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Procedia Engineering 00 (2014) 000–000
www.elsevier.com/locate/procedia
“APISAT2014”, 2014 Asia-Pacific International Symposium on Aerospace Technology,
APISAT2014
Effect of melting temperature on the microstructure stability of a Nibased single crystal superalloy
Zhenxue Shi*, Shizhong Liu, Xiaoguang Wang, Xiao-dai Yue, Jia-rong Li
Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials, Beijing
100095, China
Abstract
The single crystal superalloy with [001] orientation were prepared by screw selecting method in the directionally solidified
furnace. Three different melting temperature 1570℃, 1620℃ and 1670℃ were used in the melting process, respectively. The long
term aging of the specimens after full heat treatment were performed at 1100℃ for 800h. The effect of melting temperature on the
microstructure stability of the alloy was investigated. The results showed that the primary dendrite arm spacing of alloy
decreased with the increasing of melting temperature. The size of γ′ phase particles in these two regions decreases and cubic
shape of those turns a little regular with increasing of melting temperature. The difference of γ′ size between the dendrite core
and the interdendritic region reduces. The size of γ′ phase particles after heat treatment decreases with increasing of melting
temperature. The extent of elements segregation decreases with increasing of melting temperature. The needle shaped TCP
phases have already been observed in all of specimens and the amount of TCP phases decreases with increasing of melting
temperature. Increase melting temperature of the single crystal superalloy with high refractory content can increase high
temperature phase stability.
© 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).
Keywords: single crystal superalloy; melting temperature; microstructure stability; TCP phase
1. Intruduction
Nickel based single crystal superalloys have superior mechanical properties at elevated temperature, which makes
* Corresponding author. Tel.: +86 10 62498312; fax: +86 10 62498306.
E-mail address: shizhenxue@126.com
1877-7058 © 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA)
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Zhen-xue Shi / Procedia Engineering 00 (2014) 000–000
them the most suitable materials for the manufacture of turbine blades in aero engines. The temperature capability of
the turbine blade has increased significantly for the past several decades. Some of the advances have been achieved
through improved the content of refractory alloying elements [1-4]. However, the superalloys with high fraction
refractory elements are susceptible to the precipitation of deleterious topologically close packed (TCP) phases [5, 6],
and the creep properties decrease as a result of the TCP formation [7, 8]. In recent years, many investigations have
revealed that the melt characteristics have strong influence on the solidification processing and finally on the
microstructure and property of solidified materials. Wang indicated that the melt superheating treatment enhances
the solidification interface stability and has important effect on the solidification characteristics [9]. Zou found in
DD3 alloy that the primary dendrite arm spacing and γ′ phase are decreased with the increasing of superheating
temperature [10]. Zhang concluded that the γ′ rafts become more stable and the stress rupture with the elevated
superheating temperature [11]. However, there is no research about the influence of the melt characteristic on the
microstructure stability of Ni-based single crystal superalloy. In this study, the effect of melting temperature on the
dendrite arm spacing, γ′ phase, dendrite segregation and microstructure stability of a Ni-based single crystal
superalloy was investigated. The purpose of the investigation is to promote the development of new generation
single crystal superalloy with high microstructure stability.
2. Experimental procedure
A Ni-Cr-Co-Mo-W-Ta-Nb-Re-Al system single crystal with [001] orientation were cast by means of crystal
selection method in the directionally solidified furnace. Three different melting temperature were used in this
experimental study. The sample was first heated to the temperature of 1570℃, 1620℃ and 1670℃, respectively.
The melt was kept at for 40 min and then rapidly cooled down to the same temperature of 1560℃ before pouring.
The single crystal with [001] orientation were cast by means of crystal selection method. The crystal orientations of
the specimens were determined with Laue X-ray back reflection method, and the crystal orientation deviations of the
specimens were maintained within 7 degree from the [001] orientation. The single crystal specimens received a
standard heat treatment comprising of a solution treatment (1340 ℃/ 5 h, AC) and a two-step aging treatment (1 120
℃/4 h, AC + 870 ℃/24 h, AC). The long term aging of the specimens after full heat treatment were performed at
1100℃ for 800h. The samples were etched with 5 g CuSO 4+25 ml HCl+20 ml H2O+5 ml H2SO4 which dissolves
the γ′ phase. The composition of the dendritic core and interdendritic region was determined in the electron probe
micro analyzer (EPMA). Microstructures of the samples were examined by using S4800 scanning electron
microscope.
3. Results
3.1. As-cast microstructure
The as-cast dendritic micostructure of alloy with different melting temperature are shown in Fig.1. The
relationship between primary dendrite arm spacing of alloys and melting temperature is shown in Fig.2. It exhibites
the typical dendritic structure with a large amount of γ/γ′ eutectic in the interdendritic regions. The primary dendrite
arm spacing of alloy with different melting temperature is 283µm, 267µm and 256µm. It can be seen that the
dendritic microstructure is refined with the increasing of melting temperature.
The as-cast γ′ morphologies in the dendrite core and in the interdendritic region of alloy are shown in Fig.3. The
size and the morphology of γ′ phase formed during solidification process will influence the high temperature
mechanical properties of single crystal superalloy even after the heat treatment. It can be seen from Fig.3 that the
shape and size of γ′ particles in the dendrite core are more cubic and smaller than that in the interdendritic region. It
has also been found that the size of γ′ phase particles in these two regions decreases and cubic shape of those turns a
little regular with increasing of melting temperature. Also, the difference of γ′ size between the dendrite core and the
interdendritic region reduces.
Zhen-xue Shi / Procedia Engineering 00 (2014) 000–000
(a)
(b)
(c)
Fig. 1 As-cast dendritic micostructure of the alloy with different melting temperature (a) 1570℃; (b) 1620℃; (b) 1670℃
Fig. 2 Relationship between primary dendrite arm spacing of alloys and melting temperature.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3 As-cast γ′ morphologies of the alloy with different melting temperature
(a) dendritic core, 1570℃; (b) interdendritic region, 1620℃; (c) dendritic core, 1670℃;
(d) interdendritic region, 1570℃; (e) dendritic core, 1620℃; (f) interdendritic region, 1670℃.
3
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Zhen-xue Shi / Procedia Engineering 00 (2014) 000–000
3.2. Dendritic segregation
All tables should be numbered with Arabic numerals. Every table should have a caption. Headings should be
placed above tables, left justified. Only horizontal lines should be used within a table, to distinguish the column
headings from the body of the table, and immediately above and below the table. Tables must be embedded into the
text and not supplied separately. Below is an example which the authors may find useful. The dendrite segregation
of the alloy element plays an important role in the formation of the TCP phase and the solidification defects such as
stray grain and freckle. The degree of microsegregation between the interdendritic regions and dendrite cores can be
better characterized with the use of the chemical partitioning term, k′. By definition, k′ is taken to be the ratio of the
interdendritic composition over the dendrite core composition level. The compositional analysis of the alloy with
different melting temperature was carried out with standard electron microprobe analysis (EPMA) methods. The
results of segregation ratio for each alloy element are presented in Fig.4. Every datum is the mean value of five tests.
It can be seen from the results that elements such as Cr, Re, Mo and W all tend to segregate towards the dendrite
core during solidification. This indicates that these elements are the first to solidify within the dendrite cores during
the single crystal superalloy withdrawal process. The γ′ forming elements such as Al, Hf, Nb and Ta all tend to the
interdendritic region. These elements are present in greater quantity in the last liquid phase to solidify and result in
large eutectic γ/γ′ phases to form within the interdendritic regions. The extent of elements segregation decreases
with increasing of melting temperature. These results are similar to previous report on a experimental single crystal
superalloy [12].
Fig. 4 The element segregation ratio of the alloy with different melting temperature.
3.3. Microstructure after heat treatment
Fig.5 shows the heat treatment γ′ morphologies of the alloy with different melting temperature. It can be seen that
the primary γ′ and γ/γ′ eutectic dissolved completely after the high temperature solution treatment. They contain
more than 60% γ′ phase in cubic shape and γ matrix channel. The cubical γ′ phase is regularly arranged along [100]
direction. The size of γ′ phase particles decreases with increasing of melting temperature.
(a)
(b)
(c)
Fig. 5 Heat treatment γ′ morphologies of the alloy with different melting temperature (a) 1570℃; (b) 1620℃; (b) 1670℃.
Zhen-xue Shi / Procedia Engineering 00 (2014) 000–000
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3.4. Microstructure after long term aging
The microstructure stability at elevated temperatures is a key concern for the single crystal superalloys. Fig. 6
illustrates the microstructures of (001) plane of the alloy with different melting temperature after long term aging at
1100℃ for 800 h. It can be seen that the needle shaped TCP phases had already been observed in all of specimens.
The TCP phases initially precipitated locally within the dendrite core and gradually spread into the interdendritic
area. Furthermore, TCP phases precipitated and grew along fixed direction in all of specimens. The crystal
structures of TCP phases are extremely complex and the size of the unit cell is much larger than the lattice of the γ
and γ′ phases. A large nucleation barrier serves to prevent the formation of TCP phases in the microstructures of
single crystal superalloys [13]. When TCP phases form in the alloys, they nucleate preferentially on close-packed
planes, forming a semicoherent interface, and exhibit distinctive orientation relationships with parent crystal [14].
The amount of TCP phases decreases with increasing of melting temperature. The width of TCP phases was
influenced by melting temperature too. It indicates that increase melting temperature of the single crystal superalloy
with high refractory content can increase high temperature phase stability.
The chemical composition of TCP phases in the alloy after long term aging at 1100℃ for 800 h using SEM
quipped with an energy-dispersive X-ray spectroscope (EDS) is shown in Table 1. Every datum is the mean value of
five tests. In all of the samples, Re, W, Co, and Mo are enriched in the TCP phase.
(b)
(a)
(c)
Fig. 6 TCP precipitates in the alloy with different melting temperature after long term aging at 1100℃ for 1000h.
(a) 1570℃; (b) 1620℃; (b) 1670℃
Table 1 Chemical composition of TCP phase in the alloy with different melting temperature (mass fraction, %).
Melting temperature
Al
Cr
Co
Mo
Re
W
Ni
1570℃
4.71
2.32
7.63
3.25
24.03
13.21
Bal.
1620℃
5.22
2.04
8.26
2.82
25.24
12.61
Bal.
1670℃
4.82
1.89
8.04
2.49
22.65
12.78
Bal.
4. Discussions
Lots of investigations have revealed that the melt characteristics have strong influence on the solidification
microstructure and property of solidified[9-12]. So it is believed that the microstructure variation of the
experimental alloy with the elevated melting temperature can be attributed to the internal melt characteristic change
in this investigation. The melt just above the liquidus temperature is usually in the non-equilibrium state consisting
of different kinds of atom clusters. With the increasing of melting temperature, these clusters can decrease in their
sizes and even disappear, which results in the composition uniformity enhancing. The melt at this kind of state has
increased supercooling degree and compressed temperature range for crystallization [9]. The melt will remain its
high temperature characteristic to a certain extent even when it is cooled down owing to the so-called ‘heredity
effect’ [11]. Therefore, the melt characteristic change caused by melting at high temperature will be partially
reserved to pouring temperature, which consequently results in the change of solidification characteristic of single
crystal superalloys with refined dendrite and γ′ phase and decreased dendrite segregation.
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Zhen-xue Shi / Procedia Engineering 00 (2014) 000–000
In all of the specimens, the TCP phases initially precipitated locally within the dendrite core and gradually spread
into the interdendritic regions when long term aging at 1100℃ for 800h. It is because that the TCP phases forming
elements such as Re, Mo and W all tend to segregate towards the dendrite core. Moreover, the microsegregation is
still existing even after fully heat treatment due to the low diffusivity of Re, W refractory alloying elements. So
controlling the extent of dendritic segregation behavior is critical to microstructure stability of the single crystal
superalloys. In this study, melting temperature increase can reduces the severity of segregation of Re and other
refractory elements. Moreover, the primary dendrite arm spacing decrease with the increasing of melting
temperature. The extent of dendrite microsegregation should decrease too with the increasing of melting temperature
after the specimens receive the same heat treatment. Therefore, increase melting temperature of the alloy can
increase high temperature phase stability.
5. Conclusions
(1) The primary dendrite arm spacing of alloy decreased with the increasing of melting temperature. The size of
γ′ phase particles in these two regions decreases and cubic shape of those turns a little regular with increasing of
melting temperature. The difference of γ′ size between the dendrite core and the interdendritic region reduces. The
size of γ′ phase particles after heat treatment decreases with increasing of melting temperature.
(2) The extent of elements segregation decreases with increasing of melting temperature. The needle shaped TCP
phases have already been observed in all of specimens and the amount of TCP phases decreases with increasing of
melting temperature. Increase melting temperature of the single crystal superalloy with high refractory content can
increase high temperature phase stability.
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