DEVELOPMENT MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM PROCESSED BY EQUAL CHANNEL ANGULAR PRESSING AND COLD DRAWING

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1 DEVELOPMENT MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COMMERCIALLY PURE TITANIUM PROCESSED BY EQUAL CHANNEL ANGULAR PRESSING AND COLD DRAWING Miroslav GREGER a, Václav MAŠEK b a VSB - Technical University Ostrava, Ostrava, Czech Republic, EU, miroslav.greger@vsb.cz b TRADING UNIVERSAL s.r.o., Pilsen, Czech Republic, EU, TRADE25@seznam.cz Abstract Equal channel angular pressing has been used to refine the grain size of commercially pure titanium (grade 2) as well as other metals and alloys. Titanium grade 2 is usually processed at about 350 C because it lacks sufficient ductility at lower temperatures. The warm processing temperature limits the capability of the equal channel angular pressing technique in improving the strength of titanium grade 2. We have employed cold drawing following warm equal channel angular pressing to further refine the grains and improve the strength of titanium grade 2. Ti billets were first processed for eight passes via route B C, with a clockwise rotation of 90 between adjacent passes. They were further processed by successive cold drawing to an accumulative reduction in cross-section area by 50 %. This paper reports the surface quality, microstructures and mechanical properties of these titanium billets processed by a combination of equal channel angular pressing and drawing. Keywords: ultra-fine grain titanium, ECAP, drawing 1. INTRODUCTION Mechanical properties of commercially pure titanium (CP Ti) at low temperatures depend significantly on the contents of impurities. Oxygen and nitrogen have big influence. Each tenth of percent of oxygen increases the value of ultimate strength and yield strength approx. by 150 MPa. Yield strength of CP Ti varies in dependence on contents of impurities between 250 to 650 MPa, strength within the interval 350 to 700 MPa, see Table 1. Notch toughness is an important indicator of titanium quality. Tab. 1. Chemical composition of individual grades of CP titanium and titanium alloys, wt. % Contents of individual elements, wt.% CP Ti and alloys C max N max O max Fe max H max Pd max Nb max Zr max Mo max Al max Grade 1 < < < < 0.03 < Grade Grade 3 < 0.08 < 0.05 < 0.35 < 0.30 < Grade 4 < 0.08 < 0.05 < 0.40 < 0.50 < Grade 7 < 0.08 < 0.05 < 0.25 < 0.30 < Ti-13Nb-13Zr Ti-15Mo-5Zr-3Al Notch toughness is influenced mainly by hydrogen. At hydrogen contents around 0.001% notch toughness varies around 100 J [1, 2[. When hydrogen contents increases to 0.01% notch toughness drops down to 40 J [3]. Titanium gets strengthened by cold forming. Fig. 1 shows Titanium strengthening in dependence on magnitude of deformation. 1

2 Engineering stress (R m, R p 0,2 ) [MPa] Vickers hardness HV , Brno, Czech Republic, EU Rm R m = e e e R p 0.2 = E-05e e e e HV30 = e e e Rp0.2 HV Engineering strain [%] 150 Fig. 1. Strengthening of titanium Grade 2 in dependence on magnitude of cold deformation Development of new titanium alloys for implants has to strive for replacement of toxic elements in alloys by non-toxic elements. Special attention is focused namely on the alloys TiTa, TiMo, TiNb and TiZr. Simultaneously single-phase Ti alloys are being developed, which are characterised by low value of elastic modulus [4]. Development of metallic bio-materials is complicated not only by their real or possible toxicity. For these reasons pure titanium remains to be the most preferred material. Development trend in the case of CP Ti is oriented on preservation of low value of elastic modulus and on an increase of mechanical properties, particularly of strength [5]. Mechanical properties of CP titanium are given in Tab. 2 Strength properties of CP Ti can be enhanced by grain refining [6]. That s why it is appropriate to use for implants instead of titanium with usual grain size rather fine-grained titanium (nano-titanium). Tab. 2. Basic mechanical properties of CP titanium and titanium alloys CP Titanium and Ti alloys R m [MPa] R p 0.2 [MPa] Elong. [%] E [GPa] Grade Grade Grade Grade Grade Ti-13Nb-13Zr Ti-15Mo-5Zr-3Al PREPARATION OF ULTRA-FINE GRAINED TITANIUM BY SEVERE PLASTIC DEFORMATION Increase of the level of strength properties of polycrystalline metallic materials with preservation of sufficient toughness can be achieved by grain refining. Dependence between grain size and level of yield strength can be described by the Hall-Petch relation: y = +kd - 1 / / 2 (1) 2

3 where y is stress on the yield strength, o and k are constants, d is grain size. Search of possibilities of efficient refining of structure of technical materials lead to important modifications of technology of thermal-mechanical treatment, which make it possible to obtain grain size on the level of micrometers. Further grain refining requires application of extreme values of plastic deformation of material. Equal channel angular pressing (ECAP) is one of such methods. Its principle is based on extrusion of the samples through a tool, in which two channels intersect usually at the angle of 90. Equivalent strain at one pass through the tool varies around to 1.18, in dependence on the angle between the channels. Total strain may achieve the value 10 or even higher [7]. The ECAP method enables obtaining the grain size from several hundreds or dozens of nanometers. It is very difficult to obtain by the ECAP method materials with nanostructure, i.e. materials with grain size less than 100 nm. For understanding the mechanisms of development of material structure at application of methods of severe plastic deformation it is very important to characterise the share of sub-grains formed by recovery mechanisms and the share of grains with high angle boundaries, which are created by recrystallisation. Definition of the difference between sub-grains and grains is not rigid. Usually the values of are considered as limit angle of disorientation. The grains separated by high angle boundaries have generally much more important influence on the level of mechanical properties than sub-grains separated by low angle boundaries. In the region of grain size less than 300 nm the dislocation mechanism of plastic deformation is replaced by other mechanisms. It is possible to include among the most important causes of this effect the increasing surface if grain boundaries per unit of material volume, drop in density of dislocations inside grains with size less than 100 nm, and localisation of deformation into shear bands. 3. PROCESSING OF CP TITANIUM BY ECAP PROCESS Investigation was performed with use of commercial CP Ti Grade 2, its chemical composition is given in table 1, its mechanical properties are summarised in Table 2. The supplied material was after annealing 600 C/2h. Initial material was used for fabrication of cylindrical samples with diameter of 30 mm and length 125 mm. The angle between the channels in the ECAP tool was 105. The samples were prior to extrusion reheated to the temperature of 300 C, temperature in the channels varied around 290 C. Maximal number of passes through the ECAP tool was 8. Samples for tensile test, which was made at room temperature, were prepared from individual deformed samples. For the purposes of structural analysis scratch patterns parallel to the axis of samples were prepared after 4 passes (equivalent strain e = 3.5) and after 8 passes through the ECAP tool (e = 7.l) [8]. Influence of severe plastic deformation applied by the ECAP method on strength characteristics and on structure of pure titanium was investigated with use of transmission electron microscope (TEM) and scanning electron microscope (SEM). 4. RESULTS AND DISCUSSION Results of tensile test of the supplied CP Ti and that processed by the ECAP method were processed and their tabular and graphic presentation is shown in Fig. 2. Severe plastic deformation of the evaluated titanium has lead to distinct increase of strength properties. The biggest increase of strength properties was determined after the first two passes, next passes lead to gradual increase of strength parameters. Initial micro-structure of CP titanium was formed by equiaxed grains, see Fig. 3. 3

4 Stress, Rm [MPa] Grain size, d z [ mm ] , Brno, Czech Republic, EU Rm d z True strain Fig. 2. Strengthening and grain refining of CP titanium in dependence on applied equivalent strain Fig. 3. Initial structure of CP titanium Micro-structure of CP Ti after 4 ECAP passes is non-homogenous, initial grains are distinctly deformed [9]. Fig. 4 shows strongly deformed grains and directed bands. It was proven by the TEM analysis, that original equiaxed grains were replaced by elongated sub-grains/grains of variable size. Sub-grains/grains usually formed directed parallel bands, see Fig. 5 and Fig. 6. Important local difference in diffraction contrast indicates that disorientation angles between individual sub-grains/grains are very variable. 4

5 Fig. 4. Microstructure of CP titanium after equivalent strain 3.5 Fig. 5. Microstructure of CP titanium after equivalent strain 7.1 Fig. 6. Microstructure of CP titanium after equivalent strain 7.1 Dislocation density inside individual elongated sub-grains/grains is usually high, or an arrangement of dislocations into dislocation walls was observed. It may be assumed that formation of fine-grained structure was influenced not only by mechanism of fragmentation of deformed grains, but also by recrystallisation processes. It was established by TEM analysis of the sample after 8 ECAP passes that increase of number of passes resulted in improvement of uniformity and brought better final fine-grained structure. This is result of synergic 5

6 effect of applied extrusion temperature, total true strain and latent heat generated by severe plastic deformation. Dislocation density inside grains was usually very low, grain boundaries were well defined. Majority of grains was equiaxed, however, distinctly elongated grains were observed in some areas. Results of TEM analysis indicate that formation of ultra-fine grained structure was significantly influenced by recrystallisation processes. Size of some grains is less than 100 nm, other are bigger than 500 nm. 5. CONCLUSIONS It is possible to summarise the results obtained by analysis of influence of severe plastic deformation applied by the ECAP method on structure and properties of CP titanium Grade 2 in the following manner: Deformation of the investigated titanium by ECAP method at the temperature of approx. 290 C lead to significant increase of strength properties. The biggest increase in strength was determined after the first two passes through the ECAP tool. Deformation after 8 passes through the ECAP tool resulted in formation of ultra-fine grained structure with small share of globular particles, which were usually present on grain boundaries. Dislocation density inside the grains was very low. Majority of grains was formed by mechanism of recrystallisation of the deformed metallic matrix. Average grain size with high angle boundary after 8 passes through the ECAP tool was 320 ± 120 nm. The achieved refining of grain size was by better by two orders in comparison with the initial structural state. ACKNOWLEDGEMENTS This paper was created within the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. LITERATURE [1] RAAB, G.I., SOSHNIKOVA, E.P., VALIEV, R.Z. Influence of temperature and hydrostatic pressure during equalchannel angular pressing on the microstructure of commercial-purity Ti. Materials Science and Engineering, 2004, vol. A , p [2] KIM, I., JEONG, W.S., KIM, J. et al. Deformation structures of pure Ti produced by equal channel angular pressing. Skripta Materialia, 2001, vol. 45, p [3] SHIN, D.H., KIM, I., KIM, J. et al. Microstructure development during equal-channel angular pressing of titanium. Acta Materialia, 2003, vol. 51, p [4] LI, S.J., CUI, T.C., LI, Y.L. et al. Ultrafine-grained β-type titanium alloy with nonlinear elasticity and high duktility. Appl. Phys. Lett., 2008, vol. 92, p [5] ZHAO, X.C., FU, W.J., YANG, X.R. et al. Microstructure and properties of pure titanium processed by equalchannel angular pressing at room temperature. Scripta Materialia, 2008, vol. 59, p [6] FAN, Z.G., JIANG, H., SUN, X.G. et al. Microstructures and mechanical deformation behaviors of ultrafinegrained commercial pure (grade 3) Ti produced by two-step severe plastic deformation. Materials Science and Engineering, 2009, vol. A527, p [7] GREGER, M, ČERNÝ, M., KANDER, L. et al. Structure and properties of titanium for dental implants. Metalurgija, 2009, vol. 48, no. 4, p [8] GREGER, M., WIDOMSKÁ, M., MAŠEK, V. et al. Ultrafine grained titanium for biomedical applications. Materials Engineering, 2009, vol. 16, no.3, p [9] GREGER, M., KURSA, M. Grain refinement of commercially pure titanium by ECAP. Hutnické listy, 2010, vol.64, no.6, p