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" C Mo ) 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 (C Mo = 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.

The Effect of the Forging Process on the Microstructure and Mechanical Properties of a New Low-Cost Ti-5Al-1.5Mo-1.8Fe Alloy

“…Ref. [4] also showed a similar conclusion, where the micro-level fracture surfaces had dimpled relief, indicating the ductile nature of the fracture. In contrast, cracks are visible in the single-phase region in Figure 8b, propagating along the grain boundaries.…”

“…Recently, researchers have focused on developing low-cost titanium alloys. Among them, Ti-Al-Mo-Fe systems have shown a high, dependable, and competitive mechanical property balance in the hot deformed, annealed, and thermally strengthened stages [ 4 ]. Moreover, Ti-Al-Mo-Fe alloy systems have been proven to be a promising alternative for developing complete β-Ti alloys with a low cost, high strength, and low elastic modulus, because a low-cost Mo-Fe master alloy can be used to stabilize the β phase at room temperature [ 5 ].…”

Effect of Electron Beam Welding on the Microstructure and Mechanical Properties of Low-Cost Titanium Alloys

“…In general, to improve the mechanical properties of welded joints and the base metal, it is necessary to recommend additional heat treatment -hardening or annealing, which will allow for low-cost pseudo-β alloys to further reduce the amount of β-phase [25].…”

“…Thus, electron beam welding does not allow to obtain more homogeneous uniform structure in all areas of the welded joint, due to higher percentage of metastable β-phase in weld metal and HAZ of studied low-cost titanium alloys. So, these welded joints are required to be subjected to post-welding furnace heat treatment [24,25].…”

Effect of Pre-Heating and Post-Weld Local Heat Treatment on the Microstructure and Mechanical Properties of Low-Cost β-Titanium Alloy Welding Joints, Obtained by EBW

“…Over the past few years, many more low-cost titanium alloys were developed, which also includes iron and has the stabilizing effect of the β-phase [13,14]. Nevertheless, due to the high density of iron, high quantity of β-stabilizing elements and the formation of TiFe-based brittle intermetallic phases (as per the binary Ti-Fe phase diagram) welding joints of low-cost titanium alloys are prone to formation of cold cracks which is very important limiting factor for obtaining welded joints with a strength of at least 90% compared to the strength of base material [15,16].…”