Abstract
Even though TIMETAL-54M (Ti-5Al-4V-0.6Mo-0.4Fe or Ti54M) has been commercially available for over 10 years, further study of its superplastic properties is still required in order to assess its applicability within the aerospace industry as a potential replacement for other commercial titanium alloys such as Ti-6Al-4V (Ti64). Ti54M is expected to obtain superplastic characteristics at a lower temperature than Ti64 due to its lower beta-transus temperature. The superplastic forming (SPF) capability of alloys that can be formed at lower temperatures has always attracted the interest of industry as it reduces the grain growth and alpha-case formation, leading to longer life for costly high temperature resistant forming tools. In this work, the SPF characteristics of both Ti54M and Ti64 have been examined by conducting tensile tests according to the ASTM E2448 standard within a range of temperatures and strain values at a fixed strain rate of 1 × 10-4/S. A high strain rate sensitivity and uniform deformation at high strains are key indicators in selecting the optimum superplastic temperature. This was observed at 815˚C and 925˚C for Ti54M and Ti64 respectively. The tensile samples were water quenched to freeze their respective microstructure evolution following superplastic deformation and SEM images were captured for grain size and volume fraction of alpha-phase analyses. A slightly higher alpha-grain growth rate was observed during superplastic deformation of Ti64. The initial fine-grain microstructure of Ti54M (~1.6 micron) resulted in a final microstructure with an average grain size of ~3.4 micron and optimum the alpha/beta ratio. Both the fine-grained microstructure and increased amount of beta-volume fraction promotes the superplastic behaviour of Ti54M by grain boundary sliding (GBS). Thus superplastic properties were observed for Ti54M at a lower temperature (~100˚C) than for Ti64.
Highlights
The ability of a polycrystalline material to exhibit very high tensile elongations prior to failure is referred as superplasticity
The dominant deformation mechanism typically operating during superplastic deformation is Rachinger grain boundary sliding (GBS)
The positive gradient on the curve was evidence of the work hardening flow behaviour of the material. This behaviour was observed in previous studies of Ti54M that the material investigated previously had a different average grain size and volume fraction of primary α phase [7]
Summary
The ability of a polycrystalline material to exhibit very high tensile elongations prior to failure is referred as superplasticity. A Ti54M sheet with 1.6 mm thickness and an average grain size of ~1.5 μm was thermo-mechanically processed and its microstructural evolution was investigated during superplastic forming and compared to a Ti64 sheet with similar thickness, though with a larger initial grain size. Both the materials were supplied by Timet
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