Металознавство та обробка металів (Sep 2020)
Structural and chemical inhomogeneities in the sintered titanium alloys after severe plastic deformation
Abstract
The results of studies of porosity, microhardness, microstructure, phase composition, and distribution of alloying elements in samples of VT6 and VT8 titanium alloys are presented. They were synthesized from a mixture of powder components with various modes and subjected to severe plastic deformation by torsion under high pressure with multiple regimes. The effects of sintering time and the regimes of subsequent severe plastic deformation on the microhardness, and it is dispersion through the cross-section of the samples were identified. It was shown that, regardless of the sintering regimes, the microhardness of the samples across the cross-section and does not depend on the processing regimes. Consequently, this serves as an indication of a saturation effect of fragmentation and hardening. The analysis of the porosity of the initial samples concerning both the sintering time and the samples subjected to high pressure torsion was conducted. It was demonstrated that irrespective of the initial value of porosity, severe plastic deformation enables the complete elimination of porosity across the entire cross-section of the samples. It was concluded that the formation of new phases does not occur in the samples of the studied titanium alloys during the process of the severe plastic deformation. It was shown that the samples after sintering obtain a significant chemical heterogeneity, which is effectively eliminated in the process of deformation. The beneficial role of the generated in the samples vortex flows, which facilitate efficient mass transfer was confirmed after adding slowly diffusing chemical elements into the composition of the powder mixture. Considering the physical similarity of the processes which occur in a thin deformable layer of samples during the severe plastic deformation by twist extrusion and high pressure torsion, conclusions on the effectiveness of using twist extrusion to homogenize structural and chemical inhomogeneities in bulk billets of the sintered titanium alloys were drawn.
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