Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5653

HEAT TREATMENT EFFECT ON MICROSTRUCTURE AND

PROPERTIES OF SINGLE CRYSTAL CMSX-4® NICKEL-BASED

SUPERALLOY

Andrzej Nowotnik(*)

, Paweł Rokicki, Grzegorz Jakubowicz, Daniel Kurkowski, Grażyna

Mrówka-Nowotnik, Małgorzata Wierzbińska, Jan Sieniawski, Jacek Nawrocki

Department of Materials Science, Rzeszow University of Technology, Rzeszow, Poland (*)Email: nowotnik@prz.edu.pl

ABSTRACT

Single crystal CMSX-4® nickel-based superalloy is one of the most popular representatives

commonly used for manufacturing of aircraft engines hot-zone turbine blades. Although there

are plenty of described procedures concerning heat treatment and final properties, literature

presents loss of data in aim of description of microstructure modelling while the heat

treatment process. Presented paper describes microstructure changes through different stages

of heat treatment process namely: as-cast, annealed, precipitation hardened and aged.

Additionally impression of mechanical properties change has been presented in form of

micro-hardness measurements and creep testing.

Keywords: heat treatment, single crystal, CMSX-4, superalloys, vacuum furnace.

INTRODUCTION

Gas turbine engine designers seek improved fuel efficiency power to weight performance,

improved hot section durability and lower life cycle costs. To achieve the improved

performance and efficiencies, designers tend to utilize increasingly capable single crystal

alloys. Most popular in the described field is CMSX-4® . It is an ultra-high strength, single

crystal alloy developed by the Cannon Muskegon Corporation. This second generation

rhenium-containing, nickel-base single crystal alloy is capable of higher peak

temperature/stress operation of at least 1163°C. Additionally it is capable to operate in long

multiple cycles f.e. Solar® Turbines report blade lives to overhaul of 25,000 - 30,000 hours in

their 15,000 hp Mars 100 industrial gas turbine. The aim of presented work is to investigate

heat treatment effect on microstructure and properties of nickel-based single crystal

superalloy CMSX-4®. Nickel-based single crystal superalloy CMSX-4® is commonly used

for aircraft engines hot-zone turbine blades manufacturing. In the aircraft industry limitations

there are no norms describing proper hat treatment procedures to obtain desired final

parameters. Very problematic, especially in the aerospace industry, is phase transformation in

high temperature and thus lowering of melting point of the alloy. Current paper presents

complex microstructure investigation of CMSX-4® alloy after different stages of heat

treatment process.

RESULTS AND DISCUSSION

To obtain most suitable surrounding parameters, different fixture materials and mounting

methods have been investigated and their influence on the interface with the specimens and

possibility for partial or full melt has been estimated. Most suitable heat treatment conditions

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(without any influence on the surface of the treated material) have been obtained for CMSX-

4® alloy used as the fixture material (Fig. 1a).

(a) (b)

Fig. 1 - (a) Fixture with instrumentation for CMSX-4 superalloy specimen mounting and a reference

thermocouple in a Monotherm vacuum furnace chamber, (b) microstructure of the CMSX-4 superalloy in as-cast

state

CMSX-4 alloy in the as-cast state has a non-uniform microstructure consisting of γ-phase

dendrites, γ/γ' eutectic areas and γ'-phase precipitates of different morphology (Fig. 1b).

Additionally no chemical composition difference has been observed in the dendritic and

interdendritic regions. Most of the carbides in the microstructure are of the MC type. They are

mostly based on hafnium and tantalum. Figure 2a present microstructure of as-cast state of the

material in which dendritic type structure can be observed. Primary dendrite arms are

approximately parallel to the axis of crystal growth. Presence of the carbides has been also

proven by the metallographic observations. γ-γ’ eutectic has been noted in the interdendritic

areas (fig. 2b). Precipitation hardening phase γ’ appears in the matrix in bimodal particle size

(larger sizes γ' phase particles are observed in interdendritic areas while smaller in the

dendrites).

(a) (b)

Fig. 2 - As-cast state of single crystal CMSX-4 nickel-based superalloy structure details: a)

γ-γ´ eutectic and porosity on dendrites boundaries (LM 100x), b) γ-γ´ eutectic and γ´

precipitates with bimodal size distribution (SEM 1700x)

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

On the basis of literature analysis and own research

aging process has been divided into two stages in

properties by precipitation of

carried out at 1140°C for 6 h.

specimens were prepared: P

solution heat treatment followed by

heat treatment and two stages of aging process.

Fig. 3 presens metallographic observations of microstructure from annealed

figure obtained by light microscope present some residual dendrites not fully

annealing process (fig. 3a). Based on the metallographic observations one can con

the annealing process presented in the paper highly influenced the homogeneity of the

microstructure of CMSX-4® nickel based single crystal superalloy in mean of size, shape and

distribution of the precipitation hardening

Fig. 3 - Microstructure of CMSX

a)

c)

Fig. 4 - (a) Microstructure of single crystal CMSX

diffraction using transmission electron microscope, (c)

a)

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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On the basis of literature analysis and own research, involving several heat

aging process has been divided into two stages in order to obtain required

properties by precipitation of γ’-phase dispersed in a γ matrix. The first aging stage was

1140°C for 6 h. The followed second stage at 871°C for 20 h. Three sets of

P – specimens after solution heat treatment, S1

followed by first stage of aging process, S2 - sample

treatment and two stages of aging process.

metallographic observations of microstructure from annealed

figure obtained by light microscope present some residual dendrites not fully

. Based on the metallographic observations one can con

the annealing process presented in the paper highly influenced the homogeneity of the

4® nickel based single crystal superalloy in mean of size, shape and

precipitation hardening γ’ phase.

Microstructure of CMSX-4 alloy in annealing state

b)

(a) Microstructure of single crystal CMSX-4 superalloy after first stage of aging S1

diffraction using transmission electron microscope, (c) indexation of diffraction pattern.

b)

involving several heat-treatment tests,

required mechanical

. The first aging stage was

second stage at 871°C for 20 h. Three sets of

treatment, S1 - samples after

samples after solution

metallographic observations of microstructure from annealed state. Pictures in

figure obtained by light microscope present some residual dendrites not fully dissolved in the

. Based on the metallographic observations one can conclude that

the annealing process presented in the paper highly influenced the homogeneity of the

4® nickel based single crystal superalloy in mean of size, shape and

after first stage of aging S1, (b) electron

indexation of diffraction pattern.

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The γ’ phase size and morphology can be precisely controlled by careful precipitation

strengthening heat treatments. The strengthening γ'-phase precipates are initially spherical and

rounded cubic in shape and grow into cubic shape only (Fig. 4). γ'-phase morphological

changes have important effect on the strength, toughness, high-temperature creep and fatigue

property of the heat treated alloy. One can see in the microstructure of CMSX-4 superalloy after first

stage of aging S1 that γ matrix contains cube-like γ' precipitates (Fig. 3a).

Electron diffraction confirmed the presence of the γ and γ ' phases (Fig. 4b and 4c). Third step

of single crystal CMSX-4® nickel-based superalloy heat treatment assumes precipitation

hardening. Figure 5 presents chemical composition analysis from this state of the material.

The investigation proves presence of both γ and γ' phases in fine structure morphology. Fig. 6

presents microstructure observation. Gentle porosity can be observed on the grain boundaries

of γ phase (fig 6a). SEM pictures present cubic and elongated shapes of γ’ precipitates to

compare with annealed state (fig. 6b). Additionally gas porosity observed in light microscopy

can be investigated in more details.

Fig. 5 - Chemical composition analysis of microstructure regions 1 and 2 for precipitation

hardened state of single crystal CMSX-4® nickel-based superalloy: a) structure with

pointed regions for measurement, b) region 1 and 2 measurements presenting both γ and γ'

phase constitution, c) chemical composition measured in regions 1 and 2 in weight and

atomic percentage

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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Fig. 6 - Precipitation hardened state of single crystal CMSX-4 nickel-based superalloy structure details: a)

porosity formed on the grain boundaries (LM 100x), b) homogeneous structure with fully eliminated dendrites

(LM 500x).

CONCLUSION

The heat treatment process applied in the paper resulted in significant changes in the

microstructure and mechanical properties of single crystal CMSX-4® nickel-based

superalloy. Metallographic observation analysis results in the as-cast state of the material

showed dendritic structure of γ’ phase with MC carbides. In interdendritic areas separation of

γ- γ’ eutectic could be observed. Analysis of the chemical composition presented no

significant differences in the content of chemical elements between interdendritic areas and

dendrites. Metallographic observations of annealed state of the material presented

homogenization of the structure in meaning of γ’ dendrites dissolution in the matrix. Both γ -

γ’ eutectic and MC type carbides have been fully dissolved. No secondary carbides of M23C5

or MC6 type appeared. Chemical composition analysis and microscopic observation of the

CMSX-4® nickel-based superalloy after two-step aging process showed slight precipitation

strengthening γ’ particles growth. Additionally shape change from pure cubic to elongated

ones could be observed. Besides shape change no significant difference in the microstructure

or hardness values between the precipitation hardened and aged stage of the material was

observed. Hardness testing proved mechanical properties standard behaviour through

dissolving of γ’ phase in the annealing process and precipitation hardening in further stages of

the heat treatment. Creep test has additionally showed increase of creep resistance and

decrease of plasticity.

ACKNOWLEDGMENTS

This work was supported by National Centre for Research and Development (NCBR) in the

frame of Applied Research Programme (Contract No PBS1/A5/10/2012).

REFERENCES

[1]-Lortrakul P, Trice RW, Trumble KP, Dayananda MA. Investigation of the mechanisms of

type-II hot corrosion of superalloy CMSX-4. Corros Sci. 2014;80:408-15.

[2]-Pytel, M. , Nowotnik, A., Szeliga, D., Sieniawski, J. Microstructural investigations of

nickel-based superalloys with different structure, Key Engineering Materials, Volume 592-

593, 2014, Pages 557-560

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[3]-Szeliga, D., Kubiak, K. Burbelko, A., Motyka, M., Sieniawski, J. Modeling of directional

solidification of columnar grain structure in CMSX-4 nickel-based superalloy castings,

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