546
Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
PHYSICAL-METALLURGICAL CHARACTERISTICS OF NICKEL SUPER-ALLOYS
OF INCONEL TYPE
Jonšta Z., Jonšta P., Vodárek V., Mazanec K.
1
VSB – Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 17.
listopadu 15/2172, CZ 708 33 Ostrava-Poruba, Czech Republic, e-mail: [email protected]
Ě
FYZIKÁLN METALURGICKÉ CHARAKTERISTIKY NIKLOVÝCH SUPERSLITIN
TYPU INCONEL
Jonšta Z., Jonšta P., Vodárek V., Mazanec K.
1
VŠB – Technická univerzita Ostrava, Fakulta metalurgie a materiálového inženýrství, 17.
listopadu 15/2172,708 33 Ostrava-Poruba, Česká republika, e-mail: [email protected]
Abstrakt
asto používá
V leteckém pr myslu se pro lopatky turbín tryskových motor
niklových superslitin. Je to proto, že tento materiál je schopen splnit adu extremních
požadavk , jako je nap . žárupevnost za vysokých teplot, odolnost proti únavovému poškození,
odolnost proti únavovému p sobení spalin aj. Požadavkem je zajišt ní vysoké provozní
spolehlivosti a bezpe nosti p i exploataci za vysokých teplot. Dlouhodobá životnost a
spolehlivost materiálu je p ímo spojena s mikrostrukturou, resp. s její stálostí p i dlouhodobé
exploataci. P edm tné materiály bývají komplexn legovány a jsou ze strukturního hlediska
zna n komplikované [1].
P edložená práce se zabývá rozborem fyzikáln metalurgických charakteristik litých
variant niklových superslitin, konkrétn typ INCONEL 713LC a INCONEL 792 – 5A..
V daných typech superslitin se uplat uje n kolik zpev| ujících mechanizm . Hlavním je
precipita ní zpevn ní koherentními precipitáty intermetalické fáze Ni3Al, resp. Ni3 ( Ti,Al ).
Vlastní analýza je založena na hodnocení mikrostrukturních parametr p edm tných superslitin
a to jednak aplikací sv telné mikroskopie, jednak za použití elektronové mikroskopie v etn
chemické mikroanalýzy.
Strukturní analýza pomocí sv telné mikroskopie byla provedena na sv telném
mikroskopu OLYMPUS IX 71 a elektronomikroskopický rozbor pak na
ádkovacím
elektronovém mikroskopu JEOL JSM – 5510 v režimu sekundárních elektron . Lokální
chemická analýza pak byla realizována na elektronovém mikroanalyzátoru JCXA 733,
vybaveném energiov dispersním analyzátorem Sapphire.
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Abstract
Aerospace industry often uses nickel super-alloys for blades of jet engine turbines.
The reason is that this material can satisfy numerous extreme requirements, such as e.g. strength
even at very high temperatures, resistance to fatigue damage, resistance to fatigue effect of
combustion gases, etc. The main requirement is assurance of high operational reliability and
safety at exploitation under high temperatures. Long-term service life and material reliability is
directly linked to its micro-structure, or with its stability at long-term exploitation. These
materials are usually alloyed in a complex manner and they are very complicated from structural
viewpoint [1].
Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
547
Presented article deals with analysis of physical-metallurgical characteristics of cast
variants of nickel super-alloys, namely of the types INCONEL 713LC and INCONEL 792 – 5A.
Several strengthening mechanisms are applied in these types of super-alloys. Principal
mechanism is precipitation strengthening by coherent precipitates of Inter.-metallic phase Ni3Al,
or Ni3 (Ti, Al). The analysis as such is based on evaluation of micro-structural parameters of
these super-alloys by application of light microscopy, as well as electron microscopy and
chemical micro-analysis.
Structural analysis with use of light microscopy was performed on the light
microscope OLYMPUS IX 71, and electron-microscopic analysis was realised on the scanning
electron microscope JEOL JSM – 5510 in the mode of secondary electrons. Local chemical
analysis was then made on the electron micro-analyser JCXA 733, equipped with energydispersive analyser Sapphire.
Key words: nickel super–alloys, as–cast condition, structural – phase analysis
1. Introduction
Strict requirements to materials working in extreme conditions create a space for use
of nickel-based super-alloys. Use of these materials in hot parts of turbines belong to the most
demanding applications. Important position of super-alloys in this area is manifested by the fact,
that they represent at present more than 50 % of mass of advanced aircraft engines. Extensive
use of super-alloys in turbines, supported by the fact that thermo-dynamic efficiency of turbines
increases with increasing temperatures at the turbine inlet, became partial reason of the effort
aimed at increasing of the maximum service temperature of high-alloyed alloys [1].
This increase was enabled particularly by two factors. The first factor represents
advanced techniques of processing, which improved cleanness of alloy and thus enhanced its
reliability, and also mastering of technology of directional crystallisation and subsequently also
technology of products made of single crystals. The second factor represents development of
alloys with higher service temperature, achieved by alloying particularly by Re, W, Ta and Mo
[2].
Nickel super-alloys are used in Czech Republic mostly for structural parts of aircraft
engines, gas turbines and turbo blower. They are used in smaller extent also in glass industry and
scarcely also in other areas for structural parts, where working conditions do not allow use of
less alloyed materials.
Alloys of INCONEL type hold important position among nickel super-alloys
INCONEL. They are used for already mentioned components of aircraft engines, gas turbines
and turbo blowers. It is cast material which was subjected to complex alloying on the basis of Cr
– Al – Mo – Ti – Nb – Zr.
Presented work gives an evaluation of micro-structure of two of these cast superalloys, namely INCONEL 713LC and INCONEL 792 – 5A. The alloy INCONEL 792 – 5A
represents due to higher contents of Co and Ta higher type of these alloys.
2. Basic material characteristics of evaluated nickel super-alloys
Chemical composition of evaluated nickel super-alloys INCONEL 713LC ( IN
713LC ) and INCONEL 792 – 5A ( IN 792 – 5A ) is given in the Tables 1 and 2.
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Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
Table 1 Chemical composition of nickel super-alloy INCONEL 713LC (at. %)
C
Cr
Al
Mo
Ti
Nb
0.05
9.3
6.5
4.7
0.8
2.1
Zr
0.1
Table 2 Chemical composition of nickel super-alloy INCONEL 795-5A (at. %)
C
Mn(max)
Si(max)
Cr
Ti
Al
0.06-0.10
0.15
0.20
12.0-13.0
3.85-4.50
3.15-3.60
Fe(max)
0.50
Nb
0.50
Ta
3.85-4.50
Mo
1.65-2.15
W
4.50
Co(max)
8.50-9.50
P(max)
0.015
Ni
rest
B
0.010-0.020
S(max)
0.015
Ni
rest
Apart from this complex analysis of nickel super-alloy IN 792 – 5A specifies also
contents of Zr = 0.01 – 0.5 weight %.
Mean value of electron vacancy on 3d sphere N v = max. 2.38 serves as additional
informative data about chemical constitution of this super-alloy. Similar value of mean
concentration of vacancies N v = 2.30 is specified fro the nickel super-alloy IN 713LC. In
connection to these data it is possible to state in accordance with existing experience that
evaluated nickel super-alloys will probably not be susceptible to forming of σ phase.
Due top the fact that nickel super-alloy contains precipitates of the type Ni3 (Ti, Al),
corresponding to the phase ’, carbides, or even borides, which modify chemical composition of
basic metallic matrix, the exact evaluation of conditions of forming of the σ phase would require
application of correction of chemical composition of basic matrix, formed by solid solution. It
would be necessary to correct similarly also „losses“ of carbide forming elements in carbides, or
elements bound to borides. It is necessary to deduct their thus bound concentration from basic
chemical composition of the matrix, which then serves as basis for calculation of the mean value
of concentration of vacancies N v . However, it was established in practice that σ phase was
formed in the case that N v is greater than 2.5 – 2.6, and also in the case that N v = 2.0 – 2.1.
These results lead to a conclusion that presented processing has meaning of the first
approximation and that for more precise processing it is necessary to take into account also the
mean value of electron vacancies both in matrix of the phase Ni3( Ti,Al) – phase ’, and also in
basic matrix of the solid solution .
γ
γ
γ
3. Description and discussion of results
Figures 1 to 4 show photos of micro-structures of investigated super-alloys at
comparable magnifications. Figures 5 and 6 then present photos of grain boundaries.
Fig.1 Basic micro-structure of nickel super-alloy IN 713LC Fig.2 Basic micro-structure of nickel super-alloy IN 792-5A
Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
549
Fig.3 Micro-structure of nickel super-alloy IN 713LC
Fig.4 Micro-structure of nickel super-alloy IN 792-5A
Fig.5 Micro-structure from the area of grain boundaries
(Ni-super-alloy IN 713LC)
Fig.6 Micro-structure from the area of grain boundaries
(Ni-super-alloy IN 792-5A)
It is evident from presented photos that morphology of micro-structure of both
investigated materials is substantially identical, although in case of the super-alloy IN 792-5A
the percent occurrence of precipitates in comparison with that of IN 713LC is slightly increased,
as it can be seen in Fig. 6.
It follows from the Fig. 5 (IN 713LC), that precipitates are preferentially segregated
along the grain boundaries and their concentration in the volume is distinctly lower. These
results must be understood as a case of the first approximation. It is necessary to make a detailed
analysis with use of electron microscopy evaluation techniques in order to obtain more detailed
overview.
In connection with this fact it can be stated that in micro-structure of the super-alloy
IN 713LC carbidic particles will be probably localised at the grain boundaries apart from
segregated particles of ‘ phase Ni3 (Ti,Al). Their morphology indicates that these are probably
carbides of the M23C6 type, since the types like M6C or MX usually precipitate in volume of
matrix grains and they differ morphologically from M23C6. Chemical composition of particles of
the type MX from five local analyses is given in the Table 3.
γ
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Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
Table 3 Results of semi-quantitative analysis of MX carbides (at %)
Analysis No.
C
Nb
Ti
Ni
1
53.2
34.3
9.4
3.1
2
53.5
34.5
8.1
3.9
3
52.4
36.7
7.2
3.7
4
52.5
35.4
8.7
3.4
5
52.6
35.3
8.6
3.5
This table shows also important influence of Nb and less important effect of Ti. At the
same time minority occurrence of the phase with increased contents of Mo and Cr was also
established (see the Tab. 4), which indicates a possibility of connecting this precipitation thanks
to the detected high contents of molybdenum with forming of the Laves phase.
Table 4 Results of semi-quantitative analysis of coarse particle (at %)
Analysis No.
Mo
Cr
Ni
1
53.1
35.8
11.1
Areas (phases) of different etchability were also detected in evaluated micro-structure
of the alloy IN 713LC. These are for example cases of increased etchability, when these
anomalies have in most cases elliptic shape [3]. They are characterised by higher concentration
of carbide forming elements, namely Zr, Nb and Mo. This is with high probability a
conglomerate of carbidic particles, metallic particles and metallic matrix – see Fig. 7. Fine
particles of Ni3 ( Ti,Al ) – ‘ phase are usually uniformly segregated in basic matrix. These fine
particles are segregated both in inter-dendritic areas and also in the centre of crystallisation
nuclei, as it can be seen in the Fig. 8.
γ
Fig.7 Micro-structure of matrix – detailed picture of
segregation of IN 713LC
Fig.8 Detailed picture of matrix – IN 713LC, particles of
Ni3 (Ti,Al)
It can be generalised that nickel super-alloy INCONEL 713LC demonstrates
existence of certain irregularities in character of chemical composition, which is directly linked
to the used process of casting and evaluation of the alloy in as cast condition. In this connection
it is necessary to mention formation of large crystallites, including intensive development of
segregation process.
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Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
In case of the nickel super-alloy IN 792 – 5A particles of Ta and Ti carbides were detected in
basic metallic matrix, which represent the type MX. These particles were observed mainly in
inter-dendritic areas. Figure 9 show characteristic picture realised by visualisation of reflected
electrons.
Fig.9 Primary carbides of the type MX in Ni-super-alloy
IN 792-5A (reflected electrons)
Fig.10 Micro-structure of matrix – different etchability –
IN 792-5A
Table 5 contains data about chemical composition of discussed carbidic particles.
Table 5 Chemical composition of primary carbides MX (at. %)
Analysis No.
C
Ta
Mo
Ti
1
48.9
25.5
1.9
23.7
2
47.7
23.5
1.3
27.5
3
45.5
26.0
1.3
27.3
Micro-structural anomalies formed by coarse particles of precipitate were found in
inter-dendritic spaces. Specific, micro-morphologically differed elliptic formations [4], in which
very fine particles of precipitate were observed (see Fig. 10), were linked to the above areas.
Chemical composition of mentioned coarse particles in discussed areas leads to a conclusion that
these are particles of the phase Ni3 ( Ti,Al ), as it is documented by the Table 6. Areas with
slightly reduced etchability occurred in close vicinity of these micro-structural anomalies, as it is
shown above in the Fig. 10.
Table 6 Chemical composition of coarse particles (Al, Ti)Ni (at. %)
Analysis No.
Al
Ta
Ti
Cr
Co
Ni
1
12.2
3.4
9.0
3.4
6.9
65.1
2
12.1
3.2
8.7
3.5
6.4
66.1
3
11.5
3.1
9.2
3.9
6.6
65.7
4
11.6
3.1
9.1
3.4
6.4
66.4
5
11.6
3.3
8.2
3.7
6.6
66.6
Results of micro-chemical analysis of these areas and of metallic matrix in the centre
of crystallisation nuclei are given in the Tables 7 and 8.
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Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
Table 7 Chemical composition of metallic matrix (at. %)
Analysis No.
Al
Ta
W
Mo
Ti
1
6.9
1.9
1.9
1.4
3.9
2
7.2
2.1
2.0
1.1
4.1
3
7.3
2.3
2.1
1.4
3.9
Cr
16.0
16.0
15.5
Co
10.3
10.0
10.6
Ni
57.7
57.5
56.9
Table 8 Chemical composition in the areas with impaired etchability – in inter-dendritic spaces in vicinity
particles (at. %)
Analysis No.
Al
Ta
W
Mo
Ti
Cr
Co
1
6.0
1.6
1.6
1.8
3.8
17.7
11.0
2
6.3
1.7
1.8
1.6
3.9
17.5
10.8
3
5.9
1.7
1.6
1.6
4.0
18.0
10.4
of coarse
Ni
56.5
56.4
56.8
It follows from the obtained results that the areas with impaired level of etchability
are partly enriched in chromium, at the average from approx. 16 at.% to 17.5 – 18 at.%, but on
the other hand they lack elements, which enrich the areas situated in vicinity of coarse particles
’.
Particles of the precipitate Ni3 (Ti,Al) were segregated uniformly in the basic metallic
matrix. Very fine particles were uniformly segregated also in the areas with impaired etchability,
which occurred in the vicinity of micro-structural anomalies of elliptic form with coarse
particles Ni3 (Ti, Al). Example of distribution of fine particles Ni3 (Ti, Al), the occurrence of
which was detected in inter-dendritic areas, as well as in the centre of crystallisation nuclei, is
shown in the Figure 11.
γ
Fig.11 Detailed photo of matric – IN 792-5A ( ástice Ni3(Ti,Al)
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4. Conclusion
It can be stated that although the evaluated super-alloys INCONEL 713LC and
INCONEL 792- 5A manifest certain degree of micro-structural heterogeneity, it corresponds to
the state of evaluated materials (samples in as-cast state) and to segregation characteristics of
these super-alloys.
There is no principal difference between their micro-structural characteristics.
Probably the highest contribution will consist in additional alloying of the super-alloy
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Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553)
INCONEL 792 – 5A by tantalum and by higher contents of cobalt. First one of these elements
contributes to formation larger volumetric part of carbidic particles. Higher contents of cobalt
can contribute to increase of the volumetric part of ‘ phase, and also to reduction of the
stacking fault energy, which leads to enhancement of creep properties.
Presented micro-structural analysis and evaluation of local chemical composition will
contribute to better knowledge of the given types of super-alloys from the viewpoints of their
technical applicability and possible realised subsequent heat treatment.
γ
Acknowledgements
This work was supported by the research project MSM619890013 (Ministry of
Education of the Czech Republic).
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