Intermetallics

1. Observation of a Ru-rich
Heusler phase in a multicomponent Ni-base superalloy (UM-F13)
B.Tech seminar report
by
Mainak Saha
Roll No:14/MM/65
Under the guidance of
Prof. Manab Mallik
Department of Metallurgical and Materials Engineering
National Institute of Technology(NIT) Durgapur
Durgapur-713209,West Bengal
18 February, 2018

2. Acknowledgement
With deep regards and profound respect, I would like to express my sincere and hearty gratitude
towards my guide Prof. Manab Mallik for providing me the opportunity to carry out the project
in this esteemed institute. My special thanks to him for bringing much awareness in me about
research and allowing me to express myself in research. Besides, I am also strongly indebted
to him for his valuable suggestions, both academically and non-academically which led to self-
motivation and helped me complete the seminar work successfully.
Besides, I am also thankful to all other research scholars from my department for their constant
help during the project work.
I also thank my parents all the elders for their support and encouragement to pursue higher
studies.

3. CERTIFICATE
This is to certify that the seminar report entitled “Observation of a Ru-rich Heusler phase
in a multicomponent Ni-base superalloy (UM-F13)” is a bonafide work by Mainak
Saha,(Roll No: 14/MM/65), Department of Metallurgical and Materials Engineering,
National Institute of Technology (NIT), Durgapur.
The above mentioned seminar work was carried out at NIT Durgapur , under our supervision
on 18th February 2018. In partial fulfillment of “BACHELOR OF TECHNOLOGY” in
Metallurgical and Materials Engineering , NIT Durgapur.
Prof. Manab Mallik Prof. Manas Mondal Mainak Saha
(Project guide) (Seminar Supervisor) (14/MM/65)
Prof. S. Pramanik
(Seminar Supervisor)

4. Abstract
A new, ordered L21 Heusler phase based on Ru2AlTa was identified in a heat-treated
multicomponent Ni-base superalloy containing high ruthenium. Such a phase has not
previously been observed in muticomponent Ni-base superalloys or in quaternary Ni–Al–Ta–
Ru systems. The influence of solidification-induced microsegregation on the formation and
distribution of the Heusler phase is discussed.
Keywords: Nickel alloys; Platinum group; Phase transformation; Microstructure;
Transmission electron microscopy

5. 1. Introduction
Continuous advances in the high temperature capabilities of Ni-base superalloys have been
possible through advances in the understanding of microstructure, properties and processing
of these multi-phase, multicomponent materials. Most recently, Ru-containing Ni-base
superalloys have been studied since Ru additions may further improve high temperature
properties of these materials [1–4]. To design alloys with long term microstructural stability
at high temperatures requires a detailed knowledge of phase equilibria. Nibased superalloys
contain at least two basic phases, a disordered fcc c matrix and ordered L12 c0 precipitates
based on Ni3Al. In addition to these phases, a large number of secondary phases may be
present, such as carbides, TCP phases [5], the Re(Ru)-rich d phase [4] and other ordered
intermetallic phases such as B2 NiAl (a bcc derivative structure). The existence of TCP
phases usually results in degradation of high temperature mechanical properties (creep and
fatigue). It is known that Ti additions to B2 NiAl result in the formation of a highly ordered
L21 Heusler compound, Ni2AlTi. This Heusler phase has a large unit cell composed of 8
NiAl unit cells, in which Al and Ti atoms occupy ordered sites on one sublattice. The Heusler
phase has been frequently observed in a number of Ni– Al–X ternary systems, and examples
of this phase include Ni2AlTi [6], Ni2AlTa [7] and Ni2AlHf [8]. It has also been reported
that a b0=b two-phase alloy consisting of B2 NiAl precipitates in a Heusler Ni2AlTi matrix
had an exceptionally high creep resistance [9]. A B2 solid solution based on RuAl exists in
the Ru– Al binary [10] and Ru–Ni–Al ternary systems [11]. There is limited knowledge of
the equilibrium Ru–Al–Ta phase diagram and the existence of ternary phases in this system
has not yet been explored in any detail. The purpose of the current research is to report the
existence of a new Heusler phase, Ru2AlTa, as an equilibrium phase in a multicomponent Ni-
base superalloy containing high levels of Ru. Phase identification was conducted by the
combination of X-ray diffraction and electron diffraction analyses. The morphology and
distribution of this Heusler phase in a superalloy has been investigated. The distribution of
the Heusler precipitates is discussed with regard to the solidification path and the resultant
elemental segregation in the dendritic microstructure of the material.
2. Experiment
The nominal chemical composition of the experimental alloy, designated UM-F13, is listed in
Table 1.
Table 1. Nominal composition of alloy UM-F13 (wt.%)
Element Ni Al Ru Re Ta W Co
Wt% 59-60 5.6 14.1 3.7 6.3 4.3 6.8

6. The alloy was vacuum induction melted (VIM) at Sophisticated Alloys, Inc. and cast into a
cylindrical ingot weighing approximately 2.5 kg. Heat treatment experiments were performed
in a tube furnace in two stages: (1) solution treatment at 1300C for 4 h; (2) aging at 1100 C
for 100 h. Both stages were followed by water quenching. The as-cast and heat-treated
specimens were studied with an optical microscope and a Philips/FEI XL30FEG scanning
electron microscope (SEM). The phase composition was determined with a Cameca SX100
EDS. Electrolytic phase extraction experiments were conducted to isolate the unknown
phases from γ and γ ‘phases. This extraction technique has been described in more detail
elsewhere [12]. X-ray diffraction patterns were obtained from the extracted residues with a
Rigaku diffractometer using CuKa radiation. The lattice parameters of the phases were
calculated from the X-ray results by using Cohen’s method, described in [13]. TEM
specimens were prepared from 3 mm disks mechanically ground to 90 lm thickness.
Subsequently, they were electro-polished in a solution of 10% perchloric acid, 9% distilled
water, 13% butyl cellusolve and 68% methanol at )40 C and 20 V. The TEM studies were
conducted in a JEOL 4000EX electron microscope with the accelerating voltage at 400 kV.
3. Results
3.1. Microstructures and morphology of the Heusler phase
A typical dendritic microstructure with a low volume fraction of cþc0 eutectic (2.8%) was
observed in the as-cast polycrystalline material. It contained a twophase cc0 structure with no
additional phases present. Fig. 1(a) shows a γ+ γ’ eutectic pool in the interdendritic region.
The eutectic was completely dissolved into the γ matrix after solution treatment at 1300 C for
4 h and no other phases appeared after water quenching from 1300 C. Fig. 1(b) is a back-
scattered electron (BSE) image, showing the typical microstructure of the alloy after an aging
treatment at 1100 C for 100 h. Following this aging treatment, the microstructure consisted of
c matrix (gray), γ’ precipitates (dark) and Heusler (H phase) precipitates (white), which are
clearly shown in a higher magnification BSE image (Fig. 1(c)). The determination of the H
phase is described in more detail in the following section. It was found that the H phase
precipitated in both dendritic and interdendritic regions, and was enveloped by the c matrix.
The volume fraction of γ’ precipitates was low at approximately 29 vol.% in this alloy and
distinctly decreased from the interdendritic regions to the dendrite core regions. The size of
the H precipitates was much larger than the γ’ precipitates. There was some degree of
clustering of H precipitates within the interdendritic regions, marked by arrows in Fig. 1(b).
Interestingly, the size of the clusters was similar to that of former eutectic pools. This
suggests that the clusters precipitated in the previous eutectic regions. The three-dimensional
morphology of H precipitates was investigated by examining a bulk sample, in which the
electrolytic extraction had been carried out to partially remove γ -γ’matrix, leaving the
precipitates protruding. Fig. 2(a)–(c) shows the morphology of H precipitates viewed at
different magnifications in an aged sample. The H precipitates were found to grow
anisotropically, resulting in a distinctive faceted rod-like and blocky appearance (Fig. 2(b)
and (c)). The clusters of H precipitates were also observed and are marked by arrows in Fig.
2(a)–(b).

8. Fig. 1. (a) A secondary electron (SE) image showing a γ+ γ’ eutectic pool of as-cast UM-F13
alloy; (b) a BSE image showing the typical microstructure of UM-F13 alloy after solution
treatment at 1300 C for 4 h and aging at 1100 C for 100 h, which consists of c (gray contrast),
c0 (dark contrast) and Heusler precipitates (white contrast); the clusters of Heusler
precipitates are marked by arrows; (c) a higher magnification image of (b).
3.2. Heusler phase identification
The results of microprobe analyses on larger H precipitates are listed in Table 2. The H
precipitates contained high levels of Ru, Ta and Al, and were strongly depleted in Re.
Compared to the nominal alloy composition, they were enriched in Ru, Ta and Al by factors
of 5.0, 5.6 and 2.4 times, respectively. Interestingly, for the precipitates examined, the H
phase contained Ru as high as 45 at.%, which is almost equal to the total atomic percentage
of (Al+Ta).
Fig. 3 shows the diffraction intensity versus the diffraction angle 2h between 20and120 for
the precipitates extracted from the heat-treated alloy. The measured X-ray diffraction pattern
is similar to that of the B2 phase RuAl, except for the presence of an additional relatively
strong diffraction peak at 2h of 25 and some other weak peaks, for example, at 50and 67. It is
known that the lattice parameter of the H
Fig. 2. (a), (b) and (c) are SE images showing the morphology of Ru-rich Heusler precipitates
Ru2Al(Al, Ta) after an electrolytic extraction from UM-F13 aged alloy, the clusters of
Heusler precipitates are marked by arrows.
Fig. 3. X-ray diffraction pattern of extracted Ru-rich precipitates taken from the aged alloy
UM-F13 indicating these precipitates are Heusler phase.

9. Fig. 2. (a), (b) and (c) are SE images showing the morphology of Ru-rich Heusler precipitates
Ru2Al(Al, Ta) after an electrolytic extraction from UM-F13 aged alloy, the clusters of
Heusler precipitates are marked by arrows.
Table 2 Chemical composition of Heusler phase measured by a microprobe in UM-F13 heat-
treated alloy (wt.%)
Location Ni Al Ru Ta Re W Co
Eutectic 65.89 16.11 7.16 4.67 0.23 0.20 5.73
Dendritic
core
62.82 11.38 10.70 1.48 2.71 2.34 8.49
Interdendritic 65.97 14.39 8.05 2.21 0.81 1.09 7.49

11. Fig. 2. (a), (b) and (c) are SE images showing the morphology of Ru-rich Heusler precipitates
Ru2Al(Al, Ta) after an electrolytic extraction from UM-F13 aged alloy, the clusters of
Heusler precipitates are marked by arrows.
Fig. 3 shows the diffraction intensity versus the diffraction angle 2h between 20and120 for
the precipitates extracted from the heat-treated alloy. The measured X-ray diffraction pattern
is similar to that of the B2 phase RuAl, except for the presence of an additional relatively
strong diffraction peak at 25 and some other weak peaks, for example, at 50and 67. It is
known that the lattice parameter of the H phase is normally equivalent to or very close to
twice as large as that of the B2 phase, from which the H phase is derived. The extra peaks are
consistent with the existence of the H phase and were identified as (111), (311) and (331)
superlattice peaks at 25, 50 and 67, respectively, for the H phase. This phase was further
confirmed by electron diffraction analyses, described in the next paragraph. The lattice
parameter was calculated to be 0.6089 nm from six diffraction peaks ((400), (420), (422),
(440) (600) and (620) shown in Fig. 3) at high diffraction angles between 60and120. The
lattice parameters of the H phase Ni2AlTa have been determined to range from 0.5904 to
0.5949 nm [14]. This is consistent with the current measured value for the H phase Ru2AlTa
(aH ¼ 0:6089 nm), considering that Ru has a larger atomic radius (0.134 nm) than Ni (0.125
nm) and that substitution on Ni sites in this crystal structure would increase the size of the
unit cell. Fig. 4(a) is a bright field TEM image showing a large precipitate attached to a thin
layer of γ+ γ’ matrix in a specimen after aging. TEM diffraction analyses were carried out in
order to further confirm the structure of the precipitates. The diffraction patterns of H phase
(aH ¼ 0:6089 nm, the current result) and B2 phase (aB2 ¼ 1=2aH ¼ 0:3044 nm) were
simulated using the program of Stadelmann [15]. At a fixed camera constant, two B2 and H
patterns are identical for the beam directions of <001> and <111>. However, the patterns are
not the same for <011> and <112>, which were indexed and are shown in Fig. 4(b), (c), (e)
and (f). The superlattice diffraction vectors of {111} and {311} of H phase are correlated
with d-spacing and do not exist in the B2 pattern. The electron diffraction patterns taken from
the precipitate, shown in Fig. 4(d) and (g), matched the simulated patterns of H phase in both
Æ011æ and Æ112 æ zones, respectively. The combined results of X-ray diffraction and
electron diffraction analyses indicate that these precipitates are the H phase, not the B2
phase. More detailed X-ray and electron diffraction analyses on the current alloy and a
ternary Ru–Al–Ta alloy to be reported elsewhere confirm that this phase is a L2 1 Heusler
phase, rather than the closely related D03 structure. Considering the precipitate composition,
this ordered H phase can be described as Ru2AlTa.
3.3. Solidification-induced microsegregation
The results of microprobe analyses on the as-solidified alloy are given in Table 3, in terms of
the average composition at the center of the dendrite core and in the interdendritic region as
well as the composition of the eutectic. Segregation of individual alloying elements occurred
from dendritic to interdendritic regions in this multicomponent superalloy. The most strongly
segregating elements included Re, W and Ta. The Re and W strongly partitioned to the
dendrite core regions in the early stages of solidification. Ruthenium also partitioned to the
dendrite core, but only weakly in comparison to Re and W. The Ta and Al segregated to the
interdendritic regions with very strong partitioning of Ta to the γ+ γ’ eutectic pools, where
the clustering of the H precipitates appeared in the heat-treated microstructure.

12. 4. Discussion
The alloy investigated is a high Ru superalloy, containing 9.0 at.% (14.1 wt.%) Ru. The total
refractory
Fig. 4. (a) A bright field TEM image showing a large precipitate attached to a thin layer of γ-
γ’matrix after aging; (b), (c), (e) and (f) are simulated diffraction patterns of B2 [011],

13. Heusler [011], B2 [112] and Heusler [112] zone axes, respectively; (d) and (g) are
experimental diffraction patterns of <011> and <112> zone axes, respectively, indicating
again that it is a Heusler phase.
The alloy investigated is a high Ru superalloy, containing 9.0 at.% (14.1 wt.%) Ru. The total
refractory alloy content (Ru+Re+W+Ta) is about 28.4 wt.%. The as-cast microstructure and
microsegregation indicates that the solidification behavior is similar to other conventional
superalloys [12,16]. The microstructure following solution treatment suggests that no H
phase (Ru2AlTa) exists above 1300 C in this alloy. The microstructure after aging at 1100 C
for 100 h contained three phases: γ – γ’ and the H phase. In our previous studies on Ru-
containing multicomponent superalloys [4], only the γ and γ’ phases were observed after heat
treatment. The major chemistry difference among these alloys is overall Ru content. Those
reported on previously contained 3.5–6 at.% Ru, lower than that of the current alloy. The
increased Ru-content provided the chemical driving force for the transformation from
twophase (cþc0) to three-phase (γ + γ ‘+H). To the best of our knowledge, such a Ru-rich
Heusler phase has not been reported previously in multicomponent Ni-base superalloys.
However, the existence of Ru2AlTa phase is not altogether surprising, considering that H
phases have been observed in a number of Ni–Al–X ternary systems including Ni2AlTa and
Ni2AlTi. Another Ru-rich H phase Ru2AlNb has also been reported in the Ru–Al–Nb ternary
system [17]. Since this H phase has been observed in the present Ni-base alloy, it is expected
that it is also stable in the Ni–Al–Ru–Ta quaternary system. Unfortunately, this quaternary
system has not yet been studied in any detail. There are four ternary systems associated with
this quaternary system: Ni–Al–Ru, Ni–Al–Ta, Ni–Ru–Ta and Al–Ru–Ta. Phase equilibria in
Ni–Al–Ta system at 1000 and 1250 C were investigated recently by Palm et al. [18]. The H
phase Ni2AlTa was stable in this temperature range and the phase stability range increased
with increasing annealing temperature. There was no three-phase (γ + γ ‘+H) equilibrium
region identified in the Ni–Al–Ta system. Isothermal sections of the Ni–Al–Ru system at
1000 and 1250 C were determined by Chakravorty and West [11]. No H phase was observed
and the B2 solid solutions based on NiAl and RuAl (designated b1 and b2, respectively) were
stable in this ternary system. Interestingly, a miscibility gap between the b1 and b2 phases
was reported. Recent X-ray studies on ball-milled materials and first principle type
calculations suggest a single B2 phase field between RuAl and NiAl [19–21]. Such
discrepancies should be clarified for further understanding the phase equilibria in the Ni–Al–
Ru ternary system and multicomponent Ni-base superalloys. To date, limited research has
been conducted on the phase equilibria in Ni–Ru–Ta and Al– Ru–Ta ternary systems [22].
Unlike the Re-rich TCP phases [23,24] and the Re(Ru)-rich d phase [25] which precipitate
preferentially in dendritic core regions, the Ru-rich Heusler Ru2AlTa
precipitates showed no location preference in the current alloy besides clustering of H
precipitates in eutectic pools (Fig. 1(b)). These observations may be explained by the
elemental segregation resulting from the dendritic solidification. The current EDS studies
clearly indicated that Re and W strongly partitioned to the dendrite core regions in the as-cast
condition. The residual chemical enrichment of Re and W still remained significant after
solution treatment in the dendrite core regions. This segregation has been observed to
promote TCP phases and d phase formation locally after aging [23,25]. This is difficult to
suppress considering interdiff usion coefficients, which indicate that the atomic mobility of Re
and W is very low [26]. The EDS results also indicate that Ru has a preference for

14. partitioning to the dendrite core region, although the segregation behavior of Ru is much
weaker than that of Re and W. Following heat treatment, the element Ru showed minor
segregation compared with Re and W [27]. Apparently, Ru interdiff usion in nickel is faster
than Re and W, which is in agreement with the most recent report [28]. Hence, reduced
segregation and the tendency for diffusional homogenization result in a fairly uniform
distribution of H precipitates. However, as mentioned previously, there were some clusters of
H precipitates in regions occupied by previous eutectic pools. The EDS results revealed that
Ta strongly segregated in these eutectic pools. Due to low Ta diffusivity, the residual
enrichment of Ta in the eutectic still existed following solution heat treatment. Thus, the Ta
profiles have the strongest influence on the location preference of Heusler precipitation. It is
interesting that the H phase was surrounded by the c phase after aging, unlike the Re-rich
TCP phases [23] and Re(Ru)-rich d phase [25], which are typically enveloped by the γ’
phase. Considering the composition of the H phase, precipitation of this phase results in a
depletion of the surrounding material in Ru, Al, Ta and enrichment of Re during aging. While
Ta and Al are strong c0 formers, on the other hand, Re is a strong stabilizer of the γ phase.
Detailed atomic site occupation preference studies for Ru2AlTa phase were not conducted in
the present study. However, considering the composition of the present H phase and site
preferences in other H phases, including Ni2AlTa and Ni2AlTi, it is highly likely that Ta
atom has a strong preference for the Al sites in B2 structure of RuAl. Wilson and Howe have
suggested that large elements have a tendency to locate on Al site and enhance the H phase
precipitation [29]. This is consistent with our data since the atomic radius of Ta (0.147 nm) is
slightly larger than that of Al (0.143 nm). However, this is not in agreement with the
theoretical calculations by Bozzolo et al., which indicate that Ta prefers Ru site in both Ru-
rich and Al-rich RuAl alloys [30]. Further detailed studies on this aspect would clearly be
useful.
5. Conclusions
An ordered L21 Heusler phase based on Ru2AlTa has been identified in a high Ru-containing
multicomponent superalloy. The major characteristics of this phase can be summarized as
follows:
The H phase co-existed with the c and c0 at 1100 C, but was not stable above 1300 C. The
precipitation of this phase in a model Ni-base superalloy is promoted by high levels of Ru
additions (9 at.%). (2) The lattice parameter of the current Heusler phase Ru2Al(Ta, Al) was
measured to be 0.6089 nm. The extracted H precipitate morphologies were either faceted rod-
like or blocky. (3) In the aged condition, the H phase precipitated fairly uniformly from the
dendrite cores to the interdendritic regions, due to the weak segregation behavior of Ru.
However, some small clusters of Heusler precipitates appeared in the former eutectic
solidification regions due to strong residual microsegregation of Ta in these regions.
Acknowledgements
The author would like to express a hearty gratitude to the Ministry of Steel, Govt. of India for
funding the project and also to the guide Prof. Manab Mallik, Department of Metallurgical and
Materials Engineering, NIT Durgapur, for his great support during the seminar report work.

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