Influence of Ultrafine-Grained Structure on the Kinetics and Fatigue Failure Mechanism of VT6 Titanium Alloy

Abstract

The kinetics and mechanism of fatigue failure of the VT6 titanium alloy (composition, wt %: 5.95 V, 5.01 Al, 89.05 Ti) in the initial (hot-rolled) coarse-grained (CG) state and after equal-channel angular pressing (ECAP) in the ultrafine-grained state (UFG) are investigated. ECAP is performed using billets of the mentioned alloy 20 mm in diameter and 100 mm in length preliminarily subjected to homogenizing annealing. Then, quenching in water is performed from 960°C with holding for 1 h, tempering at 675°C for 4 h, and ECAP at 650°C (route Bс, φ = 120°, number of passes n = 6). The fine alloy structure after ECAP is investigated by transmission electron microscopy at an accelerating voltage of 200 kV. To determine alloy hardness, a Time Group TH 300 hardness meter is used. Static tensile tests are performed for round samples 5 mm in diameter using a Tinius Olsen H50KT universal testing machine. The extension velocity is 5 mm/min. Fatigue tests are performed using prismatic samples 10 mm in thickness at 20°C according to the three-point bending test using an Instron 8802 installation. It is shown that, under the same loading conditions, the fatigue life of alloy samples (the number of cycles before failure) in the initial CG state is higher than that of the alloy samples in the UFG state. It is shown that the number of cycles before fatigue-crack nucleation was at a level of 19–23% of the total sample longevity, regardless the alloy state. The straight-linear segment in kinetic diagrams of the alloy fatigue failure is approximated by the Paris equation. It is revealed that the propagation rate of the fatigue crack in the alloy with an UFG structure is somewhat higher than in the alloy with a CG structure. The microrelief of fatigue cleavages of the VT6 alloy both in the CG and UFG state can be characterized as scaly with fatigue grooves on the scale surface. A low-relief region 4–6 μm in length can be observed in the failure region of the samples with an UFG structure. The microrelief of the rupture region is pit, irrespective of the alloy state.