The Kinetics of Primary Alpha Plate Growth in Titanium Alloys

Abstract

The kinetics of primary α-Ti colony/Widmanstätten plate growth from the β are examined, comparing model to experiment. The plate growth velocity depends sensitively both on the diffusivity D(T ) of the rate-limiting species and on the supersaturation around the growing plate. These result in a maxima in growth velocity around 40 K below the transus, once sufficient supersaturation is available to drive plate growth. In Ti-6246, the plate growth velocity was found to be around 0.32 µm min −1 at 850 • C, which was in good agreement with the model prediction of 0.36 µm min −1 . The solute field around the growing plates, and the plate thickness, was found to be quite variable, due to the intergrowth of plates and soft impingement. This solute field was found to extend to up to 30 nm, and the interface concentration in the β was found to be around 6.4 at.% Mo. It was found that increasing O content will have minimal effect on the plate lengths expected during continuous cooling; in contrast, Mo approximately doubles the plate lengths obtained for every 2 wt.% Mo reduction. Alloys using V as the β stabiliser instead of Mo are expected to have much faster plate growth kinetics at nominally equivalent V contents. These findings will provide a useful tool for the integrated design of alloys and process routes to achieve tailored microstructures.

In-situ alloying of beta titanium for implant application via laser powder bed fusion

“…Ta exhibits slower diffusion rate in high temperature β-Ti as the concentration of Ta increases [182]. Such sluggish diffusion of Ta can become a limiting factor for the growth rate of lath/acicular phase which subsequently leads to finer microstructure [183]. The observed microstructure refinement is consistent with a similar Ti-Ta study previously done [123].…”

Comparison of Microstructure Refinement in Wire-Arc Additively Manufactured Ti–6Al–2Sn–4Zr–2Mo–0.1Si and Ti–6Al–4V Built With Inter-Pass Deformation

“…However, Ti6242 is alloyed with molybdenum, which has a much slower solid-state diffusivity than vanadium, the slowest diffusing species in Ti64. [26] When compared to Ti64, this difference in diffusivity is known to impede the b-grain growth kinetics, [56,58,59] and the growth rate of a laths during the b fi a transformation [60] ; so it is perhaps not surprising that Ti6242 was found to have an overall more refined microstructure. Given that both materials were built with near-identical thermal histories, with multiple-heating cycles above and below the b transus, where the b grains and a + b transformation microstructure have ample opportunity to coarsen, the results demonstrate that the presence of slower-diffusing alloying additions in titanium alloys like Ti6242 appear to be advantageous for refining the as-deposited microstructure in a high-deposition-rate AM process, which have higher heat inputs and lower cooling rates than more conventional AM platforms, such as powder bed.…”

“…[28,36] The growth rate of a laths at a given undercooling is dependent on partitioning of the slowest diffusing species in each alloy between a and the b matrix, which will try to maintain equilibrium at the a/b interface. [60] The dominant elements that control the a growth kinetics are thus vanadium for Ti64 and molybdenum for Ti6242. [26,27,52,53,68] Given that molybdenum has the slowest diffusivity by far in titanium, the a plates in Ti6242 will grow more slowly than in Ti64, [60] allowing more nucleation and thus an overall finer basketweave microstructure.…”

“…[60] The dominant elements that control the a growth kinetics are thus vanadium for Ti64 and molybdenum for Ti6242. [26,27,52,53,68] Given that molybdenum has the slowest diffusivity by far in titanium, the a plates in Ti6242 will grow more slowly than in Ti64, [60] allowing more nucleation and thus an overall finer basketweave microstructure.…”

Crystallographic Features of Phase Transformations during the Continuous Cooling of a Ti6Al4V Alloy from the Single-Phase β-Region

“…Some authors have declared [ 42 ] that the features of both shear and diffusion mechanisms are typical of titanium alloys and high-alloyed steels. Other studies have described each transformation mechanism in detail and strictly divide diffusion and shear [ 8 , 10 , 22 , 43 , 44 ]. However, it has been demonstrated that the BOR is followed for all transformation mechanisms [ 18 , 23 , 43 , 45 ].…”