properties of comercial pure titanium after equal channel angular

METAL 2009
19. – 21. 5. 2009, Hradec nad Moravicí
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PROPERTIES OF COMERCIAL PURE TITANIUM AFTER EQUAL
CHANNEL ANGULAR PRESSING
VLASTNOSTI TECHNICKY ČISTÉHO TITANU PO PROTLAČOVÁNÍ
UHLOVÝMI KANÁLY
Miroslav Gregera, Radim Kocicha, Martin Černýa
Ladislav Kanderb
a
VŠB – TU Ostrava, 17. listopadu 15, 708 00 Ostrava - Poruba, ČR,
E-mail : [email protected]; radim [email protected]; [email protected]
b
Material & Metallurgical Research Ltd, Pohraniční 20, 706 02 Ostrava - Vítkovice,
ČR, E-mail: [email protected]
Abstrakt
Pro náhrady jsou vyvíjeny nové materiály, které lépe vyhovují současným
požadavkům, zejména z hlediska medicíny. Čistý titan je preferovaným materiálem
pro uvedené aplikace. Je bioinertní, neobsahuje toxické ani alergenní přísady.
Vývojový trend u tohoto materiálu je zaměřen na zachování nízké hodnoty modulu
pružnosti, zvýšení mechanických vlastností, především pevnosti. Podle HallPetchova vztahu lze zjemněním zrna výrazně zvýšit pevnostní vlastnosti kovů.
Perspektivním materiálem je nano-strukturní titan. V překládaném článku je popsána
výroba jemnozrnného titanu, je uvedena jeho struktura a vlastnosti. Dosažené
mechanické vlastnosti jsou porovnávány s vlastnostmi ostatních materiálů
používaných pro dentální implantáty. Jemnozrnný titan má vyšší měrné pevnostní
vlastnosti než běžný (hrubozrnný) titan. Titan byl zpracován protlačováním úhlovými
kanály (ECAP procesem). Vlastní výzkum byl zaměřen na fyzikální podstatu
zpevňovacích a odpevňovacích procesů a dějů probíhajících na hranicích zrn při
ECAP procesu za polotepla. Pevnost jemnozrnného titanu se pohybuje kolem 950
MPa, velikost zrn kolem 300 nm.
Abstract
New materials, which better satisfy the current requirements, namely from medical
viewpoints, are being developed for implants. Pure titanium is the preferred material
for bio applications. It is bio-inert, it does not contain any toxic or allergenic
admixtures. Development trend in case of this material is oriented on preservation of
low value of the modulus of elasticity, increase of mechanical properties, particularly
strength. According to the Hall-Petch relation it is possible to increase substantially
the strength properties of metals by grain refinement. Ultra-fine grain titanium is a
perspective material. This paper describes manufacture of ultra-fine grain titanium, its
structure and properties. Obtained mechanical properties are compared with the
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properties of other materials used for implants. Ultra-fine grain titanium has higher
specific strength properties than ordinary (coarse-grained) titanium. Ultra-fine grain
titanium was produced by the Equal Channel Angular Pressing (ECAP process). The
research itself was focused on physical base of strengthening and softening
processes and developments occurring at the grain boundaries during the ECAP
process at half-hot temperature. Strength of Ultra-fine grain titanium varies around
950 MPa, grain size around 300 nm.
1. INTRODUCTION
It is required that material for implants is bio compatible, it must not be toxic
and it may not cause allergic reactions. It must have high ultimate strength Rm and
yield value Rp 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 bio-material 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 viewpoint among materials
used fort dental implants.
In the past years 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 [1, 2]. Typical representative of these
alloys is duplex alloy (α a β) Ti6Al4V [3, 4]. After application of implants made of
these alloys toxicity of vanadium was confirmed. Aluminium, too, can be categorised
among potentially toxic elements.
During the following development of dental implants the efforts were concentrated
on replacement of titanium alloys the toxic and potentially toxic elements by non-toxic
elements. That’s why new alloys of the type TiTa, TiMo, TiNb and TiZr began to be
used [5]. Single phase β Ti alloys were developed at the same time, which are
characterised by the low value of the modulus of elasticity [6]. Ti alloys with elements
with very different density and melting temperature (TiTa, TiMo) require special
9technology of manufacture, by which they significantly increase production costs
and price of semi-products for dental implants.
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 is 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. Sensitivity of population to Ni is increasing.
For these reasons pure titanium still remains to be a preferred material for bio
applications. Development trend in case of this material is oriented on preservation of
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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 implants rather fine-grained Ti instead of coarse-grained Ti.
Use of ultra-fine grains materials concerns numerous fields including medicine. Bulk
ultra fine grains metallic materials are used for bio applications. These are materials
with the grain size smaller than approx. 100 to 300nm [7,8]. High-purity titanium is
used for implants. Chemical composition of CP Ti for implants must be within the
following interval.
2. STRUCTURE AND PROPERTIES OF COMMERCIALLY PURE TITANIUM
Commercially pure titanium (CP) bars and sheets were used in this study. The
average grain size of the as-received CP titanium is ASTM no. 4. Tensile specimens
with a gauge of 50 mm length, 10 mm width and 3.5mm thickness were machined
with the tensile axis oriented parallel to the final rolling direction [9, 10]. The
specimens were deformed at room temperature with different initial strain rates. After
testing, the deformed specimens in order to preserve the microstructure Fig. 1, 2.
Fig. 1. Initial microstructure of titanium
Obr. 1. Mikrostruktura výchozích vzorků
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Obr. 2. Mikrostruktura titanu po deformaci za studena (ε = 50 %)
Fig. 2. Microstructure of titanium after cold forming (ε = 50 %)
Specimens were sectioned along the gauge and grip parts of the deformed sample.
The samples were then polished etched using 10% HF, 10%HNO3 and 80% H2O for
20 second. Chemical analysis and mechanical properties CP titanium Table 1 - 3.
Tabulka 1. Chemické složení technicky čistého titanu
N
O
C
Fe
Al
Cr
Ti
0.004
0.068
0.008
0.03
0.01
0.01
Rest.
Table 1. Chemical analysis commercially pure (CP) titanium, (weight %)
Tabulka 2. Mechanické vlastnosti technicky čistého titanu po žíhání 649 oC/1 h
Rm
Rp0,2
Tažnost
Zúžení
[MPa]
[MPa]
[%]
[%]
365
212
51
71
Table 2. Tensile results (CP) titanium after condition annealed 649 oC/1 hour
(ASTM E8)
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Tabulka 3. Počáteční tvrdost ( HV) titanu a tvrdost po deformaci za studena
(ε = 50 %)
Label
Sample nr. 1
Sample nr. 2
Sample nr. 3.
Diagonal of indention d
[ µm]
659
632
652
658
655
658
527
525
535
HV30
128
139
131
128
130
128
200
202
194
Table 3. Initial hardness of titanium and hardness after cold forming (ε = 50 %)
3. PROPERTIES OF ULTRA-FINE GRAINS TITANIUM
Ultra-fine grains titanium (UFG) is characterised by exceptional mechanical
properties, among which high strength and high yield value are of utmost importance
[11]. Strength properties of UFG titanium must have the following values: Rm ∼ 1000
MPa, Rp0.2 ∼ 850 MPa [12]. Apart from the strength another important properties of
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. In case of classical coarse-grained titanium the relation
(Rm/ρ) varies around 70 to 120 (Nm/g), for the alloy Ti6Al4V [13] it varies around 200
(Nm/g), and for (n)Ti it is possible to predict the values Rm/ρ = 270 (Νµ/ρ). As a
matter of interest it is possible to give the specific strength also for some other bio
materials: steel AISI 316L - Rm/ρ = 65 (Νµ/ρ) [14], cobalt alloys - Rm/ρ = 160 (Νµ/ρ),
βTi (Ti15Mo5Zr) - Rm/ρ = 180 (Νµ/ρ). Disadvantage of implants based on steel or
cobalt alloys is their high tensile modulus of elasticity: E = 200 to 240GPa, while in
case of titanium and its alloys this value varies around 80 to 120 GPa.
At present only few companies in the world manufacture commercially bulk UFG
materials.
The main objective of experiments was manufacture of ultra-fine grain Ti,
description and optimisation of its properties from the viewpoint of their biocompability, resistance to corrosion, strength an d other mechanical properties from
the viewpoint of its application in implants. Chemical purity of semi products for Ti
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
ECAP technology [15]. The output was ultra-fine grain titanium with strength around
1050 MPa). The obtained Ti was further processed by technology (of rotation forging)
to the shape suitable for implants.
4. OBTAINED RESULTS AND THEIR ANALYSIS
Semi products from individual heats were processed according to modified programs by the
ECAP technology and rotation re-forging. Wire diameter varied around 10 - 11 mm.
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ECAP technology and cold forming was made in several variants :
a)
b)
c)
d)
2 passes ECAP at the temperatures of 450oC.
10 passes ECAP at the temperatures of 280oC; with process annealing
between individual passes.
Rotation re-forging to the diameter of 10 mm (cold forming : e = 2.2).
The samples for mechanical tests and for micro-structural analyses were
prepared from individual variants of processing. On the basis of the results,
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 Ti after application
of the ECAP process is shown in the Fig. 3 and Fig. 4. The structure was analysed
apart from light microscopy also by the X-ray diffraction. Table 4 summarises the
obtained basic mechanical properties.
Tabulka 4. Mechanické vlastnosti titanu po ECAP a po rotačním kování
Forming
processed
ECAP
(2 passes)
ECAP
(10 passes)
Rotation
re-forging
(Dd = 10 mm)
Rp0,2
516,5
873,2
927
to
945
Rm
[MPa]
580,1
A
[%]
16,7
E
[GPa]
98,6
960,5
12,3
100
1030
to
1050
9,1
100
dz
[nm]
800
100
to
300
100
to
300
Table 4. Mechanical properties titanium after ECAP and rotation re-forging
Obr. 3. Mikrostruktura titanu po dvou průchodech ECAP
Fig. 3. Microstructure of Ti after ECAP (2 passes)
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Obr. 4. Mikrostruktura titanu po 10 průchodech ECAP
Fig. 4. Microstructure of titanium after ECAP (10 passes)
5. CONCLUSION
Technology of manufacture of ultra fine grain titanium was proposed and
experimentally verified. Grain refinement in input materials was obtained by the
ECAP process. In conformity with the Hall-Petch relation the strength properties of Ti
increased significantly as a result of grain refinement. The obtained mechanical
properties correspond with the declared requirements. Ultra fine grain titanium has
higher specific strength properties than ordinary titanium. Strength of ultra fine grain
titanium varies around 1050 MPa, grain size around 300 nm.
ACKNOWLEDGEMENT
Authors of the paper would like top express their gratitude for financial support
of the GAČR 106/091598
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