MECHANICAL BEHAVIOUR AND MICROSTRUCTURE EVOLUTION OF SEVERELY DEFORMED TITANIUM

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 properties of other materials used for implants. Ultra-fine grain titanium has higher specific strength properties than ordinary (coarsegrained) 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.


INTRODUCTION
It is required that material for dental implants is bio-compatible, it must not be toxic and it may not cause allergic reactions.It must have high ultimate tensile strength R m and yield strength R p at low density  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 [1].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 [2].Titanium is at present preferred to stainless steels and cobalt alloys namely thanks to its excellent biocompatibility [3].Together with high bio-compatibility of Ti its resistance to corrosion evaluated by polarisation resistance varies around the value 10 3 R/Ωm [4].
It therefore occupies a dominant position from this viewpoint among materials used for 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.Typical representative of these alloys is duplex alloy (α a β Ti6Al4V [5].After application of dental 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 containing 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.Single phase  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 technology of manufacture, by which they significantly increase production costs and price of semiproducts 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 [7][8].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.Sensitivity of population to Ni is increasing.
For these reasons pure titanium 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.Use of ultra fine-grained concerns has numerous fields including medicine.Bulk ultra fine-grained structural metallic materials are used for dental applications.These are materials with the grain size smaller than approx.100 to 300 nm.High-purity titanium is used for dental implants.

STRUCTURE AND PROPERTIES OF TITANIUM
Commercially pure titanium (CP) bars and sheets were used in this study.The average grain size of the asreceived CP titanium is ASTM no. 4. Tensile specimens with a gauge length of 50 mm, 10 mm width and   while in case of titanium and its alloys this value varies between 80 and 120 GPa.At present only few companies in the world manufacture commercially bulk nano-materials.

The technology for manufacture of ultra fine-grained Titanium
The main objective of experiments was manufacture of ultra fine-grained 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.Chemical purity of semi products for ultra fine-grained 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 ECAP technology.The output was ultra fine-grained titanium with strength of approx.1050 MPa.The obtained ultra fine-grained titanium was further processed by technology of rotation forging and drawing to the shape suitable for dental implants.

OBTAINED RESULTS AND THEIR ANALYSIS
Semi products from individual heats were processed according to modified programs by the ECAP The following technology of drawing was realised at increased temperatures.
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 microstructure at application of the ECAP and subsequent drawing after heat treatment.Structure of ultra fine-grained titanium after application of the ECAP process is shown in the Fig. 3.The microstructure was analysed apart from light microscopy also by the X-ray diffraction.Table 4 summarises the obtained basic mechanical properties.

CONCLUSION
Technology of manufacture of ultra-grained titanium was proposed and experimentally verified.Grain refinement in input materials was obtained using 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-grained titanium has higher specific strength properties than ordinary titanium.Strength of ultra fine-grained titanium around 1250 MPa, grain size around 300 nm.

Tab. 3 . 1
Počáteční tvrdost titanu a tvrdost po válcování za Properties of ultra fine-grained titaniumUltra fine-grained titanium is characterised by exceptional mechanical properties, among which high ultimate strength and high yield strength are of utmost importance.Strength of ultra-grained titanium must have the following values: R m > 1000 MPa, R p0.2 > 850 MPa[9].Apart from the 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.In case of classical coarse-grained titanium the relation (R m /ρ) varies around 70 to 120 (N•m/g), for the alloy Ti6Al4V it varies around 200 (N•m/g)[10 -12], and for ultra-grained titanium it is possible to predict the values R m /ρ = 270 (Ν•m/g.As a matter of interest it is possible to give the specific strength also for some other dental materials: steel AISI 316L -R m ρ = 65 (Ν•m/g)[13], cobalt alloys R m ρ = 160 (Ν•m/g Disadvantage of dental implants based on steel or cobalt alloys is their high modulus of elasticity: E = 200 to 240 GPa, technology and then drawn to a wire.Wire diameter varied about 4 -6 mm [14-16].ECAP technology and drawing was made in variants: a) 8 passes ECAP at temperatures of 280 o C; with annealing between individual passes, b) Rotation re-forging to a diameter of 10 mm (cold forming : e = 2.2), c)

Table 1 .
Chemical analysis of the used commercially pure titanium (CP), (weight %)

Table 3 .
Hardness of commercially pure Ti before and after cold rolling.

Table 4 .
Mechanical properties of fine-grained titanium after ECAP and drawing