Processing-microstructure relationships for Ti-6Al-2Sn-4Zr-2Mo-0.1Si

Abstract

The detailed relationships between thermal-mechanical processing parameters and resulting microstructures for Ti-6Al-2Sn-4Zr-2Mo-0.1 Si (Ti-6242) have been established through compression testing and heat treatment. Beginning with either an equiaxed alpha or Widmanstätten alpha preform microstructure, isothermal compression tests were run at strain rates typical of isothermal forging (10⁻³ to 10⁻¹ s⁻¹) and conventional forging (1 to 100 s⁻¹). Metallographic investigation of these test specimens in as-deformed and heat treated conditions was used to characterize deformation-induced microstructures and transformations. For the equiaxed alpha microstructure, it was shown that deformation, as well as post-deformation heat treatment, were more effective in promoting microstructures close to the expected equilibrium ones than heat treatment alone, a finding similar to that for other alloy systems. For the metastable Widmanstätten alpha microstructure, the deformation and heat treatment parameters that promote the development of an equilibrium, equiaxed alpha microstructure have been determined. For this microstructure, two separate temperature-strain rate regimes have been identified, and the resulting microstructures correlated with the measured flow stress behavior. For the low temperature regime, deformation is highly nonuniform, and the microstructural features are shown to be similar to those in pearlitic steels and other lamellar alloys. In the higher temperature regime, on the other hand, deformation is much more uniform. The results presented can be applied to select hot forging parameters for the control of final microstructure and properties in Ti-6242 and similarα/β titanium alloys.