PSI - Issue 6

Ivan Smirnov et al. / Procedia Structural Integrity 6 (2017) 196–200 Ivan Smirnov et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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3. Results and discussion

The test results are shown in Fig. 1-3 and Table 2. After processing by the ECAP-Conform method, the tensile strength of the material increased by 1.5 times, but the total elongation decreased by more than a third, Fig. 1. However, the total elongation of the UFG material remains greater than the minimum allowable elongation for commercially pure titanium Grade 4 - 15%. The selected sequence of processing regimes of SPD, heat treatment and drawing allowed obtaining the UFG material with higher impact toughness than its coarse-grained counterpart, Fig. 2. The impact toughness of the UFG titanium is 15% greater than the impact toughness of its original coarse-grained analogue. At the same time, the fracture of coarse-grained material requires 1.5 times more energy.

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CG UFG

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Strain (%)

Fig. 1. Typical diagrams in tensile tests of titanium Grade 4. CG - coarse-grained state. UFG - ultrafine-grained state.

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t*

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Fig. 2. Typical diagrams in impact tests of titanium samples with a U-shaped notch. CG - coarse-grained state; UFG - ultrafine-grained state; W is the displacement work; S is the area of a sample cross section in the point of impact; t* is the start time of macro fracture; t ff is the time of full fracture. Fig. 3 shows the typical diagrams in fracture toughness tests of the compared materials. Calculations of the stress intensity factor were carried out according to the formulas presented in the ASTM1820 (2011). The fracture toughness of the UFG titanium has decreased by 30% in comparison with the coarse-grained analogue, see Table 2. However, it is important to note that the requirements for sample dimensions in fracture toughness tests, where the width and height of the working part of a sample should be greater than the ratio of 2.5( KI C / σ 0.2 ) 2 ( σ 0.2 is the 0.2 percent offset yield strength and KI C is the critical stress intensity factor), were satisfied only for the UFG titanium. These conditions ensure that the plastic zone size is much less than the relevant dimension, which ensures that plane