Effect of ultrafine-grained structure on kinetics and mechanism of VT6 titanium alloy fatigue failure

The kinetics and mechanism of fatigue fracture of VT6 titanium alloy (composition, wt.%: 5,95 V, 5,01 Al, 89,05 Ti) in the initial (hot-rolled) coarse-grained state and after equal channel angular pressing (ECAP) in the ultrafine-grained state were studied. Blanks used for ECAP were made of the specified alloy 20 mm in diameter and 100 mm in length previously subjected to homogenization annealing. Then, quenching was carried out in water from 960 °C (1 hour), tempering at 675 °C for 4 hours, ECAP at 650 °C (Вс route, ф = 120°, n = 6 passes). The fine structure of the alloy after ECAP was studied using transmission electron microscopy at accelerating voltage 200 kV. The Time Group TH 300 hardness tester was used to determine alloy hardness. Static tension of round samples with a diameter of 5 mm was carried out on the Tinius Olsen H50KT universal test machine. Tension speed was 5 mm/min. Fatigue tests of 10 mm thick prismatic samples were carried out at 20 °C using the three-point bending test at the Instron 8802 unit. It was shown that under the same loading conditions, sample durability (the number of cycles before failure) from the alloy in the initial coarse-grained state is slightly higher than in the ultrafine-grained state. The number of cycles before fatigue crack formation, regardless of the alloy state, is at the level of 19—23 % of the total durability of samples. The straight section in kinetic diagrams of alloy fatigue fracture is approximated by the Paris equation. It was found that the rate of fatigue crack propagation in an alloy with an ultrafine-grained structure is somewhat higher than in an alloy with a coarse-grained structure. The microrelief of VT6 alloy fatigue fractures both in coarse-grained and ultrafine-grained state can be characterized as «scaly» with fatigue grooves on the surface of flakes. The region of fracture of the alloy with the ultrafine-grained structure contain a low-relief area 4—6 pm in length. The break, irrespective of the alloy state, has a pit microrelief.

1. Chung C.S., Kim J.K., Kim H.K., Kim WJ. Improvement of high—cycle fatigue life in a 6061 Al alloy produced by equal channel angular pressing. Mater. Sci. Eng. A. 2002. Vol. 337. P. 39—44.

2. Vinogradov A., Nagasaki S, Patland V, Kitagawa K, Kawa-zoe M. Fatigue properties of 5056 Al—Mg alloy produced by equal—channel angular pressing. Nanostruct. Mater. 1999. Vol. 11. P. 925—934.

3. Furuya Y., Matsuoka S, Shimakura S, Hanamura T, To-rizuka S. Effects of carbon and phosphorus addition on the fatigue properties of ultrafine-grained steels. Scripta Mater. 2005. Vol. 52. P. 1163—1167.

4. Okayasu M, Sato K, Mizuno M, Hwang D.Y., Shin D.H. Fatigue properties of ultra-fine grained dual phase ferrite/ martensite low carbon steel. Int. J. Fatigue. 2008. Vol. 30. P. 1358—1365.

5. Meyer L.W., Sommer K, Halle T, Hockauf M. Crack growth in ultrafine grained AA6063 produced by equal-channel angular pressing. J. Mater. Sci. Eng. 2008. Vol. 43. P. 7426-7431.

6. MeyerL.W, SommerK, Halle T, Hockauf M. Microstructure and mechanical properties affecting crack growth behaviour in AA6060 produced by equal—channel angular extrusion. Mater. Sci. Forum. 2008. Vol. 584-586. P 815—820.

7. Estrin Y., Vinogradov A. Fatigue behaviour of light alloys with ultrafine grain structure produced by severe plastic deformation: An overview. Int. J. Fatigue. 2010. Vol. 32. P 898—907.

8. Goto M, Yamamoto T, Kitamura J., Iwamura T, Han S.Z., Ahn J.-H, Kim S, Lee J. Crack growth rate of inclined and deflected surface-cracks in round-bar specimens of copper processed by equal channel angular pressing under cyclic loading. Eng. Fract. Mech. 2017. Vol. 182. P. 100—113.

9. Estrin Y., Vinogradov A. Extreme grain refinement by severe plastic deformation: A wealth of challenging science. Acta Mater. 2013. Vol. 61. P. 782—817.

10. Mughrabi H, Hoppel H.W., Kautz M. Fatigue and microstructure of ultrafine—grained metals produced by severe plastic deformation. Scripta Mater. 2004. Vol. 51. P. 807—812.

11. Wang K, Tao N.R., Liu G, Lu J, Lu K. Plastic strainin-duced grain refinement at the nanometer scale in copper. Acta Mater. 2006. Vol. 54. P. 5281—5291.

12. Millett P.C., Selvam R.P., Saxena A. Stabilizing nanocrystalline materials with dopants. Acta Mater. 2007. Vol. 55. P. 2329—2336.

13. Khalaj G, Khalaj M.J., Nazari A. Microstructure and hot deformation behavior of almg6 alloy produced by equal-channel angular pressing. Mater. Sci. Eng. A. 2012. Vol. 542. P. 15—20.

14. Skryabina N. U, Pinyugzhanin V.M., Frushar D. Features of formation of texture of deformation in magnesian AZ31 alloy in the course of equal-channel angular pressing. Perspektivnye materialy. 2013. No. 1. P. 33—42 (In Russ.).

15. Islamgaliyev R.K., Nesterov K.M., Hafizova E.D., Ganeev A.V, Golubovsky E.R., Volkov M.E. Durability and fatigue of ultrafine-grained AK4-1 aluminum alloy. Vestnik UGATU. 2012. No. 8. P. 104—109 (In Russ.).

16. Valiev R.Z., Semenova I.P., Latysh V.V., Shcherbakov A.V, Yakushina E.B. Nanostructural titanium for biomedical applications: New developments and prospects of commercialization. Rossiiskie nanotekhnologiiy. 2008. No. 9-10. P. 80—90 (In Russ.).

17. Klevtsov G. V, Valiyev R.Z., Botvina L.R., Klevtsova N.A., Semyonova I. P, Kashapov M. R, Fesenyuk M. V, Solda-tenkov A.P. Kinetics of fatigue failure of the titan in a submicrocrystalline state. Vestnik OGU. 2012. No. 9. P. 123—125 (In Russ.).

18. Valiev R.Z., Zhilyaev A.P., Langdon T.G. Bulk nanostructured materials: Fundamentals and applications. TMS, Wiley, 2014.

19. Valiyev R.Z., Raab G.I., Gunderov D.V, Semyonova I. P, Murashkin M.Yu. Development of methods of intensive plastic deformation for receiving volume nanostructural materials with unique mechanical properties. Nano-tekhnika. 2006. No. 2. P. 32—42(In Russ.).

20. Klevtsov G.V, Bobruk E.V, Semyonova I.P, Klevtsova N.A., Valiyev R.Z. Durability and mechanisms of destruction of the volume nanostructured metal materials. Ufa: RIK UGATU, 2016 (In Russ.).

21. Klevtsov G. V, Botvina L.R., Klevtsova N.A., Limar L.V. Fractography of destruction of metal materials and designs. Moscow: MISIS, 2007 (In Russ.).