10_2011_t1_BinczykSledzionaGradon.pdf

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

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 +

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

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

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

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

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.

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.

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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-

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