Strength and fracture mechanism during torsion of ultrafine-grained austenitic steel for medical applications

The article considers evaluation of torsional strength and fracture of austenitic corrosion-resistant steel 08Kh18N9 with an ultrafine-grained (UFG) and coarse-grained (CG) structure, widely used in medicine for the production of plates, screws, rods for bone osteosynthesis and other medical products. The structure of the CG steel was studied using an Axiovert 40 MAT metallographic microscope, and the fine structure of the UFG steel was investigated with a JEM-2100 transmission electron microscope. Torsion tests of the cylindrical samples with a diameter of 10 mm were carried out at a temperature of 20 °C on MK-50 installation. JEOL JCM-6000 scanning electron microscope was used for the microfractographic studies of fracture surfaces. The analysis of the “Torque - torsion angle” diagrams showed that the torsional ultimate strength (τ_t) and yield strength (τ_0.3) of UFG steel increase by 1.3 - 3.8 times, and the relative shear (g) decreases by 2.4 times in comparison with CG steel. High values of torsional strength properties of UFG steel make it possible to provide high torque without destroying the product. Consequently UFG steel 08Kh18N9 in comparison with CG steel is a more promising material for the manufacture of medical screws and other medical products that experience significant loads during the torsion process. Three areas were identified on the surface of all fractures: fibrous central part, transitional (middle) part, and a relatively smooth peripheral part. Fracture begins with the formation of shear pits in the middle and peripheral parts, which, with further rotation of the sample, are completely rubbed out (in case CG steel), or remain (in case of UFG steel). Final failure occurs under the action of normal stresses in the central part of the sample.

1. Semenova I.P., Klevtsov G.V., Klevtsova N.A, Dyakonov G.S., Matchin A.A., Valiev R.Z. Nanostructured titanium for maxillofacial mini-implants. Advanced Engineering Materials. 2016, vol. 18, no. 7, pp. 1216-1224.http://doi.org/10.1002/adem.201500542

2. Popkov A.V. Biocompatible implants in traumatology and orthopedics. Genii Ortopedii. 2014, no. 3, pp. 94-99. (In Russ.).

3. Fujishiro T., Moojen D.J., Kobayashi N., Dhert W.J., Bauer T.W. Perivascular and diffuse lymphocytic inflammation are not specific for failed metal-on-metal hip implants. Clinical Orthopaedics and Related Research. 2011, vol. 469, no. 4, pp. 1127-1133. http://doi.org/10.1007/s11999-010-1649-1

4. Valiev R.Z., Zhilyaev A.P., Langdon T.G. Bulk Nanostructured Materials: Fundamentals and Applications. Hoboken, New Jersey: John Wiley & Sons, 2014, 440 p.

5. Valiev R.Z. Design of nanostructured metals and alloys with unique properties using severe plastic deformation, Rossiiskie nanotekh-nologii. 2006, vol. 1, no. 1-2, pp. 208-217. (In Russ.).

6. Klevtsov G.V., Bobruk E.V., Semenova I.P., Klevtsova N.A., Valiev R.Z. Strength and Fracture Mechanisms of Bulk Nanostructured Metallic Materials. Ufa: USATU, 2016, 240 p. (In Russ.).

7. Tsuji N., Ueji R., Minamino Y., Saito Y. A new and simple process to obtain nanostructured bulk low-carbon steel with superior mechanical property. Scripta Materialia. 2002, vol. 46, no. 4, pp. 305-310. https://doi.org/10.1016/S1359-6462(01)01243-X

8. Hohenwarter A., Pippan R. Fracture of ECAP-deformed iron and the role of extrinsic toughening mechanism. Acta Materialia. 2013, vol. 61, no. 8, pp. 2973-2983. http://doi.org/10.1016/j.actamat.2013.01.057

9. Song R., Ponge D., Raabe D., Speer J.G., Matlock D.K. Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels. Materials Science and Engineering: A. 2006, vol. 441, no. 1-2, pp. 1-17. https://doi.org/10.1016/j.msea.2006.08.095

10. Estrin Y., Vinogradov A. Fatigue behavior of light alloys with ultrafine grain structure produced by severe plastic deformation: An overview. International Journal of Fatigue. 2010, vol. 32, no. 6, pp. 898-907. https://doi.org/10.1016/j.ijfatigue.2009.06.022

11. Pickering F.B. Physical Metallurgy and the Design of Steels. Applied Science Publishers, 1978, 275 p. (Russ. ed.: Pickering F.B. Fizicheskoe metallovedenie i razrabotka stalei. Moscow: Metallur-giya, 1982, 182 p.)

12. Odnobokova M., Belyakov A., Enikeev N., Molodov D.A., Kaibyshev R. Annealing behavior of 304L stainless steel processed by large deformations of cold and warm rolling. Materials Science and Engineering: A. 2017, vol. 689, pp. 370-383. https://doi.org/10.1016/j.msea.2017.02.073

13. Bondarenko A.V., Peleganchuk V.A., Raspopova E.A., Peche-nin S.A. Destruction of implants after extraosseous osteosynthesis of long bones fractures. N.N. Priorov Journal of Traumatology and Orthopedics. 2004, vol. 11, no. 2, pp. 41-43. (In Russ.). https://doi.org/10.17816/vto200411241-43

14. Lau T.W., Leung F., Chan C.F. Wound complication of minimally invasive plate osteosynthesis in distal tibia fractures. International Orthopaedics. 2008, vol. 32, no. 5, pp. 697-703. http://doi.org/10.1007/s00264-007-0384-z

15. Livani B., Belangero W.D., Castro de Medeiros R. Fractures of the distal third of the humerus with palsy of the radial nerve: management using minimally-invasive percutaneous plate osteosynthesis. The Journal of Bone and Joint Surgery. British volume. 2006, vol. 88-B, no. 12, pp. 1625-1628. http://doi.org/10.1302/0301-620X.88B12.17924

16. Xia L., Zhou J., Zhang Y., Mei G., Jin D. Meta-analysis of reamed versus unreamed intramedullary nailing for the treatment of closed tibial fractures. Orthopedics. 2014, vol. 37, no. 4, pp. 332-338. http://doi.org/10.3928/01477447-20140401-52

17. Zolotarevskii V.S. Mechanical Properties of Metals. Moscow: MISiS, 1998, 400 p. (In Russ.).

18. Meisner S.N., Kotenko M.V., Kopysova V.A., Yaremenko A.I., Ratkin I.K. Features of implants damage from metal alloys. ENI Zabaikal'skii meditsinskii byulleten'. 2016, no. 1, pp. 59-68. (In Russ.).

19. Kleweno C.P., Jawa A., Wells J.H., O'Brien T.G., Higgins L.D., Harris M.B., Warner J.J. Midshaft clavicular fractures: Comparison of intramedullary pin and plate fixation. Journal of Shoulder and Elbow Surgery. 2011, vol. 20, no.7, pp. 1114-1117. https://doi.org/10.1016/j.jse.2011.03.022

20. Zehir S., Zehir R., §ahin E. Comparison of novel intramedullary nailing with miniinvasive plating in surgical fixation of displaced midshaft clavicle fractures. Archives of Orthopaedic and Trauma Surgery. 2015, vol. 135, no. 3, pp. 339-344. https://doi.org/10.1007/s00402-014-2142-1