Fatigue tests were carried out on samples of titanium VT1−0 and zirconium alloy Zr−1 wt % Nb in the ultrafine-grained, fine-grained and coarse-grained states in a gigacycle fatigue regime. It was found that the formation of an ultrafine-grained structure led to an increase in the fatigue limit in the gigacyclic region (10 9 cycles) by 1.3 times for titanium and 1.7 times for zirconium alloy when compared to the fine-grained and coarse-grained states. An evolution of the temperature field for titanium and zirconium alloy samples in various structural states in the process of cyclic loading was studied by the method of infrared thermography. It was shown that the process of cyclic deformation in all types of structural states was accompanied by an initiation and expansion of a heat source in a local volume of samples which has a significant impact on the fatigue strength. The increment of the maximum temperature on the surface of ultrafine-grained samples of titanium VT1−0 and zirconium alloy Zr−1 wt % Nb is significantly lower than that for the fine-grained and coarse-grained states. This fact indicates a qualitative change in the mechanism of energy dissipation which is associated with characteristic features of the ultrafine-grained state. When comparing the dynamics of thermal fields for the titanium and zirconium alloy samples in coarse-grained, fine-grained and ultrafine-grained states, it was found that the energy dissipation zone covered a considerable volume of the sample in the process of fatigue tests in case of ultrafine-grained state, whereas in case of coarse-grained and fine-grained states the growth of thermal energy was localized in the gauge area of the sample.
Bioinert Zr-1Nb alloy, which is a prospective material for the fabrication of implants for different applications, is studied. Annealed billets of the alloys are subjected to severe plastic deformation including multi-cycle abc-pressing and multipass rolling in grooved rolls. The abc-pressing stage involves three cycles of pressing within the temperature range 500 -400°C with one pressing in each cycle at a given temperature. In the second stage, the billets are deformed through rolling in grooved rolls at room temperature. Rolling in grooved rolls provided the formation of a homogeneous structure throughout the bulk billet volume and additional grain refinement. After annealing the alloy had a fine-grained structure consisting of 2.8 µm sized equiaxial α-Zr matrix grains and 0.4 µm sized β-Nb particles distributed on the boundaries and interiors of α-Zr matrix grains. As a result of severe plastic deformation, a binary ultrafine-grained alloy with 0.2 µm size of structural elements was obtained. Transmission electron microscopy shows that the microstructure of the alloy consists of α-Zr grains, while β-Nb phase grains are not identified structurally or via X-ray diffraction. Only the diffraction identification analysis reveals the presence of β-Nb in the alloy. Ultrafine-grained structure enhances the mechanical properties of the alloys: yield stress 450 MPa, ultimate tensile strength 780 MPa, and microhardness 2800 MPa are obtained while keeping a low value of Young's modulus (51 MPa) comparable to the Young's modulus of bone tissue.
The results of studies on the evolution of the microstructure and phase composition of ultrafine-grained Ti-40 wt.% Nb alloy during annealing in the temperature range of 673 -1073 K are presented. The ultrafine-grained structure in the Ti-40 wt.% Nb alloy was formed by a combined severe plastic deformation (SPD) method, which includes three-cycled abc-forging with a sequential temperature decrement in the range of 773 -673 K, multi-pass rolling in grooved rollers at room temperature, and subsequent recrystallization annealing at 573 K. After SPD, the Ti-40 wt.% Nb alloy had a microstructure represented by the β-phase subgrains with ellipsoidal particles of the ω-phase localized in the bulk of the β-grains, and the α-phase subgrains. The average size of structural elements (grains, subgrains and fragments) was 0.28 μm. After annealing in the range of 673 -873 K, the microstructure consisted of the dispersion-strengthened ω-phase, β-subgrains and α-subgrains, similarly to the initial UFG state. At the same time, a redistribution of the volume fraction of the α-phase occurred. In the range of 773 -973 K, the transformation of the ultrafine-grained (β + α + ω)-structure into a fine-grained structure consisting of β-and ω-phases with phase transformation according to the α → β scheme was observed. At temperatures above 973 K, active recrystallization occurred, which was accompanied by the rapid growth of a dispersive-strengthened β-phase grain size. This was also accompanied by the transformation of the alloy into a сoarse-grained state and a significant decrease of microhardness. Change in the concentration of niobium in the range of 40 -45 wt.% for the titanium-niobium alloy in the ultrafine-grained state does not have a significant effect on the structural-phase transformations during annealing.
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