Microscopic approaches to isothermal transformation of incommensurate omega phase zones in Ti-20wt%Mo alloy studied by XDS, HREM and EXAFS

Hida, Moritaka; Sukedai, Eiichi; Terauchi, Hikaru

doi:10.1016/0001-6160(88)90211-8

Cited by 23 publications

(10 citation statements)

References 40 publications

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“…Along with an increase in the elastic modulus, the ω phase increases the microhardness of β-Ti alloys. Multiple studies have reported an increase in microhardness after precipitation of the isothermal ω phase, regardless of alloy composition [74,76,87,89,97,104,128,[131][132][133][134]. Although Jones et al [69] attribute the microhardness increase to precipitation of the nanoscale α phase, Coakley et al [74] counter that the observed hardness increase from 280 Hv to 400 Hv was due to the rapid precipitation of ω iso during a 400°C heat treatment.…”

Section: Effect Of the ω Phase On Mechanical Propertiesmentioning

confidence: 99%

“…As a result of the small size of ω ath , local strain fields around ω ath , and/or short-range correlated displacements in the β lattice, ω ath is observed to lead to diffuse streaking in SAED patterns rather than discrete diffraction spots [34,68,69,76,77]. Diffuse streaking in selected area diffraction patterns are also observed when there is an incomplete collapse of the (111) β planes during the β-to-ω transformation.…”

Section: Classification Of ω Phasesmentioning

confidence: 99%

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

Ballor

Prima

et al. 2022

International Materials Reviews

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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.

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Section: Effect Of the ω Phase On Mechanical Propertiesmentioning

confidence: 99%

Section: Classification Of ω Phasesmentioning

confidence: 99%

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

Ballor

Prima

et al. 2022

International Materials Reviews

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“…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].…”

mentioning

confidence: 99%

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

et al. 2015

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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...

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“…Important applications of the method in materials science are studies of decomposing alloys [1,2]. SAS can be used to characterize the time-evolution of such systems; i. e. the development of size, shape and crystallographic orientation of precipitates.…”

Section: Introductionmentioning

confidence: 99%

“…In b.c.c. Ti-Mo alloys particles with small aspect ratio were investigated: in this material the decomposition process starts via precipitation of metastable, coherent w-phase particles of ellipsoidal shape [2]. After long-time or high-temperature annealing plate-like particles of a-Ti appear, at the expense of w-particles [5].…”

Section: Introductionmentioning

confidence: 99%

Shape and size of non-spherical particles in single crystals, investigated by SAXS

1993

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The analysis of small-angle scattering (SAS) from single crystalline alloys provides a high quality of information on decomposing systems: size, shape and orientation of particles can be determined from three dimensional SAS data of single crystals. Adequate evaluation procedures for SAS data from single crystals are, e. g. , simulations of two dimensional (2D) scattering patterns and evaluation of slices along special directions in reciprocal space. These techniques, which allow the full exploitation of the instrumental resolution, are demonstrated with the examples of ellipsoidal and plate-like precipitates in alloys. In favourable cases the upper sensitivity limit can be increased up to 4000 A at a sample to detector distance of only lm.

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Microscopic approaches to isothermal transformation of incommensurate omega phase zones in Ti-20wt%Mo alloy studied by XDS, HREM and EXAFS

Cited by 23 publications

References 40 publications

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

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

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

Shape and size of non-spherical particles in single crystals, investigated by SAXS

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