Dynamic Spheroidization Mechanism and Its Orientation Dependence of Ti-6Al-2Mo-2V-1Fe Alloy during Subtransus Hot Deformation

Abstract

The dynamic spheroidization mechanism and its orientation dependence in Ti-6Al-2Mo-2V-1Fe alloys during subtransus hot deformation were studied in this work. For this purpose, hot compression tests were carried out at temperatures of 780–880 °C, with strain rates of 0.001–0.1 s−1. Based on SEM, EBSD and TEM characterization, the results showed that the aspect ratio of the α phase decreased with increasing deformation temperatures and decreasing strain rates. At 880 °C/0.001 s−1, the aspect ratio of the α phase was the smallest at 2.05. The proportion of HAGBs decreased with increasing temperatures and strain rates, which was different from the trend of the spheroidization; this indicated that the formation of HAGBs was not necessary for the spheroidization process. Furthermore, the formation of the α/α interface was related to the evolution of dislocations and twin boundaries at high (880 °C) and low temperatures (780 °C), respectively. Moreover, the dependence of lamellar spheroidization on the crystallographic orientation tilt from the compression direction (θ) was clarified: when θ was between 45° and 60°, both the prism <a> slip and basal <a> slip systems were activated together, which was more favorable for spheroidization. This study could provide guidance for titanium alloy process designs and microstructure regulation.

Effect of Initial Microstructure on the Temperature Dependence of the Flow Stress and Deformation Microstructure under Uniaxial Compression of Ti-407

“…As the temperature increases above 790 °C but below Tβt, the elongated and kinked α laths suffer a change in morphology to globular α grains with aspect ratios < 3. A similar transformation of the α phase towards an equiaxed morphology upon high-temperature deformation close to Tβt has also been reported for Ti-6Al-4V and Ti-6Al-2Mo-2V-1Fe, as a result of spheroidization [41][42][43]. Finally, for deformation at T > Tβt, the α phase observed after cooling to room temperature exhibits a lamellar morphology, regardless of the magnitude of the strain (ε = −0.8) applied during uniaxial compression testing.…”