A study on microstructure and porosity of NiTi alloy implants produced by SHS

Article abstract of DOI:10.1016/j.jallcom.2009.08.023

In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the different pressures (50, 75 and 100 MPa). Then, the porous NiTi alloy compacts obtained were synthesized by SHS (self-propagating high-temperature synthesis)

Full text of DOI:10.1016/j.jallcom.2009.08.023

Journal of Alloys and Compounds 487 (2009) 605–611
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
A study on microstructure and porosity of NiTi alloy implants produced by SHS
a,∗
b
c
d
Gul Tosun , Latif Ozler , Mehmet Kaya , Nuri Orhan
a
Firat University, Technical Vocational School, 23119 Elazig, Turkey
Firat University, Engineering Faculty, Mechanical Engineering Department, 23119 Elazig, Turkey
Adiyaman University, Vocational School, 02040 Adiyaman, Turkey
b
c
d
Firat University, Technical Education Faculty, Metal Education Department, 23119 Elazig, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, Ni and Ti with 50.5 at.% Ni powders were blended for 12 h and cold pressed in the different
pressures (50, 75 and 100 MPa). Then, the porous NiTi alloy compacts obtained were synthesized by SHS
Received 14 January 2009
Received in revised form 4 August 2009
Accepted 8 August 2009
(
3
self-propagating high-temperature synthesis) at the different preheating temperatures (200, 250 and
00 C) and heating rates (30, 60 and 90 C/min). The effects of the pressure, preheating temperature
◦
◦
Available online 15 August 2009
and heating rate were investigated on the porosity and the microstructure. NiTi was seen as the dom-
inant phase in the microstructure with other secondary intermetallic compounds. The porosity of the
synthesized products was in the range of 50.7–59.7 vol.%.
Keywords:
NiTi
SHS
©
2009 Elsevier B.V. All rights reserved.
Microstructure
Porosity
1
. Introduction
Porous NiTi SMAs have been fabricated with powder metallurgy
PM) processes such as self-propagating high-temperature syn-
(
Biomaterial is a nonviable material used in medical devices and
thesis (SHS), metal injection molding (MIP), hot isostatic pressing
(HIP) and spark plasma sintering (SPS) [3,4,6–9]. These processes
can avoid the problems associated with casting, like segregation
or extensive grain growth and have the added advantages of pre-
cise control of composition and easy realization of complex part
shapes [8]. As a result, compared with traditional methods, self-
propagating high-temperature synthesis (SHS) has the advantages
of time and energy savings, and is being studied extensively for the
fabrication of ceramic and intermetallic compounds [10].
In this study, porous NiTi alloy implants with 50.5 at.% Ni was
produced by SHS under the different process parameters such as
preheating temperature, heating rate and pressure. The effect of
these process parameters on the microstructure and porosity of
NiTi alloy implants were experimentally investigated.
artificial organs to interact with biological systems. With develop-
ments in medicine area, biomaterials are widely used to standby
organ instead of certain organs of human body not to function
with reasons such as various illness, accidents and old age. The
biomaterials industry worldwide has an annual turnover of $2.3
billion in the field of hard-tissue repair and replacement. There
has been and will continue to be a growing need for such bio-
materials. However, common biomaterials can result in various
problems usually related to the mismatch between the implant and
the replaced bone. When an implant due to its physicochemical and
mechanical properties is rejected, it is not in good and permanent
contact with body tissues [1]. Porous NiTi shape memory alloys
(
SMAs) are materials widely used in numerous biomedical applica-
tions (orthodontics, cardiovascular, orthopedics, urology, etc.) due
to their good biocompatibility, unique shape memory properties,
mechanical properties, superior damping capability, excellent cor-
rosion resistance and wear resistance [2–5]. Moreover, the porous
NiTi alloy shows promising potential in the application of bone
implantation because the porous structure allows the ingrowths
of new bone tissue along with the transport of body fluids, thus
ensuring a harmonious bond between the implant and the body
2
.
Experimental procedure
Titanium and nickel powders were used to produce porous NiTi alloy implants.
The characteristic features of powders used are shown in Table 1. The mixed pow-
ders of Ni and Ti with 50.5 at.% Ni were blended for 12 h and then cold pressed in
a cylindrical die with 10 mm diameter under different compaction pressures (50,
75 and 100 MPa) using a hydraulic press. The cold compacted samples with green
◦
[
3].
porosity were heated under different heating rates (30, 60 and 90 C/min) with the
protection of high purity argon gas (99.9%) of about 0.1 MPa in a furnace (Fig. 1).
The samples were ignited under different preheating temperatures (200, 250 and
◦
3
00 C) using electrical discharge pulse (14 kV and 30 mA). Once ignited, combus-
tion waves could self-propagate along the axis to the other end of the compact in a
very short time, and then porous NiTi implants were obtained by synthesizing.
∗ Corresponding author. Tel.: +90 424 2370000; fax: +90 424 2415526.
E-mail address: gultosun@firat.edu.tr (G. Tosun).
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.08.023

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G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
Fig. 1. Experimental setup.
Table 1
challenging task. Interestingly, this aspect too has received little
attention in the literature so far [7].
Characteristics of Ti and Ni powders [11].
Feature of material
Nickel
Titanium
Purity (%)
Specific gravity (g/mol)
Powder dimension (mesh)
Melting temperature ( C)
Specific weight (g/cm )
99.8
58.71
−325
1453
8.9
99.5
47.9
−325
1680
4.507
3260
3
.1. Effect of heating rate
◦
3
From SEM and optical micrographs and EDX analyses
(Figs. 2 and 3), it can be seen that the amount of NiTi phase
◦
Boiling temperature ( C)
2832
increases with increasing heating rate [3]. Generally, the main
phase microstructure in the samples heated at low heating rate
◦
(
30 C/min) is NiTi. But, these porous products contained NiTi2
phase apart from the major phase, NiTi. Other binary phases could
not be avoided except at higher temperature of combustion and at
the expense of the product shape and porosity. In the micrographs
of the specimens, NiTi matrix was seen grey, Ni Ti or Ni Ti phases
were seen as light grey and NiTi2 phase was seen as cornered shape
in dark grey.
The general porosity of NiTi products was calculated by the following formulae:
ꢀ
ꢁ
ꢀ
ε = 1 − ꢀ
× 100
(1)
0
3
2
4
3
where ꢀ and ꢀ0 are the density of the specimen and its corresponding theoretical
density, respectively. The density of the specimen was determined by measuring its
3
weight and dimension. The theoretical density of Ti–50.5 at.% Ni alloy is 6.21 g/cm .
In order to maintain the integrity of the porous skeletal struc-
ture during synthesis, the rate of generation of liquid should be
equal to the rate of consumption of the liquid. Therefore, the heat-
ing rate is to be proportionate with this requirement. This heating
rate is defined as the threshold heating rate for melting. It was
The NiTi products were etched by a mixture of 10% HF, 5% HNO3 in water after
polishing [12]. The microstructure and the pore characteristics of the NiTi specimens
were analyzed with optical microscope and the scanning electron microscope (SEM,
LEO Evo-40VP). The chemical composition of the phases was determined by energy
dispersive spectroscopy (EDS) coupled with the SEM.
◦
determined that threshold-heating rate was 60 C/min in this study
3
. Results and discussion
(Fig. 4).
The porous NiTi was obtained by keeping the heating rate
below the threshold rate. When the heating rates were kept low
(30 C/min), the absorption rate of liquid could match the liquid
Even though the SHS technique successfully produces NiTi with
◦
the prosthesis-quality porous structure, these products are never
single phase. The as-reacted porous NiTi alloy always contains other
phases like NiTi , Ni Ti, Ni Ti or in some cases even elemental
generation rate. Thus, the low heating rate could achieve the chem-
ical homogenization and retention of the porous structure at the
same time [7].
2
3
4
3
Ni in varying proportions. Full austenitic NiTi material generally
has suitable properties for surgical implantation and superelas-
ticty. Therefore, other undesirable phases need to be eliminated.
In a recent work [13], porous NiTi with adjustable porosity and size
were successfully fabricated by SHS and single phase austenitic NiTi
was also obtained by solution treatment under load. But solution
treatment decreased porosity. The treatment should be done with-
out reducing porosity so that ingrowth of the bone tissue is not
affected. Normally, the product shows single phase microstructure
when the combustion temperature or the sintering temperature
is sufficiently high. But, that usually leads to melting of the prod-
uct and in turn loss of porosity. So, to achieve complete conversion
to the single phase NiTi and maintain its porosity too remains a
◦
In the samples fabricated by the heating rate of 60 C/min, NiTi
phase quantity increased whereas NiTi2 phase decreased. More-
over, Ni Ti2 phase also formed.
3
When the heating rate is low, the time needed for synthesis gets
longer. Faster rates ofheatingincreasedthe rateofliquidgeneration
beyond the absorption limit, as a result of which the pores started
collapsing [7].
◦
The samples at the higher heating rates (90 C/min) underwent
melting and resulted in the loss of both the shape and the porosity.
In this case, NiTi phase quantity much more increased in the sam-
ples at the higher heating rate and NiTi phase vanished. Instead of
2
NiTi2 phase, Ni Ti2 was seen (Fig. 3).
3

G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
607
Fig. 2. Microstructure photographs of NiTi alloys produced at 100 MPa pressure,
Fig. 3. SEM photographs and EDX analysis of NiTi alloys produced at 100 MPa
pressure, 250 ◦C preheating temperature and 90 ◦C/min heating rate.
◦
◦
2
6
50 C preheating temperature and three different heating rates: (a) 30 C/min; (b)
◦
◦
0
C/min; (c) 90 C/min.
teristics associated with the formation of NiTi intermetallics by
SHS depend on both the initial sample density and the preheating
temperature [3]. In the specimens with low green density (50 MPa
cold compaction pressure) the main phase was NiTi and secondary
phases such as NiTi and Ni Ti were also seen (Figs. 5 and 6). When
Minimum porosity was obtained at the heating rate of
0 C/min, and maximum porosity was determined at 60 C/min.
◦
◦
3
On the contrary, minimum density was obtained at the heating rate
◦
of 60 C/min (Fig. 4b). When heating rate was more increased up to
2
3
2
◦
9
0 C/min, it can be seen that porosity decreased again (Fig. 4a).
the cold compaction pressure or green density was increased, the
amount of NiTi phase also increased. The quantity of secondary
3.2. Effect of cold compaction pressure
phases decreased but small amount of Ni Ti2 phase was observed.
3
Fig. 7a shows variation of porosity with pressure. It can be seen
that porosity decreased with increasing compacting pressure and
therefore density also increased (Fig. 7b). The initial (green) density
of the compacted samples was increased with the increasing com-
SEM and EDX analyses were performed to determine the influ-
ence of green density on the microstructure (Figs. 5 and 6). Previous
experimental observations indicated that the combustion charac-

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G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
Fig. 4. (a) Porosity and (b) sintered density obtained at different heating rates.
pacted pressure (Fig. 7c). Porosity in the specimens compacted by
low pressure is high and its density low (Fig. 7a and b). Each pore
in the structure is a barrier for the continuation of flame propaga-
tion. In other words, the synthesis is interrupted by the pores. As
a result, the synthesis cannot be completed and secondary phases
are present in microstructure. It indicated that not only the product
composition and morphology, but also the combustion charac-
teristics were strongly affected by the initial sample density and
preheating temperature [3].
Fig. 5. Microstructure photographs of NiTi alloys produced at heating rate of
◦
◦
3.3. Effect of preheating temperature
60 C/min, preheating temperature of 250 C and different pressures: (a) 50 MPa;
b) 75 MPa; (c) 100 MPa.
(
On account of the weak exothermic heat and low combustion
temperature of the Ni + Ti reaction, the combustion can only be
initiated by preheating the compacts [1].
SEM and EDX analysis in Figs. 8 and 9 show the influence of the
preheating temperature on the microstructure. It can be seen from
the figures that the NiTi phase quantity increases with increasing
preheating temperature. NiTi as the main phase and some sec-
observed, it can be concluded that SHS process can be controlled by
adjusting the preheating temperature, which influences the phase
amount and the macro distribution of pores in the sintered prod-
ucts.
ondary phase, NiTi , were seen in the microstructure of NiTi alloy
Li et al. [1] noted that the combustion does not propagate at a
2
◦
◦
produced at 200 C preheating temperature. The microstructure of
preheating temperature of 150 C. Considering this, the preheating
◦
◦
NiTi alloy produced at 250 C preheating temperature consisted of
temperatures used in this study were chosen higher than 150 C.
NiTi main phase and secondary phases, Ni Ti and less NiTi . The
microstructure of NiTi alloy produced at 300 C preheating temper-
Fig. 10a and b shows the porosity and sintered density obtained at
different preheating temperatures, respectively. The sample poros-
ity increased when the preheating temperature was increased
3
2
2
◦
ature contained both NiTi main phase and NiTi , secondary phase.
2
◦ ◦
from 200 C to 250 C. However, when preheating temperature was
The mechanism of SHS of porous NiTi SMAs varies with the
SHS process parameters [1]. Based upon the experimental results
◦
◦
increased from 250 C to 300 C, porosity decreased [3].

G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
609
Fig. 7. (a) Porosity; (b) sintered density and (c) green density obtained at different
pressures.
Even if some secondary phases appeared beside NiTi, the major
◦
phase, threshold preheating temperature is determined as 250 C
from the porosity results
Fig. 6. SEM photographs and EDX analysis of produced NiTi alloys in 50 MPa pres-
sure, 250 C preheating temperature and 60 C/min heating rate.
◦
◦
4. Conclusions
This study presents a comprehensive description of the SHS
process associated with the formation of NiTi intermetallic com-

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G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
◦
Fig. 8. Microstructure photographs of produced NiTi alloys at 60 C/min heating
◦
rate, 100 MPa pressure and (a) 200; (b) 250; (c) 300 C preheating temperature,
respectively.
pounds from elemental powder compacts. It was found that not
only the product composition and morphology, but also the com-
bustion characteristics are strongly affected by the initial sample
density, preheating temperature, and heating rate.
◦
When the heating rates were kept very low (30 C/min), the
rate of absorption of liquid could match the liquid generation rate.
Thus, the low rate of heating could achieve the chemical homoge-
nization and retention of the porous structure at the same time.
Fig. 9. SEM photograph and EDX analyses of produced NiTi alloys at 75 MPa pres-
◦
◦ ◦
sure, 250 C preheating temperature and 60 C/min heating rate.
In microstructure of heating rates of 60 C/min, in microstruc-
ture of samples, NiTi phase quantity increased and NiTi2 phase
decreased. Ni Ti phase formed in case of threshold-heating rate
3
2
◦
was 60 C/min in this study. However, the higher rates of heating
90 C/min) products underwent melting and resulted in the loss of
◦
(
both the shape and the porosity. In microstructure of samples at of

G. Tosun et al. / Journal of Alloys and Compounds 487 (2009) 605–611
611
Green density affects the phases formed. Secondary phases such
as NiTi and Ni Ti were also seen beside the major phase NiTi when
2
3
2
the green density was low. The increase in green density causes NiTi
phase to increase, secondary phases to decrease and little amount
of Ni Ti2 phase to form
3
Preheating temperature has a considerable effect on poros-
ity in some temperature ranges. Porosity increases and density
◦
◦
decreases at the preheating temperatures between 200 C to 250 C
and porosity decreases and density increases at the preheating tem-
◦
◦
peratures between 250 C and 300 C.
◦
Threshold heating rate is 60 C/min for porosity.
NiTi SMA with highest porosity was obtained at a heating rate
◦
◦
of 60 C/min, a preheating temperature of 250 C.
It is inevitable formation of undesirable phases such as NiTi and
2
Ni Ti phases with the parameters used in this study.
3
2
Acknowledgement
The authors would like to acknowledge the Firat University Sci-
entific Research Projects Foundation (FUBAP-1137) for financial
support for this study.
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◦
9
0 C/min heating rates, NiTi phase quantity much more increased
and NiTi2 phase vanished but Ni Ti2 phase appeared
3