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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Table 2. Strength parameters of coarse-grained (CG) and ultrafine-grained (UFG) titanium Grade 4. Structure/ Parameter CG UFG Tensile strength  t , MPa 685 1004 Impact toughness KCU , kJ/m 3 172 198 Fracture toughness K IC , MPa  м 0.5 47 32

Despite the near-quasi-brittle macro-behavior of the UFG material, the fracture surfaces have a similar dimpled character in all tests of both material states. For example, Fig. 4 shows the fracture surfaces in the impact tests and the fracture toughness tests. Such dimpled fractography was observed in fracture toughness tests of Ti Grade 2 in the paper of Sabirov et al. (2010) and impact toughness tests of Ti Grade 5 in the paper of Semenova et al. (2017). In this work, the average size of the dimples (~0.9 μm for UFG and 10 μm for CG materials) is fairly close to the average grain size (Table 1). However, in the case of the UFG material, the apexes of the dimples are more elongated, and the dimples of the CG material have more angular shapes. The results show that optimizing the processing regimes of commercially pure titanium by SPD in combination with heat treatment and drawing can lead not only to high tensile strength, but also to impact toughness which is not worse than that of the material prior to processing. Nevertheless, ways to increase fracture toughness of the UFG material remain in question. Understanding the fracture processes of titanium can help develop a special microstructure that will improve the fracture toughness. However, optimization of SPD and understanding of the fracture processes remain at the level of empirical observations and assumptions, and therefore they are attractive for further research. The studies of the influence of SPD on the multifunctional mechanical properties of titanium were supported by the Ministry of Education and Science of the Russian Federation (Grant №14.B25.31.0017). The study of fracture toughness and impact toughness of UFG titanium was supported by the Grant of the President of the Russian Federation for young scientists (MK-2587.2017.1). The SEM images were obtained in the Interdisciplinary Center for Nanotechnology at the Research Park of Saint Petersburg University. The tests were conducted in the Centre for Extreme States of Materials and Constructions at the Research Park of Saint Petersburg University. ASTM E1820, 2011. Standard Test Method for Measurement of Fracture Toughness. GOST 9454, 1978. Metals. Method for Testing the Impact Strength at Low, Room and High Temperature. Pippan, R., Hohenwarter, A., 2016. The importance of fracture toughness in ultrafine and nanocrystalline bulk materials. Materials Research Letters 4:3, 127-136. Polyakov, A.V., Semenova, I.P., Raab, G.I., 2012. Peculiarities of ultrafine-grained structure formation in Ti Grade-4 using ECAP-Conform. Rev. Adv. Mater. Sci. 31, 78-84. Polyakov, A.V., Semenova, I.P., Valiev, R.Z., Huang Y., Langdon, T.G., 2013. Influence of annealing on ductility of ultrafine-grained titanium processed by ECAP-conform and drawing. MRS communications 3, 249-253. Sabirov, I., Valiev, R.Z., Semenova, I.P., Pippan, R., 2010. Effect of equal channel angular pressing on the fracture behavior of commercially pure titanium. Metall. Mater. Trans. A 41, 727 – 733. Semenova, I.P., Valiev, R.Z., Yakushina, E.B., Salimgareeva, G. H., Lowe T.C., 2008. Strength and fatigue properties enhancement in ultrafine grained Ti produced by severe plastic deformation. J. Mater. Sci. 43, 7354 – 7359. Semenova, I.P., Polyakov, A.V., Polyakova, V.V., Grishina, Yu.F., Huang, Yi, Valiev, R.Z., Langdon, T.G., 2017. Mechanical behavior and impact toughness of the ultrafine-grained Grade 5 Ti alloy processed by ECAP. Mat. Sci. Eng. A 696, 166-173. Sergueeva, A.V. , Stolyarov, V.V., Valiev, R.Z., Mukherjee, A.K., 2001. Advanced mechanical properties of pure titanium with ultrafine grained structure. Scripta Mater. 45, 747 – 752. 4. Conclusions Acknowledgements References