Abstract
To grasp age hardening and phase transformation behaviors in a β rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, during aging at 300-500°C after the solution treatment at high α+β temperature. Vickers hardness and microstructure changes during aging were closely investigated using XDR and TEM/EDS. Vickers hardness rapidly increased with increasing holding time and reached about 440HV by aging at 450°C for only 5 min. It further increased to 510HV in 8h of aging time. The initial stage of age hardening is extremely fast compared to that in other conventional α+β and β type titanium alloys. After aging for only 5min, extremely fine acicular products of about 2 to 10 nm in width were formed in the transformed β phase. TEM/EDS analysis revealed that all substitutional alloying elements, Al, Fe and Mo, homogeneously distributed after the aging, indicating that the transformation is diffusionless as far as substitutional elements are concerned just like martensite transformation although it has time dependency. To explain the mechanism of this unique phase transformation having features of isothermal martensite transformation, we propose bainitic transformation where interstitials such as O diffuse without conspicuous diffusion of substitutional elements.
Highlights
Ti-5Al-2Fe-3Mo is a β rich α+β type titanium alloy and has been used for automotive engine parts such as intake valves by making use of its excellent properties such as high strength, high fatigue strength and good hot workability[1].Figure 1 is a schematic illustration of the phase diagram of titanium alloys, in which a horizontal axis is concentration of the β phase stabilizing elements [1,2]
The fine acicular products were formed in the retained β phase during aging at 450 ̊C Ti and substitutional elements such as Al, Fe and Mo homogeneously distributed according to the transmission electron microscopy (TEM)/EDS analysis
The transformed β phase in which oxygen is excessively contained as a solid solution is formed by solution treatment at α+β high temperature region followed by water quenching
Summary
Mechanical properties can be changed by solution treatment at high temperatures in the α+β region followed by rapid cooling. Specimens solution treated at 900 - 930 ̊C, which is just below the β transus (around 955 ̊C), followed by water quenching show extremely low 0.2% proof stress, quite high tensile strength over 1250MPa, and low Young’s modulus around 80GPa, equivalent to β titanium alloys[2]
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