Phase composition, structure and microhardness of the VT23 titanium alloy after deformation in a Bridgman chamber

The authors have studied for the first time the phase composition, microhardness and fine structure of the VT23 (α+β)-titanium alloy, with stable and metastable β-phase, after torsional deformation in a Bridgman chamber under a pressure of 4 GPa at room temperature. It has been found that the alloy microhardness, depending on the true degree of deformation under high hydrostatic pressure, changes along a curve with a maximum. The role of stress-induced βm→α" martensitic transformation in the formation of alloy structure, and microhardness under high-pressure torsion was revealed. The highest microhardness of the alloy with stable β-phase was 395 HV 0.05, and with metastable – 470 HV 0.05. At the same time, the maximum microhardness of metastable alloy, compared to stable alloy, was shifted to the region of lower true strain е=2.6. Using X-ray diffraction analysis, and transmission electron microscopy methods, made it possible to trace the evolution of alloy structure under high-pressure deformation consisting in grinding of α-, and α"-phase plates compared to the quenched state, as well as in the development of deformation βm→α", and α"→βm martensitic transformations. An increase in the degree of deformation by high-pressure torsion to е=7.7...7.9, regardless of the deformation stability of the β-phase, leads to a decrease in the alloy microhardness to a level of 185...205 HV 0.05. This is associated with the development of the dynamic recrystallisation process, and the formation of equiaxed α-phase nanoparticles with a size of 20...50 nm. The differences in the loading-unloading curves revealed by kinetic indentation, corresponded to the nature of the change in the VT23 alloy microhardness, depending on the quenching temperature and the true deformation degree.