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

Abstract

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.