HEAT TREATMENT EFFECT ON MICROSTRUCTURE AND ......of heat treatment process namely: as-cast,...
Transcript of HEAT TREATMENT EFFECT ON MICROSTRUCTURE AND ......of heat treatment process namely: as-cast,...
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: [email protected]
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
-269-
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
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[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
(a) (b)
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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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