PSI - Issue 23

A. Eremin et al. / Procedia Structural Integrity 23 (2019) 233–238 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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without any replacement and their life should be long enough. Thus it raises requirements for higher mechanical properties and corrosion resistance, which are relatively low for pure titanium. Addition of doping elements enhances mechanical properties but results in a reduction of biocompatibility and possible release of toxic elements into the human body (Browne and Gregson (2000)). An alternative approach consists in nanostructuring via severe plastic deformation (SPD) which results in an efficient increase in mechanical properties without alloying ( Ovid’ko and Langdon (2012); Valiev et al. (2012)). Fatigue durability after SPD could be different depending on the loading procedure and range (Cavaliere (2009) ; Fintová et al. (2017); Panin and Egorushkin (2009)). The purpose of the study is to investigate the grain refinement effect upon microstructure, mechanical properties (including deformation behavior and fracture) and fatigue crack growth kinetics of coarse-grain (CG) and ultrafine grain (UFG) grain titanium grade 2. Presented results and conclusions are based on the data obtained via loading diagrams, acoustic emission (AE), digital image correlation (DIC) and scanning electron microscopy (SEM) images of the fracture surface. For the grain refinement of the initial CG roll-formed stock, the three-stage severe plastic deformation (SPD) technique was implemented. At the first stage, the blanks were subjected to ABC-pressing at the temperature of 500°C and the strain rate was 10 -3 – 10 -1 s -1 . Maximum strain at each pressing cycle was 40-45%. At the second stage, the blanks were deformed using rolling (five times) with the total strains about 90%. The third stage was annealing at T=350 ° С during 1 hour which was aimed at decreasing internal stresses caused by severe deformation. The final blanks had the shape of bands with dimens ions of 1.3×5×70 mm (th ickness × width × length). The microstructure of the samples was investigated by optical microscopy (Carl Zeiss Axio Observer) and transmission electronic microscopy (TEM) using JEOL JEM 2100. The average size of structural elements (grains, subgrains, fragments) was calculated using the intercept length method according to ASTM E1382-97 (2015). It is shown that the CG titanium (Fig. 1a) has  -phase grains (average grain size of 25 µm) . The TEM microstructure of UFG titanium after SPD is presented in Fig. 1b and shows equiaxial subgrains and fragments with the average size of  0.2 µm . The subgrain boundaries are barely discernible and could be found inside the grains. Moreover, the SPD material has a high dislocation density. Reflexes at the microdiffraction pattern (Fig. 1b) indicate the presence of high-angle boundaries, while azimuth diffusion testifies for a high level of internal stresses. 2. Severe plastic deformation technique and microstructure investigation

Fig. 1. Optical image of coarse- grained titanium (а) and bright -field TEM image of ultrafine-grained titanium (b). The inset in (b) illustrates the microdiffraction pattern.

3. Experimental procedure: static tensile and fatigue tensile tests

CG and UFG titanium were tested by using electromechanical testing machine Instron 5582 for static tests and servo-hydraulic testing machine BISS Nano-15 for fatigue tests.