The indirect influence of the ω phase on the degree of refinement of distributions of the α phase in metastable β-Titanium alloys

Abstract

Non-conventional transformation pathways have been exploited to determine the degree of refinement of the microstructure of a metastable -Ti alloy. This has involved a set of heattreatments where samples, initially quenched to room temperature (RT) from above the  transus are slowly heated to 350°C, then isothermal annealed at 350°C for various times prior to upquenching to 600°C and again isothermally annealed at that temperature, followed by quenching to RT. Interrupted treatments have also been performed so that the factors influencing the development of the final microstructure could be determined. Microstructures have been characterized by scanning and transmission electron microscopy, and atom probe tomography. It has been found that a distribution of the  phase may be formed whose scale is intermediate between the "super refined" and "refined" versions observed in previous research. The results are consistent with precursory isothermal  precipitates playing an indirect role in the formation of the refined distribution of the  phase via a non-conventional transformation pathway.

Influence of oxygen on omega phase stability in the Ti-29Nb-13Ta-4.6Zr alloy

“…On the other hand, isothermal ω structure develops after subsequent annealing at a temperature below ω solvus. After prolonged aging, isothermal ω is known to promote α phase either directly by acting as heterogeneous nucleation sites or indirectly [17][18][19][20]. It is known that ω structure is formed due to collapse of {111} planes in β phase [14,15].…”

Characterizing nanoscale precipitation in a titanium alloy by laser-assisted atom probe tomography

“…Typically, the -phase is undesirable as it embrittles the alloy [10]; it is, however, generally believed to be a precursor to the formation of stable nanoscale -precipitates that highly strengthen the alloy [8,9,13,14]. This precipitation sequence is not fully understood, and a vast amount of APT research has been published recently to help further our insight into the precipitation mechanisms of the high-strengthto-weight -Ti alloys: Ti-5Al-5Mo-5V-3Cr wt.% (Ti-5553); and Ti-6Cr-5Mo-5V-4Al (wt.%) (Ti-6554) [9,[11][12][13][14][15][16][17]. Figure 1(a) illustrates the deleterious effects on mechanical properties associated with i precipitation in 24 h aged Ti-24-Nb-4Zr-8Sn wt.% (Ti-2448) [10].…”

Investigation of Novel Phase Transformation Mechanisms in Titanium Alloys Using Atom Probe and Aberration-Corrected Scanning Transmission Electron Microscope

“…In our recent studies, coupling atom probe and aberration-corrected scanning transmission electron microscope, several novel phase transformation mechanisms have been systematically explored in a beta titanium alloy, Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) [1][2][3][4]. It has been clearly shown that the size, morphology and number density of hcp structure alpha precipitates in Ti-5553 can be significantly influenced by the nano-scale structural and compositional instability, more specifically the metastable hexagonal structure isothermal omega phase, present in parent bcc structure beta phase.…”