme - Innovative Economy (IE
OP)
financed from the European Regional Develop-
ment Fund - Project No. POIG.0101.02-00-015/08 is
gratefully acknowledged.
REFERENCES
[1] Zupanic F., Boncina T., Krizman A, Tichelaar F.G., Struc
ture of continuously cast Nibased superalloy Inconel 713C,
J. Alloys and Compounds 2001, 329, 12, 14, 290297.
[2] Tabuchi M., Kubo K., Yagi K., Yokobori A.T., Results of
a Japanese round robin on creep crack growth evaluation
methods for Nibase superalloys, Engineering Fracture Me
chanics 1999, 62, 1, 4760.
[3] Smith A., Mainwood A., Watkins M, The kinetics of the
capture of nitrogen by nickel defects in diamond, Diamond
and Related Materials 2002, 11, 36, 312315.
[4] Lachowicz M., Charakterystyka zmian mikrostrukturalnych
i mechanizmów pękania występujących w trakcie spawania
i obróbki cieplnej superstopu Inconel 713C, Praca doktorska
Politechnika Wrocławska, Wrocław 2006.
[5] Hartman D., Muerrle U., Reber G., The effects of electron
beam refining on the castability of IN 713 C, Metal 1992, 5,
443.
[6] Xiong Y., Yang A., Guo Y, Liu W., Liu L, Grain refinement
of superalloys K3 and K4169 by the addition of refiners,
Science and Technology of Advanced Materials 2001, 2,
1317.
[7] Pucka G., Technologia otrzymywania łopatek turbin gazo
wych o regulowanej wielkości ziarna, Inżynieria Materiało
wa 1984, 45, 115119.
[8] Zielińska M., Sieniawski J., Wierzbińska M., Effect of
modification on microstructure and mechanical properties of
Fig. 10. Hypothetical model of nickel alloy modification due to nuclei
forming power of Co particles
Rys. 10. Hipotetyczny model modyfikowania stopów niklu w wyniku
zarodkotwórczego działania cząstek Co
CONCLUSIONS
The study clearly shows that the combination of sur-
face modification (modifying coating on the mould
surface) with bulk modification (modifier as a compo-
nent of the filter) is greatly responsible for the for-
mation of equiaxial crystals within the entire volume of
the casting.
The bulk modification alone leads to a mixed struc-
ture of equiaxial and columnar crystals. The conse-
Kompozyty 11: 1 (2011) All rights reserved
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