REGULARITIES OF FORMATION AND DEGRADATION OF THE MICROSTRUCTURE AND PROPERTIES OF NEW ULTRAFINE-GRAINED LOW-MODULUS Ti–Nb–Mo–Zr ALLOYS

Abstract

Regularities of the formation of ultrafine-grained (UFG) and submicrocrystalline (SMC) structures in new nickel-free low-modulus Ti–Nb–Mo–Zr titanium β alloys under the action of plastic deformation were studied. Temperature-time ranges of the development of dynamic recrystallization processes under the simultaneous action of temperature and plastic deformation were determined. The recrystallization diagram of II type of the Ti–28Nb–8Mo–12Zr alloy was constructed and analyzed. It was shown using scanning electron microscopy and electron backscatter diffraction method that the UFG structure with an average grain size of no more than 7 μm and high fraction of high-angle grain boundaries is formed in the investigated alloys as a result of longitudinal rolling followed by annealing for quenching. It was found that the formation of the UFG structure leads to a significant increase in the strength and plastic characteristics of these alloys. The regularities of the formation of UFG and SMC structures in the titanium β alloys Ti–28Nb–8Mo–12Zr and VT30 widely used in industry under the action of plastic deformation by the helical rolling method were studied. It was shown that the helical rolling of the VT30 alloy leads to the formation of the homogeneous UFG state as opposite to the developed Ti–28Nb–8Mo–12Zr β alloy where this method causes structure softening with micropores and microcracks formed in the central region. It is possible to form a nanostructured state with an average grain size of about 100 nm in Ti–Nb–Mo–Zr titanium β alloys using high-pressure torsion method.