Microstructure and Tensile Properties of Cost-Efficient Thermally Hardenable α + β Alloys of Ti–Al–Mo–Fe and Ti–Al–Mo–Cr Systems

Abstract

Four new compositions of titanium alloys of Ti–Al–Mo–Fe and Ti–Al–Mo–Cr systems with content of β-phase stabilizing elements in a range of 3.8–9.8 wt.% (in terms of “molybdenum equivalent” CMo) were studied. The alloys were melted by single-melt electron-beam cold hearth technique, thermomechanically processed, and subjected to conventional (in furnace) or continuous rapid (resistant heating by electric current) heat treatments with different regimes. The microstructure, phase composition, and mechanical properties both in as-deformed state and after heat treatments were studied in detail. It is found that the proposed thermomechanical processing allows to obtain a transformed microstructure characterized by nearly equiaxed α-phase particles with relatively small aspect ratio and weak crystallographic texture of a basal type. Subsequent annealing in the α + β field led to completion of the α + β microstructure transformation; a microstructure of globular type was obtained in the low-alloyed (CMo = 3.8%) material. The phase transformations at various stages of strengthening heat treatments and their influence on the final microstructure and properties were studied, depending on the content of β-stabilizing elements and processing route. Particular attention was paid to the influence of the content of β-stabilizing element on the mechanical properties, as well as to the difference between the alloying systems with iron or chromium. It is shown that the alloys of proposed compositions provide high strength and reliability; they are competitive with conventional commercial alloys in terms of the balance of mechanical properties in hot deformed, annealed, and thermally strengthened states. Strengthening treatment based on rapid heating allows to achieve strength above 1500 MPa with sufficient ductility.