MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM … · 4. CONCLUSIONS Technology of manufacture...

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18. - 20. 5. 2011, Brno, Czech Republic, EU 1 MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM GRADE 2 AFTER SEVERE PLASTIC DEFORMATION Miroslav Greger a , Václav Mašek b , Václav Snášel c , a VŠB – Technical University Ostrava, FMMI, 17. listopadu 15, 708 33 Ostrava – Poruba, ČR, [email protected] b ŠKODA TRANSPORTION a.s. Tylova 57, 301 28 Plzeň, ČR, trade[email protected] c SE-MI Engineering s.r.o., Matuškova 1929/10, 710 00 Ostrava, ČR, [email protected] Abstract Mechanical properties and microstructures of commercially pure titanium (CP-Ti) developed by a newly designed multi-pass equal channel angular pressing (ECAP) process and cold drawing were investigated. The ECAP apparatus was developed in order to improve the efficiency of common ECAP and the performance of finished products. A much stronger CP-Ti billet was produced by using the proposed processes. As shown in the results, a severe plastic deformation occurred in the material by means of continuous shear deformation during the radical bending process through a repeated route. The resulting mechanical properties, such as yield and tensile strength, were improved thanks to the grain refinement that is connected with the Hall-Petch theory. Moreover, cold drawing was conducted to produce a cylindrical type bulk material by using the ECAP. Microstructural refinement with improvement of mechanical properties was attributed to this additional plastic forming process. It was found out that the finished products developed by the multi-pass ECAP and cold drawing processes had therefore remarkably hardened, and achieved the ultimate tensile strength of CP-Ti 397.2 MPa, compared to initial strength of 716.3 MPa, with significant refinement of the microstructure. Keywords: titanium, mechanical properties, equal channel angular pressing 1. INTRODUCTION It is required that material for dental implants is bio compatible, it must not be toxic and it may not cause allergic reactions. 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 [1-3]. 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 the requirements to higher strength properties. For these reasons CP Ti 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 [4,5]. 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 materials concerns numerous fields including medicine. Bulk ultra-fine 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. Chemical composition of CP Ti for dental implants must be within the following interval (Table 1.)

Transcript of MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM … · 4. CONCLUSIONS Technology of manufacture...

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18. - 20. 5. 2011, Brno, Czech Republic, EU

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MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM GRADE 2 AFTER SEVERE PLASTIC DEFORMATION

Miroslav Greger a, Václav Mašek b, Václav Snášel c,

a VŠB – Technical University Ostrava, FMMI, 17. listopadu 15, 708 33 Ostrava – Poruba, ČR, [email protected]

b ŠKODA TRANSPORTION a.s. Tylova 57, 301 28 Plzeň, ČR, [email protected] c SE-MI Engineering s.r.o., Matuškova 1929/10, 710 00 Ostrava, ČR, [email protected]

Abstract

Mechanical properties and microstructures of commercially pure titanium (CP-Ti) developed by a newly designed multi-pass equal channel angular pressing (ECAP) process and cold drawing were investigated. The ECAP apparatus was developed in order to improve the efficiency of common ECAP and the performance of finished products. A much stronger CP-Ti billet was produced by using the proposed processes. As shown in the results, a severe plastic deformation occurred in the material by means of continuous shear deformation during the radical bending process through a repeated route. The resulting mechanical properties, such as yield and tensile strength, were improved thanks to the grain refinement that is connected with the Hall-Petch theory. Moreover, cold drawing was conducted to produce a cylindrical type bulk material by using the ECAP. Microstructural refinement with improvement of mechanical properties was attributed to this additional plastic forming process. It was found out that the finished products developed by the multi-pass ECAP and cold drawing processes had therefore remarkably hardened, and achieved the ultimate tensile strength of CP-Ti 397.2 MPa, compared to initial strength of 716.3 MPa, with significant refinement of the microstructure.

Keywords: titanium, mechanical properties, equal channel angular pressing

1. INTRODUCTION

It is required that material for dental implants is bio compatible, it must not be toxic and it may not cause allergic reactions. 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 [1-3].

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 the requirements to higher strength properties.

For these reasons CP Ti 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 [4,5]. 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 materials concerns numerous fields including medicine. Bulk ultra-fine 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. Chemical composition of CP Ti for dental implants must be within the following interval (Table 1.)

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2. STRUCTURE AND PROPERTIES OF PURE TITANIUM

Commercially pure titanium 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 3.5 mm thickness were machined with the tensile axis oriented in parallel to the final rolling direction. The specimens were deformed at room temperature with different initial strain rates. Microstructure of titanium is shown Figure 1 and Figure 2. 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 seconds [6,7]. Chemical analysis and mechanical properties CP titanium are given in the Table 1. Table 1. Chemical analysis commercially pure titanium (CP), weight %

CP Titanium Content of individual elements (wt.%)

Cmax Nmax Omax Femax Hmax Pd Ti

Grade 1 < 0.008 < 0.004 < 0.068 < 0.03 < 0.015 -

Rest Grade 2 0.06 0.04 0.18 0.22 0.012 - Grade 3 < 0.08 < 0.05 < 0.35 < 0.30 < 0.015 - Grade 4 < 0.08 < 0.05 < 0.40 < 0.50 < 0.015 - Grade 7 < 0.08 < 0.05 < 0.25 < 0.30 < 0.015 0.12 – 0.25

a) b)

Fig. 1. Initial microstructure of commercial pure titanium Grade 2: a) longitudinal direction, b) transverse direction

2.1. Properties of ultra-fine grained titanium

Ultra-fine grained titanium is characterised by exceptional mechanical properties, among which high ultimate strength and high yield value are of utmost importance, table 2. Table 2. Mechanical properties of CP titanium after annealing 649°C /1 hour (ASTM E8)

CP Titanium UTS

[MPa] YS

[MPa] El. [%]

Grade 1 280 - 410 170 - 310 24 - 30

Grade 2 397.2 275.4 22.8

Grade 3 460 - 590 380 - 550 18 - 20

Grade 4 540 - 740 485 - 655 15 - 16

Grade 7 390 - 540 275 - 400 20 - 22

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a) b)

Fig. 2. Microstructure of titanium after: a) ECAP (2 passes) and cold rolling b)

2.2. 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-compatibility, resistance to corrosion, strength and other mechanical properties from the viewpoint of its application in dental implants. Chemical purity of semi products for titanium was ensured by technology of melting in vacuum and by zonal remelting [8-10]. 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 [11].

3. OBTAINED REsULTS AND THEIR ANALYSIS

Semi products from individual heats were processed according to the modified programs by the ECAP

technology and then drawn to a wire (ε = 42 %). Wire diameter varied around 6 mm.

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 ultra-fine grained titanium after application of the ECAP process is shown in Figure 3. The structure was analysed apart from light microscopy also by the X-ray diffraction. Table 3 summarises the obtained basic mechanical properties. Tab. 3. Mechanical properties of ultra-fine grained titanium Grade 2 after ECAP and drawing

Forming processes UTS

[MPa] A

[%] E

[GPa] dz

[nm]

initially 397.2 26.5 98.0 520

ECAP (2 passes) 506.6 14.7 80.1 430

ECAP (4 passes) 612.7 15.3 99.7 -

ECAP (8 passes) 716.3 12.1 100.2 250 to 300

Drawing (ε = 42 %) 1023,2 7.3 100.2 -

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a) b)

Fig. 3. Microstructure of ultra-fine grained titanium after: a) ECAP 8 passes, b) cold rolling ε = 42 %

3.1. SEM analysis of fracture surfaces

SEM JEOL JSM 6490L was used for detailed investigation of the samples after tensile test. Details of fracture areas at selected grain size are shown in Figures 4 a) and b). We are only at the beginning of understanding the evolution of damage and final fracture in ultra-fine grained titanium. The absence of substantial macroscopic tensile ductility in ultra-fine grained 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 cold rolling (Fig. 4b), 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. Furthermore, the dimple size is uniform and extends across major part of the specimen

cross-section. When the grain size is reduced to 0.1 µm or less, and in the case of titanium after 8 passes ECAP, 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 Figure 4a.

a) b)

Fig. 4. Fracture area of the sample after: a) 8 passes ECAP, b) cold rolling

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4. CONCLUSIONS

Technology of manufacture of nano-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. Nano-titanium has higher specific strength properties than ordinary titanium. Strength of nano-titanium varies around 1023.2 MPa, grain size around 300 nm.

ACKNOWLEDGEMENT The authors are grateful to the Czech Grand Agency - Grant No.106/091598.

REFERENCES

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