On the nature of the omega tri-layer periodicity in rapidly cooled Ti-15Mo

Abstract

Scanning/transmission electron microscopy (STEM) High angle annular dark field (HAADF) Electron energy loss spectroscopy (EELS) a b s t r a c t High angle annular dark field (HAADF) images of the omega phase in metastable beta titanium alloys exhibit tri-layered periodicity. However, it is unclear if this indicates preferential site occupation, or is related to the structural modification of omega formation. Here, the periodicity was studied using a combination of HAADF imaging and electron energy loss spectroscopy. The results show that there is no preferential site occupancy or ordering and that the observed intensity variations are related to the imaging conditions.Additions of transition metal elements to titanium are known to stabilise the high temperature beta phase, enabling its metastable retention at room temperature following rapid cooling. However, such material is often reported to contain the omega phase, which forms through the collapse of consecutive pairs of {1 1 1} b [1-3]. This formation mechanism suggests that the parent beta phase has a soft phonon mode with respect to {1 1 2} b shear [4], and that the omega phase present following rapid cooling should be compositionally indistinct from the parent matrix at the point of quenching. However, there is evidence to suggest that a chemical alteration accompanies the structural modification [5,6]. Thus, determining the mechanism of omega formation remains an intriguing question.The nature of the omega transformation has been studied extensively through lattice imaging using a transmission electron microscope (TEM) [7][8][9][10][11][12]. However, phase contrast effects prevented early studies from directly imaging the {1 1 1} b collapse and compositional information was unobtainable due to the dominance of elastically scattered electrons [13]. Improved atomic column resolution and contrast have been achieved by sampling only the incoherently scattered electrons using a high angle annular detector in a scanning-TEM (STEM) [5,13,14]. Recently, interest in the omega phase and its effect on subsequent phase transformations [15][16][17][18][19][20][21] have led to new observations through high angle annular dark field (HAADF) imaging [13,[22][23][24].In HAADF images of Ti-5Al-5Mo-5V-3Cr (wt.%), intensity variations have been observed, with the collapsing planes exhibiting a lower intensity than the non-collapsing planes [24]. This intensity difference has been attributed to variations in the average atomic number of each column of atoms, suggesting preferential site occupation and, possibly, ordering within the omega phase. Intensity variations between the collapsing and non-collapsing planes have also been observed in rapidly cooled Ti-18Mo (wt.%) [13]. Similarly, tri-layered structures have been reported in HAADF images of Gum metal (Ti-31.9Nb-2.0Ta-2.7Zr-0.3O wt.%) [22]. However, multi-slice scattering simulations suggest that the observed intensity variations could also be related to the structural modification of the omega phase rather than compositio...

Phase equilibria in the Fe-Mo-Ti ternary system at 1000 °C

“…However, additional reflections were also present, highlighted by the dashed circle. Similar patterns have been reported previously in water quenched Ti -22% Fe [29] and other commercial alloys [30,31], which have been attributed to the incommensurate omega phase. The dark field (DF) image shown in Figure 3c was formed using the reflections within the dashed circle in Figure 3b.…”

Transformation Pathway upon Heating of Metastable β Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction

“…After quenching from a temperature above the β transus, the microstructure of these alloys often consists of small and densely distributed particles of the athermal ω (ωath) phase in the metastable β phase matrix. The ω phase forms through consecutive collapse of pairs of {111}β as a result of a soft phonon mode in the β phase, and the particles are compositionally indiscernible from the surrounding matrix [7,8]. It is widely accepted that these particles can influence the nucleation of the α phase during subsequent heat treatments, thereby providing a method for microstructural control [9].…”

The origin of striation in the metastable β phase of titanium alloys observed by transmission electron microscopy

“…Structure instability of the phase has also been a topic attracting attention (Nag et al, 2011;Bennett et al, 2015;Zheng et al, 2016;Abdel-Hady et al, 2007;Ng et al, 2011;Barriobero-Vila et al, 2015;Li et al, 2016;Williams et al, 1971;Sukedai et al, 1997). Atomic scaled lattice modulations have been revealed in some binary and ternary Ti-Nb (Tahara et al, 2011;Zheng et al, 2016) and Ti-Mo (Nag et al, 2011;Devaraj et al, 2012) alloys through examining the diffuse streaks in TEM SAED patterns and the atomic correspondences in highresolution TEM.…”