UDK 669.245:57.012.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek Mater. Tehnol.
X. ZHANG et al.: ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110) AND (111) PLANES ...
891–896
ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110)
AND (111) PLANES IN SRR99 SINGLE-CRYSTAL SUPERALLOY
ANIZOTROPNI RAZVOJ MIKROSTRUKTURE NA RAVNINAH
(001), (110) IN (111) V MONOKRISTALNI SUPERZLITINI
TIPA SRR99
Xiaoli Zhang1,a, Kuan Lei1,a, Tingzhen Xin1, Yishan Wu2, Guiqun Liu1*
1College of Material Science and Engineering, North Minzu University, Ningxia, Yinchuan 750021, China
2Ningxia Coal Industry Co., Ltd., Yinchuan 750011, China
aThese authors contributed equally to this work
Prejem rokopisa – received: 2025-07-11; sprejem za objavo – accepted for publication: 2025-10-10
doi:10.17222/mit.2025.1522
We investigate plane-dependent microstructures in an SRR99 single-crystal superalloy in the as-cast condition. Using optical
and scanning electron microscopy, we examine sections parallel to the (001), (110), and (111) planes. Dendrite traces display
"+" symmetry on (001), asymmetric "+" symmetry on (110), and an "×" pattern on (111). The ' precipitates appear as irregular
cube-like, triangular-prism-like, and triangular-pyramid-like shapes on (001), (110), and (111), respectively, whereas eutectic
pools and carbides show no marked plane-dependent differences under the present conditions. These plane-specific observations
provide a concise basis – limited to the as-cast state – for discussing anisotropic microstructural features that may be relevant to
hot-cracking susceptibility and for clarifying how sectioning orientation influences apparent morphology in single-crystal super-
alloys.
Keywords: single-crystal superalloy,  phase, ' phase, dendrite morphology, eutectic, carbide
Avtorji v ~lanku opisujejo raziskavo odvisnosti mikrostrukture od kristalografskih ravnin v liti monokristalni superzlitini vrste
SRR99. V tej raziskavi so avtorji preiskovali preseke vzporedne ravninam (001), (110) in (111) s pomo~jo vrsti~nega
elektronskega mikroskopa. Sledovi dendritov so kazali "+" simetrijo na ravnini (001), asimetrijo "+" na ravnini (110) in vzorec
"×" na ravnini (111). Izlo~ki ' faze se pojavljajo kot nepravilna kubi~na, trikotna prizma in trikotna piramida na ravninah (001),
(110) in (111). Pri tem evtekti~ni »bazen~ki« (meddendritni prostori) in karbidi v teh stanjih ne ka`ejo nobenih pomembnih
razlik, odvisnih od kristalografskih ravnin. Opisana od ravnin odvisna specifi~na stanja lite mikrostrukture superzlitine
omogo~ajo osnovno razlago njenih anizotropnih zna~ilnosti, ki bi bile lahko pomembne za nastanek oziroma nagnjenost litine k
tvorbi razpok v vro~em in razjasnitev, kako orientacija sekcije kristalografskih ravnin vpliva na navidezno morfologijo v litih
monokristalnih superzlitinah.
Klju~ne besede: monokristalna superzlitina v litem stanju (litina), fazi  in ', dendritna morfologija, evtektik, karbidi
1 INTRODUCTION heat-flow direction. Thus, the molten material solidifies
along with the unidirectional temperature gradient. Dur-
Superalloys exhibit excellent corrosion resistance and ing the solidification process, the latent heat of crystalli-
high-temperature performance, enabling them to operate zation increases the temperature of the solid-liquid inter-
above 600 °C while withstanding considerable stress. face. This leads to a negative temperature gradient and a
Based on their composition, superalloys can be catego- supercooled state in the liquid phase. In this case, the
rized into iron-based, cobalt-based, and nickel-based solid-liquid interface tends to be non-flat. As a result,
superalloys. Nickel-based superalloys account for ap-
numerous dendrites begin to grow, with secondary den-
proximately 40 % of the market share and offer the best
performance, making them widely used in the aerospace drites branching from the primary ones. Until now the
industry, e.g., for aeroengines and missiles.1 dendrite structures have been extensively studied and
Nickel-based single crystal (SC) superalloys are typi- have been shown to play a crucial role in the material
properties.2–4
cally produced using directional solidification tech-
niques. These techniques involve creating a unidirec- SC materials are known for their anisotropy. For
tional temperature gradient between the solidified nickel-based SC superalloys with a face-centered cubic
material and the unsolidified molten material. The crys- (FCC) crystal structure, the [001] direction is the fast-
tal growth direction must be parallel and opposite to the est-growing and highest-performance direction. During
the production of nickel-based SC superalloys, primary
dendrites typically grow along the [001] direction.5–8
*Corresponding author's e-mail:
gqliu10b@alum.imr.ac.cn (Guiqun Liu) Therefore, numerous studies have been conducted on the
© 2025 The Author(s). Except when otherwise noted, articles in this jour- structure and properties along the [001] crystal orienta-
nal are published under the terms and conditions of the Creative Com-
mons Attribution 4.0 International License (CC BY 4.0). tion.
Materiali in tehnologije / Materials and technology 59 (2025) 6, 891–896 891
X. ZHANG et al.: ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110) AND (111) PLANES ...
So far, few studies have been conducted on different constant speed of 6 mm/min. Once the mold cooled to
crystal planes of nickel-based SC superalloys. In this room temperature, it was broken to obtain the SRR99
study, three different crystal planes were cut from the SC.
SRR99 SC superalloy. In addition, (001), (110) and Three sets of crystal planes, (001), (110), and (111),
(111) crystal plane samples were cut with a precision were cut into cubes of (10 × 10 × 10) mm using wire
wire cutting machine. The findings related to dendrite electrical discharge machining (Suzhou Baoma Numeri-
morphologies, ' phase morphologies, eutectic structure, cal Control Equipment Co. Ltd., BM400C-CT). Follow-
and carbide content are presented herein. ing the metallographic process, three sets of samples
with different crystal planes were prepared. The corro-
sion solution consisted of 100 mL HCl + 5 mL H2SO4 +
2 EXPERIMENTAL MATERIALS AND 20 g CuSO4 + 100 mL H2O. The dendrite structures of
METHODS the three crystal planes were observed using an optical
microscope (Zeiss, Axio vert. A1). To study the /'
The SRR99 nickel-based single crystal (SC) superal- phase, carbide and eutectic structure, three different crys-
loy used in this experiment is a first-generation material. tal planes were observed using a scanning electron mi-
The nominal composition of SRR99 is shown in Table 1. croscope (Zeiss, Sigma500).
Table 1: Nominal compositions of SRR99 alloy (w/%)
3 RESULTS AND DISCUSSION
Cr Co W Al Ti Ta C Ni
8.4 5.0 9.5 5.5 2.1 2.8 0.015 Bal. 3.1 Dendritic structure
Two methods are used to produce SC superalloys: the Figure 1 shows the dendritic morphologies of three
seeding method and the spiral selection method. The spi- different crystal planes of SRR99 nickel-based SC super-
ral selection method is efficient for producing SC super- alloy. It was found that the dendritic morphologies of the
alloys but cannot accurately control the orientation of the three crystal planes were obviously different. As shown
crystal. The seeding method allows precise control of in the (001) crystal plane on Figure 1a, the dendrites are
crystal orientation. Thus, the seeding method was se- symmetrical "+", and their centers correspond to the pri-
lected to produce the SRR99 nickel-based SC superalloy mary axis along the [001] direction. The four symmetri-
in this experiment. cal directions of the dendrites correspond to the second-
To define the section geometry, the following steps ary dendrite arms. The dendrites extending from the
were taken. The (110) section was obtained by rotating secondary arms are the tertiary dendrite arms. Due to the
the reference (001) sample by 45° about an in-plane existence of the tertiary dendrite arms, the "+" shape of
[110] axis. The (111) section was prepared by rotating dendrites is less distinct. The (110) section was obtained
(001) by 54.7° about an in-plane 110 axis followed by rotating the reference (001) sample by 45° about the
by azimuthal alignment. Angles were stated between the in-plane [110] axis and using wire cutting. As shown in
normals to the planes unless otherwise noted. the (110) crystal plane on Figure 1b, the dendrites of the
In the seeding method, a seed with the desired orien- (110) crystal plane exhibit an asymmetric "+" shape. The
tation was placed at the bottom of a ceramic mold to pre- two secondary dendrite arms along one direction are par-
vent random grain nucleation. The ceramic mold con- ticularly long, while the two dendrite arms along the
taining the required seed was heated to 1550 °C. The other direction are particularly short. As shown in the
SRR99 alloy was superheated to 1550 °C and held at this (111) crystal plane on Figure 1c, the dendritic morphol-
temperature for 5 min to homogenize the melt. The mol- ogy of the (111) crystal plane is symmetrical "×".
ten alloy was then poured into the preheated mold. After In Figures 1a-b-c, the black area between adjacent
a holding time of 10 min, allowing the system to reach dendrites is the interdendritic region, and the bright
the thermal equilibrium, the mold was withdrawn at a white areas within the interdendrite are the eutectic4.
Figure 1: Dendritic structures of the three crystal planes at 50× magnification: a) (001) crystal plane; b) (110) crystal plane; c) (111) crystal plane
892 Materiali in tehnologije / Materials and technology 59 (2025) 6, 891–896
X. ZHANG et al.: ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110) AND (111) PLANES ...
3.2 /'phase Due to the energy barrier associated with planar fault
formation, known as the anti-phase boundary (APB), dis-
As shown in Figures 2 and 3, the white gray grid locations in the ' phase must move in pairs. The disloca-
area represents the  phase matrix. The black sunken re- tions and their paired counterparts are referred to as
gions correspond to the ' phase of the secondary precip- superpartials and superdislocations, respectively. For in-
itation. The ' phase morphologies of the three crystal stance, the resolved shear stress measured under uniaxial
planes exhibited significant differences. The ' phase of compression increased by more than a factor of four at
the (001) crystal plane appeared as an irregular cubic the peak temperature of 1000 K compared to room tem-
shape. Since the (110) sample was obtained by rotating perature in Ni3(Al, Nb).13 This anomalous behavior was
the (001) sample by 45°, the ' phase on the (110) crystal attributed to the thermally activated cross-slip of
plane exhibited an irregular triangular prism shape. Simi- superdislocations from the {111} planes to the {100}
larly, since the (111) sample was obtained by rotating the planes with lower APB energy, which acted as a
(001) sample by 54.7°, the ' phase on the (111) crystal self-trapping mechanism known as Kear-Wilsdorf (KW)
plane exhibited an irregular triangular pyramid shape. locks.16–18 Consequently, the inverse temperature depend-
The irregular ' phases observed on the three crystal ence of ' strength is recognized as one of the key
planes were attributed to the cast samples not being heat strengthening mechanisms for Ni-based superalloys.10
treated. If they had been subjected to heat treatment, the
' phases would have been more regular. 3.3 Eutectic structure
A nickel-based SC superalloy typically consists of a 
phase with a disordered face-centered cubic crystal struc- To frame the discussion, we treat the eutectic influ-
ture and ' phase with an ordered face-centered cubic ence factor as a conceptual, qualitative index defined as
crystal structure (L12). The ' phase is a crystal structure E = Ve × Se, where Ve denotes the apparent eutectic frac-
strengthened by solid solution elements.9,10 Due to the tion (e.g., area fraction in micrographs) and Se denotes
strengthening ' phase, a nickel-based SC superalloy ex- the characteristic size of eutectic pools (e.g., equiva-
hibits excellent strength and creep resistance. The lent-circle diameter). All statements below are limited to
strength of the most materials decrease with increasing the as-cast condition and visual inspection under compa-
temperature, but the ' phase shows the opposite temper- rable fields of view; rigorous plane-by-plane quantifica-
ature dependence in the process of uniaxial tension.11–15 tion is beyond the scope of this work.
This phenomenon is attributed to the movement of During the process of directional solidification, den-
superdislocations. The Burgers vector required for a fault dritic trunks solidified first and interdendritic regions so-
in the ' lattice to revert to its original state is twice that lidified later. For the solidification of the as-cast SRR99
of a single dislocation in the  phase. nickel-based SC superalloy, the formation of eutectic oc-
Figure 2: /' phases of the three crystal planes at 10000× magnification: a) (001) crystal plane; b) (110) crystal plane; c) (111) crystal plane
Figure 3: /' phases of the three crystal planes at 30000× magnification: a) (001) crystal plane; b) (110) crystal plane; c) (111) crystal plane
Materiali in tehnologije / Materials and technology 59 (2025) 6, 891–896 893
X. ZHANG et al.: ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110) AND (111) PLANES ...
Figure 4: Eutectic structures of the three crystal planes at 4000× magnification: a) (001) crystal plane; b) (110) crystal plane; c) (111) crystal
plane
curred later than that of dendritic trunks. The eutectic factor on the hot cracking sensitivity of the (110) and
was formed from the co-crystallization of  and ' phases (111) planes is higher than in the case of the (001) plane.
in the interdendritic regions. As shown in Figure 4, the
eutectic structure was composed of a large quantity of
the black ' phase and a small amount of the white-gray  3.4 Carbides
phase.
There were no significant differences between the Four types of carbide structures with distinct
eutectic morphologies of the three crystal planes. The ex- morphologies were observed, namely acicular, spherical,
istence of the eutectic structure could reduce the stability massive and Chinese character.21 As shown in Figure 5,
of the alloy and lead to hot cracking. Thermal cracking the carbide morphology of the three crystal planes exhib-
sensitivity was closely related to eutectic characteristics. ited the Chinese-character form.
It occurred because eutectic characteristics reflected the The possible types of carbides in nickel-based SC
solidification process, segregation behavior and coales- superalloys are MC, M6C and M23C6. According to the
cence behavior of dendrite. Apart from the other factors, previous studies, different metal elements are enriched in
the size and volume fraction of the /' eutectic structure different carbides. Ta and Ti are rich in the MC carbide.
were not sufficient to evaluate the effect of eutectic char- W is rich in the M6C carbide. Furthermore, Cr is rich in
acteristics on hot cracking sensitivity.19 the M23C6 carbide. In this experiment, the energy spec-
Therefore, we use the eutectic influence factor (E) as trum compositions of carbides were analyzed, as shown
the qualitative index in the as-cast condition, defined as: in Figure 6. The diffraction pattern analysis indicated
that Ta and Ti were rich in carbide. Thus, the type of car-
E = Se + Ve bide was the MC carbide.
In the early nickel-based SC superalloys, carbon was
where Ve denotes the apparent eutectic fraction (i.e., the not added, so carbides generally did not appear. How-
area fraction in micrographs) and Se denotes the charac- ever, with the development of technology and continuous
teristic size of eutectic pools (e.g., equivalent circle di- change in the composition of alloy elements, the addition
ameter). of carbon could lead to the formation of carbides. Car-
The E value is consistent with hot cracking sensitiv- bide could strengthen the grain boundary. Thus, carbon
ity.20 From our results (Figure 1), the volume fractions was a grain-boundary strengthening element. The size
of eutectic of the (110) and (111) planes were larger than and volume fraction of carbides increased with the in-
that of the (001) plane. Therefore, the influence of this crease in the carbon content.20–25
Figure 5: Carbide structures of the three crystal planes at 4000× magnification: a) (001) crystal plane; b) (110) crystal plane; c) (111) crystal
plane
894 Materiali in tehnologije / Materials and technology 59 (2025) 6, 891–896
X. ZHANG et al.: ANISOTROPIC MICROSTRUCTURAL EVOLUTION OF (001), (110) AND (111) PLANES ...
Figure 6: Composition energy spectra of the carbides: a) selected area diffraction patterns at their interface (SEM); b) map of selected area 1; c)
map of selected area 2
5
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