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Materials Science and Engineering A 527 (2010) 1016–1021
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Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea
Effects of heat treatments on the microstructure of IN792 alloy
Jinxia Yang ∗ , Qi Zheng, Hongyu Zhang, Xiaofeng Sun, Hengrong Guan, Zhuangqi Hu
Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history:
Received 23 June 2009
Received in revised form 5 September 2009
Accepted 13 October 2009
Keywords:
Superalloy
Heat treatment
Carbides
␥ phase
a b s t r a c t
The microstructure of IN792 alloy were investigated at the as-cast state, annealed state at 845 ◦ C for 24 h,
HS1220 and HS1120 heat treatment states (the material was solution heat treated at 1220 and 1120 ◦ C,
respectively, followed by a two-stage ageing process with 4 h at 1080 ◦ C and 24 h at 845 ◦ C, which were
named HS1220 and HS1120, respectively). The incipient points of eutectic and solution temperature of
carbide are 1230 and 1260 ◦ C, respectively, which are measured by metallographic examination. Chinese
script MC-type carbides, rose-shaped ␥ /␥ eutectic and cubic ␥ phase are observed in the as-cast IN792
alloy. The annealed microstructure is the same as that of as-cast alloy because the annealing treatment
only removes the residual cast stress. After HS1120 solution treatment, MC-type carbides and roseshaped ␥ /␥ eutectic partly dissolves into ␥ matrix, and M23 C6 carbide that has a cube–cube orientation
relationship with ␥ matrix forms near ␥ /␥ eutectic and MC-type carbides. After HS1220 solution treatment, rose-shaped ␥ /␥ eutectic completely dissolves into ␥ matrix, but morphology of carbides does not
change. The subsequent aging treatments make ␥ phase arrange regularly, and induce re-precipitation
of profuse fine ␥ particles throughout ␥ matrix channels.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
IN792 alloy is widely used in industrial and aircraft turbine
because of its high strength and excellent hot corrosion resistance
in high temperature. However, IN792 as the directional solidification (DS) alloy often suffers from poor cast-ability due to hot
cracking, i.e. crack formation in the final stage of solidification
[1,2]. The hot cracking deteriorates the mechanical properties of
DS IN792 alloy although it can be decreased by controlling the concentrations of Ti, Ta, B, Zr and Hf [1,2]. The conventionally cast (CC)
process can improve or decrease greatly the tendency of hot cracking because of the refinement of grains and the effects of grain
boundaries. However, CC IN792 alloy has some shortages, also. For
example, the tensile strength, especially at room temperature, is
much lower than those of DS IN792 alloy. Recently, we found that
CC IN792 alloy has the great potential because its strength can be
increased by heat treatment. In addition, it is found that cold crack
that is different from the hot cracking is easier to form at grain
boundaries when solution temperature is increased to the temperatures above 1120 ◦ C. Therefore, the improper solution temperature
may have crucial effects on the microstructure and properties of
CCIN792 alloy. The present work focuses on the influences of heat
treatments on the microstructure of CC IN792 alloy in the as-cast
∗ Corresponding author. Tel.: +86 24 83970809; fax: +86 24 23971807.
E-mail address: [email protected] (J. Yang).
0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2009.10.026
state, annealed state, and solution heat treatment (HS1220 and
HS1120) states.
2. Experimental
The chemical composition of Ni-base superalloy IN792 is listed
in Table 1. Tested specimens are made from the ingots obtained by
vacuum induction melting process. Differential scanning calorimetry (DSC) is employed to confirm the critical temperature of phase
formation during solidification, which guides the metallographic
examination. A cooling rate of 5 ◦ C/min is employed in all measurements. The temperatures of phase formation are used to select
the solution temperature. According to the incipient melting temperature of 1221 ◦ C of ␥ /␥ eutectic and the solution temperature of
1261 ◦ C of carbides shown in the DSC pattern (Fig. 1a), the metallographic examination is conducted with 30 minutes at 1260, 1245,
1240, 1230, 1220, 1120, 1080 ◦ C, respectively. The metallographic
examination showed that the remarkable re-melting of ␥ /␥ eutectic occurs at and above 1230 ◦ C (Fig. 1b), and carbides are dissolved
into ␥ matrix at 1260 ◦ C (Fig. 1c). Furthermore, cold cracking is
more prone to produce at grain boundaries and dendritic boundaries at 1260, 1245, 1240, 1230, 1220 ◦ C holding for 4 h than at 1120,
1080 ◦ C holding for 4 h (Fig. 2). In addition, the annealing treatment
is chosen in order to remove the residual stress and avoid crack
formation, but the alloy cannot be strengthened without the reprecipitation of ␥ phase during solution treatment. The solution
temperature, 1080 ◦ C, is too low to keep the re-solution of most
J. Yang et al. / Materials Science and Engineering A 527 (2010) 1016–1021
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Fig. 1. DSC pattern of as-cast IN792 alloy (a), incipient melting of eutectic at 1230 ◦ C on SEM (b) and re-formed carbides during cooling after solution of at 1260 ◦ C on optical
microscope (c).
H3 PO4 , 20 pct H2 SO4 and 20 pct H2 O and chemically by the solution of 44 pct CuSO4 + 33 pct HCl + 23 pct H2 O. High magnification
observation is carried out using transmission electron microscope
(TEM).
3. Results and discussion
Fig. 2. Crack morphology after solution treatment.
␥ particles while the higher temperature, 1220 ◦ C, may cause the
cold crack seriously. Hence, IN792 alloy is selected to solution heat
treat at 1120 ◦ C and follow by a two stage ageing process with 4 h
at 1080 ◦ C and 24 h at 845 ◦ C in the presented work.
The microstructures of IN792 alloy are examined by scanning
electron microscope (SEM) with energy dispersive X-ray analysis
(EDAX) and optical microscope. The samples for SEM observation are etched electrolytically in a solution consisting of 60 pct
Table 1
The chemical composition of IN792 alloy.
C
Co
Cr
W
Mo
Al
Ti
Ta
B
Zr
Ni
0.13
9.06
12.41
3.97
2.04
3.47
3.97
3.88
0.01
0.093
Bal
The as-cast microstructure of IN792 alloy is composed of carbides, ␥ /␥ eutectic, ␥ phase and ␥ matrix as shown in Fig. 3.
Carbides are observed in interdendritic area and presented as
block shape and rod shape in Fig. 3a–c, but only MC-type carbides
are identified by SAD spectrum (Fig. 3d and e). EDAX pattern in
Fig. 3f proves that it is rich in Ta (29.19 at.%) and Ti (45.83 at.%),
probably TaC and TiC, which is similar to that in IN792 + Hf alloy
(Ta–32.91 at.%, Ti–47.59 at.%) and other superalloys [1–11]. Due to
its light weight, alloying element C cannot be detected correctly
in EDAX analysis. A rose-shaped ␥ /␥ eutectic is distributed at the
adjacent region to carbides in interdendritic area (Fig. 3b). As shown
in Fig. 4, the ␥ phase has an irregular cubic morphology with nonuniform distribution of size, and may have a coherent relationship
with ␥ matrix.
As-cast IN792 alloy is annealed at 845 ◦ C for 24 h in order to
remove the residual stress, hence the annealed microstructure is
the same as that of as-cast IN792 alloy.
HT1120 heat treatment leads to a great change of microstructure of IN792 alloy. The solution treatments at 1120 ◦ C for 4 h
induces the disperse distribution of carbides, and MC-type carbides are partly dissolved into ␥ matrix as shown in Fig. 5a. Another
phase appears grey color, while MC-type carbides are rather light
and presented as well-distributed Chinese script morphology in
backscattered electron image (Fig. 5b). Such a Chinese script like
MC carbide is also found in many alloys such as K465, Mar-M200,
Mar-M247LC and so on [3–11]. The new phase is found to nucleate
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J. Yang et al. / Materials Science and Engineering A 527 (2010) 1016–1021
Fig. 3. Microstructure of as-cast IN792 alloy (a) interdendritic area, (b) carbide and ␥ /␥ eutectic, (c) TEM image and (d) SAD pattern and (e) EDAX of carbides.
Fig. 4. ␥ morphology in dendritic arm (a) and interdendritic area (b) in the as-cast microstructure of IN792 alloy.
Fig. 5. Microstructure of IN792 alloy after solution treatment at 1120 ◦ C for 4 h (a) disperse distribution of carbides in interdendritic area, (b) backscattered image of M23 C6
(grey) and MC (bright) carbides, (c) SAD pattern of M23 C6 carbide and (d) SAD patterns of M23 C6 and ␥.
J. Yang et al. / Materials Science and Engineering A 527 (2010) 1016–1021
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Fig. 6. ␥ /␥ eutectic (a) at 1120 ◦ C for 4 h and (b) after HS1120 heat treatment.
either at MC-type carbides or around ␥ /␥ eutectic, and is identified
as Cr23 C6 carbide by SAD pattern (Fig. 5c). However, in IN792 + Hf
alloy, such a phase is identified as M5 B3 , M3 B2 borides [4,5]. It has
been reported that Cr, Mo and W are the major alloying elements
comprising M23 C6 , M6 C and sigma phase in Ni-base superalloy.
M23 C6 carbide distributes near ␥ /␥ eutectic and MC carbide in
interdendritic region. It has been reported that it also appears preferentially on grain boundaries and sub-grain boundaries in some
Fig. 7. Size distribution and morphology of ␥ particle at (a) dendritic arm and (b) interdendritic area at 1120 ◦ C for 4 h, (c) dendritic arm and (d) interdendritic area at 1120 ◦ C
for 4 h + 1080 ◦ C for 4 h, and (e) dendritic arm and (f) interdendritic area after HS1120 heat treatment.
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J. Yang et al. / Materials Science and Engineering A 527 (2010) 1016–1021
Fig. 8. Size distribution and morphology of ␥ particle at (a) interdendritic area and (b) dendritic arm after HS1220 heat treatment
Fig. 9. ␥ particles (a) after HS1120 heat treatment and (b) at as-cast state.
superalloys [3,11]. This carbide has the fcc structure with a lattice
constant which is nearly three times greater than that of matrix, and
has a cube–cube orientation relationship with matrix as shown in
Fig. 5d:
{1 0 0} M23 C6 //{1 0 0}␥ , 0 0 1
M23 C6 //0 0 1␥
These are the characteristic features of the chromium-rich
M23 C6 carbide [3]. As the unevenly precipitation of M23 C6 carbide is closely related to the decomposition of MC and dissolution
of ␥ /␥ eutectic during solution treatment, the subsequent aging
treatments hardly change the distribution, type and morphology
of carbides. As seen in Fig. 6a, ␥ /␥ eutectic dissolved partly into
␥ matrix after solution treatment. The following aging at 1080 ◦ C
for 4 h and aging at 845 ◦ C for 24 h has no effect on ␥ /␥ eutectic
(Fig. 6b), also. The above aging treatments are conducted to make
␥ phase arrange more regular in dendritic arm and interdendritic
area (Fig. 7a–f), and cause ␥ phase to distribute more homogeneous
than those after solution treated at 1120 ◦ C (Fig. 7e and f).
The as-cast microstructure is completely changed by HT1220
heat treatment (Fig. 8a and b). Carbides are still observed in interdendritic area and still presented as block shape and rod shape as
shown in Fig. 8a. ␥ /␥ eutectic disappears because it is dissolved into
␥ matrix. In contrast to those of HS1120 heat treatment (Fig. 7), the
distribution of ␥ phase is more homogeneous in dendritic arm and
interdendritic area, furthermore, its arrangement is more regular,
and its size becomes finer.
TEM image displays profuse fine ␥ re-precipitate throughout
␥ matrix channels during cooling (Fig. 9a) which will improve the
alloy properties. The profuse fine ␥ particles do not form in as-cast
conditions (Fig. 9b). In addition, ␥ size is increased a little, but its
cubic shape hardly changes after the aging treatment.
4. Conclusions
The as-cast microstructure of IN792 alloy is composed of Chinese script MC-type carbides, rose-shaped ␥ /␥ eutectic and cubic
␥ phase. After HT1120 solution treatment, MC-type carbides and
rose-shaped ␥ /␥ eutectic partly dissolves into ␥ matrix, and M23 C6
carbide forms near ␥ /␥ eutectic and MC-type carbides having
cube–cube orientation relationship with ␥ matrix. After HS1220
solution treatment, rose-shaped ␥ /␥ eutectic completely dissolves
into ␥ matrix, but morphology of carbides do not changed in contrast to that of HS1220. Comparing with HS1120 heat treatment,
the ␥ distribution treated by HS1220 heat treatment is more
homogeneous in dendritic arm and interdendritic area, furthermore, its arrangement is more regular and its size becomes finer.
The two-stage aging treatments make ␥ phase arrange regularly,
and the re-precipitation of profuse fine ␥ throughout ␥ matrix
channels.
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