Proceedings of the 12 th World Conference on Titanium – Author’s Copy 1137 High cycle fatigue strength of hydrostatically extruded nanocrystalline CP titanium Halina Garbacz 1 , Maciej Motyka 2 , Waldemar Ziaja 2 , Małgorzata Lewandowska 1 , Jan Sieniawski 2 , Krzysztof Topolski 1 1 Warsaw University of Technology, Faculty of Materials Science and Engineering, ul. Narbutta 85, 02-524 Warszawa, Poland 2 Rzeszow University of Technology, Department of Materials Science, ul. W. Pola 2, 35-959 Rzeszow, Poland Hydrostatic extrusion is an efficient method of grain refinement to the nanometric scale in metallic materials. Nanocrystalline pure titanium is characterized by about three times higher strength that the microcrystalline one. It is known that grain refinement via severe plastic deformation methods (ECAP, HPT) leads to an increase in fatigue properties of ultra-fine and nanocrystalline metallic materials but may have deleterious effect on fatigue crack growth behaviour. High cycle fatigue behaviour of pure titanium processed by hydrostatic extrusion was examined. Stress controlled tensile and bending tests were carried out on smooth and notched specimen. It was found that hydrostatic extrusion leads to increase of fatigue limit for smooth specimens. Keyword: SPD methods, hydrostatic extrusion, nanocrystalline metallic materials, fatigue strength, CP titanium 1. Introduction The growing interest in ultra fine grained (having the grain size in the range of 100nm-1m) and nanocrystalline materials (average grain size below 100nm) stems from their exceptional properties comparing to conventional polycrystalline materials 1-2) . They may exhibit superior mechanical and physical properties i.e. increased strength/hardness, improved toughness, reduced elastic modulus and ductility, enhanced diffusivity, higher specific heat and enhanced thermal expansion coefficient (CTE). Due to that fact nanocrystalline metallic materials have been the subject of widespread research over the past couple of decades 1-5) . A number of techniques have been applied over the years for producing ultra fine grained and nanostructured materials. These include equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB), multistep forging (MF), continuous confined strip shearing (C2S2) 6) . However many of them are limited to synthesis in small quantities 2) . Hydrostatic extrusion is one of the Severe Plastic Deformation methods (SPD), which allows to obtain a significant grain refinement, down to the nanometric scale. This method is developed in collaboration between the Warsaw University of Technology and Institute of High Pressure Physics, Polish Academy of Sciences in Warsaw. In comparison to the standard SPD methods, the hydrostatic extrusion (HE), as a method of grain refinement, usually requires a significant smaller total strain. A number of commercially pure metals and alloys have been processed by HE at ambient temperature to evaluate the potential of this method for achieving grain size refinement 5, 7) . For pure metals, the highest degree of grain size refinement was achieved in the case of titanium 8) . The parameters of nanocrystalline titanium production using hydrostatic extrusion have been previously described elsewhere 9) . The behaviour of ultra fine grained and nanocrystalline materials under cyclic loads attracts much interest 10-13) . Fatigue strength of SPD processed pure titanium may exceed values obtained for conventional titanium alloys 14) . However high mechanical properties under static strain cannot be directly related to its fatigue resistance 6, 15) . In addition, the fatigue tests are often very sensitive to the structural state and its stability under the action of load. Also the effect of mean stress on fatigue of ultra fine grained metals and alloys is not fully understood as the majority of fatigue experiments were performed under tension–compression loading with the stress ratio R=-1 6) . The aim of current study was to investigate fatigue properties of hydrostatically extruded CP titanium using smooth and notched specimen. 2. Material and experimental The material investigated was commercially pure titanium Grade 2. A rod 50mm in diameter was subjected to a multi-pass hydrostatic extrusion at room temperature at the maximum strain rate of the order of 10 2 s −1 . Two sequences of the extrusion process were applied giving the final diameter of the rods Ø=25mm (HE1) and 10mm (HE2). The parameters of the hydrostatic extrusion process are given in Table 1. Table 1. Parameters of the hydrostatic extrusion process applied to Grade 2 titanium Hydrostatic extrusion process Number of HE passes Total cross-section reduction Accumulated strain (ε) Ø50mm → Ø25mm (HE1) 2 4 1.38 Ø50mm → Ø10mm (HE2) 7 25 3.21 The microstructure of the material in the initial state and after hydrostatic extrusion was examined by light and transmission electron microscopy. Quantitative analysis was carried out using digital image analysis software. Grain size was characterized by the mean value of the equivalent grain diameter, E(d 2 ). Mechanical properties of the material were evaluated in the static tensile tests. Round specimens 3mm in diameter were tested at room temperature at the constant strain rate of 1.33·10 -4 s -1 using MTS 858 universal testing machine and MTS extensometer with the gauge length of 5mm. Additionally hardness measurements were performed on the cross-sections of the rods (HV0.2). Two series of fatigue tests were carried out to characterize fatigue properties of the CP titanium. Stress