M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
97
MICROSTRUCTURE EVOLUTION OF PURE
TITANIUM DURING ECAP
Miroslav Greger1,*, Ladislav Kander2, Václav Snášel3, Martin Černý1
1
VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava Poruba, Czech Republic
Material & Metallurgical Research Ltd., Pohraniční 693/31, 706 02 Ostrava Vítkovice, Czech Republic
3
SE-MI Engineering s.r.o., Matuškova 1929/10, 710 00 Ostrava, Czech Republic
*
corresponding author: Tel.: +420 596994434, e-mail: [email protected]
2
Resume
The main goal of the presented paper was to describe deformation
behaviour of the commercial purity titanium during the ECAP method.
Attention was paid particularly on reached mechanical properties of
above mentioned material. Design of experiments rested in extrusion at
temperature in range from room temperature up to 280°C. The way of
approach was planned in investigation of imposed strain accumulation
ability. Among used methods for determination of intended aims were
tensile tests, TEM, SEM. Mechanical properties were evaluated using
standard tensile tests as well as small punch tests. Correlation between
mechanical properties from tensile test and punch test was developed.
Depending on imposed strain (e = 2 up to 8) was found that mechanical
properties (namely tensile strength) have increased up to 960 MPa.
Developed ECAP process enables controlling morphology of
microstructural constituents and workability of titanium Grade 1.
Available online: http://fstroj.uniza.sk/PDF/2011/17-2011.pdf
1. Introduction
It is required that a material for dental
implants is bio compatible, it must not be toxic
and it may not cause allergic reactions. It must
have high ultimate strength UTS and yield value
YS at low density ρ and low modulus of
elasticity E. Metallic materials used for dental
implants comprise alloys of stainless steels,
cobalt alloys, titanium (coarse-grained) and
titanium alloys. Semi-products in the form of
coarse-grained Ti or Ti alloys are used as biomaterial for medical and dental implants since
the second half of the sixties of the last century.
Titanium is at present preferred to stainless
steels and cobalt alloys namely thanks to its
excellent bio-compatibility. Together with high
bio-compatibility of Ti its resistance to
corrosion evaluated by polarisation resistance
varies around the value 103 R/Ωm. It therefore
occupies a dominant position from this
Article info
Article history:
Received 12 July 2011
Accepted 17 August 2011
Online 2 September 2011
Keywords:
ECAP
Ultra-Fine Grain Titanium
Structure
Mechanical Properties
ISSN 1335-0803
viewpoint among materials used for dental
implants. In the past years a higher attention was
paid also to titanium alloys due to requirements
to higher strength properties. The reason was the
fact that titanium alloys had higher strength
properties in comparison with pure titanium.
Typical representative of these alloys is duplex
alloy (α and β) Ti6Al4V. After application of
dental implants made of these alloys toxicity of
vanadium was confirmed. During the following
development of dental implants the efforts were
concentrated on replacement of titanium alloys
the toxic and potentially toxic elements by nontoxic elements. That’s why new alloys of the
type TiTa, TiMo, TiNb and TiZr began to be
used. Single phase β Ti alloys were developed at
the same time, which are characterised by the
low value of the modulus of elasticity [1-3]. Ti
alloys with elements with very different density
and melting temperature (TiTa, TiMo) require
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M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
special technology of manufacture, by which
they significantly increase production costs and
price of semi-products for dental implants [4,5].
The problem at the development of
metallic bio-materials consists not only in their
real or potential toxicity, but also in their
allergenic potential.
Sensitivity of population to allergies
keeps increasing. Allergies to metals are caused
by metallic ions which are released from metals
by body fluids. Share of individual metals on
initiation of allergies is different. What concerns
the alloying elements for dental implants special
attention is paid namely to Ni and Co, as their
allergenic effect varies around (13.5%) and Cr
(9.5%). Some titanium alloys also contain the
elements classified as allergens. These are e.g.
the following alloys: Ti13Cu4.5Ni; Ti20Pd5Cr;
Ti20Cr0.2Si [6,7]. Sensitivity of population to
Ni is increasing.
For these reasons commercially pure
titanium (CP) still remains to be a preferred
material for dental applications. Development
trend in case of this material is oriented on
preservation of low value of the modulus of
elasticity and on increase of mechanical
properties, especially strength. According to the
Hall-Petch relation it is possible to increase
considerably strength properties of metals by
grain refinement. That’s why it is appropriate to
use for dental implants rather fine-grained Ti
instead of coarse-grained Ti.
references taking into consideration the remarks
included in the section “References”. It is
necessary to present the aim of the research
included in the paper and clearly point out the
originality of solutions and content-related
approach to the issue worked out and described
by authors.
2. Structure and properties of commercial pure
titanium
The average grain size of the as-received CP
titanium is ASTM E 1181 no. 4. Tensile
specimens with a gauge of 50 mm length, 10
mm width and 3.5 mm thickness were machined
with the tensile axis oriented parallel to the final
rolling direction. The specimens were deformed
at room temperature with different initial strain
rates. Microstructure of deformed specimens
after testing is shown in Fig. 1-4.
Fig. 1. Initial microstructure of CP titanium
(longitudinal direction)
Use of ultra-fine grain materials concerns
numerous fields including medicine. Bulk ultrafine grain materials are used for dental
applications. These are materials with the grain
size smaller than approx. 100 to 300 nm [8].
High-purity titanium is used for dental implants.
Chemical composition of CP Ti for dental
implants must be within the following interval.
The paper should begin with the
introduction in which the present state of the
issue relevant to the paper will be presented
generally and concisely. It is necessary to quote
Fig. 2. Initial microstructure of CP titanium
(transverse direction)
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M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
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2.1. Properties of ultra-fine grain titanium and
dental implants
Fig. 3. Microstructure of CP titanium after cold
rolling (longitudinal direction)
Fig. 4. Microstructure of CP titanium after cold
rolling (transverse direction)
Specimens were sectioned along the gauge and
grip parts of the deformed sample. The samples
were then polished and etched using 10 % HF,
10 % HNO3 and 80 % H2O for 20 second. Same
procedure has been used for preparing of all
metallography specimens in these work
(included ECAP process).
Table 1
Chemical analysis of titanium Grade 1 (in wt. %)
N
O
C
Fe
Al
Cr
Ti
0.004
0.068
0.008
0.03
0.01
0.01
balance
Table 2
Tensile properties of titanium Grade1 after
annealing 650 °C /1 hr. (ASTM E8)
UTS
YS
(MPa)
375
(MPa)
230
Elong.
(%)
51
Red. in
area
(%)
71
Chemical analysis and mechanical properties of
commercially pure (CP) titanium are given in
the Table 1 and Table 2, respectively.
Ultra-fine grain titanium is characterised by
exceptional mechanical properties, among
which high ultimate strength and high yield
value are of utmost importance. Strength
properties of ultra-fine grain titanium must
have the following values: UTS > 1000 MPa,
YS > 850 MPa [9]. Apart from the tensile
strength, another important property of dental
implants is their so called specific strength
(strength related to density). Mechanical
properties of metallic material for implants are
evaluated in relation to its density as so called
specific properties (see Table 3). In case of
classical coarse-grained titanium the relation
(UTS/ρ) varies around 70 to 120 (Nm/g), for
the alloy Ti6Al4V it varies around 200
(Nm/g), and for titanium it is possible to
predict the values UTS/ρ = 270 (Nm/g). As a
matter of interest it is possible to give the
specific strength also for some other dental
materials [10]:
Table 3
Specific strength for some dental materials
Material
UTS/ρ (Nm/g)
AISI 316L steel
65
Cobalt alloys
160
βTi (Ti15Mo5Zr)
180
Disadvantage of dental implants based on
steel or cobalt alloys is their high tensile
modulus of elasticity: E = 200 to 240 GPa, while
in case of titanium and its alloys this value
varies between 80 and 120 GPa [11]. At present
only few companies in the world manufacture
commercially bulk ultra-fine grain materials.
2.2 The technology for manufacture of ultrafine grain titanium for dental implants
The main objective of experiments was
manufacture of ultra-fine grain Titanium,
description and optimisation of its properties
from the viewpoint of their bio-compability,
resistance to corrosion, strength and other
mechanical properties from the viewpoint of its
application in dental implants.
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M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
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Chemical purity of semi products for
titanium was ensured by technology of melting
in vacuum and by zonal remelting. The obtained
semi-product was under defined parameters of
forming processed by the Equal Channel
Angular Pressing (ECAP) technology.
The output was ultra-fine grain titanium
with strength about 1050 MPa. The obtained
ultra-fine grain titanium will be further
processed by technology (of rotation forging)
and drawing to the shape suitable for dental
implants.
On the basis of the results of standard tensile
tests (Fig. 5), particularly the obtained strength
values, several variants were chosen for more
detailed investigation of developments occurring in
the structure at application of the ECAP and
subsequent drawing after heat treatment. Structure
of ultra-fine grain titanium after application of the
ECAP process is shown in the Figs. 6 - 10. The
structure was analysed apart from light microscopy
also by the X-ray diffraction.
3. Results
Samples from individual heats were
processed by the ECAP. The samples for
mechanical tests and for micro-structural
analyses were prepared from individual variants
of processing.
Fig. 6. Microstructure of ultra-fine grain CP titanium
after 2 ECAP passes + 630 °C /0.2 hr.,
a) initial sample
Fig. 7. Microstructure of ultra-fine grain CP
titanium after 4 ECAP passes,
b) after 8 ECAP passes
Fig. 5 Stress – strain curve of CP titanium
Fig. 8. Microstructure of ultra-fine grain CP
titanium after 5 ECAP passes
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is clearly seen, that origin correlation for steel
can not be used for evaluation of mechanical
properties of titanium by punch test, new
developed correlation changed significantly
probably due to friction coefficient effect or
lattice effect. Work is still continuing to evaluate
reason of this effect as well as to get a more
results for titanium and Ti alloys.
Fig. 9. Microstructure of ultra-fine grain CP
titanium after 5 ECAP passes + 630 °C /0.2 hour
and drawing,
Fig. 11. Effect of number of passes on mechanical
properties of CP titanium
Fig.10. Microstructure of ultra-fine grain CP
titanium after 12 ECAP passes
Results were correlated with tensile test
results and are presented in Fig. 12 for yield
stress and in Fig. 13 for tensile strength of
titanium. Correlation are presented in the form
of dependence of SPT test characteristics (load
corresponding to yield stres Pe, maximum load
Pm respectively devided by thickness (t) or
deflection correspond to maximum load (dm) on
YS or UTS. This calculation is based on [12]. In
the Figures 12 and 13 is also presented
correlations for steels (dashed line), that have
been developed for 12 years. From both figures
Ti
Fig. 12. Correlation for titanium Yield Stress and
steel
800
750
UTS
700
steel
650
UTS [MPa]
Fig. 11 summarises effect of number of
passes on mechanical properties. From samples
after ECAP were also manufactured specimens
for small punch tests (8 mm in diameter, 0.5 mm
in thickness) to evaluate new correlation
between standard tensile tests and small punch
tests for titanium.
steel
600
550
500
Ti
450
400
350
300
1300
1400
1500
1600
Pm/t/dm [MPa]
Fig. 13. Correlation for titanium Tensile Strength
and steel
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M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
3.1 SEM a TEM analyse of fracture surfaces
For detailed investigation of the samples
after tensile test SEM JEOL JSM 6490L was
used. Details of fracture areas at selected grains
size are shown in Fig. 14 up to 16. The
evolution of damage and final fracture in ultrafine grains titanium is only beginning to be
understood. The absence of substantial
macroscopic tensile ductility in ultra-fine grains
titanium together with the observation of
dimpled rupture on fracture surfaces leads to the
hypothesis that deformation is localised).
Fracture surfaces resulting from tensile
tests have frequently shown dimpled rupture in
microcrystalline titanium. Further, it has been
shown that the dimple size is significantly larger
than the average grain size; in addition (Fig. 14),
a pair of mating fracture surfaces was shown
that clearly illustrated the presence of significant
stretching of the ligaments between the dimples
that was taken to be indicative of appreciable
local plasticity. An example of a fracture surface
obtained from a tensile specimen of ultra-fine
grained titanium with a grain size of around 250
– 300 nm is shown Fig. 10. It reveals dimpled
rupture with the dimple depth (3-4 µm) being an
order of magnitude larger than the grain size
(Fig. 15).
Fig. 14. Fracture area of the CP titanium after
2 ECAP passes, SEM
Fig. 15. Fracture area of the sample after 4 ECAP
passes, SEM
Fig. 16. Fracture area of the CP titanium after 8
ECAP passes, SEM
Fig. 17. Substructure of CP titanium in the initial
state, TEM
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M. Greger, L. Kander, V. Snášel, M. Černý: Microstructure evolution of pure titanium during ECAP
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Furthermore, the dimple size is uniform
and extends across most of the specimen crosssection (Fig. 16).When the grain size is reduced
to 0.1 µm or less an in the case of titanium after 8
passes ECAP (Fig. 16) [13]. The resulting
fracture surface from a tensile specimen still
continues to show what appears to be dimpled
rupture with the important difference that the
dimple diameter on an average is finer in size
relative to those seen in Fig. 17 and Figs. 18 - 21.
Fig. 21. Substructure of the CP titanium after 8
ECAP passes, TEM
4. Conclusions
Technology of manufacture of ultra-fine
grain titanium was proposed and experimentally
verified. Grain refinement in input materials was
obtained using the equal channel angular pressing
process.
Fig. 18. Substructure of the CP titanium after 3
ECAP passes, TEM
Fig. 19. Substructure of the CP titanium after 4
ECAP passes, TEM
In conformity with the Hall-Petch, relation
the strength properties of commercially pure
titanium increased significantly as a result of
grain refinement. The obtained mechanical
properties correspond with the declared
requirements. Ultra-fine grain has higher specific
strength properties than ordinary titanium. To
evaluate mechanical properties of Ti standards
tensile tests were carried out as well as very
potential method (small punch test) were used.
New correlation has been found for
titanium that differs significantly from correlation
using for steels. Further work is necessary to get
more data for new developed correlation on
titanium and Ti alloys.
Based on the experimental results can be
seen that small punch tests can be using to
evaluate for titanium especially in the case, when
small amount of experimental materials is
expected.
Acknowledgements
Fig. 20. Substructure of the CP titanium after 6
ECAP passes, TEM
The authors are grateful to the Grand
Agency of the Czech Republic, project No.
106/09/1598 and the Czech Ministry of
Education, Project No. VS MSM 619 891 0013
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