Materials Transactions, Vol. 46, No. 10 (2005) pp. 2129 to 2134
#2005 The Japan Institute of Metals
Improvement in Recrystallization Temperature and Mechanical Properties
of a Commercial TZM Alloy through Microstructure Control
by Multi-Step Internal Nitriding
Masahiro Nagae1 , Tetsuo Yoshio1 , Jun Takada2 and Yutaka Hiraoka3
1
Division of Sustainability of Resources, Graduate School of Environmental Science, Okayama University,
Okayama 700-8530, Japan
2
Division of Chemical and Biological Technology, Graduate School of Natural Science and Technology, Okayama University,
Okayama 700-8530, Japan
3
Department of Applied Physics, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan
In order to develop a molybdenum alloy with high recrystallization temperature and excellent mechanical properties, multi-step internal
nitriding of a commercial TZM alloy (Titanium–Zirconium–Molybdenum alloy) was studied through optical and transmission electron
microscopy, fractographic analysis and three-point bending tests. Two types of nitriding processes, a two-step and another four-step process,
were carried out at temperatures between 1423 and 1873 K. Recrystallization was completely suppressed, even at the center of a 1-mm-thick
specimen through four-step nitriding, whereas recrystallization proceeded at the specimen center in the case of two-step nitriding. The
recrystallization temperature of the TZM alloy in vacuum was raised above 1973 K by four-step nitriding. The ductile-to-brittle transition
temperature (DBTT) of the nitrided TZM alloy depended strongly on the thickness of the deformed microstructure. DBTT of the four-step
nitriding specimen was below 130 K, which is about 70 K lower than that of the two-step nitriding specimen. The yield stress of the four-step
nitriding specimen at 1773 K was about 2.6 times as large as that of the recrystallized specimen.
(Received June 17, 2005; Accepted August 3, 2005; Published October 15, 2005)
Keywords: TZM alloy (Titanium–Zirconium–Molybdenum alloy), multi-step internal nitriding, recrystallization temperature, ductile-to-brittle
transition temperature, fractographic analysis, high-temperature strength
1.
Introduction
TZM (Titanium–Zirconium–Molybdenum) is one of the
most popular commercial molybdenum-based alloys. Its
typical areas of application include components for heat
treatment furnaces, such as hot isostatic pressing, and moulds
for light-alloy casting. The TZM alloy was developed by
Climax Molybdenum, Co., U.S.A., in 1954.1) Its nominal
chemical composition in mass% is Mo–0.5Ti–0.08Zr–0.03C.
TZM is strengthened mainly through the formation of
carbides of the alloying metals. As is evident from its
chemical composition, the amount of alloying metals is
somewhat in excess of that required to entirely consume the
carbon in the formation of precipitates. These excess alloying
metals contribute to solid-solution strengthening. As well as
being strengtheners, carbide particles that precipitate during
the manufacturing process act as recrystallization inhibitors.
Thus, the recrystallization temperature of TZM is about
400 K higher than that of pure molybdenum.2) However, the
maximum use temperature is limited to 1673 K or less
because the strength of TZM decreases notably above the
recrystallization temperature.3)
Our recent studies on internal nitriding of dilute Mo–Ti
alloys4,5) have shown that the multi-step internal nitriding
technique6) is effective in raising the recrystallization
temperature of these alloys. Using this technique, it is
possible to disperse fine TiN particles in these alloys, with the
deformed microstructure retained owing to the gradual, stepby-step increase in nitriding temperature from below the
recrystallization temperature. These TiN particles act effectively as pinning points for grain boundary migration. As a
result, the recrystallization temperature of dilute Mo–Ti
alloys is elevated above 1873 K. Since, as mentioned above,
the TZM alloy contains excess titanium and zirconium,
multi-step nitriding is expected to have the same effect in the
case of the TZM alloy.
In the present study, we subjected a commercial TZM
alloy to a multi-step internal nitriding process in an attempt to
improve its recrystallization temperature and mechanical
properties. TZM has a higher recrystallization temperature
than Mo–Ti alloys. Therefore, the multi-step internal nitriding can be started from a higher temperature than in the case
of Mo–Ti alloys. We assessed the effectiveness of the multistep internal nitriding technique for a TZM alloy. We paid
particular attention to the recrystallization temperature, the
ductile-brittle transition behavior, and high-temperature
strength of the TZM alloy after multi-step internal nitriding.
2.
Experimental
Commercial TZM alloy (Mo–0.5%Ti–0.08%Zr–0.03%C)
sheets with a thickness of 1 mm were used as starting
materials. Mechanical and electrolytic polishing were performed on rectangular specimens (w2:5 t1:0 l25 mm)
cut out from the as-rolled sheets. Nitriding was performed at
1423 to 1873 K in flowing N2 at 0.1 MPa. The multi-step
internal nitriding process employed in this study is schematized in Fig. 1. Two kinds of specimens were prepared under
different nitriding conditions: two-step nitriding at 1423 K
for 4 h (primary nitriding) and at 1873 K for 25 h (secondary
nitriding), and four-step nitriding at 1423 K for 64 h (primary
nitriding), at 1473 K for 25 h (secondary nitriding), at 1573 K
for 25 h (tertiary nitriding) and at 1873 K for 25 h (quaternary
nitriding). A specimen subjected to two-step or four-step
2130
M. Nagae, T. Yoshio, J. Takada and Y. Hiraoka
(a)
(a)
1873 K for 25 h
Temperature
1423 K for 4 h
100µm
(b)
1873 K for 25 h
(b)
1423 K for 64 h
1573 K for 25 h
1473 K for 25 h
Time
Fig. 1 Schematic of the multi-step internal nitriding process used in this
study. (a) two-step nitriding, (b) four-step nitriding.
nitriding is hereafter called a two-step or four-step nitriding
specimen, respectively. For comparison, recrystallized specimens were prepared by heating as-rolled specimens at
1873 K for 1 h in a vacuum of about 1:4 104 Pa. Some
recrystallized specimens were subjected to four-step nitriding
for comparison. The recrystallization temperature was
investigated by heating nitrided specimens at temperatures
between 1873 and 2173 K in a vacuum of about 1:4 104 Pa.
The cross-section of the specimens was examined by
optical microscopy. The deformed microstructure and precipitates were studied in detail by transmission electron
microscopy (TEM; Topcon EM-002B). Thin films for TEM
observation were prepared by mechanical polishing, dimpling and electrolytic polishing in a solution of methyl
alcohol–15 vol% H2 SO4 . In order to determine the ductile-tobrittle transition temperature (DBTT), three-point bending
tests were performed at temperatures from 77 K to room
temperature at a crosshead speed of 0.39 mm/min (initial
strain rate was 3:9 104 s1 ). The span of the jig for the
bending tests and diameter of the load pin were 10 mm and
2.5 mm, respectively. The crack initiation area was investigated by fractographic analysis using a scanning electron
microscope (SEM; JEOL JSM-6330FS). High temperature
three-point bending tests were performed at 1773 K in a
vacuum of about 5:0 104 Pa at a crosshead speed of
0.2 mm/min (initial strain rate was 1:7 104 s1 ). The span
of the jig and diameter of the load pin were 11 mm and
3.5 mm, respectively.
3.
Results and Discussion
3.1 Microstructural observations
The primary nitriding temperature is important in the
multi-step internal nitriding process because the rate of
internal nitriding increases with increasing nitriding temper-
100µm
Fig. 2 Optical micrographs of the cross-sections of the two-step (a) and
four-step nitriding specimens (b).
ature. In order to determine the primary nitriding temperature, a 25-h nitriding run was performed at temperatures
below the recrystallization temperature of the as-rolled alloy
(about 1673 K). The center region was recrystallized as a
result of the 25-h nitriding run at 1573 K. Thus, the primary
nitriding temperature was fixed at 1423 K by considering a
margin for recrystallization due to long-time nitriding. This
temperature is 200 K higher than that for Mo–Ti alloys.6)
Figure 2 shows optical micrographs of the cross-section of
a two-step (a) and four-step nitriding specimen (b). It is found
that recrystallization was completely suppressed across the
entire cross-section by four-step nitriding, while the thickness of the deformed microstructure that persisted through
two-step nitriding was about 170 mm. Thus, the final microstructure varied with the nitriding conditions. In order to
retain a thicker deformed microstructure, the nitriding
temperature was raised step by step following primary
nitriding. Mechanical properties of nitrided specimens
strongly depended on the thickness of the deformed microstructure, as discussed below.
In order to investigate the multi-step internal nitriding
behavior of the TZM alloy, its microstructure was observed
by TEM after each step of the four-step nitriding process.
Figure 3 shows TEM images taken near the side surface of
specimens viewed in the transverse direction: as-rolled (a),
subjected to secondary nitriding (b), and to quaternary
nitriding (c). Average grain sizes in the thickness direction, as
estimated from low-magnification TEM images, are listed in
Table 1. A number of disk-shaped fine precipitates with a
coherent strain field were clearly visible in the molybdenum
grains after secondary nitriding. The precipitates on the grain
boundaries differed in shape from the disk-shaped precipitates [see enlargement in Fig. 3(b)]. These precipitates grew
into rod-like precipitates [shown in Fig. 3(c)]. Notwithstanding the slight increase in grain size observed after four-step
Improvement in Recrystallization Temperature and Mechanical Properties of a Commercial TZM Alloy
2131
(a)
(a)
100µm
100nm
(b)
(b)
100µm
Fig. 4 Optical micrographs of the cross-section of the four-step nitriding
specimens annealed in vacuum for 1 h at 1973 K (a) and 2073 K (b).
100nm
(c)
100nm
Fig. 3 TEM images taken near the side surface viewed in the transverse
direction of specimens: as-rolled (a), after secondary nitriding at 1473 K
(b) and quaternary nitriding at 1873 K (c).
Table 1 Average grain sizes in the thickness direction of the specimens
shown in Fig. 3.
Average grain size (mm)
Figure 3(a) (as-rolled)
0.190
Figure 3(b) (secondary nitriding)
0.164
Figure 3(c) (fourth nitriding)
0.274
nitriding (see Table 1), which suggests a certain degree of
recovery, the fine-grained deformed microstructure persisted
in the case of the four-step nitriding specimen. (The
difference in grain size between the as-rolled and secondary
nitriding specimens is a scattering on measurement.) Thus,
precipitates can be grown by multi-step internal nitriding
without recrystallization of the molybdenum matrix. The
electron diffraction analysis showed that these precipitates
were TiN with an f.c.c structure. A solid solution with ZrN
may be present, however, no evidence for this was obtained
from the electron diffraction analysis because of the low
zirconium content.
The recrystallization temperature was studied through 1-h
anneals in a vacuum of about 1:4 104 Pa. Figure 4 shows
optical micrographs of the cross-section of a four-step
nitriding specimen annealed in vacuum for 1 h at 1973 K
(a) and 2073 K (b). It was found that recrystallization was
completely suppressed up to 1973 K in vacuum. This is 100 K
higher than the recrystallization temperature of Mo–Ti
alloys6) subjected to multi-step internal nitriding. According
to thermodynamic data on nitrides,7) the standard free energy
of formation of ZrN is lower than that of TiN. Since the TZM
alloy contains zirconium in addition to titanium, the TiN
particles that precipitated as a result of multi-step internal
nitriding of TZM can be assumed to be solid solutions with
ZrN. The recrystallization temperature after multi-step
internal nitriding is higher in the case of TZM than in the
case of Mo–Ti alloys, presumably because the TiN particles
in TZM are thermodynamically more stable than the pure
TiN in Mo–Ti alloys. Thus, multi-step internal nitriding was
also effective in improving the recrystallization temperature
of the TZM alloy. It is notable that abnormal grain growth
was identified in the specimen heated at 2073 K, as shown in
Fig. 4(b). This recrystallized structure is similar to that of a
La2 O3 -doped Mo alloy8) having an elongated coarse-grain
structure after complete recrystallization. A detailed study on
the abnormal grain growth behavior of the TZM alloy
subjected to multi-step internal nitriding is in progress and
will be discussed in a future paper.
3.2 Mechanical properties
Stress-displacement curves obtained at 298 K for an as-
2132
M. Nagae, T. Yoshio, J. Takada and Y. Hiraoka
2000
(a) as-rolled
100
(e) 4 step nitriding
after recrystallization
(d) 4 step nitriding
(c) 2 step nitriding
1000
(b) recrystallized
500
0
0
0.5
1
Displacement, x / mm
1.5
Fig. 5 Stress-displacement curves obtained at 298 K for the as-rolled (a),
recrystallized (b), two-step (c) and four-step nitriding specimens (d). The
data for a specimen subjected to four-step nitriding after recrystallization
(e) are also shown for comparison.
rolled specimen (a), a recrystallized specimen (b), a two-step
nitriding specimen (c) and a four-step nitriding specimen (d)
are shown in Fig. 5. The data for a specimen subjected to
four-step nitriding after recrystallization (e) was also shown
for comparison. The yield stresses of the multi-step nitriding
specimens, yðcÞ and yðdÞ , were about 1.5 times as large as
that of the recrystallized specimen, yðbÞ . It is found that the
yield stress of the as-rolled specimen, yðaÞ , decreased by
about 200 MPa after multi-step internal nitriding. The yield
stress of the specimen nitrided after recrystallization, yðeÞ ,
was approximately equal to that of the multi-step nitriding
specimens even though its deformation strain prior to fracture
was much smaller. These results led to the following three
conclusions: 1) the difference between yðbÞ and yðeÞ
(yðbÞ yðeÞ ) is due to precipitation hardening by internal
nitriding, 2) the grain size effect on yðdÞ can be ignored and
3) the difference between yðaÞ and yðdÞ (yðaÞ > yðdÞ ) stems
from the difference between work hardening and precipitation hardening. That is, the yield stress after multi-step
internal nitriding is explained mainly by precipitation hardening.
Figure 6 shows the temperature dependence of the bend
angle for as-rolled specimens (a), recrystallized specimens
(b), two-step nitriding specimens (c), and four-step nitriding
specimens (d). The data for specimens subjected to four-step
nitriding after recrystallization (e) were also shown for
comparison. The DBTT values obtained by bending tests are
listed in Table 2. It is found that the four-step nitriding
specimen exhibited sufficient ductility even at about 130 K.
In the case of the recrystallized grain structure, internal
nitriding embrittled the specimen substantially. In contrast, in
the case of the fine-grained deformed microstructure, internal
nitriding induced almost no effect on the ductility of the
specimen. The superior ductility of the four-step nitriding
specimen can be explained by the observation that the
deformed microstructure of the as-rolled specimen was
80
Bend Angle, θ / deg.
Stress, σ / MPa
1500
(a) as-rolled
(b) recrystallized
(c) two-step nitriding
(d) four-step nitriding
(e) four-step nitriding after recrystallization
60
40
20
0
50
100
150
200
250
300
350
Temperature, T / K
Fig. 6 Temperature dependence of the bend angle as-rolled (a), recrystallized (b), two-step (c) and four-step nitriding specimens (d). The data for a
specimen subjected to four-step nitriding after recrystallization (e) are also
shown for comparison.
Table 2 DBTTs obtained by bending tests. DBTT was defined as the
temperature at which the yield stress and maximum stress become equal,
i.e., the point at which the specimen fractured immediately after it yielded.
DBTT (K)
(a) as-rolled specimen
(b) recrystallized specimen
<77
135
(c) two-step nitriding specimen
145
(d) four-step nitriding specimen
<130
(e) recrystallized specimen
subjected to four-step nitriding
230
maintained by four-step nitriding. On the other hand, the
two-step nitriding specimen exhibited a temperature dependence of bend angle similar to that of the recrystallized
specimen, both of which rapidly lost ductility below 180 K.
Fractographic analysis was performed in order to investigate
the difference between the four-step and two-step nitriding
specimens.
Figure 7 shows SEM images of typical fracture surfaces
obtained by bending tests of a two-step (a) and four-step
nitriding specimen (b). S and I indicate the specimen surface
and the interface between the deformed microstructure and
recrystallized region, respectively. The white arrows indicate
the crack initiation area (see enlargement of the area enclosed
by the white rectangle). It is found that the crack initiation
area of the four-step nitriding specimen was the specimen
surface, while that of the two-step nitriding specimen was a
recrystallized region near the interface I. In order to explain
these findings, we will discuss the relationship between
microstructure and stress distribution in a three-point bending
Improvement in Recrystallization Temperature and Mechanical Properties of a Commercial TZM Alloy
2133
500
(a)
(e) 4 step nitriding
I
after recrystallization
Stress, σ / MPa
400
500µm
300
(d) 4 step nitriding
200
100
(a) recrystallized
S
(b)
0
0
0.5
1
Displacement, x / mm
1.5
Fig. 9 Stress-displacement curves obtained at 1773 K for the recrystallized
(a) and four-step nitriding (b) specimens. The data for the specimen
subjected to four-step nitriding after recrystallization (c) are also shown
for comparison.
500µm
Fig. 7 SEM images of typical fracture surfaces obtained by bending tests
of two-step (a) and four-step nitriding specimens (b).
Load pin
Surface
Neutral plane
Undersurface
0
Compressive stress
Tensile stress
Fig. 8 The schematic stress distribution in the elastic deformation range of
a three-point bending specimen.
test. Figure 8 schematizes the stress distribution in the elastic
deformation range. A specimen under an applied load
experiences compressive and tensile stresses on its surface
and undersurface, respectively. A neutral plane where the
stress is equal to zero exists between the surface and
undersurface. The tensile stress below the neutral plane is
important for the bending test and increases gradually toward
the undersurface. The four-step nitriding specimen fractured
at the undersurface when the tensile stress on the undersurface reached the fracture stress of the deformed microstructure because the four-step nitriding specimen is macroscopically uniform in microstructure. However, the crosssectional microstructure of the two-step nitriding specimen is
a layered structure that consists of two microstructures with
different fracture strengths. If the stress applied at the
recrystallized region near the interface with the deformed
microstructure reaches the fracture stress of the recrystallized
microstructure before the stress on the undersurface reaches
the fracture stress of the surface deformed microstructure, the
two-step nitriding specimen will be fractured at the internal
recrystallized region. Thus, the difference in ductile-to-brittle
transition behavior between the two-step and four-step
nitriding specimens results from the difference in their
cross-sectional microstructure. Therefore, DBTT would
decrease with increasing thickness of the deformed microstructure. Details of the relationship between the thickness of
the deformed microstructure and mechanical properties will
be reported later.
Stress-displacement curves obtained at 1773 K for the
recrystallized specimen (a) and the four-step nitriding specimen (b) are shown in Fig. 9. The data for the specimen
subjected to four-step nitriding after recrystallization (c) are
also shown for comparison. The yield stress of the four-step
nitriding specimen was about 2.6 times as large as that of the
recrystallized specimen, but somewhat lower than the yield
stress of the specimen nitrided after recrystallization. This
can presumably be explained by grain boundary sliding.
Takida et al.9) reported that an alloy having a fine-grained
equiaxed molybdenum matrix containing 0.8 mol% ZrC
prepared by mechanical alloying and hot isostatic pressing
exhibited a large uniform elongation of 551% at 1770 K due
to superplastic deformation. Although the four-step nitriding
specimen is a fine-grained elongated molybdenum matrix, its
microstructure resembles that of the above-mentioned alloy
given the fine-grained molybdenum matrix and distribution
of ultra-fine precipitates along its grain boundaries. These
observations suggest the possibility that the four-step nitriding specimen shows a large elongation due to superplastic
deformation. This is a topic for future research.
2134
4.
M. Nagae, T. Yoshio, J. Takada and Y. Hiraoka
Conclusion
The multi-step internal nitriding behavior of a commercial
TZM alloy and the mechanical properties of the resultant
nitrided specimens were studied. The results can be summarized as follows.
(1) Primary nitriding of the TZM alloy could be performed
at a temperature 200 K higher than that used in a
previous study on Mo–Ti alloys.
(2) The thickness of the non-recrystallized deformed
microstructure depended on the nitriding conditions. It
was important to raise the nitriding temperature step by
step after primary nitriding in order to retain a thicker
deformed microstructure.
(3) The four-step nitriding specimen showed a higher
resistance to recrystallization compared with Mo–Ti
alloys after multi-step internal nitriding. The recrystallization temperature in vacuum was about 2073 K.
(4) The four-step nitriding specimen exhibited a higher
yield stress and ductility than the recrystallized specimen. Yield stresses at 298 and 1773 K were, respectively, about 1.5 and 2.6 times as large as those of the
recrystallized specimen. This is mainly explained by
the precipitation hardening effect.
(5) DBTT decreased with increasing thickness of the
deformed microstructure retained after multi-step internal nitriding. DBTT of the four-step nitriding specimen was lower than 130 K.
Thus, we can conclude that multi-step internal nitriding is
highly effective in the suppression of recrystallization
embrittlement and strengthening of the present TZM alloy.
Acknowledgements
The authors are grateful to T. Fukumitsu and T. Hidaka for
their assistance with experiments and Dr. H. Kurishita
(International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University,
Oarai, Japan) for helpful discussions. We are greatly indebted
to Sunric Co., Ltd. for supplying the TZM alloy. This study
was partly supported by a Grant-in-Aid for Encouragement of
Young Scientists and a Grant-in-Aid for JSPS Fellows from
Japan Society for the Promotion of Science.
REFERENCES
1) J. L. Ham, F. P. Bens, A. J. Herzig and G. A. Timmons: U.S. Patent
2678271 (1954).
2) J. A. Shields: Adv. Mater. and Proc. 142 (1992) 28–36.
3) M. G. Manzone and J. Z. Briggs: Less-Common Alloys of Molybdenum,
(Climax Molybdenum Company, New York, 1962), 177–182.
4) M. Nagae, A. Kunisada, J. Takada, Y. Hiraoka, Y. Takemoto, M. Hida
and H. Kuwahara: J. Jpn. Soc. Powder Powder Metal. 45 (1998) 190–
195.
5) M. Nagae, S. Okada, M. Nakanishi, J. Takada, Y. Hiraoka, Y. Takemoto,
M. Hida, H. Kuwahara and M. K. Yoo: Int. J. Refr. Metals & Hard
Mater. 16 (1998) 127–132.
6) M. Nagae, Y. Takemoto, T. Yoshio, J. Takada and Y. Hiraoka: Mater.
Sci. Eng. A, in press.
7) J. F. Elliott and M. Gleiser: Thermochemistry for Steelmaking vol. I,
(Addison-Wesley Publishing Co., Reading, Mass., 1969).
8) T. Takida, K. Takebe and T. Igarashi: J. Jpn. Soc. Powder Powder Metal.
45 (1998) 453–458.
9) T. Takida, H. Kurishita, M. Mabuchi, T. Igarashi, Y. Doi and T. Nagae:
Mater. Trans. 45 (2004) 143–148.