Metastable Beta TitaniumAlloys: Tuning Their Beta Phase Stability and Low-Temperature Martensitic Trans- formation

This research focuses on the stability of three metastableβ titanium alloys – Ti-10V-1Fe-3Al (T i −1013), Ti-10V-2Cr-3Al (T i −102Cr 3) and Ti-10V-2Fe-3Al-B (T i −1023B). In the first stage of the work, new β+ (α+β) heat treatment schemes were designed on the basis of Thermo-Calc® simulations and previous research work and carried out to such an extent that the β phase stability is tuned to the right level and a sufficient amount of retained β phase is obtained at room temperature available for further transformation. At the second stage, the effectiveness of the thermal driving force in triggering a martensitic transformation in the retained β phase was investigated. Both in-situ and ex-situ transformations were studied via DSC and further quenching in self-made cooling baths, respectively. The starting temperatures of low-temperature martensitic transformations were determined for each alloy. It was found that the low-temperature martensitic transformation for the three alloys was of an isothermal character. Through XRD testing, the phases of T i −1013 before and after low-temperature martensitic transformation were determined; as in the commercial Ti-10V-2Fe-3Al (T i −1023) alloy, the type of martensite formed in response to quenching from the two-phase region isα′′ in stead ofα′ martensite, due to oversaturation of the martensite. EPMA testing proved the formation of Martensite-Free Zone (MFZ) observed in the case of T i −102Cr 3 when quenched to a temperature just below the starting temperature of the low-temperature martensitic transformation does not relate to the diffusion behavior of alloying elements. Electrical Discharge Machining (EDM) was found to be detrimental in this research as the machining process changed the designed microstructure of the three metastable β titanium alloys. Finally, a proposal is made to produce tailored α+β titanium metallic composite via SPS.