New α+β type titanium alloy with Ti-4.5Al-6Nb-2Mo-2Fe was developed
on the basis of using biocompatible elements and eliminating the cytotoxic ones such
as Vanadium, while achieving the desirable mechanical properties such as appropriate
strength, cold workability and low superplastic forming (SPF) temperature. The
present study was conducted to investigate the effect of yttrium addition of less than
0.05% into this alloy on static and under superplastic deformation grain growth
behavior. The new alloy bar manufactured by α+β processing and annealed at 1073K
yielded extremely fine two-phase microstructure with α grain size around 2μm.
Specimens were heated at temperatures of 1048, 1073 and 1098K and kept for times
between 3.6 to 172.8KS. Yttrium forms in-situ Y2O3 particles, and the presence of
these particles yield finer two phase microstructure due to their retardation effect on β
phase grain growth. Grain growth behavior during hot deformation was investigated
by hot compression test in use of a hot working simulator of THERMEC-Master Z.
Strain rate was varied from 2×10-2 to 2×10-4S-1 and strain was 0.69. Grain size of
both α and β phases increased with a reduction of strain rate, and Y2O3 particle was
also effective to retard grain growth under hot deformation. It was confirmed from
comparison of grain growth during isothermal heating with and without hot
deformation that grain growth was much accelerated by deformation. All of these
results were discussed based on grain growth mechanism or model for two-phase
microstructures as well as superplastic deformation mechanism.