Heat Treatment of Metastable Beta Titanium Alloys

Abstract

Heat treatment of metastable beta titanium alloys involves essentially two steps-solution treatment in beta or alpha+beta phase field and aging at appropriate lower temperatures. High strength in beta titanium alloys can be developed via solution treatment followed by aging by precipitating fine alpha (α) particles in a beta (β) matrix. Volume fraction and morphology of α determine the strength whereas ductility is dependent on the β grain size. Solution treatment in (α + β) range can give rise to a better combination of mechanical properties, compared to solution treatment in the β range. However, aging at some temperatures may lead to a low/nil-ductility situation and this has to be taken into account while designing the aging step. Heating rate to aging temperature also has a significant effect on the microstructure and mechanical properties obtained after aging. In addition to α, formation of intermediate phases such as omega, beta prime during decomposition of beta phase has been a subject of detailed studies. In addition to covering these issues, the review pays special attention to heat treatment of beta titanium alloys for biomedical applications, in view of the growing interest this class of alloys have been receiving.

Tailoring a Low Young Modulus for a Beta Titanium Alloy by Combining Severe Plastic Deformation with Solution Treatment

“…In contrast to the influence of α" precipitation, the possible formation of the athermal ω phase during ST is correlated with a rise in strength, reduced ductility, and, in most cases, an unsatisfactory increase in elastic modulus [42]. However, knowing that the addition of about 5-7 wt.% of Zr to Ti-Nb/Mo-Fe alloys represents a sufficient quantity to suppress the formation of athermal ω precipitates during ST (water quenching) [47], it can be concluded that the Zr content (7.6 wt.%) from the present case, alongside other alloying elements, would be sufficient to ensure this desideratum.…”

“…The β-transus temperature (T β ) represents an important parameter for selecting the heat treatment temperatures. The T β value strongly depends on the alloying elements: α-stabilizers increase T β , β-stabilizers decrease T β , while the neutral elements do not change T β [42]. Equation (1) to find the T β value is given below [32]:…”

“…For the present studied alloy, the calculated T β is: Figure 2 represents the schema of the applied SPD and ST processes for the studied alloy with established parameters for both V-1 and V-2 variants. According to [42], the super transus solution treatment for β-Ti alloys comprises a heating of the alloy with about 30-60 • C above T β (for the present case it means of about 900 • C). Meanwhile, for similar alloys with almost the same amount of Nb, a ST above T β has been successfully applied with 950 • C as the heating temperature, followed by quenching [12,[54][55][56][57].…”

“…The reason is that the single β-phase facilitates the down-stream cold working operations [12], hence the metastable β-alloys were supplied in the ST-ed condition. For the microstructural analysis, it will be also useful to find the temperature for starting the martensitic transformation (Ms), and to calculate the Mo eq parameter, which represents a generally accepted parameter to characterize the stability of the β-phase for a disposed chemical composition [42,54].…”

“…This approach seems to be very effective considering that, first of all, SPD is a powerful reducer of the grain size until ultrafine or even nanometer dimension [39][40][41]; by this, the Young's modulus can strongly decrease with yield strength enhancing. Second, ST can keep the single βphase if applied in the super β-transus variant [42]. From this perspective, there are some encouraging reports, but on particular alloy compositions.…”