Download Report

15. - 17. 5. 2013, Brno, Czech Republic, EU
STRUCTURE AND MECHANICAL PROPERTIES OF NICKEL ALLOYS
Martin POHLUDKAa, Jitka MALCHARCZIKOVÁa, Vít MICHENKAb, Miroslav KURSAa,
Tomáš ČEGANa, Ivo SZURMANa
a
VŠB – Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava Poruba, Czech Republic
b
VÚHŽ a.s., 739 51 Dobrá 240, Czech Republic
[email protected], [email protected], [email protected], [email protected],
[email protected], [email protected]
Abstract
Three different nickel alloys – IC221M, IC396 and IC438 – were prepared by induction melting followed by
centrifugal casting. Metallographic samples were made of bar castings. The samples were used for
microstructure documentation, for porosity evaluation and for chemical composition verifying by scanning
electron microscope. The bars of nickel alloys were machined to tensile specimens which were strained at
standard conditions. Dendritic structure of cast nickel alloys proved that it was unsuitable because it
contained large shrinks which caused premature fracture of tensile specimens. This result was opposed to
tensile tests of directionally solidified samples because directional solidification orientates a structure and
also reduces the presence of shrinks. Therefore, it is important to continue in searching of the processes
which reduce a dendritic structure of nickel alloy castings.
Keywords: nickel alloys, centrifugal casting, porosity, tensile test
1.
INTRODUCTION
Materials based on nickel aluminides are used in high-temperature applications [1]. Ni3Al intermetallic
compound is a base of nickel alloys and it exhibits a positive dependence of deformation stress on
temperature [2]. Thereby, strength of nickel alloys increases together with temperature up to 800 °C.
Unfortunately, polycrystalline Ni3Al is brittle at room temperature. Brittleness of Ni3Al can be successfully
reduced by boron alloying in small quantities [3]. Chrome inhibits a corrosion cracking at high temperatures.
Addition of molybdenum and zirconium provides ductility.
To carry out tensile tests of commercial nickel alloys with modified composition and to examine effect of
chemical composition on mechanical properties being related to these types of test, these were the primary
work aims. But they failed, therefore author decided to examine a cause of failure and to propose a
treatment which will inhibit the failure in future.
2.
EXPERIMENT
IC-396, IC-221M and IC-438 nickel alloys were prepared by induction melting in vacuum after which molten
alloys were centrifugally cast. All was made in casting apparatus Supercast – Titan which is the property of
Regional materials science and technology centre in VŠB – Technical University of Ostrava. Melting
conditions are in Tab. 1.
15. - 17. 5. 2013, Brno, Czech Republic, EU
Tab. 1 Preparation conditions of nickel alloys
Alloy
Casting No.
Melting
Casting
Mould
IC-396
N01
vacuum
argon
graphite
IC-221M
N02
vacuum
argon
graphite
IC-438
N03
vacuum
argon
graphite
Castings were four oval bars connected by riser. Diameter of the bar was 18 mm and length was 160 mm.
After separating of the riser, the bars were not heat-treated. One bar of each nickel alloy was used for cutting
of transversal section and for machining of two tensile specimens. Metallographic sample for documentation
of microstructure was made from the transversal section.
Chemical composition of the IC-396, IC-221M and IC-438 nickel alloys used in this article is modified and it
differs from the composition of commercially applied nickel alloys [1]. Tab. 2 gives the modified composition.
Tab. 2 Nominal composition of nickel alloys
Alloy
Ni
Al
Cr
Mo
Zr
B
(wt. %)
(wt. %)
(wt. %)
(wt. %)
(wt. %)
(wt. %)
IC-396
80.42
7.98
7.72
3.02
0.85
0.01
IC-221M
81.16
8.00
7.70
1.43
1.70
0.01
IC-438
79.51
8.10
5.23
7.02
0.13
0.01
3.
3.1
RESULTS
Structure and chemical composition
All three nickel alloys had a typical cast microstructure. They consisted of long grains oriented in a direction
from mould wall to casting centre. The middle of castings was full of pores. Etching of metallographic
samples by Marble’s etchant revealed that individual coarse grains contained many narrow dendrites with
phases in interdendritic space (Figs. 1 till 3). Documentation of microstructure was taken place by inverse
metallographic microscope OLYMPUS GX51 equipped with digital camera OLYMPUS DP12.
Fig. 1 Microstructure of the IC396 alloy
Fig. 2 Microstructure of the IC221M alloy
Fig. 3 Microstructure of the IC438 alloy
Chemical composition of nickel alloys was at first confirmed by optical emission spectrometer
2
SPECTROMAXx from pure and ground surfaces of the castings. Analysed area had dimension of 12 mm
with minimal depth after sparking (≈ 100 μm). Measurement was carried out several times on different
accurately defined places of the sample. Final average values of OES analysis are written in Tab. 3.
15. - 17. 5. 2013, Brno, Czech Republic, EU
Tab. 3 Results of OES and EDS analyses
Alloy
Analysis
method
Ni
Al
Cr
Mo
Zr
B
(wt. %)
(wt. %)
(wt. %)
(wt. %)
(wt. %)
(wt. %)
IC-396
OES
80.61
7.84
7.98
2.80
0.57
0.025
IC-396
EDS
79.45
9.19
7.18
3.13
1.05
–
IC-221M
OES
81.34
7.72
8.06
1.35
1.31
0.026
IC-221M
EDS
80.36
8.81
7.34
1.60
1.89
–
IC-438
OES
80.12
7.80
5.16
6.64
0.12
0.022
IC-438
EDS
78.45
9.10
4.77
7.33
0.35
–
Independently of OES analysis, chemical composition of nickel alloys was examined with help of scanning
electron microscope QUANTA FEG 450 with a probe EDAX APOLLO X. The electron microscope has one
disadvantage – the analysis of elements lighter than carbon is impossible. Final average values of EDS
analysis of nickel alloy chemical composition are in Tab. 3. The results of both chemical composition
analyses are not too different from nominal values from Tab. 2.
Chemical analysis of phases present in nickel alloy castings revealed that all nickel alloys contained small
phases whose diameter was units of micrometer. The phases were mainly situated in interdendritic space
and were formed from the Ni-Zr and the Zr-Mo elements, chrome stayed dissolved in alloy matrix. Sulphur
was also identified. It combined with the Zr-Mo phase. Sulphur had an origin in Ni3S2 which contaminated
charge of nickel [4]. There were also small carbides in the nickel alloys.
3.2
Tensile tests
Two tensile specimens were machined from each nickel alloy. The specimens were strained at room
temperature and standard conditions. Tensile specimens had circular cross-section with non-threaded grip
sections. Diameter of gauge section was 5 mm and its length was 25 mm. The tests were carried out
consistent with the ČSN EN ISO 6892-1 norm in VÚHŽ a.s. on tensile apparatus Tira Test 2300. The results
of tensile tests are concluded in Tab. 4.
Tab. 4 Tensile test results of nickel alloy
Alloy
Tensile
specimen No.
YS
UTS
E
AR
(MPa)
(MPa)
(%)
(%)
IC-396
N01.A
–
490
0.1
0.4
IC-396
N01.B
–
176
0.1
0.1
IC-221M
N02.A
–
179
0.1
0.1
IC-221M
N02.B
–
190
0.1
0.1
IC-438
N03.A
–
42
0.1
0.1
IC-438
N03.B
non-carried out
non-carried out
non-carried out
non-carried out
Unfortunately, all tensile specimens exhibited so low ductility that it was not possible to reach deformation of
0.2 % which is important for determining of nominal yield strength, YS. Therefore, values of elongation, E,
15. - 17. 5. 2013, Brno, Czech Republic, EU
and area reduction, AR, have only an informative character. In the case of sample No. N03.B, it fractured
during marking of the gauge section.
Observation of tensile specimen fracture surfaces on scanning electron microscope (Figs. 4 till 6) revealed
that all tensile samples were premature fractured in a place with enhanced concentration of shrinks. This
conclusion was confirmed by following defectoscopy analysis.
Fig. 4 Fracture surface of the IC396 alloy
Fig. 5 Fracture surface of the IC221M alloy
Fig. 6 Fracture surface of the IC438 alloy
Cast IC-396, IC-221M and IC-438 alloys prepared by above mentioned approach have insufficient
mechanical properties for using in commercial practice and they need additional treatment. But even though
the alloys were annealed at the conditions of 1100 °C/1.5 h/cooling in air, dendritic structure of the alloys
with shrinks was not reduced. That was a process of directional solidification which positively affected alloy
structure with shrinks [5]. Another solution of casting treatment can be represented by HIP method.
3.3
Porosity
By reason of unsuccessful tensile tests, attention was paid to statistical and morphological description of
pores and shrinks. Cast structures used in this work were compared with the ones directionally solidified in
[5]. Procedure is explained in [4].
Tab. 5 Porosity of nickel alloys after casting and directional solidification
Alloy
Preparation
P
n
d
(%)
(-)
(m)
IC-396
casting
0.0534  0.0259
519
7.08
IC-396
directional solidification
0.0500  0.0167
637
6.18
IC-221M
casting
0.0662  0.0341
695
6.81
IC-221M
directional solidification
0.0420  0.0146
461
6.66
IC-438
casting
0.0642  0.0317
960
5.71
IC-438
directional solidification
0.0513  0.0175
950
5.13
Porosity of nickel alloys was quantified with help of the same microscope as their microstructure. Ten
photographic images of different places of non-etched sample surface were documented at two hundredfold
magnification. These images were evaluated by analySIS auto, the computer program for image analysis.
Measured parameters are given in [4]. There are the results of porosity, P, together with amount of identified
15. - 17. 5. 2013, Brno, Czech Republic, EU
pores, n, and average pore diameter, d, in Tab. 5. Porosity and average pore diameter of directionally
solidified nickel alloys are smaller although the alloys can contain more pores than cast alloys (e.g. IC-396).
That was identified from 461 to 960 pores in prepared samples of nickel alloys. Diameter of these pores was
in range from 1.5 to 21.0 m. Majority of pores was situated in interdendritic space. Predominant character of
pore diameter distribution is log-normal. That holds for castings and directionally solidified nickel alloys (Figs.
7 and 8) but the directionally solidified ones contain less pores greater than 10 m.
Fig. 7 Pore distribution in nickel alloys after casting
Fig. 8 Pore distribution in nickel alloys after
directional solidification
Results of pore morphology comparison in cast, C, and directionally solidified, DS, nickel alloys are
contradictory (Fig. 9). Fully positive effect of directional solidification can be seen in the case of IC-438 alloy
whose directionally solidified alloy contains more circular pores with smoother surface than cast alloy. In the
case of IC-396 alloy, directional solidification affected positively only pore circularity. Pores in IC-221M alloy
after directional solidification had worse morphology than the ones in its casting.
Fig. 9 Pore morphology in nickel alloys after casting and directional solidification expressed by medians
15. - 17. 5. 2013, Brno, Czech Republic, EU
4.
CONCLUSION
Cast samples of the IC-396, IC-221M and IC438 nickel alloys were strained by tension at standard
conditions. Their fracture happened before reaching of 0.2 % deformation. The cause of premature facture
was coarse-grained dendritic structure containing many shrinks. Following heat treatment did not lead to
structure refinement and to shrink reduction. This problem was successfully resolved by directional
solidification which had an effect on amount and morphology of pores. In future, HIP method can be
promised in the challenge of shrink reduction.
ACKNOWLEDGEMENT
The presented results were obtained within the frame of solution of the research project
TA 01011128 “Research and development of centrifugal casting technology of the
Ni-based intermetallic compounds” and the project CZ.1.05/2.1.00/01.0040
“Regional materials science and technology centre”.
LITERATURE
[1]
DEVI, S. C., SIKKA, V. K. Nickel and iron aluminides: an overview on properties, processing and applications.
Intermetallics, 1995, Volume 4, Issue 5, Pages 357-375.
[2]
MASAHASHI, N. Physical and mechanical properties in Ni3Al with and without boron. Materials
Science and Engineering: A, 1997, Volume 223, Issues 1-2, Pages 42-53.
[3]
CHAKI, T. K. Boron in polycrystalline Ni3Al – mechanism of enhancement of ductility and reduction of
environmental embrittlement. Materials Science and Engineering: A, 1995, Volume 190, Issues 12, Pages 109-116.
[4]
POHLUDKA, M. et al. Porosity of Ni3Al-based alloys prepared by gravity and centrifugal casting. In
st
rd
th
Metal 2012: 21 International Conference on Metallurgy and Materials: May 23 – 25 2012. Brno,
Hotel Voroněž I, Czech Republic [CD-ROM]. Ostrava: Tanger, May, 2012, pp. 1547-1553. ISBN 97880-87294-31-4.
[5]
MALCHARCZIKOVÁ, J. et al. Structural and fracture characteristics of nickel-based alloys. In Metal
nd
th
th
2013: 22 International Conference on Metallurgy and Materials: May 15 - 17 2013. Brno, Hotel
Voroněž I, Czech Republic [CD-ROM]. In press.