physical-metallurgical characteristics of nickel … · physical-metallurgical characteristics of...

Acta Metallurgica Slovaca, 13, 2007, 4 (546 - 553) 546

PHYSICAL-METALLURGICAL CHARACTERISTICS OF NICKEL SU PER-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] FYZIKÁLN

Ě 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 V leteckém prů myslu se pro lopatky turbín tryskových motorů č asto používá niklových superslitin. Je to proto, že tento materiál je schopen splnit ř adu extremních požadavků , jako je např . žárupevnost za vysokých teplot, odolnost proti únavovému poškození, odolnost proti únavovému pů sobení spalin aj. Požadavkem je zajiště ní vysoké provozní spolehlivosti a bezpeč nosti př i exploataci za vysokých teplot. Dlouhodobá životnost a spolehlivost materiálu je př ímo spojena s mikrostrukturou, resp. s její stálostí př i dlouhodobé exploataci. Př edmě tné materiály bývají komplexně legovány a jsou ze strukturního hlediska znač ně komplikované [1]. Př edložená práce se zabývá rozborem fyzikálně metalurgických charakteristik litých variant niklových superslitin, konkrétně typů INCONEL 713LC a INCONEL 792 – 5A.. V daných typech superslitin se uplatň uje ně kolik zpev|ň ujících mechanizmů . Hlavním je precipitač ní zpevně ní koherentními precipitáty intermetalické fáze Ni3Al, resp. Ni3 ( Ti,Al ). Vlastní analýza je založena na hodnocení mikrostrukturních parametrů př edmě tných superslitin a to jednak aplikací svě telné mikroskopie, jednak za použití elektronové mikroskopie vč etně chemické mikroanalýzy. Strukturní analýza pomocí svě telné mikroskopie byla provedena na svě telném mikroskopu OLYMPUS IX 71 a elektronomikroskopický rozbor pak na ř ádkovacím elektronovém mikroskopu JEOL JSM – 5510 v režimu sekundárních elektronů . Lokální chemická analýza pak byla realizována na elektronovém mikroanalyzátoru JCXA 733, vybaveném energiově dispersním analyzátorem Sapphire.

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].


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 energy-dispersive 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 super-alloys, 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.


Table 1 Chemical composition of nickel super-alloy INCONEL 713LC (at. %) C Cr Al Mo Ti Nb Zr Ni

0.05 9.3 6.5 4.7 0.8 2.1 0.1 rest

Table 2 Chemical composition of nickel super-alloy INCONEL 795-5A (at. %)

C Mn(max) Si(max) Cr Ti Al Fe(max) B

0.06-0.10 0.15 0.20 12.0-13.0 3.85-4.50 3.15-3.60 0.50 0.010-0.020

Nb Ta Mo W Co(max) P(max) S(max) Ni

0.50 3.85-4.50 1.65-2.15 4.50 8.50-9.50 0.015 0.015 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 spherevN = max. 2.38 serves as additional informative data about chemical constitution of this super-alloy. Similar value of mean concentration of vacancies vN = 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 γ ’, 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 vN . However, it was established in practice that σ phase was formed in the case that vN is greater than 2.5 – 2.6, and also in the case that vN = 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 γ ’, and also in basic matrix of the solid solution 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


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)

Fig.3 Micro-structure of nickel super-alloy IN 713LC Fig.4 Micro-structure of nickel 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 γ ‘ 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.


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)

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 ) – γ ‘ 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.

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.


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

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.

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.


Table 7 Chemical composition of metallic matrix (at. %) Analysis No. Al Ta W Mo Ti Cr Co Ni

1 6.9 1.9 1.9 1.4 3.9 16.0 10.3 57.7

2 7.2 2.1 2.0 1.1 4.1 16.0 10.0 57.5

3 7.3 2.3 2.1 1.4 3.9 15.5 10.6 56.9

Table 8 Chemical composition in the areas with impaired etchability – in inter-dendritic spaces in vicinity of coarse particles (at. %)

Analysis No. Al Ta W Mo Ti Cr Co Ni

1 6.0 1.6 1.6 1.8 3.8 17.7 11.0 56.5

2 6.3 1.7 1.8 1.6 3.9 17.5 10.8 56.4

3 5.9 1.7 1.6 1.6 4.0 18.0 10.4 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 γ ’. 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.

Fig.11 Detailed photo of matric – IN 792-5A (č ástice Ni3(Ti,Al)

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


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 γ ‘ 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.

Acknowledgements This work was supported by the research project MSM619890013 (Ministry of Education of the Czech Republic).

Literature [1] Hernas A., Jonšta Z.,et al.: Žárupevné oceli a slitiny [Refractory steels and alloys]. ZUSI

Žilina, 2002 [2] Hrbáč ek K.: Souč asný stav výzkumu a vývoje slitin na bázi niklu používaných v naší

republice na č ásti plynových turbín, turbodmychadel a odlitky pro sklář ský prů mysl [Current state of research and development of nickel based alloys used in our country for components of gas turbines, turbo blowers and castings for glass industry], Research Report , Brno 2004

[3] Dubiel B., Blackler M., Barnard P.M., Czyrska-Filemonowicz A.: TEM Microstructural Analysis of Crč el Deformed CM186LC Single Crystal Ni-Base Superalloy. In: Materials for Advanced Power Engineering 2006, Band/Vol.53, Part I, pp.485-494.

[4] Hou J.S., Guo J.T., Zhou L.Z., Li Z.J., Qin X.Z., Li Y.L., Ye H.Q.: Influence of Solution Heat Treatment Conditions on Gamma Prime Characteristics in a Hot Corrosion Resistant Superalloy. In: Materials for Advanced Power Engineering 2006,Band/Vol.53,Part.I,pp.435- 444.

physical-metallurgical characteristics of nickel … · physical-metallurgical characteristics of...

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