A review of the metastable omega phase in beta titanium alloys: the phase transformation mechanisms and its effect on mechanical properties

Abstract

Since its discovery in 1954, the omega (ω) phase in titanium and its alloys has attracted substantial attention from researchers. The β-to-ω and ω-to-α phase transformations are central to β-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the β-to-ω transformation and key nucleation driving forces for the ω-assisted α phase formation are discussed. Additionally, the influence of ω phase on the mechanical properties of β-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed.

Plasticity and brittleness of the ordered βo phase in a TNM-TiAl alloy

“…darkest parts marked o() exhibit the so-called tweed contrast typical for the o-phase containing -type precipitates. Figure 2a depicts a diffraction pattern comparable to that previously reported for the bcc phases of conventional disordered Ti alloys [13,15,17,19] or intermetallic TiAl alloys. [10,11].…”

“…Looking at the (222 ̅̅̅̅̅ )o direction in particular, several spots are believed to originate from the variant for which the c axis is parallel to the (222 ̅̅̅̅̅ )o direction: one fundamental reflexion and two superstructure spots, resulting from the long range ordered structure. However, it may be argued that similar patterns have been seen in Ti and Zr alloys exhibiting a disordered  structure [13,15,17,19].…”

“…The o phase is known to transform into -type phases at temperatures ranging between 500 °C and 1100 °C, depending on alloy composition and thermal treatments [10,11]. This transformation of -type hexagonal phases in bcc phases is similar to that commonly observed in commercial  or near- Ti base alloys [12][13][14][15][16][17], or more generally in alloys of group IV elements such as Ti, Zr and Hf [17][18][19]. The transformation occurs through local chemical rearrangements associated with the collapse of {111} bcc planes, leading to the formation of double layers, the latter alternating with single layers.…”

Double Diffraction or ω-Fe Diffraction in Body-Centered Cubic {112}<111>-Type Twinned Martensite

“…The latter has a primitive hexagonal crystal structure with its a ω = 2 0.25em × a bcc and c ω = 3 /2 × a bcc, − as shown in Figure . The ω phase mentioned here is quite similar to those existing in other BCC metals and alloys, such as Ti and Zr alloys. ,− The existence of an ideal BCC {112}<111>-type twin structure has also been discussed in the present investigation.…”

Effect of Structure and Hydrogen on the Short-Term Creep of Titanium Ti-2.9Al-4.5V-4.8Mo Alloy

“…Titanium alloys have high specific strength and corrosion resistance under atmospheric conditions and with a number of strong chemicals. This unique combination of properties allows for using titanium alloys in various branches of industries and medicine [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Titanium two-phase (α + β type) Ti-2.9Al-4.5V-4.8Mo alloy is widely used for the manufacturing of implants and medical instruments [ 2 , 8 ], as well as fastening elements in the aerospace industry and shipbuilding [ 2 , 9 ].…”